We are in a Schwarzschild black hole-T or F?

[marcus]

We are in a Schwarzschild black hole--T or F?

What I am wondering is WHO HERE THINKS WE ARE IN A SCHWARZSCHILD BLACK HOLE where the black hole event horizon coincides with one of the two well-known cosmology horizons?

There are a couple of well-known horizon radii that we hear about a bunch:

the Hubble radius
this is c/H₀ and is the distance at which normal recession speed is c.
If I remember right, something around 13.5 billion LY, current distance.
(the radius of the Hubble sphere, as sometimes called)

the radius of the cosmological event horizon
In past years we've discussed this at PF quite a lot. I recall reading about it in Lineweaver's excellent 2003 paper, where one of the figures shows it as around 16 billion LY. Events that occur today outside the cosmological event horizon cannot ever affect us.
We are out of causal contact with current events at that distance---ASSUMING the LCDM model with its constant positive Lambda.

If Lambda is really zero and the present small positive measured value is an artifact, then the cosmological event horizon would not exist---events that occur today at arbitrarily large distances could eventually affect us, light from them could eventually reach us etc. But the LCDM model has this interesting feature (which Lineweaver 2003 presents in a nice clear treatment.)

I guess either radius could be called a "cosmic horizon" although this runs a risk of confusion because it wouldn't necessarily be clear which of the two was meant.
There were a couple of recent papers by Melia where he used that term. My impression was that he means the Hubble radius, but I could be wrong.

Anyway, I get the impression that some people think the universe inside one of these horizons is a Schwarzschild black hole and that the horizon, whichever one is meant, is the BLACK HOLE EVENT HORIZON of the black hole that we are in. This never occurred to me to imagine, and it simply does not make sense to me. But because similar WORDS are used I guess people can get the idea. Or maybe there is more to it, that I don't understand!

So here's the poll. Are we in a black hole?

 

[marcus]

Well Chris for goodness sake please register a "No" on the poll
We actually have people here at PF who think Yes, and the poll got a yes vote within 1 minute of being posted.

 

[jal]

I voted yes! ... Even though it goes against what I have learned.
The theoretical work implies it. The observations of some papers are inclined to say yes.
In fact http://arxiv.org/abs/0711.4810
Dark Energy in Light of the Cosmic Horizon
Authors: Fulvio Melia
(Submitted on 29 Nov 2007)
Says that we cannot tell. YET...
It must be worth while not to reject the model.
Sooooo, how is an amateur able to decide?
jal

 

[jal]

If I would not be studying the papers, I would not be able to answer.

 

[patty144]

Hi all. I wrote to Prof. Melia over the weekend about this very issue, and this is what he wrote:

Hi Patti:

thanks for writing. I don't mind at all. I may not always be able to
answer right away, but I try to get to all of my e-mail, so do feel
free to write whenever...

Yes, the Cosmic Horizon is not very difficult to understand, nor why
it arises in the first place. It helps if you know electrodynamics,
because this effect is similar (though not the same!) as what one
encounters there.

If you have a uniform, infinite (or effectively infinite) medium,
then if you cut out a spherical cavity in that medium, there is
absolutely no gravitational field/acceleration within the cavity.
The symmetry provides a perfect cancellation of the field created
by all of the sources outside of the cavity.

Thus, if you now place a mass, say an apple, at the center of that
cavity, then the gravitational field (or curvature, if you prefer
to think in those terms) produced by that apple inside the cavity
is as if there were nothing else outside---as if the apple were
the only source in the whole universe.

Now imagine gradually filling the cavity while you move out
to larger radii. Eventually, you reach the radius at which the
enclosed mass produces a Schwarzschild surface there.

Using the term "black hole" is not appropriate here because a
black hole, as we define it, is an object surrounded by vacuum.
But what is true is that light signals reaching us at the origin
of our coordinates from that radius, let's now call it the cosmic
horizon, are infinitely redshifted.

Does that mean there's nothing "on the other side"? No, of course
not. The universe is probably infinite. But any light that would
be approaching us from beyond the Cosmic Horizon is infinitely
redshifted, and therefore carries no signal or information.

Please note that this does not mean we live inside a black hole.
It's important to get that straight, because that term has come
to mean something else. But it does mean that our Cosmic Horizon
is as far as we can ever get information from events occurring
in our realizable universe. Whatever happens outside is not
communicable to us.

Also, please note that this Cosmic Horizon is not necessarily
static. It is only fixed for all time in a so-called de Sitter
universe, because in such a universe the density does not change,
so the horizon radius itself does not change. In a more realistic
universe, containing matter and radiation, as well as possibly
a vacuum energy density, this radius changes. In fact, it increases
with time. So as the universe ages, we get to see more and more
of it.

But this too must end, if the universe contains a cosmological
constant. In that case, eventually matter and radiation wither
away to zero, while the vacuum energy stays constant forever.
So our future would then be in a de Sitter universe, and the
Cosmic Horizon would then approach the de Sitter limit and
stay fixed at that value forever thereafter.

Best wishes,
Fulvio======================================================
Newly Released: "The Galactic Supermassive Black Hole"
http://press.princeton.edu/titles/8453.html

Fulvio Melia
The University of Arizona
Department of Physics & Steward Observatory
Rm 447, PAS #81 (520) 621-9651
http://www.physics.arizona.edu/~melia
======================================================

 

[marcus]

Thanks Patty, everything he says agrees with my understanding as well.
I think what he calls the "cosmic horizon" is the sphere at Hubble radius and he says it is a "Schwarzschild surface" which does not mean there is a black hole but simply that we don't get info from outside that surface.

so if something is ANALOGOUS to a black hole, it is what is OUTSIDE that spherical surface (our part of the universe is not the analog, it is all the rest that is the analog---and the analogy is very weak)

 

[Wallace]

Thanks Patty, everything he says agrees with my understanding as well.
I think what he calls the "cosmic horizon" is the sphere at Hubble radius and he says it is a "Schwarzschild surface" which does not mean there is a black hole but simply that we don't get info from outside that surface.

