# Are we in a black hole?

1. Jan 15, 2006

### ubavontuba

Hello,

I'm new here and I have a question I hope you might help me with...

Might "dark energy" be a natural consequence of relativity?

It seems to me that if the matter at the edge of the "known universe" is moving away from us at nearly the speed of light... that relative to us it must have nearly infinite mass.

Or in other words, could the universe be apparently "falling" outward toward a black hole shell, rather than simply accelerating from some invisible internal force like "dark energy?"

The weird part about this idea is that from every observational point in the universe, you'd observe the same thing (meaning there is no real shell, only a relative shell).

I think this might account for the time lag from the first expansion to the current expansion too do to matters of scale. What do you think?

ubavontuba

2. Jan 15, 2006

### Ivan Seeking

Staff Emeritus
IIRC, the boundary conditions for the universe are, or can be described as that of a black hole. I know that Kaku says something close to this: "If you want to know what it looks like inside of a black hole, look around your room".

I'm in no position to vouch for this though, and it may be out of date.

3. Jan 15, 2006

### Garth

The universe is like a BH only in that it is impossible to 'get out of' either.

The BH is one particlular solution of the spherically symmetric solution of Einstein's GR field equation, $\rho = \rho(r)$, $p = p(r)$, cosmology arises out of the cosmological solution $\rho = \rho(t)$, $p = p(t)$.

They are different and should not be confused. A BH is something within the universe, it is not the unverse itself, even in the BB singularity - that expands whereas the BH solution does not.

Garth

4. Jan 15, 2006

### SpaceTiger

Staff Emeritus
This is a common enough question that we might want to consider putting it in a sticky (along some of the other common questions). Anyway, here's a link with more information:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html" [Broken]

Last edited by a moderator: May 2, 2017
5. Jan 15, 2006

### ubavontuba

Relating it more to "dark energy."

Hey guys,

Thanks for the responses.

I'm more intersted in how this relates to dark energy, rather than black holes.

My concept is that to the observer, the universe having seemingly been expanding at relativistic speed from any given point, the perceived black hole "shell" should not act inside like the interior gravitational effects of a normal gravitational sphere. This is because the mass is receding at near light speed in either perceived direction (sort of like shining two flashlights in opposite directions).

That is that from the standpoint of the observer, the energy and mass at the perceived edge of the universe on his left, let's say, is in a noncontiguos reference frame from the matter and energy on his right.

So, all the matter on his right, let's say, is going to be effected more and more by the matter at the perceived edge on his right, in a relationship to distance. The closer it is to the observer, the less it is thusly affected and conversely, the farther away it is, the faster it will seem to be accelerating outward (generally speaking).

As the universe expands, this effect should be perceived by the observer as an acceleration of the expansion of the universe. Ergo, as "dark energy." This is because the receding walls allow there to be more space for this effect to take place in, and therfore everything (including perceived motion) must seemingly get bigger (faster) too.

So, is dark energy an as yet unquantified force pushing outward? Or is dark energy merely a gravitational effect of relativity?

Last edited: Jan 15, 2006
6. Jan 15, 2006

### pervect

Staff Emeritus
Some, but not all, of your issues are addressed in the FAQ

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/universe.html" [Broken]

Basically the singularity of the big bang is just in the wrong place for the universe to be a black hole - it's possible the universe is a white hole, but this is not a standard model and it' not particularly likely.

For more detail, see the FAQ.

Last edited by a moderator: May 2, 2017
7. Jan 15, 2006

### SpaceTiger

Staff Emeritus
Also, if I'm understanding you correctly, here you're relying on a common misconception about relativity:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_fast.html" [Broken]

Our models of the universe already use general relativity, so it wouldn't make sense to say that dark energy is a "natural consequence of relativity" unless:

1) General relativity is modified at large scales
2) We use a distribution of matter different from that observed (i.e. reject the cosmological principle)
3) We add an extra component of "dark energy"

There are also some folks who speculate that the acceleration could be explained by a back-reaction from large-scale inhomogeneities, but that appears to be quite different from what you're suggesting.

Last edited by a moderator: May 2, 2017
8. Jan 16, 2006

### ubavontuba

pervect and SpaceTiger:

Pervect, I guess I shouldn't have used the phrase "in a black hole," but rather; "Might that which is beyond our known universe be perceived by us as a sort of black hole?"

