arXiv subjects

Jeremy Sakstein

Publications and source records attributed to Jeremy Sakstein.

At least 37 records · Page 2Linked to original sources

arXiv2025

Neutron Stars in Aether Scalar-Tensor Theory

Aether Scalar-Tensor theory is a modification of general relativity proposed to explain galactic and cosmological mass discrepancies conventionally attributed to dark matter.~The theory is able to fit the cosmic microwave background and the linear matter power spectrum.~In this work, we derive the Tolman-Oppenheimer-Volkoff equation in this theory and solve it for realistic nuclear equations of state to predict the mass-radius relation of neutron stars.~We find solutions that are compatible with all current observations of neutron stars.

gr-qc↗

arXiv2024

First Constraints on a Pixelated Universe in Light of DESI

Pixelated dark energy is a string theory scenario with a quantum mechanically stable cosmological constant. The number of pixels that make up the universe slowly increases, manifesting as a time-dependent source of dark energy. DESI has recently reported evidence for dynamical dark energy that fits within this framework. In light of this, we perform the first cosmological analysis of the pixelated model. We find that the simplest model where the pixel growth rate is constant is able to accommodate the data, providing a marginally better fit than $Λ$CDM; and we show that models where the pixel growth rate is increasing and of order the Hubble constant today could provide better fits. Our analysis helps to clarify the features of UV constructions of dark energy necessary to accommodate the data.

astro-ph.CO↗

arXiv2024

Cosmology and Astrophysics of CP-Violating Axions

We study the cosmology and astrophysics of axion-like particles (ALPs) with CP-violating Yukawa couplings to nucleons. At finite nucleon density, the ALP's dynamics is governed by an effective potential which is the sum of the bare periodic potential and a linear potential whose strength depends on the nucleon density. We identify a critical nucleon density $ρ_c$ controlling the dynamics. At densities smaller than $ρ_c$ the effective potential is a tilted sinusoidal curve and the field is displaced from its zero-density minimum. At densities larger than $ρ_c$ the minima (and maxima) are absent, and the ALP is destabilized. Astrophysically, this implies that neutron stars can source a radial ALP field, providing a complementary probe to equivalence principle tests. Cosmologically, the ALP may have been destabilized in the early Universe and could have made large field excursions. We discuss model-building applications of our results for such early universe scenarios.

hep-ph↗

arXiv2024

Dark Matter Annihilation and Pair-Instability Supernovae

We study the evolution of heavy stars ($M\ge40{\rm M}_\odot$) undergoing pair-instability in the presence of annihilating dark matter. Focusing on the scenario where the dark matter is in capture-annihilation equilibrium, we model the profile of energy injections in the local thermal equilibrium approximation. We find that significant changes to masses of astrophysical black holes formed by (pulsational) pair-instability supernovae can occur when the ambient dark matter density $ ρ_{\rm DM} \gtrsim10^9 \rm \, GeV \, cm^{-3}$. There are two distinct outcomes, depending on the dark matter mass. For masses $m_{\rm DM}\gtrsim1$ GeV the DM is primarily confined to the core. The annihilation increases the lifetime of core helium burning, resulting in more oxygen being formed, fueling a more violent explosion during the pair-instability-induced contraction. This drives stronger pulsations, leading to lighter black holes being formed than predicted by the standard model. For masses $m_{\rm DM}\lesssim0.5$ GeV there is significant dark matter in the envelope, leading to a phase where the star is supported by the energy from the annihilation. This reduces the core temperature and density, allowing the star to evade the pair-instability allowing heavier black holes to be formed. We find a mass gap for all models studied.

astro-ph.HE↗

arXiv2024

Multi-Generational Black Hole Population Analysis with an Astrophysically Informed Mass Function

