What if the universe doesn't expand at all? What if we're completely wrong and redshift is caused by something else entirely, like some yet-undiscovered phenomenon that occurs to spacetime or electromagnetic waves? How can we be so sure it's space that's expanding, not time?
The more I read about this, the more it feels like phlogiston theory[1]. Works great for describing observations at first, but as more observations are made, some contradict the theory, so exceptions are made for these cases (phlogiston must have negative mass sometimes/there must be extra matter or energy for galaxies to spin as fast as they do), and then finally someone discovers something (oxygen/???) that explains all observations much simpler and requires no weird exceptions.
> What if the universe doesn't expand at all?
Not possible. Redshift is not the only observation we have. The totality of all the observations we have cannot be explained in any other way than an expanding universe.
> How can we be so sure it's space that's expanding, not time?
Our best current model does not say "it's space that's expanding, not time". It says that in one particular frame (the comoving frame), the overall spacetime geometry can be described using a "time" that always corresponds to the time of comoving observers and a "space" whose scale factor increases with that time.
> The more I read about this, the more it feels like phlogiston theory
This is an extremely unjustified comparison. Phlogiston theory never accounted well for actual observations.
> as more observations are made, some contradict the theory
None of the observations being discussed contradict the general model of an expanding universe. They only pose problems for the indirect methods we use to convert our direct observations into model parameters.
Surely there are infinite other possible explanations that fit the finite number of data points available to us. Probably what you meant is that the expanding universe theory is the simplest of them all and creates less problems then others.
> Surely there are infinite other possible explanations
If you think there are others, please exhibit one.
> Probably what you meant is that the expanding universe theory is the simplest of them all and creates less problems then others.
There are no other theories that I'm aware of that account for all the data we have, even approximately.
If everyone like you attempts to rail road the imaginative process at the beginning of hypothesis formation, then we'll never get to the point of being able to exhibit one, should one be possible.
The demand for rigour at this point in a discourse - which was pretty clearly signalled by the commenter to be offered at a stage prior to substantive hypotheis formation - just shuts down the imaginative process. It's not constructive.
When a random techie comes up with a completely novel hypothesis that contradicts a broad range of theories accepted by the vast majority of practicing physicists, the proper response is not to stop and say "Hmmm. I wonder if he's right. Let's talk about it."
Why not? Seems perfectly normal to just talk about it.
I learn things all the time by wondering if somebody else is right. Much better than just thinking everything is simply the way I think it is now.
Even if you know somebody is wrong, talking about it is absolutely harmless.
The issue is that these physics threads always end up the same, with commenters having only popular-level background offering suggestions they came up in five minutes, mirroring obvious thoughts that actual physicists have of course already thought of decades ago, and in much more detail. It is really hard and quite unlikely to come up with novel ideas that haven't already been discussed and played out ad nauseum in that field.
I’m not suggesting the techie is correct, I just don’t think the right answer is complete dismissal instead of communication. Ok, so you know it’s obviously wrong, but there’s no obligation to then go and stifle their curiosity or imagination. Just don’t say anything or let them talk to somebody who has the time and care to indulge.
The original commenter I’m replying to, taken at their word, is ready to dismiss anything somebody says, regardless of merit with no further discussion, just because they think there’s “no way” that person could be right. Which is hilariously close-minded way to conduct oneself.
This is fine the first few times, but after a few dozens it gets exhausting. This is also not about stifling their curiosity or imagination. It's about understanding that in a field as advanced as physics and cosmology, how incredibly unlikely it is for some layman to come up with a worthwhile idea that hasn't already be tackled. To even be able to explain why an idea is impractical or beside the point, a solid knowledge and understanding of the field is often aready necessary. Articles like from Quanta Magazine dress the topics up in language that make them seem substantially simpler, and closer to human intuition, than they actually are.
Once we have open source AI training data and AI historians they could help make real science directly available instead of pop magazines.
Quanta Magazine generally write very well written, well researched and pretty comprehensive articles. They cite their sources and they're very careful to get science as correct as they possibly can (I'm a scientist, and they once contacted me to fact-check one of their articles).
Comparing their work to the dross that AI produces is insulting
Insulting? Sorry! I didn't mean to say "pop mag" in a bad way like "ai" in a bad way.
I'm actually excited for computers getting big enough to comprehend human language and science.
I hope one day Dross becomes Dos! AI needs more XP. VR AI and VR AI Science edu is what I'm most hyped for.
Sorry, all this twaddle is like a culture war ... it's noise. Take the cosmologists and put them out of mind.
Now, exhibit a testable hypothesis. Better, try to explain the 67 to 75ish mpc/s ranges in other methods.
Then we can talk. Nuff said!
I had the idea that each of these phenomena were being influenced by our local gravity well, in a different way. Then I remembered my physics. Then I read a few of their papers. This is not just good science, it's great science.
I withdraw my idea, but continue to wave lengths of wire 11.8 inches long.
I do wish I could read a website called cosmogony news, every day.
Dismissing uninformed ideas does everyone a service. If you're a complete outsider to a field and have no training it it, decide to come up with random ideas about difficult unsolved problems, and then feel stifled when an expert dismisses your ideas... well, that's a level of arrogance and hubris that I think is more than a little infuriating.
That is a hilariously uncharitable interpretation of what they said.
If you're not doing math, you're not doing physics.
Lookup what General Relativity actually is, what it looks like. The mass-energy tensor and the extremely complicated underlying partial differential equations it is actually encoding.[1]
Every parseable language explanation is irrelevant: the mathematics works. If you have an alternative idea...then the mathematics needs to work. What that means is irrelevant, provided it makes useful predictions and does not contradict established observations.
[1] https://en.m.wikipedia.org/wiki/Stress%E2%80%93energy_tensor
Has anyone ever tested the theory of matter that is repelled by gravity instead of attracted? They would zoom away and seperate like helium escaping the Earth.
What theory is that? How would it be tested? Gravity is not even a force, it is a consequence of how space deforms around mass&energy. So, even if something like "negative mass" existed, it wouldn't repel, it would just cancel out.
Every day.
This would be way cool. A way to rid science of randos and their theories. For the fourth time today, I have to admit being facetious.
Also
… is this really an issue?
I speculate you’ve taken something you don’t want to see personally and dressed it up as “the problem” when you could instead just find a way to be okay with it.
But maybe there’s something genuinely problematic with that behavior which I don’t know about.
If you've spent any amount on HN reading physics posts, you will see it is absolutely an issue.
I remember the halcyon room temp superconductor days.
Yes, it's an issue. I come here as a layperson who is interested in the universe; it's exhausting to read plausible-sounding but completely meritless theories brought up by people here with little more training than I have, and try to decide if these ideas are actually useful, or are just uninformed things some rando thought up in five minutes after reading a pop-sci cosmology book.
I'd much rather hear plausible theories made by people who actually know what they're talking about.
They will eventually die and a younger generation will enter the field who don't know why those ideas were dismissed. For all you know you're shutting down a 14-year-old (either directly or someone observing) who is actually interested and may become one of those physicists.
It's roughly equivalent to https://xkcd.com/1053/
No, the younger generations are constantly entering the field, slowly learning and building up the tools required to think about these sorts of things. By the time they've done that, they don't need to have their silly ideas shut down, because they do that themselves, using the knowledge they've built.
And when they do have novel ideas, they have the mathematical and scientific tools to actually argue why their novel ideas deserve expensive, scarce telescope time, unlike the armchair pop-sci wannabe cosmologists (myself included) on HN.
Because it's noise, and noise is distracting, and there's a LOT of noise out there.
Part of being an expert is knowing how to filter out the noise so that you can actually get some work done.
If one wanted to deal with noise all day, they'd join SETI. Or parliment.
If someone has a novel theory, let them come up with evidence to support it, and clear identification of how the theory can be invalidated.
