Imprint of Distortions in the Oort Cloud on the CMB Anisotropies

TL;DR

This paper investigates how close stellar encounters can distort the Oort Cloud, creating measurable anisotropies in the CMB, and discusses how these distortions could inform us about past stellar fly-bys.

Contribution

It introduces a model linking stellar fly-bys to specific CMB anisotropies caused by Oort Cloud distortions, providing a new method to study stellar encounters.

Findings

Quadrupole moments in CMB fluctuations due to stellar encounters are quantified.

Comparison shows these distortions are smaller than cosmological fluctuations.

Potential to use CMB spectral distortions to trace past stellar fly-bys.

Abstract

We study the effect of a close encounter of a passing star on the shape of the inner Oort Cloud, using the impulse approximation. The deviation of the perturbed Oort Cloud from sphericity adds angular fluctuations to the brightness of the Cosmic Microwave Background (CMB) due to thermal emission by the comets. An encounter with a solar-mass star at an impact parameter of $1750 \au$, as expected based on the abundance and velocity dispersion of stars in the solar neighborhood, leads to a quadrupole moment in the square of the fractional CMB intensity fluctuation of $C_2 = 4.5 \times 10^{-15}, 6.7 \times 10^{-12}, 1.1 \times 10^{-9}$ at $\nu = 30, 353, 545 \GHz$ (these being the frequency bands of the upcoming Planck satellite). We also quantify the quadrupole spectral distortions produced by the Scattered Disc, which will exist regardless of any perturbation and the subsequent shape of the Oort Cloud. For comparison, the square fractional temperature fluctuation quadrupole moment predicted by the current cosmological model is $C_2 = 1.76 \times 10^{-10}$, which corresponds to fluctuations in the CMB intensity of $C_2 = 2.9 \times 10^{-10}, 6.8 \times 10^{-9}, 1.6 \times 10^{-8}$ at $\nu = 30, 353, 545 \GHz$. Finally, we discuss how a measurement of the anisotropic spectral distortions could be used to constrain the trajectory of the closest stellar fly-by.