How do we know that the Universe is expanding?

[MikeeMiracle]

TL;DR Summary

How do we know that the universe is expanding?

Summary: How do we know that the universe is expanding?

Am I correct in my understanding that we believe the universe is expanding due to the objects we can see, i.e. galaxy's moving away from us, and that the further away from us they are, the faster they apear to be receding away from us.

The basic point I am trying to understand here is, how do we know that it is indeed space that is being expanded and "carrying" galaxy's further away along with it, as opposed to space possibly being "fixed" and that something else might be responsible for celestial objects receding from each other.

I am not looking to debate / disagree / propose an alternate theory, just an understanding of why we are so sure it is indeed space that is expanding.

 

[weirdoguy]

how do we know that it is indeed space that is being expanded and "carrying" galaxy's further away along with it

By definition. "Space expansion" (metric expansion) means that objects that are not gravitationaly bound move apart.

 

[MikeeMiracle]

I agree, assuming space is expanding it would carry along the galaxy's not gravitationaly bound with it. That was not my question though :)

My question was how do we know it is indeed space specifically that is expanding and there is not another potential explanation. For example, assuming for a moment a non-expanding space / spacetime, that still does not discount the current theory of "dark energy." Dark energy could still be responsible for "pushing everything apart" as opposed to "expanding the universe." For example if we look at red shifts of distance galaxy's, could that also be explained potentially just by the galaxy moving away from us by an as yet unknown force.

What is it that makes us all believe it is the universe itself which is expanding?

 

[weirdoguy]

assuming space is expanding it would carry along the galaxy's not gravitationaly bound with it

No, you didn't grasp the thing. It's the very definition of "expansion of space". Don't think about expansion and galaxy clusters moving apart as a cause and effect. These are the same thing. Space (which overall is an ambiguous thing) is not a rubber that stretches. There are quite a lot threads about what expansion really is, use "search" option.

 

[MikeeMiracle]

I feel from being a member of this forum that I have a reasonably good understanding of the theory of expansion and how this expansion is occurring i.e. each point is expanding in all directions etc. Well as much a someone who has not stutied astrophysics but sees it as a hobby / interest can reasonably be expected to understand.

The theory of expansion and how it is occurring explains our observations. That does not necesarily imply that it may have another cause instead which could also explain what we see without having expansion. In my head though, I feel this could also be explained by some kind of repulsive force that does not require the actual space to expand.

Am I at least correct that we cannot "see" space expanding and only see it's effects on matter we can see?

I don't want to go on and on and so unless someone says something specific that I feel I want to query, i wll not carry on posting the same thing endlessly. Just in my head I do not feel my question has been answered.

Thanks

 

[PeroK]

In my head though, I feel this could also be explained by some kind of repulsive force that does not require the actual space to expand.

What sort of force? Why would a force produce a recession rate that is linear with distance? A force normally produces a defined acceleration, not a defined speed with distance.

It's very easy to have an idea in your head. The difficult thing is to construct a theory that meets the observational data. It's only when you calculate things - using a formulation of the theory and some data - that you have any test of your theory.

The trump card that GR has is that, before expansion was known about, the theory implied that we should expect an expanding universe.

 

[jbriggs444]

Am I at least correct that we cannot "see" space expanding and only see it's effects on matter we can see?

That sounds correct. We cannot see space expanding because "space" is just part of a coordinate system -- mental lines drawn within a shared abstraction, a "foliation" of space-time into a sequence of time-tagged spatial slices.

As I understand it:

We deduce that the distance between objects is increasing over time when using a standard of synchronization tied to a "co-moving" coordinate system against which the universe is seen to be isotropic. That deduction is based on local measurements using parallax, red shifts and various standard candles along with some standard background assumptions such as the large scale isotropy and homogeneity of the universe.

When using co-moving coordinates, the description of "space is expanding" matches the deduced results.

