Before the big bang the entire universe must have been a black hole.

[mdmaaz]

Before the big bang the entire universe was squeezed down into an incredibly hot and small ball. If so much matter is squeezed down into such a small space it will become a black hole. Nothing can escape the gravitational pull of a black hole, so how did this tiny universe before the big bang explode? The way I'm thinking the entire universe must have been a black hole before the big bang. After all, if lot's of matter is squeezed down a lot, it becomes a black hole. But nothing can explode out of a black hole. But I think that's just what happened during the big bang. Please share your ideas regarding this topic.

 

[marcus]

Before the big bang the entire universe was squeezed down into an incredibly hot and small ball. ...
...Please share your ideas regarding this topic.

EDIT: Ben Crowell kindly pointed out that I didn't at first answer your question! My response must have really looked dumb. I will add this brief beginning paragraph that is more responsive:
================
If expansion is fast enough it can defeat the tendency of concentrated matter to collapse.

And according to standard model, in the very early universe expansion was millions of times faster.

Indeed ordinary black hole models are constructed in non-expanding geometry---and only apply where expansion is slow enough to be neglected. So they are irrelevant here. What they describe would not work in the context of rapidly expanding geometry. There is no need to worry about this in discussing early universe.
=========================

Now you can reasonably ask, if expansion of the very high density U was so fast, then how did that very rapid expansion get started? What are some reasonable conjectures about conditions right at the start?

That is the province of quantum cosmology (cosmology at extreme density where quantum effects can be expected to dominate.)

Now here is what I said earlier, without that introduction:

There is a new area of cosmology called Quantum Cosmology (QG) which studies what could have occurred around the start of expansion.

I'll try to get some links for you.

Here are several hundred QC research papers that have appeared in 2008 or later:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=dk+quantum+cosmology+and+date+%3E+2007&FORMAT=WWW&SEQUENCE=citecount%28d%29

Let me know if the site is slow, it gets a lot of traffic. (This is the German mirror of a Stanford U. site that is hopelessly slow and I'm worried this mirror site will get slow also.)

The papers here are not for you to read (there are 330 of them! ), just to glance at the list and see what the field looks like now. It is a fast growing field.

They construct models which may be testable by observations of the CMB (cosmic microwave background). These models are of conditions before during and immediately after what used to be called "the big bang". The aim is to try to understand it (using quantum theory) and to replace the singularity with something more realistic.

In physics a singularity is not considered a real thing in nature but a symptom that something is wrong with the theory that suffers from the singularity. So one tries to fix it.

Here is a popular treatment of QC:
http://www.einstein-online.info/spotlights/big_bangs
It is an essay called "A Tale of Two Big Bangs"
at the public outreach website of a branch of a research outfit called Max Planck Institute, the branch of the institute that studies quantum gravity, cosmology and related stuff. Here's more from them:
http://www.einstein-online.info/spotlights/cosmology/?set_language=en

 

[Waxbear]

well since not only all of the matter/energy, but also the space it exists within, was compressed, I don't think it makes any sense calling it a black hole. Black holes occur when a lot of mass is compressed into a tiny region of space, but here we have that space itself is compressed into a tiny region of... well what really?... It never makes any sense talking about the pre big-bang universe :)

Also your post implies that the universe actually did exist before big bang, but that it was compressed into a point. I have certainly heard that theory before, but i imagine it to be somewhat controversial.

I think it would be more precise to say, like the wikipedia article does, that the universe was a place of infinite temperature and density, as you would imagine it to be if everything existed at the same infinitely small point in space. But saying that the universe was a point is dangerous, since it forces you to think of it as a point in some larger space.

 

[marcus]

... like the wikipedia article does, that the universe was a place of infinite temperature and density, ...

be careful Waxbear, either the Wippy article is sloppy or it has some fine print. The vintage 1915 theory of geometry GR (that cosmo has traditionally been based on) predicts infinite temperature and density. That is considered to be an unphysical prediction that shows GR fails to apply in extreme conditions.

