arXiv subjects

Charuhas Shiveshwarkar

Publications and source records attributed to Charuhas Shiveshwarkar.

8 recordsLinked to original sources

arXiv2026

Connecting the Dots: Prospects for Detecting Multi-Source Inflation through Scale-Dependent and Stochastic Bias of Little Red Dots

Recent work has proposed using the spatial clustering of Little Red Dots (LRDs) as a high-redshift cosmological probe. In this paper, we investigate the feasibility of obtaining a LRD sample with a future space experiment, and forecast LRD constraints on primordial local-type non-Gaussianity, $f^{\rm loc}_{\rm NL}$, and the collapsed-limit trispectrum amplitude, $τ^{\rm loc}_{\rm NL}$. LRDs are ideally suited to probe these inflationary parameters because they span a large cosmological volume, have high bias and are easily identifiable. We perform idealised joint $f_{\rm NL}^{\rm loc}$--$τ_{\rm NL}^{\rm loc}$ Fisher forecasts assuming the universality relation, Gaussian covariance, Poisson shot-noise and a normalised redshift dispersion, $σ(z) / \left( {1+z} \right)= 0.03$. We quantify the significance at which one would detect multi-source inflation under the Suyama--Yamaguchi relation, $τ^{\rm loc}_{\rm NL} > \left( 6/5 f_{\rm NL}\right)^2$, across the $f_{\rm NL}^{\rm loc}$--$τ_{\rm NL}^{\rm loc}$ plane. We find that a LRD analysis could robustly detect multi-source inflation at high-significance over a large region of parameter space, and would be more constraining than all other past or present experiments.

astro-ph.CO↗

arXiv2026

PSFSim: PhySics First Simulations for the Point Spread Function of the Roman Space Telescope

We present a new tool for generating simulated point spread functions (PSFs) in the Wide Field Instrument (WFI) of the Nancy Grace Roman Space Telescope. The simulation tool is physics forward, and performs a raytrace of the optical system of the instrument to compute the PSF, while also accounting for various detector effects. The tool also has the capability to add perturbations to the optical system and detectors. We explore several applications of the tool that may arise during the commissioning of the telescope, including optical ghosts, measuring the optical aberrations from diffraction spikes, and studying the effect of mirror polarization.

astro-ph.IM↗

arXiv2026

Modeling the impact of filter-substrate refraction in the Roman point spread function

For broadband imaging surveys, filter-substrate refraction causes light at different wavelengths to follow slightly different paths through the filter substrate before reaching the detector. This effect produces two chromatic perturbations to the point spread function (PSF): a shift in the effective focal position along the optical axis (longitudinal shift), which manifests as a defocus-like perturbation, and a wavelength-dependent displacement of the image position in the focal plane (lateral shift), which manifests as image decentering. Using image simulations, we provide the first study of these two effects independently across all eight Roman imaging bands and over the full focal plane. We compute the resulting PSF and photometric errors from images with and without the effect included, and compare the magnitude of the effect to the Roman science requirements. We find that the lateral shift is the dominant contribution, producing PSF size and ellipticity residuals in most bands of order ~0.3-0.4%. These exceed the Roman science requirements for weak lensing by roughly an order of magnitude. The effect is also strongly field dependent, increasing toward the edges of the focal plane. By contrast, flux residuals remain below one third of the 1% requirement for most bands, except in R062 and W146. We find the longitudinal shift to be subdominant and negligible in most bands, including the weak lensing bands. Finally, we implement the dominant lateral-shift effect in a framework suitable for large-scale image simulations and validate that the resulting PSF size and shape changes are accurately reproduced. Overall, we find that filter-substrate refraction is a relevant chromatic effect for Roman PSF modeling, and we provide tools to model and incorporate it in large-scale image simulations.

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arXiv2025

The kSZ optical depth degeneracy and future constraints on local primordial non-Gaussianity

Recent reconstructions of the large-scale cosmological velocity field with kinetic Sunyaev Zeldovich (kSZ) tomography have returned an amplitude that is low with respect to the halo model prediction, captured by the kSZ velocity reconstruction bias $b_v <1$. This suggests that common choices for modeling the galaxy-electron cross correlation have systematically overestimated the true power, at least over scales and redshifts used in the velocity reconstruction measurements. In this paper, we study the implications of this overestimation for constraints on local-type primordial non-Gaussianity in current and near-future cosmological surveys. For concreteness, we focus on kSZ velocity reconstruction from a Vera Rubin Observatory-like survey in tandem with contemporary cosmic microwave background measurements. Assuming standard choices for the fiducial model of the small-scale galaxy-electron cross correlation, we find that upcoming kSZ tomography measurements can significantly improve constraints on local primordial non-Gaussianity via measurement of scale-dependent galaxy bias, in broad concordance with previous studies of the application of kSZ tomography to primordial non-Gaussianity. However, when we instead modify the assumed galaxy-electron cross-spectrum to be consistent with recent measurements of the velocity reconstruction bias, this picture can change appreciably. Specifically, we find that if the inferred suppression of galaxy-electron power persists at higher redshifts $z\gtrsim 1$, kSZ-driven improvement in local primordial non-Gaussianity constraints may be less significant than previously estimated. We explore how these conclusions depend on various modeling and experimental assumptions and discuss implications for the emerging program of kSZ velocity reconstruction.

astro-ph.CO↗

arXiv2025

Where does non-Universality in Assembly Bias come from?

