arXiv subjects

Robert J. Scherrer

Publications and source records attributed to Robert J. Scherrer.

At least 19 recordsLinked to original sources

arXiv2026

Dark Energy in the $w-c_s^2$ Plane

We introduce a unified framework for dark energy diagnostics based on the joint phase space of the equation of state $w$ and the sound speed $c_s^2$. The resulting $w-c_s^2$ plane provides a minimal extension beyond background cosmology, capturing both the expansion history and perturbative properties within a single representation. Building on this framework, we define the microphysical flow parameter $F=dc_s^2/dw$, which encodes the dynamical relation between background evolution and perturbative response. We derive a direct connection between the present-day value $F_0$, $H_0$, and $σ_8$, and show that the microphysical flow parameter enables a hierarchy of increasingly stringent consistency tests that substantially reduce the viable dark energy theory space. We further demonstrate how trajectories in the $w-c_s^2$ plane distinguish models that are nearly degenerate at the level of $w(a)$, including canonical quintessence, Chaplygin gas models, and noncanonical scalar field realizations. This framework provides a compact phenomenological bridge between dark energy microphysics and future perturbation-sensitive observations, establishing $F_0$ as a useful discriminator of the kinetic structure underlying cosmic acceleration.

astro-ph.CO↗

arXiv2026

Exact general solutions for cosmological scalar field evolution in a vacuum-energy dominated expansion

We derive exact general solutions (as opposed to attractor particular solutions) for the evolution of a scalar field $ϕ$ in a universe dominated by a background fluid with equation of state parameter $w_B = -1$, extending earlier work on exact solutions with $w_B > -1$. Straightfoward exact solutions exist when the evolution is described by a linear differential equation, corresponding to constant, linear, and quadratic potentials. In the nonlinear case, exact solutions are derived for $V = V_0\ln ϕ$, $V = V_0 ϕ^{1/2}$ and $V = V_0/ϕ$, and the logarithmic potential also yields an exact first integral. These complicated parametric solutions are considerably less useful than those derived previously for a universe dominated by a barotropic fluid such as matter or radiation with $w_B > -1$. However, we generalize the slow-roll approximation and show that it applies to all sufficiently flat potentials in the case of a vacuum-dominated expansion, while it never applies when the universe is dominated by a background fluid with $w_B > -1$.

gr-qc↗

arXiv2026

Solar System Probes for Scalar Field Dark Matter

Scalar field dark matter provides us with a well motivated alternative to conventional particle dark matter, especially when ultralight fields form coherent oscillations or compact self gravitating clumps. Here we develop three complementary Solar System and local Galactic level probes of such models. These probes pertain to ADAF-like flares from scalar clump encounters with Kuiper Belt Objects, atomic clock searches for oscillatory variations of fundamental constants and astrometric microlensing by compact scalar clumps. We derive simple sensitivity estimates and null detection bounds on the scalar clump fraction, clock couplings and compact lens abundance. Our results show that Gaia-like astrometry can probe compact scalar clumps at the percent level near $M_s\gtrsim10^{-2}M_\odot$, while future astrometric and clock experiments can extend the reach to lower masses and weaker couplings.

astro-ph.CO↗

arXiv2026

Reconstructing the slope of a nearly flat quintessence potential from cosmography

We revisit thawing quintessence models with nearly flat scalar-field potentials using a cosmographic framework. Earlier work indicates that the cosmographic reconstruction of the slope $λ=-(dV/dϕ)/V$ of the quintessence potential in the general case requires the knowledge of the cosmographic paremeters up to the jerk parameter $j$. In this work we show that the slow-roll conditions $[(dV/dϕ)/V]^2 \ll 1$ and $|(d^2V/dϕ^2)/V| \ll 1$ allow the reconstruction of the slope of a nearly flat potential with knowledge of only the deceleration parameter $q$ (and the density parameter $Ω_ϕ$). Confronting the assumption of near-flatness with the cosmographic data after DESI DR2, however, reveals possible tension between the two. We further show that these models exhibit attractor behaviour in the $w$--$Ω_ϕ$ and $w$--$w'$ phase planes, corresponding to a universal thawing evolution with $w \approx -1$ at early times. We also derive the corresponding relation in the cosmographic $q$--$j$ plane and show that different cosmological expansion histories can produce the same thawing evolution. Nevertheless, all viable trajectories remain close to the $Λ$CDM limit $j=1$.

