arXiv subjects

John T. Giblin Jr

Publications and source records attributed to John T. Giblin Jr.

At least 37 records · Page 2Linked to original sources

arXiv2018

Gravitational waves from gauge preheating

We study gravitational wave production during Abelian gauge-field preheating following inflation. We consider both scalar and pseudoscalar inflaton models coupled directly to Abelian gauge fields via either a dilatonic coupling to the gauge-field kinetic term or an axial coupling to a Chern-Simons term. In both cases gravitational waves are produced efficiently during the preheating phase, with a signature louder than most cosmological signals. These gravitational waves can contribute to the radiation energy budget of Universe at a level which will be probed by upcoming cosmic microwave background experiments through $N_{\rm eff}$. For axially coupled fields the resulting gravitational wave spectrum is helically polarized---a unique feature that can be used to differentiate it from other stochastic gravitational wave backgrounds. We compute the gravitational topological charge and demonstrate that gauge preheating following axion inflation may be responsible for the matter-antimatter asymmetry of the Universe via gravitational leptogenesis.

astro-ph.CO↗

arXiv2018

Non-Abelian gauge preheating

We study preheating in models where a scalar inflaton is directly coupled to a non-Abelian $SU(2)$ gauge field. In particular, we examine $m^2 ϕ^2$ inflation with a conformal, dilaton-like coupling to the non-Abelian sector. We describe a numerical scheme that combines lattice gauge theory with standard finite difference methods applied to the scalar field. We show that a significant tachyonic instability allows for efficient preheating, which is parametrically suppressed by increasing the non-Abelian self-coupling. Additionally, we comment on the technical implementation of the evolution scheme and setting initial conditions.

hep-ph↗

arXiv2017

General Relativistic Corrections to the Weak Lensing Convergence Power Spectrum

We compute the weak lensing convergence power spectrum, $C^{κκ}(θ)$, in a dust-filled universe using fully non-linear general relativistic simulations. The spectrum is then compared to more standard, approximate calculations by computing the Bardeen (Newtonian) potentials in linearized gravity and utilizing the Born approximation. We find corrections to the angular power spectrum amplitude of order ten percent at very large angular scales, $\ell ~ 2-3$, and percent-level corrections at intermediate angular scales of $\ell ~ 20-30$.

astro-ph.CO↗

arXiv2017

A cosmologically motivated reference formulation of numerical relativity

The application of numerical relativity to cosmological spacetimes is providing new insights into the behavior of Einstein's equations, beyond common approximations. In order for simulations to be performed as accurately and efficiently as possible, we investigate a novel formulation of Einstein's equations. This formulation evolves differences from a "reference" solution describing the dominant behavior of the spacetime, which mitigates error due to both truncation and approximate finite difference calculations. We find that the error in solutions obtained using the reference formulation can be smaller by an order of magnitude or more, with the level of improvement depending on how well the reference solution approximates the exact solution.

gr-qc↗

arXiv2016

Gauge-preheating and the end of axion inflation

We study the onset of the reheating epoch at the end of axion-driven inflation where the axion is coupled to an Abelian, $U(1)$, gauge field via a Chern-Simons interaction term. We focus primarily on $m^2ϕ^2$ inflation and explore the possibility that preheating can occur for a range of coupling values consistent with recent observations and bounds on the overproduction of primordial black holes. We find that for a wide range of parameters preheating is efficient. In certain cases the inflaton is seen to transfer all its energy to the gauge fields within a few oscillations. In most cases, we find that the gauge fields on sub-horizon scales end preheating in an unpolarized state due to the existence of strong rescattering between the inflaton and gauge-field modes. We also present a preliminary study of an axion monodromy model coupled to $U(1)$ gauge fields, seeing a similarly efficient preheating behavior as well as indications that the coupling strength has an effect on the creation of oscillons.

astro-ph.CO↗

arXiv2016

Observable Deviations from Homogeneity in an Inhomogeneous Universe

How does inhomogeneity affect our interpretation of cosmological observations? It has long been wondered to what extent the observable properties of an inhomogeneous universe differ from those of a corresponding Friedman-Lemaitre-Robertson-Walker (FLRW) model, and how the inhomogeneities affect that correspondence. Here, we use numerical relativity to study the behavior of light beams traversing an inhomogeneous universe and construct the resulting Hubble diagrams. The universe that emerges exhibits an average FLRW behavior, but inhomogeneous structures contribute to deviations in observables across the observer's sky. We also investigate the relationship between angular diameter distance and the angular extent of a source, finding deviations that grow with source redshift. These departures from FLRW are important path-dependent effects with implications for using real observables in an inhomogeneous universe such as our own.

