The Duration of the Big Bang: Exploding Myths and Misconceptions

[PeterDonis]

what is the meaning of an expanding, yet infinite universe?

It means that the distance between objects in the infinite universe is increasing with time. You have to be very careful with infinities; they don't work the way many of our intuitions about finite things would indicate. An infinite universe (more precisely, a spatially infinite one) with objects moving apart with time is perfectly consistent.

 

[PeterDonis]

Does the 380 million years really mean anything in today’s terms?

Yes. When I said there isn't one clock that applies to the entire universe, I was referring to the fact that observers moving on different worldlines through spacetime can experience different elapsed times between events. I was not saying that the concept of "time" has no meaning at all. I was only saying that you need to specify which observers are measuring the time (note that the "observers" can be hypothetical and don't have to be conscious beings, hypothetical atomic clocks or light clocks work just fine).

In the case of the 380 thousand years (per your correction from 380 million in a later post), that means 380 thousand years as a hypothetical observer on a "comoving" worldline (meaning, an observer who always sees the universe as homogeneous and isotropic) would measure time. This is perfectly well-defined; but it applies only to that particular family of observers. Observers in different states of motion would measure different elapsed times.

 

[PeterDonis]

Could someone please clarify what we mean by “infinite” in this context?

It means "spatially infinite"--the spatial volume of the universe is infinite at any instant of time. Here "time" means "comoving time", i.e., the time experienced by "comoving" observers who always see the universe as homogeneous and isotropic.

 

[PeterDonis]

In the first inflation period, which lasted a fraction of a second, there was an incredible number of events

What do you mean by "number of events"? It is true that the universe was expanding much, much faster, but how does that translate into "more events"? Remember that, according to classical GR (we are leaving out speculations about quantum gravity in this discussion), spacetime is a continuum, so there are exactly as many points in a line segment one femtometer long as there are in all of spacetime (namely, $C$ of them, where $C$ denotes the cardinality of the continuum).

 

[PeterDonis]

I have reopened the thread and posted some clarifications on points that it seemed were causing confusion. Please keep the discussion on the topic of the original question (which I think has been answered by now, but there may be further follow-up questions).

 

[Buzz Bloom]

Those happen at a point. The BB singularity did not.

Hi phinds:

If the universe is finite, then its volume is and always has been finite. If we ignore the Planck period as special with respect to the GR gravitational model, the scale factor, a, approaches zero as time approaches zero. The volume of a finite universe is always proportional to a³. Therefore the volume approaches zero as time approaches zero. Linguistically, it seems reasonable to say that a volume that approaches zero approaches a spacially geometric point.

Your statement that the BB did not happen at a point seems reasonable only from a point of view that explicitly leaves time equals zero out of the discussion. But if the discussion includes a singularity, then for a finite universe it seems reasonable to say the singularity occurred at time equal to zero, and for a finite universe the volume at that moment is zero. Therefore it is reasonable to say the singularity was a point singularity. I think it is also reasonable to say that this singularity happened at a point in 3+1D spacetime.

Regards,
Buzz

 

[PeterDonis]

Your statement that the BB did not happen at a point seems reasonable only from a point of view that explicitly leaves time equals zero out of the discussion.

And it is correct to leave time = zero out of the discussion, because that time--the initial singularity--is not part of spacetime. Pop science presentations often fail to mention this, because it takes a lot of the fun out of talking about the initial singularity. In the actual models (the FRW spacetimes, and in fact spacetimes in general), singularities are not part of the manifold.

Also, the term "Big Bang" as it is standardly used in cosmology, does not refer to the initial singularity; it refers to the hot, dense, rapidly expanding state that was present at the end of the inflationary era. That is the earliest point at which our knowledge is reasonably reliable. We certainly do not know enough to say that that hot, dense, rapidly expanding state arose from an earlier state that approached infinite density. In fact, most cosmologists do not believe that is the case.

 

[Buzz Bloom]

it refers to the hot, dense, rapidly expanding state that was present at the end of the inflationary era.

Hi Peter:

I thought that the inflationary period was not "officially" part of the standard model, but remains a still controversial proposal for a possible extension of the model. If the inflationary period is excluded, what other event becomes the beginning of "the hot, dense, rapidly expanding state" of the universe? Could it be the end of the Planck era?

