Early Cosmology Constrained

TL;DR

This paper constrains possible additional energy components in the early universe using a largely model-independent approach, finding tight bounds on extra fluids and radiation, and analyzing the impact on the standard ruler, the sound horizon.

Contribution

It introduces a method to separate early- and late-universe information, reducing model dependence in constraining early universe energy components.

Findings

95% credible bounds: $ ext{Ω}_{ m MR} < 0.006$ and $2.3 < N_{ m eff} < 3.2$

Standard ruler length $r_{ m s} = 147.4 ext{ Mpc}$ with standard components

Uncertainty in $r_{ m s}$ increases when allowing non-standard dark radiation

Abstract

We investigate our knowledge of early universe cosmology by exploring how much additional energy density can be placed in different components beyond those in the $\Lambda$CDM model. To do this we use a method to separate early- and late-universe information enclosed in observational data, thus markedly reducing the model-dependency of the conclusions. We find that the 95\% credibility regions for extra energy components of the early universe at recombination are: non-accelerating additional fluid density parameter $\Omega_{\rm MR} < 0.006$ and extra radiation parameterised as extra effective neutrino species $2.3 < N_{\rm eff} < 3.2$ when imposing flatness. Our constraints thus show that even when analyzing the data in this largely model-independent way, the possibility of hiding extra energy components beyond $\Lambda$CDM in the early universe is seriously constrained by current observations. We also find that the standard ruler, the sound horizon at radiation drag, can be well determined in a way that does not depend on late-time Universe assumptions, but depends strongly on early-time physics and in particular on additional components that behave like radiation. We find that the standard ruler length determined in this way is $r_{\rm s} = 147.4 \pm 0.7$ Mpc if the radiation and neutrino components are standard, but the uncertainty increases by an order of magnitude when non-standard dark radiation components are allowed, to $r_{\rm s} = 150 \pm 5$ Mpc.