That's not closed-mindedness; that's pragmatism. Could they actually be right? Yes, in the same way that a baby could beat Muhammad Ali - but you won't see anyone lining up to buy tickets.
You only have so much time in your life, so no need to waste it on peoples' 10-second "theories", like "How about achieving faster-than-light communication by stuffing so many photons down the fibreoptic cable that they "push" each other faster?". Some ideas are just plain dumb and obviously not worth a trained person's time.
On the one hand, I agree that those people are usually wrong and generally pretty annoying.
On the other hand, who cares? This is a random internet forum, not the Proceedings of the National Academy of Sciences, so maybe there is no such thing as a proper response?
What are you busy with? Are there any particular other subjects you would want to talk about instead?
Thanks, but I don't think it's a fair description of what happened here. I'm a mathematician who noticed that the statement "The totality of all the observations we have cannot be explained in any other way" is obviously false.
Explaining is neither hard nor useful and it's not what science is normally concerned with. The goal is to predict new observations not to explain known ones.
There's a difference between being closed-minded and saying "yes, we've obviously thought about this thing that you, someone with no apparent background in our field, thought of in ten seconds". And if you're an expert in any field that gets a lot of people who are interested, but not a lot of people who are experts, you hear these kinds of half-baked theories all the time, often with this exact "oh you orthodox experts just can't handle my field-disrupting free-thinking!" kind of framing.
I'm a mathematician by education, and I cannot tell you how many people insist on things like 0.999... < 1 without an understanding of (a) what the left side of that expression even means, (b) what a real number is, or (c) what basic properties the real numbers have. Going "no, you're wrong, and it would take me a couple of full lectures to explain why but trust me we're pretty sure about this" is a reasonable answer to that, provided that you have indeed established that to your own satisfaction, at least.
Whoa! What? Not a mathematician in any way (in case that isn't obvious), but I'd have totally thought 0.999... asymptomatically _approaches_ 1, but never reaches it, and so is <1. Is there a short-form explanation (that I might have a chance of understanding, lol) of why that's incorrect? I'd love to have my mind blown.
Sequences can approach things. The sequence 0.9, 0.99, 0.999, 0.9999 and so on asymptotically approaches 1. The difference between 1 and the Nth term in the sequence is 1e-N, which goes to 0 with N.
0.999...[forever] is not a sequence, it is a number. Numbers have values, they don't approach things. The misleading part is that 'forever' is not something about evolution or the passage of time. It's not 'happening' or 'sequential' like the sequence. There is no 'and then another 9'. All the 9s are really there, at once. And it is closer to 1 than any term in the sequence. Since the sequence gets closer and closer to 1, converging to it asymptotically, 0.999...[forever] cannot differ from 1; if it did the sequence wouldn't converge.
Thank you, and everyone else who answered (I hope they see this reply). Your distinction between "sequence" and "number", along with the mathematics of 0.333... = 1/3, convinced me - and my mind is successfully blown.
Follow-up: is it the same for other repeating sequence-looking numbers? As in, would 0.9333... = 0.94?
0.9333... is equal to 9/10 + 1/30. To get 9/10 + 4/100 I think what you're aiming at is 0.93999...
Its true of numbers whose with a decimal representation which ends in an infinite string of 9s, so 0.939999... = 0.94. This is because we write numbers in base 10. If you write numbers in base 2 its equal to numbers whose binary representation ends with an infinte number of 1s e.g. 0.11111... (base 2) = 1.
There are a lot of proofs of this, but they all rely on a certain level of rigor about what a real number is - and that, it turns out, is a much more difficult question than it sounds like. You don't typically get a rigorous definition of the real numbers until well into a college-level math education.
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First, you're making a category error. "0.9999..." is a single value, not the sequence of values 0.9, 0.99, 0.999, 0.9999... Single values cannot "asymptotically approach" anything, any more than the value 2 or the value 7 can asymptotically approach anything. It's just a number like any other.
To show what value 0.9999... takes on, we need to do two things. First, we need to show that this notation makes sense as a description of a real number in the first place, and second, we need to show what that real number is (and it will happen to be 1).
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So, why is it a real number?
Well, remember what we mean by place value. 0.9 means "0 ones, 9 tenths[, and zero hundredths, thousandths, and so on]". 0.99 means "0 ones, 9 tenths, 9 hundredths, [and zero of everything else]". Another way to say this is that 0.9 is the value 0 * 1 + 9 * 0.1 [plus 0 times 0.01, 0.001, and so on], and that 0.99 is the value 0 * 1 + 9 * 0.1 + 9 * 0.01 + [0 of everything else].
What that means is that if 0.9999... means anything, it means 9 tenths, plus 9 hundredths, plus 9 thousandths, plus 9 ten-thousandths, plus 9 hundred-thousandths, and so on and so forth forever. In other words, 0.9999... is the value of an infinite sum: .9 + .09 + .009 + .0009 + ...
Infinite sums, in turn, are by definition the limit of a sequence. This is where that "asymptotic" thing comes back, but notice the distinction. 0.9999... is not the sequence, it is the LIMIT OF the sequence, which has a single value.
To show that it's a real number, then, we need to show that the limit of the sequence 0.9, 0.99, 0.999, 0.9999... does in fact exist. But this sequence is clearly increasing, and it is clearly not greater than 1, so we can (among other things) invoke the Monotone Convergence Theorem [1] to show that it must converge (i.e., the limit exists). Alternately, you can think back to your algebra 2 or calculus classes, and notice that this is the geometric series [2] given by sum 9 * 10^-n, and this series converges.
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Now, why is it equal to 1?
Well, there's a few ways to prove that, too. But the simplest, in my book, is this: given any two different real numbers x and y, I'm sure you would agree that there is a value z in between them (this is not a difficult thing to prove rigorously, provided you've done the hard work of defining the real numbers in the first place). The average of x and y will do. But we can flip that statement around: if there is NOT a value between two real numbers, those two real numbers MUST be equal.
In more symbolic terms, we claim that for all real numbers x and y such that x < y, there exists z such that x < z < y. So if there ISN'T such a z, then we must not have x < y in the first place. (This is the contrapositive [3], if you're not up on your formal logic.)
So consider the values of 0.9999... and 1. What value lies between them? Can you find one? As it turns out, no such value exists. If you pick any real number less than 1, your 0.99[some finite number of nines]9 sequence will eventually be bigger than it - and therefore, since the sequence is increasing, its limit must be bigger than that value too.
Since there are no numbers between 0.9999... and 1, they must be equal.
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[1] https://en.wikipedia.org/wiki/Monotone_convergence_theorem [2] https://en.wikipedia.org/wiki/Geometric_series [3] https://en.wikipedia.org/wiki/Contraposition
This is my (new) favorite answer. You've made something counter-intuitive seem simple and obvious. Fantastic!
(I've never done this before, but: a pox on those down-voting my original question. Learning is the very essence of "hacker"-dom. Thank you to all who have seen this and taken their time to teach me something.)
This needs to be repeated time and time again for people who deny the basic tools of Calculus, and will suffer the misuse of them. ( specifically the sum of the sequence of the reciprocals of the natural numbers is equal to 1/12. I get that it is a useful tool for quantum chromodynamics, but it makes my skin crawl. )
0.xxx... is just a notation for certain fractions (specifically, the fraction x/9). If we set x = 9, then the notation 0.9999... is just a notation for 9/9 = 1. So it's just a silly notation for 1.
This actually makes the most sense to me. It’s an artifact of our chosen numerical notation system.
The simplest argument I can think of is to ask yourself: "Are there any numbers between 0.999... and 1?"
If not then it's logical to conclude they wind up at the same "place", that is, the same number. Or equivalently: it can't have any other value besides 1.
If you care about such things, then you're a mathematician.