I must admit I'm not entirely sure what he means by his use of the term "Schwarzschild surface" in this context? His apple-in-a-cavity thought experiment implies that there is a requirement for some critical amount of enclosed matter for the horizon to appear, but it is not clear how that critical requirement relates to anything from the Schwarzschild Solution. Well, it's not clear to me at this point anyway.

I must admit I still haven't read the first Melia paper in the recent pair that came out, and I'm sure it is explained in more detail in there.

 

[jal]

I must admit I'm not entirely sure what he means by his use of the term "Schwarzschild surface" in this context? His apple-in-a-cavity thought experiment implies that there is a requirement for some critical amount of enclosed matter for the horizon to appear, but it is not clear how that critical requirement relates to anything from the Schwarzschild Solution. Well, it's not clear to me at this point anyway.

I was thought that Schwarzschild radius implied that nothing could get out... it's a "brick wall"... as a result ... anything inside can only bounce around.
It a good way to get conservation of energy... nothing can escape.

 

[George Jones]

Can any stuff here and now escape to future null infinity (scri +)?

 

[cristo]

Can any stuff here and now escape to future null infinity (scri +)?

That depends on the metric that we're taking, doesn't it?

I don't understand the idea proposed by people modelling the universe as schwarzschild: How can there be a global schwarzschild geometry when there is an assortment of matter; i.e. the matter is not confined to one specific location (or centre)?

 

[George Jones]

Can any stuff here and now escape to future null infinity (scri +)?

Do this even make sense?

What does the conformal diagram for our \LambdaCDM universe look like?

 

[Wallace]

Do this even make sense?

What does the conformal diagram for our \LambdaCDM universe look like?

Check the figures from Davis & Lineweaver. They have a very clear figure or two of the conformal representation of several cosmologies, including LCDM.

 

[jonmtkisco]

Prof Melia's description of a spherical cavity sounds like the result Peebles describes from Birkhoff's theorem. Which is why it's possible to consider any reasonably sized spherical subset of an expanding universe without regard to all of the mass/energy outside the sphere.

Also, as I understand it, the typical density of a black hole is about equal to the density of water. Obviously, our universe currently is far less dense. At some early time it was that dense, but I don't think there's any explanation how a black hole could ever get as un-dense as our observable universe.

 

[chronon]

Too many horizons

There seem to be a lot of candidates for what can be called a cosmic horizon, and it's important not to get them mixed up. (see http://www.chronon.org/Articles/cosmichorzns.html)

1) Hubble Sphere: This has no physical significance whatsoever.

2) Particle horizon: This is the limit of what can have had any effect on us since the big bang. It occurs in most models of the universe which have gravitating matter.

3)Cosmological Event horizon: This occurs when the expansion of the universe is accelerating. It has some similarities to the event horizon of a black hole (see http://www.chronon.org/articles/Cosmological_Event_Horizon.html)

4) Now Melia seems to have invented another horizon, which is the radius at which the matter around us would form a black hole. I'm very suspicious about this, since if you go back in time, this horizon encompasses a smaller and smaller part of the universe, and yet we have somehow got beyond the 'black hole' we were in then. Its interesting to look for what the problem is with Melia's horizon. I would guess that as long as his horizon lies outside the particle horizon, the matter won't be able to form a black hole. If we lived in a closed universe which was destined to recollapse to a singularity then it might be reasonable to say that we were in a black hole.

Hopefully Chris Hillman will be along in a short while to set us straight on this matter.

 

[George Jones]

Check the figures from Davis & Lineweaver. They have a very clear figure or two of the conformal representation of several cosmologies, including LCDM.

I have somehow misplaced my hardcopy of this article, but looking online last night, I didn't see what I was looking for. Now, I have roughly the same spacetime coordinates as my books, so I have looked in Hawking and Ellis, which has a conformal diagram for de Sitter spacetime, and, in the future, our \LambdaCDM spacetime looks like this, i.e., future null infinity is spacelike. In the past, however, \LambdaCDM has a Big Bang singularity.

Here's what I was hinting at.

For a spacetime M, define a black hole to be the region B = M - J^-(scri^+). Here scri^+ is future null infinity and J^- denotes causal past. A particle (photons included) at any event p in B cannot escape to infinity, since p isn't in the past of infinity.

For our \LambdaCDM spacetime, which seems to model observations well, everything is in the past of future null infinity, so B is empty; our \LambdaCDM spacetime is not a black hole spacetime.

(No, I'm not saying that black holes don't exist in our universe.)

While there are lots of horizons, including event horizons for particles (since future null infinity is spacelike), none of these is a black hole event horizon, since the boundary of the (causal) past of future null infinity is empty.

 

[Vast]

I chose no, because the dynamics of a BH horizon and the cosmological event horizon seem to behave very differently.

But then again I could be wrong.

 

[jal]

A review by Ruth Gregory. A theoretical physicist at Durham University, UK

http://wwwphy.princeton.edu/~steinh/
Paul J. Steinhardt
Endless Universe: Beyond the Big Bang
Paul J Steinhardt and Neil Turok

Inflation was designed to solve some problems which can also be solved by the cyclic universe.
Steinhardt and Turok the universe is simply a slice (known as a brane) through these extra dimensions, and the Big Bang was a collision of branes — a huge cosmic thunderclap. This model builds on an idea called M-theory, in which the strings live on two walls at the end of an 11D space–time. Applying the usual rules of string theory leads to a general picture in which these walls can move across the canyon separating them, and occasionally (every trillion years or so according to Steinhardt and Turok) slam into each other. It is this slamming together that is responsible for what we see as the Big Bang, although from a higher-dimensional point of view it is a collision rather than a singularity.

One message the authors communicate clearly is that we should never accept something simply because most people say it is true, but should constantly challenge and look for alternatives to any picture that cannot be rigorously proven.
--------
present vote …
yes, … 2
no, … 12
-------
hehehe

 

[jal]

If you have been reading the papers then it is obvious that The Cosmic Horizon
by Fulvio Melia has got as much observational info for it to be considered a serious candidate as any other model.
If you support colliding branes then they would create a Cosmic Horizon. There is no reason to assume that our universe was the only one created by colliding branes. Therefore, the logic would be to assume that the “bulk” or “cosmos" is populated with 10^500 universes each having their own Cosmic Horizon. All would be irrelevant … until … they meet and mearged.

