arXiv subjects

Eric D. Carlson

Publications and source records attributed to Eric D. Carlson.

18 recordsLinked to original sources

arXiv2026

Comparison of $f(R,T)$ Gravity with Multiple Datasets

We examine $f(R,T)=R+\lambda T^\epsilon$ gravity models by finding the best fit parameters for various values of $\epsilon$. This is accomplished by analyzing data from the cosmic microwave background, baryon acoustic oscillation observations, cosmic chronometer, and Type Ia supernovae introducing correlations and radiation effects to our previous work. We find the probability distribution for the best fit model around the value of $\epsilon=0.010^{+0.013}_{-0.021}$, consistent with the standard cosmological model value of $\epsilon = 0$.

gr-qc↗

arXiv2025

Exploration of Parameters in f(R,T) Gravity and Comparison with Type Ia Supernovae Data

The expansion of the Universe in $f(R,T)$ gravity is studied. We consider functions of the form $f(R,T)=R+\lambda T^\epsilon$ where $\epsilon<1$. We find that for all models with $\epsilon<0$, the Universe transitions to exponential growth at late times, just as it does in the standard cosmological model, which corresponds to $\epsilon=0$. It also fits the type Ia supernova data slightly better than the standard cosmological model, without increasing the number of parameters of the theory. In contrast, the fits for $\epsilon >0$ rapidly become worse than the standard cosmological model.

gr-qc↗

arXiv2025

Comparison of $f(R,T)$ Gravity with Type Ia Supernovae Data

The expansion of the universe in $f(R,T)$ gravity is studied. By focusing on functions of the form $f(R,T)=f_1(R)+f_2(T)$, we assert that present day acceleration can be achieved if the functional form of $f_2(T)$ either grows slowly or falls as a function of $T$. In particular, we demonstrate that when $f_2(T) \propto T^{-1}$, the universe transitions to exponential growth at late times, just as it does in the standard cosmological model. A comparison of predictions of this model, with Type Ia supernovae shows that this model fits the data as well or even slightly better than the standard cosmological model without increasing the number of parameters.

gr-qc↗

arXiv2022

Argument for the radiation-dominated behavior of matter fields in the preinflationary era

We investigate the leading-order behavior of matter fields in the preinflationary era using the semiclassical approximation. Many inflationary models assume without supporting arguments that the Universe was radiation dominated prior to inflation, leading to modifications of cosmological observables, such as the Cosmic Microwave Background power spectrum. In previous work, we demonstrated that conformally coupled scalar fields do have a radiation-like contribution to the stress-energy tensor at sufficiently early times. In this work, we extend these arguments to apply to massless spin-1 fields and massive or massless spin-$\frac{1}{2}$ fields. We find massless spin-1 fields always have a radiation-like contribution. For spin-$\frac{1}{2}$ fields, we find the contribution at early times is radiation-like assuming this is the dominant contribution to the stress-energy tensor.

gr-qc↗

arXiv2021

Nuclear Limits on Non-Minimally Coupled Gravity

We explore alternate theories of gravity where the gravitational term in the Lagrangian $\frac{R}{8πG}$ is replaced by a function $f_1(R)$ and the matter Lagrangian is multiplied by a function $f_2(R)$. We argue that nuclear physics can provide strong experimental constraints on such theories. In particular using energy conditions on the pressure in the $^4$He nucleus, for $f_1(R)=\frac{R}{8πG}$ and $f_2(R)=1+λR$, we find a limit of $|λ| < 5\times 10^{-12}\, \hbox{m}^2$, more than thirty orders of magnitude stronger than the previous limit.

gr-qc↗

arXiv2021

Semiclassical predictions regarding a pre-inflationary era and its effects on the power spectrum

An investigation is undertaken into the properties and effects of a preinflationary era during at least part of which semiclassical gravity was valid. It is argued that if the Universe (or our part of it) was approximately homogeneous and isotropic during that era, then the Universe was likely to have been radiation dominated. A simple model in which the Universe contains classical radiation and a cosmological constant is used to investigate potential effects of such a preinflationary era on the cosmic microwave background. The power spectrum is computed using the mode functions of a quantized massless minimally coupled scalar field. Various choices of state for this field are considered, including adiabatic vacuum states of various orders and the vacuum state that would naturally occur if the Universe made a sudden transition from being radiation dominated to de Sitter space. In all cases investigated, there is a suppression of the power spectrum at large angles, and, when plotted as a function of the momentum parameter, there are always oscillations with state-dependent amplitudes.