It is a limit. A very powerful tool.
You can choose any arbitrary finite number in the sequence, and I can find a number greater. So the value of 1 exists, and its a continuous function, so therefore the limit exists.
I do care about such things, and yes, a degree in math for me was just slogging through every math course the college had, with a long string of high grades, and honor roll achievements.
I think the simple one is x=0.999..., 10x=9.9999..., 10x-x=9.999...-0.999..., 9x=9, x=1
There's a rigorous proof on Wikipedia, but there's simpler ways to show it.
For example, we know that 1/3 = 0.333...
3 * 1/3 = 3 * 0.333...
1 = 0.999...
You can also do it with subtraction. For example, 1 - 0.999... = x. Assuming x is greater than 0, then it should evaluate to 0.000...1.
But we can't have the digit 1 after an infinite number of zeros. If there truly were a "1" after infinite zeros, it implies reaching the end of infinity, which is a logical contradiction. So x can't be greater than 0.
1/3 + 1/3 + 1/3 = 3/3 = 1
.3… + .3… + .3… = .9… = 1
https://en.m.wikipedia.org/wiki/0.999...
An alternative to the sibling comments:
In this context, the notation 0.999... does not represent a process. It represents a fixed number.
Which number? Well, if you reason through it you'll find that it has to be the same number as that represented by the notation 1.
An insight that is crucial (and pretty obvious in hindsight, though many people seem not to be exposed to it) is to distinguish between a number and its representations. "1" is not a number, it is merely the representation of a number. And numbers have many different representations. As a member of this forum, you can probably appreciate that "12" and "0xC" are two different representations of the same number. In the same way, 0.999... and 1 are different representations of the same number.
It's just that 0.999... is an awful notation, in the sense that it invites people to complete these allusive ellipsis with whatever fit their intuition, possibly even different meaning depending on the context.
If we mean 9×10^-i for i from 1 to infinity, then let's just state so. Let's not blame people to interpret towards other direction when they are provided misguiding hints.
Regarding infinity, there is a constructive proof that it doesn't exists which work perfectly provided that there is an infinite amount of resources allocated to it's execution.
I don't blame people for finding it counterintuitive. Lots of things in math are counterintuitive. I spent like three months learning Galois theory and I'm still pretty sure someone just snuck in there and changed a 1 to a 0 to make that sorcery work.
My point is that it's not closed-minded of me if I fail to provide a complete answer to someone making such a claim, particularly if that person hasn't done any of the research or rigor to handle what is - by the standards of an expert - a pretty easy question to answer. Outsiders can occasionally result in great insights, but they do that through very hard work, not from ten seconds of thinking about a field they haven't learned anything but the pop-science version of.
Most of the theories being speculated about in this thread are veering into "not even wrong" territory, in that they're not even necessarily well-defined. When you're talking about cosmology you'd better bring your general relativity game, which means you better bring your differential geometry game, which means you better have a graduate-level mathematics education. I have a graduate-level mathematics education and on a good day I could half explain to you what a metric tensor is and what the hell it has to do with curvature ("it's, uh, it's kinda like a Jacobian I guess, except the dx and dy are local vectors that can't be translated globally around the space").
Without those tools, you don't even have meaningful notions of what "distance" even is on a cosmological scale, much less how it changes with time! It's like speculating about biology without knowing what a protein is, or speculating about computer science without knowing what a for loop is. It's just not going to get you anywhere.
Like everywhere else in the intertubes, people think their experience domain is relative to every other specialty domain. We live in a conspiracy world now, where RTFM or "do the work" is a micro aggression.
https://en.m.wikipedia.org/wiki/0.999...
From Wikipedia, an intuitive explanation of an elementary proof:
And then:
Sure, but we are talking about a physical theory here not a mathematical one. There are always alternative physical theories, infinitely many. They may not be good theories, but they clearly exist. This has nothing to do with cosmology but more is a fundamental principle of logic.
To put another way, there is a big difference between saying some specific alternative theory is wrong/unlikely/bad, and claiming there exists no alternative theories at all regardless of quality.
BS, nobody has to listen to your imaginative process. Imagine away, build something that conforms with the data, then show it !
For now, we dont know any other way to explain than to say it expands, except maybe imaginative fantasies from amateurs on Hackernews, but does it count ?
What if it expanded from anti-mass? Couldn't we form testable hypotheses from that?
I looked it up on the Wikipedia page for Exotic Matter and there isn't much exploration or tested theories on it.
You certainly could try. But don’t expect experts in the field to bother to do any of the legwork for you, and that includes learning about all the evidence that your model has to account for to be remotely as good as our existing ones.
It's exciting to me just even being able to ask experts if anyone has tried before! progress = legwork + new direction!
No one prevents new ideas from being presented, but simply suggesting the universe does not expand without giving any arguments for this position nor trying to explain the observed red shifts, contributes exactly nothing to the discourse. This, is not constructive.
It seems pretty silly to think that we are collectively “at the beginning of hypothesis formation” about the structure of the universe today, in 2024.
But the expansion of the universe has been thoroughly studied for over a century. We're past the brainstorming phase.
I generally think people should brainstorm to generate ideas and then filter them down. And it's true that filtering too early can significantly decrease the quality of ideas.
And it's also true that in a place like Hacker News there will be smart people from all sorts of backgrounds getting to experience the joy of exploring a new topic that they're not fully up to speed on yet.
The risk though is that somebody who thoroughly understands the field is reading your comment. So for that reason I think it's a good practice to always be aware that technical fields we're not expert in are usually more subtle than we initially think.
Benevolent giant omnitech space squid manipulate EM radiation incident on the solar system to fool our primitive meat brains and sand computers into thinking we're alone in the universe so we don't go venture our and embarrass our local cluster.
See? There exist infinite theories explaining any set of data points. Parsimony, choosing the simplest theory that explains the facts, is what lets us make progress understanding and predicting the world around us. It's hard to predict space squid.
Thats like saying cars run because they want to, and only make us perfect the engine, and processes that happen inside it, to fool us into thinking we have a say in whether a car will run.
Sure, it is a hypothesis, but thanks to the Scientific Method the majority of people with knowledge in the field knows its most likely BS.
Going the same "it can be for ANY reason, guys!" in any field, will not get you much far, regardless if you feel justified in your ignorance or not.
The funny thing is, though, you can easily dismiss a crazy idea like "omnitech space squid", but a majority of humans on Earth today have entire worldviews based on equally silly ideas, and it's considered wrong somehow to mock these belief systems. In fact, many scientists subscribe to these silly belief systems, but then get offended if you make up an equally silly idea about space squid or flying spaghetti monsters.
Of course not. The point is logical correctness: as a matter of logic, infinite theories explain any set of facts. Are almost all of these theories useless? Of course. We should restrict our attention to plausible theories.
But how do we decide which theories are plausible? We look for the ones that require the fewest assumptions.
Try that with computer technology. You are now not just scary, but really really scary.
"That is like saying computers run because they want to, and only make us perfect the CPU, and secret processes inside it, to fool us into thinking we have a say in whether a computer will run."
I am going to quietly turn off my computer.
I am just thinking about Steven Wolfram's causal networks.
It’s not a scientific theory though. Which is a super important distinction in this discussion
Whence cometh the squid? By what mechanism does it manipulate EM radiation? And what experiment could be devised to detect the squid?
That's not a theory, nothing has been explained, merely further convoluted.
I could easily do that (god tinkering with our measurements? we live in a simulation?) but explanations by themselves are worthless. Any set of facts can be "explained" but it doesn't really help.
The science is concerned with theories that don't just explain known facts and measurements but also predict new ones. These are also known as falsifiable theories because a theory is falsifiable IFF it predicts at least one new observation that can be actually made and tested.