We analyze the population statistics of black holes in the LIGO/Virgo/KAGRA GWTC-3 catalog using a parametric mass function derived from simulations of massive stars experiencing pulsational pair-instability supernovae (PPISN). Our formalism enables us to separate the black hole mass function into sub-populations corresponding to mergers between objects formed via different astrophysical pathways, allowing us to infer the properties of black holes formed from stellar collapse and black holes formed via prior mergers separately. Applying this formalism, we find that this model fits the data better than the powerlaw+peak model with Bayes factor $ 9.7\pm0.1$. We measure the location of the lower edge of the upper black hole mass gap to be $M_{\rm BHMG}=84.05_{-12.88}^{+17.19}{\rm M}_{\odot}$, providing evidence that the $35{\rm M}_{\odot}$ Gaussian peak detected in the data using other models is not associated with the PPISN pile-up predicted to precede this gap. Incorporating spin, we find that the normalized spins of stellar remnant black holes are close to zero while those of higher generation black holes tend to larger values. All of these results are in accordance with the predictions of stellar structure theory and black hole merger scenarios. Finally, we combine our mass function with the spectral siren method for measuring the Hubble constant to find $H_0=36.19_{-10.91}^{17.50}$ km/s/Mpc and discuss potential explanations of this low value. Our results demonstrate how astrophysically-informed mass functions can facilitate the interpretation of gravitational wave catalog data to provide information about black hole formation and cosmology. Future data releases will improve the precision of our measurements.

astro-ph.HE↗

arXiv2024

DESI Constraints on Exponential Quintessence

The DESI collaboration have recently analyzed their first year of data, finding a preference for thawing dark energy scenarios when using parameterized equations of state for dark energy. We investigate whether this preference persists when the data is analyzed within the context of a well-studied field theory model of thawing dark energy, exponential quintessence. No preference for this model over $Λ$CDM is found, and both models are poorer fits to the data than the Chevallier-Polarski-Linder $w_0$--$w_a$ parameterization. We demonstrate that the worse fit is due to a lack of sharp features in the potential that results in a slowly-evolving dark energy equation of state that does not have enough freedom to simultaneously fit the combination of the supernovae, DESI, and cosmic microwave background data. Our analysis provides guidance for constructing dynamical dark energy models that are able to better accommodate the data.

astro-ph.CO↗

arXiv2024

Parameterized Post-Tolman-Oppenheimer-Volkoff Framework for Screened Modified Gravity with an Application to the Secondary Component of GW190814

The secondary component of GW190814 has mass in the range $2.5$--$2.67{\rm M}_\odot$, placing it within the lower mass gap separating neutron stars from black holes. According to the predictions of general relativity and state-of-the-art nuclear equations of state, this object is too heavy to be a neutron star.~In this work, we explore the possibility that this object is a neutron star under the hypothesis that general relativity is modified to include screening mechanisms, and that the neutron star formed in an unscreened environment. We introduce a set of parameterized-post-Tolman-Oppenheimer-Volkoff (post-TOV) equations appropriate for screened modified gravity whose free parameters are environment-dependent. We find that it is possible that the GW190814 secondary could be a neutron star that formed in an unscreened environment for a range of reasonable post-TOV parameters.

gr-qc↗

arXiv2024

An Attractive Proposal for Resolving the Hubble Tension: Dynamical Attractors that Unify Early and Late Dark Energy

Early dark energy is a promising potential resolution of the Hubble tension. Unfortunately, many models suffer from the need to fine-tune their initial conditions to ensure that the epoch of early dark energy coincides with matter-radiation equality. We propose a class of attractive early dark energy models where this coincidence arises naturally as a saddle point of a dynamical system that attracts a large volume of phase-space trajectories regardless of the initial conditions. The system approaches a global dark energy attractor at late-times. Our framework therefore unifies early and late dark energy using a single scalar degree of freedom. We analyze a fiducial attractive early dark energy model and find that it is disfavored by cosmological data due to the presence of a long-lived saddle point in the matter era where the scalar plays the role of an additional component of (non-clustering) dark matter. Our investigations provide lessons for future model-building efforts aimed at constructing viable attractive early dark energy models.

astro-ph.CO↗

arXiv2023

Prediction of Multiple Features in the Black Hole Mass Function due to Pulsational Pair-Instability Supernovae

Using high-resolution simulations of black hole formation from the direct collapse of massive stars undergoing pulsational pair-instability supernovae (PPISN), we find a new phenomenon which significantly affects the explosion and leads to two peaks in the resulting black hole mass function (BHMF). Lighter stars experiencing the pair-instability can form a narrow shell in which alpha ladder reactions take place, exacerbating the effect of the PPISN. The shell temperature in higher mass stars ($>62 {\rm M}_\odot $ at the onset of helium burning for population-III stars with metallicity $Z=10^{-5}$) is too low for this to occur. As a result, the spectrum of black holes $M_{\rm BH} (M_i)$ exhibits a shoulder feature whereby a large range of initial masses result in near-identical black hole masses. PPISN therefore predict two peaks in the mass function of astrophysical black holes -- one corresponding to the location of the upper black hole mass gap and a second corresponding to the location of the shoulder. This shoulder effect may explain the peak at $35_{-2.9}^{+1.7}{\rm M}_\odot$ in the LIGO/Virgo/KAGRA GWTC-3 catalog of merging binary black holes.