gr-qc↗

arXiv2026

Apocalypse When? Solar System Constraints on an Imminent Big Rip

Phantom dark energy models with an equation of state parameter $w < -1$ lead generically to a future big rip singularity, in which the dark energy density becomes infinite in a finite time. Current limits on dark energy constrain $w$ to be close to $-1$, and if $w$ is assumed constant, then a future big rip cannot occur in less than the order of a Hubble time in the future. However, many models allow $w$ to decrease rapidly with time. In that case, or if one assumes an additional phantom component with current energy density far below the dark energy density and $w << -1$, it is possible to achieve an imminent big rip, which we define to be a future singularity occuring in much less than the Hubble time. Such a possibility cannot be constrained by any cosmological measurements, as these are all based on light emitted billions of years in the past. Indeed, it is not possible, on the basis of cosmological observations, to rule out a future big rip tomorrow. However, solar system dynamics are sensitive to the behavior of phantom dark energy on timescales of decades rather than billions of years. Using solar system measurements, we are able to derive limits on the timescale for a future big rip independent of the dynamics of the phantom component. We obtain $t_{rip} - t_0 > 30$ years. While admittedly a poor limit, these results are likely to be improved by future more precise measurements of solar system dynamics. Our results also show that evidence for an imminent big rip would show up first in solar system data, rather than in any cosmological observation.

astro-ph.CO↗

arXiv2026

Dark Matter from Holography

Previous studies have examined the holographic principle as a means of producing dark energy. Here we propose instead the possibility of holographic dark matter. In this case, dark matter does not arise in the framework of particle physics but is derived from the infrared cutoff set by the horizon scale. Using the Ricci cutoff, and a universe containing only baryons and radiation, we can account for the dark matter and naturally explain the coincidence between baryonic and nonbaryonic contributions to the density. In the presence of a pre-existing vacuum energy density our model reverses the sign of this density, thus accounting for the fact that certain string theories generically predict a negative vacuum energy, but observations require a positive value.

astro-ph.CO↗

arXiv2026

Smoking Gun Signatures of Quasilocal Probability in Black Hole Ringdowns

Building on recent work introducing the idea of Quasilocal Probability in curved spacetime, we develop its observational implications for black hole ringdown in detail. We show that horizon-induced probability flux leads to an effective non-Hermitian dynamics producing three distinctive signatures, which are correlated multi-mode deviations, weak amplitude dependence and a mismatch between waveform damping and energy accounting. These effects arise from a single boundary-flux mechanism and therefore exhibit a constrained, low-dimensional structure not expected in generic modified gravity scenarios. We demonstrate that while individual deviations may be mimicked, their combined pattern provides a robust discriminator of quasilocal probability. We further argue that upcoming gravitational wave observations can probe these signatures at meaningful precision. We also establish that black hole ringdown is a novel arena to test whether quantum mechanical Hermiticity is really a fundamental property or an emergent symmetry in quantum gravity.

gr-qc↗

arXiv2026

Direct cosmographic reconstruction of the quintessence potential

We derive expressions for the first and second derivatives of the quintessence potential $V(ϕ)$, in terms of $λ= -V^{\prime}/V$ and $Γ= (V^{\prime \prime}/V)/(V^\prime/V)^2$, as functions of the quintessence density fraction $Ω_ϕ$ and the cosmographic parameters $q$, $j$, and $s$. Our mapping is not explicitly a function of the equation of state parameter $w$. We use these results, along with recent observational data, to derive expansions of $V(ϕ)$ about the present-day value of the scalar field, $ϕ_0$.

gr-qc↗

arXiv2026

Probing Dark Energy on the Moon

The effective field theory (EFT) of cosmic acceleration provides a model-independent framework for describing dark energy and modified gravity, yet many of its defining operators remain weakly constrained by existing observations. We show that measurements of horizon-scale metric fluctuations with a lunar laser interferometer can directly probe the kinetic sector of the EFT of dark energy, enabling constraints on operators governing scalar perturbation dynamics rather than only the background expansion history. In particular, we demonstrate sensitivity to the EFT kinetic coefficient $M_2^4$ and the associated sound speed of dark energy, $c_s^2$. This establishes a qualitatively new observational handle on the microphysical consistency conditions of late-time acceleration models, allowing broad regions of EFT parameter space to be probed, constrained, or potentially discovered.

