Planck Stars: Carlo Rovelli & Francesca Vidotto

[marcus]

Nugso, I'll give you an idea of where I'm coming from. The root assumption here (WHICH COULD BE WRONG!) is that gravitational collapse can rebound instead of forming a "singularity" (which could simply be a mathematical error, something that happens where manmade theories fail, and does not really occur in nature.)

This idea of quantum effects coming into play at high density and making gravity repel, resising further compression, has gradually attracted a lot of research interest. "Quantum cosmology" is the research field where researchers study the very early universe and what might have started the expansion. At present ROUGHLY HALF of "quantum cosmology" research is now using a bounce model. For example Loop quantum cosmology has the bounce as a robust prediction. It comes out in all or most of the cases studied. And that alone accounts for about half of the QC research papers written.

These listings are not to read, just to get an idea of numbers of people and amount of research activity. They are ranked by citation count which gives a rough idea of a paper's importance/influence---how much it gets cited or reference in other research.

"quantum cosmology" since 2009, Inspire search:
http://inspirehep.net/search?ln=en&...search=Search&sf=&so=d&rm=citation&rg=25&sc=0 (652 found as of 20 Feb 2014)

"quantum cosmology" and not "loop" since 2009, Inspire search:
http://inspirehep.net/search?ln=en&...search=Search&sf=&so=d&rm=citation&rg=25&sc=0 (323 as of 20 Feb)

So you can see that about half are Loop, and the a lot of people are working with "rebound" models.

But that is so far not about black holes! That is about the early universe, from what conditions expansion got started. when they run those models back in time they find an earlier contracting phase, and a bounce.

What Rovelli and Vidotto and a few others are doing is trying to carry over that general idea to a model of black holes, and see if it works. To a large extent all I can do is wait and see. I'm interested, but I don't know what will happen with this research initiative. I want to understand better and be prepared if it gains credibility.

Ooops have to go! My wife has an errand for me to do. :^D

 

[MTd2]

This is interesting:

http://arxiv.org/pdf/1402.3055v1.pdf

Cosmic censorship as the source of hawking radiation.

 

[marcus]

Recalling something said earlier about black hole lifetimes, according to Planck star model, a hole with one fifth gigaton mass has a lifespan just under 14 billion years, i.e. comparable to the time since expansion began: the current cosmic age. So a fifth gigaton initial mass implies that the rebound is almost done and ready to break out of its time-dilation shell and end in a gamma ray burst finale.

This gives a convenient handle or benchmark case to remember. So I suggested this nursery rhyme as an aid to memory.

Hey Planck star,
you dark rebounder,
how long before you burst?
I'm almost done.

But your time
goes by so slowly...
What mass were you at first?
Fifth gigaton.

A black hole which formed with that mass in the early universe would in fact, according to the model, be "almost done" and ready to end in a burst of gamma rays. And we can calculate lifespans for other initial masses: The lifespan of a black hole, using this rebound model, is proportional to the cube of the initial mass. So for example if the initial mass were one gigaton then the lifespan would be five-cubed or 125 times the current 14 billion year expansion age.

For concreteness sake, a gigaton, one billion metric tons, is the mass of a one kilometer wide cube with the density of water. It would not be an unusual mass for a moderate-size asteroid

 

[MTd2]

That's a tiny energy, considering that the explosion is isotropic. We would just get a tiny area angle of all explosion. We'd do better looking for interference for much bigger explosions at CMB spetrum... The mass is smaller, but likely a log log mass distribution would make it far more common.

 

[marcus]

You raise an interesting question, MTd2! At what distance would a Planck star model explosion be detectable, assuming 1/5 gigaton initial mass?

According to the rebound model, the final mass is equals the initial mass divided by √2. So it is about 70% of the initial.
So in the case of the BH that takes 14 billion year to bounce, the final mass is 1/7 gigaton and when this is converted to energy how many ergs or joules is the explosion?

0.14e12 kg*c^2

This is 1.26 x 10²⁸ joules, or 1.26 x 10³⁵ ergs.

I seem to recall that the output of the Sun is about 10²⁶ watts

So if that explosion of 10²⁸ joules was released in 1 second it would be roughly 100 times more powerful than the Sun. But it would presumably be in gamma ray wavelengths. It would have to be observed by the telescopes like "Fermi-LAT" which look for gamma ray bursts (GRB).

