arXiv subjects

Jared Fier

Publications and source records attributed to Jared Fier.

7 recordsLinked to original sources

arXiv2026

Dynamics of Reheating in Modified Loop Quantum Cosmologies: mLQCI-I

We study the reheating epoch and total bounce-to-present expansion in modified loop quantum cosmology using current observational determinations of the scalar amplitude and spectral index. We consider the power-law plateau potential, whose normalization is determined independently from the observed values of the scalar amplitude and spectral index. We find that, for the kinetic-energy-dominated initial conditions, the quantum-bounce regime is remarkably insensitive to this normalization, while the subsequent inflationary and reheating evolution shows a more appreciable dependence. We then investigate the sensitivity of the results to the inflationary potential by considering the Starobinsky and polynomial chaotic potentials. The reheating e-fold number and reheating temperature are found to depend strongly on both the form of the potential and, in the polynomial case, the branch selected. In contrast, the total bounce-to-present expansion satisfies the conservative bound N > 129 over the ranges considered here and remains close to the characteristic scale found in LQC. Moreover, the difference between mLQC-I and LQC is remarkably stable for all three potentials. These results suggest that reheating is strongly model dependent, whereas the small systematic shift in the total expansion is primarily associated with the different background quantum geometries of LQC and mLQC-I.

gr-qc↗

arXiv2026

Effects of the Ekpyrotic Mechanism on Inflationary Phase in mLQC-II

Bouncing cosmological models replace the big bang singularity with a regular bounce, but face a well-known obstacle: the anisotropic shear grows as a^(-6) during contraction, where a is the average expansion factor, threatening to dominate before a homogeneous, isotropic universe can emerge from the bounce. A standard remedy is a scalar field with an ekpyrotic-like potential that turns negative near the bounce, pushing its effective equation of state above unity so it outgrows the shear. Here we examine how this mechanism affects the subsequent inflationary phase within mLQC-II, a modified loop quantum cosmology (LQC) model in which inflation is otherwise generic. Taking a potential combining ekpyrotic and inflationary components, we numerically evolve the dynamical equations across the bounce for various parameter choices. We find that while some choices of the free parameters of the theory allow the ekpyrotic potential to dominate near the bounce and resolve the shear problem, it substantially reshapes the post-bounce evolution. In particular, parameter choices that yield sufficient inflation without the ekpyrotic mechanism now often fail to do so. Sufficiently long inflation remains possible but appears to require fine-tuning. As our results are numerical, a more systematic analysis will be needed to establish their generality.

gr-qc↗

arXiv2026

Probing the Distribution and Nature of Dark Matter Around Supermassive Black Holes from EMRI and IMRI Gravitational Waves

The distribution of dark matter in the immediate vicinity of supermassive black holes remains poorly understood despite its importance for galaxy evolution and precision tests of gravity. Future space-based gravitational-wave observatories offer a unique opportunity to probe this relativistic regime through the inspiral of compact objects into supermassive black holes. Building upon our previously constructed exact Einstein-cloud solutions within General Relativity, we develop a fully relativistic framework to investigate the gravitational-wave signatures of collisionless dark-matter halos surrounding supermassive black holes. The framework provides a unified treatment of orbital dynamics, adiabatic inspiral, accumulated gravitational-wave cycles, waveform phase evolution, signal-to-noise ratio, and waveform mismatch for extreme- and intermediate-mass-ratio inspirals (EMRIs/IMRIs). As a representative application, we specialize the formalism to Model I. We show that relativistic dark-matter halos can produce measurable modifications to the accumulated gravitational-wave cycles, waveform phase, signal-to-noise ratio, and waveform mismatch, leading to consistent conclusions regarding detectability. By separating conservative modifications of the spacetime geometry from dissipative effects due to relativistic dynamical friction, we find that the observable signatures are dominated by the former, while the latter remains negligible for the halo models considered. These results demonstrate that future gravitational-wave observations by LISA and similar missions may provide a powerful probe of the relativistic distribution and physical nature of dark matter around supermassive black holes.

gr-qc↗

arXiv2026

Effects of the ekpyrotic mechanism on inflationary phase in loop quantum cosmologies

In bouncing cosmological models, either classical or quantum, the big bang singularity is replaced by a regular bounce. A challenging question in such models is how to keep the shear under control in the contracting phase, as it is well-known that the shear grows as fast as $1/a^{6}$ toward the bounce, where $a$ is the average expansion factor of the universe. A common approach is to introduce a scalar field with an ekpyrotic-like potential which becomes negative near the bounce, so the effective equation of state of the scalar field will be greater than one, whereby it dominates the shear in the bounce region. As a result, a homogeneous and isotropic universe can be produced after the bounce. In this paper, we study how the ekpyrotic mechanism affects the inflationary phase in both loop quantum cosmology (LQC) and a modified loop quantum cosmological model (mLQC-I), because in these frameworks inflation is generic without such a mechanism. After numerically studying various cases in which the potential of the inflaton consists of two parts, an inflationary potential and an ekpyrotic-like one, we find that, despite the fact that the influence is significant, by properly choosing the free parameters involved in the models, the ekpyrotic-like potential dominates in the bounce region, during which the effective equation of state is larger than one, so the shear problem is resolved. As the time continuously increases after the bounce, the inflationary potential grows and ultimately becomes dominant, resulting in an inflationary phase. This phase can last long enough to solve the cosmological problems existing in the big bang model.

