Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

A recent article by Galtier and Nazarenko (2017 Phys. Rev. Lett. 119 221101) proposed that weakly nonlinear gravitational waves could result in a turbulent cascade, with energy flowing from high to low frequency modes or vice versa. This is an interesting proposition for early universe cosmology because it could suggest some 'natural' initial conditions for the gravitational background. In this paper we use the ADM formalism to show that, given some simple and, arguably, natural assumptions, such initial conditions lead to expansion (or collapse) of the spacetime on a timescale much faster than that of the turbulent cascade, meaning that the cascade is unlikely to have sufficient time to develop under general conditions. We suggest possible ways in which the expansion could be mitigated to give the cascade time to develop.

Abstract:

Motivated by neutrino astronomy, we consider a plane wave of coupled and massive flavors, scattered by a static black hole, and describe analytically and numerically the corresponding oscillation probability in the surrounding space. Both the interpretation as particles traveling along geodesics and as scattered waves are studied, and consistently show a nontrivial and potentially long range interference pattern, in contrast to the spatially uniform transition probability in a flat spacetime. We introduce a numerical method for studying the oscillations around black holes, which accounts for the full curved geometry and flavor wave mixing. Whilst limited to the region immediately around the black hole, this numerical approach has the potential to be used in more general contexts, revealing the complex interference patterns which defy analytic methods.

Abstract:

In a ΛCDM cosmology, galaxy formation is a globally inefficient process: it is often the case that far fewer baryons are observed in galaxy discs than expected from the cosmic baryon fraction. The location of these ‘missing baryons’ is unclear. By fitting halo profiles to the rotation curves of galaxies in the SPARC data set, we measure the ‘missing baryon’ mass for individual late-type systems. Assuming that haloes initially accrete the cosmological baryon fraction, we show that the maximum energy available from supernovae is typically not enough to completely eject these ‘missing baryons’ from a halo, but it is often sufficient to heat them to the virial temperature. The energy available from supernovae has the same scaling with galaxy mass as the energy needed to heat or eject the ‘missing baryons’, indicating that the coupling efficiency of the feedback to the ISM may be constant with galaxy virial mass. We further find that the energy available from supernova feedback is always enough to convert a primordial cusp into a core and has magnitude consistent with what is required to heat the ‘missing baryons’ to the virial temperature. Taking a census of the baryon content of galaxies with 109 < Mvir/M⊙ < 1012 reveals that ∼86 per cent of baryons are likely to be in a hot phase surrounding the galaxies and possibly observable in the X-ray, ∼7 per cent are in the form of cold gas, and ∼7 per cent are in stars.