The Universe without Cosmic Inflation?

  • #51
JMz
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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.)
 
  • #52
timmdeeg
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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.
 
  • #53
JMz
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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.
 
  • #54
JMz
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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.
 
  • #55
JMz
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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".)
 
  • #56
PeterDonis
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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.
 
  • #57
JMz
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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.
 
  • #58
PeterDonis
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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.
 
  • #59
JMz
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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.)
 
  • #60
PeterDonis
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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.
 
  • #61
JMz
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AFAIK nobody has a quantum field theory that predicts this, but nobody has an ironclad argument ruling it out either.
Not surprised: Either this follows directly from some general theorem on fields (related to Noether's, maybe), or else it's just speculation at this point, and not appropriate for PF.

(Tantalizing, though. It seems that, given enough time, gravity can organize a lot of structure from such "decay" particles. ;-)
 
  • #62
bapowell
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Look up quintessential inflation and see if it’s related to what you’re thinking.
 
  • #63
JMz
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Look up quintessential inflation and see if it’s related to what you’re thinking.
Thanks for the reference. Not clear how closely, but it's clearly related, since it deals with inflation and the scalar-field cosmological "constant" -- or non-constant in these theories.
 
  • #65
JMz
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How about this: https://en.wikipedia.org/wiki/False_vacuum#Vacuum_metastability_event? People have talked about the possibility that our universe is currently in a metastable false vacuum, and will one day decay to the true vacuum. Typically this phase transition would be 1st order, making it a modern-day version of the "old" primordial inflation models based on the Coleman-deLuccia transition.

Yes, indeed. As I recall, nobody has been able to estimate a half-life time scale for such a collapse that is any longer than a microsecond, or some such interval that is too short by an embarrassing number of orders of magnitude (like, 25): Every microsec, the local universe would have an independent choice of decaying or not, and the probability of lasting 1 millisec would be ~10^-300. So if the reasoning is sound, this leads to the obvious question, Why are we still here? ;-)
 
  • #66
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Inflation solves some problems in cosmology, but is there some other reason to think it happened? And what is the mechanism? Its just too convenient.
 
  • #67
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Inflation solves some problems in cosmology, but is there some other reason to think it happened? And what is the mechanism? Its just too convenient.
If you are asking for evidence of inflation I don't think there is any at the moment, but there could be.
It explains the earliest state of the Universe, and pushes that dreadful singularity away, though it doesn't get rid of it.
 
  • #68
Ibix
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Inflation solves some problems in cosmology, but is there some other reason to think it happened?
It correctly predicted the spectrum of fluctuations in the CMB.
And what is the mechanism?
Not yet known.
 
  • #69
JMz
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Inflation solves some problems in cosmology, but is there some other reason to think it happened? And what is the mechanism? Its just too convenient.
I am curious what "too convenient" means. Aren't good scientific ideas supposed to agree with observations and be based on other, already established science (as inflation does)? In fact, aren't those among the key properties that make an idea "good"? The inflation idea is incomplete, but it is not pseudo-science.
 
  • #70
nearc
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I am curious what "too convenient" means. Aren't good scientific ideas supposed to agree with observations and be based on other, already established science (as inflation does)? In fact, aren't those among the key properties that make an idea "good"? The inflation idea is incomplete, but it is not pseudo-science.

obviously i cannot speak for mr smith, however, i like the use of his phrase "too convenient" as it gets the point across without being as blunt as i would have been, moreover, the phrase is accomplishing its goal by keeping people questioning the validity of using inflation
 
  • #71
JMz
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I am skeptical that there are very many cosmologists who fail to question inflation on its merits.

Inflation is the best working hypothesis at the moment, and I believe most of them would (a) feel comfortable developing further theoretical insights based on it, including observable consequences (tests!) but (b) readily accept an alternative idea that agrees well with current observations -- possibly even agreeing a little less well with some, if it had other advantages (such as not requiring an entirely new "inflaton" field of unknown properties).
 
  • #72
bapowell
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I think it’s unfortunate that people view the consideration of “new fields” to be a weakness of a model or hypothesis. Unless one considers the Standard Model complete, there *must* be additional fields operating in the universe at the same energy scales as inflationary phenomena. Indeed, failing to consider additional fields and new physics when attempting to understand the early universe, precisely when any new physics is expected to be especially relevant, is a bad way to do science .
 
  • #73
bapowell
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If you are asking for evidence of inflation I don't think there is any at the moment, but there could be.
There are several lines of evidence. See Wikipedia, e.g.
 
  • #74
JMz
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I think it’s unfortunate that people view the consideration of “new fields” to be a weakness of a model or hypothesis. Unless one considers the Standard Model complete, there *must* be additional fields operating in the universe at the same energy scales as inflationary phenomena. Indeed, failing to consider additional fields and new physics when attempting to understand the early universe, precisely when any new physics is expected to be especially relevant, is a bad way to do science .
I won't argue the point strongly, since I tend to agree, and I think inflation is likely to be the right story. My only point was that any theory that can explain a phenomenon by relying only on known fields is stronger than than one that needs a new field: That's just parsimony. The BB provides an especially hard case because there are few feasible experiments or predictions that can nail down the attributes of the new field. Conversely, positing a newfangled electromagnetic field 150 years ago provided lots of feasible experiments and unified several physical laws at once.
 
  • #75
JMz
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Unless one considers the Standard Model complete, there *must* be additional fields operating in the universe at the same energy scales as inflationary phenomena.
BTW, someone on WP believes that such new fields are not needed for inflation: https://en.wikipedia.org/wiki/Chronology_of_the_universe.

I can't speak to the validity of that claim. (It's only WP, after all.) But if true, then I will withdraw my earlier parenthetical remark about alternative theories having an advantage if they do not require a new inflaton field: They could have advantages, but avoiding a new field is not one of them.
 

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