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Cheap Astronomy
2013-Nov-09, 09:43 AM
Given that we don't even know what dark matter is - it is puzzling that multi-million dollar projects are searching for 'signatures' of dark matter annihilation (e.g. the Alpha Magnetic Spectrometer aboard the ISS).

Can anyone explain the grounds for thinking that dark matter particles will annihilate upon encountering each other. Is it just that a DM particle (e.g. a neutralino) is its own anti-particle?

What then is the assumed physics underlying these DM particles regularly bumping into and annihilating each other?

More importantly, why is dark matter, with a known tendency to 'clump', so prevalent in the universe today if it has such a propensity for self-annihilation??

Furthermore, how will we recognise what the positronic signature such an annihilation will look like - or is it just a case of searching for a 'signature' that we have never seen before?

Hope that's not too many questions within a question ;)

Thanks,

Steve

online365
2013-Nov-09, 10:21 AM
Good questions - although I am not qualified to answer you, I would like to know a little more on the subject of dark matter myself.


Given that we don't even know what dark matter is - it is puzzling that multi-million dollar projects are searching for 'signatures' of dark matter annihilation (e.g. the Alpha Magnetic Spectrometer aboard the ISS).

I think the overall consensus is that we know what it is, because we can observe it's effects, and there are several lines of evidence now that leads us to believe it's there. Which may explain all the resources being dumped into the project.

NoChoice
2013-Nov-09, 01:34 PM
Not only do we not know what DM is, we don't even know that it actually exists.

All we have is data (e.g. the rotational speeds of galaxies) that does not fit the current models.
The rest is speculation.

We have absolutely no direct evidence for DM.
We have some data that can be interpreted to be an indicator for DM. So far this is all indirect evidence, however.

The AMS on the ISS can not provide direct evidence either, since we cannot rule out other explanations for an excess of positrons (which the AMS is designed to detect).
In fact, in April this year the AMS has detected an excess of positrons (http://physicsworld.com/cws/article/news/2013/apr/04/ams-confirms-positron-excess) but we are really none the wiser as far as DM is concerned.

Btw: We are not looking for a specific signature of positrons (since we do not know what WIMPs are - or if they even exist - and have therefor no model to make any such detailed predictions). We are simply looking for an excess of positrons in a wide energy range.

The waters are very murky here, full of unsubstantiated assumptions and speculations. We cannot even explain why there is so much more matter in the universe than antimatter, i.e. we only have a very rudimentary (and possibly very wrong) understanding of matter and antimatter.
How confident can we then be to speculate about possible annihilation events of particles we know virtually nothing about since we have never detected them?
Who is to even say that WIMPs annihilate in the way we know other particles behave?

Is the money for AMS money well spent?
Given the price tag of $1.5 billion (same link) it makes one wonder.
On the other hand that amount of money corresponds to about 15 crappy blockbuster movies...

Personally, I cannot shed the feeling that we are looking in the wrong direction altogether but it makes sense to look in that direction first.
We do need more data - that much is certain. Even if all it does is rule out the WIMP idea (or maybe even confirm it).

Shaula
2013-Nov-09, 01:47 PM
All we have is data (e.g. the rotational speeds of galaxies) that does not fit the current models.
The rest is speculation.
There are several lines that suggest an excess of non-baryonic matter in the universe that we only observe interacting via gravity. Structure formation in the early universe (large scale, filamentary) looks far more realistic with it in. Nucleosynthesis works far better with it in. The Cosmic Microwave Background Radiation has features that are most easily explained by including it. Galactic cluster dynamics requires it.

You paint a very bleak picture of it, but this is really no different from what happened with neutrinos. We saw 'missing energy', tracked back and eventually found the particle. Dark matter is harder, and you are right to say that it may turn out to be the wrong explanation. But it is on a stronger footing than you suggest and at the moment it is the best fit to multiple observations and modelling problems.

One of the reasons there is interest in the positron flux and its relationship to dark matter is that supersymmetry is rather on the ropes, at least the minimal versions. These positrons, if they are associated with dark matter, could be a signature of certain kinds of supersymmetric candidates for dark matter.

online365
2013-Nov-09, 09:34 PM
There are several lines that suggest an excess of non-baryonic matter in the universe that we only observe interacting via gravity. Structure formation in the early universe (large scale, filamentary) looks far more realistic with it in. Nucleosynthesis works far better with it in. The Cosmic Microwave Background Radiation has features that are most easily explained by including it. Galactic cluster dynamics requires it.