 

[Robert Friz]

I am a lay person, with a strong physics interest (disclaimer). In the past I have postulated that space itself is expanding, caused by either (1) inflation has not yet ended, or (2) the existence of energy in all its forms results in entropy that expands space itself. I would welcome feedback on these thoughts. Dr. Perlmutter's research that resulted in a Nobel prize demonstrates that very distant objects are observed receding at an accelerating rate, which would be the outcome of space itself expanding within our universe.

 

[metastable]

I am personally wondering what other kinds of evidence exists (aside from observed redshifts proportional with distance) that tends to prove expansion is accelerating.

 

[MikeeMiracle]

I saw someone on this forum posulate that dark energy could be a property of a vacuum. If that were the case then the recesion force would be linear with distance as we currently observe and isotropic. As far as I can see this is still consistent with the red-shift, standard candle observations.

In either case, we are getting off topic. Like I said I did not want to propose an alternate theory even though I feel my explanation still works theoretically.

If GR predicted expansion before we observed it then I am content to accept that as you guys are more kowledgable on these matters than myself.

 

[MikeeMiracle]

By the way, thank you for the replies. I can almost feel the groans when an established members sees a topic like this and sighs thinking "not another one on the same topic" :)

 

[Torbert]

I am personally wondering what other kinds of evidence exists (aside from observed redshifts proportional with distance) that tends to prove expansion is accelerating.

The observed redshifts and brightness are proportional to distance until approximately five billion years ago. The ratio of dimness and distance changed in our observations before and after that point. The interpretation that best fits the data is a change in the expansion rate.
The data base is larger than can be simplified here but is consistent with accelerated expansion.

 

[kimbyd]

I am personally wondering what other kinds of evidence exists (aside from observed redshifts proportional with distance) that tends to prove expansion is accelerating.

The most concrete evidence is the ISW effect:
https://en.wikipedia.org/wiki/Sachs–Wolfe_effect#Integrated_Sachs–Wolfe_effect

 

[PeterDonis]

In the past I have postulated that space itself is expanding, caused by either (1) inflation has not yet ended, or (2) the existence of energy in all its forms results in entropy that expands space itself.

Please review the PF rules on personal speculation.

 

[PeterDonis]

I saw someone on this forum posulate that dark energy could be a property of a vacuum.

"Postulate" is a strange word to use. The simplest explanation of dark energy (which just means "whatever it is that is causing the expansion of the universe to accelerate") is a cosmological constant, which can be viewed as "a property of a vacuum". There is no currently known evidence against this simplest explanation. So whoever you are referring to was just describing our best current model.

Like I said I did not want to propose an alternate theory

You might not have wanted to, but you did. Please review the PF rules on personal speculation.

even though I feel my explanation still works theoretically.

I don't see how since your "explanation" has no math and doesn't make any specific predictions. It's just throwing some words together.

One of the reasons for the PF policy about personal speculation that I have asked you to review is that it is much, much, much, much, much harder than most people imagine to actually make a theoretical proposal that (a) actually makes any predictions at all (which requires it to be stated using precise math, not vague ordinary language), (b) matches the predictions of our current, experimentally verified theories in every case where those theories have been experimentally verified, and (c) makes some different prediction from our current theories in some case where experiments have not yet been done, but could be done sometime in the reasonably near future. Doing this successfully is the kind of thing that gets people Nobel prizes. Even doing it unsuccessfully (i.e., reach item c above but then have the prediction falsified) is hard enough that it's rare for even professional scientists to do it.

 

[MikeeMiracle]

@PeterDonis Appreciate that in the end i did propose an alternative theory, but not because I wanted to push that or discuss that alternative. I was really just trying to ask how SURE are we that our current understanding is correct as opposed to something else. The answer to which seems to be "very sure," so in that respect I have my answer, and I again thank everyone for their input and clarification.

Feel free to close / lock this thread.

 

[PeterDonis]

Feel free to close / lock this thread.

Thread closed.