 

[pervect]

While the majority of the posters have already gotten the right answer (no), some of the answers were quite technical.

I would like to point out that this question is addressed in less technical terms in the sci.physics.faq Is the Big Bang a black hole?

What is the distinction between the big bang model and a black hole?

The standard big bang models are the Friedmann-Robertson-Walker (FRW) solutions of the gravitational field equations of general relativity. These can describe open or closed universes. All these FRW universes have a singularity at the origin of time which represents the big bang. Black holes also have singularities. Furthermore, in the case of a closed universe no light can escape which is just the common definition of a black hole. So what is the difference?

The first clear difference is that the big bang singularity of the FRW models lies in the past of all events in the universe, whereas the singularity of a black hole lies in the future. The big bang is therefore more like a white hole which is the time reversal of a black hole. According to classical general relativity white holes should not exist since they cannot be created for the same (time-reversed) reasons that black holes cannot be destroyed. This might not apply if they always existed.

But the standard FRW big bang models are also different from a white hole. A white hole has an event horizon which is the reverse of a black hole event horizon. Nothing can pass into this horizon just as nothing can escape from a black hole horizon. Roughly speaking, this is the definition of a white hole. Notice that it would have been easy to show that the FRW model is different from a standard black or white hole solution such as the static Schwarzschild solutions or rotation Kerr solutions, but it is more difficult to demonstrate the difference from a more general black or white hole. The real difference is that the FRW models do not have the same type of event horizon as a white or black hole. Outside a white hole event horizon there are world lines which can be traced back into the past indefinitely without ever meeting the white hole singularity whereas in a FRW cosmology all worldline originate at the singularity.
Could the big bang be a black or white hole all the same?

In the previous answer I was careful to only argue that the standard FRW big bang model is distinct from a black or white hole. The real universe may be different from the FRW universe so can we rule out the possibility that it is a black or white hole?

The short version is that the big bang is definitely not a black hole. The question "Is the big bang a white hole" is more interesting, and the FAQ talks about this in more depth than the section I quoted above, but while this is IMO a more interesting question, it is not what was asked and I don't want to derail the thread.

 

[hellfire]

Of course the FRW solution is not the Schwarzschild solution. Prof. Baez answer seems to me like 'both solutions are not the same because they are two different solutions'. To my eyes the interesting question is rather how could the experimental data fit to such a proposal.

The best agreement with all the cosmological experimental data is provided by the standard model of cosmology. However, it could be a pedagogic exercise to try to figure out how to explain some basic facts assuming a Schwarzschild geometry. For example, is it possible to have redshift, time dilation and variations of brightness according to data in a Schwarzschild solution? If yes, with what constraints or conditions? What then about other cosmological tests such as the CMB or the ratios of light elements?

 

[George Jones]

Of course the FRW solution is not the Schwarzschild solution. Prof. Baez answer seems to me like 'both solutions are not the same because they are two different solutions'.

This is the tack that I, too, took in my posts.

To my eyes the interesting question is rather how could the experimental data fit to such a proposal.

I've been wondering when someone would say this.

The best agreement with all the cosmological experimental data is provided by the standard model of cosmology. However, it could be a pedagogic exercise to try to figure out how to explain some basic facts assuming a Schwarzschild geometry. For example, is it possible to have redshift, time dilation and variations of brightness according to data in a Schwarzschild solution? If yes, with what constraints or conditions? What then about other cosmological tests such as the CMB or the ratios of light elements?

I suspect that the answers to some (many?) are no, but it would be nice to see the answers (yes or no) worked out for extended Schwarzschild, and deep inside extended Kerr (without Poisson/Israel).

 

[jal]

It appears that Fulvio Melia does not have to worry about "the inquisitors". Let the data speak. (pedagogic exercise?)
jal

 

[lightbeing]

yes

If proven to exist (you make the call i guess) it would facilitate a necessity to be true that indeed we are. Also, if this the case, only a shallow outlook would consider the obverse. xlated--> Meaning short sighted realization or an overlooking based on what resolute level you have modeled the obsrevation upon.

 

[jonmtkisco]

To my eyes the interesting question is rather how could the experimental data fit to such a proposal.

... However, it could be a pedagogic exercise to try to figure out how to explain some basic facts assuming a Schwarzschild geometry.

This exercise sounds suspiciously like Arthur Eddington's (supposedly tongue-in-cheek) proposition in the 1930's that the universe is not expanding, instead all of the matter is shrinking. Such a theory can easily explain redshift (shrinking measuring rods of the observer), but messes up all sorts of other things, such as the speed of light and quantum mechanics.

Jon

 

[jal]

I think that the "thought of "edge" is the scarry part.
Depending on who you talk to there are different "edge".
Here is a partial list commencing by the most accepted.
1. Infinite -- no edge
2. Light cone --- speed of light
3. Hubble --- Expansion size
4. Cosmic Horizon/white hole/black hole/schwartchild radius --- gravity (this discussion)
5. dimensions --- 3d ---> more dimensions
6. multi-universes, --- vacuum energy ---> 10^500 bubble universes
---------
Here is some interesting reading
http://en.wikipedia.org/wiki/Brans-Dicke_theory
In theoretical physics, the Brans-Dicke theory of gravitation (sometimes called the Jordan-Brans-Dicke theory) is a theoretical framework to explain gravitation. It is a well-known competitor of Einstein's more popular theory of general relativity. It is an example of a scalar-tensor theory, a gravitational theory in which the gravitational interaction is mediated by a scalar field as well as the tensor field of general relativity.
-----------
http://en.wikipedia.org/wiki/Self-creation_cosmology
Self-creation cosmology (SCC) theories are gravitational theories in which the mass of the universe is created out of its self-contained gravitational and scalar fields, as opposed to the theory of continuous creation cosmology or the steady state theory which depend on an extra 'creation' field.
As an alternative gravitational theory SCC is a non-standard cosmology in which the Brans-Dicke theory (BD) has been modified to allow for mass creation. It relaxes the requirement of the conservation of energy-momentum (or four-momentum) so the scalar field may interact directly with matter.