astro-ph.CO↗

arXiv2026

Non-Hermitian Quantum Mechanics with Applications to Gravity

Hermiticity is usually treated as a foundational axiom of quantum mechanics, guaranteeing real spectra and unitary time evolution. In this work we argue that Hermiticity is more naturally understood as a symmetry law arising from the global conservation of an inner product current. We show that in spacetimes admitting complete Cauchy surfaces without boundary flux this conservation reduces to the familiar Hermiticity condition of the canonical inner product. However, in the presence of causal horizons, most strikingly in black hole geometries, this conservation law becomes obstructed for restricted observers. Tracing over inaccessible degrees of freedom then inevitably yields completely positive trace preserving dynamics with an effective non-Hermitian generator. Using quantum thermodynamics and the monotonicity of relative entropy, we demonstrate that the generalized second law may be reinterpreted as an entropy balance that compensates precisely for the flux of inner product charge through the horizon. The structure of Einstein equations, through the Bianchi identity and the Raychaudhuri focusing equation, provides the geometric mechanism underlying this balance. We also show that black hole ringdown can serve as a realistic observational probe of this idea and may provide quantitative upper bounds on the strength of horizon-induced inner product flux. In this way gravity, entropy production, and effective non-Hermiticity are unified under a single structural principle, with Hermiticity emerging as the special case of globally conserved inner product symmetry.

gr-qc↗

arXiv2026

Partial Relief of the Hubble Tension and a Natural Self-Interacting Dark Matter Candidate From Staged Symmetry Breaking

The values of the Hubble constant ($\rm{H_0}$) inferred from the cosmic microwave background (CMB) and local measurements via the distance ladder exhibit a $\sim5σ$ tension. In this work we propose that the tension might be partially alleviated if a subcomponent of the dark matter undergoes decays triggered by spontaneous symmetry breaking in the dark sector, so that the equation of state parameter of the subcomponent shifts from $w \approx 0$ at early times to $w \approx -1/3$ at late times. We provide an effective field theory whose structure is partially motivated by the desire for a plausible UV completion. We find that such a construction naturally produces a possible self-interacting dark matter candidate with a velocity-dependent scattering cross section as a by-product of gauge invariance. This is relevant for addressing tensions between the predictions of $Λ$CDM and observations of small-scale structure, such as the core-cusp problem.

astro-ph.CO↗

arXiv2026

Probing the Sound Speed of Dark Energy with a Lunar Laser Interferometer

The sound speed of dark energy encodes fundamental information about the microphysics underlying cosmic acceleration, yet remains essentially unconstrained by existing observations. We demonstrate that a lunar-based laser interferometer, such as the proposed Laser Interferometer Lunar Antenna (LILA), can directly probe the sound speed of dark energy by measuring the real-time evolution of horizon-scale gravitational potentials. Operating in the ultra-low-frequency gravitational band inaccessible from Earth, LILA is sensitive to scalar metric perturbations sourced by dark energy dynamics. Using both fluid and effective field theory descriptions, we develop a complete framework linking dark energy sound speed to observable strain signatures. We construct a likelihood pipeline and Fisher forecasts, showing that LILA can either detect clustering dark energy or exclude broad classes of models with unprecedented sensitivity. This establishes lunar interferometry as a novel and powerful probe of the physics driving cosmic acceleration.

astro-ph.CO↗

arXiv2025

DESI-Like Hubble Expansion From Staged Symmetry Breaking

The Dark Energy Spectroscopic Instrument (DESI) second data release shows a moderate preference for dark energy with a time-varying equation of state parameter, suggesting that the standard $Λ$CDM model may need to be revised. In particular, DESI favors dark energy whose equation of state parameter can drop below $-1$, violating the null energy condition. Chen and Loeb have recently suggested that this violation may be avoided if a subcomponent of the dark matter possesses a time-dependent equation of state. In this work, we present a realization of that idea which can be regarded as a more natural effective field theory. We show that such a construction not only yields dark matter with a time-varying equation of state parameter, but also naturally produces a self-interacting dark matter candidate with a velocity-dependent cross section as a consequence of gauge invariance. The second feature is relevant for addressing tensions between $Λ$CDM and observations of small-scale structure, particularly the diversity of galactic rotation curves.