gr-qc↗

arXiv2020

Response to Comment on "Reexamining $f(R,T)$ Gravity''

Harko and Moraes claim that in $f(R,T)$ gravity with $f(R,T)=f_1(R)+f_2(T)$, the term $f_2(T)$ cannot be incorporated in the matter Lagrangian ${\cal L}_m$. A careful examination of their Comment finds that they have made several dubious assumptions without indicating any errors in our work. Most notably, they have unjustifiably claimed that the two terms ${\cal L}_m$ and $f_2(T)$ are of ``different origin,'' and their inference that only the first contributes to the energy momentum tensor is arbitrary. Also significant, their derivation of equations of motion from a Lagrangian formulation, imposing conservation constraints {\it ad hoc} rather than via Lagrange multipliers, leads to inconsistent conclusions.

gr-qc↗

arXiv2019

Reexamining $f(R,T)$ gravity

We study $f(R,T)$ gravity, in which the curvature $R$ appearing in the gravitational Lagrangian is replaced by an arbitrary function of the curvature and the trace $T$ of the stress-energy tensor. We focus primarily on situations where $f$ is separable, so that $f(R,T) = f_1(R) + f_2(T)$. We argue that the term $f_2(T)$ should be included in the matter Lagrangian ${\cal L}_m$, and therefore has no physical significance. We demonstrate explicitly how this can be done for the cases of free fields and for perfect fluids. We argue that all uses of $f_2(T)$ for cosmological modeling and all attempts to place limits on parameters describing $f_2(T)$ are misguided.

gr-qc↗

arXiv2019

Limits on $f(R,T)$ Gravity from Earth's Atmosphere

We investigate changes in Earth's atmospheric models coming from the $f(R,T)$ modified theory of gravity, in which the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar and the trace of the stress-energy tensor. We obtain a generic form for the gravitational field equations and derive the hydrostatic equation for Earth's atmosphere for leading order terms $f(R,T) = R + 2χT.$ Based on the apparent accuracy of the 1976 U.S. Standard Atmosphere model, which varies no more than $10\%$ from observations, we find limits of $-1.6\times 10^{-13} \lesssim χ\lesssim 1.8\times 10^{-13}$.

gr-qc↗

arXiv2017

Future singularities if the universe underwent Starobinsky inflation in the past

The effects which quantum fields and an $α_0 R^2$ term in the gravitational Lagrangian have on future singularities are investigated. While all values of $α_0$ are considered, an emphasis is placed on those values which are compatible with the universe having undergone Starobinsky inflation in the past. These are also values which lead to stable solutions to the semiclassical backreaction equations in the present universe. The dark energy is modeled as a perfect fluid, and the focus is on type I-IV singularities and little rips which result when the classical Einstein equations are solved with various types of dark energy as a source. First, evidence is provided that the energy densities of massive conformally coupled scalar fields approach that of the conformally invariant scalar field as a type III singularity is approached. Then, solutions to the semiclassical backreaction equations are investigated when conformally invariant fields and the $ α_0 R^2$ term in the gravitational Lagrangian are present. General proofs regarding the behaviors of the solutions are given. The proofs are illustrated by analytic and numerical calculations in specific cases.

gr-qc↗

arXiv2011

Semiclassical Gravity in the Far Field Limit of Stars, Black Holes, and Wormholes

Semiclassical gravity is investigated in a large class of asymptotically flat, static, spherically symmetric spacetimes including those containing static stars, black holes, and wormholes. Specifically the stress-energy tensors of massless free spin 0 and spin 1/2 fields are computed to leading order in the asymptotic regions of these spacetimes. This is done for spin 0 fields in Schwarzschild spacetime using a WKB approximation. It is done numerically for the spin 1/2 field in Schwarzschild, extreme Reissner-Nordstrom, and various wormhole spacetimes. And it is done by finding analytic solutions to the leading order mode equations in a large class of asymptotically flat static spherically symmetric spacetimes. Agreement is shown between these various computational methods. It is found that for all of the spacetimes considered, the energy density and pressure in the asymptotic region are proportional to 1/r^5 to leading order. Furthermore, for the spin 1/2 field and the conformally coupled scalar field, the stress-energy tensor depends only on the leading order geometry in the far field limit. This is also true for the minimally coupled scalar field for spacetimes containing either a static star or a black hole, but not for spacetimes containing a wormhole.