Now even these can obviously be constructed ad infinitum. For your question of alternatives to expansion, take a theory from the same field (e.g. tired light), then take facts it doesn't fit and make specific ad-hoc carve-outs for these facts in the theory. Sure it makes it ugly and complex but it remains a scientific falsifiable theory. Furthermore this exact things happens very often in the scientific community because theories don't fall immediately when the first contradiction is found. They only fall when a better theory appears to supersede them and until then they just develop a "protective belt of ad-hoc assumptions" as Lakatos called it. No need to mention that the same thing will happen to the "better theory" in due time.
I write all of this not because I have a good cosmological theory sitting in my closet but rather because your statement "The totality of all the observations we have cannot be explained in any other way than an expanding universe" is outright false. As I show above it not only can be explained in an infinite number of ways, but you can also construct and infinite number of scientific theories that fit the totality of observations. Because this "totality" is alas finite.
This reminds me of my school math teacher who once told me that "the sequence 1, 2, 6, 24, 120, 720 cannot be explained other way than by n!". A pity I didn't know about Lagrange Polynomials at that age and had to spend the whole evening to construct a fitting polynomial by hand.
A big problem of the modern cosmology is not only that we have observations that do not fit models but that we do not know if the observed discrepancies are the real problems with the models or are artifacts of calculations.
For example, to simulate a galaxy one should use a model based on General Relativity. But mathematics behind GR is too complex to allow simulations on the scale of the galaxy even with all that modern computation power. So instead of GR the calculations use Newtonian mechanics with minimal corrections for the light speed limit. Plus there are a lot of other simplifications like replacing star systems with hard balls that reflects when hit each other with no notion of matter transfer or ejection in the process.
Then we see that the simulation does not fit data. A typical explanation that is used is that of dark matter hypothesis. But this is unjustified.
We have no proof that numerical simplifications are mathematically valid and do not lead to a big error on a galactic scale. Moreover, there were recent papers that tried to account at least for some effect of General Relativity. Apparently it was enough to explain at least some effects previously attributed to the dark matter.
So it can be that the dark matter is just an artifact of inaccurate simulations.
Do you maybe have links for the papers you mention?
See references on Wikipedia page https://en.m.wikipedia.org/wiki/Galaxy_rotation_curve under “alternatives to dark matter”.
Not a cosmologist, so I'll defer to any simulators who are more up-to-date with the field than I am, but the papers linked from wikipedia are about the fitting of models to existing data, and by using a more complete model, they can better constrain values in the models; whereas the massive 3D simulations are looking at a different problem, and go through a rigorous level of validation and cross-checking, with different microphysics tested and examined. Both dark matter and dark energy fall out of GR—they can both be zero, but they can also be non-zero.
There are a fair number of extensions to GR that can reproduce the standard cosmology, typically avoiding or recasting DM, DE, or both. Examples include generalized teleparallel gravity and Cotton gravity. In such theories, any solution of the EFEs are solutions of the (sometimes very different) field equations of these families of theories, although the field content may have a somewhat different physical interpretation from GR.
However, generically, these extensions tend to have an under-determination problem frustrating attempts to arrive at a unique spacetime for a given distribution of matter or a unique distribution of matter which can exist in a given spacetime (or both problems). That makes them less attractive than GR, or even possibly outright unsuitable bases for initial-values formulations (and thus are unlikely to overthrow numerical relativity soon).
One can construct such alternative theories quite easily: Everything is exactly as the "expanding universe theory" predicts except in a phone booth sized volume in a specific part of space 10 lightyears away from Earth where the universe is contracting.
Does not explain anything, and it is not testable in any practical sense. So it is not a good theory in any way, but it is a different one and it matches all the current observations.
One easy process for generating infinite explanations that fit a finite number of data points is taking the simplest theory you have, and adding extra rules that don't affect any of the existing data points.
e.g., if the standard explanation for observations like red shift, CMB, the abundance of light elements, etc. is H² = (ȧ/a)² = (8πG/3)ρ - kc²/a² + Λc²/3, One alternate explanation that fits all the data is H² = (ȧ/a)² = (8πG/3)ρ - kc²/a² + Λc²/3 + T, where T is the number of teacups in the asteroid belt. No observation has yet been made which would falsify this theory, and it fits the data just as well as the standard explanation. We reject it on the grounds of parsimony, not falsification.
Well..? What are those other observations that point to expansion?
> What are those other observations that point to expansion?
The apparent brightness and apparent angular size of distant galaxies, and more importantly, the relationship between the three observables of redshift, apparent brightness, and apparent angular size. No known model other than the expanding universe predicts the actually measured relationship between those three observations.
That’s just redshift. Redshift alone wouldn’t be evidence of expansion, just relative speed, when people say redshift evidence they mean the relationship between redshift and brightness of a standard candle. And regardless of whether you call it redshift or the relationship between redshift and something else, it would be impacted by a change to redshift.
Its not just redshift. If you look at very distant galaxies you see their apparent angular size is larger than you expect.
https://en.wikipedia.org/wiki/Angular_diameter_distance
You know how distant objects appear smaller, well in an expanding universe that isn't completely true, very distant objects start looking bigger again. Roughly speaking this happens because the distance between paths different photons take to get to us gets stretched by the expansion.
Theres also the very obvious observation of the cosmic microwave background, which isn't explained by any non-expanding universe model.
To expand on the CMBR bit here for readers who aren't as familiar with cosmology: the current temperature of the CMBR is a result of redshift, but that's not what establishes the need for expansion. No, the problem is that the CMBR seems to be in thermal equilibrium (or more properly to have been in thermal equilibrium at the time of its emission a few hundred thousand years after the Big Bang).
The problem with that is that, if the Universe had remained the same size and has the finite age it appears to have, there would not have been time for information from the "north" side of our observable Universe to reach the "south" side. After all, the "north" side's light is only just now reaching us, at the center of our observable Universe, and would have to travel a very long way again to reach the south side. In cosmological/relativity terms, we say they're not causally connected.
The obvious explanation for this is that the early Universe expanded from a very small region that was causally connected for long enough to reach thermal equilibrium, and then expanded. So while the "north" side of the observable Universe and the "south" side of the observable Universe cannot communicate from their current positions (and in an expanding Universe will never be able to do so again), they were able to communicate in the past for long enough to establish equilibrium.
Without expansion, you need a way for two patches of sky that have never been able to communicate with one another to somehow "agree" to be the same temperature. And that's pretty hard to do.
Couldn't this be explained by suggesting that the speed of light was faster in the past?
"Couldn't this be explained by suggesting that the speed of light was faster in the past?"
I wanted to address this in a few ways: 1. The speed of light is absolutely invariant,
however the space it travels through, can not only be varient, but it can be varient in ways that we are failing to understand In two ways, one way is that we are reaching to understand it, and the other is that we may never understand it.
Is the speed of atoms invarient? I.e.is the temperature invarient?
Occams razor is one of the most powerful tools along with the belief in the both the elegance of the universe, and it's nature to make things very tricky to uncover that elegance.
There are some very deep thinkers here.
I don't know if it's an accurate description, but I found this passage from Wikipedia intuitive:
> Because the universe is expanding, a pair of distant objects that are now distant from each other were closer to each other at earlier times. Because the speed of light is finite, the light reaching us from this pair of objects must have left them long ago when they were nearer to one another and spanned a larger angle on the sky.
> That’s just redshift.
No, it isn't. I explicitly described two other direct observations that are not redshift.
> when people say redshift evidence they mean the relationship between redshift and brightness of a standard candle.
No, they don't. Redshifts of distant objects are directly observed. We don't need a "standard candle" to measure them.
Observations of apparent brightness are used to estimate distances to objects by comparing apparent brightness to the absolute brightness of a "standard candle" that is the same kind of object. However, such distance estimates are model-dependent; before they can even be made, the model parameters first have to be estimated using the observed relationship between redshift, apparent brightness, and apparent angular size.