If the inflationary period is included in the model, then the time at which the BB happened is definitely not t=0.

Regards,
Buzz

 

[phinds]

Hi phinds:

If the universe is finite, then its volume is and always has been finite. If we ignore the Planck period as special with respect to the GR gravitational model, the scale factor, a, approaches zero as time approaches zero. The volume of a finite universe is always proportional to a³. Therefore the volume approaches zero as time approaches zero. Linguistically, it seems reasonable to say that a volume that approaches zero approaches a spacially geometric point.

Your statement that the BB did not happen at a point seems reasonable only from a point of view that explicitly leaves time equals zero out of the discussion. But if the discussion includes a singularity, then for a finite universe it seems reasonable to say the singularity occurred at time equal to zero, and for a finite universe the volume at that moment is zero. Therefore it is reasonable to say the singularity was a point singularity. I think it is also reasonable to say that this singularity happened at a point in 3+1D spacetime.

Regards,
Buzz

I disagree w/ your conclusion. The problem here is that you are taking the limit seriously and as Peter said, that makes no sense. Yes, the universe gets smaller and smaller as you approach zero but since you can't take the zero part seriously, you have to stop somewhere and wherever you stop there is a current universe size that corresponds to that initial size and that initial size can be any arbitrary size since there is always a current size that corresponds to it. If we knew the size of the current universe we could extrapolate all the way back to one Plank Time and say what it was then, but it would not be zero and could well but enormous.

If infinite now then it was infinite then but I get that you already realize that.

 

[PeterDonis]

I thought that the inflationary period was not "officially" part of the standard model, but remains a still controversial proposal for a possible extension of the model.

The inflationary period itself is still a matter of investigation, because we don't know exactly which inflationary model is the right one. But the "BIg Bang" state at the very end of inflation is part of the standard model, and the fact that that state was produced by "reheating" when inflation ended is also, AFAIK, part of the standard model.

If the inflationary period is excluded, what other event becomes the beginning of "the hot, dense, rapidly expanding state" of the universe?

The hot, dense, rapidly expanding state itself is the "beginning". Even if, for some reason, we discovered that none of the inflationary models would work, that wouldn't change the fact that the hot, dense, rapidly expanding state existed, and that it can serve as the starting point for our model of the universe since that state. I referred to it as happening at "the end of inflation" because, as I said above, the end of inflation is part of the standard model of cosmology, even if the details of how inflation happened are not because we don't know for sure which model is right.

Could it be the end of the Planck era?

No. The hot, dense state we know to have existed, and which we call the Big Bang, was still many orders of magnitude less hot and dense that an end of Planck era state would be.

If the inflationary period is included in the model, then the time at which the BB happened is definitely not t=0.

It's not even if, for some reason, we discovered that no inflationary model would work. The time at which the BB happened is the time at which the BB happened--the time at which the hot, dense state we call the "Big Bang" happened. That time is not "t = 0" (i.e., the notional time coordinate assigned to the "initial singularity") in any cosmological model.

 

[Buzz Bloom]

Hi Peter:

Thanks for your post. I much appreciate all your answers to my questions.

Is it possible to associate the event you have identified as

The hot, dense, rapidly expanding state

with any particular event other than the end of inflation? I am thinking of such candidate events as the following.

(1) The first creation of the Higgs boson
(2) The weak-EM symmetry breaking
(3) The GUT symmetry breaking
(4) The electron positron annihilation
(5) The quark anti-quark annihilation
(6) Other?

Regards,
Buzz

 

[Buzz Bloom]

I disagree w/ your conclusion. The problem here is that you are taking the limit seriously and as Peter said, that makes no sense.

Hi phinds:

Thanks for your post.

I am not sure which of my conclusions you disagreed with. I think you may have misundefrstood my point.

But if the discussion includes a singularity, then for a finite universe it seems reasonable to say the singularity occurred at time equal to zero, and for a finite universe the volume at that moment is zero. Therefore it is reasonable to say the singularity was a point singularity. I think it is also reasonable to say that this singularity happened at a point in 3+1D spacetime.

