Evolving beyond z=0: insights about the future of stars and the intergalactic medium

TL;DR

This study uses extended cosmological simulations to explore the future evolution of the universe's structure, star formation, and intergalactic medium properties beyond the present epoch, revealing key behaviors and potential limitations.

Contribution

It provides novel insights into the future cosmic evolution by extending simulations beyond current limits and analyzing the behavior of the halo mass function, IGM EOS, and star formation history.

Findings

Halo mass function freezes out at z≈-0.6

IGM temperature sharply declines after z=0

Future star formation peaks then declines, with some simulations showing a turnaround

Abstract

We present results from seven cosmological simulations that have been extended beyond the present era as far as redshift $z=-0.995$ or $t\approx96\,{\rm Gyr}$, using the Enzo simulation code. We adopt the calibrated star formation and feedback prescriptions from our previous work on reproducing the Milky Way with Enzo with modifications to the simulation code, chemistry and cooling library. We then consider the future behaviour of the halo mass function (HMF), the equation of state (EOS) of the IGM, and the cosmic star formation history (SFH). Consistent with previous work, we find a freeze-out in the HMF at $z\approx-0.6$. The evolution of the EOS of the IGM presents an interesting case study of the cosmological coincidence problem, where there is a sharp decline in the IGM temperature immediately after $z=0$. For the SFH, the simulations produce a peak and a subsequent decline into the future. However, we do find a turnaround in the SFH after $z\approx-0.98$ in some simulations, probably due to the limitations of the criteria used for star formation. By integrating the SFH in time up to $z=-0.92$, the simulation with the best spatial resolution predicts an asymptotic total stellar mass that is very close to that obtained from extrapolating the fit of the observed SFR. Lastly, we investigate the future evolution of the partition of baryons within a Milky Way-sized galaxy, using both a zoom and a box simulation. Despite vastly different resolutions, these simulations predict individual haloes containing an equal fraction of baryons in stars and gas at the time of freeze-out ($t\approx30\,{\rm Gyr}$).