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unclebob
2010-Feb-07, 02:29 PM
Idle speculation. What if there were clouds of micron sized black holes? If I've done my math right (doubtful) then:


Each would have a mass of ~10^12kg (about a km^3 of water)
There might be 1 in 10^27m^3 or a mean separation of 10^6km
Evaporation rate ~= 200 billion years.


Cosmic rays, CMB, and just plain starlight will add miniscule mass to these little holes. I wonder how close they are to equilibrium. What size black hole evaporates at the same rate as it grows due to incident radiation and the occastional H atom?

Would little holes like this pick up charge from cosmic ray protons? Or are there enough free electrons to counteract that? If not, would the electrostatic repulsion help to balance the self gravitation?

Collisions between holes or other stray matter would be very rare so I presume such clouds would not be very active emitters. There might be a low intensity xray glow from the occasional H atom getting swallowed.

I have to guess that a cloud like this would appear non-baryonic.

Cougar
2010-Feb-07, 05:58 PM
Welcome to the board, unclebob.


What if there were clouds of micron sized black holes?

How big a cloud? I doubt it would be gravitationally stable.



Evaporation rate ~= 200 billion years.


You might check your math on that. I thought small black holes evaporate pretty quickly...


I have to guess that a cloud like this would appear non-baryonic.

I'd think that a large such cloud would block or scatter the light from objects behind it. It would look like a cloud, which is baryonic.

unclebob
2010-Feb-07, 11:21 PM
I checked the math. I was off by an order of magnitude. (pesky decmial points!) The actual evaporation rate is two trillion years.

If you assume the density of one H per cc then there would be one hole in 10^27 cubic meters. The average separation would be a million km. Given that each hole is .000001m in diameter, I don't think such a cloud would be opaque.

Let's bump the density by 1000X, which is denser than many interstellar clouds, then the average separation is still ~100,000 km. Molecular clouds of that density are gravitationally stable. Why not a cloud of little black holes?

Would a cloud of holes like this be self-gravitating?

eburacum45
2010-Feb-08, 08:26 AM
They would resemble globular clusters dynamically rather than dust clouds, I think. The stars in a globular cluster are closely packed and gravitate towards each other, but they rarely collide. The same would happen in a cloud of tiny BHs. However if such small holes ever did collide they would release a flash of energy; dark matter made of tiny black holes might be detectable by looking for these flashes.

sabianq
2010-Feb-08, 04:27 PM
it seems to me that if little tiny black holes followed quantum dynamics, they would not last longer than a femtosecond, rather they would pop in and out of existence like virtual particles...

Cougar
2010-Feb-10, 03:16 PM
I checked the math. I was off by an order of magnitude. (pesky decmial points!) The actual evaporation rate is two trillion years.



...when the mass got down to about 1014 g the number of different species of particles being emitted might be so great that the black hole radiated away all its remaining rest mass on a strong interaction time scale of the order of 10-23 s.

Particle Creation by Black Holes (http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.cmp/1103899181) - S. W. Hawking

neilzero
2010-Feb-11, 04:06 AM
According to the now closed thread on Hawking's radiation, the entire concept is subject to reasonable doubt, so two trillion years to evaporate is as good as any other number for trillion KG black holes, if there are any that size. I think the assumptions of Unclebob are as reasonable as any others, and I agree the cloud would be invisible, collisions extremely rare, and the energy output too small to detect unless such a cloud was within a few million kilometers of Earth. If there are 1000 trillion black holes in a cube with about 100,000 black holes along each edge then the total mass of the cloud is 1000 trillion times a trillion KG = 10^27 KG = about the mass of Earth?, but spread throughout much of the solar system including the Oort cloud. Because of very distributed mass the orbits of the planets would be modified only slightly? Would one of the black holes a few meters away (near sealevel) look like ball lightning? It would have an accretion disk (accretion sphere?) in Earth's atmosphere, but perhaps rarely capture an atom due to it's extremely small event horizon? Neil

Grey
2010-Feb-11, 04:57 PM
A black hole of that size would be putting out about 350 megawatts of power from Hawking radiation. If they are spread more or less evenly every million km, there ought to be some within that distance from Earth. At a million km, something putting out 350 MW has a magnitude of about 7.5. Not quite naked eye visible, but we should easily see these little guys roaming around with binoculars and backyard telescopes. To fit the models of dark matter, it needs to be fairly "cold", so it shouldn't be moving very fast, but they should still have a large enough proper motion that we'd notice them as something unusual, especially since the radiation will be a rather distinctive blackbody spectrum at a temperature of something like 12 million Kelvins.

If you make them bigger and rarer, the energy output goes down while the typical distance goes up, so they might be less likely to be noticed; I'm not sure where the threshold is where we wouldn't spot them easily in professional observatories. Hubble has a limit of about 30th magnitude. If you make the black holes about 10,000 times bigger, the mean distance is then about 20 times farther and the power output drops to about 3.5 watts, and that drops them below Hubble's range of detection. Of course, now we're talking about black holes with the mass of Phobos; I'm not sure whether we'd notice them in other ways or not.

