Primordial Nucleosynthesis in the Rh = ct cosmology: Pouring cold water on the Simmering Universe

TL;DR

This paper examines primordial nucleosynthesis within the Rh = ct cosmology, revealing significant discrepancies with observed element abundances due to slow cooling and extended nucleosynthesis, challenging the model's viability.

Contribution

It analyzes how the Rh = ct universe's slow cooling affects element formation, highlighting its inability to match observed primordial abundances without new physics.

Findings

Predicted helium abundance is too low compared to observations.

Extended nucleosynthesis leads to almost no light elements besides hydrogen and helium.

Significant modifications are needed for the model to align with empirical data.

Abstract

Primordial nucleosynthesis is rightly hailed as one of the great successes of the standard cosmological model. Here we consider the initial forging of elements in the recently proposed Rh = ct universe, a cosmology that demands linear evolution of the scale factor. Such a universe cools extremely slowly compared to standard cosmologies, considerably depleting the available neutrons during nucleosynthesis; this has significant implications for the resultant primordial abundances of elements, predicting a minuscule quantity of helium which is profoundly at odds with observations. The production of helium can be enhanced in such a "simmering universe" by boosting the baryon to photon ratio, although more than an order of magnitude increase is required to bring the helium mass fraction into accordance with observations. However, in this scenario, the prolonged period of nucleosynthesis results of the efficient cooking of lighter into heavier elements, impacting the resultant abundances of all elements so that, other than hydrogen and helium, there are virtually no light elements present in the universe. Without the addition of substantial new physics in the early universe, it is difficult to see how the Rh = ct universe can be considered a viable cosmological model.