How did the big bang ever stop being a black hole?

[Tanelorn]

I presume you meant didn't. But yes, that is what makes the FRW universe work. One thing I haven't done, but might be interesting, is see what happens when you have, for instance, a uniform spherical distribution of matter out to some very large radius (much greater than the Hubble radius), so that the Schwarzschild radius for the total mass is larger than the size of the matter distribution, and see how it evolves in time as given by General Relativity.

Chalnoth, this would be an interesting though experiment/computer simulation. Myself, I think something would still need to be pulling the surface of a finite sized sphere outwards to overcome gravitational collapse. I suspect I have no hope of seeing this for myself because I have not covered GR at all yet.

 

[Chalnoth]

Chalnoth, this would be an interesting though experiment/computer simulation. Myself, I think something would still need to be pulling the surface of a finite sized sphere outwards to overcome gravitational collapse. I suspect I have no hope of seeing this for myself because I have not covered GR at all yet.

Well, spherically-symmetric inhomogeneous universes are one of the few solutions you can do analytically. It may be a bit of a challenge to actually do, but it should be doable on paper.

 

[amaruq]

The solution for the whole Universe is simply different from a solutions of White/Black hole.

The main difference is the momentum matter has in the expanding universe. Gravity depends not only on mass!

Mass, distance... that's all, right?

 

[Chalnoth]

Mass, distance... that's all, right?

For Newtonian gravity, this is the case. General Relativity, on the other hand, also depends upon velocity, pressure, and shear.

 

[CosmicVoyager]

The reason the early universe wasn't a black hole and didn't become one was because of the distribution of matter/energy/pressure. It was uniform throughout all of space and therefore there was no concentration to create an interior event horizon. It's very much like the way there is no gravity at the center of the earth.

The early universe may have packed everything into a golf ball but there's nothing outside the golf ball. No concentration, no black hole.

Aha! This post helped me understand the problem. Especially the sentence, "It's very much like the way there is no gravity at the center of the earth."

*If* the concentration of mass/energy at the big bang were sitting in space surrounding it, *then* it would not spread out. Nothing could achieve escape velocity, like with a black hole. But there was no space around it. There was nothing to move into. The gravitational pull of any particle A on a particle B was balanced by another particle C on the other side of B. It was as though every particle was at the center because there was no center like being at the center of the earth. (And there still is no center to the universe.)

Of course, this leads to the question of how can the universe be like the surface of an expanding balloon if there is nothing outside the balloon? I hope this is answered somewhere.

 

[CosmicVoyager]

Okay. I thought this had been resolved by the fact that all space was wrapped up with all the matter/energy at the beginning of the universe, so that it could not be like a black hole sitting in space because there was no space outside to sit in. All particles had equal gravity acting on them from all directions.

Now, in another thread people are saying the universe in probably infinite. Either this thread is wrong or that one is. Which is it?

Let's make some progress here.

https://www.physicsforums.com/showthread.php?p=3173067

 

[edgepflow]

I would condense some of the arguments to the following:

-- The Schwarzschild solution is a vacuum solution to the field equations (stress energy tensor = 0).

-- The Friedmann solution is not a vacuum solution but has stress energy tensor = diag [Density, Pressure, Pressure, Pressure]

 

[marcus]

...
Now, in another thread people are saying the universe in probably infinite. Either this thread is wrong or that one is. Which is it?

Let's make some progress here.

https://www.physicsforums.com/showthread.php?p=3173067

I believe you are seriously mistaken, Cosmic.
Show us where you saw people in that thread say that the U is "probably infinite."

I know I posted in that thread and explicitly allowed for both possibilities without saying probably one or the other. I would strongly object to being misinterpreted in such a gross fashion. Better be specific about who you mean by "people". Others might feel the same way about their statements being misrepresented.

 

[marcus]

The black hole solution is static. The expanding universe solution is not static. Comparing apples and oranges here, people.

By assuming the existence of a Schwarzschild radius, you have already made an error. The Schwarzschild radius is a feature of the static (or stationary) black hole solution. An expanding universe solution has no such feature.

Look. Nicksauce has already given a concise accurate response to the question.

 

[Trenton]

The title says it all. With all that mass in such a small space it must have been one, but then everything would have to stay inside it. If it was still a black hole we'd have a closed universe but nobody believes that anymore. Does this mean the whole universe started out as Hawking radiation or what?

Adrian.

Naturally this is conjecture but sensibly speaking if there was a big bang then it must have been the result of a singularity disruption - an event for which, under presently accepted theory, there is no cause.

There has been much debate about weather or not light excerts gravity. We all know it is influenced by gravity but does it actually exert it and if so how? Certainly if a photon's gravitational field expanded out from the photon in a gravitational wave, this could not move ahead of the photon. I can't get to grips with the maths for how it might influence another parallel photon but even rudimentry attempts seem to suggest that it's field would be diminished.

A sigularity disruption would be possible then, if there came a point when matter's collapse to ever smaller parts could only continue by being converted to pure energy.