I don't know how far away such a GRB could be and still be detected. Maybe you do MTd2, or somebody else who looks in here. I'll try to find out. I would guess that such a thing could be detected if it were in the Solar neighborhood, in our Milkyway galaxy of course, and in our general vicinity within the galaxy.

 

[MTd2]

100x the sun output is too small. Remember, that we'd get only a small solid angle of it. I don't think we'd capture it. That's the usual out put of a star not much bigger than the sun.

As I said, I think the distribution should follow the distribution of crater size or volcanic explosions. Something log-log. It should be better to look at the CMB. Or perhaps this was responsible for reionization.

 

[marcus]

That's a tiny energy, considering that the explosion is isotropic. We would just get a tiny area angle of all explosion...

For comparison, I looked up "supernova" and http://en.wikipedia.org/wiki/Supernova#Energy_output apparently the normal output of Type 1A is 1.5 x 10⁴⁴ joules. That is isotropic and we are talking about 10²⁸ joules.

So suppose we ignore the difference in wavelength and just compare energy. The factor is 10¹⁶.

So the Planck star explosion would have to be 10⁸ times closer in order for us to get the same energy as we do from a Type 1A supernova.

Of course the visible energy production of a supernova is at least hundred-fold more spread out in time. The GRB lasts on the order of one second, a supernova lasts several days. So that would give the Planck star GRB at least a hundred-fold advantage. So as a conservative estimate, let's say that to be detectable the Planck star explosion has to be a MILLION TIMES closer than a detectable SN-1A.

So as a rough estimate, if you can see a SN-1A at a distance of a BILLION light years, then you can see a Planck star GRB at a distance of a THOUSAND light years.

That is certainly very rough. Maybe I can come up with a better estimate later. But it gives some idea.

I'm glad you raised the issue! I'll keep working on it.

 

[marcus]

About the energetics and beaming of GRB's
==quote http://en.wikipedia.org/wiki/Gamma-ray_burst#Energetics_and_beaming ==
Observations suggest significant variation in the jet angle from between 2 and 20 degrees.[68]
Because their energy is strongly focused, the gamma rays emitted by most bursts are expected to miss the Earth and never be detected. When a gamma-ray burst is pointed towards Earth, the focusing of its energy along a relatively narrow beam causes the burst to appear much brighter than it would have been were its energy emitted spherically. When this effect is taken into account, typical gamma-ray bursts are observed to have a true energy release of about 10⁴⁴ J, or about 1/2000 of a Solar mass energy equivalent[68]—which is still many times the mass energy equivalent of the Earth (about 5.5x1041 J). This is comparable to the energy released in a bright type Ib/c supernova and within the range of theoretical models. Very bright supernovae have been observed to accompany several of the nearest GRBs.[27] Additional support for focusing of the output of GRBs has come from observations of strong asymmetries in the spectra of nearby type Ic supernova[69] and from radio observations taken long after bursts when their jets are no longer relativistic.[70]
Short (time duration) GRBs appear to come from a lower-redshift (i.e. less distant) population and are less luminous than long GRBs.[71] The degree of beaming in short bursts has not been accurately measured, but as a population they are likely less collimated than long GRBs[72] or possibly not collimated at all in some cases.[73]
…
…
Numerous other models have also been proposed to explain short gamma-ray bursts, including the merger of a neutron star and a black hole, the accretion-induced collapse of a neutron star, or the evaporation of primordial black holes.[80][81][82][83]
==endquote==

We have to remember that Planck star GRB are MUCH more powerful than conventional evaporation of primordial BH because the conventional model only allows a gamma flash when the mass is almost gone. E.g. 200 ton remnant. Planck star explosion happens when there is still something like 200 Million tons.
Planck star model predicts explosions which are MILLION-FOLD more powerful than conventional primordial BH end-of-life explosion.

We have also to remember that SHORT GRB are a different phenomenon from the longer GRB. Short and very short GRB are recognized as a separate category and separate explanations are offered as to what the mechanism could be.

 

[MTd2]

But Marcus, the point I am trying to raise it is not only the power, but the frequency. How this would affect the CMB. Think about crater distribution size. And think about these explosions should be tremendously more common in the beginning of the universe.

