High School The Universe without Cosmic Inflation?

[PeterDonis]

I was just referring to the flat universe by saying "stable"

Why do you think a flat universe is the only kind that can be "stable"?

 

[PeterDonis]

I’m aware of that. Please see post #27.

Ah, got it.

 

[Arman777]

Why do you think a flat universe is the only kind that can be "stable"?

Because flat case happens only $Ω=1$ and for all $Ω≠1$ universe have two different possibilities. It kind of seemed special "stable point". But now I am thinking $Ω=1$ as a local maximum point of possible $Ω$.

I was assuming that you are discussing our universe (mentioning the flatness problem), not a arbitrary FRW model. Then if you drop the inflation which produces matter wherelse should matter density come from?

That is a good point. Hmm well, it's hard to imagine maybe we will not have standard model particles but still, we can have some sort of radiation energy density, which can determine the fate of the universe?

 

[PeterDonis]

for all Ω≠1 universe have two different possibilities.

What are you referring to here?

 

[Arman777]

What are you referring to here?

$Ω<1$ a universe with hyperbolic spatial geometry
For $Ω>1$ a universe with spherical spatial geometry

 

[PeterDonis]

Ω<1a universe with hyperbolic spatial geometry
Ω>1 a universe with spherical spatial geometry

Ok. But why are these not "stable"?

 

[Arman777]

Ok. But why are these not "stable"?

They are "stable" in their own type of spatial geometry.

The first I said stable was here,

I don't think so. One of the crucial points in the inflation theory is that it solves flatness problem. Without the inflation Even there's CC I think the universe cannot be stable.

By that I mean this, I'll quote from a book

"The flatness problem states that such finely tuned initial conditions seem extremely unlikely. Almost all initial conditions lead to either closed universe that recollapses after immediately, or to an open Universe that very quickly enters the curvature-dominated regime and cools down to below 3K within in seconds. For this reason, the flatness problem also phrased as an age problem- how did our Universe get to be so old ?"

In this case by saying "cannot be stable" I was referring to this part, Depending on the initial conditions the universe can evolve in some direction.For example (quoting again)

"In the nucleosynthesis, when the universe was around 1s old we require that for a flat universe the density should be $\left|Ω_{nuc}-1\right|≤10^{-16}$

It's such a low number and that is why we need a tuning.

(Andrew Liddle, Cosmological Inflation and Large-Scale Structure page 37)

 

[Arman777]

I was assuming that you are discussing our universe (mentioning the flatness problem), not a arbitrary FRW model.

Well kind of yes, by mentioning flatness problem I was actually referring to this

"The flatness problem states that such finely tuned initial conditions seem extremely unlikely. Almost all initial conditions lead to either closed universe that recollapses after immediately or to an open Universe that very quickly enters the curvature-dominated regime and cools down to below 3K within in seconds. For this reason, the flatness problem also phrased as an age problem- how did our Universe get to be so old ?"
as I said in my previous post.

In this case, I was just trying to mention that without the inflation it's hard to say something about the universe spatial geometry and hence about its future. But you are right I think, in any case, we wouldn't have matter density (since we can't make an experiment for other possibilities) and the universe would have been evolved in de Sitter or Milne Model as you referred.

But now I am also thinking where the CC came from? Will it be there even there would be no inflation?

 

[timmdeeg]

But now I am also thinking where the CC came from? Will it be there even there would be no inflation?

We are quite sure that there exists repelling gravity which is usally called dark energy. But according to the data the latter could be also a CC. We don’t know the nature of this kind of gravity. And as was pointed out by others the inflation is independent of that. CC or dark energy to exist does not require inflation.

 

[Arman777]

We are quite sure that there exists repelling gravity which is usally called dark energy. But according to the data the latter could be also a CC. We don’t know the nature of this kind of gravity. And as was pointed out by others the inflation is independent of that. CC or dark energy to exist does not require inflation.

Is there any other dark energy theories other than CC? Is it a good idea to treat the CC a kind of repelling gravity?

Inflation cannot be caused by CC but I am not sure. If CC is there inflation happens or not then CC becomes a natural instinct property of the space-time and nothing else.

In that case, CC never can be zero. Since its a property of ST.

 

[timmdeeg]

Is there any other dark energy theories other then CC ? Is it a good idea to treat the CC a kind of repelling gravity ?.

I woudn’t call CC a dark energy theory. My advise is to google DE to see the difference. The observed accelerated expansion of our universe is due to something which is sometimes called a bit sloppy “repelling gravity”. You could have look to the 2. Friedmann equation and see what happens to the sign of the second derivative of the scale factor in case $\Lambda$ dominates $(\rho+3p)$.

Inflation cannot be caused by CC but I am not sure. If CC is there inflation happens or not then CC becomes a natural instic properity of the space-time and nothing else.

In that case CC never can be zero. Since its a properity of ST.

Inflation is independent of CC, please see my previous post. What is ST?

 

[PeterDonis]

Inflation cannot be caused by CC but I am not sure.

Inflation itself could be caused by a CC, but the value of the CC would have to be much, much larger than the one we actually measure.

