Nov 20, 2012

Dark Matter I: Introduction

I was asked to write a review on dark matter, as the final report of General Relativity (GR) class. I'd like to share my reading here.

Fig. 1: Dark matter forms a halo around a galaxy.
In astronomy and physics, dark matter (abbr. DM) is a term for non-luminous matters in the Universe. It is proposed to explain the anomalous mass-to-light ratio (abbr. M/L). Dark matter can not be observed but leaves their trace in galaxy motion among others. It is generally postulated dark matter forms a halo around a galaxy. They initiate the formation of galaxies and galaxy clusters, dominates the mass of large scale structures. At present, dark matter is part of the concordant cosmology model, the $\Lambda$-CDM model, where CDM is abbreviation for cold dark matter. It is generally believed dark matter comprises about 23% of the total mass of the Universe. Together with dark energy 72%, they dominate the present Universe.

Fig. 2: Left: The contents of the Universe according to $\Lambda$-CDM model. Only 4.6% of the Universe is ordinary matter. Dark matter comprises 23% of the Universe. 72% of the Universe, is composed of dark energy. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion. Right: Timeline of the Universe according to $\Lambda$-CDM model.

However, the nature of dark matter is still unclear. It has been shown, none of the elementary particles in the standard model (abbr. SM) could be dark matter. Perhaps some suspicion remains for neutrinos, given their masses indeterminate. Therefore, the existence of dark matter naturally postulates physics beyond the particle physics standard model. In fact, for various theoretical speculations, particle physicists tend to believe standard model is merely an effective theory of an more advanced theory. The demands of dark matter, whether truly relevant or not, has been as a strong motivation for extension of standard model in particle physics. Of course not all new particles are dark matter candidates. It is generally plausible to assume dark matter are some weakly interacting massive particles (abbr. WIMPs), raising from some TeV scale new physics. The favored WIMP candidates include neutralinos from supersymmetric (abbr. SUSY) models and the first Kaluza-Klein excitations from universal extra dimension (abbr. UED) models among others.
Fig. 3: The standard model elementary particle zoo
On the other hand, dark matter may also merely be a misleading paradigm. The existence of dark matter, inferred from large gravitational mass to luminosity ratio, is based on the assumption that general relativity with its flat space-time approximation Newtonian gravity holds up to cosmological scale. Milgrom and others has shown that it is possible to modified Newtonian gravity to explain the large mass-to-light ratio in galaxies. If confirmed, however, mankind's understanding of the cosmos will be completely overthrown.

Just as Richard P. Feynman had said, "Experiment is the sole judge of scientific truth". Some of the proposals lies within the current experimental and observational scope. People have conducted various experiments and observations to detect possible dark matter directly or indirectly, from space telescope to ground based detector and colliders. Current results, mainly null results with some suspicious signals, have excluded a large class of theories.

In this paper, I review the physics of dark matter. The aim is to give a pedagogical introduction to general interested readers, like myself. In the next few posts, I will introduce the evidences and motivations for dark matter; then review the candidates and their properties. Focus will be put on popular models such as WIMPs, gravitino, axions and sterile neutrinos. After that, I will talk about the undergoing experiments and observations in astronomy and physics. Their results and the constraints on models will be discussed. In the end, I will visit alternative theories and other speculations.

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