# Thread: How Do We Know (Big Bang Initial Size)... ?

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## How Do We Know (Big Bang Initial Size)... ?

How do we know the big bang originated from something the size of a singularity and not from something the size of our moon or our planet for example.

I heard the explanation once a long time ago, but can't remember from where or how it was known.

Thanks!

2. Hi mdpalow, welcome to CQ.

I've moved your thread from Astronomy to Q&A; I think you'll get a response faster here.

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Edit: That is to say we don't know it came from a singularity

Our models only run back to about a million-millionth of a second prior to the point where a singularity would be if you just extrapolate back (we know that this extrapolation is not valid because we know our current theories of physics break before this point). At that point the entire observable universe was (from memory) of the order of metres across.

So the things to bear in mind are that the current models don't say much about anything outside the observable universe. We suspect it is like what we see around us but we don't know - because we don't know how big the entire universe is. Also the singularity is not actually part of the current models. You get one by extrapolating back beyond the point at which we know they are not applicable.

4. I can't add much to what Shaula said but that I thought the size of the observable universe was much smaller. At first thought one way to look at it is that at 10−37 the observable universe could be no bigger then the distance light can travel in 10−37s. A proton is ~10−15m. Light speed is ~3x108...if I wasn't up all night with a massive tooth ache I might actually get the fairly straightforward math right in my head. But now that I think about it ... this is all dependent on opposite sides of the observable universe being at thermal equilibrium. Which if it was the size it was for a long time wouldn't be an issue either. Either way I thought the observable universe was on the size of a proton back then.

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I am taking the end of inflation as the point where we have sufficient confidence that we can predict a size - current models of inflation only put bounds on how much of an effect it could have had rather than giving us exact numbers. Hopefully when/if we pin down a mechanism for it we'll get better numbers out. So I was going from about 10e-32s (when the observable universe was between the size of a grapefruit and a beachball).

6. Ah got you.

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Originally Posted by mdpalow
How do we know the big bang originated from something the size of a singularity and not from something the size of our moon or our planet for example.

I heard the explanation once a long time ago, but can't remember from where or how it was known.

Thanks!
We don't know that "big bang" even happened

8. Originally Posted by mdpalow
...something the size of a singularity...
Besides, a singularity is not a thing. It's a point where the mathematics - in this case, the mathematics of general relativity - become undefined, or inapplicable, or incomplete. That's because quantum processes become very significant at such an early stage of the universe's evolution, and general relativity does not incorporate quantum processes.

Further, to talk about size, you have to define a metric, i.e., a way to measure size. This is not so clear cut when considering the state of the universe so close to the beginning.

9. Originally Posted by effingham
We don't know that "big bang" even happened
effingham,

Do not advocate non-mainstream positions in Q&A. Q&A is only for mainstream answers.

If you have not reviewed our rules (link in my signature), I suggest you do so soon.

10. Originally Posted by effingham
We don't know that "big bang" even happened
We know that the "big bang" in the sense of the universe expanding from an earlier hot, dense state happened - and is still happening. That is what the term origianlly described.

Since then the popular press and some science writers have used it to refer to some sort of "creation event"; it is probably true that we don't know if such a thing happened or not.

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Relatively minor disagreements with Shaula and Wayne:

General relativity plus the observation that galaxies are
moving apart unambiguously predicts a singularity. That
singularity was the beginning of the Universe. GR has no
problem with that, all the way back to t=0.

It so happens that close to the singularity, the energy
density approaches infinite. At t=0 it would have been
infinite. Neither of those fits in with the known laws of
quantum mechanics. So either there was no singularity,
or the laws of QM need to be added to, or GM needs to
be revised, or some combination of the above.

I think it is clear that QM has to be extended in order to
account for the Big Bang. That could result in elimination
of the singularity without any change to GR. Or it could
simply alter the conditions near the singularity such that
the density did not approach infinite.