 

[marcus]

But Marcus, the point I am trying to raise it is not only the power, but the frequency. How this would affect the CMB. Think about crater distribution size. And think about these explosions should be tremendously more common in the beginning of the universe.

MTd2, you can try to produce some numbers to show that primordial BH final explosions should have an effect on the CMB. That would be counterintuitive for me because simple intuitive reasoning suggests (to me) that they would not. But then at least we would have some concrete numbers that we might discuss.

The point I think I've explained is that if there are primordial BH which are bursting NOW within a radius say on the order of 1000 light years, then using the Rovelli Vidotto Planck star model they would be visible as a type of GRB called "short GRB" or by some people (Cline et al) "very short GRB."
So there is at least that observational possibility. It clearly cannot be dismissed. A certain known type of Gamma Ray Burst can be studied to see if some of them are in line with Planck star model explosions.

You are arguing that there is ALSO a possibility to study past Planck star explosions in the MICROWAVE BACKGROUND THAT DATES FROM AROUND YEAR 370,000.

The reason we don't see EM radiation from earlier than that is that the gas that filled space was effectively OPAQUE, including to visible light and also to gamma radiation.

But the REDSHIFT from year 370,000 up to present is only about z=1000, or z=1090 more exactly. So a GRB explosion AFTER year 370,000, which produces, like R&V say, GeV photons would, after redshift, be sending us MeV photons . That is not microwave. It would be part of the X-ray astronomy background. Not CMB. A different kettle of fish altogether.

Again, Rovelli Vidotto suggest a representative wavelength for the Planck star gamma burst could be 10^-14 cm. After a redshift of z=1000 that is still not millimeter microwave!
It is still very short: 10^-11 centimeter.

For a primordial BH to last until after 370,000 before it blows up, it must (according to Planck star model) start with an initial mass of over 5 million tons. This is not all that different from the 200 million tons a BH needs in order to last up until the present day---what I was calling "a fifth gigaton" (200 million is a fifth of a billion). So as a first approximation I'm using R&V estimates of wavelength and photon energy

 

[Berlin]

Plenty of more room at the bottom?

The paper on Planck Stars clearly states that the onset of quantum-gravitational effects is governed by energy density -not by size-.

Let's take the above statement into the low mass regime. For example the electron rest mass would have an energy density comparable to the Planck density only at a size ~10^-42 m. This is about 10^-7 of the Planck length.

If we assume that particle mass can only be described properly in a quantum-gravitational setting, would this imply that, there will be "plenty of more room at the bottom", paraphrasing Feynman? With this I mean, could the scale relevant for physics be extended from the usual Planck scale of 10^-35 m to the much lower scale of 10^-42 m ?

Of course I know that an electron is not a black hole, but does it really make any difference? It is a matter where classical gravity meets the quantum world.

berlin

 

[marcus]

...If we assume that particle mass can only be described properly in a quantum-gravitational setting, would this imply that, there will be "much more room at the bottom"?

berlin

Hi Berlin! I was interested by your comment! I don't feel I understand your idea of "much more room at the bottom". Could you use a few more words and make it more obvious?

 

[Berlin]

I edited my response, see above.

berlin

 

[marcus]

A propos of density and length scales, you may be familiar with John Baez physics FAQ explanations which are often particularly nicely written and clear. Here's his website's entry on Compton wavelength
==excerpt http://math.ucr.edu/home/baez/lengths.html#compton_wavelength ==
2 - The Compton wavelength of the electron

The Compton wavelength of a particle, roughly speaking, is the length scale at which relativistic quantum field theory becomes crucial for its accurate description. A simple way to think of it is this. Trying to localize an electron to within less than its Compton wavelength makes its momentum so uncertain that it can have an energy large enough to make an extra electron-positron pair! This is the length scale at which quantum field theory, which describes particle creation, becomes REALLY important for describing electrons. The Compton wavelength of the electron is the characteristic length scale of QED (quantum electrodynamics).

