New Light Species and the CMB

TL;DR

This paper investigates how new light particles affect the Cosmic Microwave Background, providing a framework to connect theoretical models with observational data to constrain their properties.

Contribution

It offers a comprehensive mapping between model parameters and the effective number of relativistic degrees of freedom, enabling interpretation of CMB data in terms of new light species.

Findings

Constraints on models with new light species from Planck data

Numerical calculations of each model's contribution to relativistic degrees of freedom

A framework for future CMB measurements to further constrain these models

Abstract

We consider the effects of new light species on the Cosmic Microwave Background. In the massless limit, these effects can be parameterized in terms of a single number, the relativistic degrees of freedom. We perform a thorough survey of natural, minimal models containing new light species and numerically calculate the precise contribution of each of these models to this number in the framework of effective field theory. After reviewing the relevant details of early universe thermodynamics, we provide a map between the parameters of any particular theory and the predicted effective number of degrees of freedom. We then use this map to interpret the recent results from the Cosmic Microwave Background survey done by the Planck satellite. Using this data, we present new constraints on the parameter space of several models containing new light species. Future measurements of the Cosmic Microwave Background can be used with this map to further constrain the parameter space of all such models.