# Expanding universe needs a big bang?

• I
• CHOP
In summary: Grinkle, sorry I didn't see what you were linking to. I'll check it out and thank youIn summary, the "Big Bang Theory" is a consequence of matter in motion + something (dark energy) making that matter accelerate. It is not something that was created in a particular moment, but rather a straightforward consequence of expansion. The need for this expansion with dark energy + big bang is not something that is explained by the "bang."
PeterDonis said:
The density of ordinary matter and dark matter in our universe (radiation is negligible now) combined is much less than the critical density, so the ordinary and dark matter in our universe is much less than the amount that would be needed to make it possible for our universe's expansion to stop and reverse direction, even if there were no dark energy.

Right, I get that. But I only get that -- under the idea that they are also moving at some (minimum) velocity.

That is, suppose we stopped their relative motions right now and then let go of them.

Would expansion commence? Apparently it couldn't, because PAllen said no DE (which is acceleration), and for motion to commence is motion to accelerate.

Or is my hypothetical situation missing something?

Oh hang on here, I shouldn't specify the need of a minimum velocity because the 'minimum' would depend on the ratio of separation to mass, right? So, I can, in a sense, dip below the minimum, by increasing the separation. So that, in effect, the result (expansion) is necessary given the ratio mass to spacing, alone. Is that right?

CHOP said:
suppose we stopped their relative motions right now and then let go of them.

Would expansion commence?

Meaning, if there were no dark energy, just ordinary/dark matter and radiation? Then an initial state where relative motion was zero would lead to contraction. (The initial state would basically be a closed universe at the instant of maximum expansion.) This is one way of seeing why inertia, as I have been calling it, is among the factors that are needed to explain why the expansion rate now is what it is.

CHOP said:
for motion to commence is motion to accelerate.

Deceleration is also "motion commencing", and, as above, that is what would happen if you had a universe in a state of zero relative motion at some instant, containing only ordinary/dark matter and radiation.

CHOP said:
the result (expansion) is necessary given the ratio mass to spacing, alone.

It's the ratio of actual density to critical density (assuming we're only considering situations with zero dark energy/cosmological constant), where the critical density depends on the Hubble constant. That dependence on the Hubble constant is where the "spacing" comes in, since the Hubble constant ##H = \dot{a} / a##, so it depends on both the "speed" and the "spacing".

PeterDonis said:
Then an initial state where relative motion was zero would lead to contraction. (The initial state would basically be a closed universe at the instant of maximum expansion.)

Okay, got it.
PeterDonis said:
Deceleration is also "motion commencing".

Ah, very true!
PeterDonis said:
that is what would happen if you had a universe in a state of zero relative motion at some instant, containing only ordinary/dark matter and radiation.

Deceleration would happen given some instant of zero relative motion? First quote says contraction happens.
So, are you saying that we are to think of that contraction as deceleration?

Vaclav Vavrycuk said:
and speculative

CHOP said:
are you saying that we are to think of that contraction as deceleration?

Not quite. "Deceleration" as I was using the term means ##\ddot{a} / a < 0##. "Contraction" means ##\dot{a} / a < 0##.

In the case of a closed universe with zero cosmological constant at the instant of maximum expansion, the first (deceleration) is true (since it's always true in a closed universe with zero cosmological constant) and the second (contraction) starts being true an instant later.

In the case we've been discussing for much of this thread, a universe that is expanding but with zero cosmological constant, the first (deceleration) is true, but the second (contraction) is not.

PeterDonis said:
In the case we've been discussing for much of this thread, a universe that is expanding but with zero cosmological constant, the first (deceleration) is true, but the second (contraction) is not.

So, given no relative motion at an instant, and given no DE (as PAllen specified), each body would decelerate between the time of that instant and time 2? And just to check, the separation between two neighboring bodies is increasing or decreasing?

CHOP said:
So, given no relative motion at an instant, and given no DE (as PAllen specified), each body would decelerate between the time of that instant and time 2?

Which case are you talking about? The quote you gave from me is about a universe that is expanding but with zero cosmological constant. Since that universe is expanding, "given no relative motion at an instant" is not true for that universe.

If you are talking about the other case (a closed universe with zero cosmological constant at the instant of maximum expansion), then "no relative motion" is true at that instant. For that universe, "deceleration" (##\ddot{a} / a < 0##) is always true, not just at that instant, as I said before.

PeterDonis said:
Which case are you talking about? The quote you gave from me is about a universe that is expanding but with zero cosmological constant. Since that universe is expanding, "given no relative motion at an instant" is not true for that universe.