So we have white dwarf -> neutron star -> quark star? -> WIMP star? -> photons ??

 

[marcus]

Naturally this is conjecture but sensibly speaking if there was a big bang then it must have been the result of a singularity disruption ...

A sigularity disruption would be possible then, if there came a point when matter's collapse to ever smaller parts could only continue by being converted to pure energy.

So we have white dwarf -> neutron star -> quark star? -> WIMP star? -> photons ??

It sounds like you are thinking about what in Quantum Cosmology is called the "Big Bounce".
You are considering stages of collapse and conjecturing about something that might happen at high density that turns the collapse around and starts expansion.

This as recently become a hot area of research, a recent survey paper by one of the experts is "The Big Bang and the Quantum"
http://arxiv.org/abs/1005.5491

It is getting researcher's attention for two main reasons:

A) Quantizing the main cosmology equation in a fairly simple natural way introduces quantum corrections that take effect only at very high density and make gravity effectivey repel instead of attract, thus causing a bounce

B) Observational testing experts have begun to see how the big bounce theory can be tested using Cosmic Background Radiation data (a CBR sky map that includes polarization as well as temperature fluctuations).

C) The bounce makes a satisfactory episode of cosmic inflation easier to happen without parameters being finely adjusted. It makes the universe we actually see more likely than many alternatives that might otherwise have occurred.
A recent paper on that is
http://arxiv.org/abs/1103.2475

What you are describing as a collapse followed by "singularity disruption" sounds to me like the collapse rebound caused by quantum effects at high density which the QC people call "bounce".
=============================

In any case the universe at the start of expansion forming is out of the question.
Nicksauce already explained that.
https://www.physicsforums.com/showthread.php?p=3174272#post3174272
Hopefully everybody got the message.

What I see you doing, Trenton, is going on beyond that to ask "what could have caused the start of expansion? What preceded it?" That is a good question. Definitely!

 

[nnunn]

So we have white dwarf -> neutron star -> quark star? -> WIMP star? -> photons ??

Hmm, given the hypothetical binding energies implied by preon models, are you saying:

neutron star -> quark star -> preon star -> [unzip binding energy] -> [gamma ray burst?]​

Cool. Been trying to solve that grb riddle for a while.

 

[Trenton]

It sounds like you are thinking about what in Quantum
Cosmology is called the "Big Bounce".

I am very baffled by quantum mechanics and although I have heard of corrections that make gravity effectivey repel instead of attract, I can't pretend to understand it! I arrived at my suspicions through a much simpler thought process. My starting position is that all boson and fermions are orbitals (wave particle duality). Certain values (of mass/energy) form stable particles, other values form unstable particles. The particles may exist in certain states (eg electrons in different energy states) but apart from that, every value in-between must be a photon. This pretty much sums up my scant knowledge and perhaps distorted interpretation of quantum theory.

So quanta of certain values will under certain conditions form particles or self-circling orbitals. Everything else can't self-circle and so flys off at a tangent (becomes a photon). In both cases the electric and magnetic fields are propagating at c in their frame of reference. Inertia can then be thought of as the resistance of circular orbitals to having their frame of reference changed. Gravity is connected to this in some way but I can't for the life of me, figure out how! I feel though, that negative gravity is a non starter. If it were to exist there would have to be such thing as negative inertia. For something to resist having it's frame of reference conserved has weird implications. Ultimately it would move infinitely fast or infinitely slow. I suspect the equation solutions that predict this are invalid. Not all solutions to all equations are valid as we know.

The 'certain conditions' applying to orbitals refer to the pressence of fields. At all points equilibrium (lowest energy) is sought. In the case of collapse from a white dwarf to a neutron star, at the point this occurs, the electron and proton orbitals find it cheaper to merge with each other than continuing to self-circle in their original form. What happens when neutron orbitals become unviable in the face of pressure is all conjecture. We don't know exactly at what mass this happens and so it is quite possible that modest sized black holes are really just cloaked nuetron stars. It seems reasonable to conject that in the end, weather there are such things as quark stars or as I suggested, WIMP stars, ultimately there will come a point where there are no viable orbitals and so everything must become a photon.

The implications are quite radical. In particular the singularity that is supossed to lie at the center of a black hole can't exist as a point of absurdly small size (such as the de-broglie wavelength derived from the mass). Instead, at the point of final collapse, which will occur at a finite, non-zero size, everything becomes light.

It is this weird property of light, that it always travels at c, that gives us a clue as to what may happen next. I will withdraw ny earlier conjecture that photons may not be able to exert gravity or be less effective at doing so than matter, but will replace it with something just as controversial:- The light can move outwards, away from the center of gravity, much further than matter could because it sheds it's mass equvilence without compromising speed. It starts out with energies in the far gamma (inflation?), slows to the gamma and starts making particle/anti-particle pairs (end of inflation?). The light that did not make particles can just kept on getting redder the further out it gets. And unless there is some law I have never heard of, even at one billionth of a hertz, it will still be traveling at c. This is how I prefer to comprehend Hawkins radiation - that actually light does escape from black holes, albeit red-shifted by a factor with a large number of noughts on the end (to 2.7K maybe?)

