Expanding universe needs a big bang?

[CHOP]

PeterDonis,

My original question was: Why is the big bang needed to explain the expanding universe?

When I asked that, I was not talking about the expanded universe. I was talking about the expanding universe.

And an expanding universe is one happening now.

Your position seems to be: my question is based on false premise. That is, the big bang is not needed to explain the expanding universe.

That ends the story if that's the consensus. My impression (of what the consensus was) was simply wrong.

I didn't get to know that until you made the post you just made. So thank you very much.

So, its DE. That's how I have always been thinking.
And it was said here that DE is a property of GR, so it is not a mystery at all, correct? If incorrect, what is the mystery?

 

[PeterDonis]

My original question was: Why is the big bang needed to explain the expanding universe?

And the answer is what I've already said: if you don't include the inertia from the big bang, you will make an incorrect prediction for what the expansion rate should be now. So since explaining the expanding universe means explaining why the expansion rate now is what it is, you need to include the big bang in the explanation to get the right answer.

If this answer does not satisfy you, then I think you're confused about what question you're asking, because the answer I've just given is the answer to the question quoted above.

Your position seems to be: my question is based on false premise. That is, the big bang is not needed to explain the expanding universe.

No, that's not what I'm saying. See above.

That ends the story if that's the consensus. My impression (of what the consensus was) was simply wrong.

No, it wasn't. See above.

So, its DE.

Not by itself. See above.

 

[rootone]

Observations are that the Universe is expanding,
It is expanding even quicker than we thought it was 50 years ago.
Nobody can say why it is expanding, but it definitely isn't shrinking.

 

[PeterDonis]

it was said here that DE is a property of GR, so it is not a mystery at all, correct?

I'm not sure it's accurate to say DE is not a mystery. It's easy to model using GR (just include a cosmological constant), but that's not the same as understanding why that model works. I don't think we currently have a good understanding of why including DE in the model works.

 

[Comeback City]

Unfortunately it is not. The way that objects recede from each other in the expansion of the universe is not like the expansion of fragments away from an explosion. The only way for the two to be the same would be for the big bang to have taken place in a single location, and for that location to have been exactly where our galaxy is located at now. That's a very unlikely coincidence.

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

 

[phinds]

Observations are that the Universe is expanding,
It is expanding even quicker than we thought it was 50 years ago.
Nobody can say why it is expanding, but it definitely isn't shrinking.

That is not correct. It is expanding due to inertia from the original expansion at the end of inflation (assuming inflation is correct). This is well understood.

What is also understood is that the expansion is accelerating. WHY it is accelerating is attributed to "dark energy" which is a thing that we know the effects of but not the cause.

 

[rootone]

That is not correct.

Only if you assume inflation.
Which is mathematically sound, but is not proof of anything.

 

[phinds]

Only if you assume inflation.
Which is mathematically sound, but is not proof of anything.

No, I don't think inflation is required for the universe to be expanding, although without inflation I assume it would have expanded more slowly originally and perhaps contracted again by now. The Big Bang Theory does not include inflation but assumes an expanding hot plasma as its beginning.

 

[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?"

That's very interesting. I am going to think about that and I will get back to you.

Thank you.

 

[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?"

What other question would you map "Why is the universe expanding?" to in a physics forum? Physics is about making quantitative predictions.

 

[PAllen]

Worth stating explicitly:

Assume homogeneity, isotropy, that the universe is not vacuum, and that the simplest form of the Einstein field equations hold (no cosmological constant), then expansion is mathematically required. Further, it follows that there is no upper bound on energy density observed by a comoving geodesic as it followed backwards in time. It is this last feature that is called a ‘big bang’. Thus, the Big Bang in cosmology based on GR is simply unavoidable.

Inflation and cosmological constant are only needed to explain further details of what we observe, e.g. the degree of uniformity in CMB, and the accelerated rate of expansion.

 

[PeterDonis]

Assume homogeneity, isotropy, that the universe is not vacuum, and that the simplest form of the Einstein field equations hold (no cosmological constant), then expansion is mathematically required

More precisely, expansion or contraction (except at a single instant if the universe is closed). We know from observation that the universe is expanding; math alone can't actually tell us that given the assumptions.

It is this last feature that is called a ‘big bang’.

In an idealized model such as you describe, yes, the term "big bang" can refer to the initial singularity. But I think this usage can be confusing, and I think it's better to try to reserve the term "big bang" for the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence.

 

[PAllen]

In an idealized model such as you describe, yes, the term "big bang" can refer to the initial singularity. But I think this usage can be confusing, and I think it's better to try to reserve the term "big bang" for the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence.

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

 

[PAllen]

More precisely, expansion or contraction (except at a single instant if the universe is closed). We know from observation that the universe is expanding; math alone can't actually tell us that given the assumptions.

True, but an open manifold with contraction is mathematically indistinguishable from one with expansion. The distinction only comes from coordinates assigned. I think entropic arguments can be used to choose with is the correct physical interpretation of the manifold.

 

[PeterDonis]

an open manifold with contraction is mathematically indistinguishable from one with expansion

Yes, fair point, the choice of which timelike direction is "future" is arbitrary, mathematically speaking.

 

[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.