arXiv subjects

T. Prokopec

Publications and source records attributed to T. Prokopec.

33 records · Page 2Linked to original sources

arXiv2008

Infrared Propagator Corrections for Constant Deceleration

We derive the propagator for a massless, minimally coupled scalar on a $D$-dimensional, spatially flat, homogeneous and isotropic background with arbitrary constant deceleration parameter. Our construction uses the operator formalism, by integrating the Fourier mode sum. We give special attention to infrared corrections from the nonzero lower limit associated with working on finite spatial sections. These corrections eliminate infrared divergences that would otherwise be incorrectly treated by dimensional regularization, resulting in off-coincidence divergences for those special values of the deceleration parameter at which the infrared divergence is logarithmic. As an application we compute the expectation value of the scalar stress-energy tensor.

gr-qc↗

arXiv2008

Stochastic Inflationary Scalar Electrodynamics

We stochastically formulate the theory of scalar quantum electrodynamics on a de Sitter background. This reproduces the leading infrared logarithms at each loop order. It also allows one to sum the series of leading infrared logarithms to obtain explicit, nonperturbative results about the late time behavior of the system. One consequence is confirmation of the conjecture by Davis, Dimopoulos, Prokopec and Tornkvist that super-horizon photons acquire mass during inflation. We compute a photon mass-suqared of about 3.2991 H^2. The scalar stays perturbatively light with a mass-squared of about 0.8961 3 e^2 H^2/8pi^2. Interestingly, the induced change in the cosmological constant is negative, of about -0.6551 3 G H^4/pi.

gr-qc↗

arXiv2008

Two Loop Scalar Bilinears for Inflationary SQED

We evaluate the one and two loop contributions to the expectation values of two coincident and gauge invariant scalar bilinears in the theory of massless, minimally coupled scalar quantum electrodynamics on a locally de Sitter background. One of these bilinears is the product of two covariantly differentiated scalars, the other is the product of two undifferentiated scalars. The computations are done using dimensional regularization and the Schwinger-Keldysh formalism. Our results are in perfect agreement with the stochastic predictions at this order.

gr-qc↗

arXiv2006

Creation of cosmological magnetic fields in a bouncing cosmology

We show (in a completely analytical and exact manner) that an efficient magnetic field amplification method is operative during the bounce in a time-dependent gauge coupling model. The cosmological magnetic fields so generated have particular spectral features, and may be observed by future CMB measurements and by direct cluster measurements.

astro-ph↗

arXiv2006

MSSM Electroweak Baryogenesis and Flavour Mixing in Transport Equations

We make use of the formalism developed in Ref. [1], and calculate the chargino mediated baryogenesis in the Minimal Supersymmetric Standard Model. The formalism makes use of a gradient expansion of the Kadanoff-Baym equations for mixing fermions. For illustrative purposes, we first discuss the semiclassical transport equations for mixing bosons in a space-time dependent Higgs background. To calculate the baryon asymmetry, we solve a standard set of diffusion equations, according to which the chargino asymmetry is transported to the top sector, where it biases sphaleron transitions. At the end we make a qualitative and quantitative comparison of our results with the existing work. We find that the production of the baryon asymmetry of the Universe by CP-violating currents in the chargino sector is strongly constrained by measurements of electric dipole moments.

hep-ph↗

arXiv2006

Production of Massless Fermions during Inflation

We compute the one loop self energy, in a locally de Sitter background, for a massless fermion which is Yukawa-coupled to a massless, minimally coupled scalar. We then solve the modified Dirac equation resulting from inclusion of the self energy. We find faster-than-exponential growth in the fermion wave function, consistent with the production of fermions through a process in which a scalar and a fermion-anti-fermion pair are ripped out of the vacuum by inflation.

astro-ph↗

arXiv2004

Transport equations for chiral fermions to order \hbar and electroweak baryogenesis: Part II

This is the second in a series of two papers. While in Paper I we derive semiclassical Boltzmann transport equations and study their flow terms, here we address the collision terms. We use a model Lagrangean, in which fermions couple to scalars through Yukawa interactions and approximate the self-energies by the one-loop expressions. This approximation already contains important aspects of thermalization and scatterings required for quantitative studies of transport in plasmas. We compute the CP-violating contributions to both the scalar and the fermionic collision term.

hep-ph↗

arXiv2004

Transport equations for chiral fermions to order \hbar and electroweak baryogenesis: Part I

This is the first in a series of two papers. We use the Schwinger-Keldysh formalism to derive semiclassical Boltzmann transport equations for massive chiral fermions and scalar particles. Our considerations include complex mass terms and mixing fermion and scalar fields, such that CP-violation is naturally included, rendering the equations particularly suitable for studies of baryogenesis at a first order electroweak phase transition. In part II we discuss the collision terms.