-- Jeff, in Minneapolis

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The big bang was more a point in time than space. It could have been any size, including infinite. It is problematic to dimensionalize the big bang. We know, for example, the surface of last scattering appeared about 400,000 years after the BB, but, cannot assign it a size - only a scale factor relative to 'today'. We would need to know the entire expansion history of the universe, which is unknown, to even make an approximation.

13. Originally Posted by mdpalow
How do we know the big bang originated from something the size of a singularity and not from something the size of our moon or our planet for example.

I heard the explanation once a long time ago, but can't remember from where or how it was known.

Thanks!
Bah didn't notice this was an old post...that I already replied to.

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We don't. In fact saying the big bang started from a size of zero volume is not really science. It is a pop sci answer that extrapolate further back then the models allow. The observable universe started out very small and we can calculate that back with high energy physics to smaller then a proton but there is a point before that were our models break down.

The big bang assumes as non zero volume of space to begin with. Until we get a proper quantum theory of gravity we probably won't be able to say much on what happened before the point where our current models fail.
Now the why was the observable universe smaller then a proton and not the size of the moon? I believe the answer to that lies more in that there is no reason it should stop at the size of the moon. Our current theories work many orders of magnitude beyond that size. The trick seemed to be how did the early universe get big enough that it didn't immediately crunch back down on itself and that seem to because the initial inflation was fast enough to get over that limit.

14. Originally Posted by effingham
We don't know that "big bang" even happened
We do know that it happened. We have multiple lines of evidence that it happened. If it didn't happen we wouldn't be here :P
Seriously the Big Bang is the best explanation for the observations we have and we have a ton of observations that all point to the same thing. That our observable universe was in a very hot dense state and that it had a rapid period of inflation...blah blah blah...current day cosmic expansion is not only happening but also increasing in its rate.

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Originally Posted by Jeff Root
GR has no
problem with that, all the way back to t=0.
We should emphasize that "back to t=0" does not include t=0 because that is where GR fails. Better stated as "back toward t=0".
Replace GR and QM with a new theory (quantum loop gravity, string theory, etc.) that incudes them and the expectation is that the singularity goes away. Of course quantum field theories have plenty of singularities of their own (see renormalization).
But this is not GR or for that matter QM. The singularity remains in GR.

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I still don't see any evidence that GR fails at t=0. That
has been asserted many times by many posters here on
CosmoQuest, but almost nowhere else.

As far as I can see, GR works just fine all the way back to
t=0, as I said. The problem arises only when other physics
is applied -- namely quantum mechanics. GR predicts infinite
density at t=0, which QM can't handle. QM can't handle
extremely high densities shortly after t=0, either, so the
actual conditions shortly after t=0 can't be modelled with
current theory.

The prediction by GR of a singularity at t=0 was possible
because GR *does* work all the way back to t=0. If it
failed, it would not be able to predict the singularity.

And to be boringly repetative, that doesn't mean there was
a singularity at t=0 or that the density was infinite -- it just
means that QM isn't complete.

-- Jeff, in Minneapolis

17. Originally Posted by Jeff Root
I still don't see any evidence that GR fails at t=0. That
has been asserted many times by many posters here on
CosmoQuest, but almost nowhere else.
A quick check on Arxiv finds several papers saying the same thing.

If it failed, it would not be able to predict the singularity.
In what way is predicting something that almost certainly doesn't exist not a failure?

it just means that QM isn't complete.
While QM may not be complete, I assume you mean GR in this context?

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No, as I've said maybe six or seven times in various different
threads, including previously in this one, I meant QM, not GR.

And as I've said several times, we know for sure that QM has
to be extended to account for whatever happened at the Big
Bang. As it stands now, it can't do that. Extending it in that
way might, for example, describe how all the matter-energy
of the Universe came into existence over a period of time
(maybe a second, or a picosecond), eliminating the infinite
density predicted by GR. No change needs to be made to GR
to change the result of GR's prediction. GR's prediction is
calculated with the assumption that the mass-energy of the
Universe doesn't change.