astro-ph.CO↗

arXiv2016

Integration of inhomogeneous cosmological spacetimes in the BSSN formalism

We present cosmological-scale numerical simulations of an evolving universe in full general relativity (GR) and introduce a new numerical tool, {\sc CosmoGRaPH}, which employs the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formalism on a 3-dimensional grid. Using {\sc CosmoGRaPH}, we calculate the effect of an inhomogeneous matter distribution on the evolution of a spacetime. We also present the results of a set of standard stability tests to demonstrate the robustness of our simulations.

gr-qc↗

arXiv2016

Departures from the FLRW Cosmological Model in an Inhomogeneous Universe: A Numerical Examination

While the use of numerical general relativity for modeling astrophysical phenomena and compact objects is commonplace, the application to cosmological scenarios is only just beginning. Here, we examine the expansion of a spacetime using the Baumgarte-Shapiro-Shibata-Nakamura (BSSN) formalism of numerical relativity in synchronous gauge. This work represents the first numerical cosmological study that is fully relativistic, non-linear and without symmetry. The universe that emerges exhibits an average Friedmann-Lemaïtre-Robertston-Walker (FLRW) behavior, however this universe also exhibits locally inhomogeneous expansion beyond that expected in linear perturbation theory around a FLRW background.

gr-qc↗

arXiv2014

Gravitional radiation from first-order phase transitions in the presence of a fluid

First-order phase transitions are a source of stochastic gravitational radiation. Precision calculations of the gravitational waves emitted during these processes, sourced by both the degrees of freedom undergoing the transition and anisotropic stress of the coupled, ambient constituents, have reached an age of maturity. Here we present high-resolution numerical simulations of a scalar field coupled to a fluid and parameterize the final gravitational wave spectrum as a function of the ratio of the energies of the two sectors and the coupling between the two sectors for a set of models that represent different types of first-order phase transitions. In most cases, the field sector is the dominant source of gravitational radiation, but it is possible in certain scenarios for the fluid to have the most important contribution.

astro-ph.CO↗

arXiv2014

Vacuum Bubbles in the Presence of a Relativistic Fluid

First order phase transitions are characterized by the nucleation and evolution of bubbles. The dynamics of cosmological vacuum bubbles, where the order parameter is independent of other degrees of freedom, are well known; more realistic phase transitions in which the order parameter interacts with the other constituents of the Universe is in its infancy. Here we present high-resolution lattice simulations that explore the dynamics of bubble evolution in which the order parameter is coupled to a relativistic fluid. We use a generic, toy potential, that can mimic physics from the GUT scale to the electroweak scale.

hep-th↗

arXiv2013

Preheating with Non-Minimal Kinetic Terms

We present the first 3+1-dimensional numerical simulations of scalar fields with non-minimal kinetic terms. As an example, we examine the existence and stability of preheating in the presence of a Dirac-Born-Infeld (DBI) inflaton coupled to a canonical matter field. The simulations represent the full non-linear theory in the presence of an expanding Universe. We show that parametric resonance in the matter field, along with self-resonance in the inflaton, repopulate the Universe with matter particles as efficiently as in traditional preheating.

astro-ph.CO↗

arXiv2013

Gauge Field Preheating at the End of Inflation

Here we consider the possibility of preheating the Universe via the parametric amplification of a massless, U(1) abelian gauge field. We assume that the gauge field is coupled to the inflaton via a conformal factor with one free parameter. We present the results of high-resolution three-dimensional simulations of this model and show this mechanism efficiently preheats the Universe to a radiation-dominated final state.

astro-ph.CO↗

arXiv2012

Long-lived Positronium and AGN Jets

We suggest that stable states of positronium might exist in the jets of active galactic nuclei (AGN). Electrons and positrons are created near the accretion disks of supermassive black holes at the centers of AGN and are accelerated along magnetic field lines while within the {\sl Alfvén radius}. The conditions in this region are ideal for the creation of bound states of positronium which are stable against annihilation. Traveling at relativistic speeds along the jet, the helical magnetic field enables the atoms to survive for great distances.