hep-ph↗

arXiv2003

Vacuum polarization and photon mass in inflation

We give a pedagogical review of a mechanism through which long wave length photons can become massive during inflation. Our account begins with a discussion of the period of exponentially rapid expansion known as inflation. We next describe how, when the universe is not expanding, quantum fluctuations in charged particle fields cause even empty space to behave as a polarizable medium. This is the routinely observed phenomenon of vacuum polarization. We show that the quantum fluctuations of low mass, scalar fields are enormously amplified during inflation. If one of these fields is charged, the vacuum polarization effect of flat space is strengthened to the point that long wave length photons acquire mass. Our result for this mass is shown to agree with a simple model in which the massive photon electrodynamics of Proca emerges from applying the Hartree approximation to scalar quantum electrodynamics during inflation. A huge photon mass is not measured today because the original phase of inflation ended when the universe was only a tiny fraction of a second old. However, the zero-point energy left over from the epoch of large photon mass may have persisted during the post-inflationary universe as very weak, but cosmological-scale, magnetic fields. It has been suggested that these small seed fields were amplified by a dynamo mechanism to produce the micro-Gauss magnetic fields observed in galaxies and galactic clusters.

astro-ph↗

arXiv2002

One Loop Vaccum Polarization in a Locally de Sitter Background

We compute the one loop vacuum polarization from massless, minimally coupled scalar QED in a locally de Sitter background. Gauge invariance is maintained through the use of dimensional regularization, whereas conformal invariance is explicitly broken by the scalar kinetic term as well as through the conformal anomaly. A fully renormalized result is obtained. The one loop corrections to the linearized, effective field equations do not vanish when evaluated on-shell. In fact the on-shell one loop correction depends quadratically on the inflationary scale factor, similar to a photon mass. The contribution from the conformal anomaly is insignificant by comparison.

gr-qc↗

arXiv2001

Natural Magnetogenesis from Inflation

We consider the gravitational generation of the massive Z-boson field of the standard model, due to the natural breaking of its conformal invariance during inflation. The electroweak symmetry restoration at the end of inflation turns the almost scale-invariant superhorizon Z-spectrum into a hypermagnetic field, which transforms into a regular magnetic field at the electroweak phase transition. The mechanism is generic and is shown to generate a superhorizon spectrum of the form B~1/L on a length-scale L regardless of the choice of inflationary model. Scaled to the epoch of galaxy formation such a field suffices to trigger the galactic dynamo and explain the observed galactic magnetic fields in the case of a spatially flat, dark energy dominated Universe with GUT-scale inflation. The possibility of further amplification of the generated field by preheating is also investigated. To this end we study a model of Supersymmetric Hybrid Inflation with a Flipped SU(5) grand unified symmetry group.

astro-ph↗

arXiv2000

Baryogenesis from `electrogenesis' in a scalar field dominated epoch

Scalar fields can play a dominant role in the dynamics of the Universe until shortly before nucleosynthesis. Examples are provided by domination by a kinetic mode of a scalar field, which may be both the inflaton and the late time `quintessence', and also by more conventional models of reheating. The resultant modification to the pre-nucleosynthesis expansion rate can allow solely an asymmetry in right handed electrons to produce a net baryon asymmetry when reprocessed by the anomalous B+L violating processes of the standard model. The production of such a source asymmetry - what we term `electrogenesis' - requires no additional B or L violation beyond that in the standard model. We consider a specific model for its generation, by a simple perturbative out of equilibrium decay of Higgs like scalar fields with CP-violating Yukawa couplings to the standard model leptons. We show that, because of the much enhanced expansion rate, such a mechanism can easily produce an adequate asymmetry from scalars with masses as low as 1 TeV. Kinetic mode domination is strongly favoured because it evades large entropy release which dilutes the asymmetry. We also discuss briefly the effect of the abelian hypercharge anomaly.

hep-ph↗

arXiv1994

Constraints and Transport in Electroweak Baryogenesis

In unconstrained thermal equilibrium a local potential for total or fermionic hypercharge does not bias electroweak anomalous processes. We consider two proposed mechanisms for electroweak baryogenesis in this light. In `spontaneous' baryogenesis, which was argued to apply in the `adiabatic' limit of thick, slow walls, a non-zero result was obtained by setting globally conserved charges to be zero {\it locally}. We show that this is a poor approximation unless the walls are very thick. For more realistic wall thicknesses the local equilibrium approached as the wall velocity $v_w\rightarrow 0$ has zero baryon number violation and nonzero global charges on the wall. In the `charge transport' mechanism, argued to apply to the case of thin fast walls, calculations of the magnitude of the asymmetry also involve the same error. In corrected calculations the local values of global charges should be determined dynamically rather than fixed locally to zero.

hep-ph↗

arXiv1992

The Entropy of the Gravitational Field

We derive a formula for the nonequilibrium entropy of a classical stochastic field in terms of correlation functions of this field. The formalism is then applied to define the entropy of gravitational perturbations (both gravitational waves and density fluctuations). We calculate this entropy in a specific cosmological model (the inflationary Universe) and find that on scales of interest in cosmology the entropy in both density perturbations and gravitational waves exceeds the entropy of statistical fluctuations of the microwave background. The nonequilibrium entropy discussed here is a measure of loss of information about the system. We discuss the origin of the entropy in our cosmological models and compare the definition of entropy in terms of correlation functions with the microcanonical definition in quantum statistical mechanics.

gr-qc↗

arXiv1992

Entropy of a Classical Stochastic Field and Cosmological Perturbation

We propose a general definition of nonequilibrium entropy of a classical stochastic field. As an example of particular interest in cosmology we apply this definition to compute the entropy of density perturbations in an inflationary Universe. On the scales of structures in the Universe, the entropy of density perturbations dominates over the statistical fluctuations of the entropy of cosmic microwave photons, indicating the relevance of the entropy of density fluctuations for structure formation.

astro-ph↗