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Abstract Weyl fermions of spin 1 2 $$ \frac{1}{2} $$ minimally coupled to Einstein’s gravity in 4 dimensions cannot be produced purely gravitationally in an expanding Universe at tree level. Surprisingly, as we showed in a recent letter [1], this changes at gravitational 1-loop when cosmic perturbations, like a gravitational wave background, are present. Such a background introduces a new scale, thereby breaking the fermions’ conformal invariance. This leads to a non-vanishing gravitational self-energy for Weyl fermions at 1-loop and induces their production. In this paper, we present an extended study of this new mechanism, explicitly computing this effect using the in-in formalism. We work in an expanding Universe in the radiation-dominated era as a fixed background. Gravitational wave-induced fermion production has rich phenomenological consequences. Notably, if Weyl fermions eventually acquire mass, and assuming realistic — and potentially detectable — gravitational wave backgrounds, the mechanism can explain the abundance of dark matter in the Universe. More generally, gravitational-wave induced freeze-in is a new purely gravitational mechanism for generating other feebly interacting fermions, e.g. right-handed neutrinos. We show that this loop level effect can dominate over the conventional — tree-level — gravitational production of superheavy fermions in a sizable part of the parameter space ( https://github.com/koppj/GW-freeze-in/ ).

Abstract The Laser Interferometer Space Antenna (LISA) has two scientific objectives of cosmological focus: to probe the expansion rate of the universe, and to understand stochastic gravitational-wave backgrounds and their implications for early universe and particle physics, from the MeV to the Planck scale. However, the range of potential cosmological applications of gravitational-wave observations extends well beyond these two objectives. This publication presents a summary of the state of the art in LISA cosmology, theory and methods, and identifies new opportunities to use gravitational-wave observations by LISA to probe the universe.

Abstract This work introduces the chiral memory effect on the celestial sphere that measures the permanent change of electromagnetic fields by spin-dependent processes in bulk. Unlike the conventional memory effect based on the permanent soft shift in the gauge field itself, it is a permanent change in its spin angular momentum. The concept underlying the chiral memory (conventional memory) effect is optical spin torque (optical force) induction in bulk. Photons and EM radiation carry angular momentum, which is conserved without interactions. Chiral interactions with matter, medium, curvature, and theories with parity violation, i.e., axion-QED, transfers spin angular momentum to EM fields. In nature, such phenomena occur either on EM radiation (chiral memory) or in the vacuum of QED (vacuum chiral memory). It can be parametrized in terms of the photon’s topological (axial) current at null infinity. To elude the gauge ambiguity of the topological current, we use the transverse gauge and show it is the physical part of the current suggested by its cohomology structure.

Abstract Upon embedding the axion-inflation in the minimal left-right symmetric gauge extension of the SM with gauge group SU(2) L × SU(2) R × U(1)B−L, [1] proposed a new particle physics model for inflation. In this work, we present a more detailed analysis. As a compelling consequence, this setup provides a new mechanism for simultaneous baryogenesis and right-handed neutrino creation by the chiral anomaly of W R in inflation. The lightest right-handed neutrino is the dark matter candidate. This setup has two unknown fundamental scales, i.e., the scale of inflation and left-right symmetry breaking SU(2) R × U(1)B−L → U(1) Y . Sufficient matter creation demands the left-right symmetry breaking scale happens shortly after the end of inflation. Interestingly, it prefers left-right symmetry breaking scales above 1010 GeV, which is in the range suggested by the non-supersymmetric SO(10) Grand Unified Theory with an intermediate left-right symmetry scale. Although W R gauge field generates equal amounts of right-handed baryons and leptons in inflation, i.e. B − L = 0, in the Standard Model sub-sector B − LSM ≠ 0. A key aspect of this setup is that SU(2) R sphalerons are never in equilibrium, and the primordial B − LSM is conserved by the Standard Model interactions. This setup yields a deep connection between CP violation in physics of inflation and matter creation (visible and dark); hence it can naturally explain the observed coincidences among cosmological parameters, i.e., η B ≃ 0.3P ζ and ΩDM ≃ 5ΩB. The new mechanism does not rely on the largeness of the unconstrained CP-violating phases in the neutrino sector nor fine-tuned masses for the heaviest right-handed neutrinos. The SU(2) R -axion inflation comes with a cosmological smoking gun; chiral, non-Gaussian, and blue-tilted gravitational wave background, which can be probed by future CMB missions and laser interferometer detectors.

Abstract Remarkably, if CP was spontaneously broken in the physics of inflation, fermions would notice and remember it. Based on that, we present a new (non-thermal) mechanism for generating self-interacting dark Dirac fermions prior to the Hot Big Bang. The non-Abelian gauge fields and axions are well-motivated matter contents for the particle physics of inflation. In this background, we analytical study Dirac fermion doublets charged under the SU(2) gauge field and use point-splitting technique to regularize the currents. We show that the non-trivial CP-violating vacuum structure of SU(2)-axion models naturally leads to an efficient mechanism for generating massive fermions during inflation. The size of the fermionic backreaction and the density fraction of dark fermions put upper bounds on the fermion’s mass. For a GUT scale inflation, the generated dark fermions, only gravitationally coupled to the visible sector, can be as heavy as m ≲ 10 TeV.