arXiv subjects

Evangelos I. Sfakianakis

Publications and source records attributed to Evangelos I. Sfakianakis.

46 records · Page 3Linked to original sources

arXiv2016

Magnetogenesis from axion inflation

In this work we compute the production of magnetic fields in models of axion inflation coupled to the hypercharge sector of the Standard Model through a Chern-Simons interaction term. We make the simplest choice of a quadratic inflationary potential and use lattice simulations to calculate the magnetic field strength, helicity and correlation length at the end of inflation. For small values of the axion-gauge field coupling strength the results agree with no-backreaction calculations and estimates found in the literature. For larger couplings the helicity of the magnetic field differs from the no-backreaction estimate and depends strongly on the comoving wavenumber. We estimate the post-inflationary evolution of the magnetic field based on known results for the evolution of helical and non-helical magnetic fields. The magnetic fields produced by axion inflation with large couplings to $U(1)_Y$ can reach $B_{\rm eff} \gtrsim 10^{-16}\, {\rm G}$, exhibiting a field strength $B_{\rm phys} \approx 10^{-13}\, {\rm G}$ and a correlation length $λ_{\rm phys}\approx10\, {\rm pc}$. This result is insensitive to the exact value of the coupling, as long as the coupling is large enough to allow for instantaneous preheating. Depending on the assumptions for the physical processes that determine blazar properties, these fields can be found consistent with blazar observations based on the value of $B_{\rm eff}$. Finally, the intensity of the magnetic field for large coupling can be enough to satisfy the requirements for a recently proposed baryogenesis mechanism, which utilizes the chiral anomaly of the Standard Model.

astro-ph.CO↗

arXiv2016

Higgsed Chromo-Natural Inflation

We demonstrate that Chromo-Natural Inflation can be made consistent with observational data if the SU(2) gauge symmetry is spontaneously broken. Working in the Stueckelberg limit, we show that isocurvature is negligible, and the resulting adiabatic fluctuations can match current observational constraints. Observable levels of chirally-polarized gravitational radiation ($r\sim 10^{-3}$) can be produced while the evolution of all background fields is sub-Planckian. The gravitational wave spectrum is amplified via linear mixing with the gauge field fluctuations, and its amplitude is not simply set by the Hubble rate during inflation. This allows observable gravitational waves to be produced for an inflationary energy scale below the GUT scale. The tilt of the resulting gravitational wave spectrum can be either blue or red.

hep-th↗

arXiv2016

Leptogenesis from left-handed neutrino production during axion inflation

We propose that the observed matter-antimatter asymmetry can be naturally produced as a byproduct of axion-driven slow-roll inflation by coupling the axion to standard-model neutrinos. We assume that GUT scale right-handed neutrinos are responsible for the masses of the standard model neutrinos and that the Higgs field is light during inflation and develops a Hubble scale vacuum expectation value (VEV). In this set up, the rolling axion generates a helicity asymmetry in standard-model neutrinos. Following inflation, this helicity asymmetry becomes equal to a net lepton number as the Higgs VEV decays and is partially re-processed by the $SU(2)_{L}$ sphaleron into a net baryon number.

hep-ph↗

arXiv2015

A Density Spike on Astrophysical Scales from an N-Field Waterfall Transition

Hybrid inflation models are especially interesting as they lead to a spike in the density power spectrum on small scales, compared to the CMB, while also satisfying current bounds on tensor modes. Here we study hybrid inflation with $N$ waterfall fields sharing a global $SO(N)$ symmetry. The inclusion of many waterfall fields has the obvious advantage of avoiding topologically stable defects for $N>3$. We find that it also has another advantage: it is easier to engineer models that can simultaneously (i) be compatible with constraints on the primordial spectral index, which tends to otherwise disfavor hybrid models, and (ii) produce a spike on astrophysically large length scales. The latter may have significant consequences, possibly seeding the formation of astrophysically large black holes. We calculate correlation functions of the time-delay, a measure of density perturbations, produced by the waterfall fields, as a convergent power series in both $1/N$ and the field's correlation function $Δ(x)$. We show that for large $N$, the two-point function is $<δt({\bf x})\,δt({\bf 0})>\,\proptoΔ^2(|{\bf x}|)/N$ and the three-point function is $<δt({\bf x})\,δt({\bf y})\,δt({\bf 0})>\,\proptoΔ(|{\bf x}-{\bf y}|)Δ(|{\bf x}|)Δ(|{\bf y}|)/N^2$. In accordance with the central limit theorem, the density perturbations on the scale of the spike are Gaussian for large $N$ and non-Gaussian for small $N$.

astro-ph.CO↗

arXiv2014

Multifield Inflation after Planck: Isocurvature Modes from Nonminimal Couplings

Recent measurements by the {\it Planck} experiment of the power spectrum of temperature anisotropies in the cosmic microwave background radiation (CMB) reveal a deficit of power in low multipoles compared to the predictions from best-fit $Λ$CDM cosmology. The low-$\ell$ anomaly may be explained by the presence of primordial isocurvature perturbations in addition to the usual adiabatic spectrum, and hence may provide the first robust evidence that early-universe inflation involved more than one scalar field. In this paper we explore the production of isocurvature perturbations in nonminimally coupled two-field inflation. We find that this class of models readily produces enough power in the isocurvature modes to account for the \emph{Planck} low-$\ell$ anomaly, while also providing excellent agreement with the other {\it Planck} results.