To be in equilibrium with, say, the cosmic microwave background, so that they're neither growing nor shrinking, you'd want them to have roughly the same temperature, 2.7 K. For that, you need something on the order of 1023 kg. That's something like the mass of the moon; I'm sure we'd notice black holes that big if they were floating around the solar system by the gravitational anomalies. But even the smaller ones have long enough lives that I don't think we'd need to worry about whether they're stable enough.

kzb
2010-Feb-13, 06:04 PM
Actually dark matter is baryonic, and it consists of masses of free planets. They are in clusters which means the MACHO microlensing search software rejects their microlensing signatures. They CAN be detected by microlensing of quasars.

The need for dark energy evaporates, because observations can be explained by the greater optical depth caused by the fog of small bodies and the gas that evaporates from them.

According to R Schild anyway. I've only just started reading his publications but I am hooked already.

cjl
2010-Feb-14, 12:47 AM
Actually dark matter is baryonic, and it consists of masses of free planets. They are in clusters which means the MACHO microlensing search software rejects their microlensing signatures. They CAN be detected by microlensing of quasars.

The need for dark energy evaporates, because observations can be explained by the greater optical depth caused by the fog of small bodies and the gas that evaporates from them.

According to R Schild anyway. I've only just started reading his publications but I am hooked already.
The problem with that theory is that Ωbaryon is constrained to 0.04 (+/- 0.01) by observations of the ratio of deuterium to hydrogen in intergalactic gas clouds. Specifically, the deuterium to hydrogen ratio at the end of big bang nucleosynthesis is strongly dependent on the baryon to photon ratio, and therefore, if the deuterium to hydrogen ratio can be measured, the baryon to photon ratio can be extrapolated with reasonable accuracy. As most photons in existence are CMB photons, the number of photons is not difficult to find, and the number of baryons can be determined from this. Ωmatter however is observed to be at least 0.2 (actual value is estimated at roughly 0.3, with Ωnon-baryonic matter at 0.26). This requires that most of the dark matter is non-baryonic.

kzb
2010-Feb-15, 12:55 PM
cjl wrote:
the number of photons is not difficult to find

The trouble with arguments like yours is that there is the danger there could be circular reasoning. I'm not saying there IS, but there is the possibility, because I just bet some of the numbers are manipulated in some way which is "model dependent".

For example, just how is the number of photons found ? What correction factors are applied and are they dependent on the current model of the universe?

Cougar
2010-Feb-15, 02:48 PM
Actually dark matter is baryonic...

Actually, as cjl pointed out, yours is a very non-mainstream position. Healthy skepticism is usually a good thing. In this case, it appears to be misplaced.


The rate at which primordial nucleosynthesis proceeded didn't depend only on temperature. It also depended on the density of baryons. The fewer the nucleons, the less likely the reactions that built up deuterium, helium, and lithium.

If there were one nucleon for every 100 million photons at the time of primordial nucleosynthesis, then there would only be 0.00008 parts per million of deuterium in baryonic material today. If the photon to nucleon ratio is a billion to one, there would be 16 deuterium nuclei per million. And if the ratio were 10,000 million to one, there would be 600 deuterium nuclei per million.

Observations of the oldest stars show a deuterium abundance of between 16 and 20 per million nuclei, corresponding to a photon to baryon ratio of just over a billion to one.

In the late 1960s and 70s, the evidence from the background radiation and the observed abundances of the lightest elements in the oldest stars implied the density of baryons in the Universe today lay between 0.01 and 0.1 (between 1% and 10% of the critical density). By 2005, improved observations show, as cjl mentioned, that about 4% of the critical density is in the form of baryons. "There is no escape from this conclusion."

Paraphrased from The Origins of the Future [2006] - Gribbin

cjl
2010-Feb-15, 09:42 PM
cjl wrote:
the number of photons is not difficult to find

The trouble with arguments like yours is that there is the danger there could be circular reasoning. I'm not saying there IS, but there is the possibility, because I just bet some of the numbers are manipulated in some way which is "model dependent".

For example, just how is the number of photons found ? What correction factors are applied and are they dependent on the current model of the universe?
The vast majority of photons in existence come from the CMB, and are fairly evenly distributed. By knowing the luminosity and spectrum of the CMB, you know (to a fairly close approximation) the number of photons in the universe.

kzb
2010-Feb-17, 06:35 PM
Actually Cougar, I'm very much on the fence. I was putting forward the idea from some papers that I am reading now. The papers seem to explain pretty much everything in the universe without non-baryonic matter of the sort presently mainstream. Also without funny adaptations of gravitational theory.

One point they don't address so far is this H/D ratio thingy, and perhaps that is their fatal weakness. We shall have to see. I've not read everything yet.