In these statements I was not talking about physics. I was talking about discussions. I get that the singularity is not part of the physics. However, a discussion about where or when the singularity occurred is then not physics. It's just about the math. When discussing math it is OK to say that

f(x) = 1/x has a singularity at x=0.​

Regards,
Buzz

 

[phinds]

Hi phinds:

Thanks for your post.

I am not sure which of my conclusions you disagreed with. I think you may have misundefrstood my point.

In these statements I was not talking about physics. I was talking about discussions. I get that the singularity is not part of the physics. However, a discussion about where or when the singularity occurred is then not physics. It's just about the math. When discussing math it is OK to say that

f(x) = 1/x has a singularity at x=0.​

Regards,
Buzz

Sorry, I thought we were discussing physics. If you want to discuss unicorns, that's perfectly reasonable, just not something I have any interest in.

 

[PeterDonis]

I am thinking of such candidate events as the following.

(1) The first creation of the Higgs boson
(2) The weak-EM symmetry breaking
(3) The GUT symmetry breaking
(4) The electron positron annihilation
(5) The quark anti-quark annihilation

All of these events happened after the Big Bang--that is, they happened after the hot, dense state was already formed. They are all particular things that happen at various temperatures as the hot, dense state expands and cools.

 

[Buzz Bloom]

Hi Peter:

Thanks for your post. Your answer helps clarify my confusion, but raises another question.

Earlier I asked, "Could it be the end of the Planck era?" where "it" refers to the beginning of "the hot, dense state". You answered:

No. The hot, dense state we know to have existed, and which we call the Big Bang, was still many orders of magnitude less hot and dense that an end of Planck era state would be.

Why was the much hotter temperature at the end of the Planck era state incompatible with the Big Bang hot, dense state? In what way would the higher temperature at the end of the Planck era interfere with some defining attribute of the Big Bang state?

Regards,
Buzz

 

[Orodruin]

He never said it was incompatible. He said we cannot extrapolate our theories to the Planck era and be certain that they will be valid. This is simply because we have not made observations at the energies required to tell what goes on in such extreme situations.

 

[Buzz Bloom]

He said we cannot extrapolate our theories to the Planck era and be certain that they will be valid.

Hi Orodruin:

Thanks for your post. I confess I had misinterpreted what Peter meant.

Regards,
Buzz

 

[Buzz Bloom]

All of these events happened after the Big Bang--that is, they happened after the hot, dense state was already formed.

Hi Peter:

The quote above answered my question about 5 candidate events. I have found in the Wikipedia article

https://en.wikipedia.org/wiki/Chronology_of_the_universe​

some "dates" that seem inconsistent with this answer, and

Also, the term "Big Bang" as it is standardly used in cosmology, does not refer to the initial singularity; it refers to the hot, dense, rapidly expanding state that was present at the end of the inflationary era.

(1) The "Inflationary epoch" had an "unknown duration", but ended "10⁻³²(?) second after the Big Bang."
(2) The "Grand Unification" epoch occurred "between 10⁻⁴³ second and 10⁻³⁶ second after the Big Bang."
(3) The "Electroweak epoch" occurred "between 10⁻³⁶ second (or the end of inflation) and 10⁻³² second after the Big Bang."​

If I am interpreting this correctly, the Grand Unification epoch and the Electroweak epoch occurred before the BB. Please comment on this.

Regards,
Buzz

 

[PeterDonis]

I have found in the Wikipedia article
https://en.wikipedia.org/wiki/Chronology_of_the_universesome "dates" that seem inconsistent with this answer

These times are based on a non-inflationary cosmology. Note this statement: "In inflationary cosmology, times before the end of inflation (roughly $10^{−32}$ second after the Big Bang) do not follow the traditional big bang timeline." In other words, the numbers you are seeing for the Grand Unification and Electroweak epoch are not valid in inflationary cosmology.

Nor, in fact, is the number $10^{-32}$ seconds for when the inflationary epoch ended really valid; it's just a notional number which is itself based on what the time "would have been" in a non-inflationary model. That estimate, in turn, is based on an estimate of what the temperature and density was of the hot, dense state that was formed by reheating at the end of inflation--that temperature and density is then plugged into a non-inflationary model, which pops out a notional "time after the initial singularity" when the universe would have had that temperature and density. In other words, all of these numbers are just notional labels and don't really mean anything physically.