astro-ph.HE↗

arXiv2023

Light Axion Emission and the Formation of Merging Binary Black Holes

We study the impact of stellar cooling due to light axion emission on the formation and evolution of black hole binaries, via stable mass transfer and the common envelope scenario. We find that in the presence of light axion emission, no binary black hole mergers are formed with black holes in the lower mass gap ($\rm M_{\rm BH} < 4 M_\odot $) via the common envelope formation channel. In some systems, this happens because axions prevent Roche lobe overflow. In others, they prevent the common envelope from being ejected. Our results apply to axions with couplings $ g_{a γ} \gtrsim 10^{-10}\, \rm GeV^{-1}$ (to photons) or $α_{ae} \gtrsim 10^{-26} $ (to electrons) and masses $ m_a \ll 10 \, \rm keV$. Light, weakly coupled particles may therefore apparently produce a mass gap $\rm 2 M_\odot < M_{\rm BH} < 4 M_\odot $ in the LIGO/Virgo/KAGRA data, when no mass gap is present in the stellar remnant population.

astro-ph.HE↗

arXiv2023

Tip of the Red Giant Branch Bounds on the Neutrino Magnetic Dipole Moment Revisited

We use a novel method to constrain the neutrino magnetic dipole moment ($μ_ν$) using the empirically-calibrated tip of the red giant branch I-band magnitude that fully accounts for uncertainties in stellar physics. Our method uses machine learning to emulate the results of stellar evolution codes. This reduces the I-Band magnitude computation time to milliseconds, which enables a Bayesian statistical analysis where $μ_ν$ is varied simultaneously with the stellar physics, allowing for a complete exploration of parameter space. We find the region $μ_ν \leq 6\times10^{-12}μ_{\textrm{B}}$ (with $μ_{\textrm{B}}$ the Bohr magneton), previously believed to be excluded, is unconstrained after accounting for degeneracies with stellar physics. It is likely that larger values are similarly unconstrained. We discuss the implications of our results for future neutrino magnetic dipole moment searches and for other astrophysical probes.

hep-ph↗

arXiv2023

Constraining the chameleon-photon coupling with atomic spectroscopy

We compute bounds from atomic spectroscopy on chameleon fields that couple to the photon. Chameleons are a wide class of scalar field models that generically lead to screened fifth forces and a host of novel phenomenologies, particularly when the photon coupling is included. We account for perturbations to the atomic energy levels from both the scalar field "fifth force" and the scalar field's correction to the electric field. We also account for the electromagnetic interaction's contribution to the scalar charge of the proton, which enables a considerably wider class of models to be tested than without this effect. We find bounds that cover different areas of chameleon parameter space. Some regions are redundant with existing experiments, particularly $g - 2$, confirming that those models are ruled out. Other regions were previously unconstrained, and a range of models spanning approximately four orders of magnitude in chameleon coupling parameters are excluded for the first time.

hep-ph↗

arXiv2023

Dark Matter-Induced Stellar Oscillations

It has been hypothesized that dark matter is comprised of ultra-light bosons whose collective phenomena can be described as a scalar field undergoing coherent oscillations. Examples include axion and fuzzy dark matter models. In this ultra-light dark matter scenario, the harmonic variation in the field's energy-momentum tensor sources an oscillating component of the gravitational potential that we show can resonantly-excite stellar oscillations. A mathematical framework for predicting the amplitude of these oscillations is developed, which reveals that ultra-light dark matter predominantly excites p-modes of degree $l=1$. An investigation of resonantly-excited solar oscillations is presented, from which we conclude that dark matter-induced oscillations of the Sun are likely undetectable. We discuss prospects for constraining ultra-light dark matter using other stellar objects.

hep-ph↗

arXiv2023

Neutrino-Assisted Early Dark Energy is a Natural Resolution of the Hubble Tension

