What now about Dark Matter?

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A new experiment (it's true a small scale one with the possibility of being brought to a much larger scale) has failed to find any sign of Dark Matter (DM), like all the other experiments before (at least the ground based one). AMS seems to have found some important extra anti-protons and positrons, that might be consistent with DM annihilation, but even so, it could be alternative explanations (and this wouldn't count as a direct detection, anyway). I was wondering what the Physics Forums community thinks about this lack of evidence for DM despite numerous and different approaches to find it (DAMA/LIBRA experiment claims to have found it, but no one was able to reproduce their results). Do we need a fundamentally new detection method (after all, even colliders won't be able to go too much higher in energy, for sure nowhere close to Plank scale anytime soon, so we do need a new approach for high energies)? Do we need some totally new theoretical ideas to describe Dark Matter, in a complete opposite direction from the current models? Do we need to give up on DM completely and find a new explanation for the observed gravitational behavior of the galaxies (MOND?)? What do you think?
 

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  • #2
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MOND is not a viable alternative, regardless of any perceived problems with DM.

I wouldn’t rush to get rid of DM. It has been observed gravitationally and looks like it doesn’t interact much otherwise. So it will be inherently difficult to detect.

Science isn’t room service where you can order a result to your liking to be delivered by the end of the sitcom you are watching. It is a difficult enterprise and honest science always has a real risk of null results.
 
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  • #3
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MOND is not a viable alternative, regardless of any perceived problems with DM.

I wouldn’t rush to get rid of DM. It has been observed gravitationally and looks like it doesn’t interact much otherwise. So it will be inherently difficult to detect.

Science isn’t room service where you can order a result to your liking to be delivered by the end of the sitcom you are watching. It is a difficult enterprise and honest science always has a real risk of null results.
I totally agree, but if so many experiments failed to detect it, why don't we try a new approach? We know that we have something that interacts gravitationally, but why would interact otherwise with the visible matter? Maybe this is the only way, so we would need to build a detector able to detect that (which goes back to my point that we are nowhere near close to do that with the type of detectors we have so far, so maybe we need a totally new type of detector). Honestly, I feel the same goes for SUSY. It is a beautiful mathematical framework, but if so many experiments failed to detect any SUSY particle (and basically the hierarchy problem for which it was mostly invented it's hard to be solved anymore with the current boundary on SUSY), why do people keep working so hard on it (both theoretically and experimentally) instead of trying something new? And I am aware that there are many other models, but what I mean why is the scientific community so focused on something specific, when we can try many other things?
 
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I totally agree, but if so many experiments failed to detect it, why don't we try a new approach? We know that we have something that interacts gravitationally, but why would interact otherwise with the visible matter? Maybe this is the only way, so we would need to build a detector able to detect that (which goes back to my point that we are nowhere near close to do that with the type of detectors we have so far, so maybe we need a totally new type of detector). Honestly, I feel the same goes for SUSY. It is a beautiful mathematical framework, but if so many experiments failed to detect any SUSY particle (and basically the hierarchy problem for which it was mostly invented it's hard to be solved anymore with the current boundary on SUSY), why do people keep working so hard on it (both theoretically and experimentally) instead of trying something new? And I am aware that there are many other models, but what I mean why is the scientific community so focused on something specific, when we can try many other things?
Like what kind of other things ?
 
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Like what kind of other things ?
Oh i don't know, that's my point. People are focusing so much on SUSY, that other approaches barely exist or if they exist they are not given enough importance such that experiments can get funding to test them. My point is, how long do you stick to a theory (which gave absolutely no experimental evidence of being right) before you try testing other theories or start coming up with new theories? I am not an expert so I don't really know if and what other models exist out there, I am just wondering when one decides that a model doesn't describe the actual reality and tries building a new model? For example string theory makes predictions mostly at energy scale we can't test yet. So there is no solid reason to claim that String theory doesn't describe nature. But SUSY should have given experimental evidence, but it didn't. So probably it's not the right thing.
 
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People are focusing so much on SUSY, that other approaches barely exist or if they exist they are not given enough importance such that experiments can get funding to test them.
This is simply untrue. While SUSY may get a lot of attention and SUSY searches are relatively well funded, this does not mean that no other approaches exist. The most prominent type of such searches probably being axion searches, but there are others too.

Also, generic WIMP models tend to be the models that are relatively easy to test.

