arXiv subjects

Eashwar N. Sivarajan

Publications and source records attributed to Eashwar N. Sivarajan.

3 recordsLinked to original sources

arXiv2026

Dark Matter with a Drag at Low Redshift

Recent analyses of $fσ_8$ and weak-lensing data indicate that the linear growth rate at $z\lesssim 1$ may be lower than predicted by $Λ$CDM. This motivates models of dark matter in which large scale structure growth slows relative to $Λ$CDM at late times. We construct particle models in which dark matter experiences a drag with dark radiation that grows at late times, unlike conventional DM--DR interactions, which fade as the universe expands. A key ingredient is that the radiation interacting with the dark matter is produced at late times from dark matter decay. An explicit model, interacting Decaying Cold Dark Matter (iDCDM), adds two parameters beyond $Λ$CDM while leaving the background, BBN, and primary CMB intact. But it predicts a step-shaped suppression of the linear growth rate $f(k,z)$, a distinctive target for DESI, Euclid, and Rubin. Confronted with current data, iDCDM shows a modest preference over $Λ$CDM, driven by $fσ_8$, with $Δχ^2$ between $-2.7$ and $-7.6$ depending on the assumed scaling of the drag with redshift and on neutrino masses. The decisive test will come from upcoming $k$- and $z$-resolved growth measurements.

astro-ph.CO↗

arXiv2024

Stepping into the Forest: Confronting Interacting Radiation Models for the Hubble Tension with Lyman-$α$ Data

Models of interacting dark radiation have been shown to alleviate the Hubble tension. Extensions incorporating a coupling between dark matter and dark radiation (DM-DR) have been proposed as combined solutions to both the Hubble and $S_8$ tensions. A key feature of these extended models is a break in the matter power spectrum (MPS), suppressing power for modes that enter the horizon before the DM-DR interactions turn off. In scenarios with a massless mediator, modes that enter before matter-radiation equality get suppressed, whereas for massive mediators, the break is determined by the mediator mass, a free parameter. In this work, we test these models against probes of LSS: weak lensing, CMB lensing, full-shape galaxy clustering, and eBOSS measurements of the 1D Ly$α\,$ forest flux power spectrum. The latter are the most constraining since they probe small scales where many models predict the largest deviations. In fact, already within $Λ{\rm CDM}\,$, the eBOSS Ly$α\,$ data are in significant tension with Planck CMB data, with the Ly$α\,$ data preferring a steeper slope of the MPS at $k \sim h \mathrm{Mpc}^{-1}$. We find that the simplest dark radiation models, which improve the Hubble tension, worsen the fit to the Ly$α\,$ data. However, models with DM-DR interactions can simultaneously address both tensions.

astro-ph.CO↗

arXiv2023

A Step in Understanding the $S_8$ Tension

Models of dark sectors with a mass threshold can have important cosmological signatures. If, in the era prior to recombination, a relativistic species becomes nonrelativistic and is then depopulated in equilibrium, there can be measurable impacts on the cosmic microwave background as the entropy is transferred to lighter relativistic particles. In particular, if this ``step'' occurs near $z\sim 20,000$, the model can naturally accommodate larger values of $H_0$. If this stepped radiation is additionally coupled to dark matter, there can be a meaningful impact on the matter power spectrum as dark matter can be coupled via a species that becomes nonrelativistic and depleted. This can naturally lead to suppressed power at scales inside the sound horizon before the step, while leaving conventional cold dark matter signatures for power outside the sound horizon. We study these effects and show such models can naturally provide lower values of $S_8$ than scenarios without a step. This suggests these models may provide an interesting framework to address the $S_8$ tension, both in concert with the $H_0$ tension and without.

astro-ph.CO↗