I've asked this question before, but I don't think I received a good answer, so figured I'd try asking again.
How do we know that galaxies are accelerating away from us and not moving at constant speed? People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.
However, wouldn't we expect to see the same observation even without any acceleration? Imagine there are some objects in space all moving in random directions and speeds, relative to Earth. After long enough time, all objects will appear to be moving away from Earth, even if they were moving towards it initially. And after long enough time, the objects that move fastest should be farthest away, by the simple definition of speed!
In short, even if galaxies weren't accelerating, we would still see that the further away a galaxy is, the faster it recedes.
How do we know that galaxies are accelerating away from us and not moving at constant speed?
There's a more basic question: How do we know the galaxies are moving? It seems (I haven't seen any other response, like... ever) that we have one and only one way to measure the speed of galaxies: the red shift.
It's impossible to triangulate those huge distances and the time scale would also be a barrier, so no way of confirming the red shift calculations with a different method. That means that if the red shift was caused by any other effect, say the light "degrading" after millions of years of travelling the void, all the calculations would be invalid.
I've asked about this many times and the answers are in the line of "we don't know any other reason for the light to red shift" and "the current theoretical frame is consistent", even if there isn't any other measure to be consistent with.
There was a prediction (expansion is related to Big Bang) that the far away galaxies, being younger, would have a different composition. This prediction seems to be failing, but advances in instrumental could give us a more precise answer in the future.
“Degrading” sounds very intuitive to me. Can the frequency of the light waves simply slow down over a very long distance/time? Or maybe the speed of light simply slows down over an unimaginably long distance? We don’t have any model to describe such behavior, but everyday objects around us all slow down one way or another, what makes light so different?
IIRC, this was one of the explanations proposed when the existence of a red shift was first noted: that the light is somehow slowly losing its energy over very long distances, becoming “redder” as it did so. It ultimately lost out to the dark energy / space-time expansion theory, although I do not recall why. Presumably there was some observation that precluded “degrading” light from being the sole explanation.
There are several challenges for "tired light", or indeed any theory that's an alternative to expansion of the universe.
The theory has to explain why the light gets redshifted, but does not get blurred, and the spectral lines do not get broadened. This severely restricts the type of interactions possible. Also the theory has to explain the consistency between redshifts within our own galaxy, to that of far-away galaxies.
My theory is that redshifting caused by gravitational background noise. I did calculation somewhere on HN or Youtube few months ago and numbers are of same magnitude.
https://en.wikipedia.org/wiki/Tired_light
>It's impossible to triangulate those huge distances
Galaxies are BIG. Andromeda is faint, but the same angular size as the moon. It's 2.5Mly away, but it's also 150kly across. Over a long enough time line you could do triangulation on it. In fact it's moving toward our galaxy, but very, very slowly compared with its diameter at 110kps. But yeah, in theory you could do triangulation on it over a very long period of time.
For triangulation to work you need to move, not your target. Triangulation is only used for objects within about 1000 parsecs, where we can triangulate using the movement of the Earth along its orbit.
The American astronomer, Halton Arp, had a theory he called "intrinsic redshift". My limited understanding is that he saw evidence of "very close" and "very far away" structures that are connected to each other in space, which makes no sense. He theorized that redshift may also be indicative of the age of a galaxy, rather than only indicating velocity.
The interpretation of redshift as velocity is also the primary reason cosmologists think the universe is expanding.
His book Seeing Red which raises several interesting and troublesome questions for the standard big bang model is worth a read.
Degrading light sounds like an interesting idea to me, you could call it the “cosmological damper” if you will. Thought experiment: imagine you have light particles in a perfectly circular orbit around a black hole. Does the light ever fall into the black hole or does it orbit for eternity?