> And regardless of whether you call it redshift or the relationship between redshift and something else, it would be impacted by a change to redshift.
I have no idea what you mean here.
Sit Fredrick Hoyles wimper theory and Joe Halddmens sawtooth theory.
It’s not just phlogiston, it’s the lifecycle of all scientific theories that they’re used for as long as they make accurate predictions, then we start seeing things they mis-predict, then they’re revised or replaced. You seem to think the expanding universe theory can still be saved by some data artifact or parameter tweaking, but that’s been hunted for years and we’re still at “we just can’t make it match everything we’re seeing”. Historically, that’s what precedes significant revision or replacement.
What else is there?
> it’s the lifecycle of all scientific theories that they’re used for as long as they make accurate predictions, then we start seeing things they mis-predict, then they’re revised or replaced.
No, that's not what always happens with scientific theories. For example, Newtonian mechanics is still used, even though we now know it's not exactly correct; it's an approximation to relativity that works reasonably well for weak gravitational fields and relative speeds that are small compared to the speed of light.
The "mechanics" that Newtonian theory (and its predecessors, Galilean mechanics and Kepler's model of the solar system) replaced were indeed replaced--nobody uses Aristotelian physics or Ptolemaic cosmology any more, not even as approximations. But that does not always happen.
Galileo’s principle of relativity is used all the time.
Yeah, about that... A not-insignificant number of people would have a hard time explaining why the moon has phases, let alone be able to do something like explain the sidereal day.
The relatively CMB, which points to a past when the universe was more homogenous than it could/should be given SoL limitations on communication between distant locations. The only answer is that those locations were at some point in the past much closer together.
If someone has a theory that incorporates "the totality of all observations" then physics is over. Redshift explains most observations, no other concept even comes close, but there are certainly things out there that remain unknown that are not explained by redshift. Dark energy is such a monumental observation that every theory in cosmology must remain caveated.
Isn't it the case that we don't actually know if the universe is expanding, we only know that from our POV things are moving away from us and from each other, based on models and observations that are approximations at best?
In that frame an expanding universe seems to be the simplest and more elegant solution, but it's entirely possible it's not the correct one.
for example: what if, on the antipodes of the universe (assuming something like that exists), things appear to move closer to the relative POV? we'll never know
I agree with you on the overall point, but this statement.
In all honesty, Cosmology rests on the principal that physics is the same in all directions, over all translations, and over time translation. While this is a good assumption (good luck testing alternatives!!). There are a variety of papers exploring the topic of how much these assumptions would need to be violated to mirror observations.
A good example being
what if the electron was more massive in the past?
All Redshift would then be explained away ;)
P.S.
There are very good reasons to believe that the electron was not more massive in the past.
We can create and observe doppler shift by making things move towards/away from us. Thus it is proven that if something is moving away from us, it will produce a redshift. In the absence of evidence that something else is causing the redshift, the assumption should be that it is a result of things moving away from us.
As an obvious example, doppler shift often needs to be accounted for to communicate with spacecraft.
X causes Y does not mean that Y implies X. It’s reasonable to suspect X given Y and an absence of other such causal relations, but it’s not necessarily reasonable to spend decades building layers and layers of models that assume X at the most basic level.
Everyone knows this.
But without looking at the direct rules of the system, this is the best you can do.
It’s not like you can just open the source code of the universe. You observe and make a theory that explains the observations, then the theory holds at least until a new observation contradicts the theory.
Is the current theory wrong? Maybe. But everything can be wrong and the world is always welcome to hear a new theory that completely explains all current observations.
But to just say a theory is wrong without providing a completely explained new one adds nothing.
Everybody knows it, but the principle is selectively applied.
For instance, our observations imply both general relativity and quantum field theory are necessary to model various aspects of the world. That’s an example of a Y. The only known X that’s ever been discovered that can encompass all aspects of that Y at all energy levels is string theory. Yet we are rightly careful to assume string theory is correct and enshrine it into the core body of scientific consensus. That does not mean we cannot or should not investigate it or even theory build on top of it, but it does mean we should refrain from assuming it must be true just because nobody can find anything better.
The vast majority of science is not disproving a theory but adding nuance to it. Newtonian physics wasn't disproven by quantum physics, but quantum physics showed that Newtonian physics has limitations as a model. It's not unreasonable to assume our best model is true until there is a reasonable amount of data to the contrary.
As others have said, your point adds little to the conversation until you bring good data to the argument.
I don’t accept that I owe anyone any kind of additional data on any of this. Just like I don’t have to have a proven solution to the problem of finding a theory of everything to suggest that string theory may not be the truth of the universe while still acknowledging that it can be a worthwhile thread of inquiry and study.
Suggesting that we avoid possible dogma has intrinsic value. Let us step back and consider the fact that the combination of general relativity and the Standard Model already cannot explain our most basic cosmological observations. We cannot explain the stability of even our own galaxy based on current models. This situation clearly calls for some basic caution before we enshrine possible unproven explanations into humanity’s view of the universe. There’s a lot of evidence, both long established and newly growing, which shows that our models can’t consistently explain many of the basic things we see around us when we look up at the sky.
If you are trying to make a scientific argument, you should bring data since that's the cornerstone of science. You seem to be implying that people treat these models as gospel. I suspect most of those who are deep enough in the field understand they are models and respect the limitations. To that end, it's not dogma. Saying "this is the best model we currently have" is not the same as dogma. The article is specifically about using data to either support or reject a model so I don't know where you get the idea that anything is being "enshrined" and above reproach. Ironically, saying you don't need to bring data to support your point pushes your position closer to dogma.
Your original point says very little. Yes, science acknowledges that you can never 100% say "X causes Y". Science is about getting closer and closer to that 100% with better models and better data while acknowledging it's impossible to get there completely. That's why people are saying your point is a nothing-burger. It's stating the obvious based on a strawman position.
Strong reactions like yours to what should be a very mild and uncontroversal statement that you evidently don’t even disagree with are exactly why these things are increasingly viewed by many as having elements of dogma.
What I wrote needed to be said, despite evidently containing very little interesting content, precisely because of how severely it provokes certain people who claim not to even disagree with it. The degree of the provocation proves the value of the statement.
What makes you think I have a strong reaction?
There are apparently plenty of people who disagree (myself included) based on the comments. I think the reaction you're getting is because it's not a particularly fruitful comment because it adds nothing to the conversation, while being veiled as a profound statement.
Except the response isn't a response to the claim, it's in response to the absence of one. If a researcher publishes some incomprehensible word-salad and lots of people write to the editor saying it's a worthless article, it doesn't somehow translate value to the original work. I think what you're experiencing is people being protective of HN in terms of having meaningful debate and what you said isn't particularly meaningful despite the wordsmithing.
As of the writing of this comment, every single comment I wrote here was upvoted. So, no. I can’t control the future, but what you wrote here was absolutely false when you wrote it.
And that’s great — that means it’s thought provoking enough to generate debate with complex views on both sides. In other words, yet again proving the value of the statement.
If internet points are how you’re measuring the validity of a point, there’s all kinds of things going wrong.
Again, people pushing back doesn’t mean you’re fostering fruitful debate anymore than flat-earthers are generating debate “with complex views.”
You are the one who brought up reception here, not me. I merely responded. So if you don’t like it, don’t choose it yourself first. All that happened is you selected a metric that you thought was favorable to yourself, but you miscalculated and are now claiming the metric was never even valid.
Please point to where I first used forum upvotes as a useful metric.
You used reception here to argue against me, going so far as to claim that people are “protecting HN” from my comment. Given that, I pointed out another form of measuring said reception. Suddenly, you were up in arms about using reception and continue to protest it.