What cannot be caused by a CC is the end of inflation, where the energy stored in the inflaton field gets transferred to the Standard Model fields. The energy density of a CC is constant--that's what "cosmological constant" means. So it can't get transferred to anything else.

So whatever caused inflation, it has to be something that can give up energy to something else; the simplest assumption is a scalar field. According to our best current model, during inflation, the energy density of this field was so large compared to the CC (which had the same value as it does today) that the effect of the CC on the dynamics of the universe was negligible, since both the energy density of the scalar field during inflation and the CC had constant values.

Once inflation ended, and all of that energy got transferred to the Standard Model fields, the energy density in those fields--what we usually call "matter" and "radiation"--started to decrease as the universe expanded. A few billion years ago, the energy density in matter and radiation became smaller than the energy density of the CC; that's when the CC started to be the major factor determining the dynamics of the universe.

In that case CC never can be zero. Since its a properity of ST.

What are you referring to here?

 

[Arman777]

Inflation is independent of CC

Yes, I am aware of that. As I said

If CC is there, inflation happens or not, then CC becomes a natural instinct property of the space-time and nothing else.

In that case, CC never can be zero. Since its a property of ST.

What are you referring to here?

ST means space-time.

Since CC was there even in the period of cosmic inflation then we can conclude that CC was an instinct property of the space-time. Which indeed it is. So in this sense, we can say that even there's nothing (no matter and no radiation) we would have only the cosmological constant.

If the universe was created without inflation, it would be the "de Sitter" universe. That's the only answer then.

 

[nearc]

I was assuming that you are discussing our universe (mentioning the flatness problem), not a arbitrary FRW model. Then if you drop the inflation which produces matter wherelse should matter density come from?

for my own clarity, are you saying inflation is required to produce matter or inflation affects the production of matter?

 

[timmdeeg]

for my own clarity, are you saying inflation is required to produce matter or inflation affects the production of matter?

According to the inflationary theories matter is produced at the end of inflation, see Reheating. So inflation is required and affects the production of matter.

 

[nearc]

According to the inflationary theories matter is produced at the end of inflation, see Reheating. So inflation is required and affects the production of matter.

unless I'm very confused [which is often] i thought matter forms when the temperature drops. there might also be some need for expansion or even complicated cooling/heating but ultimately once things are cool enough and there is enough room matter can form? we tie matter formation into inflation as a nice gift wrapped package, but if there were some other way that allows the universe to cool and spread out matter should still form?

 

[bapowell]

unless I'm very confused [which is often] i thought matter forms when the temperature drops. there might also be some need for expansion or even complicated cooling/heating but ultimately once things are cool enough and there is enough room matter can form? we tie matter formation into inflation as a nice gift wrapped package, but if there were some other way that allows the universe to cool and spread out matter should still form?

By "matter forms when the temperature drops" are you referring to the broken symmetries that confer mass to the various elementary particles, or are you thinking about something more generic?

We don't *need* inflation to create matter, since that would be a circular argument (i.e. what created the inflaton?) The novelty of reheating is that inflation effectively erases the energy density in the inflated region, and without it we get a cold, empty universe after inflation.

Also, I'll mention that the universe cools as it expands, whether inflation happened or not.

 

[nearc]

By "matter forms when the temperature drops" are you referring to the broken symmetries that confer mass to the various elementary particles, or are you thinking about something more generic?

We don't *need* inflation to create matter, since that would be a circular argument (i.e. what created the inflaton?) The novelty of reheating is that inflation effectively erases the energy density in the inflated region, and without it we get a cold, empty universe after inflation.

Also, I'll mention that the universe cools as it expands, whether inflation happened or not.

that's what i thought thanks for the clarification. i was tempted to use the analogy about how babies are produced during sex but babies can also be produced without sex, however, in that analogy inflation would be intercourse so there are some obvious crass images plus the very short duration of inflation might hit too close to home for some.

 

[JMz]

It has been pointed out that we are in the midst of a new BB, due to the cc. (That does not mean that the previous BB was related in any way to the cc.)

BTW, there is no a priori reason that the cc (vacuum energy density) should be positive. We do not currently know of any reason it could not have been negative, in which case the expansion would eventually reverse regardless of how low the mass density was.

 

[timmdeeg]

It has been pointed out that we are in the midst of a new BB, due to the cc.

By whom?

BTW, there is no a priori reason that the cc (vacuum energy density) should be positive. We do not currently know of any reason it could not have been negative, in which case the expansion would eventually reverse regardless of how low the mass density was.

We do currently have a reason to believe that the CC is positive. Otherwise we couldn't explain why we observe that the universe expands accelerated.

 

[JMz]

By whom?

We do currently have a reason to believe that the CC is positive. Otherwise we couldn't explain why we observe that the universe expands accelerated.

Who: By Neil de Grasse Tyson, as I recall. Not that it matters: We are in a period of exponential expansion with no apparent or near-term upper limit. This is just as much as Bang as the one 14 GY ago, and just as Big.

cc: What part of "no a priori reason that the cc ... should be positive", and "[no] reason ... it could not have been negative" wasn't clear? Our current understanding of physics does not predict a necessarily positive value, or any particular value. We believe it's positive solely because the observations say it's positive, not because of GR. Unless you are aware of theoretical analysis that is broadly accepted in a cosmological context that I am unaware of...? (Always possible.)