There a whole set of theories, called "Tired light" theories, which tries to explain Cosmological Red Shift by degradation of photons with time. Buy they require whole set of different cosmological principles: a medium for light propagation is required to dump lost energy into, no Big Bang. But even with a tired light theory, galaxies are accelerating toward attractors, see https://arxiv.org/pdf/1702.02483v1.pdf https://www.youtube.com/watch?v=NpV0GQo3P0c
https://en.wikipedia.org/wiki/Cosmic_distance_ladder
https://en.wikipedia.org/wiki/G%C3%B6del_metric
There are versions of this which do provide another reason for red shift.
Personally I keep this in mind as a means to free my thinking from a single narrative.
You're not going to get a simple answer because the answer is quite complex.
Astronomy is the paleontology of photons. You should take an astronomy course if you really want to know, but essentially a "ladder" was built of distances, starting with the very near and slowly building outward using various techniques and discoveries of physics as they became available. This is called the cosmic distance ladder. You start with stellar parallax, then after that you go farther with "standard candles" (particular types of variable stars). But then you have to get even further out, where you can no longer see an individual star, and then you rely on specific breeds of supernovae. Only then do you get to redshift, and compounding tons of data from step three seems to verify the redshift estimates. By the time you get to the Hubble constant, it was a huge rift between two communities over what was still a factor of two difference.
It's quite fascinating, but I can't really dump out an entire book into a comment.
Their light is more red shifted the farther away they are. I'm no expert on this, but I believe in a constant-speed scenario they would have equal red shift no matter the distance
The assumption made here is that relative velocity is the only method that would redshift light. Gravitational redshift is a thing and our model of gravity is incomplete.
> The assumption made here is that relative velocity is the only method that would redshift light.
Not in our actual model of the universe, no. The redshift of light is determined by the spacetime geometry and the worldlines of the emitter and receiver. That is a general formula that works in any spacetime.
> Gravitational redshift is a thing
Not for the universe as a whole, no. Gravitational redshift is only meaningful in certain kinds of spacetimes, namely stationary spacetimes (which, roughly speaking, describe objects that either don't change with time at all, or which are periodic, like a rotating planet or star). The spacetime that describes our universe as a whole is not stationary and there is no meaningful concept of gravitational redshift.
> our model of gravity is incomplete
In the sense that we do not have a quantum theory of gravity, yes. But that does not affect anything under discussion here. Our current theory of gravity, GR, works fine for treating the expansion of the universe and whether or not it is accelerating.
Then why are there phantoms in the data that need dark matter and dark energy to make the supposed working model fit them?
I'm not sure what you mean by "phantoms in the data". The distribution of stress-energy is a free parameter in GR; it has to be inferred from observations.
The terms "dark matter" and "dark energy" are just names for, respectively, "stress-energy that acts like the matter we can see, but we can't see it", and "stress-energy that acts like a cosmological constant". Neither of those things poses any problem for GR, since both types of stress-energy are allowed for in the theory.
"Dark matter" poses a problem for particle physicists, who have so far been unable to find any fundamental particles that would produce the observed properties. "Dark energy" only poses a problem if for some reason you don't like having a nonzero cosmological constant.
It's clear to me why we haven't made any new discoveries in cosmology in the past two decades. It's this exact attitude of "the model is the truth". All models are wrong. The data can help you improve it, but you have to at least want to improve it.
What do you think they do now?
How do you propose they do it differently?
What evidence do you have that what they are doing now doesn't work, and does the all the evidence of how they work support your hypothesis?
Be detailed, because your comment just has some motivational speaker nonsense but no depth. For example, in the last 20 years cosmology has:
+ Refined its model of stellar formation based on observational data of the number of planets found observationally, and used this to validate and invalidate several model adjustments.
+ Observed galaxies that appear not to have dark matter, and by their existence and behavior validate some theories of dark matter, and validated others, which predict such galactic behavior. (e.g. some theories attempting to update gravity).
+ Run simulations of stellar and glactic formation that predicted structures in the universe that were later observed.
Everywhere they look they are finding things the models don't explain well, and refining the models - that is literally using the data to improve the models.
If you think you can come up with something better, then do it - all you gotta do is make up some mumbo jumbo and write down any old equation. It probably should:
- provide the same results as were observed when the plugging in the experimental parameters of existing experiments.