gr-qc↗

arXiv2003

Stress-Energy Tensor for the Massless Spin 1/2 Field in Static Black Hole Spacetimes

The stress-energy tensor for the massless spin 1/2 field is numerically computed outside and on the event horizons of both charged and uncharged static non-rotating black holes, corresponding to the Schwarzschild, Reissner-Nordstrom and extreme Reissner-Nordström solutions of Einstein's equations. The field is assumed to be in a thermal state at the black hole temperature. Comparison is made between the numerical results and previous analytic approximations for the stress-energy tensor in these spacetimes. For the Schwarzschild (charge zero) solution, it is shown that the stress-energy differs even in sign from the analytic approximation. For the Reissner-Nordstrom and extreme Reissner-Nordstrom solutions, divergences predicted by the analytic approximations are shown not to exist.

gr-qc↗

arXiv2002

Method to compute the stress-energy tensor for the massless spin 1/2 field in a general static spherically symmetric spacetime

A method for computing the stress-energy tensor for the quantized, massless, spin 1/2 field in a general static spherically symmetric spacetime is presented. The field can be in a zero temperature state or a non-zero temperature thermal state. An expression for the full renormalized stress-energy tensor is derived. It consists of a sum of two tensors both of which are conserved. One tensor is written in terms of the modes of the quantized field and has zero trace. In most cases it must be computed numerically. The other tensor does not explicitly depend on the modes and has a trace equal to the trace anomaly. It can be used as an analytic approximation for the stress-energy tensor and is equivalent to other approximations that have been made for the stress-energy tensor of the massless spin 1/2 field in static spherically symmetric spacetimes.

gr-qc↗

arXiv2000

Black hole evolution by spectral methods

Current methods of evolving a spacetime containing one or more black holes are plagued by instabilities that prohibit long-term evolution. Some of these instabilities may be due to the numerical method used, traditionally finite differencing. In this paper, we explore the use of a pseudospectral collocation (PSC) method for the evolution of a spherically symmetric black hole spacetime in one dimension using a hyperbolic formulation of Einstein's equations. We demonstrate that our PSC method is able to evolve a spherically symmetric black hole spacetime forever without enforcing constraints, even if we add dynamics via a Klein-Gordon scalar field. We find that, in contrast to finite-differencing methods, black hole excision is a trivial operation using PSC applied to a hyperbolic formulation of Einstein's equations. We discuss the extension of this method to three spatial dimensions.

gr-qc↗

arXiv1995

Pseudoscalar Conversion and X-rays from the Sun

We investigate the detection of a pseudoscalar $ϕ$ that couples electromagnetically via an interaction ${1\over4}g ϕF {\tilde F}$. In particular, we focus on the conversion of pseudoscalars produced in the sun's interior in the presence of the sun's external magnetic dipole field and sunspot-related magnetic fields. We find that the sunspot approach is superior. Measurements by the SXT on the Yohkoh satellite can measure the coupling constant down to $g=0.5$--$1 \times 10^{-10}\,\rm GeV^{-1}$, provided the pseudoscalar mass $m < 7{\times} 10^{-6}\,$eV, which makes it competitive with other astrophysical approaches.

hep-ph↗

arXiv1993

The Breaking of the SU(3)^3 Gauge Group

We discuss why the SU(3)^3 supersymmetric model with the most general superpotential can naturally break to the standard model if gauge singlets and a discrete symmetry are included. This mechanism does away with the need for fine-tuning in the form of the assumed absence of certain terms in the superpotential. It also automatically guarantees that any abelian discrete phase symmetry of the GUT will survive the symmetry breaking. Such a discrete symmetry, also known as the matter parity, is needed to suppress both proton decay and the flavor changing neutral current (FCNC), and may help solve the hierarchy problem.

hep-ph↗

arXiv1992

Will discrete symmetries help solve the hierarchy problem ?

We find that massless Higgs doublets at the GUT scale can be the natural result of a discrete symmetry. Such a mechanism does not require elaborate fine tuning or complicated particle content. The same discrete symmetry will also protect against proton decay and flavor changing neutral currents. However, this mechanism always predicts non-minimal standard models. An explicit example of how this mechanism works is also included.

hep-ph↗

arXiv1992

Trinification and the Strong P Problem

Models with spontaneously broken parity symmetry can solve the strong $CP$ problem in a natural way. We construct such a model in the context of $\SU3^3$ unification. Parity has the conventional meaning in this model, and the gauge group is unified.

hep-ph↗