Dark Matter Mike Brotherton Professor of Astronomy, University of Wyoming Author of Star Dragon and Spider Star The Father of Dark Matter • In 1933, Fritz Zwicky checked out the Coma Cluster. The galaxies were flying around too fast (as measured by the Doppler effect) for their visible mass to keep them together, so he proposed dark matter was present. The Mother of Dark Matter • A few decades later, Vera Rubin started to notice FLAT rotation curves in spiral galaxies. Flat Rotation Curves – so what? • Following Rieke, images from Bennett and Pryke The Nature of Dark Matter Can dark matter be composed of normal matter? If so, then its mass would mostly come from protons and neutrons = baryons The density of baryons right after the big bang leaves a unique imprint in the abundances of deuterium and lithium. Density of baryonic matter is only ~ 4 % of critical density, total is 30%. Most dark matter must be non-baryonic! WIMPs??? • “WIMP” = Weakly interactive massive particle – Neutrinos? • Seem to have mass, but too small. – Axions? • From Wikipedia, “The axion is a hypothetical elementary particle postulated by Peccei-Quinn theory in 1977 to resolve the strong-CP problem in quantum chromodynamics (QCD).” • As yet, not detected (axions are predicted to change to and from photons in the presence of strong magnetic fields, and this property is used for creating experiments to detect axions) MaCHOs • “MaCHO” = Massive Compact Halo Object – Just trying to explain flat rotation curves with things like black holes, brown dwarfs, etc. – These are “Baryonic,” made from conventional stuff on the periodic table (like people, planets, etc.) – Can be probed via gravitational microlensing Baryonic Dark Matter Nature of baryonic dark matter still very uncertain and speculative. Brightness of background star One component: Massive Compact Halo Objects = “MACHOs”: Small compact objects (e.g., brown dwarfs, small black holes) acting as gravitational lenses. Time Earth MACHO Distant background star Are we sure Dark Matter is real? • Astronomers have argued that dark matter can explain the strange motions of galaxies in clusters and stars in galaxies, but that it can’t be normal stuff • Could we just have gotten gravity wrong on large scales? – “MoND” = Modified Newtonian Dynamics • Viable alternative, until 2006… – Need to look more into the phenomenon of gravitational lensing on larger scales Probing Dark Matter with Distant Quasars: Gravitational Lensing Light from a distant quasar is bent around a foreground galaxy two images of the same quasar! Light from a quasar behind a galaxy cluster is bent by the mass in the cluster. Use to probe the distribution of matter in the cluster. Gravitational Lensing of Quasars Gravitational Lensing The huge mass of gas in a cluster of galaxies can bend the light from a more distant galaxy. Image of the galaxy is strongly distorted into arcs. Hot Gas in Clusters of Galaxies Space between galaxies is not empty, but filled with hot gas (observable in X rays) That this gas remains gravitationally bound, provides further evidence for dark matter. Visible light Coma Cluster of Galaxies (From Horizons by Seeds) X rays The Bullet Cluster • Given what we know about gravitational lensing (tracing the total mass in blue), hot X-ray gas in (the dominant baryonic mass, red), we can show that dark matter exists in at least one system: Images from Clowe et al. 2006 and the Chandra press release The Bullet Cluster • Lensing of background galaxies seen in the optical images lets the mass distribution be mapped. • The X-rays trace the hot gas, the dominant source of baryons in this cluster merger. • They don’t line up! Why? Dark Matter seems to not interact with itself the way diffuse gas does during a cluster collision. The Bullet Cluster Summary • That Zwicky Bastard was right! • Dark Matter does indeed seem to be real, thank you Bullet Cluster. • The majority of matter, dark or otherwise, is “non-baryonic” exotic stuff, and we don’t know for sure what it is. It’s likely flying through this room right this instant in huge amounts. • WIMPs, not MaCHOs.