- explain "wierd stuff" in the data that existing models couldn't.
- predict future observations of the known phenomena with the same or better accuracy as the old model
- predict currently unobserved and unpredicted phenomena
Go ahead and take a stab real quick - I'm sure you can do it. I mean Gallieo did it, so did Newton and Einstein. Next up is willis936.
People can be right about problems with a process or way of thinking without being the next Einstein. There's no need to get personal.
But in this case you and your friend are not right about the process. There was a claim that "the models aren't being updated based on the new data" which is categorically false. It's not that they pointed out problems in the process, it's that they flat out lied about things - such dishonesty doesn't help solve any problems you imagine you see, its just trolling.
Is that true?
At least for my hobbyist understanding of the progress of cosmology, quite a lot seems to have happened in the past two decades. Confirmation of the Higgs Boson at CERN [0] kept me up all night to watch the press conference; I found it extremely exciting. (Maybe you count this strictly as observational particle physics and not cosmology, but I might appeal for it to be allowed in the context of your critique).
And what of TFA? Isn't what we're reading now a new discovery in cosmology?
What about the rush of exoplanet discoveries?
What about the dramatically different galactic properties now observed in increasingly strange corners of the observable universe (including some which perhaps give insight into some of the properties of "dark matter" or whatever it ends up being)?
0: https://home.web.cern.ch/news/news/physics/new-results-indic...
Yes, it's true. Mainstream science refuses to accept anything radically new because of huge baggage. Nobody wants to look stupid, then relearn, recalculate, republish, reteach everything, or lose their tenures, grants, etc. It's why science advances in small incremental steps. AFAIK, there is a team of scientists secretly working on radically new set of theories (I got contact but cannot join because of war).
That notion is ridiculous. Finding something radically new is every scientist's dream. Look at how Einstein is perceived, who arguably found one of the most radically new theories. Nobody thought he looked stupid or lost tenure. No one goes into science hoping to simply confirm what everybody already thought was true.
The true reason we have made little progress in the past 20 or so years (and a 20 year slouch is historically nothing unusual) is that pretty much all data we collected in that time frame confirmed the standard model. It's the one big dilemma cosmology has. The standard model (LambdaCDM) works unreasonably well. Our problems with it are largely theoretical. New data is also hard to come by. Look at how long it took to plan, build and launch Euclid, cosmology's big hope of finding new physics. The hubble tension from the OP's article is already the most interesting discovery since 1998 when evidence for dark energy was first seen.
And trust me, all scientists know that all models are wrong. This isn't some unique insight that is beholden to amateur scientists on the sidelines.
As I understand it, if the expansion was constant, farther away stuff would still be more red shifted. Stuff twice as far away appears to be moving twice as fast. It helps me to imagine the expansion of a metal cookie sheet, where the two edges are moving apart faster relative to each other compared to the speed that they're moving away from the center.
The surprising bit is that the far away stuff seems to be even more red shifted than that, so we're not just expanding, but the rate of expansion seems to be accelerating.
It's because the movement is ascribed to the expansion of space itself, not the individual galaxies. We don't have any reason to believe galaxies are moving in random directions at random speeds (not at scales that explain the redshifts we call the expansion of the universe).
In your explanation, I think we'd expect to see some very distant, very slow-moving galaxies moving toward us. And there may be some very fast-moving galaxies close to us that just started really far away. Objects would be entering our local universe from outside it, and that simply doesn't happen.
Thank you, but I suppose I'm not really questioning the big bang piece. My question was mostly in regards to the continued acceleration piece. Feel free to disregard the "in random directions" part of my original post.
I'm picturing more of an explosion in empty space. A firework or granade of sorts. Any individual dust/shard of the explosion still sees all other objects moving away from it and the rest of my question stands. But I suppose this would imply a "center" to the explosion, which I've also heard is not the case.
Theres a few other comments offering more clarity to the acceleration piece. Thank you everyone!