What I'm getting at is that it seems to me that the "edges" of the known universe should essentially behave (from our viewpoint) essentially the same as a black hole, only concave rather than convex/spherical. That is, that there is an event horizon, intense gravity and even Hawking radiation (we'd see this as the cosmic background radiation").

SpaceTiger,

I do not mean that we are a black hole because we are traveling at relativistic speeds as compared with other arbitrary masses.

In my concept, every observer notes that they are centrally located (as far as they can tell) and everything else is redshifting away from them (generally, as a matter of distance).

To us, an arbitrary point at the edge of the perceivable universe is moving at relativitic speeds and therfore any mass in that point will have relativistic properties (in the perspective of our reference frame). One of these properties being increased mass (relative to us).

Our mass doesn't increase because of this. And were we to move over to that far away point, we'd see that the matter there was normal and that the matter where we were (very far away now) has these same relativistic properties.

Wherever we are, the local reference frame seems normal. It's only in looking out and far away that we'd see the apparent effects (they're not local).

9. Jan 16, 2006

### SpaceTiger

Staff Emeritus
If you mean beyond the observable universe, then no, read the first link. Most models of the universe extend beyond the limits of our observational capabilities (i.e. beyond those points at which galaxies appear to be moving at relativistic speeds from us).

I feel like you're only reading the titles or something, since both of your questions are answered in the text of the links we provided. If you don't understand a particular point, please ask for clarification. Matter that is moving at relativistic speeds does not form a black hole, even "apparently". There are different kinds of cosmological horizons, beyond which our observational capabilities are limited in various ways, but there are no black holes involved. I'll be happy to elaborate if you're interested in horizons.

Again, models of the universe are fully general relativistic, so there is no room for the kind of error you're talking about.

10. Jan 16, 2006

### jhe1984

On the horizons issue, can things (if they exist) beyond the universe's horizon have an effect on the way things inside the horizon (in our universe) behave?

I guess light effects are out the question, but are gravitational effects out of the question?

More generally, can our universe be connected (via what?) to areas beyond our horizon?

Eww, wasn't planning on getting into multiverse issues, but I guess just did...

11. Jan 16, 2006

### ubavontuba

Right, I've read that. Referring to the portion that states:

Doesn't the edge of the perceivable universe lie in the future? Isn't it in the future for mass to move toward it?

I've read that too. Referring to:
I am not stating that anything forms a black hole due to its relative kinetic energy/momentum. I am asking if objects receding away from us at relativistic speeds would appear to us as if they are entering a black hole horizon.

Like an object entering a black hole horizon, the object should appear compressed and highly red-shifted. In other words, the edges of the universe (if we could see them) would apppear to us like a concave black hole. Again, I'm not stating it is a black hole.
Eventually, mass moving away from us should appear infinitely red shifted (it disappears into the apparent event horizon).

So, supposing that any of this makes sense, isn't it possible to expect black hole behavior from the boundaries of the perceivable universe?

12. Jan 16, 2006

### Leonardo Sidis

Just wondering, but have we proven that we can't escape from the universe? I think I read somewhere that if we created some massive amount of energy, we could rip a hole in the fabric of space-time. Also, we can't rule out wormholes.

13. Jan 17, 2006

### Garth

We will know that is possible when you do it, enjoy the trip!

Garth

14. Jan 17, 2006

### Mike2

Both Black Holes and the Observable Universe have an Event Horizon. You already know about the Event Horizon of Black Holes. The Observable Universe has a Cosmological Event Horizon. This is the distance from us where the expansion of space has caused objects at that distance to recede away from us at the speed of light so that we can no longer see them. And just like a BH, galaxies that approach the Cosmological Event Horizon appear to red shift and slow down in their motion; they freeze just before they disappear.

Some have suggested that just as in the case of BH's, an entropy can be calculated for the area of the Cosmological Event Horizon with the same formula for calculating the entropy of a BH using the area of its event horizon. And they suggest that the area of the Cosmological Event Horizon constraines the entropy inside it just as in the case for a BH. The area is emmense for the Cosmological Event Horizon. But that suggests some interesting consequences for an accelerating universe.