 

[Garth]

---------
Here is some interesting reading
-----------
http://en.wikipedia.org/wiki/Self-creation_cosmology
Self-creation cosmology (SCC) theories are gravitational theories in which the mass of the universe is created out of its self-contained gravitational and scalar fields, as opposed to the theory of continuous creation cosmology or the steady state theory which depend on an extra 'creation' field.
As an alternative gravitational theory SCC is a non-standard cosmology in which the Brans-Dicke theory (BD) has been modified to allow for mass creation. It relaxes the requirement of the conservation of energy-momentum (or four-momentum) so the scalar field may interact directly with matter.

Cough, cough.

If you read further on under "Falsifiable tests of the theory" you will find the statement:

One of them, the Gravity Probe B geodetic precession, which measures the precessions of four accurate orbiting gyroscopes, is being evaluated in 2007; SCC predicts 2/3 that of the GR N-S precession, i.e. 4.4096 arcsec/yr. whereas the frame-dragging or gravitomagnetic E-W precession prediction is the same as that of GR i.e. 0.0409 arcsec/yr. The first results of this experiment were published at the American Physical Society Meeting on the 14th April 2007. While unforeseen errors are still being determined through 2007 the geodetic precession measurement of 6.6 arcsec/yr, which is within 1% of the GR prediction, is fatal to the present form of SCC.

A general version of the theory in which \lambda \ne 1 is being prepared, watch this space...

You may also be interested in the latest posts to the thread Alternative theories being tested by Gravity Probe B...

That http://einstein.stanford.edu/cgi-bin/highlights/showpic.cgi?name=GR-85-day_result.jpg is showing that to a 1 sigma error confidence level the results for the geodetic precession are inconsistent with GR.

This is at about a 68% confidence level, we wait for the 3\sigma 4-gyro results next year, but so far it does look interesting!

Garth

 

[Garth]

You may also be interested in the latest posts to the thread Alternative theories being tested by Gravity Probe B...

That diagram is showing that to a 1 sigma error confidence level the results for the geodetic precession are inconsistent with GR.

This is at about a 68% confidence level, we wait for the 3LaTeX graphic is being generated. Reload this page in a moment. 4-gyro results next year, but so far it does look interesting![

Now that is interesting, the GP-B website has withdrawn that diagram and replaced it with one that makes no such claims!

Garth

 

[Chronos]

Of course you can manipulate all the necessary variables to create a 'universe' that looks like the inside of a black hole. The exercise is, however, meaningless. You can also model the universe as a hydrogen atom - an equally meaningless, but amusing proposition.

 

[jal]

Is the bounce approach meaningless, in your opinion?
If not... once the 10^80 particles have been "made" ... how do you remove the gravity so that you can start the next phase wthout having a cosmic horizon already in place.

 

[marcus]

Of course you can manipulate all the necessary variables to create a 'universe' that looks like the inside of a black hole. The exercise is, however, meaningless...

I agree it would be meaningless. And I wouldn't know how to manipulate variables so as to even fake it---if the aim is to get an expanding universe (expanding at infinity). Because the Schw. solution (on which basis things like the Schw. radius and event horizon are defined) is a STATIC solution. the geometry does not change. In Schw. model, outside space is not expanding.

To illustrate, near the "bigbang" onset of expansion you don't have a static solution, you have a dynamic solution to EFE which is expanding so fast that essentially it doesn't matter how many matter particles you have briefly packed into how small a space, you still don't get a black hole (even at density approaching Planck!).

I don't see how there can be a difference of opinion. Even Fulvio Melia says clearly and emphatically that we are not in the interior of a black hole. And he is not a cosmologist---he does observational astrophysics and writes popularization books, if I recall correctly---so he could be forgiven if he used some terminology in an unconventional way that would give a naive reader the wrong idea. Cosmology is not his field. But he is definite (see Patty's letter) about our not being inside a black hole.
Experts please correct me if I'm mistaken but in my experience the Schwarzschild radius formula 2GM/c^2 can come up in other contexts to give other distances, and in the black hole situation it does NOT GIVE THE DISTANCE FROM THE central singularity out to the event horizon as an observer inside the event horizon would likely measure it. If we were actually inside a black hole of mass M, we would NOT estimate the distance to the event horizon as 2GM/c^2. I wouldn't anyway! What the formula gives is half the diameter of the event horizon as measured by an outside observer, or the circumference divided by 2pi. It does not give the radius as a person inside would be apt to see it (poor guy!).

So the fact that this same formula happens to come up defining a certain distance from our galaxy out to a certain kind of Melia horizon does in no way indicate that we are in the interior of a black hole. If we were in the interior of a black hole things would look very very very different from what we see. It totally doesn't fit the observational data. Particularly if we were near the CENTER of the spherical horizon, as we are in Fulvio Melia's picture.

Picture it. We'd be at the frikkin singularity!

So Melia doesn't say it and no competent cosmologist says it, and a moment's visualization---if you think visually---makes it obvious that it can't be. I'm puzzled as to how there can be any difference of opinion about this. But thanks to everybody who responded!

Hey, we have 24 responses!
I was curious what other people thought and it's great to have so many responses!
Thanks again, all.

 

[jal]

Let's see if there is a "math kid" who can extrapolate the following to the cosmic horizon.
http://arxiv.org/abs/0712.0817
Loop quantization of spherically symmetric midi-superspaces : the interior problem
Authors: Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
(Submitted on 5 Dec 2007)
------
The above was brought to our attention by marcus
--------
jal

 

[PaulR]

This is my first post but I feel strongly that we are in a BH from the perspective of an "outsider".

First - consider the density of a black hole = Mass/ Volume = 3 * C* C/ (8* G*π * R*R)
Using A value for R = 13.7 billion Light Years gives a density = 9.57 E -24 g/ m^3, which is about the actual density measured today.