It's easy to guess how big the Compton wavelength is using the knowledge that it depends only on the mass of the electron, relativity and quantum mechanics. Mass has dimension M. Length has dimension L. Time has dimension T. In relativity we have a constant, the speed of light, with dimensions L/T, and in quantum mechanics we have a constant, Planck's constant, with dimensions ML2/T = energy times time = momentum times position. These two constants enable us to express units of mass in terms of dimensions of inverse length. I.e.:

M = (ML²/T)(T/L)1/L = ℏ/c 1/L.

So in particular the Compton wavelength should be about

L_Compton = ℏ/mc.

This is about 4 × 10^-13 meters.

In fact, this is usually called the "reduced" Compton wavelength. What people usually call the Compton wavelength is 2π times as big, about 2 × 10^-12 meters. That's because the wavelength of a wave is really not the reciprocal of its frequency: it's 2π divided by the frequency. But I'm not worrying much about factors of 2π...
==endquote==

But it would be good to go back and see how we got onto the topic of DENSITY in the first place!

Look at Rovelli and Vidotto equation (1). It is a quantum corrected version of CLASSICAL Friedman equation. Basically it tells you approximately how classical behavior is modified at high MACROSCOPIC density. It is not describing things at a microscopic particle level.

So it does not apply to an individual particle, like an electron. And the density of an electron is not well-defined as far as I know.

 

[Berlin]

I don't think it matters much that it is macroscopic. To reach the Planck density with electrons for example, as defined in the paper, you need on average (Planck mass)/(electron mass) ~10^22 electrons squeezed into the space of (Planck length)^3. For every known particle mass this number is well above one. This looks strange to me. Maybe this would even mean that there is more than one bit stored in this volume, I don't know. Just naïve thinking perhaps. Maybe just GR where you cannot play with volumes like I do here..

berlin

 

[marcus]

... To reach the Planck density with electrons for example, as defined in the paper, you need on average (Planck mass)/(electron mass) ~10^22 electrons squeezed into the space of (Planck length)^3...

I don't think the paper talks about "reaching Planck density with electrons". I had always assumed that matter as we know it would not exist at such extreme energy density. So there would be no question of so-and-so many "electrons" contained in a given volume.

But I think I see what you are driving at. You know that the concept of "particle" becomes poorly defined in CURVED spacetime. The concept is more at home in flat. To me that suggests that in highly curved geometry it becomes increasingly difficult to distinguish between the matter and the geometry itself. Does that seem plausible to you? It is obviously just a guess!

Or perhaps there is a kind of supercondensate state of matter in which all particles are in the same quantum state, indeed all TYPES might become the same type of particle. Assuming something that we want to call particle exists at extreme (planckian) density.

I just wanted to indicate some possibilities, but I recently saw some research that is closer to your idea. It described a bounce in which fermions persist through the bounce. This may be more to your taste! So I will get the links. It's quite recent stuff.

 

[marcus]

Berlin, here are the two recent papers I was thinking might relate better to your picture of the bounce density:
http://arxiv.org/abs/1402.5719
Singularity avoidance in classical gravity from four-fermion interaction
Cosimo Bambi, Daniele Malafarina, Antonino Marciano, Leonardo Modesto
(Submitted on 24 Feb 2014)
We derive the dynamics of the gravitational collapse of a homogeneous and spherically symmetric cloud in a classical set-up endowed with a topological sector of gravity and a non-minimal coupling to fermions. The effective theory consists of the Einstein-Hilbert action plus Dirac fermions interacting through a four-fermion vertex. At the classical level, we obtain the same picture that has been recently studied by some of us within a wide range of effective theories inspired by a super-renormalizable and asymptotically free theory of gravity. The classical singularity is replaced by a bounce, beyond which the cloud re-expands indefinitely. We thus show that, even at a classical level, if we allow for a non-minimal coupling of gravity to fermions, black holes may never form for a suitable choice of some parameters of the theory.
5 pages

http://arxiv.org/abs/1402.5880
Fermi-bounce Cosmology and scale invariant power-spectrum
Stephon Alexander, Cosimo Bambi, Antonino Marciano, Leonardo Modesto
(Submitted on 24 Feb 2014)
We develop a novel non-singular bouncing cosmology, due to the non-trivial coupling of general relativity to fermionic fields. The resolution of the singularity arises from the negative energy density provided by fermions. Our theory is ghost-free because the fermionic operator that generates the bounce is equivalent to torsion, which has no kinetic terms. The physical system is minimal in that it consists of standard general relativity plus a topological sector for gravity, a U(1) gauge field reducing to radiation at late times and fermionic matter described by Dirac fields with a non-minimal coupling. We show that a scale invariant power-spectrum generated in the contracting phase can be recovered for a suitable choice of the fermion number density and the bare mass, hence providing a possible alternative to the inflationary scenario.
Comments: 6 pages

