STRINGS, GRAVITY AND PARTICLES COLLOQUIUM

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• Title and abstract
  Title: Compact objects in and beyond the Standard Model.

  Abstract: Compact objects are unique probes of the strong gravity regime and may be the key to understanding long-standing puzzles in fundamental physics. These include the nature of dark matter, the possible extension of Einstein's gravity, and the fate of spacetime singularities. The advent of gravitational-wave astronomy provides new observations with present and future interferometers and is a great opportunity to address such foundational issues. We consider a theory in which a real scalar field is Yukawa-coupled to a fermion and has a potential with two non-degenerate vacua. If the coupling is sufficiently strong, a collection of N fermions deforms the true vacuum state, creating energetically-favored false-vacuum pockets in which fermions are trapped. We embed this model within General Relativity and prove that it admits self-gravitating compact objects where the scalar field acquires a non-trivial profile due to non-perturbative effects. We discuss some applications of this general mechanism in and beyond the Standard Model.

Upcoming colloquia

Past colloquia

• May 3rd, 2024. Sergei V Ketov (TMU and Kavli IPMU, Japan)
  Title: Primordial black holes as a probe of high-scale inflation and supergravity
  
  Abstract: The basic models of cosmological inflation (Starobinsky vs. Higgs) will be reviewed and compared. The production mechanisms of primordial black holes in single-field vs. multi-field models of inflation will be briefly described, together with their extensions to supergravity. Possible connections to dark energy, dark matter, and the Minimal Supersymmetric Standard Model will be outlined.

• April 12th, 2024. Anna Negro (Leiden Univ.)
  Title: Regularization and Renormalization of Divergences in Primordial Observables -- Vacuum GWs and N_eff Bounds Revisited

  Abstract: In the literature, constraints on primordial vacuum gravitational waves (GWs) -- tensorial counterpart of scalar quantum fluctuations that seed galaxies -- are often based on upper bounds on the effective number of species N_eff derived from CMB data. During my talk, I will examine the validity of this derivation. To that end, I will start by reviewing how "quantum corrections" should be included in computing observables, and show how UV divergences can be consistently renormalized in cosmological settings. Building on the example of a finite inflationary background evolution, I will demonstrate that while physical scales corresponding to the beginning and end of inflation can regulate IR divergences, they do not regulate UV divergences. In addition, I will derive a new formula for the energy density of GWs that does not assume a prior scale separation (as opposed to that of Isaacson). With this final ingredient at hand, I will rederive the contribution of primordial vacuum GWs to N_eff, and address the question: can we use N_eff bounds to constrain primordial vacuum GWs? This talk is based on: arXiv:2402.10008 , arXiv:2403.16806.

• March 22nd, 2024. Julián Rey (DESY, Hamburg)
  Title: Baryon asymmetry, gravitational waves and non-Gaussianities from primordial black hole formation

  Abstract: We consider black hole formation due to the gravitational collapse produced by large density fluctuations generated during inflation. We show that non-Gaussian tails in the probability density function of curvature perturbations arise in these models from the delta N formalism, without resorting to stochastic inflation. Assuming the black holes form during an epoch of reheating with a stiff equation of state, we calculate the induced gravitational wave spectrum and constrain the parameter space by considering existing bounds on the total energy density of gravitational waves today. Finally, we calculate the lepton asymmetry generated by metric perturbations via the chiral gravitational anomaly and find that the large spectrum of scalar perturbations responsible for black hole formation induces a peak in the baryon asymmetry fluctuations on small scales.

• February 9th, 2024. Alex Vikman (CEICO, Prague)
  Title: DM Melting Domain Walls and NANOGrav Gravitational Waves

  Abstract: I will discuss cosmological domain walls which are described by tension red-shifting with the expansion of the Universe so that this network eventually fades away completely. These melting domain walls emit gravitational waves with the low-frequency spectral shape corresponding to the spectral index γ=3 favoured by the recent NANOGrav 15 yrs data. This scenario involves a feebly coupled scalar field, which can serve as a promising dark matter candidate. This ultra-light dark matter has mass below 0.01 neV which is accessible through planned observations thanks to the effects of superradiance of rotating black holes. This talk is based on recent works:  arXiv:2104.13722, arXiv:2112.12608 and arXiv:2307.04582.

• January 24th, 2024. Henning Bahl (ITP Heidelberg)
  Title: Deciphering the Nature of the Higgs boson
  
  Abstract: The Higgs sector is linked to many challenges faced by the Standard Model. This provides a strong motivation for the ongoing Higgs precision program. In this talk, I will discuss the current status of Higgs measurements focusing on what remains to be explored. In particular, I will focus on the Higgs potential and the CP nature of the Higgs boson.

• January 8th, 2024. Florian Kühnel (LMU, Munich)
  Title: Primordial Black Holes as Dark Matter
  
  Abstract: Primordial black holes are black holes that may have formed in the early Universe. Their masses potentially span a range from as low as the Planck mass up to many orders of magnitude above the solar mass. This, in particular, includes those black holes recently discovered by LIGO/Virgo, and these may conceivably be of primordial origin. Furthermore, there are now numerous recent hints for compact bodies constituting (part of) the dark matter. After a general introduction of the topic, I will talk about those, their connection and future detection possibilities.

• December 1st 2023. Mohammad Ali Gorji (University of Barcelona)
  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]

• 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)
  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.