View Poll Results: Are we in a BH with one of the cosmic horizons serving as BH event horizon? Yes, in my opinion we are. 13 14.29% No. 59 64.84% No opinion. 19 20.88% Voters: 91. You may not vote on this poll

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

by marcus
Tags: black, holet, schwarzschild
 P: 13 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?
Astronomy
PF Gold
P: 22,673
 Quote by PaulR 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 occuring at this moment of history. What you have noticed is an algebraic fact that is always true. Just not to misinterpret.
 P: 13 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?
Astronomy
PF Gold
P: 22,673
 Quote by 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. ...
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.
 P: 13 Thank you for this clarification As to R, I was referring to the 13.7 b light years based on the distance light has travelled 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.
P: 1,253
 Quote by 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 travelled 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.
Astronomy
PF Gold
P: 22,673
 Quote by 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 travelled 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 travelled 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.
P: 640
 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.”

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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.
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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
 P: 71 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?
Astronomy
PF Gold
P: 22,673
 Quote by 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?
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

 Quote by Fulvio Melia 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.
P: 640
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.
P: 137
 Quote by cristo 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.
 P: 531 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
 Sci Advisor PF Gold P: 3,273 But as you have been told jal, you are still using an inappropriate expression, $$R < \frac{2GM}{c^2}$$, the Schwarzschild solution, which only applies to a static spherical mass in otherwise empty space. Unless that is you are proposing that all the mass of the universe was concentrated into a small sphere situated in an infinite and empty space, which is not the understanding of the Big Bang. In the cosmological solution to Einstein's field equation the appropriate expression is one for density, and the question remains: "Is $$\rho > \frac{{3H_0}^2}{8\pi G}$$ or not?" i.e. "Is the universe closed - finite and unbounded - or not?" Garth
 Sci Advisor PF Gold P: 3,273 You are confusing two different solutions of the same Einstein Field Equation that are applicable to two different situations. In the first situation all the mass is concentrated in a spherically symmetric mass set in otherwise empty space. This is the One-Body or Schwarzschild Solution. The expression I quoted from you, $R < \frac{2GM}{c^2}$, is the condition on the radius of that mass for a BH. If that condition holds then the mass would concentrate at the singularity at the centre and that radius, the Schwarzschild radius, will be the radius of the Event Horizon that forms around it. In the second situation all the mass is spread out homogeneously and isotropically throughout the universe. This is the Cosmological Solution. The other expression I stated, $\rho > \frac{{3H_0}^2}{8\pi G}$, is the condition for the Critical Density above which the universe is closed in on itself, finite and unbounded. It is sometimes called the Closure Density. The universe is not, of course completely homogeneous, it has lumps in it, such as you. Some of those lumps of mass may satisfy the first condition in which case they will form a BH. When I look back to the CMB I do look at all the mass (visible and invisible) on my light cone back to around 400,000 year after BB, however I am not looking into the interior of a BH, I am looking back towards the BB naked 'singularity'. Do not confuse the two separate solutions to the Einstein Field Equation. Garth