astro-ph.CO↗

arXiv2025

Comparing the Spatial Correlation of Binary Black Hole Mergers to Large-Scale Structure through the Illustris Simulation

Gravitational waves (GWs) have provided a new lens through which to view the universe beyond traditional electromagnetic methods. The upcoming space-based gravitational wave mission, Laser Interferometer Space Antenna (LISA), will give us the first glimpse of the sky in mHz gravitational waves, a waveband that contains a rich variety of sources including massive binary black hole (MBBH) mergers. In this work, we investigate the spatial distribution of MBBH mergers versus the galaxy distribution to determine how well LISA could be used as a unique and independent probe of large-scale structure. We compare the two-point correlation function (2pt CF) of MBBH mergers to that of galaxies within the cosmological hydrodynamic simulation IllustrisTNG. Our results show that MBBH mergers exhibit stronger clustering than galaxies at scales less than 10 Mpc $h^{-1}$, particularly at higher redshifts, and that the bias is relatively constant as a function of separation. These findings imply that the spatial distribution of MBBH mergers detectable by LISA could inform the observed galaxy distribution. In addition, this implies that searches for a cosmological background in LISA data could use a prior derived from large-scale structure observations to subtract the MBBH foreground.

astro-ph.CO↗

arXiv2025

A long freeze is hard to achieve in the presence of matter

Certain holographic dark energy (HDE) models allow for the possibility of a ``long freeze,'' in which the scale factor evolves to a constant in the long-time limit. Here we extend previous calculations by adding a nonrelativistic matter component. The addition of a matter component tends to destroy the long freeze behavior, driving the universe to recollapse. Long freeze evolution is still possible, but only for a limited set of HDE models.

gr-qc↗

arXiv2025

The long freeze: an asymptotically static universe from holographic dark energy

We show that some holographic dark energy models can lead to a future evolution of the universe in which the scale factor $a$ is asymptotically constant, while $\dot a \rightarrow 0$ and the corresponding energy and pressure densities also vanish. We provide specific examples of such models and general conditions that can lead to an asymptotically static universe, which we have called the ``long freeze." In some cases, such evolution can follow an arbitrarily long exponential expansion essentially identical to the asymptotic evolution of $Λ$CDM. When nonrelativistic matter is added to the holographic dark energy, it tends to destroy the long freeze behavior, driving the universe to recollapse. We show that a long freeze evolution is still possible, but only for a more limited set of HDE models.

astro-ph.CO↗

arXiv2025

Gravitational Effects of a Small Primordial Black Hole Passing Through the Human Body

The gravitational effects of a primordial black hole (PBH) passing through the human body are examined, with the goal of determining the minimum mass necessary to produce significant injury or death. Two effects are examined: the damage caused by a shock wave propagating outward from the black hole trajectory, and the dissociation of brain cells from tidal forces produced by the black hole on its passage through the human body. It is found that the former is the dominant effect, with a cutoff mass for serious injury or death of approximately $M_{PBH} > 1.4 \times 10^{17} {\rm g}$. The number density of primordial black holes with a mass above this cutoff is far too small to produce any observable effects on the human population.

astro-ph.CO↗

arXiv2025

What do we learn by mapping dark energy to a single value of $w$?

We examine several dark energy models with a time-varying equation of state parameter, $w(z)$, to determine what information can be derived by fitting the distance modulus in such models to a constant equation of state parameter, $w_*$. We derive $w_*$ as a function of the model parameters for the Chevallier-Polarski-Linder (CPL) parametrization, and for the Dutta-Scherrer approximation to hilltop quintessence models. We find that all of the models examined here can be well-described by a pivot-like redshift, $z_{pivot}$ at which the value of $w(z)$ in the model is equal to $w_*$. However, the exact value of $z_{pivot}$ is a model-dependent quantity; it varies from $z_{pivot} = 0.22-0.25$ for the CPL models to $z_{pivot} = 0.17-0.20$ for the hilltop quintessence models. Hence, for all of the models considered here, a constant-$w$ fit gives the value of $w$ for $z$ near 0.2. However, given the fairly wide variation in $z_{pivot}$ over even this restricted set of models, the information gained by fitting to a constant value of $w$ seems rather limited.

astro-ph.CO↗