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 

#37
Dec1107, 05:31 PM

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? 



#38
Dec1107, 06:55 PM

Astronomy
Sci Advisor
PF Gold
P: 22,800

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 FriedmannLeMaitre. 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 emergencybrake 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. 



#39
Dec1107, 08:45 PM

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? 



#40
Dec1107, 10:37 PM

Astronomy
Sci Advisor
PF Gold
P: 22,800

This is the present distance at which a stationary point would now be receding at speed c due to expansion. One cannot measure this directlyor 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) 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. 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. 



#41
Dec1107, 11:43 PM

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. 



#42
Dec1107, 11:53 PM

Sci Advisor
P: 1,253





#43
Dec1207, 12:02 AM

Astronomy
Sci Advisor
PF Gold
P: 22,800

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 Hit 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 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. 



#44
Dec1207, 12:32 PM

P: 640

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 asyet 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 asyet 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 [17571840] who wrote about it in 18231826 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 redshifted 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/co...y_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. ========= 



#45
Dec1307, 12:14 PM

P: 640

 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 



#46
Dec1307, 04:05 PM

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?




#47
Dec1307, 05:11 PM

Astronomy
Sci Advisor
PF Gold
P: 22,800

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 nothis article was not intended to suggest thathe wants it clearly understood, and he says it is important to realize that. Here's part of his recent email message to Patty Personally I doubt his recent paper is publishable as iswill 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 pollglad 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. 



#48
Dec1307, 05:44 PM

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. 



#49
Dec1407, 05:28 AM

P: 137

Would this help to explain it? Suspend beleif that that is not possible for a moment. 



#50
Dec1407, 01:22 PM

P: 531

Hi PRyckman,
The Schwarzschild radius solution technically is applicable only in empty (vacuum) space around a pointsource 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 selfexpanding 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 



#51
Dec1407, 01:51 PM

P: 640

All you need to ask yourself are two questions.
“Was all the mass of the universe ever contained is a radius less than R = 2GM/c2 ?” http://en.wikipedia.org/wiki/Schwarzschild_radius The Schwarzschild radius If the mass collapses to a radius less than R = 2GM/c2, where G is the gravitational constant and c is the speed of light, then nothing (including light) can escape from inside this radius. It is called the event horizon or the Schwarzschild radius. The Schwarzschild radius of an object is proportional to the mass.  Second question to ask yourself. ”Was the black hole bigger than 3 solar masses?” http://en.wikipedia.org/wiki/Primordial_black_hole One way to detect primordial black holes is by their Hawking radiation. All black holes are believed to emit Hawking radiation at a rate inversely proportional to their mass. Since this emission further decreases their mass, black holes with very small mass would experience runaway evaporation, creating a massive burst of radiation. A regular black hole (of about 3 solar masses) cannot lose all of its mass within the lifetime of the universe (they would take about 10^60 years to do so). However, since primordial black holes are not formed by stellar core collapse, they may be of any size. A black hole with a mass of about 1012 kg would have a lifetime about equal to the age of the universe. If such lowmass black holes were created in sufficient number in the Big Bang, we should be able to observe some of them exploding today.  Conclusion: (if the answer to the above two questions was YES), then we are still in a black hole. Or if you prefer, within the “cosmic horizon”. 



#52
Dec1507, 02:29 AM

Sci Advisor
PF Gold
P: 3,273

But as you have been told jal, you are still using an inappropriate expression,
[tex]R < \frac{2GM}{c^2}[/tex], 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 [tex]\rho > \frac{{3H_0}^2}{8\pi G}[/tex] or not?" i.e. "Is the universe closed  finite and unbounded  or not?" Garth 



#53
Dec1507, 09:53 AM

P: 640

Hi Garth!
Since you are advocating a different formula than that of a black hole then I will ask a clarification. Do you support the claim, that the picture of the Cosmic Background Radiation is a picture of the universe as it was 400,000 years after the big bang? Do you support, that when applying the energy density formula for a black hole, the result is that we are looking at the interior of a black hole? Do you agree, that the picture of the Cosmic Background Radiation contains all of the matter of the universe? You said, ..."the Schwarzschild solution, which only applies to a static spherical mass in otherwise empty space".... Should not be used By your statement, How can you justify the concept of black holes within our universe? Everyone is using the Schwarzschild solution in their papers and everyone "knows" that a black hole is not static and is not in empty space. You then said, ...."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." I beg to differ ... as will others.. and I do not want to divert into a discusion, which has been done too many times, "of what does the universe expand into." 



#54
Dec1507, 10:18 AM

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 OneBody or Schwarzschild Solution. The expression I quoted from you, [itex]R < \frac{2GM}{c^2}[/itex], 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, [itex]\rho > \frac{{3H_0}^2}{8\pi G}[/itex], 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 


Register to reply 
Related Discussions  
Black hole inside a larger black hole.  Cosmology  33  
RePoll: We are in a Schwarzschild black holeT or F?  Cosmology  4  
schwarzschild black hole/kruskal extension  Special & General Relativity  12 