The fate of baryons in counterfactual universes

TL;DR

This study uses cosmological simulations to explore how varying the cosmological constant and initial fluctuation amplitude affect galaxy formation, the intergalactic medium, and star formation over cosmic time, providing insights into the small observed value of Lambda.

Contribution

It introduces a comprehensive simulation-based analysis of counterfactual universes with different Lambda and sigma_8 values, assessing their impact on cosmic evolution and the probability of observer conditions.

Findings

Larger Lambda accelerates IGM cooling and reduces star formation.

Changing sigma_8 affects IGM thermal history independently of Lambda.

The probability of observing a Lambda as small as ours is estimated at 13%.

Abstract

We present results from nine simulations that compare the standard $\Lambda$ Cold Dark Matter cosmology ($\Lambda$CDM) with counterfactual universes, for approximately $100\,{\rm Gyr}$ using the Enzo simulation code. We vary the value of $\Lambda$ and the fluctuation amplitude to explore the effect on the evolution of the halo mass function (HMF), the intergalactic medium (IGM) and the star formation history (SFH). The distinct peak in star formation rate density (SFRD) and its subsequent decline are both affected by the interplay between gravitational attraction and the accelerating effects of $\Lambda$. The IGM cools down more rapidly in models with a larger $\Lambda$ and also with a lower $\sigma_8$, reflecting the reduced SFRD associated with these changes -- although changing $\sigma_8$ is not degenerate with changing $\Lambda$, either regarding the thermal history of the IGM or the SFH. However, these induced changes to the IGM or ionizing background have little impact on the calculated SFRD. We provide fits for the evolution of the SFRD in these different universes, which we integrate over time to derive an asymptotic star formation efficiency. Together with Weinberg's uniform prior on $\Lambda$, the estimated probability of observers experiencing a value of $\Lambda$ no greater than the observed value is 13%, substantially larger than some alternative estimates. Within the Enzo model framework, then, observer selection within a multiverse is able to account statistically for the small value of the cosmological constant, although $\Lambda$ in our universe does appear to be at the low end of the predicted range.