If I have an apple (which I do), and multiply the number of
apples I have by two every day, in less than two months I'll
have over a quintillion apples. Certainly that number of
apples doesn't exist in the whole world. Multiplication must
be incorrect, mustn't it? It failed by predicting an impossible
number of apples in a perfectly reasonable period of time.

Or maybe one of the inputs into the multiplication was wrong.
Maybe it wasn't a failure of multiplication, but a failure to
account for the fact that I can't increase the number of apples
I have in the way I assumed.

a singularity and infinite density at t=0 is not a failure of GR.
It is a correct prediction by GR from the assumption that the
mass-energy of the Universe is unchanging.

-- Jeff, in Minneapolis

19. Originally Posted by Jeff Root
a singularity and infinite density at t=0 is not a failure of GR.
Well, if you don't think that dividing by zero is not a sign of failure, I don't know what is.

We probably need a theory of quantum gravity (an extension to GR, taking into account quantum-level effects) to properly describe black holes and the earliest times of the universe.

Extending it in that
way might, for example, describe how all the matter-energy
of the Universe came into existence over a period of time
(maybe a second, or a picosecond), eliminating the infinite
density predicted by GR.
Or maybe a theory of quantum gravity (an extension to GR) would not predict a singularity.

20. Originally Posted by Jeff Root
I still don't see any evidence that GR fails at t=0.... GR predicts infinite density at t=0....
Does "infinite density" make sense to you?

It is the prediction of infinite density, among other things, that demonstrates GR has gone off the tracks.

21. And wouldn't it be more accurate to say that density is undefined, as it involves a division by zero?

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Originally Posted by Strange
Originally Posted by Jeff Root
a singularity and infinite density at t=0 is not a failure of GR.
Well, if you don't think that dividing by zero is not a sign of
failure, I don't know what is.
This division by zero is essentially a division of the mass
of the Universe by the volume of the Universe. If the volume
was zero at the very beginning, then division by zero occurs
at that point.

It does not suggest any kind of failure.

Originally Posted by Strange
We probably need a theory of quantum gravity (an extension
to GR, taking into account quantum-level effects) to properly
describe black holes and the earliest times of the universe.
Saying that we "probably" need a theory of quantum gravity
goes beyond what we know. We *might* need a theory of
quantum gravity. We *know* we need a more complete
quantum theory. That alone might suffice.

Originally Posted by Strange
Originally Posted by Jeff Root
Extending it in that way might, for example, describe how
all the matter-energy of the Universe came into existence
over a period of time (maybe a second, or a picosecond),
eliminating the infinite density predicted by GR.
Or maybe a theory of quantum gravity (an extension to GR)
would not predict a singularity.
As I said in post #11, which is not all that far back. And
as I implied in the last sentence of post #16, less than
twelve hours ago.

The prediction of the singularity of the beginning of the
Universe was so profound that it gave the title to what
may be the most well-known popular science book ever
written: "A Brief History of Time" It is the notion that
time started at the Big Bang which GR predicts and labels
a "singularity".

Originally Posted by Strange
And wouldn't it be more accurate to say that density is
undefined, as it involves a division by zero?
No, the density clearly and unambiguously approaches
infinite value asymptotically as the time is asymptotically
regressed to t=0. I see no reason to avoid saying so.

-- Jeff, in Minneapolis

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Originally Posted by Cougar
Does "infinite density" make sense to you?
Rather surprisingly, yes, it does.

My own guess is that the density was never *extremely*
high in the first moments of the Big Bang, but that, as I
suggested a couple of posts back, the mass-energy of the
Universe grew from zero to a very large value over some
(possibly very, very short) period of time. But still, I have
no problem with idea of the matter in the center of a black
hole becoming more and more dense without limit as it is
continually crushed more and more by its own weight.
It would only reach infinite density in infinite time, which
does seem like a sort of cop-out... especially considering
how quickly it would reach densities QM can't describe.

-- Jeff, in Minneapolis

24. Originally Posted by Jeff Root
My own guess is that the density was never *extremely*
high in the first moments of the Big Bang, but that, as I
suggested a couple of posts back, the mass-energy of the
Universe grew from zero to a very large value over some
(possibly very, very short) period of time.
Maybe you would like to support that in ATM?