If the expansion of the universe is accelerating, then the distance to the Cosmological Event Horizon is shrinking, and the area associated with the Cosmological Event Horizon is decreasing as well. That would mean that the entropy enclosed inside the Observable Universe is decreasing. And we should expect some form of structures to emerge to compensate for the decreasing entropy of the collapsing Cosmological Event Horizon. Not only that, but we are losing material behind the Cosmological Event Horizon as galaxies recede away from us. So there seems to be a need for more complex and stable structures required of ever decreasing amounts of material. I find it interesting that life arose on earth at just about the same time that the universe started it accelerated expansion. But what can we expect of the future if this process continues? Will some people have to rise from the dead to make up for this decreasing entropy? I wonder.

15. Jan 17, 2006

### Leonardo Sidis

Thanks! Don't worry, I'll tell you how it went.

16. Jan 17, 2006

### Mike2

Just had a thought. The Unruh effect assigns a temperature to acceleration, and points in space are accelerating away from each other - the farther away points of space get, the faster they move away from us. And there must be an energy density for a given temperature. So does someone want to do the calculation: What is the energy density associated with the expansion of space? Is it anywhere close to the dark energy density? If no one else wants to venture this question, I'll try to get to it when I get time. Thanks.

Last edited: Jan 17, 2006
17. Jan 18, 2006

### pervect

Staff Emeritus
What you can correctly say is that the universe has a cosmological event horizon, if that's what you're getting at.

Note that the the redshift and apparent disappearing of an object at the horiozn is what an outside observer sees.

An observer inside a black hole would not observe these effects, he would see light from the outside observer just fine - the horizon is a one way barrier. This barrier is "outside can't see inside" for a black hole, it's "inside can't see outside" for the universe.

Thus what's similar is the existence of a horizon in both cases, but it's not right to think of the universe as being a black hole.

As far as the gravity of rapidly moving objects go, it doesn't behave like you think it does.

For instance, take the following scenario

spaceship1-------------->black hole
spaceship2

Spaceship1 heads directly towards a black hole at a high rate of speed, passig spaceship2 which is stationary with respect to the black hole.

Does the black hole appear heavier to spaceship 1 when it passes spaceship2, because of the relative motion? (Spaceship1 can consider that the black hole is moving directly towards it.)

The answer as to the black hole's apparent gravity depends on the coordinate system used, but a simple measurement that's easy to do on the two spaceships is to measure the difference in gravity along the length of the ship (the tidal force). If the black hole appeared heavier, we would expect that spaceship1 would experience more tidal force than spaceship2.

However, spaceship1 and spaceship2 will both measure the same tidal force in the above scenario.

If spaceship1 was moving away from the black hole we would have the same result.

If spaceship1 was moving up the page instead (with the black hole still to its right), it _would_ measure increased tidal forces as compared to spaceship 2.

Last edited: Jan 18, 2006
18. Jan 18, 2006

### Mike2

If you could give me the expansion rate in units of meters and seconds and the dark energy density in SI units of Joules per meter cubed, I'll see what I can do. Thanks.

19. Jan 18, 2006

### SpaceTiger

Staff Emeritus
I've been trying to find the time to put together a good summary of cosmological horizons, but I'm afraid I'm still falling short. Start by taking a look at hellfire's brief review:

The edge of the perceivable universe actually lies in the past. In theory, the maximally redshifted light would reach us from the moment of the Big Bang. In practice, we can't see anything before recombination (z ~ 1100).

There may exist an event horizon; that is, a comoving distance at which a light ray fired from earth at the present time would never reach. I don't know if that's what you mean. It's not the same as a black hole event horizon, but is, in some ways, analogous to it.

In current theory, an infinitely redshifted "object" would sit at the particle horizon at t=0 (the Big Bang). The particle horizon is always growing with time, so nothing disappears behind it. The event horizon can shrink with time and objects can "disappear" behind it, but it doesn't represent a boundary of infinite redshift.

Last edited by a moderator: Apr 21, 2017
20. Jan 18, 2006

### Mike2

So how far away is the Cosmological Event Horizon at present. I don't think this is the same as asking how far away the presently observeable horizon is because that is what it was in the past, right?

Would it be right to calculate the accelerataion of points in space from other points in space as the universe expanse as follows: Take the distance to the Cosmological Event Horizon (at present?) and say that there is a constant acceleration from zero to the speed of light over that distance over that time? Thanks.