How can dark energy comprise 74% of the Universe?

[Quotidian]

TL;DR Summary

How does dark energy contribute to, or become measured, as mass?

I have often read that dark matter and dark energy are now thought to comprise 94% of the total mass of the Universe. Just now I was reading an article which says that dark energy comprises 74% of the total. So, I'm confused about how energy can comprise mass. Dark matter, I can understand - even given that its nature is unknown, it's some kind of stuff, so presumably has mass. I also understand that matter and energy are interchangeable via the famous e=mc2 equation. But I'm still hazy on how energy can make up so much of the mass of the Universe.

 

[PeterDonis]

I'm confused about how energy can comprise mass.

When cosmologists talk about "the total mass of the universe", they mean energy (more precisely, average energy density). They don't mean "stuff that has mass the way an ordinary person would think of mass".

 

[Delta2]

This 94% is for the 94% of the observable universe right? And the rest 6% is what we call baryonic (ordinary) matter (protons, neutrons, electrons, quarks e.t.c) right?

 

[PeterDonis]

This 94% is for the 94% of the observable universe right?

No, it's 94% of the average energy density of the universe, period.

the rest 6% is what we call baryonic (ordinary) matter (protons, neutrons, electrons, quarks e.t.c) right?

Yes.

 

[Delta2]

No, it's 94% of the average energy density of the universe, period.

Can you expand a bit more on that? How do we calculate the average energy density of the universe since we don't know if it is finite or infinite? Only the observable universe is finite.

 

[PeterDonis]

How do we calculate the average energy density of the universe since we don't know if it is finite or infinite?

We calculate the average energy density of our finite observable universe, and since we have no reason to suppose that the non-observable part of the universe is different, on average, from the observable part, we use the same average for the entire universe.

 

[Delta2]

and since we have no reason to suppose that the non-observable part of the universe is different

This is a ***Huge*** assumption imho we can never be sure what is happening at something we can't observe but ok.

 

[phinds]

This is a ***Huge*** assumption imho we can never be sure what is happening at something we can't observe but ok.

One of the fairly persuasive arguments about that goes like this. From a galaxy at the edge of our observable universe, the "observable universe" includes some of our observable universe and more of what's NOT in our observable universe. How can you argue that what's in over half of their observable universe is different (on average) than what is in the overlap between theirs and ours? And of course, this extends forever. Granted, you can argue for a very gradual change in things, but that strikes me as a weak argument.

Occam's Razor argues that it's all the same.

 

[Delta2]

But if the universe is infinite, making the assumption that something infinite is similar to just a negligible fraction/sample of it (the observable universe is huge but it would be a negligible fraction of a possibly infinite universe) seems not such a safe assumption.

 

[DaveC426913]

It's called The Principle of Mediocrity and it's used a lot in Cosmology.

https://en.wikipedia.org/wiki/Mediocrity_principle

It is used when making tentatvue conclusions about
- stars we can't directly observe, assuming they're roughly spherical, as opposed to, say, a cube,
- galaxies we can't directly observe are probably disc-shaped or elliptical, as opposed to, say, pyramid shaped,
- the 115 or so elements we know of here are probably the same ones 100 billion light years away.

 

[PeterDonis]

It is used to explain why we see stars whose shape we can't directly observe and assume their shape is roughly spherical, as opposed to, say a cube. And why galaxies too small to observe are probably disc-shaped or elliptical, as opposed to, say, pyramid shaped.

I'm not sure these are valid applications of the principle, since there are physical laws that govern the shapes of stars and galaxies. It's simply not physically possible to have a cube-shaped star or a pyramid-shaped galaxy; those aren't physically stable configurations.

I'm also not sure that the mediocrity principle itself is the basis for our assumption that the non-observable parts of the universe are, on average, the same as the part we can observe. I would say that's just Occam's Razor, as @phinds said: in the absence of evidence to the contrary, it's the simplest hypothesis.

 

[DaveC426913]

I'm not sure these are valid applications of the principle, since there are physical laws that govern the shapes of stars and galaxies.

There is a principle of cosmology that makes the assumption "we are probably not special". Maybe I've got the wrong name for it. It's not Occam's Razor though.

 

[PeterDonis]

the assumption that something infinite is similar to just a negligible fraction/sample of it

That's not the assumption being made. The underlying assumption is, as @phinds said, Occam's Razor: we observe that, on large enough scales, our observable universe is homogeneous and isotropic, and we build a model based on physical laws and that assumption, and that model matches observations reasonably well. But that model also says that the rest of the universe looks, on large enough scales, like our observable universe. Any other model, that predicted a difference between our observable universe and the rest, would be more complicated than the model we are actually using; and in the absence of any evidence for such complications, the model we are actually using is the simplest, and therefore is to be preferred.