25. Originally Posted by Cougar
Does "infinite density" make sense to you?

It is the prediction of infinite density, among other things, that demonstrates GR has gone off the tracks.
"Other things" meaning what? Time? Distance? Expansion velocity? Something else?

26. Originally Posted by mkline55
"Other things" meaning what? Time? Distance? Expansion velocity? Something else?
Well, I mainly added that so as not to be overly limiting in my statement. But the "singular" conclusion typically claims infinite density along with zero volume - another prediction that makes utterly no sense at all. I think it was caveman who pointed out that it is not "the mathematics" of GR that "break down." The mathematics work just fine. But, I would add, the conclusion or prediction that the mathematics leads to involves impossible, nonsensical infinities. When this happens in physics, it basically means "back to the drawing board."

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Originally Posted by Strange
We know that the "big bang" in the sense of the universe expanding from an earlier hot, dense state happened - and is still happening. That is what the term origianlly described.

Since then the popular press and some science writers have used it to refer to some sort of "creation event"; it is probably true that we don't know if such a thing happened or not.
Until such evidence arises showing that the universe

a) is expanding (Expanding into what, exactly? Do you even realize the logical absurdity of "expanding universe"? The universe is all that exists, into what can it expand if it encompasses all that exists? Ex nihilo creation myth, anyone? Expanding universe is "fact" yet expanding earth is met with hysterical crying?)

and/or b) the universe has an origin, edge, limit, side, top, bottom, inside, outside and/or an end, both these notions may safely be deemed riotous fantasy.

Consensus of belief systems is not a requisite of science. It doesn't matter how many people believe in a creation myth, it is still a myth until some evidence arises supporting it.

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Originally Posted by Jeff Root
Relatively minor disagreements with Shaula and Wayne:

General relativity plus the observation that galaxies are
moving apart unambiguously predicts a singularity. That
singularity was the beginning of the Universe. GR has no
problem with that, all the way back to t=0.

It so happens that close to the singularity, the energy
density approaches infinite. At t=0 it would have been
infinite. Neither of those fits in with the known laws of
quantum mechanics. So either there was no singularity,
or the laws of QM need to be added to, or GM needs to
be revised, or some combination of the above.

I think it is clear that QM has to be extended in order to
account for the Big Bang. That could result in elimination
of the singularity without any change to GR. Or it could
simply alter the conditions near the singularity such that
the density did not approach infinite.

-- Jeff, in Minneapolis
Not all galaxies are moving apart, your gibberish thesis is invalid therefore.

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Originally Posted by Thanatos
The big bang was more a point in time than space. It could have been any size, including infinite. It is problematic to dimensionalize the big bang. We know, for example, the surface of last scattering appeared about 400,000 years after the BB, but, cannot assign it a size - only a scale factor relative to 'today'. We would need to know the entire expansion history of the universe, which is unknown, to even make an approximation.
"surface of last scattering" is a spurious notion with utterly no empirical referent

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Originally Posted by WayneFrancis
Bah didn't notice this was an old post...that I already replied to.

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We don't. In fact saying the big bang started from a size of zero volume is not really science. It is a pop sci answer that extrapolate further back then the models allow. The observable universe started out very small and we can calculate that back with high energy physics to smaller then a proton but there is a point before that were our models break down.

The big bang assumes as non zero volume of space to begin with. Until we get a proper quantum theory of gravity we probably won't be able to say much on what happened before the point where our current models fail.
Now the why was the observable universe smaller then a proton and not the size of the moon? I believe the answer to that lies more in that there is no reason it should stop at the size of the moon. Our current theories work many orders of magnitude beyond that size. The trick seemed to be how did the early universe get big enough that it didn't immediately crunch back down on itself and that seem to because the initial inflation was fast enough to get over that limit.
Compressing matter until it is smaller than a proton defies all known physical laws, it is sheer fantasy.