Thus it would seem that an "outside" observer measuring our observable universe would see something that has the density of a 13.7 billion light year black hole. Surely that is what is meant by saying we live in a BH.

Now consider someone inside this space with this density and radius..

I do not understand why at this density there should be a singularity at the center. By analogy, the gravity at the center of the Earth cancels out to 0. It does not go up. The same way, the gravity as you move away from the "edge" of a truly massive BH that does not have to fight matter degeneration should drop, not increase.

Thus it does not violate any rules to consider a black hole with that low a density staying low in density all over rather than having a high density in the center.

Second - Look at the relation to the concept that space curvature in our BH is equal 0.

The equation for Density to get 0 curvature is 3 * H * H / 8 * π * G

If you set the two densities equal, you get

3 * H * H / 8 * π * G = 3 * C* C/ (8* G*π * R*R)

This reduces to H = C/R, providing a value for the Hubble constant of 71.373 km/ sec/ Mpc

This is within the estimated value. Doesn't this prediction of the Hubble Constant value provide some validity to this approach? As Hubble measurements get more precise and they converge on this value, would that not prove the likelihood of this conjecture?

It would be an amazing coincidence that the Hubble Constant is the value from the Schwartzild density at the current age of the universe and the observed flatness.

Surely the almost perfect relation between these three is more than coincidence.

As an aside, don't forget the Hawking Radiation that would be occurring if this is a black hole.

 

[jal]

Hi PaulR!
Welcome!
I tried to do the calculations and kept getting my units and zeros mixed up. I'm sure that someone will check your calculations since you came up with a very interesting observation.

It would be an amazing coincidence that the Hubble Constant is the value from the Schwartzild density at the current age of the universe and the observed flatness.

As you know, the Hawking Radiation is related to the size of a black hole. The smaller the the black hole the more radiation/evaporation. Therefore, using the 17 BLY of Fulvio Melia, I would expect no observable radiation/evaporation.
Can someone do a this calculation?

 

[pervect]

Of course the FRW solution is not the Schwarzschild solution. Prof. Baez answer seems to me like 'both solutions are not the same because they are two different solutions'. To my eyes the interesting question is rather how could the experimental data fit to such a proposal.

The best agreement with all the cosmological experimental data is provided by the standard model of cosmology. However, it could be a pedagogic exercise to try to figure out how to explain some basic facts assuming a Schwarzschild geometry. For example, is it possible to have redshift, time dilation and variations of brightness according to data in a Schwarzschild solution? If yes, with what constraints or conditions? What then about other cosmological tests such as the CMB or the ratios of light elements?

I thought the FAQ was pretty clear on this point, actually - however, I didn't quote the applicable sections, because I thought this was wandering away from the original question, and I figured that interested people could read the FAQ on this point.

The answer to "is the universe a black hole" is pretty definite - it's no. It has the wrong structure to be a black hole.

While the universe can't be a black hole, a sufficiently large white hole is basically not distinguishable from a FRW cosmology. This makes the question essentially moot. (There *might* be a way to distinguish the two theories if one was willing to wait several billion years. The one thing that the FAQ might be accused of omitting is the fact that there might *not* be any way to distinguish the two theories if there is indeed some sort of cosmological constant, making the question totally moot rather than moot only in practice.)

Could the big bang be a black or white hole all the same?

In the previous answer I was careful to only argue that the standard FRW big bang model is distinct from a black or white hole. The real universe may be different from the FRW universe so can we rule out the possibility that it is a black or white hole? I am not going to enter into such issues as to whether there was actually a singularity and I will assume that general relativity is effectively correct as for as we are concerned here.

The previous argument against the big bang being a black hole still applies. The black hole singularity always lies in the future light cone whereas astronomical observation clearly indicate a hot big bang in the past. The possibility that the big bang is actually a white hole remains.

...

It follows that the time reversal of this model for a collapsing sphere of dust is indistinguishable from the FRW models if the dust sphere is larger than the observable universe. In other words, we cannot rule out the possibility that the universe is a very large white hole. Only by waiting many billions of years until the edge of the sphere comes into view could we know.

 

[chronon]

This reduces to H = C/R, providing a value for the Hubble constant of 71.373 km/ sec/ Mpc

But H=C/R just gives the radius of the Hubble sphere, and we can certainly see beyond that.

 

[jal]

Let us see if we can reduce the amount of arm waving.
1. Present interpretations starts from a big bang and a singularity. The universe started from nothing and expanded to an infinite size in only 13.7 billion years.
2. Now we change the story and say that the universe started from a minimum size of 24 units, at the big bang and expanded to infinity in only 13.7 billion years.
3. Now we change the story a little bit more and we say that the universe is repeating this contracting and big bang cycle. You are asking that the infinite size of the universe can contract to a size near the Planck scale not only once but repeatedly in a finite amount of time. Tell me, how much finite time do you want to use to have this infinite size universe go through each of these cycles?
4. Now, … let us get real. Let us use a finite size, 17 billion years, of an infinite “cosmo” and see if we get some kind of bouncing universe that correspond to observations. There is only one force, gravity, which will be able to select that finite size so that we can have these repeated cycles of bounce. Therefore, we could imagine that this cosmic horizon could have been smaller in previous bounces and that it grew with the addition of more “matter”. As a result, we are now in a universe that has 10^80 “particles” and it now has a cosmic horizon of 17 billion light years. If you want to eliminate the cosmic horizon then find a way to eliminate the gravity that caused it. If you disagree with a cosmic horizon then you got to find/invent a mechanism that will select a finite size of an infinite universe that will go through the bounce cycles in a finite amount of time.

 

[PaulR]

I apologize but I am confused.
I do not see why having a singularity in the past has any relation to the concept that a sphere of radius 13.7 B light years and a density of the actual current density of our universe would not be a black hole to an observer outside this sphere.
What other charecteristics are needed for a sphere that matches the black hole charecteristics are needed?
How does our 13.7 billion year neighborhood not qualify?
Is there something associated with its past that disqualifies a body from being a black hole?