All theories have ranges or domains of applicability. The classic 1915 theory breaks down and does not tell us meaningful stuff very close to the start. So people are working on improvements that do not break down (and do not predict infinitiies).

You might look at some of those links I just gave.

 

[Waxbear]

I'm sorry, the wippy article actually says exactly what you just said:

"Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.[32] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch."

I have always had a hard time accepting the image of the universe as having been a tiny point with an extremely high temperature, since to me, saying that something is a point also implies that this point exists within some larger frame of reference (or else the whole definition of a point make no sense). Maybe it would be more accurate to say that the universe only contained one point in spacetime, that all of spacetime was one point within the universe? Rather than viewing the universe as being a point.

 

[Dmitry67]

I have always had a hard time accepting the image of the universe as having been a tiny point with an extremely high temperature, since to me, saying that something is a point also implies that this point exists within some larger frame of reference (or else the whole definition of a point make no sense). Maybe it would be more accurate to say that the universe only contained one point in spacetime, that all of spacetime was one point within the universe? Rather than viewing the universe as being a point.

You are a victim of a popular misconception - but your intuition had correctly warned you that there was something wrong with that picture. Big Bang is not an explosion of a "ball" or a point.

In early universe density was almost the same everywhere. Big Bang happened not in a single point, but everywhere. If Universe is infinite now, it was always infinite. If Universe is closed and finite then in fact it's volume was very small, but still hot dense matter occupied all the volume of the early Universe.

 

[Waxbear]

Thank you! i did know that the big bang was not an explosion of a point or ball, because yes, everything is intuitively and logically wrong about that picture. If you accept that the "inside" of the universe is everything that exists, then logically the big bang is merely an expansion of everything. But i have heard it explained as the exploding ball (even by competent people) several times, probably because of these people's inability to describe it in a better, yet as understandable way. Then the exploding ball analogy is a lot easier, albeit very wrong.

 

[baric]

Before the big bang the entire universe was squeezed down into an incredibly hot and small ball. If so much matter is squeezed down into such a small space it will become a black hole. Nothing can escape the gravitational pull of a black hole, so how did this tiny universe before the big bang explode? The way I'm thinking the entire universe must have been a black hole before the big bang. After all, if lot's of matter is squeezed down a lot, it becomes a black hole. But nothing can explode out of a black hole. But I think that's just what happened during the big bang. Please share your ideas regarding this topic.

Why do you think the universe started out as a black hole if, by your own logic, nothing than escape it?

 

[marcus]

I'm sorry, the wippy article actually says exactly what you just said:

"Extrapolation of the expansion of the Universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past.[32] This singularity signals the breakdown of general relativity. How closely we can extrapolate towards the singularity is debated—certainly not earlier than the Planck epoch."
...

That's good. It's reassuring that Wippy made that point. So the problem is clearly with GR---the 1915 theory is inadequate to deal with events and conditions right around the start of expansion.

That is why so many people are working on improving GR (with quantum corrections) so that it does not give meaningless results.

So rather than trying to GUESS on your own what direction research is going, I'd suggest you have a look at what the professionals are doing.

Scan the list of titles. If you want to look at the abstract summary of anything just click on abstract. If for some reason you want to see the full PDF just click "PDF".
The whole professional effort at replacing the old "singularity" idea is open to view so have fun.

Or look at the popularization "A Tale of Two Big Bangs" and follow links from there.

There are several different lines of research that seem to be converging on a "bounce" picture. Observationalists seem prepared to check---look for signs of a bounce to see if one occurred or not.

Here are 40-odd papers that appeared recently (2008 or later) about the problem of testing:
http://www-library.desy.de/cgi-bin/spiface/find/hep/www?rawcmd=FIND+%28DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+DK+QUANTUM+COSMOLOGY%2C+LOOP+SPACE%29+AND+%28DK+PRIMORDIAL%2C+FLUCTUATION+OR+DK+INFLATION+OR+DK+COSMIC+BACKGROUND+RADIATION%29+AND+DATE+%3E+2007&FORMAT=www&SEQUENCE=citecount%28d%29

They describe how to recognize the "footprint" of the bounce in the CMB skymap. And things like that. Interesting line of research. A new type of satellite instrument needs to be put up to do the mapping. CMB polarization, instead of just temperature.

