In the beginning there was.... a black hole?

• I
• Warp
In summary, the universe began with all of its energy inside its own Schwarzschild radius, which is puzzling as energy doesn't seem to escape from its own event horizon. However, this may be because the universe expanded faster than gravity could propagate and so the event horizon didn't have time to form.
Warp
When matter is dense enough (most usually because its own gravity compresses it), it collapses into a black hole. The necessary density is defined by the Schwarzschild radius of that matter: If matter is inside its own Schwarzschild radius, it will collapse into a singularity (as far as we know). Famously, it's impossible for such matter to escape this radius once inside, a limit that's know as the event horizon.

At the initial moments of the universe, all of the energy in the universe was inside its own Schwarzschild radius. This is puzzling. How did the energy escape its own event horizon? Why isn't all the energy that exists in this universe simply in a singularity inside one giant black hole? How did it escape?

Being a complete layman on this subject, and having only a very cursory understanding of the physics involved, my own hypothesis is that the answer to this is that at the initial moments, the universe expanded faster (much, much faster) than gravity could propagate (which can only propagate at c). This means that the event horizon didn't actually have time to form because the metric expansion of space caused all the energy to spread out before its own gravity could "catch up" with it, if I'm allowed to use a what I'm sure is a rather informal expression. Thus, energy didn't actually escape its own event horizon, because there was no event horizon to escape from. Not yet. The universe expanded way too fast for it to have ever time to form and "enclose" all the energy.

Is this even close to correct?

The universe is infinite in extent, with more or less uniform matter density everywhere. A collapsing star is a high density region surrounded by a very much lower density region. These two situations are very, very different. Trying to apply reasoning developed in one to another isn't possible (except in the sense that you can go right back to general relativity, which underlies both).

If you want to reason about the early universe, rather than know some facts, you need to get a textbook and learn the maths, I'm afraid. Physicists don't all learn about tensors and calculus and algebra just for fun - it's the only way to describe the things we're interested in accurately. Fudging together verbal descriptions of different scenarios won't cut it. If it did, we wouldn't bother with the maths.

Dale
Warp said:
Thus, energy didn't actually escape its own event horizon, because there was no event horizon to escape from.
This part is correct, but not for the reason you suspect. The key is that the Schwarzschild spacetime is a vacuum spacetime, meaning that it describes the vacuum outside a central spherically symmetric object. It is this spacetime which has the event horizon.

The Big Bang is part of a FLRW spacetime. It is characterized by two key properties: isotropy and homogeneity. Homogeneity, in particular, distinguishes it from the Schwarzschild spacetime. A homogenous spacetime has no “outside vacuum”, everything is part of the gravitating body. This is what prevents the formation of an event horizon.

Ibix
Dale said:
This part is correct, but not for the reason you suspect. The key is that the Schwarzschild spacetime is a vacuum spacetime, meaning that it describes the vacuum outside a central spherically symmetric object. It is this spacetime which has the event horizon.

The Big Bang is part of a FLRW spacetime. It is characterized by two key properties: isotropy and homogeneity. Homogeneity, in particular, distinguishes it from the Schwarzschild spacetime. A homogenous spacetime has no “outside vacuum”, everything is part of the gravitating body. This is what prevents the formation of an event horizon.
Would a rather simplistic way of describing it be that all the energy was distributed evenly inside the (pretty much edgeless) universe, and therefore gravity was pretty much the same in all directions, and therefore there was no center of mass nor a particular direction towards which gravity would have pulled mass towards?

So rather than the universe (at the earliest moments) being like an object with had an absolutely massive gravitational force towards a "center", it was, perhaps ironically, pretty much weightless (because gravity was even towards all directions)?

Warp said:
Would a rather simplistic way of describing it be that all the energy was distributed evenly inside the (pretty much edgeless) universe, and therefore gravity was pretty much the same in all directions, and therefore there was no center of mass nor a particular direction towards which gravity would have pulled mass towards?
Yes, except that it is more than just “pretty much” edgeless. The FLRW spacetime has no boundary, it is completely edgeless.

Warp said:
and therefore there was no center of mass nor a particular direction towards which gravity would have pulled mass towards?
Yes. Since it is both isotropic and homogenous, by symmetry, there can be no location that will be the center of a pulling force.

Warp said:
So rather than the universe (at the earliest moments) being like an object with had an absolutely massive gravitational force towards a "center", it was, perhaps ironically, pretty much weightless (because gravity was even towards all directions)?
Yes.