This is a great heuristic to practice often. I just told my wife she looks fat in her new dress, and the degree of her provocation proves the value of my statement.
I’d expect a bit more nuance on Hacker News. What I wrote is not provocative by being offensive or hurtful to anyone. It’s provocative by introducing an idea that intelligent, educated people cannot come to a consensus on in a purely intellectual way.
What you’re missing is there is not a debate about your point. The debate is about whether it adds anything to the discussion.
Saying “the sky is up” and having people respond by telling you that is a trivial point shouldn’t be conflated with fostering a meaningful dialog.
And yet you yourself have been passionately engaged at length in the very debate you say doesn’t exist. The evidence abounds here from your own comments that it is not purely about the triviality of my comment. In fact, your first objection was about whether you thought I owed you additional data before I’m allowed to make the statement that I made. Moreover, there are plenty of other comments engaging in discussion and debate that clearly go beyond a mere discussion of triviality but rather actually engage with the concept.
That's fair. I'm trying to point out that the degree to which your statements illicit an emotional response from someone, by any means, has no bearing on the validity of that statement.
And I acknowledged that by providing additional clarification.
The point of science isn't to punk the researchers. So no, what you wrote didn't need to be said.
As I (and others) have repeatedly pointed out, our models are wrong. We know they are wrong. What they are is less wrong than previous models. That doesn't make them "right" or "dogma." Rather it makes them the model that currently provides the best explanation for observed reality.
That neither requires or suggests that research/investigation into modifications of our current models and/or into completely different models is unseemly or inappropriate.
What I (and presumably others, as they've expressed similar thoughts) require, if you want me to accept modified/brand new theories/models is, at a minimum, a logic-based argument as to why a modified/new model describes the universe more completely/accurately than current models. Assuming you can convince me that it's plausible, the next step is to present observational data that supports your logically argued hypothesis -- and that such data is described by your model/theory more completely/accurately than other models. I.e., that your theory/model is less wrong than our extant models which are also wrong, but less wrong than previous models/theories.
And if you can't present such data (e.g., with M-Theory[0]), then it's not science, it's just math, philosophy and/or metaphysics.
That's not to say math/philosophy/metaphysics aren't useful. They absolutely are. However, without data (or the means to collect such data), it's impossible to falsify[1] such hypotheses and, as such, aren't science.
[0] https://en.wikipedia.org/wiki/M-theory
[1] https://en.wikipedia.org/wiki/Falsifiability
He is making an epistemic argument, and epistemology is a part of proper science, though not of scientism, which is what you are bringing.
Binary is not the only form of logic available, but it is the most popular in discussions of the unknown.
You are literally mixing up subjective and objective.
Careful though: science also does the opposite. Do you know why? Because science is composed of scientists, and scientists are Humans, and Humans are famously unable to distinguish between facts and their opinion of what is a fact. In fact, doing so is almost always inappropriate, and socially punished.
It may intend to aspire to that, but what it is, is what it is. And what that is, comprehensively, is unknown, because it is unknowable. But we do know portions of what it is: there's the part you said, but there is also deceit, hyperbole, delusion, etc...again, because it is composed of Humans, and this is how Humans are. In my experience, all Humans oppose extreme correctness, I have never met a single one who does not.
The fact that your comment is downvoted speaks volumes to the close-mindedness of your critics.
[delayed]
https://en.wikipedia.org/wiki/Epistemology
I don't think anyone is claiming that science isn't biased because it's conducted by humans. Just like I don't think anyone is really claiming that the OP is incorrect in their statement. The comments I've read are merely pointing out "X causes Y does not mean that Y implies X" is a given in the context of a scientific discussion. It reads as if you and the OP are getting wrapped around the axle by treating science as an outcome rather than a process and, in doing so, fighting a claim that was never made, and one where the counterclaim is generally well understood in the scientific community. So well understood that it doesn't really need to be said.
> The only known X that’s ever been discovered that can encompass all aspects of that Y at all energy levels is string theory.
This is not correct; string theory is not the only candidate we have for a theory of quantum gravity.
I didn’t say it’s the only theory of quantum gravity. You can get a theory of quantum gravity by taking standard QFT — it just won’t work at extremely high energies. Other options like loop quantum gravity do not reproduce the rest of our models about everything else.
I said it’s the only theory of gravity and everything else at all energy levels, and that’s true. In fact, you’ll notice I did not even use the term “quantum gravity” to avoid the exact confusion you fell into anyway.
A good scientist will tell you that we don't assume it is truth. Instead, it is the closest thing to truth we can get at this time, but we are always seek better. But like a limit, we can only ever approach closer and never arrive at truth. As the other poster mentioned, we don't have a way to open up the source code of the universe.
Some scientists get a bit too attached to theories and can move them from "closest we currently have to truth" to "truth", but I think the bigger issue is that the non-scientists involve in transmitting science too often present it as truth, instead of the best approximation we currently have. Often because fake confidence beats out measured modesty, and the one claiming to have truth is more convincing than the one saying we can't know truth and only better approximate it.
A scientist will say science is true for the sake of simplifying the philosophy of science to those unfamiliar with it, but any scientist who thinks they have captured objective truth has lost the philosophical foundations of science.
It certainly does. You don't have to know how something works to be able to know how it doesn't work. And there is value in knowing how something doesn't work, even if you don't know how it works.
You are not a making insightful point at all. Nothing in the world can guarantee you that "Y implies X", after all, we can be living in a simulation. Does that mean we should shutdown all scientific discussions by repeating what you stated? Of course not.
Point out where I said we should “shutdown all scientific discussions.” You won’t be able to, and you will then realize how incredibly absurd what you just wrote is.
The parent is pointing out that the prior comment is literally meaningless, and self defeating too, since the same logic would apply to your own existence, or simulated existence. Including any possible words you could ever write.
(As far as any other HN reader could ever perceive)
Don’t you think jumping to a complete existential crisis over such a simple comment is a little extreme? That alone is a red flag that maybe dogma has taken over. No, nothing I wrote suggests one must shut down all scientific discussion or inquiry. No, it does not mean you cannot investigate X and its implications. No, it does not mean you cannot build speculative models on top of X. Yes, it does mean it’s important to be careful with language and avoid enshrining what is only an assumption as an unassailable fact of reality for generations.
Who said it’s a ‘complete existential crisis’?
It’s just a fact of reality that no one else on HN can verify with certainty whether or not you are a genuine, bonafide, human being, especially online.
Everyone who is engaging with you is just assuming you are.
I said that. If you thought I was wrong in that assertion, then all you had to do was stop questioning your entire existence here. The fact that you continued to do so lends credence to my assertion rather than refuting it.
Who is ‘I’?
If you mean ‘nilkn’, the presumably human created HN account ostensibly still operated by that same sole human being, writing their own thoughts down and not on behalf of other entities, then thanks for providing an excellent demonstration.
I think everybody would be happy if you came up with a different explanation! what's happened so far is that we have a known mechanism, and no alternative explanations have worked yet.
If we can suspect X given Y, but we shouldn't build models on top of the assumption of X, then what are we supposed to do with Y?
To me it seems like you're arguing that it was a bad idea to build on the assumption of Newton's theory of gravity because eventually it would be replaced by Einstein's theories of relativity. Which is obviously not sensible, since Einstein's theories were in part the result of trying to explain inaccurate predictions made by building on Newton's theory.
If the only option for finding out better evidence for or against X is by building those models and watching them either keep matching observations or finding a contradiction that can lead to the downfall of X as the suspect, then it is if you want to progress science any further.
Maybe there is another area that will give results faster, but much of the easy and fast science has already been science. And if someone finds a better option we missed, which does happen from time to time, add some rigor to it, verify it with testing, and they'll likely have themselves a Nobel prize.