You paint a very bleak picture of it, but this is really no different from what happened with neutrinos. We saw 'missing energy', tracked back and eventually found the particle. Dark matter is harder, and you are right to say that it may turn out to be the wrong explanation. But it is on a stronger footing than you suggest and at the moment it is the best fit to multiple observations and modelling problems.

One of the reasons there is interest in the positron flux and its relationship to dark matter is that supersymmetry is rather on the ropes, at least the minimal versions. These positrons, if they are associated with dark matter, could be a signature of certain kinds of supersymmetric candidates for dark matter.

Hi Shaula - Thanks for the info. Do we have any specific science journals that talks about these lines of evidence in more detail?

Jeff Root
2013-Nov-09, 11:31 PM
Nucleosynthesis works far better with it in.
I don't recall seeing this before, and it is shocking.
I hadn't heard of any problem without dark matter.
How does it help?

What is the reason for thinking that particles of dark
matter might "annihilate" in some way? What would
they emit when they annihilate that we would recognize
as a signature of the annihilation?

-- Jeff, in Minneapolis

Shaula
2013-Nov-10, 07:15 AM
I don't recall seeing this before, and it is shocking. I hadn't heard of any problem without dark matter. How does it help?


Deuterium and the baryon density
Unlike helium, deuterium is a very fragile element. It burns at a temperature of only 10^6 K, well below the temperature in the solar core. A considerable fraction of any primordial deuterium at the beginning of the galaxy would have been destroyed by the present time. This is confirmed by observation: interstellar clouds contain deuterium, as do protostars, stars which have not yet developed nuclear burning cores, whereas evolved stars have essentially no deuterium. By studying clouds of gas at very high redshift in the medium between galaxies one can infer the amount of deuterium relative to that of hydrogen. Comparison with the Big Bang prediction requires one to choose a density that cannot exceed about a tenth of the critical density for closure of the universe, otherwise too little primordial deuterium would have been synthesized. There is no alternative to the Big Bang for synthesizing deuterium: stars destroy it rather than produce it. The significance of this result is that most of the matter in the universe must be non baryons, for example it could consist of weakly interacting neutral particles that did not participate in the nuclear reactions that led to deuterium production.

There is an issue with this in that it is predicated in the assumption that the universe has the critical density or close to it. Until recently it seemed to be the prevailing hypothesis that this was the case. Now, however, the density of the universe is thought to be lower reducing the need for non-baryonic dark matter (but not eliminating it). This review has some fairly good bounds on what is though to be going on: http://pdg.lbl.gov/2009/reviews/rpp2009-rev-bbang-nucleosynthesis.pdf


What is the reason for thinking that particles of dark matter might "annihilate" in some way? What would they emit when they annihilate that we would recognize as a signature of the annihilation?

-- Jeff, in Minneapolis
I only spoke about MSSM superpartners. They can be constrained more than totally unknown particles. As I said, that seems to be one of the reasons people are so interested in this. The LHC has not so far shed any light on supersymmetric models (other than to exclude a lot of them) so this could be a chance to find ro exclude more models. I think, off the top of my head, they were proposing that this was a neutralino decay product.

Shaula
2013-Nov-10, 07:29 AM
Hi Shaula - Thanks for the info. Do we have any specific science journals that talks about these lines of evidence in more detail?
Honestly there are loads. Look for Baryon Acoustic Oscillations (BAO), large scale structure formation - you will find them. Most of the links here are not scholarly papers but have some useful references in.

Nucleosynthesis:
http://pdg.lbl.gov/2009/reviews/rpp2009-rev-bbang-nucleosynthesis.pdf

BAO and universe parameters:
http://www.ast.cam.ac.uk/ioa/meetings/dv10/talks/dv10_day1_simon_white.pdf
http://arxiv.org/pdf/0907.1660.pdf

Structure formation:
http://astro.berkeley.edu/~mwhite/modelcmp.html
http://ned.ipac.caltech.edu/level5/Sept09/Einasto/Einasto5.html
http://www.mpa-garching.mpg.de/galform/virgo/millennium/