Wouldn't an outside observer see that light or matter approaching our sphere acts exactly like it would approaching any other black hole? What clue/ measurement would an outside observer have to say this is not a BH?

 

[marcus]

This is my first post but I feel strongly that we are in a BH from the perspective of an "outsider".

It would be an amazing coincidence that the Hubble Constant is the value from the Schwarzschild density at the current age of the universe and the observed flatness.

Surely the almost perfect relation between these three is more than coincidence.
...

Hi PaulR, welcome to PF! You sound reasonable and able to change your mind.
We are not in a BH with the Hubble Radius as event horizon radius. That would put us at the center and things would look very different.

But if you had a lever that would ABRUPTLY HALT THE EXPANSION OF THE UNIVERSE, like on old train cars there was that rope you could pull in case of emergencies and make it screech to a halt, then we could consider what would happen.

It is a tautology, something built into the algebra, that in the flat case the Hubble radius is given by a formula which looks just like the Schwarzschild radius formula in a VERY DIFFERENT SPACETIME. The Schw. solution to the Einstein equation is a very different spacetime geometry. It is not an expanding Friedmann-LeMaitre. It is not flat inside.
It happens that the same formula gives Schw radius in the very static very unflat case of Schw geometry and also gives the Hubb radius in the very UNstatic very flat case of Friedmann geometry.

Same formula, happens to give two different things in two different geometries.

But suppose you did have an emergency-brake handle that can stop the universe expanding. It is painted red, and has a comfortable grip. You grasp the handle and think... what would happen? It is a serious question because as soon as you pull the universe is going to start collapsing! There is plenty of density for that. All over the place. Many spheres, overlapping ours and much larger, have the required density. The moment you deprive the universe of its expansion it will assume a collapsing geometry.
But that's a different geometry, a different future, a total other kettle of fish.


BTW the Hubble parameter is not constant, even though it used to be called "Hubble constant". And as it changes, the Hubble radius changes. There is no coincidence occurring at this moment of history. What you have noticed is an algebraic fact that is always true. Just not to misinterpret.

 

[PaulR]

Thank you for this explanation.

As a follow up, I have seen many estimates of the Hubble parameter and estimates of the age of the universe.
If the Hubble parameter is algebraically always C/ R, why is this relation not used to refine the estimates.
Instead, the best estimates of H and R are almost but not quite in line with this formula.

It was this lack of assuming they were algebraically related that led me to think they did not have to be related. Then when I saw how close they were I thought that had a significance.

As a second question, does this difference also apply to an expanding black hole? I.e. if a body that has the right density is expanding, does that prevent it from being a black hole.

Finally, what would an observer see when looking at a bubble that is 13.7 b Light years wide with the density we have. How would it differ from a black hole? Would the gravity not be suficient to form an event horizon from this outsider's perspective? Could the outsider easily traverse this sphere back and forth?

 

[marcus]

Thank you for this explanation.

As a follow up, I have seen many estimates of the Hubble parameter and estimates of the age of the universe.
If the Hubble parameter is algebraically always C/ R, why is this relation not used to refine the estimates.
...

You don't specify, but I think by R you mean the HUBBLE RADIUS.
This is the present distance at which a stationary point would now be receding at speed c due to expansion.

One cannot measure this directly---or determine it in any other way than by the usual means for measuring H. So one cannot use measurement of R to refine that of H. It is more the other way around.

What one does is measure H as accurately as possible, sampling recession speed at all convenient distances, then once one has a value for H then one DEFINES the Hubble radius as c/H

(all c/H means is that distance at which the recession speed is c, because H is the ratio of present recession speed to present distance)

As a second question, does this difference also apply to an expanding black hole? I.e. if a body that has the right density is expanding, does that prevent it from being a black hole.

Sure! That is what the people don't understand, who keep talking about us being in black hole. A region of spacetime which is expanding, even if has enough matter density to form hole if it were STATIC, nevertheless if it is expanding fast enough will NOT collapse to hole!

and BTW out at the Hubble radius (that Fulvie Melia was calling "cosmic horizon") stuff is receding at the speed of light so this spacetime region of ours is expanding like a bat out of hell.

and around big bang time, stuff was WAY denser than Schwarzschild requires, so why didnt the universe collapse then and there? Because it was expanding so fast.

Finally, what would an observer see when looking at a bubble that is 13.7 b Light years wide with the density we have. How would it differ from a black hole? Would the gravity not be suficient to form an event horizon from this outsider's perspective? Could the outsider easily traverse this sphere back and forth?

you mean 13.7 billion LY is the Hubble radius, so that is c/H and we are in space that is expanding at the rate H ( equals c/13.7 bLY)
you mean a bubble that has RADIUS equal to that, so it is twice that much wide.

Such a bubble is a typical chunk of our universe. To an outsider out near the bubble surface boundary it wouldn't look any different from any other similar volume. Geometrically it would be approximately flat.
CROSSING any rapidly expanding region raises more complicated issues. But suppose instead of crossing, the outsider just wants to dip in a few million LY and come out again. He could travel in and out of it just as he would venture into any other patch of space.

Remember that even though for us the boundary is receding at speed c, for him out there in the space around the boundary IT IS NOT MOVING. He is IN the space that is receding from us at speed c. So for him it is just ordinary space. there is nothing like a BH event horizon there. There is no point of no return. There is no trap. he can cruise across to our side, and be inside for a while, buy an icecream cone, and then cruise on back to the outside.

 

[PaulR]

Thank you for this clarification

As to R, I was referring to the 13.7 b light years based on the distance light has traveled since the universe began, not the Hubble Radius.
I was noting that one can use the best estimate of the age of the universe to compute the Hubble Parameter, thus providing a different independent estimate of this parameter.
I was wondering why this is not done.

 

[Wallace]

Thank you for this clarification

As to R, I was referring to the 13.7 b light years based on the distance light has traveled since the universe began, not the Hubble Radius.
I was noting that one can use the best estimate of the age of the universe to compute the Hubble Parameter, thus providing a different independent estimate of this parameter.
I was wondering why this is not done.

