Other lines of evidence for Dark Energy? (Besides supernova data)

[Cerenkov]

Hello.

My current understanding (please correct, if wrong) is that the expansion of the universe is observed to be accelerating, rather than coasting or slowing down. The tentative cause of this acceleration has been given the placeholder name of 'Dark Energy'. One line of evidence for this phenomenon is supernova data, first announced in 1998/99.

Besides any subsequent supernova data, are there any other lines of evidence that also indicate an accelerating universe?

If so, what are these and could I please be directed to sources that I can peruse to improve my understanding of this phenomenon?

Thanks in advance.

Cerenkov.

 

[mathman]

The universe appears to be flat. This requires a certain energy density. Ordinary matter plus dark matter seem to have only about 30% of what is needed. Dark energy (or something) seems necessary to fill in the gap. Google "flat universe" for more information.

 

[Cerenkov]

Hello mathman and thank you for this help.

I've Googled "flat universe", as you recommended. The information was helpful, but I think that I might be caught in some kind of misunderstanding about closed, open and flat universes and how dark energy relates to them. (This sentence, in of itself, may also be mistaken, but please bear with me.)

Here's how I understand things at the moment.

Friedmann universes are GR solutions that can fall into three types - closed, open or flat. Prior to the supernova discoveries of 1998/99, it was thought that our universe was either closed or flat. But the type 1a supernova data indicated the presence of a cosmological constant that was accelerating cosmic expansion and 'opening' the universe. So, we appear to live in a universe that isn't flat and coasting, but open and accelerating.

mathman, the above paragraph is my attempt to summarize what I understand. Now, I realize that, as you pointed out, the universe is observed to be flat. And here is where I'm getting confused.

The universe is observed to be accelerating, meaning that it should be an Open Friedmann solution.
Yet it is also observed to be Flat, meaning that it should be a coasting, Flat Friedmann solution.

Can you help me out, please? I don't understand how it can be both of these things. Ok, I do realize that these two conditions need not be mutually exclusive, but I can't quite see how and that's why I need some guidance.

Thank you.

Cerenkov.

 

[Vanadium 50]

Besides any subsequent supernova data, are there any other lines of evidence that also indicate an accelerating universe?

Wikipedia has a list. https://en.wikipedia.org/wiki/Dark_energy

 

[Cerenkov]

Thank you Vanadium 50. That's helpful.

 

[PeterDonis]

Prior to the supernova discoveries of 1998/99, it was thought that our universe was either closed or flat. But the type 1a supernova data indicated the presence of a cosmological constant that was accelerating cosmic expansion and 'opening' the universe.

That's not correct. You can have a universe with a cosmological constant that is closed, flat, or open--more precisely, that is spatially closed, flat, or open, since those terms refer to the spatial geometry of spacelike slices of constant time in standard FRW coordinates. Our best current model is of a universe that is spatially flat.

The universe is observed to be accelerating, meaning that it should be an Open Friedmann solution.
Yet it is also observed to be Flat, meaning that it should be a coasting, Flat Friedmann solution.

Can you help me out, please? I don't understand how it can be both of these things.

You have this wrong even for FRW models without a cosmological constant.

Without a cosmological constant, all models are decelerating--none of them are "coasting" and none of them are "accelerating". The difference between spatially closed, flat, and open models is only in how much they are decelerating, or, to put it in the way it is more commonly put, how the actual density compares with the critical density.

For an FRW universe with no cosmological constant, if the actual density is greater than the critical density, the universe will be spatially closed, and will end up recollapsing into a Big Crunch. If the actual density is equal to the critical density, the universe will be spatially flat and will expand forever, with its rate of expansion decreasing towards zero. If the actual density is less than the critical density, the universe will be spatially open and will expand forever, with its rate of expansion decreasing, but not towards zero, towards some positive constant.

(Note: A "coasting" universe, also called the Milne universe, is a limiting case of the open universe above, where the density is zero and we really have just flat Minkowski spacetime in unusual coordinates.)

When you add a positive cosmological constant to the model, what happens is that the above rules become limited to the spatial geometry only: a universe with density (including the cosmological constant as dark energy density) greater than critical is spatially closed, density equal to critical is spatially flat (this is our best current model), density less than critical is spatially open. However, it is no longer true that the spatially closed case will always recollapse; there will be a range of densities greater than critical for which the universe still expands forever. The upper limit of this range of densities is the density at which the universe is static--this is the Einstein static universe, where the density of ordinary matter and radiation is fine-tuned so that its attractive gravity exactly balances the repulsive gravity of the cosmological constant, so the universe does not expand or contract. For densities greater than this, the universe will recollapse.

 

[Cerenkov]

Ah, thank you Peter.

This is most helpful. I was 99% certain that I was misunderstanding some important points about Friedmann universes and you've confirmed that. Your last paragraph really helps, too.

So, our best current model is a spatially flat universe with a small, but positive cosmological constant.

Thank you.

Cerenkov.