> I'm picturing more of an explosion in empty space.
No, that's not what the big bang is.
> this would imply a "center" to the explosion, which I've also heard is not the case
That's correct. The big bang does not work like anything ordinary that you are used to imagining. The math is straightforward and unambiguous, but there is no good ordinary language description that corresponds to the math.
As I understand it, before the big bang the whole observable universe was contained in a small sphere and then it started to expand metrically. Is this interpretation correct?
Another thing: suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to? To an infinite void? Is it correct to say that stars radiate energy to the infinite then?
It's just an interpretation. Your interpretation is similar to the Big Bang model of visible Universe expansion. If you can convince us that your model is better than other models, then we will use your model, but nobody can prove than a model is correct, unless we will find a hidden recorder somewhere which was turned on for few dozens of billion years.
Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium (photon is a wave, so it waves something).
That sounds suspiciously like postulating the 'ether'. Surely what a photon 'waves' is the electromagnetic field, which is not a medium, and which fills the whole of spacetime. There is no 'end of the medium'.
"Field" means 3d array of numbers. Spacetime means 4d array of numbers. You are talking about mathematical model of Universe, while I'm talking about physics. Mountain is not just an excitement in a height field. If photon is not waving something, then it's not a wave. Physicists prove that photon is a wave.
No, you’ve simply hit the limits of needing/wanting to understand something in terms of something else similar or more familiar.
Makes the same point on a related matter: https://youtu.be/Q1lL-hXO27Q
> It's just an interpretation
No, it's not, it's our best current model's description of the actual physical reality of our universe.
> nobody can prove than a model is correct
That's true, but it's also true that we can show models to be incorrect, as in, falsified by the data. For example:
> Photon will hit something, or will travel until it will be redshifted to obvilion, or will travel until end of the medium
For the scenario that was posed, a laser beam that never hits anything, none of your statements here are true. The first is ruled out by the scenario; the second is known to be false because there is no "gravitational redshift" of light in the universe as a whole (because models in which there would be such a redshift are known not to correctly model our data on the universe as a whole), and there is no "end of the medium" (again, models in which there would be an "end of the medium", i.e., where the universe stopped containing matter and started being just vacuum, are known not to correctly model our data).
I have described what actually happens in my own response to the GP upthread.
> (photon is a wave, so it waves something).
Light is an electromagnetic wave; what "waves" is the electromagnetic field. (If you use a "photon" model, you are using the quantum electromagnetic field as opposed to its classical approximation.) There doesn't have to be any other "medium"; the electromagnetic field is present everywhere.
OK, it's our best model, but it doesn't invalidate other models, less complete or less popular, it compete with them.
Yep. The article is about the Huble Tension, which invalidates Big Bang model. We still use it.
The Big Bang model is incomplete too: galaxies with FTL speeds, different speeds of expansion, no center of bang, no flows, no source of energy, it stretches time and space, etc.
I assume that the only infinite thing in infinite Universe is Universe itselft. All other things are finite. Thus, a photon has finite life, like any other wave.
The right-hand rule in EM suggests that we are in north hemisphere of something, so south hemisphere will have symmetrical rule, unless you believe that God chose right-hand rule for the whole infinite universe. If we are in a sphere, then that sphere rotates and have a boundary.
"Field" is an array of numbers. You are mixing model and reality.
> before the big bang the whole observable universe was contained in a small sphere and then it started to expand
We have no evidence of any time when the universe was not expanding. At the earliest times we have evidence of, the universe was already expanding (extremely rapidly--much, much, much more rapidly than it is now). At those times, our observable universe was indeed contained in a very small volume.
> suppose I point a laser beam to the space and by chance this laser beam never finds any kind of matter in its way, where is this laser going to?
Since the universe is spatially infinite in our best current model, the laser beam will just keep on going forever.
> To an infinite void?
According to our best current model, no, the laser beam will never stop passing by matter, of approximately the same average density as the matter we can see.
