I Expanding universe needs a big bang?

[PeterDonis]

I was not referring to the singularity per se. Simply that energy density grow without bound.

My remarks about the term "big bang" would apply to this as well; most physicists seem to think that energy density growing without bound in a model is a sign that the model becomes invalid in that regime and new physics should be expected to show up. But we don't need to know how all that works out to deal with the hot, dense, rapidly expanding "big bang" state and the history of the universe since then.

 

[PAllen]

My remarks about the term "big bang" would apply to this as well; most physicists seem to think that energy density growing without bound in a model is a sign that the model becomes invalid in that regime and new physics should be expected to show up. But we don't need to know how all that works out to deal with the hot, dense, rapidly expanding "big bang" state and the history of the universe since then.

Seems like a dodge to me. What happened a little before what you want to call the beginning is a legitimate question, unless there is something makes it ill defined or meaningless. GR is making a prediction that current theory must break down in the early universe, and this remarkable prediction is what I call the Big Bang.

 

[CHOP]

PeterDonis,

So you are saying that there is no question "Why is the universe expanding?" unless it were asking "What are the list of things which cause the expansion rate to be precisely what it is?"
---------------------------------

Yes, that's true of course. Not sure why I was suspicious of it earlier. Something subtle, it will come to me.

Okay, so the expanding universe is thought to be caused by BB, and DE just spices it up with acceleration. But since that expands it more than it would be expanded (without DE), DE is also the cause. So those combined is why expansion happens.

Correct?

I know you said that there is a third thing that explains it, radiation and dark matter that act gravitation-ally, but how can that gravitational effect be part of what explains the opposite effect?

Thank you,

 

[CHOP]

Bandersnatch,

I think the main issue here is that it is unclear what, exactly, do you mean by 'being caused'. It is then hard to tell if there is some misconception regarding the expanding universe that needs to be addressed, or if there's a conceptual agreement and we're all just talking past each other.

Bandersnatch,

your post was very helpful to me in that it made very clear where things stand. Thank you very much for putting in the time to write that up for me. I really appreciate that.

All in all, you are pretty well correct, when you say that I may have been under the impression that DE's contribution was more significant. Pretty well correct but not exactly. I wasn't thinking of contribution in terms of what did more toward the current amount of recession; rather, where by 'more' I was thinking 'quality' of the expansion - like a more 'genuine' form of it. Not just some objects moving at different velocities, in turn causing increased spacing, but where something is causing increased spacing, in turn causing relative velocities to change.

Thanks again for being very perceptive, and your detailed post. I'll be looking at it again later.

 

[PeterDonis]

Seems like a dodge to me.

No, just a preference about terminology.

GR is making a prediction that current theory must break down in the early universe, and this remarkable prediction is what I call the Big Bang.

What makes this prediction is not GR. It's most physicists' expectation that GR will stop being an accurate theory if spacetime curvature/stress-energy density gets large enough (roughly the Planck scale). And I do not think that "big bang" is a good term for this expectation.

 

[PeterDonis]

So those combined is why expansion happens.

Correct?

Not completely. See below.

I know you said that there is a third thing that explains it, radiation and dark matter that act gravitation-ally, but how can that gravitational effect be part of what explains the opposite effect?

The gravitational effect of radiation and ordinary/dark matter makes a contribution to the rate of expansion we actually observe. The fact that the contribution is of the opposite sign to the contribution of dark energy does not mean it isn't there or doesn't count.

Note, btw, that DE acts to accelerate the expansion, but inertia does not; inertia alone would just keep the expansion rate the same. So the contribution of ordinary matter/dark matter and radiation is not "opposite" to the contribution of inertia.

 

[Bandersnatch]

I wasn't thinking of contribution in terms of what did more toward the current amount of recession; rather, where by 'more' I was thinking 'quality' of the expansion - like a more 'genuine' form of it. Not just some objects moving at different velocities, in turn causing increased spacing, but where something is causing increased spacing, in turn causing relative velocities to change.

This looks to me like a distinction between initial velocity and acceleration (in our Pioneer analogy). Which is fine, yeah. DE is the only thing actively pushing things apart. But remember that expansion of the universe is not a nebulous term, but one with a specific meaning - it's the growth of large-scale distances. So if you ask what causes expansion, then you're asking what causes the distances to grow. The answer must then be the recession velocity (by definition; it's what velocity means), which in turn has contributions from both DE and the initial impulse. One can't ignore either. And of those two, the latter is contributing more.
Again, it's like asking what causes changes in displacement in Newtonian motion (if you recall SUVAT equations). One needs to take into account both any acceleration terms and the initial velocity term.

 

[PeterDonis]

which in turn has contributions from both DE and the initial impulse

And from ordinary matter/dark matter (and radiation, strictly speaking, but the latter is negligible by several orders of magnitude now).

 

[Bandersnatch]

Yes, thanks. But as you've just discussed, those are negative.

 

[PeterDonis]

as you've just discussed, those are negative.

