Why big bang is possible?

Main Question or Discussion Point

We know that matters cannot escape from the gravity of a black hole when they are close enough to its center. Would it be the same at the start of the big bang? Why matters can escape from the huge gravity at the start of the big bang?

Answers and Replies

Good question. No it would not be the same for the big bang, because the standard picture of a 'black hole' applies to a particular space-time geometry (e.g. a point mass in an otherwise flat geometry), which is very different from the models of Big Bang / inflation.

Chronos
Science Advisor
Gold Member
The most plausible explanation at present, IMO, is the BB arose from a quantum fluctuation. It bears little resemblance to a black hole - all that black body radiation stuff and primordial elemental abundances pretty much rules out that concept.

Good question. No it would not be the same for the big bang, because the standard picture of a 'black hole' applies to a particular space-time geometry (e.g. a point mass in an otherwise flat geometry), which is very different from the models of Big Bang / inflation.
Thank you for the reply. I have difficulties in seeing the difference between the two, as if GR is the governing equation that applies to both cases, it should give the same implication for the starting point of BB where gravity should be large enough to stop any possible expansion? Really not sure.

BTW, if the density of the present universe is large enough, it should be able to halt the expansion and ultimately lead to big crunch. That's why I think in the early stage of BB, the density is so high and gravity should be large enough (don't know any calculation) to stop it.

It is also puzzling that the mass-density inside space-time governs its evolution and curvature given that mass inside space time has no degree of freedom outside space time dimension. Using the balloon surface analogy, if matters are restricted to move along the surface, how can it affect the balloon's expansion?

Thank you for the reply. I have difficulties in seeing the difference between the two, as if GR is the governing equation that applies to both cases, it should give the same implication for the starting point of BB where gravity should be large enough to stop any possible expansion? Really not sure.
GR does apply to both cases, but you get different results. Space-time itself can carry energy/momentum; in the big bang, this energy/momentum helps drive material outward in an expansion of space. That's not present in a standard black-hole.

BTW, if the density of the present universe is large enough, it should be able to halt the expansion and ultimately lead to big crunch. That's why I think in the early stage of BB, the density is so high and gravity should be large enough (don't know any calculation) to stop it.
That is good logic, but it misses some of the details of inflation and the big bang---you can think of it in terms of there just being a really strong expanding energy/momentum during the BB which was great enough to resist the gravitational pull of so much matter.

It is also puzzling that the mass-density inside space-time governs its evolution and curvature given that mass inside space time has no degree of freedom outside space time dimension. Using the balloon surface analogy, if matters are restricted to move along the surface, how can it affect the balloon's expansion?
Unfortunately, thats part of why the balloon surface is only an analogy. In GR space-time and matter are coupled together, and are coupled in a highly non-linear (and thus complex) manner. Lets try a different analogy:

Lets say you throw some object (maybe a stone, maybe a model boat, whatever) across the surface of a pond. That object will cause ripples in the surface of the water. Those ripples and waves themselves could cause the object to move---and then the new motion of object could cause further waves, motions, turbulence etc in the water.

In the case of GR the interactions are far more complex and long-ranged.

The most plausible explanation at present, IMO, is the BB arose from a quantum fluctuation. It bears little resemblance to a black hole...
That's the point of the post, the OP is asking why it bears so little resemblance to a BB, when surely a (naive) calculation of the escape velocity for the (obscenely) high primordial energy density would be greater than the speed of light.

GR does apply to both cases, but you get different results. Space-time itself can carry energy/momentum; in the big bang, this energy/momentum helps drive material outward in an expansion of space. That's not present in a standard black-hole.

That is good logic, but it misses some of the details of inflation and the big bang---you can think of it in terms of there just being a really strong expanding energy/momentum during the BB which was great enough to resist the gravitational pull of so much matter.

Unfortunately, thats part of why the balloon surface is only an analogy. In GR space-time and matter are coupled together, and are coupled in a highly non-linear (and thus complex) manner. Lets try a different analogy:

Lets say you throw some object (maybe a stone, maybe a model boat, whatever) across the surface of a pond. That object will cause ripples in the surface of the water. Those ripples and waves themselves could cause the object to move---and then the new motion of object could cause further waves, motions, turbulence etc in the water.