Because the age calculated from the observed Hubble parameter, so you can't then do it in reverse! We observe H and calculate the age. We can't observe the age of the Universe, though we can put rough lower bounds on it based on other observations, such as the age of globular clusters. There is no current significant 'age problem', since all objects in the Universe are, within the uncertainties, younger than the inferred age from the measured Hubble parameter today and the other cosmological parameters. There are some arguments about how long it would take Black Holes to form in the early Universe as well as some issues with metal abundances at high redshift, but the modelling of these is very uncertain, so it is not a very accurate way of measuring the age from which to calculated H.

 

[marcus]

Thank you for this clarification

As to R, I was referring to the 13.7 b light years based on the distance light has traveled since the universe began, not the Hubble Radius.
I was noting that one can use the best estimate of the age of the universe to compute the Hubble Parameter, thus providing a different independent estimate of this parameter.
I was wondering why this is not done.

the most precise way we estimate the age of the universe is again to first measure H and then use H to compute it.

there are some other ways to estimate the age, like getting statistics on stars and trying to guess how fast different size stars age etc etc. , or studying clusters, or abundances of elements, but those are much rougher and more iffy.

you can use star and cluster statistics and elements and the like as a CHECK on what you get from H, but the main way is to calculate from H.

therefore you cannot use age of universe to refine estimat of H-----it is more the other way around
======================

BTW, you know space is expanding, distances are increasing. You can picture this.
So why do you think that a photon of light would have traveled 13.7 bLY since beginning of expansion?

Don't you imagine it would have covered much much more ground? Because whatever distance it covered during the first part of its trip would have been way stretched out.

But you say

referring to the 13.7 b light years based on the distance light has traveled since the universe began

that can't be right! what do you guess the real figure is?

OOPS, while I was typing this I see that Wallace already replied! Well Paul, now you have two answers. I think I said much the same things as Wallace.

 

[jal]

I am aware of what our present model says:
1. If we look at the CBR we should be seeing light from 10^80 “particles” that were created and existed 400,000 years after the big bang. Protons have not decayed in that time span, therefore, due to expansion, those particles would now be, 47 billion light years away if the Universe is only 14 billion years old.
2. The size of the universe that existed 400,000 years after the big bang would contain all of those 10^80 “particles” and as a result, every direction you looked would eventually end on the surface of a star/particle, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers' Paradox.
3. The numerical value of the CMR redshift is about z = 1089 (z = 0 corresponds to present time). The highest measured quasar redshift is z = 6.4 while as-yet unconfirmed reports from a gravitational lens observed in a distant galaxy cluster may indicate a galaxy with a redshift of z = 10.. There is a big empty hole of knowledge between about z = 1089 and a redshift of z = 10.

There are a lot of particles up to a redshift of z = 10, yet they should be 47 billion light years away according to the present model.
I need a better explanation and I’m willing to examine what Fulvio Melia and others have to say.
===========


http://en.wikipedia.org/wiki/Redshift
The most distant objects exhibit larger redshifts corresponding to the Hubble flow of the universe. The largest observed redshift, corresponding to the greatest distance and furthest back in time, is that of the cosmic microwave background radiation; the numerical value of its redshift is about z = 1089 (z = 0 corresponds to present time), and it shows the state of the Universe about 13.7 billion years ago, and 379,000 years after the initial moments of the Big Bang
Currently, the highest measured quasar redshift is z = 6.4,[46] with the highest confirmed galaxy redshift being z = 7.0[47] while as-yet unconfirmed reports from a gravitational lens observed in a distant galaxy cluster may indicate a galaxy with a redshift of z = 10.
http://www.astro.ucla.edu/~wright/doppler.htm
If the Universe were infinitely old, and infinite in extent, and stars could shine forever, then every direction you looked would eventually end on the surface of a star, and the whole sky would be as bright as the surface of the Sun. This is known as Olbers' Paradox after Heinrich Wilhelm Olbers [1757-1840] who wrote about it in 1823-1826 but it was also discussed earlier. Absorption by interstellar dust does not circumvent this paradox, since dust reradiates whatever radiation it absorbs within a few minutes, which is much less than the age of the Universe. However, the Universe is not infinitely old, and the expansion of the Universe reduces the accumulated energy radiated by distant stars. Either one of these effects acting alone would solve Olbers' Paradox, but they both act at once.
http://www.weburbia.com/physics/olber.html
1. The Universe is expanding, so distant stars are red-shifted into obscurity.
2. The Universe is young. Distant light hasn't even reached us yet.

But the final two possibilities are surely each correct and partly responsible. There are numerical arguments that suggest that the effect of the finite age of the Universe is the larger effect. We live inside a spherical shell of "Observable Universe" which has radius equal to the lifetime of the Universe. Objects more than about 15 billion years old are too far away for their light ever to reach us.
http://www.astro.ucla.edu/~wright/cosmology_faq.html#ct2
If the Universe is only 14 billion years old, how can we see objects that are now 47 billion light years away?
… the most distant object we can see is bigger than 3 times the speed of light times the age of the Universe. The current best fit model which has an accelerating expansion gives a maximum distance we can see of 47 billion light years.
=========

 

[jal]

Fulvio Melia
“We will show that, with the recent WMAP results (Spergel et al. 2003), our observational limit clearly corresponds to the distance beyond which the spacetime curvature prevents any signal from ever reaching us. An observer’s worldline must therefore always be restricted to the region R < R0, i.e., to radii bounded by the cosmic horizon, consistent with the corollary to Birkhoff’s theorem.
The restrictions on an observer’s worldlines should be set by the physical radius R0, beyond which no signal can reach her within a finite time, no matter what internal structure the spacetime may possess.