> Is it correct to say that stars radiate energy to the infinite then?
Yes, as long as you recognize that "the infinite" never becomes a "void".
I find helpful this analogy of the space-time (4-dimension) expansion from the big bang: the surface (2D) of an expanding bubble. YMMV.
-Galaxies are not accelerating, space is expanding.
-No, In your scenario then end result would be a most static average distance between all objects in the universe. As an infinite number of objects come from infinite distances, there would ALWAYS be objects in the neighborhood.
I think what you're imagining is a bunch of objects in a box, give them random vector and then remove the box. If they maintain course, all will eventually move outside the original box boundaries, and away from each other. (not the way the universe is).
They know space is expanding. The primary mechanism we know this is the speed with with we measure an object (moving away) is redshifted. Objects at the same distance from Earth, but opposite regions of space are moving at the ~SAME measured velocity.
There simply is no existing theory which can account for what we are seeing besides space expanding. I'm not big on thinking we understand it all, but in this particular measurement, there is basically zero doubt. Space is expanding, which has the affect of accelerating all objects in the universe away from you, with an acceleration relative to the distance. The more space between you, the more opportunity to expand.
What does "space is expanding" mean? That the distance between objects is increasing? How can you tell the difference between "space expanding" and "objects moving in a non-expanding space"? Is there any way to tell the difference or is it just that all objects are moving away at the exact speeds that satisfies the "space expanding" explanation and nothing else?
But then, I'm back to what does "space is expanding" mean? What is doing the expansion?
space expanding means what it literally says: there is more space everywhere at once. there was less a moment ago and now there's more. the longer the distance, the more space gets added in between, thus the effect is extreme on universe scale and undetectable on planetary scales.
Well yes, but did you read my post in detail? I asked what the difference was between "space expanding" vs "objects moving away from each other".
And secondly, what is the mechanism for "space expanding"? What is "space" in this context? What actually is expanding?
Spacetime is 4D array of points. It's like a movie file, but with 3D frames instead of 2D frames (pictures). Expansion of space means that coordinates of points in a frame changed to move away from us, to match movie (model) with reality.
An array of 4D points implies some sort of construct upon which the points exist. As far as I know, the that was the concept of "the ether" and that fell out of fashion long ago.
So again, how can you tell the difference between space expanding or two things actually just moving away from each other (to keep it to a very simple example of two bodies).
Ether now known as "physical vacuum". As I told you already, expansion of spacetime is the mathematical tool, like a shader in OpenGL.
Let me try to explain- galaxies moving would be like balls floating in a pool. The galaxies are moving across the water.
Space moving would be like balls placed on a bed sheet and the sheet expanding. The galaxies aren't moving- the sheet is.
What's actually happening is more like galaxies moving around on a sheet that's being pulled further at all sides.
How can you tell the difference?
things are moving away from other things the faster the farther away they are, uniformly across the universe. everything is moving away from everything, not just two particular points.
is there a difference if there isn't a difference?
Imagine a deflated baloon with two dots drawn close to each other. Now inflate it; the dots didn't move but the plane that they were drawn/positioned on did.
I understand that concept. I'm asking, how can you materially tell the difference?
> How do we know that galaxies are accelerating away from us and not moving at constant speed?
More precisely, we see that galaxies started accelerating away from us a few billion years ago; before that they were decelerating (moving away from us but with the "speed" decreasing instead of increasing).
> People often point to the observation that the further away a galaxy is, the faster it appears to be moving away from us, implying acceleration.
That observation tells us that the universe is expanding, but by itself it does not tell us whether the expansion is accelerating or decelerating or neither. So you are correct that that observation alone is not sufficient to show that the expansion is accelerating.
What we look at to see how the expansion rate changes with time is a comparison of three pieces of observed data, galaxy by galaxy: redshift, brightness, and angular size. The relationship between these three quantities is what cosmologists use to construct a model of the expansion history of the universe, which in turn tells us things like what I said above, that the expansion has been accelerating for the last few billion years but before that it was decelerating.