There was some earlier work by Ed Wilson-Ewing where matter was included in a LQG bounce and he found that the bounce occurred at much lower density. So as the above CLASSICAL work suggests might be the case, when matter is included in a Loop gravity bounce it might indeed turn out that the bounce occurs not at some percentage like 40% of Planck density but several orders of magnitude sooner, as Ed W-E found. I don't think I am able to evaluate this work.

 

[Berlin]

I don't think the paper talks about "reaching Planck density with electrons". I had always assumed that matter as we know it would not exist at such extreme energy density. So there would be no question of so-and-so many "electrons" contained in a given volume. QUOTE]

Aha! I guess we reach the real point of discussion. If you don't know the state of matter at those density's, you don't really have a physical theory describing it! So how can Rovelli et al. conclude that the bounce takes place at that specific density? They refer to a paper of Ashtekar et al. which I do not understand. I have to check if the prediction about the radiation is based on the above assumption, but if so, it is may be poorly justified.

Thanks for the references. Only time for a quick look, but both do not seem to use any LQG scale physics. What would Rovelli has to say?

berlin

 

[marcus]

Hi Berlin,
I think it would help this thread to bring in some sense of the history of showing the Loop QC bounce happens (in combination with various matter fields). Obviously it has to be checked by computer runs in a wide variety of cases to get an idea of how robust the conclusion is that there is a bounce.
The papers on this go back to 2001, and especially back to 2006 when Ashtekar, Pawlowski, Singh introduced an improved version of LQC dynamics. For perspective, here's a paper that was posted yesterday, with Singh one of the authors, but warning: don't try to read it, too specialized and technical!. However in the first few paragraphs it reviews the history and gives references [1 - 4] to some of the circa 2006 papers:
http://inspirehep.net/record/1282592
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce and the validity of effective dynamics
Peter Diener, Brajesh Gupt, Parampreet Singh
Feb 26, 2014 - 46 pages - 26 figures

A key result of isotropic loop quantum cosmology is the existence of a quantum bounce which occurs when the energy density of the matter field approaches a universal maximum close to the Planck density. Though the bounce has been exhibited in various matter models, due to severe computational challenges some important questions have so far remained unaddressed. These include the demonstration of the bounce for widely spread states, its detailed properties for the states when matter field probes regions close to the Planck volume and the reliability of the continuum effective spacetime description in general. In this manuscript we rigorously answer these questions using the Chimera numerical scheme for the isotropic spatially flat model sourced with a massless scalar field. We show that as expected from an exactly solvable model, the quantum bounce is a generic feature of states even with a very wide spread, and for those which bounce much closer to the Planck volume. We perform a detailed analysis of the departures from the effective description and find some expected, and some surprising results. At a coarse level of description, the effective dynamics can be regarded as a good approximation to the underlying quantum dynamics unless the states correspond to small scalar field momenta, in which case they bounce closer to the Planck volume, or are very widely spread. Quantifying the amount of discrepancy between the quantum and the effective dynamics, we find that the departure between them depends in a subtle and non-monotonic way on the field momentum and different fluctuations. Interestingly, the departures are generically found to be such that the effective dynamics overestimates the spacetime curvature, and underestimates the volume at the bounce.​

Here you see they are checking in some specific cases. The bounce has been checked also in NON-isotropic and in spatially NON-flat cases, with different kinds of matter, both with and without inflation (which involves introducing an additional field). There seems to be no way all at once to verify that it happens in all possible cases. One would have to somehow vary all the parameters in every possible way in one grand number-crunching simulation.

You asked about Rovelli. AFAIK he has worked primarily on the full LQG and Spinfoam theory, rather than the application to cosmology. In the Planck star paper, Rovelli and Vidotto simply borrow the bounce, which many years' work by Loop cosmology people have made plausible, and apply it in the context of black holes.