In the case of GR the interactions are far more complex and long-ranged.
Thank you for the insights. Does it mean that in BB model there is non-conservation of energy? In other words in expanding universe the total energy content of the whole universe is increasing, isn't it?

I have an idea that the expansion of universe should in principle be measurable in laboratory. If space itself is expanding uniformly everywhere, then objects tends to move apart from each others no matter how far they are apart. Would it contribute to an observable "repulsive" force between objects? If this is so, we should be able to observe that a force is necessary to keep any two objects at the same distance apart inside a laboratory.

Thank you for the insights. Does it mean that in BB model there is non-conservation of energy? In other words in expanding universe the total energy content of the whole universe is increasing, isn't it?
Most theories manage to maintain conservation of energy... I'm sorry I don't know enough about the details to say anything else.

I have an idea that the expansion of universe should in principle be measurable in laboratory. .... we should be able to observe that a force is necessary to keep any two objects at the same distance apart inside a laboratory.
You're absolutely right, that should theoretically be the case. Unfortunately that force would be so ridiculously small that I can guarantee there is no chance that experiment would be doable anytime soon.

Most theories manage to maintain conservation of energy... I'm sorry I don't know enough about the details to say anything else.

You're absolutely right, that should theoretically be the case. Unfortunately that force would be so ridiculously small that I can guarantee there is no chance that experiment would be doable anytime soon.
Yeah, in fact I may be wrong but believe that the force of dark energy currently on a scale of, say, me and my cat doesn't actually seperate us at all. The local strength of EM/SN/GR not just very nearly but entirely 100% nullify the repulsive force of dark energy. Can anyone confirm this?

EDIT: I realize the answer gets more involved/complicated when you try and describe the hypothetical observation of sub-atomic particles and the repulsive effects of dark energy on them due to quantum uncertainty. Not as simple as saying you have 2 particles next to each other, stationary and at rest relative to each other and you can measure that there is absolutely no repulsion.

Well fwiw I dont believe the universe appeared from nowhere. We arent a single electron positron pair that appeared momentarily and apparently from nothing. Cause and effect still apply as well as conservation of energy, and energy being supplied from something and/or somewhere else.

Chronos
Science Advisor
Gold Member
Strictly speaking, energy is not conserved in GR. Einstein combined the laws of energy conservation and momentum conservation into a single law. So, you can 'lose' conservancy of one property as long as it is balanced out by the other. Here is a good discussion that is not overly mind boggling:
http://www.utdallas.edu/~parr/chm5414/54140903.html

Chronos
Science Advisor
Gold Member
A universe from 'nothing' is possible so long as it remains 'nothing'. It can be demonstrated that the net energy of the entire universe is exactly zero. The big bang arose because 'nothing' rearranged itself as a collection of things and anti-things. This weirdness is not only permitted, but, demanded by quantum physics. This is still not saying anything about what came before the big bang. That is most likely unknowable. It does, however, say a lot about what happened once the big bang got underway. The physics of what transpired is fairly well understood almost back to time zero. Here is a link to a lecture given by Lawrence Krauss:
http://topdocumentaryfilms.com/a-universe-from-nothing-lecture/

Chronos thanks for reply. I vaguely remember the argument that "a universe from 'nothing' is possible so long as it remains 'nothing'. It can be demonstrated that the net energy of the entire universe is exactly zero. The big bang arose because 'nothing' rearranged itself as a collection of things and anti-things."

I am afraid that I cant give in and accept it as canon until I have worked it through a whole lot more. It is likely that the GR and QM mathematical aspects of it will prevent understanding. Perhaps I have been an engineer too long! It hints of the fallacy of using QM as a free energy source ie. some kind of electron tunneling diode charge collection mechanism.

Not that it doesnt stop some from trying to acheive the likely impossible, free energy from ZPE:

http://psiphen.colorado.edu/Pubs/VacEnergyExtrac_Jan10.pdf

Last edited: