arXiv subjects

Kevin S. Croker

Publications and source records attributed to Kevin S. Croker.

8 recordsLinked to original sources

arXiv2026

A self-consistent analytical model for both the photoionization rate and reionization history

Recent developments at the intersection of cosmology and astrophysics have highlighted the need for improved analytical models of observables that probe the Epoch of Reionization. With few exceptions, fast analytical treatments of reionization suitable for use in Bayesian inference have been limited to modeling the reionization history, $x_i(z)$. Such models cannot take full advantage of observables that constrain $x_i$ indirectly. One such observable is the photoionization rate of neutral hydrogen, $Γ_{\rm HI}(z)$, which can be inferred from the mean transmission of the Lyman-$α$ forest of high-redshift quasars and galaxies. It has been shown by several prior works that the evolution of $Γ_{\rm HI}$ at $5 \lesssim z \lesssim 6$ is highly sensitive to the tail end of reionization, potentially providing a tight astrophysical constraint on the reionization timeline. We present a new analytical formalism, based on the cosmological radiative transfer equation, that self-consistently predicts $x_i$ and $Γ_{\rm HI}$. We test our model against detailed radiative transfer simulations and find it to be percent-level accurate in $x_i$ and $20-30\%$ accurate in $Γ_{\rm HI}$ at $z \lesssim 6$ - better than or comparable to existing observational uncertainties. Finally, we demonstrate that modest shifts in the ionizing photon output of high-redshift galaxies and/or the endpoint of reionization lead to differences in $Γ_{\rm HI}$ much larger that the model's intrinsic uncertainty, highlighting its utility for interpreting existing data. We explore the origin of modeling uncertainty in $Γ_{\rm HI}$ and comment on future pathways for improvement.

astro-ph.CO↗

arXiv2026

We Must Preserve Hubble given its Unique Complementarity to Webb, Roman, and Euclid

We present compelling arguments -- focusing on galaxy science -- for preserving the main imagers and operational modes of the Hubble Space Telescope (HST) for as long as is technically feasible, to assure maximum complementarity to the James Webb Space Telescope (JWST), Roman, and Euclid. HST was designed to work well over the 0.1-1.6 $μ$m wavelength range, and its unique UV-optical performance has fundamentally contributed to our understanding of galaxy assembly and the Cosmic Star Formation History (CSFH). While star-formation started at redshifts $z \gtrsim 10$, when the universe was less than 500 Myr old, the CSFH did not peak until $z \simeq 1.9$ (i.e., about 10 Gyr ago), and has steadily declined since that time. Hence, at least half of all stars in the universe formed it in the last 10 Gyrs where HST provides its unique rest-frame UV view of unobscured young, massive stars tracing cosmic star-formation, as well as unobscured Active Galactic Nuclei (AGN). HST thus uniquely probes (unobscured) young, hot, massive stars and AGN in galaxies, while JWST, Euclid and Roman reveal more advanced stages of older stellar populations, as well as relatively short-lived phases where galaxies produce and shed a lot of dust from intense star-formation, dusty AGN, and the very high redshift universe ($z \gtrsim 10$) not accessible by HST. HST is thus highly complementary to these other facilities, all of which took decades to build to ensure decades of operation. To maximize return on investment in these facilities, ways will need to be found to operate HST imaging instruments in all relevant modes for as long as possible into the JWST and Roman missions.

astro-ph.GA↗

arXiv2025

The CMB optical depth constrains the duration of reionization

Recently, it was pointed out that invoking a large value of the CMB optical depth, $τ_{\rm CMB} = 0.09$, could help resolve tensions between DESI DR2 BAO data and the CMB. This is larger than the value of $τ_{\rm CMB} = 0.058$ measured from the Planck low-$\ell$ polarization data. Traditionally, $τ_{\rm CMB}$ is thought of as a constraint on reionization's midpoint. However, recent observations and modeling of the Ly$α$ forest of high-$z$ quasars at $5 < z < 6$ have tightly constrained the timing of the last $10-20\%$ of reionization, adding nuance to this interpretation. Here, we point out that fixing reionization's endpoint, in accordance with the latest Ly$α$ forest constraints, renders $τ_{\rm CMB}$ a sensitive probe of the duration of reionization, as well as its midpoint. We compare low and high values of $τ_{\rm CMB}$ to upper limits on the patchy kinematic Sunyaev-Zeldovich (pkSZ) effect, another CMB observable that constrains reionization's duration, and find that a value of $τ_{\rm CMB} = 0.09$ is in $\approx 2σ$ tension with existing limits on the pkSZ from the South Pole Telescope.

astro-ph.CO↗

arXiv2024

DESI Dark Energy Time Evolution is Recovered by Cosmologically Coupled Black Holes