 

[Delta2]

I'm not sure these are valid applications of the principle, since there are physical laws that govern the shapes of stars and galaxies. It's simply not physically possible to have a cube-shaped star or a pyramid-shaped galaxy; those aren't physically stable configurations.

Because you think in terms of physical laws that hold in the observable part of the universe. How do we know that in a far unknown non-observable part of universe different physical laws hold that allow for cube shaped stars? Ok I know what you ll probably answer: The invariance of the physical laws in time and space , ok I have my doubts about this invariance but ok this is not the thread to discuss it.

 

[PeterDonis]

I know what you ll probably answer: The invariance of the physical laws in time and space

Yes, which is another instance of Occam's Razor: in the absence of any evidence to the contrary, the simplest model is that physical laws are the same everywhere in our universe. So that's the model we use.

 

[Delta2]

OK, Occam's razor, seems quite reasonable but we just can't be 100% sure.

 

[Delta2]

I have to apologize to the OP cause I kind of hijacked this thread, we have deviated from the main theme and I feel I am the main responsible for this.

 

[Quotidian]

No that’s OK. There are interesting methodological and philosophical considerations.

I think the ‘principle of mediocrity’ is indeed the same as ‘the Copernican principle’ which is that the Universe is assumed to behave consistently from any point in it.

I wasn’t aware of the ‘average energy density’ concept and will read up on that.

Thanks for the replies!

 

[PeroK]

But if the universe is infinite, making the assumption that something infinite is similar to just a negligible fraction/sample of it (the observable universe is huge but it would be a negligible fraction of a possibly infinite universe) seems not such a safe assumption.

It's a better assumption than assuming that the universe changes radically immediately beyond our current observable universe. And, it's a better assumption than assuming that because we can't see beyond the observable universe, we can't have a cosmological model.

There is no evidence for any other assumption, so that must be the default until we find direct observations that contradict it.

PS and if the universe is infinite, then that may well always be an assumption, since it's not clear how such a thing could ever be confirmed beyond any doubt. Whereas, if it's finite we may eventually find that out for sure.

 

[PeroK]

Because you think in terms of physical laws that hold in the observable part of the universe. How do we know that in a far unknown non-observable part of universe different physical laws hold that allow for cube shaped stars? Ok I know what you ll probably answer: The invariance of the physical laws in time and space , ok I have my doubts about this invariance but ok this is not the thread to discuss it.

It's one thing to keep the option open that the physical laws may change in some way across a possibly infinite universe. It's a different matter to have some physical evidence for such changes and/or a viable alternative theory to GR and the current cosmological models.

We can all imagine the possibility of cube shaped stars - any 14-year-old with a basic knowldege of physics can do that - but unless you have a viable theory to support it, such speculations are physically meaningless.

For example, I've seen a Feyman lecture where he talks about all the letters he gets along the lines of "how do we know time and space are not discrete?" He says we don't know. But that's not the point. The difficulty is fnding a viable theory that supports discrete spacetime.

It's not groundbreaking science to think what might be possible, it's groundbreaking to show how it might be possible. We can all say the words and think the thought "there must be a theory of quantum gravity". It's another matter entirely to develop such a workable theory!

 

[Vanadium 50]

This is degenerating into philosophy, and not very good philosophy at that.

We know that the visible universe has the same mix of Dark Energy, Dark Matter and Baryonic Matter everywhere. (On large enough scales, of course, not individual stars)

There are regions of space that we cannot see, and will never be able to see. Science has nothing to say about them. They could look like our neighborhood, or they could be filled with dancing pink unicorns. I doubt it, but there is no way to test.

But that's not the whole story. There are regions of space in our visible universe that are not in each other's. That is, we can see two regions, A and B today, but at the time the light was emitted from each, neither could see - or be otherwise influenced - by the other. And these both have the same mix of Dark Energy, Dark Matter and Baryonic Matter everywhere in the region. So while we don't know for sure what is going on outside our visible universe, we do know what's going outside other people's. ("People" in this case referring to the intelligent slime molds of Kelnoria XII) And it looks the same.