astro-ph.HE↗

arXiv2012

Gravitational Radiation from First-Order Phase Transitions

It is believed that first-order phase transitions at or around the GUT scale will produce high-frequency gravitational radiation. This radiation is a consequence of the collisions and coalescence of multiple bubbles during the transition. We employ high-resolution lattice simulations to numerically evolve a system of bubbles using only scalar fields, track the anisotropic stress during the process and evolve the metric perturbations associated with gravitational radiation. Although the radiation produced during the bubble collisions has previously been estimated, we find that the coalescence phase enhances this radiation even in the absence of a coupled fluid or turbulence. We comment on how these simulations scale and propose that the same enhancement should be found at the Electroweak scale; this modification should make direct detection of a first-order electroweak phase transition easier.

astro-ph.CO↗

arXiv2012

Enhanced Preheating after Multi-Field Inflation: On the Importance of being Special

We discuss preheating after multi-field inflation in the presence of several preheat matter fields that become light in the vicinity of (but not at) the inflatons' VEV, at distinct extra-species-points (ESP); this setup is motivated by inflationary models that include particle production during inflation, e.g.trapped inflation, grazing ESP encounters or modulated trapping, among others. While de-phasing of inflatons tends to suppress parametric resonance, we find two new effects leading to efficient preheating: particle production during the first in-fall (efficient if many preheat matter fields are present) and a subsequent (narrow) resonance phase (efficient if an ESP happens to be at one of several distinct distances from the inflatons' VEV). Particles produced during the first in-fall are comprised of many species with low occupation number, while the latter are made up of a few species with high occupation number. We provide analytic descriptions of both phases in the absence of back-reaction, which we test numerically. We further perform lattice simulations to investigate the effects of back-reaction. We find resonances to be robust and the most likely cause of inflaton decay in multi-field trapped inflation if ESP distributions are dense.

astro-ph.CO↗

arXiv2011

Gravitational Waves from Global Second Order Phase Transitions

Global second-order phase transitions are expected to produce scale-invariant gravitational wave spectra. In this manuscript we explore the dynamics of a symmetry-breaking phase transition using lattice simulations. We explicitly calculate the stochastic gravitational wave background produced during the transition and subsequent self-ordering phase. We comment on this signal as it compares to the scale-invariant spectrum produced during inflation.

astro-ph.CO↗

arXiv2010

Gravitational Radiation from Preheating with Many Fields

Parametric resonances provide a mechanism by which particles can be created just after inflation. Thus far, attention has focused on a single or many inflaton fields coupled to a single scalar field. However, generically we expect the inflaton to couple to many other relativistic degrees of freedom present in the early universe. Using simulations in an expanding Friedmann-Lemaître-Robertson-Walker spacetime, in this paper we show how preheating is affected by the addition of multiple fields coupled to the inflaton. We focus our attention on gravitational wave production--an important potential observational signature of the preheating stage. We find that preheating and its gravitational wave signature is robust to the coupling of the inflaton to more matter fields.

astro-ph.CO↗

arXiv2010

How to Run Through Walls: Dynamics of Bubble and Soliton Collisions

It has recently been shown in high resolution numerical simulations that relativistic collisions of bubbles in the context of a multi-vacua potential may lead to the creation of bubbles in a new vacuum. In this paper, we show that scalar fields with only potential interactions behave like free fields during high-speed collisions; the kick received by them in a collision can be deduced simply by a linear superposition of the bubble wall profiles. This process is equivalent to the scattering of solitons in 1+1 dimensions. We deduce an expression for the field excursion (shortly after a collision), which is related simply to the field difference between the parent and bubble vacua, i.e. contrary to expectations, the excursion cannot be made arbitrarily large by raising the collision energy. There is however a minimum energy threshold for this excursion to be realized. We verify these predictions using a number of 3+1 and 1+1 numerical simulations. A rich phenomenology follows from these collision induced excursions - they provide a new mechanism for scanning the landscape, they might end/begin inflation, and they might constitute our very own big bang, leaving behind a potentially observable anisotropy.

hep-th↗