Also, note that this whole article uses the term "Big Bang" to refer to the initial singularity, not to the hot, dense state at the end of inflation. This, as I have explained, is not really correct terminology, but what can you expect from Wikipedia?

 

[Buzz Bloom]

These times are based on a non-inflationary cosmology.

Hi Peter:

Thank you for your prompt and useful answer. I think I now understand that the Wikipedia time line is based on assuming inflation never happened, except for the rather strange vague time given for inflation based on trying to be consistent with the other dates.

One thing I remember reading about inflation was that it's ending caused the first creation of matter of the kind described in the standard model. (I am unable to track down right now where I read this.) Does this seem correct to you?

I also remember reading in another thread a discussion about the existence of magnetic monopoles before inflation, and the thinning out of these primordial particles during inflation so that they can no longer be found in the observable universe.

Can you recommend any useful references that discusses (1) the time line based as assuming inflation did happen, and/or (2) attributes of the stuff in the universe before inflation?

Regards,
Buzz

 

[OCR]

This, as I have explained, is not really correct terminology, but what can you expect from Wikipedia?

Lol... looks like Buzz said about the same thing, back on September 5, 2015...

(I am unable to track down right now where I read this.)

Maybe... here ?

Good discussion... carry on.

 

[PeterDonis]

One thing I remember reading about inflation was that it's ending caused the first creation of matter of the kind described in the standard model.

Yes, this is what "reheating" is in inflationary models: the energy stored in the inflaton field gets converted into ordinary matter and radiation--standard model particles.

I also remember reading in another thread a discussion about the existence of magnetic monopoles before inflation, and the thinning out of these primordial particles during inflation so that they can no longer be found in the observable universe.

AFAIK magnetic monopoles are still only speculative. They are predicted by most Grand Unified Theories, but there is no evidence for them that I'm aware of.

Can you recommend any useful references that discusses (1) the time line based as assuming inflation did happen, and/or (2) attributes of the stuff in the universe before inflation?

Unfortunately, all the non-technical references I'm aware of use the same "notional" times that the Wikipedia article you quoted uses. Part of the problem may be that inflationary models don't really tell us anything about "how long" things took in any meaningful sense. More technical references don't talk about time at all; they talk about the number of doublings of the size of the universe that would have been required for our current observations to be consistent with the model. (Usually that number, IIRC, is somewhere around 60 doublings.)

As far as the attributes of the stuff in the universe before inflation, I don't think there is any real answer at this point. During inflation, the only "stuff" in our observable universe would have been the inflaton field itself (the field that drives inflation). The question is what the inflaton field came from, and AFAIK answers vary greatly between inflationary models.

 

[Buzz Bloom]

Unfortunately, all the non-technical references I'm aware of use the same "notional" times that the Wikipedia article you quoted uses.

As far as the attributes of the stuff in the universe before inflation, I don't think there is any real answer at this point.

Hi Peter:

Thanks again for your prompt and informative answers.

Regards,
Buzz

 

[Dave Eagan]

The big bang singularity happened everywhere at once

Can anyone tell me the mechanism for this? Or does it defy known physics and logic?

 

[phinds]

Can anyone tell me the mechanism for this? Or does it defy known physics and logic?

"Singularity" is a placeholder word for the phrase "we don't know what the ... was going on or how it happened", it's just a non-physical answer that falls out of the math. (but we know a lot about what happened starting about 1 Plank Time after it happened)

 

[rootone]

Can anyone tell me the mechanism for this? Or does it defy known physics and logic?

This questions is equivalent to saying 'what caused the big bang?', and there is no shortage of ideas, but no particular one is more convincing, (to me anyway).
However the mechanism (cause of) it is not part of the big bang theory itself.
It doesn't defy physics and logic since it's based on observational evidence.
The theory makes complete sense from a moment (1 plank time) after the actual big bang leading to the subsequent evolution of the Universe we see today.
However before this time we have one of those dreaded singularities - and yes that does defy logic.
All it really means though is that we don't know what happened, and the most likely explanation is that there is physics going on which we so far don't understand.