So there is no solid reason to claim that String theory doesn't describe nature.
More importantly, there is also no reason to claim that it does.

But SUSY should have given experimental evidence, but it didn't.
This is also untrue. What has been tested are very particular SUSY models that were constructed to be easily detectable. Of course you could question the practice of engineering models that are easily testable, but that is another question.

What has been tested (to some extent) is low scale SUSY. In superstring theory models, the natural SUSY scale is the string scale ...
 
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  • #8
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I totally agree, but if so many experiments failed to detect it, why don't we try a new approach?
We do. Each new experiment is a new approach. And each research lab has a different strategy.

I mean why is the scientific community so focused on something specific, when we can try many other things?
If you read the literature you will see that this is already happening. The scientific community is always approaching a problem multiple ways, and in stiff competition with each other.
 
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Okay, so I understand that MOND-type solutions are out-of-favour at the moment, so we won't go there. Another out-of-favour solution is MACHO-based DM. Some experiments have come to the conclusion that it can't be billions of black holes, but how do they come up with that conclusion? They come to this conclusion based on our current understanding of how black holes form, so we assume that they must all come from stellar-mass sources. What if much smaller mass black holes were produced in the aftermath of the Big Bang? It's not a mechanism for forming black holes that we're familiar with. These would have the mass of asteroids or planets, rather than stars, and they'd be so small that we couldn't possibly look for them through gravitational lensing experiments. They'd be wandering around the universe widely dispersed, barely ever encountering another such micro-black hole, ever. But in the aggregate, they'd be so common within the multi-peta cubic light-years of a galactic halo, that they'd make up the majority of the mass of these galaxies?
 
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Okay, so I understand that MOND-type solutions are out-of-favour at the moment, so we won't go there. Another out-of-favour solution is MACHO-based DM. Some experiments have come to the conclusion that it can't be billions of black holes, but how do they come up with that conclusion? They come to this conclusion based on our current understanding of how black holes form, so we assume that they must all come from stellar-mass sources. What if much smaller mass black holes were produced in the aftermath of the Big Bang? It's not a mechanism for forming black holes that we're familiar with. These would have the mass of asteroids or planets, rather than stars, and they'd be so small that we couldn't possibly look for them through gravitational lensing experiments. They'd be wandering around the universe widely dispersed, barely ever encountering another such micro-black hole, ever. But in the aggregate, they'd be so common within the multi-peta cubic light-years of a galactic halo, that they'd make up the majority of the mass of these galaxies?
I dont think that would be possible. First we need a lot of black holes. We need 5 times mass of the visible mass, and that much of black holes in the size of astroids...?

Also small black holes will evaporate faster.

Another thing is Actually DM created the galaxy structures that we see. So can these tiny black holes can create the galacy structures that we see ??

So visible matter is just affected by DM. DM also must be created in the early universe to keep expansion at critical level.
 
  • #11
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They come to this conclusion based on our current understanding of how black holes form, so we assume that they must all come from stellar-mass sources.
You really need to stop making unsubstantiated and false claims without any proper references. It is a really bad way of discussing.

What if much smaller mass black holes were produced in the aftermath of the Big Bang? It's not a mechanism for forming black holes that we're familiar with.
Yes it is. Primordial black holes are actively discussed in the dark matter community and mostly ruled out by different experiments depending on the mass range. Some people in the community argue for and against certain bounds so there may still be some windows, but overall the situation looks rather grim for PBHs.
 
  • #12
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These would have the mass of asteroids or planets, rather than stars,
Asteroids, yes, but not planets. Ongoing efforts to detect them are in progress, and the current lower threshold for detecting them is about 1/3 of the mass of the moon. So if they exist then they are smaller than that.

All of these possibilities are being pursued in the scientific community. Each group is in competition with the others to find evidence supporting a plausible mechanism.
 
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Asteroids, yes, but not planets. Ongoing efforts to detect them are in progress, and the current lower threshold for detecting them is about 1/3 of the mass of the moon. So if they exist then they are smaller than that.

All of these possibilities are being pursued in the scientific community. Each group is in competition with the others to find evidence supporting a plausible mechanism.
I want to ask something. If these are the cases then should we see a lot of black holes around us ??? I didnt understand the thoery here.
 
