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Register a new userGravitational Leptogenesis in Bounce Cosmology
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Authors
Neil D. Barrie
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We investigate whether successful Gravitational Leptogenesis can take place during an Ekpyrotic contraction phase. Two possible paths by which this can occur are coupling the Ekpyrotic scalar to a gravitational Chern-Simons term, or to a $ U(1) $ gauge field Chern-Simons term. These couplings lead to the production of chiral gravitational waves, which generate a lepton number asymmetry through the gravitational-lepton number anomaly. This lepton asymmetry is subsequently reprocessed by equilibrium sphaleron processes to produce a baryon asymmetry. We find successful Gravitational Leptogenesis to be possible in Ekpyrotic bounce cosmologies through both of these mechanisms.
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Radiatively-induced gravitational leptogenesis is a potential mechanism to explain the observed matter-antimatter asymmetry of the universe. Gravitational tidal effects at the quantum loop level modify the dynamics of the leptons in curved spacetime and may be encoded in a low-energy effective action Seff. It has been shown in previous work how in a high-scale BSM theory the CP odd curvature-induced interactions in Seff modify the dispersion relations of leptons and antileptons differently in an expanding universe, giving rise to an effective chemical potential and a non-vanishing equilibrium lepton-antilepton asymmetry. In this paper, the CP even curvature interactions are shown to break lepton number current conservation and modify the evolution of the lepton number density as the universe expands. These effects are implemented in a generalised Boltzmann equation and used to trace the dynamical evolution of the lepton number density in different cosmological scenarios. The theory predicts a potentially significant gravitationally-induced lepton-antilepton asymmetry at very early times in the evolution of the universe.
An oscillating universe cycles through a series of expansions and contractions. We propose a model in which ``phantom energy with $p < -rho$ grows rapidly and dominates the late-time expanding phase. The universes energy density is so large that the effects of quantum gravity are important at both the beginning and the end of each expansion (or contraction). The bounce can be caused by high energy modifications to the Friedmann equation, which make the cosmology nonsingular. The classic black hole overproduction of oscillating universes is resolved due to their destruction by the phantom energy.
We explore the possibility of an Ekpyrotic contraction phase harbouring a mechanism for Baryogenesis. A Chern-Simons coupling between the fast-rolling Ekpyrotic scalar and the Standard Model Hypercharge gauge field enables the generation of a non-zero helicity during the contraction phase. The baryon number subsequently produced at the Electroweak Phase Transition is consistent with observation for a range of couplings and bounce scales. Simultaneously, the gauge field production during the contraction provides the seeds for galactic magnetic fields and sources gravitational waves, which may provide additional avenues for observational confirmation.
We show that the f(T) gravitational paradigm, in which gravity is described by an arbitrary function of the torsion scalar, can provide a mechanism for realizing bouncing cosmologies, thereby avoiding the Big Bang singularity. After constructing the simplest version of an f(T) matter bounce, we investigate the scalar and tensor modes of cosmological perturbations. Our results show that metric perturbations in the scalar sector lead to a background-dependent sound speed, which is a distinguishable feature from Einstein gravity. Additionally, we obtain a scale-invariant primordial power spectrum, which is consistent with cosmological observations, but suffers from the problem of a large tensor-to-scalar ratio. However, this can be avoided by introducing extra fields, such as a matter bounce curvaton.
We propose a scenario of spontaneous leptogenesis in Higgs inflation with help from two additional operators: the Weinberg operator (Dim 5) and the derivative coupling of the Higgs field and the current of lepton number (Dim 6). The former is responsible for lepton number violation and the latter induces chemical potential for lepton number. The period of rapidly changing Higgs field, naturally realized in Higgs inflation during the reheating, allows large enhancement in the produced asymmetry in lepton number, which is eventually converted into baryon asymmetry of the universe. This scenario is compatible with high reheating temperature of Higgs inflation model.
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