This is wrong on several levels:
1. As other commenter said, X causes Y does not mean that Y implies X. There can be another cause for the Doppler.
And surprisingly, 2. There is at least one known mechanism that cause Doppler WITHOUT moving: when the observer is in a gravity well (ex: earth) and observing a stationary object outside the gravity well (ex: some fixed point in outer space)
As I've mentioned in another post, that leads to questioning one of the most well tested theories in physics, so you need extraordinary evidence to prove it over something as elementary as doppler shift. Like, if it's Earth's gravity well causing us to see things differently, then things that are further from Earth should observe things differently.
There is already extraordinary evidence that something in physics is behaving differently at larger scales: the behavior of galaxies (spin, gravity pull) doesn't match their mass. Any mass. There is no single mass value that predicts their entire behavior correctly. Thus dark matter, dark energy etc competing theories, which are so far untestable.
It wouldn't surprise me if we discover someday another Doppler mechanism that occurs at those same large scales
> There is at least one known mechanism that cause Doppler WITHOUT moving: when the observer is in a gravity well
This is gravitational redshift, not Doppler shift. Doppler shift specifically means redshift due to an object moving away--but to really be correct that definition has to be limited to flat spacetime, or to a small enough region in a curved spacetime that the curvature can be ignored.
> In the absence of evidence that something else is causing the redshift, the assumption should be that it is a result of things moving away from us.
But that is not what our best current model of the universe actually says. Our best current model of the universe says that the observed redshift of a distant object tells us by what factor the universe expanded between the time the light was emitted and now (when we see the light). Viewing it as a Doppler shift is an approximation that only works for small redshifts (much less than 1).
There is a very old theory called the "Tired Light Hypothesis" which supposes that for some unknown reason light loses energy as it travels over cosmological distances. This would reproduce the observed redshifts, but it has issues predicting pretty much every other cosmological observation.
In particular it doesn't explain observed reductions in surface brightness (expansion has the effect of "defocusing" collimated light). And it doesn't explain observed time dilation effects.
I've always wanted to play a game based on defunct theories. I'm a fan of luminiferous aether myself. What are the impacts on a spacefaring civilization?
Sci-fi already grants alternative physics to enable FTL and other magic. What about hard sci-fi, but wrong-hard sci-fi?
Extra credit: go back to Zeno and all motion is paradoxical, what would you even do in the game?
It feels like all space combat games I've seen rely on Aristotle's theory that objects prefer to be at rest.
And in old Trek it seemed that if the engines ever lost power the ship was in danger of deorbiting and crashing within hours for some reason.
My theory was always that ships hardly ever orbited unpowered, they usually went into much lower powered "orbits" or even just hovered "in place" using the (immense) power of their engines.
But to fly so low as to slow the ship down if unpowered, they'd generate enormous heat from atmospheric friction. They could use shields but then the ship would glow, alerting the natives below and violating the Prime Directive. And they called it "standard orbit".
Orbiting doesn't require power. Even a satellite in a very low orbit only needs a slight boost once in a while to counter the drag.
There's no reason for the shields to cause friction though. They're not made of ordinary matter so an extended, very angular shield could probably cut through atmosphere seamlessly.
Maybe they need a very low orbit to keep the planet's surface in range of the transporters.
They're usually in geosynchronous orbit over the interesting area of the planet. That requires power to maintain if you're not just staying over the equator. Even over the equator it only works without power at a certain altitude.
Try Frontier: Elite II if you want to try a "realistically modeled" view of space combat - things are pretty much strictly Newtonian.
Children of a Dead Earth is my suggestion https://store.steampowered.com/app/476530/Children_of_a_Dead...
Terminus seemed to have pretty decent physics too, but it's been a long time.
Not space combat ... Flight Of Nova
https://www.youtube.com/@flightofnova5746
If you haven’t read Greg Egan’s Orthogonal Trilogy, you might like it. https://en.m.wikipedia.org/wiki/The_Clockwork_Rocket
Luminiferous aether in what sense, what physics? Relativity doesn't exactly disprove it, it shows that everything distorts in a way that would make any aether unmeasurable. So if you just say aether:yes by itself I don't think anything happens.
> This would reproduce the observed redshifts, but it has issues predicting pretty much every other cosmological observation.
Not to mention contradicting the laws of conservation of energy and momentum.
To be fair we do already know that energy is not globally conserved over cosmological timescales. (Energy conservation is a consequence of time invariance, but cosmological expansion breaks that symmetry.)
Fritz Zwicky attempted to propose a mechanism of tired light that was caused by Compton scattering off of the intergalactic medium. But these kinds of scattering mechanisms produce far too much blurring in the expected images of distant galaxies and galaxy clusters.
> To be fair we do already know that energy is not globally conserved over cosmological timescales.
No, what we know is that there is no invariant global concept of "energy" at all except in a special class of spacetimes (the ones with a timelike Killing vector field), to which the spacetime that describes our universe as a whole does not belong.
However, "tired light" (at least the versions of it that aren't ruled out the way the Zwicky model you describe was) violates local energy-momentum conservation, which is a valid conservation law in GR (the covariant divergence of the stress-energy tensor is zero).
Somehow I learnt about Riemannian manifolds and Killing vector fields in a geometry class that didn't mention physics at all.
I am always entertained when some nonsense I learned actually has something to do with the real world.
That is a good clarification.
I like your theory more than the current setup.
I have an interesting addition to it:
Time dilation could be that going very fast in the space makes you relatively faster in one direction.
The thing is, atoms also have to travel; so the atoms (and matter in general) have a slightly longer distance to travel, to achieve the same chemical reaction. Which means interactions between atoms is slower, giving illusion of a slower time due to slower inter-atoms reactions.
I don’t think this would match the observations that can be made from earth of things on earth?
Though the phrasing seems a bit ambiguous. Could you put some math behind those words?
I think you just described relativity from the perspective of atoms. It's still the same old relativity though.
How would you explain the CMB? We can literally see that the universe used to be much denser.
And if the universe was much denser, doesn't that imply that all that matter affected its surroundings gravitationally? And as we know, time runs slower near large masses. And when something falls into a black hole, according to our very own theories, it would also red-shift because of the black hole's gravitational pull without anything having to expand.
No, it implies it expanded in the meantime. We can see that it was a hot plasma up until 300k years after the big bang. This isn't some redshifted illusion, the matter was literally packed so densely and thus so hot that it was in another aggregate state.
Don't get hung up on redshifts for evidence of the big bang. The CMB is the real smoking gun. Read up on it, it's entirely worth it. I can recommend Simon Singh's book "Big Bang".
There is also a plethora of other probes that in concordance all point to the same thing: that the universe is almost 14 billion years old and expanded from a very hot, dense state. It's settled science, really.
Speaking of the big bang, how did time work back then? :)
It's cool to say "in the first milliseconds of the existence of the universe X and Y happened", but how did time supposedly run as usual while everything else was on the fringe of our understanding of reality? There don't seem to be any answers to this (or I haven't looked thoroughly enough) but it feels like a very important question that's always overlooked by everyone talking about this.
Yeah, it is overlooked because the real answers are 'hidden' behind a lot of graduate level math. And most people don't really want to learn a bookcase worth of math first to talk about it, but they talk all the same.
Like, if you'd like to really dive into it then you're going to need to go through a lot of textbooks first.
If you are moderately familiar with multi-variable calc, then here is a good book to get started down the GR hole: https://www.amazon.com/Mathematical-Methods-Physicists-Compr...
Suffice to say, yes, there have been a lot of grad students that have the exact same questions and issue that you currently have. Further, once they have reached the end of the mathematical education required to understand how space time works in the first few minutes of the universe, they focus those questions into the issues we have with inflation. Those issues mostly come from our lack of understanding about how GR and QM interact, so the first 10e-43 seconds or so. At least, that is my understanding. Physicists are welcome to tell me how dumb I am right now!