It was stated by PAllen that the universe would expand regardless of DE. That's the universe I am talking about. I found this 'necessity of expansion' interesting. I wanted to see whether the existing velocities have anything to do with this necessity being one. So, I gave a hypothetical -- let's say we go out and find two neighboring objects whose separation is expanding, and we stop their relative motion with force. Then we let them be. My question is, what will happen to that separation after the instant that we let them be? Will it increase again? If my hypothetical is not allowed, why is that?

Comeback City said:
I'm assuming GR rules this possibility out anyways, yes? (regardless of its likelihood, that is)

That's right. GR breaks down everywhere at t=0, not at a single location. In other words, the big bang singularity occurs everywhere in GR.

Comeback City
Please note, I said assuming homogeneity and isotropy (and no DE). Stopping two objects in isolation violates these assumptions. It was also clarified in discussion with Peter that you could run a Big Bang expansion forever backwards (contraction from the infinite past) and that the math of GR doesn’t care which interpretation of the solution you adopt. Physical plausibility, though, picks out the expansion as the physical time direction.

As Peter has already explained, any homogenous, isotropic situation with a moment of no expansion must have expansion from Big Bang before this, followed by Big Crunch after.

CHOP said:
It was stated by PAllen that the universe would expand regardless of DE.

I believe he meant that our universe, with the density of matter and radiation that it has, would keep expanding even without DE, because the combination of inertia and the effect of matter/radiation would not be sufficient to cause the expansion to ever stop; the density of matter and radiation is less than the critical density, so the universe would keep expanding forever even without DE.

CHOP said:
let's say we go out and find two neighboring objects whose separation is expanding, and we stop their relative motion with force.

Then the objects will no longer be comoving, so their motion will in general not be the same as the motion we describe as the "expansion of the universe", which is defined with respect to comoving objects, and won't tell you much about that expansion.

PeterDonis, PAllan
got it, thank you for your clarifications,

Ken G said:
So it is for dark energy and the Big Bang-- we think of the Big Bang as being akin to an initial condition (that is not explained by any physics at present), and then the physics kicks in and tells us what happened next. So which is the "cause" of the expansion we now see? Which is the cause of a home run, the batter who hit it or the pitcher who pitched it? Both are, together.

Right, i read all you said in that post and agree. Thanks for all your posts Ken.
I was thinking of expansion in a certain way, not as 'what is in fact happening', but as a possible circumstance between two material points (regardless that it's happening).
And I was focused on a certain 'mechanism' that would give rise to expanding, because I felt it generated a more authentic kind of expanding. That mechanism is DE.
Thanks again,

Drakkith said:
That's right. GR breaks down everywhere at t=0, not at a single location. In other words, the big bang singularity occurs everywhere in GR.
And if I'm not mistaken, that is the problem with the "bang"...if it happened "everywhere" how could there not be a center? I've been reading a lot of articles that are seriously questioning the "big bang theory"...

RandyD123 said:
And if I'm not mistaken, that is the problem with the "bang"...if it happened "everywhere" how could there not be a center? I've been reading a lot of articles that are seriously questioning the "big bang theory"...
You have that backwards. "If it happened everywhere how COULD there be a center" is the right question and of course that makes sense since there WAS no center.

RandyD123
RandyD123 said:
And if I'm not mistaken, that is the problem with the "bang"...if it happened "everywhere" how could there not be a center? I've been reading a lot of articles that are seriously questioning the "big bang theory"...

The big bang simply describes the fact that our current model, the Lambda-Cold-Dark-Matter (LCDM) model, suffers from infinities when we run time back to a certain point, which we call t=0. And that after this point in time it rapidly expands, going from a very hot, very dense state to a much less dense and cooler state. Crucially, the model describes and predicts a great many things in cosmology with very good accuracy, such as the distribution of matter, the ratio of different elements, the CMB, among others. The LCDM model is in no danger of being replaced any time soon. Especially since we keep finding more and more evidence supporting it.

I'm not sure which articles you are reading, but unless they are reputable scientific papers then they simply don't matter. You can find thousands of articles written by news sites, amateur scientists, and others that say all sorts of things which seemingly goes against the standard view of science. This is because they aren't professional scientists and don't always know which ideas are well accepted and which ideas are either fringe or obsolete.

Ken G
Also, bear in mind that one should determine the "Big Bang theory" from a more colloquial meaning of the Big Bang as a singular event happening everywhere. The theory is built from two basic postulates, general relativity and the cosmological principle. The latter says that the universe is more or less the same everywhere on large scales at a given age. So since general relativity is a dynamical theory, you can take the situation that we currently observe and check that the dynamical story works out. Then you can extrapolate it back before we can see (before about 400,000 years old when the opacity became to great to see though), and check that that story also works out. With some key caveats (dark matter, dark energy, inflation) it does.

