Andreas Burkert

Whereas cold dark matter (CDM) simulations predict central dark matter cusps with densities that diverge as $\rho$(r)$\sim$ 1/r observations often indicate constant density cores with finite central densities $\rho_0$ and a flat density distribution within a core radius r$_0$. This paper investigates whether this core-cusp problem can be solved by fuzzy dark matter (FDM), a hypothetical particle with a mass of order m$\approx$10$^{-22}$eV and a corresponding de Broglie wavelength on astrophysical scales. We show that galaxies with CDM halo virial masses M$_{vir} \leq 10^{11}$M$_{\odot}$ follow two core scaling relations. In addition to the well known universal core column density $\Sigma_0 \equiv \rho_0 \times$r$_0$ = 75 M$_{\odot}$pc$^{-2}$ core radii increase with virial masses as r$_0 \sim$ M$_{vir}^{\gamma}$ with $\gamma$ of order unity. Using the simulations by Schive et al. (2014) we demonstrate that FDM can explain the r$_0$-M$_{vir}$ scaling relation if the virial masses of the observed galaxy sample scale with formation redshift z as M$_{vir}\sim$(1+z)$^{-0.4}$. The observed constant $\Sigma_0$ is however in complete disagreement with FDM cores which are characterised by a steep dependence $\Sigma_0 \sim$r$_0^{-3}$, independent of z. More high-resolution simulations are now required to confirm the Schive et al. simulations and explore especially the transition region between the soliton core and the surrounding halo. If these results hold, FDM can be ruled out as the origin of observed dark matter cores and other physical processes are required to account for their formation.

Anson Hook, Gustavo Marques-Tavares, Davide Racco

The low frequency part of the gravitational wave spectrum generated by local physics, such as a phase transition or parametric resonance, is largely fixed by causality, offering a clean window into the early Universe. In this work, this low frequency end of the spectrum is analyzed with an emphasis on a physical understanding, such as the suppressed production of gravitational waves due to the excitation of an over-damped harmonic oscillator and their enhancement due to being frozen out while outside the horizon. Due to the difference between sub-horizon and super-horizon physics, it is inevitable that there will be a distinct spectral feature that could allow for the direct measurement of the conformal Hubble rate at which the phase transition occurred. As an example, free-streaming particles (such as the gravity waves themselves) present during the phase transition affect the production of super-horizon modes. This leads to a steeper decrease in the spectrum at low frequencies as compared to the well-known causal $k^3$ super-horizon scaling of stochastic gravity waves. If a sizable fraction of the energy density is in free-streaming particles, they even lead to the appearance of oscillatory features in the spectrum. If the universe was not radiation dominated when the waves were generated, a similar feature also occurs at the transition between sub-horizon to super-horizon causality. These features are used to show surprising consequences, such as the fact that a period of matter domination following the production of gravity waves actually increases their power spectrum at low frequencies.

David Garfinkle, Frans Pretorius

We perform numerical simulations of the approach to spacetime singularities. The simulations are done with sufficient resolution to resolve the small scale features (known as spikes) that form in this process. We find an analytical formula for the shape of the spikes and show that the spikes in the simulations are well described by this formula.