(former) Physicist with focus on cosmology here. Your reply is one of the sanest in this thread.
There's a lot of attempts at investigating those questions. Here's a couple of pages I'd recommend to peruse:
https://en.wikipedia.org/wiki/Cosmic_time
https://en.wikipedia.org/wiki/Chronology_of_the_universe
If you're into podcasts at all, I'd strongly recommend Crash Course Pods: The Universe. The first (full) episode goes into detail on that first fraction of a second in our universe and it's pretty enlightening without being to thick on the math.
> doesn't that imply that all that matter affected its surroundings gravitationally?
It did; it caused the expansion to decelerate. That was true until a few billion years ago, when the matter density became smaller than the dark energy density and dark energy started to dominate the dynamics.
It's really trippy to think about how hawking radiation becomes 'real' once its sufficiently 'far' away from a 'strong' gravitational well, and how this can be thought of as a Doppler shift giving real physical presence (in that we can interact and be affected by ) to what was once a 'virtual' particle
I think scienceclic does a good job visualizing this, but end of the day I can't see a way to distinguish event horizons regardless if they're a black hole or the distant past/big bang.
https://youtu.be/isezfMo8kWQ?si=9wGliV-Qo1bXCTRy
Specifically look at the relativity of the vacuum section, which builds to a great insight at around 5:45
The universe is a 4 dimensional sphere, so everything could be moving away without increasing 3 dimensional space. Eventually in trillion or quadrillions of years everything would start to blue shift as things move towards us on the other side of the sphere.
So how do I travel backwards in the 4th dimension? Or conversely, where's the source of the force that propels us through the 4th dimension?
I'll leave this comment here since I'm about to get rate limited: I read/heard a great idea recently, what if gravity is an emergent quantity like heat? Maybe dense fermions just radiate gravitons just like a hot mass radiates photons?
Gravity is the result of spacetime curvature, if I remember correctly.
Yes, but it's irreconcilable with quantum mechanics. So what I described was one of many attempts to do so.
In that case, wouldn't it have to be more complex than just a particle which creates gravity? Something which is compatible with the finding of spacetime curvature?
Expanding universe and Big Bang Theory go hand-in-hand. There are multiple independent observations besides the red shift that make it nearly certain there had to be a BBT event to explain what we see. The universe is too hot, chaotic and clumpy for there not to have been a massive explosion to kick it all into motion. Since there is good confidence BBT happened, transitioning from that event to a steady-state non-expanding universe would require some sort of mechanism to slow then freeze the expansion. Not aware of any support for that model.
The name “big bang” was a pejorative epithet coined by Fred Hoyle, who believed in a steady-state universe that was expanding (as Hubble had argued convincingly) but had some hypothetical mechanism for creating galaxies, such that the universe could have an unbounded past and future.
So, historically, they did not go hand-in-hand.
https://en.wikipedia.org/wiki/Fred_Hoyle#Rejection_of_the_Bi...
True, but with the BBT-affirming discovery of the cosmic microwave background in 1965 they've been hand-in-hand ever since.
Easiest method is to simply take your idea at face value.
In our first version, imagine all of the stars at rest. Now, we emphatically know this not to be true locally due to all kinds of measurements, but let's go with it. What happens? The moment you let these stars "go," they begin to draw toward one another due to gravity. You would have gravitational collapse. We do not see that.
Next iteration: we throw the stars, and the galaxies, and the galactic clusters away from one another. No expansion required. Here we have two options. In the first, we did not throw with enough speed, they expand out ... slow to a halt ... and the gravitational collapse again. Again, unobserved. Option two, you have thrown at escape velocity and what you would see is an asymptotically decreasing speed, never quite hitting zero, since gravity works "forever away." Also unobserved.
What you're suggesting is basically the Steady State concept, a kind of static universe. This is a very old idea. So old it was given a kind of courtesy term in general relativity, which would eventually be set to zero.
Here is a rule for any armchair astrophysicists: whatever you think of, that was most likely an idea at one point and was eventually ruled out.
The relevant XKCD is Astrophysics - https://xkcd.com/1758/ ( https://www.explainxkcd.com/wiki/index.php/1758:_Astrophysic... )
The transcript from explain:
I suppose that's possible. Does that hypothesis adequately explain our observations?
Is the model we currently have completely "correct"? Almost certainly not. But it appears to be less wrong[0] than earlier models.
If you (or anyone) can show how the above describes our observations better and more completely than our current models, then it's likely "less wrong."
But you offer no evidence or even logical argument to support your hypothesis. As such, it's not much more than idle speculation and essentially equivalent, from a scientific standpoint, as suggesting the universe is a raisin dropped into a sugar syrup solution[1] and absorbing the liquid -- hence the expansion of the universe.
[0] https://en.wikipedia.org/wiki/The_Relativity_of_Wrong
[1] https://en.wikipedia.org/wiki/Compote
The observation of the Hubble constant requires us to measure distance to an object in space. This is very hard to do at the extreme distances required (https://en.wikipedia.org/wiki/Parallax). In the end, the variation in the Hubble constant might be only due to our limited accuracy in measurement.
I remember reading, a long, long time ago, a paper where the authors suggested if the universe was slightly hyperbolic, it would also cause a redshift effect. I can't seem to find it (and as far as I remember it was purely theoretical), but at the time I thought it was an neat idea.
Not that I have the background to know what else they might not have accounted for to reach this conclusion.
For example the entire atomic composition of the stars in the observable universe depends exquisitely on the expansion parameters at the big bang. The ratios can be traced back through the expansion to the quark-gluon soup stage. Changing the expansion rates changes the delicate balance between the particles that form at that stage, and when the various particle fractions "freeze out" during expansion when the temperature cools (btw we're talking about seconds from the big bang here :) which can be subsequently observed in stars all over the universe by spectroscopy. It's pretty beautiful.
There are so many intricate dependencies between these pathways that it's pretty unrealistic to postulate anything else than a big bang + cooldown process IMHO.
I got an idea! Anti-mass!
Anti-mass is mass where Gravity goes Out instead of In.
The anti-mass and mass accelerated away from eachother at the start and the redshift is it's repulsion away.
This is the equivalent of finding your keys an inch off from where you remember setting them down and concluding that someone broke into your home, stole your keys, took your car for a joyride, and broke back in to place them there.
I suspect, in a few decades, when the smoke clears and the very latest sub-infrared, stadium-sized space telescope finds fully formed galaxies several billion years "before" the alleged moment of creation, then the astronomical community will finally start to question the logic of prevailing cosmological theory, from the ground up.
The big bang was first postulated by an agent of the vatican, and scientists raised in any religious context tend to generate experiments that confirm their beliefs.
The _conclusion_ that the Universe is expanding is based on the long accepted premise that the Universe is _flat_. And this premise can not be proven or disproved unless we travel great distances to actually _observe_ if the Universe does, in fact, look the same from any point you look.
The Copernican principle is, indeed, attractive to the modern mind because its neutrality. But it's not neutral. It's just as loaded as any other principle, no matter how crazy it may sound today, philosophical, religious, or merely personal.
Whispers: Halton Arp
My guess is that scientists are considering this, but until now no better theory has been presented.
Part of this is the distinction between what is happening and why the model says is happening. Does any physicist believe they have the perfect model? Or is it that they use the model that best fits the observations and are open to any other model, as long as it is either simpler or produces fewer contradictions than the current model (and is just as testable).
I think too often we hear reports of "science says X is what happens" when the reality is more like "science says that the current model based on X happening is what best describes current data and observations".
While I don't necessarily think at lot of alternative ideas proposed are correct, I always love seeing alternative concepts being considered. Very cool to see ideas that could solve standing issues even if they themselves could have issues.