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Q & A: fundamental cosmology questions

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Most recent answer: 07/05/2018
Q:
I do not understand how the big bang was possible given the state of the obversable universe. Consider the workthing theory of black holes, where the gravitional field of too much matter/energy within a small region prevents anything from escaping. If this is so, and black holes are common. And matter/energy is distributed of 13+ billion years of expanding space. Then how could all of the energy/matter present at the big bang possible have overcome the gravity field it would have necessarily created. We say nothing, not even light can escape the graviational well. The energy/mass densities at the big bang should have effectively been infinitely strong, thus preventing anything from leaving the region. Is this why I hear speculation that the universe may itself be inside an event horizon? Next, how can the size of black hole grow? And what is the justification for the amount of energy/mass that must have existed to start the big bang since given laws of conservation all of it must have been present then. Finally, has anyone worked out the entropy and information theory address the complexity of the observable universe (billions of years and increasing complexity just seem strange).
- Eric (age 37)
Urbana, IL USA
A:

Although we've answered all these questions before, it may be useful to have them grouped together here.

The black hole solutions of the General Relativity equations assume that outside the event horizon the mass density is comparatively very low. Obviously in the near-uniform starting conditions, that assumption isn't close to true. So with an entirely different mass distribution than in the black hole picture, the solution of the GR equations is entirely different. You can get a little feel for that from a classical picture. Near  a black hole, in the Newtonian approximation, you have a very strong field pointing inward. In contrast, in the middle of a uniform sea of mass the field is zero.

"Next, how can the size of black hole grow?" I'm not sure how that connects with the rest of the questions. Formally, from our outside point of view a black hole can't grow because the in-falling matter takes forever (due to gravitational redshift) to travel the last tiny, tiny bit of distance to the event horizon. In fact, by the same token a black hole can't ever quite form. If you make a more practical definition of a black hole, including everything that in practice won't leave until after some enormous Hawking time, it just grows by things falling in.

We do not know how much energy the universe has. If the universe happens to be finite, in one standard way of accounting, the total energy is zero because negative gravitational energy just cancels the positive energy. If the universe is infinite, the question has no obvious meaning. You can find a more sophisticated discussion of the issue here: http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html.

What I suspect you're really trying to get at there is "how could it start from nothing?". Probably it didn't, but knowing how to connect up our universe with a broader structure requires having some sort of quantum gravity theory to replace the starting singularity that would be there if gravity weren't quantized.

You ask something about "entropy" and about "complexity". Physics has no universal laws about or even an established definition of "complexity", so I'll skip the complexity part. With regard to entropy, there are two somewhat mysterious aspects. The first is how, given that entropy of an isolated system is conserved according to the Liouville theorem, entropy could be increasing everywhere now. The answer to that is that the entropies that we consider when we say that the total entropy increases are local entropies. These must increase as the (negative) entropy of quantum entanglement between remote regions also increases. That raises the second question: why the starting local entropy and quantum entanglement of the universe was so low. I don't think we know the answer.

Mike W.


(published on 04/01/2014)

Follow-Up #1: big bang and black holes

Q:
2. Why/How did the Big Bang singularity not become a Black Hole? Based on the definition of a black hole that is all that could have happen when the Big Bang took place; not spread out and become the universe. If a very massive collapsing star has enough stellar mass to become a black hole. Then something with infinite density/gravity like the Big Bang singularity would have no of option but to become a black hole.
- Randy (age 40)
Knoxville TN USA
A:

I've put this in a thread with an earlier answer. The key is that the black hole solution to the general relativity equations arises for a dense mass surrounded by more or less empty space. The almost uniformly dense mass of the Big Bang gives a completely different solution to the same equations.

Mike W.


(published on 09/23/2014)

Follow-Up #2: big bang and black holes

Q:
Back closer to the big bang, the universe was smaller and smaller. It seems that at some point, all the mass would have been inside its own Schwarzschild radius. So how did it get out?
- Bill (age 46)
Champaign IL
A:

See the thread above.

The black hole solution of the general relativistic equations assumes low mass density outside the horizon. A uniform density gives a completely different solution.

Mike W.


(published on 07/05/2018)

Follow-up on this answer.