2nd December 2021 – London Cosmology Discussion Meeting

(Follow links for relevant talk slides)

2.05 – 2.35 Pedro Fernandes (QMUL)
Title: The 4D Einstein-Gauss-Bonnet theory of gravity, and PBH remnants as a dark matter candidate

In this talk, I will review the topic of 4D Einstein-Gauss-Bonnet gravity, which has been the subject of considerable interest over the past two years. I then move on to consider the gravitational physics of well-defined theories that enact it, showing that the final state of evaporation of black holes is a remnant with a finite size determined by the Gauss-Bonnet coupling constant. Finally, I outline a scenario in which remnants of this kind from primordial black holes could act as dark matter.

2.40 – 3.10 Eloy De Jong (KCL)
Title: Primordial black hole formation with full numerical relativity.

I will talk about studying the formation of black holes from subhorizon and superhorizon perturbations in a matter dominated universe with 3+1D numerical relativity simulations. We find that there are two primary mechanisms of formation depending on the initial perturbation’s mass and geometry — via direct collapse of the initial overdensity and via post-collapse accretion of the ambient dark matter. In both cases, the duration of the formation the process is around a Hubble time, and the initial mass of the black hole is $M_{BH}∼10^{−2}H^{−1}M_{Pl}$. Post formation, we find that the PBH undergoes rapid mass growth beyond the self-similar limit $M_{BH}\propto H^{−1}$, at least initially. We argue that this implies that most of the final mass of the PBH is accreted from its ambient surroundings post formation.

3.25 – 3.55 Matthew Davies (QMUL)
Title: A Primordial Power Spectrum Toolkit for PBHs in Inflationary Scenarios

The primordial power spectrum contains a wealth of information on the dynamics of the early universe during inflation. Of particular interest are power spectra with a large peak, since these are capable of producing both primordial black holes (PBHs) and scalar induced gravitational waves (SIGWs). In this talk I present work that has been done to describe some of the key features of these power spectra, such as their tilt and non-Gaussianity. Modelling inflation as a series of “phases” characterised by distinct values of the second slow-roll parameter, we find that the important features of the spectrum can be inferred from the evolution of this parameter. This provides a simple toolkit in which power spectra with desired features can be constructed.

4.00 – 4.30 Andreas Mantziris (ICL)
Title: Cosmological implications of electroweak vacuum instability: constraints on the Higgs curvature coupling from inflation

The current experimentally measured parameters of the Standard Model (SM) of particle physics suggest that our Universe lies in a metastable electroweak vacuum, where the Higgs field could decay to a lower vacuum state with catastrophic consequences. Our observations dictate that such an event has not happened yet, despite the many different mechanisms that could have triggered it thoughout our cosmological history. Via this observation, we can establish a promising link between cosmology and particle physics to constrain important parameters of our fundamental theories. Exploring the possibility of vacuum decay during inflation, we can obtain robust bounds on the last unknown renormalisable SM parameter $\xi$, which couples the Higgs field with space-time curvature. In our study, we considered three inflationary models, quadratic and quartic chaotic inflation, and Starobinsky-like power-law inflation, and accounted for the time-dependence of the Hubble rate both in the geometry of our past light-cone and in the Higgs effective potential, which is approximated with three-loop renormalisation group improvement supplemented with one-loop curvature corrections. 

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