Cosmology

Observations of the Milky Way by the SPI/INTEGRAL satellite have confirmed the presence of a strong 511 KeV gamma-ray line emission from the bulge, which require an intense source of positrons in the galactic center. These observations are hard to account for by conventional astrophysical scenarios, whereas other proposals, such as light DM, face stringent constraints from the diffuse gamma-ray background. I will describe how light superconducting strings could be the source of the observed 511 KeV emission. The associated particle physics, at the ~ 1 TeV scale, is within reach of planned accelerator experiments, while the scenario has a distinguishing spatial distribution, proportional to the galactic magnetic field. I will also discuss how cosmic magnetic fields of nano-Gauss strength today could have been created at the time of baryogenesis. In addition to being astrophysically relevant, such magnetic fields, which are helical, can provide an independent probe of baryogenesis and CP violation in particle physics.

In this talk I will analyse the stochastic background of gravitational waves coming from a first order phase transition in the early universe. The signal is potentially detectable by the space interferometer LISA. I will present a detailed analytical model of the gravitational wave production by the collision of broken phase bubbles, together with analytical results for the gravitational wave power spectrum. Gravitational wave production by turbulence and magnetic fields will also be briefly discussed.

Many numerical studies show that dark matter halos have a plethora of substructure, down to the smallest resolved scales. However, the very bottom of the Cold Dark Matter (CDM) hierarchy at a few earth masses, where the spectral index n approaches -3 and structure begins to form simultaneously on a variety of scales, remains relatively unexplored. It is possible that the subhalo mass distribution, which appears to be described by a simple power-law down to mass scales 10^6 solar masses, remains unchanged and independent of scale and n. A few studies have indicated that this appears to be the case, which is surprising considering all other statistical indicators, such as the halo mass function, as well as the internal properties of halos, such as concentration, show a dependence on n. To explore the effect of the spectral index on the subhalo mass function we ran two large, scale-free simulations, P(k)=Ak^n with n=-1 and -2.5. We find that the subhalo mass function does depend on the spectral index, with the power-law becoming shallower as n->-3.