Big Bang theory and the known universe

phinds
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Could you explain yourself a bit more?
Just read up on the BBT(*). It's a description of the expansion of the universe from a dense hot plasma, after the age of Inflation (which is itself problematic but that's irrelevant since it's before the BBT starts) to now. It is a very successful theory.

* But watch out for pop-sci presentations which always get it completely wrong, starting with the incorrect statement that the universe started with either a point of infinite mass and zero volume, or that it started with a "singularity" which they define poorly or just, again, as a "point" which is not what that word means.
 
Chronos
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The BB theory makes no attempt to explain the origin of the universe, it only asserts that a very long time ago the universe was fantastically dense and hot. The confusion arises when you attempt to force it back beyond its realm of applicability [i.e.,, t=0]. Modern modeling suggests the big bang was preceded by a brief period of inflation. What came before inflation is still unknown - although that has never prevented a good guess.
 
No, it's not the word "observable" in that sentence that is misleading (as has already been explained), it's the word "point". While it is true that the observable portion of the universe started off as a very small volume, it did not start off as a point.
I am trying to improve some Wikipedia articles on this, and I think it's an excellent thread to seek some clarifications on Big Bang theory. In regards to "early BB", where in time current BB theories are becoming more speculative? What is known with near 100% certainty, and how this percentage goes lower and lower when we go back in time.

Let me start, and feel free to chime in:

- we are very close to 100% certain about conditions at recombination, ~380 thousand years since BB (matter and energy contents of the Universe, temperature, pressure, isotope balance, size of the volume which will become "observable Universe", etc...). The only thing we don't know is what exactly dark matter particle(s) were (at any moment in the history of Universe, not only this moment).

- BB nucleosynthesis (~10 to 1000 seconds since BB). We are almost as certain about this moment in history too.

- Neutrino decoupling (~1 second). This is still fairly well understood. Since this corresponds to about 1MeV temperature, it is well within the explored area of particle interactions. We still know all particle types which should have been present at this moment, no unknowns here. Our knowledge might become even better when (and if) primordial neutrino background from this moment will be detected, similar how study of CMB improved our knowledge in the past. (How big the radius of volume of future "Observable Universe" was at this point, was is ~15 ly? What was the average density?)

- Quark-gluon plasma epoch, followed by "hadron epoch". (Currently Wiki states that these periods were roughly at [10^−12, 10^−6] seconds and [10^−6, 1] seconds since BB. Does this look about right for you?) We have mostly good grasp of hadron physics, byt our knowledge of high-density states of hadronized matter (neutron stars, for example) and quark-gluon plasmas is relatively new and is evolving. We have qualitative understanding of it, but precise, sub-1% predictions are not currently available.

- Electroweak symmetry breaking. Temperature was about ~100 GeV. Was it at about 10^−32s "since BB"? Unknown particle flavors may be present (semi-random example - nuMSM extension of standard model says additional heavy neutrinos existed at this time, and they decayed before neutrino decoupling occurred).

- End of inflation, reheating. This is increasingly speculative territory, but I think scientists do have good ideas what temperatures should be achieved by reheating (too high temperature may cause unwanted particle types to be generated, such as magnetic monopoles. Inflation theory one of the reasons of existence is to "dilute" them to zero density and thus explain why they are not observed - can't have that ruined, right?). What are these temperatures?

- "Grand unification epoch", inflation, quantum gravity epoch. This is speculative territory. We don't know whether there _is_ a GUT. We don't know how long inflation lasted, and what field causes it. We don't know how cold Universe become during inflation, before reheating - 10^22K? 1K? Practically zero kelvins? We have no well-developed quantum gravity theory. Also, "N seconds since BB" times stop making sense here, "since BB" is the backward continuation of observed Hubble expansion and is not a hard rule, and this continuation stops being valid. Am I right about this?
 
russ_watters
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I would like to point out, that the Big Bang itself is a theory and we do not have definitive proof of its existence. We have some theories predicting its existence, but we are not 100% sure.
I wouldn't exactly say that's wrong (albeit not well worded), just overused and essentially pointless. Every theory is by definition not 100% proven. So what? I wouldn't go betting against the sun rising tomorrow because GR isn't 100% proven!
 
George Jones
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I would like to point out, that the Big Bang itself is a theory and we do not have definitive proof of its existence. We have some theories predicting its existence, but we are not 100% sure.
Hi Ivan. I see that you are young (12), and that you are interested in learning about science. What Russ wrote is important.

I wouldn't exactly say that's wrong (albeit not well worded), just overused and essentially pointless. Every theory is by definition not 100% proven. So what? I wouldn't go betting against the sun rising tomorrow because GR isn't 100% proven!
In one of his books, Robert Geroch, a top mathematical physicist, elaborated on this:

Robert Geroch said:
It seems to me that "theories of physics" have, in the main, gotten a terrible press. The view has somehow come to be rampant that such theories are precise, highly logical, ultimately "proved". In my opinion, at least, this is simply not the case - not the case for general relativity and not the case for any other theory in physics. First, theories, in my view, consist of an enormous number of ideas, arguments, hunches, vague feelings, value judgements, and so on, all arranged in a maze. These various ingredients are connected in a complicated way. It is this entire body of material that is "the theory". One's mental picture of the theory is this nebulous mass taken as a whole. In presenting the theory, however, one can hardly attempt to present a "nebulous mass taken as a whole". One is thus forced to rearrange it so that it is linear, consisting of one point after another, each connected in some more or less direct way with its predecessor. What is supposed to happen is that one who learns the theory, presented in this linear way, then proceeds to form his own "nebulous mass taken as a whole". The points are all rearranged, numerous new connections between these points are introduced, hunches and vague feelings come into play, and so on. In one's own approach to the theory, one normally makes no attempt to isolate a few of these points to be called "postulates". One makes no attempt to derive the rest of the theory from postulates. (What, indeed, could it mean to "derive" something about the physical world?) One makes no attempt to "prove" the theory, or any part of it. (I don't even know what a "proof" could mean in this context. I wouldn't recognize a "proof" of a physical theory if I saw one.)
 

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