By few billion are you talking like 3 billion? Why the change?
Meaning, why did the expansion change from decelerating to accelerating a few billion years ago? Because that was when the density of matter, which had dominated the dynamics until then, became smaller than the density of dark energy, which has dominated the dynamics since then. The dark energy density doe not change with time, but the density of matter decreases as the universe expands.
How is is that dark energy density does not change with time? Surely the total amount of dark energy has to be constant (energy can't be created or destroyed, and all that), and then as the universe expands, that's then the same amount of energy over a larger volume, right?
> How is is that dark energy density does not change with time?
Because that's how a cosmological constant works.
There are alternate models where there is "dark energy" (as in, stress-energy that causes accelerated expansion) whose density does change with time (for example, a "Big Rip" model in which the dark energy density increases with time), but such models do not match our best current data.
Dark energy may be the energy of vacuum itself, that's why it's constant. And no, energy conservation does not apply in this case. There is a good blog article on precisely this question by Sean Carroll: https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
/me not a cosmologist.
I think the story is that dark energy is indeed created, in the new emptiness resulting from the expansion of space.
<mumble> I believe it's supposed to be spacetime that expands, not 'space'. But it's beyond me to explain what that even means; as I understand it, spacetime refers to the whole Universe, across all of time. To 'expand' means 'to become larger over time'. But if the thing that's expanding includes time itself, then I'm bewildered.
If your idea was the case there would always be new things from very far away heading towards us, this is not the case. If the universe is flat and infinite there would be no end in supply of new galaxies with all velocities and you would always have the same mix as the initial mix of velocities. That’s not what we see.
I'm so glad "the infinite universe" as an idea is finally falling off. It works great in the Hitchhikers Guide, I love the floopy mattresses and planets that grow screwdrivers but nothing real is infinite.
I didn't even realize how many people held that belief til that article about how the universe isn't as big as we thought
Huh? Every evidence points to a flat infinite universe. Nothing but speculation points to anything otherwise.
I'm not an expert, but I think it's like this:
If the universe were expanding uniformly, we would see galaxies moving away from us. The further away they are, the faster they would move. Distance and velocity would have a linear relationship, with the Hubble Constant as the scaling factor.
But what we actually see is that, if we measure precisely enough, galaxies further away are moving faster than that. The conclusion is that the expansion is accelerating.
> galaxies further away are moving faster than that
No, you have it backwards. Accelerating expansion means, roughly speaking, that we see galaxies further away moving away slower than a "uniform" expansion would predict. Remember that we are seeing galaxies further away as they were a longer period of time ago--so "accelerating expansion" means the universe was expanding slower then, when the light was emitted, than it is now.
Actually, though, we don't observe the distance to a galaxy directly. We infer it from other observations. The actual observed quantities are redshift, brightness, and angular size, and the relationship between those three observed quantities is what tells us the expansion history of the universe.
Sure, but to argue that this explains what we observe today, you would need to show that as of today it has been “long enough,” which is its own can of worms to open.
You might say “obviously it has been long enough for full sorting, because we observe a fully sorted data set of speed correlated with distance.” But that would be begging the question.
One thing that is not often mentioned is that this effect only applies outside the local supergroup; within the supergroup gravity overrides the expansion of spacetime and holds us together (for now!)
You're right: the "more distant galaxies are moving away faster" point is just Hubble's original observation of an expanding universe. It's not an argument for cosmic acceleration. (If you see people making that claim, they're probably either speaking carelessly or not experts themselves.)
The conclusion that the expansion is accelerating was a quite recent result: 1990s, I believe. It's based on careful measurements of supernova explosions of a type with computable intrinsic brightness in increasingly distant galaxies, and the exact pattern seen in their apparent redshifts vs. apparent brightness. It was a shocking discovery when it came out, with two separate teams announcing the result pretty much neck and neck. There's also independent and compatible evidence for acceleration from the exact pattern of variations in the temperature of the cosmic microwave background seen at different points in the sky.