I do tacitly accept that the redening would require gravity which implies maybe photons can exert!

In my view the concept of curvature of space-time has been taken too far. Yes I am completely happy with the concept but not to the point where the curvature becomes infinite - the only way light could be prevented from escaping. I note though that curvature would be infinite if the singularity was of zero size but I don't see how it can get to be zero. Moreover if curvature was infinite nothing including the singularity's own gravity could escape (unless gravity travels faster than light which seems doubtful). There are further problems with the accepted notions of black holes. Rs is calculated as the radius at which the escape velocity reaches c - except that there are no Lorentz contration terms in the equation for Rs? Do they cancel out or were they omitted by mistake? Time does not stop either at Rs since the gravitational potential realized by moving from infinity to Rs is not infinite. So observers probably won't see objects freeze at the event horizon unless they are using a webcam with a Windows PC.

 

[bcrowell]

There has been much debate about weather or not light excerts gravity. We all know it is influenced by gravity but does it actually exert it and if so how? Certainly if a photon's gravitational field expanded out from the photon in a gravitational wave, this could not move ahead of the photon. I can't get to grips with the maths for how it might influence another parallel photon but even rudimentry attempts seem to suggest that it's field would be diminished.

FAQ: Does light produce gravitational fields?

The short answer is yes. General relativity predicts this, and experiments confirm it, albeit in a somewhat more indirect manner than one could have hoped for.

Theory first. GR says that gravitational fields are described by curvature of spacetime, and that this curvature is caused by the stress-energy tensor. The stress-energy tensor is a 4x4 matrix whose 16 entries measure the density of mass-energy, the pressure, the flux of mass-energy, and the shear stress. In any frame of reference, an electromagnetic field has a nonvanishing mass-energy density and pressure, so it is predicted to act as a source of gravitational fields.

There are some common sources of confusion about this. (1) Light has a vanishing rest mass, so it might seem that it would not create gravitational fields. But the stress-energy tensor has a component that measures mass-energy density, not mass density. (2) One can come up with all kinds of goofy results by taking E=mc^2 and saying that a light wave with energy E should make the same gravitational field as a lump of mass E/c^2. Although this kind of approach sometimes suffices to produce order-of-magnitude estimates, it will not give correct results in general, because the source of gravitational fields in GR is not a scalar mass-energy density, it's the whole stress-energy tensor.

Experimentally, there are a couple of different ways that I know of in which this has been tested. An order of magnitude estimate based on E=mc^2 tells us that the gravitational fields made by an electromagnetic field is going to be extremely weak unless the EM field is extremely intense.

One place to look for extremely intense EM fields is inside atomic nuclei. Nuclei get a small but nonnegligible fraction of their rest mass from the static electric fields of the protons. According to GR, the pressure and energy density of these E fields should act as a source of gravitational fields. If it didn't, then nuclei with different atomic numbers and atomic masses would not all create gravitational fields in proportion to their rest masses, and this would cause violations of Newton's third law by gravitational forces. Experiments involving Cavendish balances[Kreuzer 1968] and lunar laser ranging[Bartlett 1986] find no such violations, establishing that static electric fields do act as sources of gravitational fields, and that the strength of these fields is as predicted by GR, to extremely high precision. The interpretation of these experiments as a test of GR is discussed in [Will 1976] and in section 3.7.3 of [Will 2006]; in terms of the PPN formalism, if E fields did not act as gravitational sources as predicted by GR, we would have nonzero values of the PPN zeta parameters, which measure nonconservation of momentum.

Another place to look for extremely intense EM fields is in the early universe. Simple scaling arguments show that as the universe expands, nonrelativistic matter becomes a more and more important source of gravitational fields compared to highly relativistic sources such as the cosmic microwave background. Early enough in time, light should therefore have been the dominant source of gravity. Calculations of nuclear reactions in the early, radiation-dominated universe predict certain abundances of hydrogen, helium, and deuterium. In particular, the relative abundance of helium and deuterium is a sensitive test of the relationships among a, a', and a'', where a is the scale-factor of the universe. The observed abundances confirm these relationships to a precision of about 5 percent.[Steigman 2007]

Kreuzer, Phys. Rev. 169 (1968) 1007

Bartlett and van Buren, Phys. Rev. Lett. 57 (1986) 21

Will, "Active mass in relativistic gravity - Theoretical interpretation of the Kreuzer experiment," Ap. J. 204 (1976) 234, available online at http://articles.adsabs.harvard.edu//full/1976ApJ...204..224W/0000224.000.html

Will, "The Confrontation between General Relativity and Experiment," http://relativity.livingreviews.org/Articles/lrr-2006-3/ , 2006

Steigman, Ann. Rev. Nucl. Part. Sci. 57 (2007) 463