  • #14
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I want to ask something. If these are the cases then should we see a lot of black holes around us ??? I didnt understand the thoery here.
More likely just plain asteroids, IMO. Why suppose an asteroid sized black hole when you can just as easily suppose an asteroid.
 
  • #15
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More likely just plain asteroids, IMO. Why suppose an asteroid sized black hole when you can just as easily suppose an asteroid.
And I am asking where are the "astroids" ? I understand that we are trying to observe them but isn't it absurd ? I mean I dont understand why they are DM candidates.

"The visible disk of the Milky Way galaxy is embedded in a much larger, roughly spherical halo of dark matter. The dark matter density drops off with distance from the galactic center. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity. The luminous matter makes up approximately ##9 x 10^10## solar masses. The dark matter halo is likely to include around ##6 x 10^11## to ##3 x 10^12## solar masses of dark matter."

Small astroid size black holes how can there be formed, without evaporation and they can still contribute ? Since our galaxy still in one piece, then how it can be possible ?

I can understand that in the early universe they might have been formed. But I dont give any chance that they exist now or contribute to the DM that we see around.

"General relativity predicts the smallest primordial black holes would have evaporated by now, but if there were a fourth spatial dimension – as predicted by string theory – it would affect how gravity acts on small scales and "slow down the evaporation quite substantially"

"Depending on the model, primordial black holes could have initial masses ranging from ##10^{−8}## kg (the so-called Planck relics) to more than thousands of solar masses. However, primordial black holes with a mass lower than ##10^{11}## kg would have evaporated due to Hawking radiation in a time much shorter than the age of the Universe, so they cannot have survived until the present Universe.A noticeable exception is the case of Planck relics that could eventually be stable"

However this part might be true

" Primordial black holes are also good candidates for being the seeds of the supermassive black holes at the center of massive galaxies, as well as of intermediate-mass black holes"

btw, I am not trying argue here. Just with all these stuff why they think it can be the DM candidate

Yes it is. Primordial black holes are actively discussed in the dark matter community and mostly ruled out by different experiments depending on the mass range. Some people in the community argue for and against certain bounds so there may still be some windows, but overall the situation looks rather grim for PBHs.
I agree. I guess they dont see as a candidate now. Thats nice
 
  • #16
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Just with all these stuff why they think it can be the DM candidate
Why not? It is a perfectly fine dark matter candidate. You look at how they would behave and compare that to what you can observe about dark matter. Depending on who you ask, you might get the answer that they are ruled out by observations or that there are still some windows open that allow PBH dark matter.
 
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And I am asking where are the "astroids" ?
That is what the MACHO studies are trying to find out.

I mean I dont understand why they are DM candidates.
I don’t think they are likely either, but they are also not completely ruled out.
 
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  • #19
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More likely just plain asteroids, IMO. Why suppose an asteroid sized black hole when you can just as easily suppose an asteroid.
I suppose it's because asteroids couldn't contribute to the peaks in CMB power spectrum.
 
  • #20
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I dont think that would be possible. First we need a lot of black holes. We need 5 times mass of the visible mass, and that much of black holes in the size of astroids...?

Also small black holes will evaporate faster.

Another thing is Actually DM created the galaxy structures that we see. So can these tiny black holes can create the galacy structures that we see ??

So visible matter is just affected by DM. DM also must be created in the early universe to keep expansion at critical level.
I'm not talking about BH's the size (implying volume) of asteroids, but rather the mass of asteroids. For example a black hole approximately the mass of the asteroid Vesta would have a Schwarzchild radius of only 400 nm! You'll never be able to spot such a tiny object from any telescopic distance.

Yes, smaller BH's evaporate faster, but they will still last billions of years. In fact, the currently accepted limit for BH stability is about 60% the mass of the Moon. A BH with 0.6 Moon masses will have a Hawking Radiation temperature of about 2.7 K, which the same temperature as the Cosmic Background Radiation, so such a black hole would be feeding energy off of the CMBR at the same rate as it is losing to Hawking Radiation. Things much smaller than that would still be quite stable for billions or trillions of years.

As for creating the structure of the Universe, that structure was probably already created in the quantum fluctuations at the time of the Big Bang, and these primordial BH's were probably also created and placed within those fluctuations, so they are just following the pre-existing quantum structure. Dark Matter is probably just maintaining the original quantum structure, rather than having created that structure at the cosmological scale.