 

[timmdeeg]

We are in a period of exponential expansion with no apparent or near-term upper limit. This is just as much as Bang as the one 14 GY ago, and just as Big.

No, our universe is in a period of accelerated expansion and will approach exponential expansion in the far future asymptotically. And no, this isn't "just as much as Bang ..."

cc: What part of "no a priori reason that the cc ... should be positive", and "[no] reason ... it could not have been negative" wasn't clear? Our current understanding of physics does not predict a necessarily positive value, or any particular value. We believe it's positive solely because the observations say it's positive, not because of GR. Unless you are aware of theoretical analysis that is broadly accepted in a cosmological context that I am unaware of...? (Always possible.)

Yes, there is "no a priori reason that the cc ... should be positive". The sign of the CC is not determined by the theory.
You mentioned in #49 "We do not currently know of any reason it could not have been negative". This is not correct. We do not currently know of any reason it (the CC) could have been negative. Perhaps you have intended to say this because in #51 you confirm "We believe it's positive solely because the observations say it's positive". The observation is the reason.

 

[JMz]

No, our universe is in a period of accelerated expansion and will approach exponential expansion in the far future asymptotically. And no, this isn't "just as much as Bang ..."

Yes, there is "no a priori reason that the cc ... should be positive". The sign of the CC is not determined by the theory.
You mentioned in #49 "We do not currently know of any reason it could not have been negative". This is not correct. We do not currently know of any reason it (the CC) could have been negative. Perhaps you have intended to say this because in #51 you confirm "We believe it's positive solely because the observations say it's positive". The observation is the reason.

You are continuing to miss the "a priori". It could have been negative, based on everything we knew before we had observations. But now we know it isn't.

My impression is that "this isn't 'just as much a Bang'" is an assessment on your part. Rationale for your statement? (I agree that the fully exponential growth is aymptotic, but exponentials have a way of approaching their asymptotes very quickly. Exponentially quickly, one might even say. ;-) The same asymptotic behavior appears to have characterized the previous BB, except at the beginning and end of inflation.

 

[JMz]

BTW, my apologies if the grammatical tenses and moods (as in "could have been") are interfering with this discussion: I now see that you are probably a non-native speaker of English (though obviously fluent), which may be the source of the miscommunication.

 

[JMz]

A clarification: When I first said "a new BB due to the cc", I meant to say "a new period of inflation due to the cc". (I do not recall if Tyson called it a BB or not.) I suspect that is easier to accept.

However, I see no rationale for saying our impending exponential expansion is not a "bang". What should bang mean, if not very rapid expansion? (Even before the inflation theory, people used to refer to that early period as "the BB", when its expansion was modeled as a mere power law. Surely the new exponential expansion is even more worthy of the word "bang".)

 

[PeterDonis]

What should bang mean, if not very rapid expansion?

Which just means you've substituted one vague ordinary language word, "rapid", for another, "bang". This is a question of language, not physics.

For what it's worth, the term "Big Bang" was not really intended to just mean "rapid expansion"; it also meant "very hot and very dense", which our current universe is not. That, IMO, is a good reason to not use the term "Big Bang" to describe our current or future universe. But again, that's a matter of language, not physics. If you and Neil deGrasse Tyson want to call any exponential or approaching-exponential expansion a "Big Bang", be my guest. It doesn't matter to the physics. Nor does it matter if other people choose not to use that terminology. It's pointless to argue about it.

 

[JMz]

Not an argument, but from the POV of pre-expansion, both stages of the universe are/were very much hotter and denser than post-expansion. Ours is not an intrinsically cold or thin era, in any deep sense of physics or cosmology. It's just the coldest and thinnest the universe has been so far. But it's going to get a great deal colder and thinner still. We are in the hot, dense phase of the current expansion.

 

[PeterDonis]

We are in the hot, dense phase of the current expansion.

For some interpretations of the terms "hot" and "dense", yes. But again, that's a matter of language, not physics. And I doubt most people would use the terms "hot" and "dense" to describe our current universe, since our ordinary language sense of those terms is relative to average densities and temperatures here on planet Earth at this epoch.

 

[JMz]

Complete agreement. The reason to label the current era as either inflationary or a new BB is just for the perspective it provides. For example, if (as Alan Guth phrases it) the earlier inflaton scalar field eventually decayed into particles, are there quantum-physics reasons we should expect that the cc, which plays the same role, will do the same? (I suspect we would need the help of a quantum field theorist to answer that one.)

 

[PeterDonis]

if (as Alan Guth phrases it) the earlier inflaton scalar field eventually decayed into particles, are there quantum-physics reasons we should expect that the cc, which plays the same role, will do the same? (I suspect we would need the help of a quantum field theorist to answer that one.)

AFAIK nobody has a quantum field theory that predicts this, but nobody has an ironclad argument ruling it out either.