No need to feel sorry. There's every reason to be happy about this!

 

[bcrowell]

[Deleted my earlier post telling Marcus he hadn't really answered the OP's question, when in fact I was the one who misread the OP's question.]

 

[marcus]

Ben, thanks! I am glad you are working on FQs
Indeed what the OP asked is frequently asked, and I did not answer that at all. I was also reactingr a comment by someone else.

My reply to the original question should be
If expansion is fast enough it can defeat the tendency of concentrated matter to collapse.

And according to standard model, in the very early universe expansion was millions of times faster.

Indeed ordinary black hole models are constructed in non-expanding geometry---and only apply where expansion is slow enough to be neglected. So they are irrelevant. There is no need to worry about this in discussing early universe.

I'll go and insert that in my post #2.

==============
EDIT: Now I'm totally confused. I thought your post was entirely correct and relevant. But you deleted. I hope the post is saved for use elsewhere as occasion arises. I guess I have gotten us all mixed up!

But anyway I'm grateful to you for pointing out that my initial reaction was not responsive and giving me a chance to add a pre-amble.

 

[bcrowell]

EDIT: Now I'm totally confused. I thought your post was entirely correct and relevant. But you deleted. I hope the post is saved for use elsewhere as occasion arises. I guess I have gotten us all mixed up!

Hee hee...sorry! I'll repost my FAQ here, but actually I think your posts were more on topic than mine :-)

FAQ: In the early universe, matter was gathered together at very high density, so why wasn't it a black hole?

The first thing to understand is that the Big Bang was not an explosion that happened at one place in a preexisting, empty space. The Big Bang happened everywhere at once, so there is no location that would be the place where we would expect a black hole's singularity to form. Cosmological models are either exactly or approximately homogeneous. In a homogeneous cosmology, symmetry guarantees that tidal forces vanish everywhere, and that any observer at rest relative to the average motion of matter will measure zero gravitational field. Based on these considerations, it's actually a little surprising that the universe ever developed any structure at all. The only kind of collapse that can occur in a purely homogeneous model is the recollapse of the entire universe in a "Big Crunch," and this happens only for matter densities and values of the cosmological constant that are different from what we actually observe.

A black hole is defined as a region of space from which light rays can't escape to infinity. "To infinity" can be defined in a formal mathematical way,[HE] but this definition requires the assumption that spacetime is asymptotically flat. To see why this is required, imagine a black hole in a universe that is spatially closed. Such a cosmology is spatially finite, so there is no sensible way to define what is meant by escaping "to infinity." In cases of actual astrophysical interest, such as Cygnus X-1 and Sagittarius A*, the black hole is surrounded by a fairly large region of fairly empty interstellar space, so even though our universe isn't asymptotically flat, we can still use a portion of an infinite and asymptotically flat spacetime as an approximate description of that region. But if one wants to ask whether the entire universe is a black hole, or could have become a black hole, then there is no way to even approximately talk about asymptotic flatness, so the standard definition of a black hole doesn't can't even give a yes-no answer. It's like asking whether beauty is a U.S. citizen; beauty isn't a person, and wasn't born, so we can't decide whether beauty was born in the U.S.

Black holes can be classified, and we know, based on something called a no-hair theorem, that all static black holes fall within a family of solutions to the Einstein field equations called Kerr-Newman black holes. (Non-static black holes settle down quickly to become static black holes.) Kerr-Newman black holes have a singularity at the center, are surrounded by a vacuum, and have nonzero tidal forces everywhere. The singularity is a point at which the world-lines only extend a finite amount of time into the future. In our universe, we observe that space is not a vacuum, and tidal forces are nearly zero on cosmological distance scales (because the universe is homogeneous on these scales). Although cosmological models do have a Big Bang singularity in them, it is not a singularity into which future world-lines terminate in finite time, it's a singularity from which world-lines emerged at a finite time in the *past*.

[HE] Hawking and Ellis, The large-scale structure of spacetime, p. 315.