Cosmological Constraints on Tachyon Matter

TL;DR

This paper investigates whether tachyon matter can serve as dark matter, highlighting the need for fine-tuning initial conditions and demonstrating its potential to cluster gravitationally like dust, making it a viable dark matter candidate.

Contribution

It shows that tachyon matter can be a viable dark matter candidate if initial conditions are finely tuned, and it can cluster gravitationally similarly to pressureless dust.

Findings

Tachyon density today is highly sensitive to initial conditions.

Fine-tuning of the tachyon potential is necessary for viable dark matter density.

Tachyon matter can cluster gravitationally like pressureless dust.

Abstract

We examine whether tachyon matter is a viable candidate for the cosmological dark matter. First, we demonstrate that in order for the density of tachyon matter to have an acceptable value today, the magnitude of the tachyon potential energy at the onset of rolling must be finely tuned. For a tachyon potential $V(T)\sim M_{Pl}^4\exp(-T/\tau)$, the tachyon must start rolling at $T\simeq 60\tau$ in order for the density of tachyon matter today to satisfy $\Omega_{T,0}\sim 1$, provided that standard big bang cosmology begins at the same time as the tachyon begins to roll. In this case, the value of $\Omega_{T,0}$ is exponentially sensitive to $T/\tau$ at the onset of rolling, so smaller $T/\tau$ is unacceptable, and larger $T/\tau$ implies a tachyon density that is too small to have interesting cosmological effects. If instead the universe undergoes a second inflationary epoch after the tachyon has already rolled considerably, then the tachyon can begin with $T$ near zero, but the increase of the scale factor during inflation must still be finely tuned in order for $\Omega_{T,0} \sim 1$. Second, we show that tachyon matter, unlike quintessence, can cluster gravitationally on very small scales. If the starting value of $T/\tau$ is tuned finely enough that $\Omega_{T,0}\sim 1$, then tachyon matter clusters more or less identically to pressureless dust. Thus, if the fine-tuning problem can be explained, tachyon matter is a viable candidate for cosmological dark matter.