arXiv subjects

Mark P. Hertzberg

Publications and source records attributed to Mark P. Hertzberg.

At least 19 recordsLinked to original sources

arXiv2026

Perturbations of Charged Black Holes with Higher-Order Interactions

We study linear gravitational perturbations and the corresponding quasinormal-mode spectra of electrically and magnetically charged black holes in the presence of higher-order corrections to the Einstein-Hilbert action. In particular, we consider a four-derivative interaction that couples the Riemann tensor to the electromagnetic field strength in the unique (Horndeski) combination that preserves second-order equations of motion. We provide a comprehensive analysis of the stability conditions. We numerically confirm the absence of exponentially growing modes throughout the physically admissible region of parameter space where the theory remains free of pathologies. We then investigate in detail the properties of the quasinormal-mode spectra and identify several distinctive features induced by the higher-order interaction.

gr-qc↗

arXiv2026

Constraints on Stable Scalar-Tensor Dark Energy from DESI Data and Solar System Tests

We investigate the viability of scalar-tensor (quintessence) models of dark energy with a quartic polynomial nonminimal coupling to gravity and a linear scalar potential. The polynomial nonminimal coupling is used to ensure that the field is well stabilized in the early universe. We perform a systematic exploration of the parameter space spanned by the quadratic and quartic nonminimal couplings, as well as the slope of the potential. We confront the predictions of the model with the latest Dark Energy Spectroscopic Instrument (DESI) constraints on the dark energy equation of state and also with complementary bounds from local tests of gravity, including solar system constraints and limits on the time variation of the effective Newton's constant. We identify bands in parameter space where all these constraints are satisfied, finding such bands to be very narrow.

astro-ph.CO↗

arXiv2026

Theoretical and Observational Bounds on Dynamical Chern-Simons Gravity as an Effective Field Theory

Gravitational effective theories are essential for characterizing the space of deviations from General Relativity (GR). Testing these theories against fundamental principles, such as causality and unitarity, can yield constraints on the corresponding parameters. In this paper, we perform such an analysis on the very interesting dynamical Chern-Simons (dCS) gravity. This is a parity violating correction to GR wherein a new scalar field couples to the Pontryagin density $^*R\,R$. It has generated significant interest, including possible new gravitational wave shapes for LIGO/Virgo and new phenomena from cosmic inflation. In this work, we begin by deriving the dispersion relation and wave packet speed on top of a gravitational wave background in dCS gravity. This alters the corresponding Shapiro time delay (which we compute to second order), potentially giving superluminality. Causality then demands a bound on the dCS coupling constant, which we find to be moderately sharper than, but compatible with, standard estimates. We then examine a UV completion in the form of a set of $N$ fermions with a (pseudo) Yukawa coupling. By imposing perturbativity and a gravitational species bound, we find that the dCS coupling constant is constrained significantly more, depending on the choice of scale of the species bound. We also identify higher order operators generated from the UV completion. Overall, we find that any dCS corrections to gravitational dynamics should likely be very small on macroscopic systems of observational interest, such as in late-time merging black holes.

hep-th↗

arXiv2025

Effects of New Forces on Scalar Dark Matter Solitons

New long range forces acting on ordinary matter are highly constrained. However it is possible such forces act on dark matter, as it is less constrained observationally. In this work, we consider dark matter to be made of light bosons, such as axions. We introduce a mediator that communicates a new force between dark matter particles, in addition to gravity. The mediator is taken to be light, but not massless, so that it can affect small scale galactic behavior, but not current cosmological behavior. As a concrete application of this idea, we analyze the effects on scalar dark matter solitons bound by gravitation, i.e., boson stars, which have been claimed to potentially provide cores of galaxies. We numerically determine the soliton's profiles in the presence of this new force. We also extend the analysis to multiple mediators. We show that this new force alters the relation between core density and core radius in a way that can provide improvement in fitting data to observed galactic cores, but for couplings of order the gravitational strength, the improvement is only modest.

astro-ph.CO↗

arXiv2025

Unitarity, Causality, and Solar System Bounds May Significantly Limit the Use of Gravitational Waves to Test General Relativity

