Multi-Messenger Astronomy and Dark Matter
This chapter presents the elaborated lecture notes on Multi-Messenger Astronomy and Dark Matter given by Lars Bergström at the 40th Saas-Fee Advanced Course on "Astrophysics at Very High Energies". One of the main problems of astrophysics and astro-partic
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1 Preamble Astrophysics, and more specifically astroparticle physics, has been going through tremendous progress during the last two decades. Still, one of the main problems, that of the nature of the dark matter, remains unsolved. With the help of accelerator experiments (at CERN’s Large Hadron Collider (LHC) in particular, which started operation in 2010 and which is currently gathering an impressive integrated luminosity) we could soon hope to get a first indication of the mass scale for the new physics that is associated with dark matter. However, to actually prove that a particle discovered at accelerators has the right properties to constitute the astrophysical dark matter, complementary methods are needed. The fact that a candidate for dark matter is electrically neutral (as not to emit nor absorb light—that is what we mean with the term “dark”) can plausibly be determined at accelerators. However, the coupling of the dark matter particles to other matter needs to be weak, and the lifetime of the dark matter particle needs to be at least of the order of the age of the universe. This cannot be tested at accelerators—the dark matter particles would leave the detector in some 100 ns. There could be very useful information still gathered at the LHC, as possibly decays of more massive states in the “dark sector” would be observable, and the missing energy could be estimated. Fortunately, through observations of various types of messengers—radio waves, microwaves, IR, optical and UV radiation, X-rays, γ-rays and neutrinos, there is great hope that we could get an independent indication of the mass scale of dark matter. This variety of possible methods of indirect detection methods is a part of multi-messenger astronomy, and it is the second way by which we approach the dark matter problem. In particular, for models where the dark matter particles are L. Bergström Department of Physics, The Oskar Klein Centre, AlbaNova, Stockholm University, SE-106 91 Stockholm, Sweden e-mail: [email protected] F. Aharonian et al., Astrophysics at Very High Energies, Saas-Fee Advanced Course 40, DOI: 10.1007/978-3-642-36134-0_2, © Springer-Verlag Berlin Heidelberg 2013
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involved in breaking the electroweak symmetry of the Standard Model, so-called WIMP models (for weakly interacting massive particles), prospects of detection in the near future look promising. We will look in some detail on the properties of WIMP candidates, where the fact that they are massive means that they move nonrelativistically in galactic halos, and form so-called cold dark matter (CDM). One thought earlier that neutrinos could be the dark matter, but they would constitute hot dark matter (HDM), which is not favoured by observations. Due to free-streaming motion, they would only form very large structures first, which then fragment into smaller scales, like galaxies. This scenario does not agree with observations, as it gives too little power on small scales. Of course, one may also consider an in-between scenario, warm dark matter, usu
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