Also, you didn’t ask for this, but it may help anyway. There are three possibilities, all of which are consistent with the data we have today.

One is that the universe has a positive spatial curvature. That would mean that it is shaped like a 3D sphere (the surface of a ball is a 2D sphere), so we would have a universe that is spatially finite but has no boundaries.

The next two possibilities are that the universe is spatially flat or has negative spatial curvature. Both of those possibilities would be a 3D sheet where flat would be a normal sheet and negative curvature is the type of curvature like a saddle or a Pringle’s potato chip. Both of those would be spatially infinite.

The reason I mention it is that immediately after t=0 the different models are rather different. The positive curvature would be a small compact space whereas the 0 and negative curvature would already be spatially infinite even immediately after the initial singularity.

PhanthomJay
Warp said:
When matter is dense enough (most usually because its own gravity compresses it), it collapses into a black hole.

No. When matter is dense enough and is static or contracting, it collapses into a black hole.

The early universe was neither static nor contracting; it was expanding rapidly. That's why it was not a black hole.

PeterDonis said:
No. When matter is dense enough and is static or contracting, it collapses into a black hole.

The early universe was neither static nor contracting; it was expanding rapidly. That's why it was not a black hole.

I think the challenge for many folks to understand is related to a t = 0 that is absolute in some sense being rapidly expanding. Physics usually has a pretty good explanation for "what was happening the instant before that?" In the absence of a good explanation, lots of laymen picture some kind of equilibrium for what they think must have pre-existed the rapid expansion for eternities before t = 0. The idea of a singular instant of time before which science cannot look or answer questions is odd. Because physics gives folks the notion that once the laws of nature are understood, the movie can always be run backwards to know what happened before. And that may not be possible, even in principle, with an absolute t = 0 and current cosmology.

Dr. Courtney said:
I think the challenge for many folks to understand is related to a t = 0 that is absolute in some sense being rapidly expanding.

Our current best model of the universe does not claim that there was an initial singularity, and the term "Big Bang" in that model does not refer to one. It refers to the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence. In inflation models, which appear to be the current front-runners, this state occurs at the end of inflation.

Dale said:
The next two possibilities are that the universe is spatially flat or has negative spatial curvature. Both of those possibilities would be a 3D sheet where flat would be a normal sheet and negative curvature is the type of curvature like a saddle or a Pringle’s potato chip. Both of those would be spatially infinite.
I don't really understand how it's possible for the universe to be infinite (I thought that's impossible), and especially how it's possible for the universe to instantly change from a singularity of zero volume into infinite volume.

Motore
Warp said:
I don't really understand how it's possible for the universe to be infinite (I thought that's impossible),
The universe does not feel obliged to obey your sense of what is possible.
and especially how it's possible for the universe to instantly change from a singularity of zero volume into infinite volume.
The currently accepted model of the universe is the Big Bang Theory. It is silent on any such "creation event" and certainly does not include what you think it includes.

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Warp said:
how it's possible for the universe to instantly change from a singularity of zero volume into infinite volume.
This actually is the reason that people here are careful to point out that the Big Bang properly refers to a hot dense initial state of the universe and not the singularity. For some very important theoretical reasons singularities are not part of spacetime. That includes the singularity of a black hole as well as the singularity in the Big Bang’s FLRW spacetime. Neither singularity is part of the spacetime.

What this means is that if the universe is infinite now then it was infinite at all times, no matter how close to t=0. And t=0 is not part of the spacetime, so there is never anything which changes from 0 volume to infinite volume.

pinball1970 and jbriggs444
Dale said:
. . . the different models are rather different.
That's absolutely. . . golden! .

.

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Dale and Motore

1. What is a black hole?

A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape from it. It is formed when a massive star dies and its core collapses in on itself.

2. How big can a black hole be?

Black holes can range in size from a few kilometers to billions of times the mass of our sun. The size of a black hole is determined by its mass, with larger masses resulting in larger black holes.

3. How do we know black holes exist if we can't see them?

Although we cannot see black holes directly, we can observe their effects on their surroundings. For example, we can see stars orbiting around a black hole and detect the radiation emitted from matter falling into a black hole.

4. Can anything escape from a black hole?

No, once something crosses the event horizon of a black hole (the point of no return), it cannot escape. This includes light, which is why black holes appear black.

5. Could a black hole destroy the Earth?

No, a black hole would need to be extremely close to Earth in order to have a significant effect on it. The nearest known black hole is over 1,000 light years away, making it too far to pose any threat to Earth.

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