 

[PeterDonis]

our best current model is a spatially flat universe with a small, but positive cosmological constant

That's correct.

 

[Cerenkov]

That's correct.

Thank you for your help, Peter.

All the best,

Cerenkov.

 

[Buzz Bloom]

The universe appears to be flat.

Our best current model is of a universe that is spatially flat.

Hi Mathman and Peter:

I believe that the two above quoted statements are intended to be saying exactly the same thing, but I may be mistaken. The phrase that makes me unsure is "appears to be". This phrase may mean either (1) is observed to be, or (2) is deduced to be from best fit model data.

If (1) is the intended meaning, then I am very curious about what the observations are that would produce this conclusion.

If (2) is intended then I think the following might be a useful more precise statement to @Cerenkov.

Our current best model indicates that the curvature is so very close to spacially flat (that is, having the curvature value equal to zero) that it is currently not possible to distinguish any difference between it's being absolutely flat or it's possibly being almost but not quite flat and either finite-hyperspherical-closed or infinite-hyperbolic-open.​

Regards,
Buzz

 

[PeterDonis]

I believe that the two above quoted statements are intended to be saying exactly the same thing

I think they are. Our best current model is based on our best available evidence, which is what (I think--see below) "appears to be" refers to.

If (1) is the intended meaning

It's not what I intended; I suspect it's not what @mathman intended either, but he can correct me if I'm wrong.

It's not what I intended because we do not directly observe spatial curvature; we can't, since we can't directly observe a spacelike slice. We can only observe our past light cone.

 

[PeterDonis]

Our current best model indicates that the curvature is so very close to spacially flat (that is, having the curvature value equal to zero) that it is currently not possible to distinguish any difference between it's being absolutely flat or it's possibly being almost but not quite flat and either finite-hyperspherical-closed or infinite-hyperbolic-open.

This is correct as a statement of the limitations of any model; no model can ever pin down spatial curvature to infinite precision.

 

[Buzz Bloom]

It's not what I intended because we do not directly observe spatial curvature; we can't, since we can't directly observe a spacelike slice. We can only observe our past light cone.

Hi Peter:

I intuit that it is theoretically possible to observe spatial curvature, although current technology is unable to do it, although of course my intuition might be wrong. I have not seen anywhere any discussion of this "theoretical possibility". The idea is that given a view of a portion of the universe that is reasonably close to isotropic, the numerical density of galaxies would approximately vary as the square of the distance from us, if space is flat. If the observed variation of numerical density does not vary with the square of the distance, then this could demonstrate non-flatness.

Regards,
Buzz

 

[PeterDonis]

I intuit that it is theoretically possible to observe spatial curvature

As I said, we can't observe it directly because we can't directly observe a spacelike slice; we can only observe our past light cone.

The idea is that given a view of a portion of the universe that is reasonably close to isotropic, the numerical density of galaxies would approximately vary as the square of the distance from us, if space is flat.

You're thinking of us "viewing" a portion of the universe instantaneously, but we can't. As I said, we can't observe a spacelike slice. We can only observe our past light cone. Light takes a finite time to get to us from other galaxies.

Also, your intuition is ignoring the expansion of the universe; the galaxies are moving apart, so the density "now" is not the same as the density we observe in our past light cone, which is all that we can observe.

 

[timmdeeg]

we can't directly observe a spacelike slice;

Could we calculate it in principle from redshift measurements and applying the $(\Omega_M,\Omega_\Lambda)=(0.3,0.7)$ model? We calculate the "now" distance of a z = 1 galaxy as 10 Glyr.

 

[PeterDonis]

Could we calculate it in principle from redshift measurements and applying the $(\Omega_M,\Omega_\Lambda)=(0.3,0.7)$ model?

Yes, of course. But calculating it is not the same as directly observing it. For one thing, the calculation assumes that nothing outside our past light cone changes the result; but we can never actually rule that out.

 

[kimbyd]

The ISW effect.

 

[timmdeeg]

The ISW effect.

How does the ISW effect relate to a spacelike slice?

 

[Mary Conrads Sanburn]

Data from antipodal places: First use of CMB polarization to detect gravitational lensing from galaxy clusters
January 13, 2020 | Catherine N. Steffel

[snip]
In a study published in Physical Review Letters, Fermilab and University of Chicago scientist Brad Benson and colleagues use the polarization, or orientation, of the cosmic microwave background to calculate the masses of enormous galaxy clusters using a new mathematical estimator. This is the first time that scientists have measured these masses using the polarization of the CMB and the novel estimation method.

“Making this estimate is important because most of the mass of galaxy clusters isn’t even visible – it’s dark matter, which does not emit light but interacts through gravity and makes up about 85% of the matter in our universe,” Benson said.

The scientists’ work may eventually shed light on dark matter, dark energy and cosmological parameters that reveal more about structure formation in the universe.

[snip]

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https://news.fnal.gov/2020/01/data-...t-gravitational-lensing-from-galaxy-clusters/