To paraphrase in effect, I think they say something like this: look this has been tested both numerically and in the solvable equation version for many years in case after case, with quantum states of geometry that are peaked and spread-out, with closed, flat, and open spatial geometry, with various stand-ins for matter, and it looks increasingly robust, so let's SUPPOSE that the Loop cosmology people (Ashtekar, Agullo, Nelson, Singh, Wilson-Ewing, Vidotto, Pawlowski, etc) are right and that when you quantize cosmology Loop-style and run it back to the start of expansion you see a BOUNCE. So let's suppose that and take it over and apply it to black holes!

 

[marcus]

Oh, you were also asking about the two CLASSICAL papers I mentioned in post #47 (by Alexander, Bambi, Marciano, Modesto…). What should one conclude? They even get a bounce in some classical setting. Yes I think that is very encouraging. In a quantum theory when you get some result, it's highly supportive if some other people find that a similar result can arise in a non-quantum version of the problem.

Regarding black holes, one of the papers says: "...The classical singularity is replaced by a bounce, beyond which the cloud re-expands indefinitely. We thus show that, even at a classical level, if we allow for a non-minimal coupling of gravity to fermions, black holes may never form for a suitable choice of some parameters of the theory…"

... What would Rovelli have to say?

Two of the authors of that paper have co-authored/postdoc't at Marseille. I imagine CR would be pleased by the supportive classical results but to answer your question I really don't know what he would say.
Here are a dozen or so LQG papers solo or coauthored by Leonardo Modesto, most about the Lqg black hole:
http://arxiv.org/find/gr-qc/1/AND+au:+modesto_L+ti:+loop/0/1/0/all/0/1

And some Marciano solo or co-author papers:
1105.3480 Towards a Loop Quantum Gravity and Yang-Mills Unification (with Alexander on that one)
1011.5676 Coherent states for FLRW space-times in loop quantum gravity (PRD)
1010.1258 Big Bounce in Dipole Cosmology (PRD)
1003.0352 Towards inhomogeneous loop quantum cosmology: triangulating Bianchi IX with perturbations (MG12 proc.)
0911.2653 Triangulated Loop Quantum Cosmology: Bianchi IX and inhomogenous perturbations (PRD, with Rovelli)

Of course I don't know, but I'd think he would have to be pretty happy, especially about the one about black holes with fermion matter being non-singular and developing a bounce, the one titled:
http://arxiv.org/abs/1402.5719
Singularity avoidance in classical gravity from four-fermion interaction

 

[MTd2]

MTd2, you can try to produce some numbers to show that primordial BH final explosions should have an effect on the CMB. That would be counterintuitive for me because simple intuitive reasoning suggests (to me) that they would not.

But formation of these smaller black holes, either 200 tons, 1 ton, 20kton, 50Mton, are all counter intuitive since it conflicts with the isotropy and jeans instability. That is, the universe was too homogenous for stars to form. For such small masses, an incredible density and inhomogeneity in very small places would be required.

The only thing I can imagine it is that inflation was turbulent. That is, at smaller scales, it presented some kind instability. This instability would super compress matter.

Considering that energy dissipation is null, the komologorov velocity microscale (the compression element) goes with 3/4 of the temperature . The density of energy for formation of a black hole goes with 1/T, that -1 of the temperature. So, black hole formation falls with -1/4 of the temperature. But, given that we are talking about an exponential decrease of temperature, the great majority of black holes will be of small size. Even the total mass of black holes formed.

So, if something is seen exploding today, maybe a signature on CMB is not unlikely.

 

[marcus]

But formation of these smaller black holes, either 200 tons, 1 ton, 20kton, 50Mton, are all counter intuitive since it conflicts with the isotropy and jeans instability. That is, the universe was too homogenous for stars to form. For such small masses, an incredible density and inhomogeneity in very small places would be required.
...

Maybe you don't understand, MTd2. The formation of primordial BH has nothing to do with STARS.

I am talking about primordial BH with masses like 200 million tons. If primordial BH were formed with masses significantly smaller than that they would most likely already have evaporated (at least according to the Rovelli Vidotto calculation.)

I do not find it "counter-intuitive" that primordial BH would have formed, at a time when the universe was very dense, due to random density disturbances producing small regions of over-density.