astro-ph.CO↗

arXiv2014

Multifield Inflation after Planck: The Case for Nonminimal Couplings

Multifield models of inflation with nonminimal couplings are in excellent agreement with the recent results from {\it Planck}. Across a broad range of couplings and initial conditions, such models evolve along an effectively single-field attractor solution and predict values of the primordial spectral index and its running, the tensor-to-scalar ratio, and non-Gaussianities squarely in the observationally most-favored region. Such models also can amplify isocurvature perturbations, which could account for the low power recently observed in the CMB power spectrum at low multipoles. Future measurements of primordial isocurvature perturbations and the tensor-to-scalar ratio may serve to distinguish between the currently viable possibilities.

astro-ph.CO↗

arXiv2013

Primordial Bispectrum from Multifield Inflation with Nonminimal Couplings

Realistic models of high-energy physics include multiple scalar fields. Renormalization requires that the fields have nonminimal couplings to the spacetime Ricci curvature scalar, and the couplings can be large at the energy scales of early-universe inflation. The nonminimal couplings induce a nontrivial field-space manifold in the Einstein frame, and they also yield an effective potential in the Einstein frame with nontrivial curvature. The ridges or bumps in the Einstein-frame potential can lead to primordial non-Gaussianities of observable magnitude. We develop a covariant formalism to study perturbations in such models and calculate the primordial bispectrum. As in previous studies of non-Gaussianities in multifield models, our results for the bispectrum depend sensitively on the fields' initial conditions.

astro-ph.CO↗

arXiv2013

Multifield Dynamics of Higgs Inflation

Higgs inflation is a simple and elegant model in which early-universe inflation is driven by the Higgs sector of the Standard Model. The Higgs sector can support early-universe inflation if it has a large nonminimal coupling to the Ricci spacetime curvature scalar. At energies relevant to such an inflationary epoch, the Goldstone modes of the Higgs sector remain in the spectrum in renormalizable gauges, and hence their effects should be included in the model's dynamics. We analyze the multifield dynamics of Higgs inflation and find that the multifield effects damp out rapidly after the onset of inflation, because of the gauge symmetry among the scalar fields in this model. Predictions from Higgs inflation for observable quantities, such as the spectral index of the power spectrum of primordial perturbations, therefore revert to their familiar single-field form, in excellent agreement with recent measurements. The methods we develop here may be applied to any multifield model with nonminimal couplings in which the ${\cal N}$ fields obey an $SO ({\cal N})$ symmetry in field space.

hep-ph↗

arXiv2012

Density Perturbations in Hybrid Inflation Using a Free Field Theory Time-Delay Approach

We introduce a new method for calculating density perturbations in hybrid inflation which avoids treating the fluctuations of the "waterfall" field as if they were small perturbations about a classical trajectory. We quantize only the waterfall field, treating it as a free quantum field with a time-dependent $m^2$, which evolves from positive values to tachyonic values. Although this potential has no minimum, we think it captures the important dynamics that occurs as $m^2$ goes through zero, at which time a large spike in the density perturbations is generated. We assume that the time-delay formalism provides an accurate approximation to the density perturbations, and proceed to calculate the power spectrum of the time delay fluctuations. While the evolution of the field is linear, the time delay is a nonlinear function to which all modes contribute. Using the Gaussian probability distribution of the mode amplitudes, we express the time-delay power spectrum as an integral which can be carried out numerically. We use this method to calculate numerically the spectrum of density perturbations created in hybrid inflation models for a wide range of parameters. A characteristic of the spectrum is the appearance of a spike at small length scales, which can be used to relate the model parameters to observational data. It is conceivable that this spike could seed the formation of black holes that can evolve to become the supermassive black holes found at the centers of galaxies.

astro-ph.CO↗

arXiv2012

Analysis of Oscillons in the SU(2) Gauged Higgs Model

A stable oscillon was found in the (classical) analysis of the SU(2) gauged Higgs model under a 2:1 ratio between the Higgs and W masses, both analytically and numerically. We extend these results by constructing oscillons for all values of the Higgs mass and examining their stability, finding them to be stable when the Higgs mass is almost equal to twice the W mass. We also investigate the dependence of the oscillon lifetime on the mass of the Higgs, seeing different behavior for different regions of the Higgs mass. Close to the 2:1 mass ratio the interaction of the two fields leads to either a stable oscillon or to its quick decay, based on its amplitude and the exact masses of the fields. Moving further away from this ratio the fields seize to interact and the oscillon disperses. Furthermore, we show that the oscillon lifetime scales quadratically with the small amplitude parameter which is linked to the small amplitude expansion that we use to construct the oscillon. Finally, we list all families of oscillon solutions that can be formally constructed in this model and discuss their properties. The results can be easily extended to other composite oscillons in particle physics and beyond.

hep-ph↗