gr-qc↗

arXiv2025

Gravitational wave cosmology in Einstein-scalar-Gauss-Bonnet gravity

In the framework of Einstein-scalar-Gauss-Bonnet (EsGB) gravity, we systematically study gravitational waves (GWs), first produced by remote compact astrophysical sources and then propagating through the flat homogeneous and isotropic Universe at cosmic distances before arriving at detectors. Assuming that the speed $c_T$ of the spin-2 graviton is the same as that of photons, we find explicitly the stability conditions of the theory and then obtain the severest observational constraint found so far. In particular, all these conditions and constraints are satisfied, provided that $0 \leq α\dot{f}(ϕ_0) \lesssim 8.97 \times 10^{-24}$ (km), where $α{f}(ϕ)$ denotes the coupling strength between the scalar field $ϕ$ and the Gauss-Bonnet term, an over-dot represents the derivative with respect to the cosmic time, and $ϕ_0$ is the present value of $ϕ$. The trajectories for both spin-2 and spin-0 gravitons and the amplitudes of GWs along the trajectories are explicitly obtained. The amplitude of a spin-2 GW is practically indistinguishable from that of GR, while the spin-0 GWs remain almost constant during radiation- and matter-dominated epochs, and in the dark energy-dominated epoch it is proportional to the physical distance between the source and the observer. A careful analysis shows that the latter is due to the assumption $c_T = 1$. When $c_T \not= 1$ to the extent that is consistent with the stability conditions and observational constraints, the above behavior disappears.

gr-qc↗

arXiv2021

Gravitational wave cosmology I: high frequency approximation

In this paper, we systematically study gravitational waves (GWs) produced by remote compact astrophysical sources. To describe such GWs properly, we introduce three scales, $λ, \; L_c$ and $L$, denoting, respectively, the typical wavelength of GWs, the scale of the cosmological perturbations, and the size of the observable universe. For GWs to be detected by the current and foreseeable detectors, the condition $λ\ll L_c \ll L$ holds, and such GWs can be well approximated as high-frequency GWs. In order for the backreaction of the GWs to the background to be negligible, we must assume that $\left|h_{μν}\right| \ll 1$, in addition to the condition $ε\ll 1$, which are also the conditions for the linearized Einstein field equations for $h_{μν}$ to be valid, where $g_{μν} = γ_{μν} + εh_{μν}$, and $γ_{μν}$ denotes the background. To simplify the field equations, we show that the spatial, traceless, and Lorentz gauge conditions can be imposed simultaneously, even when the background is not vacuum, as long as the high-frequency GW approximation is valid. However, to develop the formulas that can be applicable to as many cases as possible, we first write down explicitly the linearized Einstein field equations by imposing only the spatial gauge. Applying the general formulas together with the geometrical optics approximation to such GWs, we find that they still move along null geodesics and its polarization bi-vector is parallel-transported, even when both the cosmological scalar and tensor perturbations are present. In addition, we also calculate the gravitational integrated Sachs-Wolfe effects, whereby the dependences of the amplitude, phase and luminosity distance of the GWs on these two kinds of perturbations are read out explicitly.

astro-ph.CO↗

arXiv2020

Singularities of plane gravitational waves and their memory effects

Similar to the Schwarzschild coordinates for spherical black holes, the Baldwin, Jeffery and Rosen (BJR) coordinates for plane gravitational waves are often singular, and extensions beyond such singularities are necessary, before studying asymptotic properties of such spacetimes at the null infinity of the plane, on which the gravitational waves propagate. The latter is closely related to the studies of memory effects and soft graviton theorems. In this paper, we point out that in the BJR coordinates all the spacetimes are singular physically at the focused point $u = u_s$, except for the two cases: (1) $α=1/2, \; \forall \; χ_n$; and (2) $α=1, \; χ_i = 0\; (i = 1, 2, 3)$, where $χ_n$ are the coefficients in the expansion $χ\equiv \left[{\mbox{det}}\left(g_{ab}\right) \right]^{1/4} = \left(u - u_s\right)^α\sum_{n = 0}^{\infty}χ_n \left(u - u_s\right)^n$ with $χ_0 \not= 0$, the constant $α\in (0, 1]$ characterizes the strength of the singularities, and $g_{ab}$ denotes the reduced metric on the two-dimensional plane orthogonal to the propagation direction of the wave. Therefore, the hypersurfaces $u= u_s$ already represent the boundaries of such spacetimes, and the null infinity does not belong to them. As a result, they cannot be used to study properties of plane gravitational waves at null infinities, including memory effects and soft graviton theorems.

gr-qc↗