Constraints on local primordial non-Gaussianity (LPnG) obtained from galaxy power spectra are limited by the perfect degeneracy between the LPnG parameter $f_{\rm NL}$ and the bias parameter $b_ϕ$ which encodes the response of galaxy clustering to a change in the amplitude of primordial curvature fluctuations. For galaxies observed by galaxy surveys, the relation between $b_ϕ$ and the galaxy bias $b_{g}$ is poorly understood and differs significantly from the universal mass function ansatz. In this paper, we investigate this non-universality in the context of dark-matter halos using the Separate Universe framework, focussing on dark-matter halos selected by mass and/or concentration. We show that the Separate Universe framework provides a natural explanation of the observed universality in the bias of dark-matter halos selected purely by their mass, independent of the spherical collapse picture of halo formation. We further propose an explanation for the observed non-universality in halos selected by concentration and corroborate it with $N$-body simulations in scale-free (EdS) and $Λ\text{CDM}$ cosmologies. In particular, we show that the relation between $b_ϕ$ and halo bias $b_{h}$ for halos selected by concentration in matter-dominated cosmologies tends towards universality at the highest halo masses due to such halos gravitationally dominating their environment throughout their evolution. We also argue that concentration-selected halos of lower masses exhibit non-universality due to their mass accretion being significantly affected by the gravitational influence of neighbouring, more massive halos. Our results suggest that any non-universality in high redshift ($z\gtrsim 3$), high-bias objects observed by realistic galaxy surveys is entirely an artifact of the associated selection function.

astro-ph.CO↗

arXiv2024

Constraining general multi-field inflation using the SPHEREx all-sky survey

We investigate how well the SPHEREx all-sky survey can constrain local primordial non-Gaussianity beyond the parameter $f_{\text{NL}}$ using galaxy power spectra. We forecast joint constraints on the parameters $f_{\text{NL}}$, $g_{\text{NL}}$ and $τ_{\text{NL}}$ obtained assuming a simple two-field curvaton model of inflation. The parameters $f_{\text{NL}}$ and $g_{\text{NL}}$ characterise the squeezed limits of the primordial bispectrum and trispectrum respectively, and lead to a characteristic scale-dependence of the galaxy bias that increases out to arbitrarily large scales. Values of the parameter $τ_{\text{NL}}> (\frac{6}{5}f_{\text{NL}})^{2}$ cause the galaxy power spectrum to have a stochastic component which also increases out to arbitrarily large scales. Our MCMC forecasts indicate that SPHEREx can provide joint constraints on any two of the three parameters $f_{\text{NL}}, \ g_{\text{NL}}$ and $τ_{\text{NL}}$. Due to strong degeneracies among these parameters, measurements of the galaxy power spectra alone may not be sufficient to jointly constrain all three. Constraints on $f_{\text{NL}}, \ g_{\text{NL}}$ and $τ_{\text{NL}}$ obtained from galaxy power spectrum observations depend on the modelling of underlying nuisance parameters. We study the robustness of our forecast constraints to modelling choices and note that even with relatively conservative modelling assumptions, SPHEREx galaxy power spectra can provide strong evidence of local non-Gaussianity, even if the particular values of $f_{\text{NL}}$ and $g_{\text{NL}}$ cannot be measured precisely.

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arXiv2023

Post-inflationary Contamination of Local Primordial Non-Gaussianity in Galaxy Power Spectra

The scale-dependent bias of galaxy density contrasts is an important signal to be extracted in constraining local primordial non-Gaussianity ($f_{\rm NL}^{\text{local}}$) from observations of large-scale structure. Constraints so obtained rely on the assumption that horizon-scale features in the galaxy power spectrum are exclusively due to primordial physical mechanisms. Yet, post-inflationary effects can induce modulations to the galaxy number density that appear as horizon-scale, scale-dependent bias. We investigate the effect of two such sources of scale-dependent bias - the free-streaming of light relics and fluctuations in the background of ionising radiation - on precision measurements of local primordial non-Gaussianity $f_{\rm NL}^{\text{local}}$ from galaxy power spectrum measurements. Using the SPHEREx survey as a test case survey reaching $σ(f_{\rm NL}^{\rm local}) \lesssim 1$, we show that ignoring the scale-dependent bias induced by free-streaming particles can negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $\sim 0.1-0.3σ$. Ignoring the effect of ionising radiation fluctuations can negatively bias the inferred value of $f_{\rm NL}^{\rm local}$ by $ \sim 1σ$. The range of biases depends on the source populations and the ranges of scales used in the analysis, as well as the value of the neutrino mass and the modelling of the impact of ionising radiation. If these sources of scale-dependent bias are included in the analysis, forecasts for $f_{\rm NL}^{\rm local}$ are unbiased but degraded.

astro-ph.CO↗

arXiv2020

Scale-dependent halo bias and the squeezed limit bispectrum in the presence of radiation

We investigate the gravitational effect of large-scale radiation perturbations on small-scale structure formation. In addition to making the growth of matter perturbations scale dependent, the free-streaming of radiation also affects the coupling between structure formation at small and large scales. We study this using Separate Universe N-body simulations to compute the (isotropized) squeezed-limit matter bispectrum and the linear halo bias. Our results show that the scale dependence in the growth of long-wavelength matter perturbations, caused by radiation, translates into these quantities acquiring a non-trivial scale-dependence at $k\lesssim 0.05$ Mpc$^{-1}$. In a universe with radiation composed of cosmic microwave background photons and three species of massless neutrinos, the bias of halos with $b = 2$ at high $k$ will decrease by $0.29\%,\ 0.45\%$ and $0.8\%$ between $k = 0.05$ Mpc$^{-1}$ and $k = 0.0005$ Mpc$^{-1}$ at redshifts $z=0,\ 1$, and $3$ respectively. For objects with $b\gg1$, these differences approach $0.43\%,\ 0.68\%$ and $1.2\%$ respectively.

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