 

[artis]

@Quotidian well I too had similar questions a while ago until I realized the true meaning of the word "dark" before matter and energy :D

Basically it's just a phrase used for something that we currently have no real physical idea or proof of apart from the fact that we had a need to explain why the expansion of the universe is accelerating and the cosmological constant.
In other words from what I gathered it's basically a gap that we had in the numbers so we realized there have to be more forces at play than the ones we already accounted for.As for mass and energy, well according to the Lambda CDM aka Big bang model the universe was even more energetic when it was young much like a kid growing up, the hot plasma state condensed to form cool matter objects that eventually turned out to be planets etc, so we kind of traded energy for mass.

If this model is correct then the parts of the universe that we don't see now shouldn't be different from the ones we see simply because those that we don't see eventually came from the same "mold" so to speak. I would bet that there is a much smaller chance that the universe is different beyond the visible horizon and instead there might be something weird going on with "dark" energy because unlike the parts of space we can't see dark energy is right here right now and yet we can't "see" it which to me seems weirder than anything that might happen outside the visible universe.

 

[andrew s 1905]

As I understand it the Hubble sphere is expanding so in the future we will be able to see further than current size of the observable Universe.

Regards Andrew

 

[PeterDonis]

the Hubble sphere is expanding so in the future we will be able to see further than current size of the observable Universe.

The Hubble sphere is not the boundary of "how far we can see" and is not the same as our observable universe.

 

[DaveC426913]

In fact, all indications are that are sky will get more and more empty as distance objects disappear out of our OU.
One can imagine a time in the very far future when only our local cluster will be visible (since it is bound by gravity and not subject to CE).

 

[andrew s 1905]

The Hubble sphere is not the boundary of "how far we can see" and is not the same as our observable universe.

I know that. I never said if was. Regards Andrew

 

[DaveC426913]

I know that. I never said if was. Regards Andrew

I guess I think the same way as PeterDonis, because I interpreted your post exactly the same as he. Moreover, upon rereading, I don't see how it could be interpreted differently.

Maybe you can clarify.

 

[phinds]

I guess I think the same way as PeterDonis, because I interpreted your post exactly the same as he. Moreover, upon rereading, I don't see how it could be interpreted differently.

I agree.

Andrew, it appears that what you had in mind and what you said are not thing same thing.

 

[andrew s 1905]

The current observable Universe is how far we can see now. That is not how far we will be able to see in the future. Which is what I said. Please point out where I equated the two.
As the Hubble sphere is expanding we will be able to see further in the future. I also know we can see things receeding faster than the speed of light
If this is wrong then please explain why?
Regards Andrew

 

[PeterDonis]

Please point out where I equated the two.

Nobody said you did. You are misunderstanding what we said is the problem.

As the Hubble sphere is expanding we will be able to see further in the future.

This is what we said is the problem: this statement equates, or at least links, the Hubble sphere at any given time with the boundary of the observable universe at that time. That is wrong. The two are not the same, nor are they linked; although it is true that both the Hubble sphere and the size of our observable universe are expanding, the size of our observable universe is not expanding because the Hubble sphere is expanding.

 

[andrew s 1905]

Ok I give up. @PeterDonis why say this

The Hubble sphere is not the boundary of "how far we can see" and is not the same as our observable universe.

You now say this

Nobody said you did. You are misunderstanding what we said is the problem.

I explicitly did not equate them at the same time. For the last time I said the expanding Hubble sphere will mean we can see further (than the currently observable Universe) in the future.

Regards. Andrew

 

[DaveC426913]

...we can see further (than the currently observable Universe) in the future.

Are you trying to say simply that, in the future, the OU will be larger, allowing us to see farther?

 

[PeterDonis]

I said the expanding Hubble sphere will mean we can see further (than the currently observable Universe) in the future.

And that is wrong. I explained why in post #30. That is the only statement of yours that I or anyone else is saying is wrong.

If you had just said "we can see further than the currently observable Universe in the future", that would have been fine. But you said more than that.

 

[andrew s 1905]

Are you trying to say simply that, in the future, the OU will be larger, allowing us to see farther?

Yes.
But how far we can see is related to...
I agree you can argue they have the same underlying cause.

I have learned my lesson.

Regards Andrew

 

[andrew s 1905]

And that is wrong. I explained why in post #30. That is the only statement of yours that I or anyone else is saying is wrong.

If you had just said "we can see further than the currently observable Universe in the future", that would have been fine. But you said more than that.

But they are related contrary to your statement. Light has to cross the Hubble sphere to reach us. See section 3.3 https://www.google.com/url?sa=t&sou...FjABegQIBRAF&usg=AOvVaw0gAqrLMM4grSnGgMqhElQs

Regards Andrew