It has very recently been claimed that the neutrino-assisted early dark energy model -- a promising resolution of the Hubble tension that can ameliorate the theoretical fine-tuning and coincidence problems that plague other theories -- does not provide natural or cosmologically interesting results. In this short paper, we show that these conclusions are incorrect for three reasons. First, we identify errors in the calculations. Second, we dispute the definition in of what constitutes an 'interesting' and 'natural' model. Finally, we demonstrate that the conclusions of were arrived at without fully exploring the full parameter space of the model. Neutrino-assisted early dark energy remains a natural and interesting potential resolution of the Hubble tension that merits further study.

astro-ph.CO↗

arXiv2022

Exploring $\boldsymbol{2+2}$ Answers to $\boldsymbol{3+1}$ Questions

We explore potential uses of physics formulated in Kleinian (i.e., $2+2$) signature spacetimes as a tool for understanding properties of physics in Lorentzian (i.e., $3+1$) signature. Much as Euclidean (i.e., $4+0$) signature quantities can be used to formally construct the ground state wavefunction of a Lorentzian signature quantum field theory, a similar analytic continuation to Kleinian signature constructs a state of low particle flux in the direction of analytic continuation. There is also a natural supersymmetry algebra available in $2+2$ signature, which serves to constrain the structure of correlation functions. Spontaneous breaking of Lorentz symmetry can produce various $\mathcal{N} = 1/2$ supersymmetry algebras that in $3 + 1$ signature correspond to non-supersymmetric systems. We speculate on the possible role of these structures in addressing the cosmological constant problem.

hep-th↗

arXiv2022

Dark Matter In Extreme Astrophysical Environments

Exploring dark matter via observations of extreme astrophysical environments -- defined here as heavy compact objects such as white dwarfs, neutron stars, and black holes, as well as supernovae and compact object merger events -- has been a major field of growth since the last Snowmass process. Theoretical work has highlighted the utility of current and near-future observatories to constrain novel dark matter parameter space across the full mass range. This includes gravitational wave instruments and observatories spanning the electromagnetic spectrum, from radio to gamma-rays. While recent searches already provide leading sensitivity to various dark matter models, this work also highlights the need for theoretical astrophysics research to better constrain the properties of these extreme astrophysical systems. The unique potential of these search signatures to probe dark matter adds motivation to proposed next-generation astronomical and gravitational wave instruments.

hep-ph↗

arXiv2022

Mapping the Weak-Field Limit of Scalar-Gauss-Bonnet Gravity

We derive the weak field limit of scalar-Gauss-Bonnet theory and place novel bounds on the parameter space using terrestrial and space-based experiments. In order to analyze the theory in the context of a wide range of experiments, we compute the deviations from Einstein gravity around source masses with planar, cylindrical, and spherical symmetry. We find a correction to the Newtonian potential around spherical and cylindrical sources that can be larger than PPN corrections sufficiently close to the source. We use this to improve on laboratory constraints on the scalar-Gauss-Bonnet coupling parameter $Λ$ by two orders of magnitude. Present laboratory and Solar System bounds reported here are superseded by tests deriving from black holes.

gr-qc↗

arXiv2022

Axion Instability Supernovae

New particles coupled to the Standard Model can equilibrate in stellar cores if they are sufficiently heavy and strongly coupled. In this work, we investigate the astrophysical consequences of such a scenario for massive stars by incorporating new contributions to the equation of state into a state of the art stellar structure code. We focus on axions in the "cosmological triangle", a region of parameter space with $300{\rm\,keV} \lesssim m_a \lesssim 2$ MeV, $g_{aγγ}\sim 10^{-5}$ GeV$^{-1}$ that is not presently excluded by other considerations. We find that for axion masses $m_a \sim m_e $, axion production in the core drives a new stellar instability that results in explosive nuclear burning that either drives a series of mass-shedding pulsations or completely disrupts the star resulting in a new type of optical transient -- an \textit{Axion Instability Supernova}. We predict that the upper black hole mass gap would be located at $37{\rm M}_\odot \le M\le 107{\rm M}_\odot$ in these theories, a large shift down from the standard prediction, which is disfavored by the detection of the mass gap in the LIGO/Virgo/KAGRA GWTC-2 gravitational wave catalog beginning at $46_{-6}^{+17}{\rm M}_\odot$. Furthermore, axion-instability supernovae are more common than pair-instability supernovae, making them excellent candidate targets for JWST. The methods presented in this work can be used to investigate the astrophysical consequences of any theory of new physics that contains heavy bosonic particles of arbitrary spin. We provide the tools to facilitate such studies.

hep-ph↗