The prospect of detecting/constraining deviations from general relativity by studying gravitational waves (GWs) from merging black holes has been one of the primary motivations of GW interferometers like LIGO/Virgo. Within pure gravity, the only possible way deviations can arise is from the existence of higher order derivative corrections, namely higher powers of the Riemann curvature tensor, in the effective action. Any observational bounds imply constraints on the corresponding Wilson coefficients. At the level of the action, one can imagine the coefficients are sufficiently large so as to be in principle detectable. However, from the point of view of some fundamental principles, namely causality and unitarity, this is much less clear, as we examine here. We begin by reviewing certain known bounds on these coefficients, which together imply a low cut off on the effective theory. We then consider a possible mechanism to generate such terms, namely in the form of many scalars, minimally coupled to only gravity, that can be integrated out to give these higher order operators. We show that a by product of this is the generation of quantum corrections to Newton's potential, whose observable consequences are already ruled out by solar system tests. We point out that over 7 orders of magnitude of improvement in interferometer sensitivity would be required to avoid such solar system constraints. We also mention further constraints from Hawking radiation from black holes.

hep-th↗

arXiv2025

Comparing Minimal and Non-Minimal Quintessence Models to 2025 DESI Data

In this work we examine the 2025 DESI analysis of dark energy, which suggests that dark energy is evolving in time with an increasing equation of state $w$. We explore a wide range of quintessence models, described by a potential function $V(φ)$, including: quadratic potentials, quartic hilltops, double wells, cosine functions, Gaussians, inverse powers. We find that while some provide improvement in fitting to the data, compared to a cosmological constant, the improvement is only modest. We then consider non-minimally coupled scalars which can help fit the data by providing an effective equation of state that temporarily obeys $w<-1$ and then relaxes to $w>-1$. Since the scalar is very light, this leads to a fifth force and to time evolution in the effective gravitational strength, which are both tightly constrained by tests of gravity. For a very narrow range of carefully selected non-minimal couplings we are able to evade these bounds, but not for generic values.

astro-ph.CO↗

arXiv2025

Solar System Constraints on Light Propagation from Higher Derivative Corrections to General Relativity and Implications for Fundamental Physics

While the two derivative action of gravitation is specified uniquely, higher derivative operators are also allowed with coefficients that are not specified uniquely by effective field theory. We focus on a four derivative operator in which the Riemann tensor couples directly to the electromagnetic field $a\,R_{μναβ}F^{μν}F^{αβ}$. We compute the corresponding corrections to the Shapiro time delay in the solar system and compare this to data from the Cassini probe. We place an observational upper bound on the coefficient $a$ at 95\% confidence $|a|<26\,(1000\,\mbox{km})^2$. By way of motivation, we also compare this to a weak gravity conjecture (WGC) prediction of a bound on the coefficients $a,\,b$ of four derivative operators involving the graviton and the photon; this includes the above term $a\,R_{μναβ}F^{μν}F^{αβ}$ as well as $b\,F^4$. We show that by using the observed value of the $b$ coefficient from measurements of light by light scattering, which arises in the Standard Model from integrating out the electron, the WGC predicted bound for $a$ is $a\lesssim 7.8\,(1000\,\mbox{km})^2$. This is consistent with the above observational bound, but is intriguingly close and can be further probed in other observations.

gr-qc↗

arXiv2025

Neutron Stars in Causal Scalar-Tensor Theories

We study static, spherically symmetric neutron stars in a class of scalar-tensor theories with non-canonical kinetic terms (K-essence) obeying all causality and hyperbolicity conditions. These models have non-trivial dynamics that lead to a type of anti-screening of the scalar. They lead to small corrections in the solar system due to a small coupling, but can lead to large corrections in regimes of high densities, especially neutron stars. We solve the modified Tolman-Oppenheimer-Volkoff equations numerically using realistic equations of state (SLy4, WFF1, MS1, MPA1). For a given central density, we find that two distinct configurations may exist, forming two separate branches of solutions. We find that above a certain critical central density solutions with the correct asymptotic behavior at spatial infinity cannot be obtained. We obtain precise predictions for the mass-radius relation for neutron stars for different values of the parameters in the model and we compare to data.

gr-qc↗

arXiv2025

Inconsistency with De Sitter Spacetime in a New Approach to Gravitational Particle Production

We study a claimed new mechanism for particle production and black hole evaporation through a spatially dependent temperature. This new temperature is comparable to the Hawking result near the black hole, but is very small far away, and therefore could be a small correction. Here we apply the proposed reasoning to the case of de Sitter space, finding that it over predicts the de Sitter temperature of a minimally coupled scalar by factor of $\approx 4.3$ and over predicts the particle production rate by a factor of $\approx 52$. For non-minimally coupled scalars, it has other various problems; it predicts a negative particle production for conformal, or nearly conformal, coupled scalars; it predicts unsuppressed productions of heavy scalars. This all demonstrates an inconsistency in the proposed formalism.