 

[MTd2]

I do not find it "counter-intuitive" that primordial BH would have formed, at a time when the universe was very dense, due to random density disturbances producing small regions of over-density.

So, you won't have problem to see that smaller masses will be vastly more abundant (even the total mass should be bigger for sets with smaller massses) than those with bigger masses.

 

[marcus]

Hi MT, check out my earlier post #40, which includes this:

...For a primordial BH to last until after 370,000 before it blows up, it must (according to Planck star model) start with an initial mass of over 5 million tons...

Remember we are using the Planck star model.

If a PBH initial mass is less than 5 megaton then it dies before year 370,000 while universe is still opaque and we never see its light.

Just as we never see ordinary CMB light emitted before 370,000, or any kind of light from before that time.

If a PBH initial mass is greater than 5 megaton then it releases a GRB after year 370,000 and the radiation is several GeV gamma! Redshift between then and now is no more than a factor of 1000. So the radiation that gets to us is between several MeV and GeV. This is not microwave.

Therefore no significant contribution to CMB, which is microwave background.

..., maybe a signature on CMB is not unlikely.

Instead, the contribution from past PBH explosions, as I've been explaining, is to the X-ray and gamma-ray background. That is pretty noisy, as we can see from the recent papers looking for emission lines that could represent Dark Matter decay. There are people assiduously studying the X-ray sky and it is a very good thing to study! And of course there is a lot of interest in the gamma-ray sky especially in GRB.

But this is very different from the studying the Cosmic Microwave Background. We should not even be talking about the CMB, but you seem to be coming back to it rather often!

 

[Berlin]

Thank you Marcus for digging up the bounce literature and surely nice that so recently as yesterday a new paper came out. What troubles me about the idea of Planck density is the amount of information stored in the Planck volume. I always had the naive idea that a Planck Volume could only contain one bit of information. I would like to see a bouncing model based on information or entropy arguments in stead of a specific approximation model like the papers you showed ('massless scalars' for example). Also there seems to be large diffeences in bouncing scale depending on the non-gravity particles and forces considered.

Berlin

 

[MTd2]

Marcus, you are forgetting the contributions from the end of inflation up to 370ky

 

[marcus]

No, MTd2, I am not forgetting those. Universe opaque to light of any kind including x-ray and gamma, before 370,000

If a PBH initial mass is less than 5 megaton then it dies before year 370,000 while universe is still opaque and we never see its light.

Just as we never see ordinary CMB light emitted before 370,000, or any kind of light from before that time.

I always had the naive idea that a Planck Volume could only contain one bit of information.

I sympathize but it is not quite that simple is it? Often people associate the bits with the AREA, the number of Planck area units, rather than the number of Planck volumes.

And that would go as the SQUARE of the mass that has fallen into the hole

I would like to see a bouncing model based on information or entropy arguments...

Berlin, for that you have to read the Rovelli Vidotto paper itself. Their argument is new and based on information/entropy. They make a big point that the even at the smallest point of the bounce the "star" is still large enough to contain all the information which has fallen in and which it must deliver at the end, when it explodes back into the rest of the world.

Are they right? I don't know. I can just wait and see if this new idea and new argument (so far just sketched, I would say) is born out by subsequent longer papers, e.g. one I believe is in prep by Rovelli with Matteo Smerlak.

 

[Chronos]

If I understand correctly, information density is like energy density - unbounded. It wouldn't make much sense to allow a mass singularity without an information singularity.

 

[MTd2]

No, MTd2, I am not forgetting those. Universe opaque to light of any kind including x-ray and gamma, before 370,000

But it would let its mark by making sounds, that is, disturbing the energy distribution on CMB,

 

[marcus]

But it would let its mark by making sounds, that is, disturbing the energy distribution on CMB,

Are you sure? Then would you like to show the numbers? The surface we see was at a distance of 42 million LY at that time (year 370,000). We see resolution on the order of one degree.
Roughly speaking one degree spread on that surface corresponds to about 1 million LY.

I wonder if the "sounds" you are talking about are short wavelength turbulence that will dissipate as noise, or whether any of them actually could have an imprint on the grand scale of the real CMB structure that is studied.

Have you done the numbers?