Yes, but that doesn't mean they aren't there. The question is not just "what is causing distances to grow"; it's "what is causing distances to grow at the rate they are observed to grow". The contribution of ordinary/dark matter to the observed rate is not negligible.

 

[Bandersnatch]

Sure. I just suspect the question in OP's mind is more about what made the chicken cross the road, and less about what tried to prevent it.

 

[CHOP]

The gravitational effect of radiation and ordinary/dark matter makes a contribution to the rate of expansion we actually observe. The fact that the contribution is of the opposite sign to the contribution of dark energy does not mean it isn't there or doesn't count.

right, that affects rate. right, both signs matter (to the topic of rate), but not (both signs matter) to the topic of why universe is expanding. Let me know if I'm wrong.

Note, btw, that DE acts to accelerate the expansion, but inertia does not; inertia alone would just keep the expansion rate the same. So the contribution of ordinary matter/dark matter and radiation is not "opposite" to the contribution of inertia.

right, understood.
actually that point is related to what i was going on about.
in reverse: 'inertia's contribution is not 'opposite' to the contribution of dark matter etc'. But DE's is.
It was in that sense that i felt DE was what was important, as far as explaining the expanding universe.
thank you for all your help with this.

 

[PeterDonis]

both signs matter (to the topic of rate), but not (both signs matter) to the topic of why universe is expanding. Let me know if I'm wrong.

I don't think it's a matter of being "right" or "wrong". It's just your personal judgment about what answer satisfies you for what question.

 

[CHOP]

I don't think it's a matter of being "right" or "wrong". It's just your personal judgment about what answer satisfies you for what question.

Well, isn't it more objective than that? To explain why something expands, doesn't include why something contracts.
Explaining why the rate is what it is, would include both.

No?

 

[CHOP]

But remember that expansion of the universe is not a nebulous term, but one with a specific meaning - it's the growth of large-scale distances. So if you ask what causes expansion, then you're asking what causes the distances to grow. The answer must then be the recession velocity (by definition; it's what velocity means), which in turn has contributions from both DE and the initial impulse.

right, i will remember that.
thank you again for your clarifications,

 

[PeterDonis]

To explain why something expands, doesn't include why something contracts.

I would say that decelerating expansion is not the same thing as contraction. But now we're just having different preferences for how we use words. To you, "explaining expansion" only includes, apparently, explaining why expansion either continues at the same rate, or accelerates. To me, it includes everything that's necessary to explain why the observed rate of expansion is what it is. We agree on the physics; we're just choosing different ordinary language to describe it.

 

[PAllen]

Note, again, a matter dominated universe with no dark energy will expand forever if there is not too much mass. It is just that the power law followed by the scale factor will have a positive exponent less than 1.

 

[PeterDonis]

To you, "explaining expansion" only includes, apparently, explaining why expansion either continues at the same rate, or accelerates.

Btw, it's worth noting in this connection that the description of the expansion as "accelerating" or "continuing at the same rate" or "decelerating", that we have been using in this thread, depends on a particular definition of what the "expansion rate" is: that it is the (normalized) second time derivative of the scale factor, $\ddot{a} / a$. But we don't directly observe this. What we directly observe is the Hubble constant $H$ and its rate of change $\dot{H}$. But $H$ is not increasing with time! What we have been calling "accelerated expansion" does not mean $H$ increases; it just means $H$ is decreasing asymptotically towards some positive constant (related to the cosmological constant) instead of towards zero.

So if we think of things in terms of $H$ as the "rate of expansion", then this rate is always decreasing, and dark energy, inertia, and ordinary/dark matter and radiation are just factors that make different contributions to the rate at which $H$ is decreasing. On this view it's not clear why any of those factors could be excluded in "explaining expansion".

 

[CHOP]

Note, again, a matter dominated universe with no dark energy will expand forever if there is not too much mass. It is just that the power law followed by the scale factor will have a positive exponent less than 1.

Hi, do you mean that the (constant) velocities are such that we've past the point that gravity could reverse the direction?

 

[PeterDonis]

do you mean that the (constant) velocities are such that we've past the point that gravity could reverse the direction?

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.

 

[CHOP]

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?

 

[CHOP]

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?

 

[PeterDonis]

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.

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.

 

[PeterDonis]

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".

 

[CHOP]

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.

Deceleration is also "motion commencing".

Ah, very true!

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?

 

[weirdoguy]

and speculative

As been told to you in your thread, it is not.

 

[PeterDonis]

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.

 

[CHOP]

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?

 

[PeterDonis]

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.

 

[CHOP]

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?

 

[Drakkith]

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.

 

[PAllen]

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.

 

[PeterDonis]

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.

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.

 

[CHOP]

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

 

[CHOP]

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,

 

[RandyD123]

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"...

 

[phinds]

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.

 

[Drakkith]

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!

 

[phinds]

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.

 

[PeterDonis]

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.

 

[RandyD123]

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!

 

[phinds]

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?

 

[Ken G]

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.

 

[phinds]

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.

 

[RandyD123]

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!

 

[Ken G]

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.

 

[phinds]

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.

 

[Ken G]

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.

 

[Michaela SJ]

ed

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.