Big Bang theory and the known universe

[icantevenn]

I just want to verify from physicists whether what I have read in this article is true: http://www.astro.ucla.edu/~wright/infpoint.html
It says:

The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point​

 

[Drakkith]

I'm under the impression that the observable universe was concentrated into a very small, but nonzero volume at the time of the big bang. However I'm certainly not an expert on this and would also like to see a good answer.

 

[Bandersnatch]

Yes, it is. You can generally take everything from Ned Wright's website to be correct.

I'm under the impression that the observable universe was concentrated into a very small, but nonzero volume at the time of the big bang. However I'm certainly not an expert on this and would also like to see a good answer.

It was a point in the limt of the singularity, just as every other finite volume. The distinction made in the linked article is between finite and infinite volume - the latter remains infinite even if you shrink all finite distances to a point.

 

[icantevenn]

Yes, it is. You can generally take everything from Ned Wright's website to be correct.It was a point in the limt of the singularity, just as every other finite volume. The distinction made in the linked article is between finite and infinite volume - the latter remains infinite even if you shrink all finite distances to a point.

Thank you!

 

[Orodruin]

It was a point in the limt of the singularity, just as every other finite volume.

A limit that most physicists would agree is not physical, but rather a result of extrapolating the theory to a regime where it is no longer valid. For the intended readership, that may not be relevant though.

 

[vanhees71]

I'd also say that the unavoidable singularities (like black holes, big bangs/crunches) of standard GR are manifestations of our ignorance rather than really describing nature. Usually one argues that for very small space-time volumes classical gravity shouldn't be valid anymore and that you need some quantum theory of gravity, but we still don't have a fully satisfactory theory, and before we don't have it, we cannot check, whether this really solves the problem of singularities in GR.

 

[russ_watters]

I'd also say that the unavoidable singularities (like black holes, big bangs/crunches) of standard GR are manifestations of our ignorance rather than really describing nature.

Maybe, but I think it is important to keep that tentative until evidence uncovers the flaws rather than assuming the way the universe has to work; like light is a wave so it has to have a medium.

 

[ameryle]

What do you think of this statement about the Big Bang: "... In reality, the expansion did not start in a single point but started in a huge number of points that appeared in a spherical shape. When scientists give up the idea of creation starting in a singularity and look for a spherical beginning, they will get their mathematical equations to work."

 

[Bandersnatch]

When scientists give up the idea of creation starting in a singularity and look for a spherical beginning, they will get their mathematical equations to work.

I think it would be very hard to give up on an idea one doesn't hold, or make work something that is not faulty. Or trivially easy, depending on how you read that.

As is often the case, before one can begin to think outside the box, first one needs to learn where the box is.

 

[phinds]

What do you think of this statement about the Big Bang: "... In reality, the expansion did not start in a single point but started in a huge number of points that appeared in a spherical shape. When scientists give up the idea of creation starting in a singularity and look for a spherical beginning, they will get their mathematical equations to work."

I think you don't understand the meaning of "singularity" because if you did you would have ignored this nonsense instead of asking about it. "Singularity" in this context means "the place where the math model gives nonphysical results and we don't know WHAT is/was happening". It does NOT mean "a point in space" as that writer mistakenly believes.

Also only pop-sci misrepresentation contend that the universe started at a point in space. The big bang happened everywhere at once. This is a bit hard to get your head around when you first encounter it.

 

[Drakkith]

What do you think of this statement about the Big Bang: "... In reality, the expansion did not start in a single point but started in a huge number of points that appeared in a spherical shape. When scientists give up the idea of creation starting in a singularity and look for a spherical beginning, they will get their mathematical equations to work."

It's wrong. Modeling the universe as starting in a spherical shape solves absolutely nothing and introduces unjustified constraints.

 

[maria_phys]

I read a very interesting article about the big bang:

https://insidetheperimeter.ca/things-didnt-go-so-smoothly-at-the-big-bang/

The Big Bang referred as the initial singularity. Mathematicians use the term "singularity" to indicate that the GR equations are failing when the temperature and energy density become infinity . It doesn't have any physical meaning. You need a more accurate theory to describe the law of physics when "singularities" appear in the equations.

 

[Ssnow]

the observable Universe was concentrated into a point

The use of the word ''observable'' is misleading... there is implicitly an observer that is no part of the Universe? ...

 

[Bandersnatch]

The use of the word ''observable'' is misleading... there is implicitly an observer that is no part of the Universe? ...

No, but there are observers who see different patches of the universe.

 

[Ssnow]

are observers who see different patches of the universe.

Ok this makes sense,
thanks

 

[phinds]

The use of the word ''observable'' is misleading... there is implicitly an observer that is no part of the Universe? ...

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 haven't done the math but I've seen various statements ranging from the size of an atom to the size of a golf ball (but I think the golf ball statement is likely way too big)

 

[Chronos]

The initial size of the observable universe depends on the number of e-folds it experienced during inflation [an e-fold roughly doubles the size of the observable universe]. The consensus choice is 50-60 e-folds, as discussed here; How long before the end of inflation were observable perturbations produced?,
https://arxiv.org/abs/astro-ph/0305263, . Of course, this assumes inflation theory and standard cosmological models are correct. Obviously, the observable universe could never been size zero under this scenario, otherwise it would have remained zero. By the same token, this does not preclude a universe of infinite size, since a universe of infinite size could have undergone any number of e-folds and still remain infinite.

 

[russ_watters]

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 haven't done the math but I've seen various statements ranging from the size of an atom to the size of a golf ball (but I think the golf ball statement is likely way too big)

Actually, I think it is the word "observable" that is at issue and by discarding it you are missing it:

The "obervable universe" would have to start at a single point, as any light cone does, because that's how it is defined. The "observable universe" is the expanding sphere of our light cone that started at whatever point we choose to define as the start, regardless of whether the *actual* universe started at a single point.

Why? Because the "start" has to be t=0, which means that light hasn't had a chance to propagate yet to some finite size of the sphere.

 

[russ_watters]

The use of the word ''observable'' is misleading... there is implicitly an observer that is no part of the Universe? ...

There is implicitly an observer that was present at the chosen point, for the event which we define as the "start" at t=0. By definition, it must be a point.

 

[Ssnow]

it's the word "point".

I think here the word ''point'' is not the same of the primitive concept in geometry.

 

[phinds]

I think here the word ''point'' is not the same of the primitive concept in geometry.

I think the difference is that Russ is describing the origin of a light cone which, as he correctly points out, IS a geometric point, and I am discussing the volume of the observable universe at ~ one Plank Time which was not a geometric point. My post had in mind the original question, which did not involve a light cone but was a question about the volume.

 

[Ssnow]

@phinds thanks for the precisation ...
Ssnow

 

[ISamson]

The Universe was not concentrated into a point at the time of the Big Bang. But the observable Universe was concentrated into a point​

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.

 

[phinds]

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.

That is incorrect. I think perhaps you misunderstand just what the Big Bang Theory IS.

 

[ISamson]

That is incorrect. I think perhaps you misunderstand just what the Big Bang Theory IS.

Could you explain yourself a bit more?

 

[phinds]

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]

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.

 

[nikkkom]

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]

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]

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:

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.)