gr-qc↗

arXiv2025

Probability of the Initial Conditions for Inflation and Slow Contraction

Some recent studies based on numerical relativity simulations claim that slow contraction/ekpyrosis is strongly preferred over inflation as the smoothing mechanism that brought the universe into the homogeneous, isotropic and flat state we observe today on large scales. In this paper, we evaluate the likelihood of the initial conditions employed in the aforementioned simulations by estimating the probability that a free scalar field dominating the universe at the beginning of inflation or ekpyrosis will be sufficiently homogeneous on scales comparable to the Hubble radius at that time. We explore the space of parameters that characterize the initial power spectrum of the scalar field, finding that either can be more likely than the other for a fixed choice of parameters. On the other hand, when we extremize over these parameters, we find that the maximal probability for inflation is much higher than that of ekpyrosis.

gr-qc↗

arXiv2025

Examining Quintessence Models with DESI Data

We examine data from the Dark Energy Spectroscopic Instrument (DESI) collaboration which has implications for the nature of dark energy. We consider classes of models that manifestly obey the null energy condition, with a focus on quintessence models. We find that hilltop potentials and exponential potentials provide modest improvement compared to a cosmological constant, but the statistical evidence is only marginal at this stage. We correct some analyses in the existing literature which attempted to compare some quintessence models to the data, giving an overly positive result.

astro-ph.CO↗

arXiv2025

Precision Unitarity Calculations in Inflationary Models

We revisit perturbative unitarity in scalar field inflation with a nonminimal coupling, with Higgs inflation serving as the most prominent example. Although such models are phenomenologically successful, it is critical to examine whether or not unitarity violations spoil their theoretical self-consistency. The analysis of these issues has so far typically relied on order-of-magnitude estimates of scattering amplitudes, which are appropriate for generic parameters. It is not evident that these methods apply to scenarios relying on a near-critical inflationary potential, for which an interplay of both small scalar self-couplings and nonminimal couplings could partially alleviate the unitarity issues. To allow for an exploration of this possibility, we consider the full $S$-matrix for the relevant scattering processes, taking into account important phase space volume factors, leading to a precise evaluation of the cut-off scale. In the single-field case, we demonstrate that near-criticality raises the cut-off scale considerably, compared to previous estimates. In the multifield case, momentum-dependent self-interactions in the kinetic sector lower the cut-off compared to the single-field case to a value comparable to but slightly larger than previous estimates. We carefully study both the single-field and multifield cases in metric and metric-affine (Palatini) formulations of gravity, as well as introduce a new phenomenologically viable model with a canonical kinetic term and a significantly raised cut-off, and discuss the importance of background field effects.

hep-ph↗

arXiv2025

Black Holes Rule Out Heavy Tachyons

We present direct observational constraints on tachyons; particles with group velocity greater than $c$ in vacuum in a Lorentz invariant theory. Since tachyons may have no direct couplings to Standard Model particles, the most robust and model independent constraints come from gravitational effects, especially black holes. We compute the Hawking radiation of tachyons from black holes, finding it to be significantly enhanced in the presence of heavy tachyons. For a black hole of mass $M$ and tachyons of mass $m$ with $g$ degrees of freedom, the black hole lifetime is found to be $t_{bh} \approx 192 π\hbar M/(g c^2 m^2)$ (or doubled for fermions). This implies that the observation of black holes of a few solar masses, with lifetime of several billion years, rules out tachyons of mass $m > 3 \times 10^9$ GeV. This means there cannot exist any tachyons associated with unification scales or quantum gravity. So while there already exists theoretical reasons to be skeptical of tachyons, our work provides a complementary direct observational constraint.

gr-qc↗

arXiv2024

Critical Analysis of Replacing Dark Matter and Dark Energy with a Model of Stochastic Spacetime

We analyze consequences of trying to replace dark matter and dark energy with models of stochastic spacetime. In particular, we analyze the model put forth by Ref. [1], in which it is claimed that ``post-quantum classical gravity" (PQCG), a stochastic theory of gravity, leads to modified Newtonian dynamics (MOND) behavior on galactic scales that reproduces galactic rotation curves, and leads to dark energy. We show that this analysis has four basic problems: (i) the equations of PQCG do not lead to a new large scale force of the form claimed in the paper, (ii) the form claimed is not of the MONDian form anyhow and so does not correspond to observed galactic dynamics, (iii) the spectrum of fluctuations is very different from observations, and (iv) we also identify some theoretical problems in these models.

gr-qc↗

arXiv2024

Constraints on an Anisotropic Universe

We analyze the possibility of global anisotropy of the universe. We consider an altered Friedmann Lemaitre Robertson Walker metric in which there are different scale factors along the three different axes of space. We construct the corresponding altered Friedmann equations. We show that any initial anisotropies decrease into the future. At late times, the difference in Hubble parameters changes as $1/\sqrt{t}$ in a radiation dominated era and as $1/t$ in a matter dominated era. We use constraints from Big Bang Nucleosynthesis and the Cosmic Microwave Background to constrain the level of anisotropies at early times. We also examine how the approach back in time to the singularity is radically altered; happening much more abruptly, as a function of density, in an anisotropic universe. We also mention improved bounds that can arise from measurements of primordial gravitons, Weakly interacting massive particles, and neutrinos.