There are ways of getting rid of the singularity, (aka mathematical nonsense) by introducing ideas such as a cyclic Universe (different versions of this), but as far as I know these kind of models also run into mathematical conundrums, just they are ones of a different kind.

 

[PeterDonis]

The theory makes complete sense from a moment (1 plank time) after the actual big bang

We don't have observational evidence for 1 Planck time after a postulated "initial singularity". We only have observational evidence for the hot, dense, rapidly expanding state at the end of inflation, and for some of the characteristics of the inflation era before that. In inflationary models, there is no "initial singularity".

 

[Dave Eagan]

This questions is equivalent to saying 'what caused the big bang?', and there is no shortage of ideas, but no particular one is more convincing, (to me anyway)...

Thanks Rootone, but I wasn't referring to the "Big Bang" but rather the idea that it happened everywhere at once. How do we know it happened everywhere at once?

 

[PeterDonis]

How do we know it happened everywhere at once?

Because we see the remnants of it to be the same everywhere at once. The most obvious remnant is the CMB, which is the same in all directions to one part in 100,000. If the process that led to the CMB didn't happen everywhere at once, it would look different in different directions (for one thing, it would have a different redshift in different directions).

Of course, the CMB was produced a few hundred thousand years after the Big Bang; but we can apply the same reasoning to observations that come from much earlier. For example, the relative abundances of light elements, which are the result of nucleosynthesis in the first few minutes after the Big Bang, are the same everywhere, as far as we can tell. If the Big Bang had happened at different times in different parts of the universe, that would not be the case.

 

[Dave Eagan]

Because we see the remnants of it to be the same everywhere at once. The most obvious remnant is the CMB, which is the same in all directions to one part in 100,000. If the process that led to the CMB didn't happen everywhere at once, it would look different in different directions (for one thing, it would have a different redshift in different directions).

Ok. That is convincing.

Of course, the CMB was produced a few hundred thousand years after the Big Bang

Hold it. Then I am not seeing how it is a remnant of the Big Bang. From what I understand the B.B. produced a universe that was huge in less than a second. After one year it was "several times huge" and I would expect 100,000 years would provide time for some sort of changes to begin. So the CMB could be due to something else maybe? For example, my source says:

"The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology. In cosmology, recombination refers to the epoch at which charged electrons and protons first became bound to form electrically neutral hydrogen atoms. Recombination occurred about 378,000 years after the Big Bang (at a redshift of z = 1100).”

“Immediately after the Big Bang, the universe was a hot, dense plasma of photons, electrons, and protons. This plasma was effectively opaque to electromagnetic radiation due to Thomson scattering by free electrons, as the mean free path each photon could travel before encountering an electron was very short. As the universe expanded, it also cooled. Eventually, the universe cooled to the point that the formation of neutral hydrogen was energetically favored, and the fraction of free electrons and protons as compared to neutral hydrogen decreased to a few parts in 10,000.

“Shortly after, photons decoupled from matter in the universe, which leads to recombination sometimes being called photon decoupling, although recombination and photon decoupling are distinct events. Once photons decoupled from matter, they traveled freely through the universe without interacting with matter, and constitute what we observe today as cosmic microwave background radiation.”

So it sounds to me that some believe the CMB was created by recombination and that recombination occurred as much as 378,000 years after the B.B.

That would mean that the cause of the CMB was already distributed in every direction and so it would not be subject to red shift resulting from the expansion of the B.B. itself.

What am I missing?

but we can apply the same reasoning to observations that come from much earlier. For example, the relative abundances of light elements, which are the result of nucleosynthesis in the first few minutes after the Big Bang, are the same everywhere, as far as we can tell. If the Big Bang had happened at different times in different parts of the universe, that would not be the case.

Ok, I'm going to have to give up on this and just draw my own conclusions I guess, because according to everything I ever learned AND according to the quotes I posted in the above paragraphs all there was for long after the B.B. was electrons, protons, and photons forming a hot plasma, and the "light elements" to which you refer didn't exist "in the first few minutes after the Big Bang". It took hundreds of thousands of years for the first, simplest element, -hydrogen, -to form.

Thanks.