However, the effects of gravity travel at the speed of light, so what matters in setting the structure of the universe within the horizon at time t is the mass-energy content within R0. The influence of these distant regions of the universe ended once their radius from us exceeded R0.
Our best fit to the Type Ia supernova data indicates that t0 would then have to be ~16.9 billion years. Though surprising at first, an older universe such as this would actually eliminate several other long-standing problems in cosmology, including the (too) early appearance of supermassive black holes (at a redshift > 6) and the glaring deficit of dwarf halos in the local group.
Our study has shown that scaling solutions not only fit the Type Ia supernova data much better than the basic _CDM cosmology, but they apparently simultaneously solve several conundrums with the standard model. As long as the time-averaged value of ! is less than −1/3, they eliminate both the coincidence and flatness problems, possibly even obviating the need for a period of rapid inflation in the early universe (see, e.g., Guth 1981; Linde 1982).
On the observational front, the prospects for confirming or rejecting some of the ideas presented in this paper look very promising indeed.”

--------------
Of course, his approach leads to speculation and other questions which he might have thought of and cannot published …. Yet!
If we can see the light (CBR) 400,000 years after the big bang then that means that those photons (EMF) have not left the universe. The universe was a 400,000 lyr sphere containing all the photons and all the particles. Something had to keep the photons from escaping or otherwise the universe would be losing energy.
It would be like dropping a rock into an ocean. The wave would keep going and never come. So, if there is no barrier, the observed (CBR) cannot be from 400,000 years after the big bang. Those photons are long gone.
Explanation #1
The expansion of the universe is always faster then the speed of light. However, that cannot be right because we would not see the light from other galaxies, gravity would not work, etc.
Explanation #2
As the 400,000 light year sphere of particles expands, at less than the speed of light, the photons (EMF) go faster than the expanding size of the particle sphere but are prevented from escaping and just go bouncing around and around within that barrier.
Therefore, the evidence of the (CMR) is the evidence of a barrier; the conservation of energy is the evidence of a barrier; neutrinos from the big bang epoch are supposed to be still around and if discovered would prove that there is a barrier keeping them here.
--------
What keeps the photons with a redshift of z = 10 to z = 1089 within our universe?
Can anyone do some explanations of red shift of neutrinos? http://conferences.fnal.gov/aspen05/talks/mena.pdf

 

[IMP]

I thought that black holes have maxiumum entropy? Looking around, I see hot areas and cold areas. Also, if we are living inside a huge black hole, how come we actually have black holes? What I mean is: Can there be black holes within black holes?

 

[marcus]

I thought that black holes have maxiumum entropy? Looking around, I see hot areas and cold areas. Also, if we are living inside a huge black hole, how come we actually have black holes? What I mean is: Can there be black holes within black holes?

Sure, black holes can fall into bigger black holes, just like stars and other stuff can fall in
But don't worry about "if we are living inside a huge black hole". Even Fulvio Melia, who keeps getting quoted, has said clearly and explicitly that we are not---his article was not intended to suggest that---he wants it clearly understood, and he says it is important to realize that. Here's part of his recent email message to Patty

Hi Patti:
...
...
Please note that this does not mean we live inside a black hole.
...

Best wishes,
Fulvio

And F. Melia is not an expert in cosmology---his research is in other stuff. Knowing his area of expertise, i certainly would not cite him as an authority on an issue in cosmology like this! But at least he is not so far out of line as to pretend we live in a black hole with event horizon coinciding with something like Hubble sphere or cosmic event horizon (which the poll question was asking).

Personally I doubt his recent paper is publishable as is---will have to be revised to eliminate the possibility that uninformed readers could misinterpret and get the idea he is saying we are in BH.

IMP I see you correctly said "no" on the poll---glad you are not confused about this. I didn't start the thread because it was an open scientific question, that you could reasonably consider either way. As I said at the start, I set up the poll be because I was interested to know if a significant number of people were confused or in doubt.

 

[jal]

There are different levels of readers reading this who have not read the papers or do not understand them.
Therefore, I will do some paraphrasing of what Fulvio Melia has done in his paper.
The picture of the Cosmic Background Radiation is a picture of the universe as it was 400,000 years after the big bang. It is a sphere of 400,000 light years. It contains all of the particles/galaxies (10^80). It contains all of the gravity and all of the photons.
He then shows (with math.) that if you take the known expansion of the universe to NOW, (13.7 billion years), then the particles/galaxies occupy a sphere of 13.7 billion light years. Gravity will occupy a sphere of 16.9 billion light years.
He then concludes that if there are any particles/galaxies outside of that 17 billion light year sphere it can be ignored since it will not have any influence on our universe.
He calls that 17 billion light year sphere the “COSMIC HORIZON”.
He did not go into the specifics of what is happening as the 400,000 light year sphere is expanding. He ends the papers with what he has observed and what he thinks.

Fulvio Melia
Our study has shown that scaling solutions not only fit the Type Ia supernova data much better than the basic _CDM cosmology, but they apparently simultaneously solve several conundrums with the standard model. As long as the time-averaged value of ! is less than −1/3, they eliminate both the coincidence and flatness problems, possibly even obviating the need for a period of rapid inflation in the early universe (see, e.g., Guth 1981; Linde 1982).

The calculation the gravity of the 10^80 particles in the 400,000 light year sphere is left up to the readers to do.

 

[PRyckman]

I don't understand the idea proposed by people modelling the universe as schwarzschild: How can there be a global schwarzschild geometry when there is an assortment of matter; i.e. the matter is not confined to one specific location (or centre)?

What if the point of observation was the center?
Would this help to explain it? Suspend beleif that that is not possible for a moment.

 

[jonmtkisco]

Hi PRyckman,

The Schwarzschild radius solution technically is applicable only in empty (vacuum) space around a point-source of gravity. For example, it does not apply to a regular star where the Schwarzschild radius would be calculated to be within the interior of the star. So it is doubtful that it could be accurately applied to our observable universe which contains a substantial amount of matter, regardless of whether you consider our matter distribution to be homogeneous or not.

I also think that the Schwarzschild solution takes no account of the Hubble scale expansion of the universe. My guess is that Schwarzschild just isn't applicable to a self-expanding region. Such a region should be expected to behave quite differently from a black hole, for reasons that simply aren't captured in the Schwarzschild equation.

Jon