Astrophysics

The Lambda Cold Dark Matter framework successfully accounts for observational constraints on large (> 1 Mpc) scales, from the clustering of galaxies to the angular dependence of the Cosmic Microwave Background to the structure and matter content of galaxy clusters. On the scale of individual galaxies and, in particular, of dwarf systems much fainter than the Milky Way, a number of apparent conflicts with LCDM expectations have been reported. These have prompted the consideration of a number of radical modifications to LCDM, such as the possibility that dark matter might be "self-interacting", or that it might not be “cold”. I will review the status of these alleged problems and will report on recent work that reevaluates the observational evidence and reexamines the role of systematic uncertainties in the comparison between observation and model predictions. In particular, I will propose a possible resolution to the “cusp vs core” problem that requires no cores; an explanation for the mass discrepancy-acceleration relation that requires no changes to LCDM halos; and a plausible tidal origin for the enigmatic population of galaxies inhabiting “extremely cold” dark matter halos, such as the recently discovered Crater 2 satellite.

The first phase of stellar evolution in the history of the Universe may be Dark Stars (DS), powered by dark matter heating rather than by nuclear fusion. Weakly Interacting Massive Particles, which may be their own antipartners, collect inside the first stars and annihilate to produce a heat source that can power the stars. A new stellar phase results, a Dark Star, powered by dark matter annihilation as long as there is dark matter fuel, with lifetimes from millions to billions of years. Dark stars are very bright diffuse puffy objects during the DS phase, and grow to be very massive. In fact, we have found they can to grow to 10^5-10^7 solar masses with luminosities 10^9-10^11 solar luminosities. Such objects will be observable with James Webb Space Telescope (the sequel to HST). Once the dark matter fuel is exhausted, the DS becomes a heavy main sequence star; these stars eventually collapse to form massive black holes that may provide seeds for supermassive black holes observed at early times as well as in galaxies today.