astro-ph.CO↗

arXiv2024

Core and Halo Properties in Multi-Field Wave Dark Matter

In this work, we compute multi-field core and halo properties in wave Dark Matter models. We focus on the case where Dark Matter consists of two light (real) scalars, interacting gravitationally. As in the single-field Ultra Light Dark Matter (ULDM) case, the scalar field behaves as a coherent BEC with a definite ground state (at fixed total mass), often referred to in the literature as a gravitational soliton. We establish an efficient algorithm to find the ground and excited states of such two-field systems. We then use simulations to investigate the gravitational collapse and virialization, starting from different initial conditions, into solitons and surrounding halo. As in the single-field case, a virialized halo forms with a gravitational soliton (ground state) at the center. We find some evidence for an empirical relation between the soliton mass and energy and those of the host halo. We use this to then find a numerical relation between the properties of the two. Finally, we use this to address the issue of alleviating some of the tensions that single-field ULDM has with observational data, in particular, the issue of how a galaxy's core and radius are related. We find that if galaxies of different masses have similar percentages of the two species, then the core-radius scaling tension is not addressed. However, more general possibilities occur if the relative abundance of species in each halo correlates with the total mass of the galaxy. If this is the case, the model predicts several other phenomenological signatures.

astro-ph.CO↗

arXiv2023

Constraints on Inflaton Higgs Field Couplings

According to the best-fit parameters of the Standard Model, the Higgs field's potential reaches a maximum at a field value $h \sim 10^{10-11}$ GeV and then turns over to negative values. During reheating after inflation, resonance between the inflaton and the Higgs can cause the Higgs to fluctuate past this maximum and run down the dangerous side of the potential if these fields couple too strongly. In this paper, we place constraints on the inflaton-Higgs couplings such that the probability of the Higgs entering the unstable regime during reheating is small. To do so, the equations of motion are approximately solved semi-analytically, then solved fully numerically. Next the growth in variance is used to determine the parameter space for $κ$ and $α$, the coupling coefficients for inflaton-Higgs cubic and quartic interactions, respectively. We find the upper bounds of $κ< 1.6 \times 10^{-5} m_ϕ\sim 2.2 \times 10^8$ GeV and $α< 10^{-8}$ to allow the Higgs to remain stable in most Hubble patches during reheating, and we also find the full two parameter joint constraints. We find a corresponding bound on the reheat temperature of $T_\text{reh} \lesssim 9.2 \times 10^9$ GeV. Additionally, de Sitter temperature fluctuations during inflation put a lower bound on inflaton-Higgs coupling by providing an effective mass for the Higgs, pushing back its hilltop during inflation. These additional constraints provide a lower bound on $α$, while $κ$ must also be non-zero for the inflaton to decay efficiently.

hep-ph↗

arXiv2023

Dark Sectors with Mass Thresholds Face Cosmological Datasets

Interacting dark sectors may undergo changes in the number of their relativistic species during the early universe, due to a mass threshold $m$ (similar to changes in the Standard Model bath), and in doing so affect the cosmic history. When such changes occur close to recombination, i.e., for $m\sim (0.1-10)~\text{eV}$, the stringent bound on the effective number of neutrino species, $N_{\text{eff}}$, can be relaxed and the value of the Hubble expansion rate $H_0$ inferred from Cosmic Microwave Background (CMB) observations raised. We search for such sectors (with and without mass thresholds) in the latest cosmological datasets, including the full-shape (FS) of BOSS DR12 galaxy power spectrum. We perform a detailed analysis, accounting for the choice of prior boundaries and additionally exploring the possible effects of dark sector interactions with (a fraction of) the dark matter. We find $ΔN_{\text{eff}}\leq 0.55\, (0.46)$ at 95% C.L. with (without) a mass threshold. While a significantly larger Hubble rate is achieved in this scenario, $H_0=69.01^{+0.66}_{-1.1}$, the overall fit to CMB+FS data does not provide a compelling advantage over the $Λ$CDM model. Furthermore, we find that dark matter interactions with the dark sector do not significantly improve the (matter fluctuations) $S_8$ tension with respect to the $Λ$CDM model. Our work provides model-independent constraints on (decoupled) dark sectors with mass thresholds around the eV scale.

astro-ph.CO↗