Recent baryon acoustic oscillation (BAO) measurements by the Dark Energy Spectroscopic Instrument (DESI) provide evidence that dark energy (DE) evolves with time, as parameterized by a $w_0 w_a$ equation of state. Cosmologically coupled black holes (BHs) provide a DE source that naturally evolves with time, because BH production tracks cosmic star-formation. Using DESI BAO measurements and priors informed by Big Bang Nucleosynthesis, we measure the fraction of baryonic density converted into BHs, assuming that all DE is sourced by BH production. We find that the best-fit DE density tracks each DESI best-fit $w_0w_a$ model within $1σ$, except at redshifts $z \lesssim 0.2$, highlighting limitations of the $w_0w_a$ parameterization. Cosmologically coupled BHs produce $H_0 = (69.94 \pm 0.81)~\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$, with the same $χ^2$ as $Λ$CDM, and with two fewer parameters than $w_0w_a$. This value reduces tension with SH0ES to $2.7σ$ and is in excellent agreement with recent measurements from the Chicago-Carnegie Hubble Program. Because cosmologically coupled BH production depletes the baryon density established by primordial nucleosynthesis, these BHs provide a physical explanation for the ``missing baryon problem'' and the anomalously low sum of neutrino masses preferred by DESI. The global evolution of DE is an orthogonal probe of cosmological coupling, complementing constraints on BH mass-growth from elliptical galaxies, stellar binaries, globular clusters, the LIGO-Virgo-KAGRA merging population, and X-ray binaries. A DE density that correlates with cosmic star-formation: 1) is a natural outcome of cosmological coupling in BH populations; 2) eases tension between early and late-time cosmological probes; and 3) produces time-evolution toward a late-time $Λ$CDM cosmology different from Cosmic Microwave Background projections.

astro-ph.CO↗

arXiv2023

Observational evidence for cosmological coupling of black holes and its implications for an astrophysical source of dark energy

Observations have found black holes spanning ten orders of magnitude in mass across most of cosmic history. The Kerr black hole solution is however provisional as its behavior at infinity is incompatible with an expanding universe. Black hole models with realistic behavior at infinity predict that the gravitating mass of a black hole can increase with the expansion of the universe independently of accretion or mergers, in a manner that depends on the black hole's interior solution. We test this prediction by considering the growth of supermassive black holes in elliptical galaxies over $0<z\lesssim2.5$. We find evidence for cosmologically coupled mass growth among these black holes, with zero cosmological coupling excluded at 99.98% confidence. The redshift dependence of the mass growth implies that, at $z\lesssim7$, black holes contribute an effectively constant cosmological energy density to Friedmann's equations. The continuity equation then requires that black holes contribute cosmologically as vacuum energy. We further show that black hole production from the cosmic star formation history gives the value of $Ω_Λ$ measured by Planck while being consistent with constraints from massive compact halo objects. We thus propose that stellar remnant black holes are the astrophysical origin of dark energy, explaining the onset of accelerating expansion at $z \sim 0.7$.

astro-ph.CO↗

arXiv2022

Well-defined equations of motion without constraint of external sources

We present a new approach to constrained classical fields that enables the action formalism to dictate how external sources must enter the resulting equations of motion. If symmetries asserted upon the varied fields can be modeled as restrictions in Fourier space, we prove that these restrictions are automatically applied to external sources in an unambiguous way. In contrast, the typical procedure inserts symmetric ansatze into the Euler-Lagrange differential equations, even for external sources not being solved. This requires ad hoc constraint of external sources, which can introduce leading-order errors to model systems despite superficial consistency between model field and source terms. To demonstrate, we consider Robertson-Walker cosmologies within General Relativity and prove that the influence of point-like relativistic pressure sources on cosmological dynamics cannot be excluded by theoretical arguments.

gr-qc↗

arXiv2021

Cosmologically coupled compact objects: a single parameter model for LIGO--Virgo mass and redshift distributions

We demonstrate a single-parameter route for reproducing higher mass objects as observed in the LIGO--Virgo mass distribution, using only the isolated binary stellar evolution channel. This single parameter encodes the cosmological mass growth of compact stellar remnants that exceed the Tolman-Oppenheimer-Volkoff limit. Cosmological mass growth appears in known solutions to General Relativity with cosmological boundary conditions. We consider the possibility of solutions with cosmological boundary conditions, which reduce to Kerr on timescales short compared to the Hubble time. We discuss complementary observational signatures of these solutions that can confirm or invalidate their astrophysical relevance.

gr-qc↗

arXiv2021

Implications of Symmetry and Pressure in Friedmann Cosmology. I. Formalism

We show that derivation of Friedmann's equations from the Einstein-Hilbert action, paying attention to the requirements of isotropy and homogeneity during the variation, leads to a different interpretation of pressure than what is typically adopted. Our derivation follows if we assume that the unapproximated metric and Einstein tensor have convergent perturbation series representations on a sufficiently large Robertson-Walker coordinate patch. We find the source necessarily averages all pressures, everywhere, including the interiors of compact objects. We demonstrate that our considerations apply (on appropriately restricted spacetime domains) to the Kerr solution, the Schwarzschild constant-density sphere, and the static de-Sitter sphere. From conservation of stress-energy, it follows that material contributing to the averaged pressure must shift locally in energy. We show that these cosmological energy shifts are entirely negligible for non-relativistic material. In relativistic material, however, the effect can be significant. We comment on the implications of this study for the dark energy problem.

gr-qc↗