Asteroids, yes, but not planets. Ongoing efforts to detect them are in progress, and the current lower threshold for detecting them is about 1/3 of the mass of the moon. So if they exist then they are smaller than that.

All of these possibilities are being pursued in the scientific community. Each group is in competition with the others to find evidence supporting a plausible mechanism.
How would they be able to detect black holes at 1/3 Moon mass? Can they already detect BH's larger than that? Well, obviously they can detect stellar mass BH's, but even those are pretty elusive if they're not orbiting and feeding on another star.
 
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As for creating the structure of the Universe, that structure was probably already created in the quantum fluctuations at the time of the Big Bang, and these primordial BH's were probably also created and placed within those fluctuations, so they are just following the pre-existing quantum structure. Dark Matter is probably just maintaining the original quantum structure, rather than having created that structure at the cosmological scale.
Please stop speculating! The research on how PBH dark matter would work is out there. Also, you cannot just refer to "original quantum fluctuations". The post Big Bang structure formation is rather well understood and requires dark matter that has certain properties. Your dark matter needs to have those properties.

Here is a few years old constraint plot on PBHs (from http://inspirehep.net/record/1630595).
ConstraintsPBH-GWB.png

There have been some updates and further discussions on each of these constraints, but generally this is how the situation looks. Your "asteroid Vesta sized" black holes are ruled out by gravitational microlensing in this paper https://arxiv.org/abs/1701.02151 to be less than 0.01 of the total dark matter content.
 

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  • #22
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For example a black hole approximately the mass of the asteroid Vesta would have a Schwarzchild radius of only 400 nm! You'll never be able to spot such a tiny object from any telescopic distance.
Not that it would be necessary. You can calculate how often such a black hole would get trapped in a star and swallow it, for example. Is the expected rate observable? If yes, does it match observations?

This is just one out of many approaches.
 
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  • #23
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How would they be able to detect black holes at 1/3 Moon mass? Can they already detect BH's larger than that?
Yes, 1/3 lunar mass is currently the lower limit for the MACHO project. Unfortunately, I cannot tell you how they are detected at that level. It is not my area of expertise.
 
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Not that it would be necessary. You can calculate how often such a black hole would get trapped in a star and swallow it, for example. Is the expected rate observable? If yes, does it match observations?

This is just one out of many approaches.
Well, first of all, what reason would there be for such a BH to get trapped inside a star? There's so much space between stars, there's no reason to get trapped by a star.

Second, a micro-BH would be nearly frictionless when entering the atmosphere of a star, because it would have such a small cross-sectional area, for example a Vesta-mass BH has a Schwarzchild Radius of only 400nm. But an actual Vesta-sized asteroid entering the atmosphere of a star would have lots of drag coefficient due to its large volume, 7.8×10^16 m^3 vs. 2.681×10^-19 m^3 for a BH of the same mass, i.e. 35 orders of magnitude larger. Even if a micro-BH entered a star (e.g. the Sun), it would emerge right out of it again, its momentum hardly changed, it would feel a drag force of only 145 mN (https://is.gd/gdHaET). Whereas an actual Vesta-sized asteroid would feel a drag force of 6.293×10^22 N. If the same micro-BH entered into the Earth it would feel a slightly larger of drag force of 561 mN. It's only if it entered into a neutron star that it would feel any sort significant drag force, 2.036×10^14 N (https://is.gd/NytkBo).

On the other hand, a Vesta-mass BH would be radiating Hawking Radiation at a temperature of 459.5 K. That would be well within the infrared range.
 
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Please stop speculating! The research on how PBH dark matter would work is out there. Also, you cannot just refer to "original quantum fluctuations". The post Big Bang structure formation is rather well understood and requires dark matter that has certain properties. Your dark matter needs to have those properties.

Here is a few years old constraint plot on PBHs (from http://inspirehep.net/record/1630595).
View attachment 233456
There have been some updates and further discussions on each of these constraints, but generally this is how the situation looks. Your "asteroid Vesta sized" black holes are ruled out by gravitational microlensing in this paper https://arxiv.org/abs/1701.02151 to be less than 0.01 of the total dark matter content.
These studies are making their own assumptions, and each study has its own set of assumptions. We won't know anything for certain until we have actual examples of micro-BH's. They can plot as many graphs as they like, but they are barely above the level of hypothesis.
 
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