STRINGS, GRAVITY AND PARTICLES COLLOQUIUM

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• Title and abstract
  Title: Primordial-tensor-induced stochastic GWs: Explaining the recent PTA signal with no PBH production
  
  Abstract: Recently, pulsar timing array (PTA) collaborations announced evidence for an isotropic stochastic gravitational wave (GW) background. The origin of the PTA signal can be astrophysical or cosmological. In the latter case, the so-called secondary scalar-induced GW scenario is one of the viable explanations, but it has a potentially serious issue of the overproduction of primordial black holes (PBHs) due to the enhanced curvature perturbation. We present a new interpretation of the PTA signal. Namely, it originated from an extra spectator tensor field that exists on top of the metric tensor perturbation. As the energy density of the extra tensor field is always subdominant, it cannot lead to the formation of PBHs. Thus our primordial-tensor-induced scenario is free from the PBH overproduction issue. 

  [based on arXiv:2302.14080 & arXiv:2307.13109]

Upcoming colloquia

Past colloquia

• October 27th 2023. Alexander Ganz (Leibniz University Hannover)
  Title: Exploring Minimally Modified Gravity

  Abstract: Minimally Modified Gravity models are a new class of modified gravity models with just two tensor degrees of freedom as in General Relativity. In this seminar I will give a overview about these models and discuss general properties such as the preferred foliation. Finally, I will discuss the application to cosmology and in particular to the early universe and address possible new phenomenological properties. Last, I will comment on potential problems arising from the constraints at the perturbation level.

• September 8th 2023. Sergey Ketov (Tokyo Metropolitan University and Kavli IPMU) (copy 1)
  Title: Production of primordial black holes in single-field models of inflation

  Abstract: Generalizations of the Starobinsky inflation model are proposed in order to accommodate production of primordial black holes during inflation at smaller scales by using the near-inflection point mechanism. One-loop quantum corrections and induced gravitational waves are also discussed. The primordial black holes may form dark matter in the current universe, whereas the induced gravitational waves may be detectable by the future space-based gravitational interferometers.

• August 25th 2023. Sumit Kumar (AEI Hannover)
  Title: Gravitational Wave Mergers and their Implications for Cosmology and Fundamental Physics

  Abstract: In recent years, with the help of LIGO-Virgo detector network, the catalog of Gravitational Waves (GW) merger events have grown to about 90 merger events. This brings us to from the era of GW astronomy. In this talk, I will briefly discuss the techniques used to detect and compile the catalog of GW mergers (from searches to source properties estimation). I will then go on to discuss cosmology with GW events and how it can help us resolve so-called Hubble tension in future.  I will also discuss our recent efforts to put constraints on the mass of graviton from the observation of the binary neutron star system GW170817. The third generation (3G) GW detectors such as Cosmic Explorer and Einstein Telescope are expected to be operational sometime during next decade and they will be an order of magnitude more sensitive compared to the current generation detectors. We will be able to probe to low frequencies such as 2-5 Hz and can detect binary black hole merger events up to very large redshift. In the end, I will discuss the forecast of cosmological studies, in context of constraining modified theories of gravity, in the era of 3G detectors.

• July 14th 2023. Guillem Domenech (LUH)
  Title: Early universe cosmology of Yukawa interactions

  Abstract: Yukawa interactions result in attractive forces. In the early universe, Yukawa forces can be much stronger than gravity, leading to an instability similar to gravitational collapse very early on. I will present recent advancements on the growth of fluctuations in the very early universe from Yukawa forces, from exact analytical solutions to the very first N-body simulations. I will focus on a simple model: heavy fermions interacting via a Yukawa coupling with a scalar field in a quartic potential. I will end with a discussion on how this mechanism potentially leads to a broad phenomenology in the dark sector.