Search Talks

The axion solution to the strong CP problem also provides a natural dark matter candidate. If the Peccei-Quinn symmetry has ever been restored after inflation, topological defects of the axion field would have formed and produced relic axions, whose abundance is in principle calculable. We study the contribution to the abundance produced by string defects during the so-called scaling regime. Clear evidence of scaling violations is found, the most conservative extrapolation of which strongly suggests a large number of axions from strings. The overall result is a lower bound on the QCD axion mass in the post-inflationary scenario that is substantially stronger than the naive one from misalignment.

The computation of transition amplitudes in Loop Quantum Gravity is still a hard task, especially without resorting to large-spins approximations. In Marseille we are actively developing a C library (sl2cfoam) to compute Lorentzian EPRL amplitudes with many vertices. We have already applied this tool to obtain interesting results in spinfoam cosmology and on the so-called flatness problem of spinfoam models. I will highlight the main aspects of our software library, describe the current applications and also introduce undergoing developments (Montecarlo over spin space, tensor contractions, GPU offloading) that will greatly enhance the library's speed and capabilities.

Most massive stars spend their lives in so close orbit with a companion star that severe mass exchange or even coalescence is inevitable as the stars evolve and swell. A third of massive stars are thus stripped of their fluffy, hydrogen-rich envelopes, leaving the compact helium core exposed. These stripped stars are so hot that most of their radiation is emitted in the ionizing regime. Using evolutionary and spectral models of stripped stars, I will show how they sometimes dominate the ionizing emission from full stellar populations and even significantly contribute to cosmic reionization. With their hard ionizing spectra, stripped stars possibly leave observable traces, for example in the nebular spectrum of distant galaxies. 

Apart from being ionizing sources, stripped stars are also interesting to consider as gravitational wave emitters. Creating a population model, we predict that several stripped stars orbiting compact objects will be detectable by LISA.