Circumventing Cosmic Variance via Remote Quadrupole Measurements

TL;DR

This paper develops a Fisher formalism to evaluate how remote quadrupole measurements via the Kamionkowski-Loeb effect can improve constraints on large-scale cosmological perturbations, potentially surpassing local CMB data.

Contribution

The authors introduce a real-space Fisher formalism for remote quadrupole surveys and demonstrate its effectiveness in constraining large-scale primordial power spectrum features.

Findings

Remote quadrupole surveys can improve constraints by over 3σ with 100 clusters.

Full-sky, low-noise surveys can enhance constraints to over 7σ.

The formalism allows flexible survey design exploration.

Abstract

A number of important cosmological questions can be addressed only by probing perturbation modes on the largest accessible scales. One promising probe of these modes is the Kamionkowski-Loeb effect, i.e., the polarization induced in the cosmic microwave background (CMB) by Thomson scattering in galaxy clusters, which is proportional to the CMB quadrupole measured at the cluster's location and look-back time. We develop a Fisher formalism for assessing the amount of new information that can be obtained from a future remote quadrupole survey. To demonstrate the constraining power of such a survey, we apply our formalism to a model that suppresses the primordial power spectrum on large scales but is poorly constrained with existing CMB data. We find that the constraints can be improved by over $3\sigma$ for a survey that measures around 100 clusters over $20\%$ of the sky with a signal-to-noise ratio of $3$. In the most optimistic case with a low-noise survey with dense full-sky coverage and only a single degree of freedom in the theory, the constraint improves to over $7\sigma$ beyond local CMB data. Our formalism, which is based in real space rather than harmonic space, can be used to explore a wide range of survey designs, and our results paint an optimistic picture for the utility of remote quadrupole measurements to probe physics on the largest observable scales in the Universe.