So all that is the "Big Bang theory." None of it refers to any creation event, or anything happening everywhere, because we're not sure how far back we can get away with our two postulates. We literally don't know if general relativity still works all the way back to the beginning, if the cosmological principle still applies, or if we have the idea right about inflation (for which there really is no confirmed theory at all). So astronomers apply "the Big Bang theory" essentially daily, without ever even mentioning how far back they imagine they can extrapolate it. For that reason, it actually isn't a theory about the beginning of the universe, though it is often mistaken for that.

By contrast, the Big Bang "event" is intended to be a creation event, but there's no scientific theory for it. It's more like a pop sci picture of what might have happened, that we really have no way to test at present. Part of the problem is that the scientific theory that we actually do have predicts an early phase that is a thermodynamic equilibrium with only tiny variations, of which we can only see a tiny part because it is thought to have expanded so much as to dwarf what we can actually see. It's hard to imagine a physical system that is better at covering its tracks than that!

Drakkith
RandyD123 said:
And if I'm not mistaken, that is the problem with the "bang"...if it happened "everywhere" how could there not be a center? I've been reading a lot of articles that are seriously questioning the "big bang theory"...
This was answered completely for you here:

https://www.physicsforums.com/threa...ue-then-how-can-there-not-be-a-center.962125/
Continuing to ask the same question is not going to change the answer. There was no center because there was no point (in space) where it began.

RandyD123 said:
I've been reading a lot of articles that are seriously questioning the "big bang theory"...

You're going to need to give specific references if you want to take this kind of position. And if they're not textbooks or peer-reviewed papers, be prepared to be told that they're not valid sources for PF discussion. There are a lot of "articles" on the Internet that are not reliable.

phinds said:
You have that backwards. "If it happened everywhere how COULD there be a center" is the right question and of course that makes sense since there WAS no center.
You're right, I had it backwards. Thought about it after the post!

RandyD123 said:
You're right, I had it backwards. Thought about it after the post!
So have you finally come to terms with the fact that there was no center?

The model has no center. The model works well to fit the observations as simply as possible. That's all a scientist can say-- we should never say there either was or was not a center, this is a lesson we have learned quite a very many times by now.

Ken G said:
The model has no center. The model works well to fit the observations as simply as possible. That's all a scientist can say-- we should never say there either was or was not a center, this is a lesson we have learned quite a very many times by now.
Good point. Thanks.

phinds said:
So have you finally come to terms with the fact that there was no center?
It's a hard concept to grasp. And then there is the "no edge"... meaning what are we expanding into! The universe is a very strange place indeed!

The "no edge" element of the model is trying to cope with the "what are we expanding into" question, by essentially doing away with the question altogether. I would say an important feature of the Big Bang model is how it "does away" with pesky questions that we have no good answer for (like what is outside the universe, what came before, etc.). Having a model that does not need to address these questions because the model renders them meaningless is a useful aspect of the model, but of course it does not mean these questions are gone for good. It is always possible that some future model with resuscitate those questions by giving them testable answers, but for now, we just don't have any testable way to deal with those questions than simply dismiss them as meaningless, which is the approach taken in the current model.

Very good points @Ken G.

I tend to be absolutist in my statements about "center" and "edge", when I should be more clear that I am talking about our model as opposed to solidly known empirical facts.

It's natural, we can always just assume that is what is meant. Sometimes it's useful to make the distinction though, because there are always room for surprises! Astronomy has rather a remarkable history of very big surprises, but then, I guess physics does too.

ed
Bandersnatch said:
Big bang is the hot and dense early state of the universe that one arrives at when one extrapolates expansion backwards in time.
Pardon my impertinence, but doesn't anything shorter than the length of Planck Time foreclose knowing what happened at the moment of the Big Bang.

Michaela SJ said:
doesn't anything shorter than the length of Planck Time foreclose knowing what happened at the moment of the Big Bang.

The "moment of the Big Bang" in our best current cosmological model is not an "initial singularity". It's the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence. In inflationary models, it's the state at the end of inflation, just after "reheating" has occurred.

Michaela SJ said:
ed
Pardon my impertinence, but doesn't anything shorter than the length of Planck Time foreclose knowing what happened at the moment of the Big Bang.

You're confusing the big bang as a single 'event' (creation of the known universe) with the big bang as a 'process' (rapid expansion from a hot, dense state). Bandersnatch is referring to the latter.

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