Dark energy and dark matter from an additional adiabatic fluid

The dark sector is described by an additional barotropic fluid which evolves adiabatically during the Universe’s history and whose adiabatic exponent $\gamma$ is derived from the standard definitions of specific heats. Although in general $\gamma$ is a function of the redshift, the Hubble parameter and its derivatives, we find that our assumptions lead necessarily to solutions with $\gamma =\text{constant}$ in a Friedmann-Lemaître-Robertson-Walker universe. The adiabatic fluid acts effectively as the sum of two distinct components, one evolving like nonrelativistic matter and the other depending on the value of the adiabatic index. This makes the model particularly interesting as a way of simultaneously explaining the nature of both dark energy and dark matter, at least at the level of the background cosmology. The $\mathrm{\Lambda }\mathrm{CDM}$ model is included in this family of theories when $\gamma =0$. We fit our model to supernovae Ia, $H(z)$ and baryonic acoustic oscillation data, discussing the model selection criteria. The implications for the early Universe and the growth of small perturbations in this model are also discussed.

Figure 1

$1\sigma$ and $2\sigma$ contours corresponding to $\gamma =0$ ($\mathrm{\Lambda }\mathrm{CDM}$) (left), and to our model with $\gamma$ free (right). See also Table 2.

Figure 2

Comparison between $\mathrm{\Lambda }\mathrm{CDM}$ (black solid lines) and our model, for a few values of $\gamma$: $-0.3$ (dotted), $-0.15$ (dot-dashed), 0.15 (dashed, small) and 0.3 (dashed, large). The grey bands correspond to $\pm 10\%$ departures from $\mathrm{\Lambda }\mathrm{CDM}$. The plotted quantities are ${\mathrm{\Delta }}_H$ [Eq. (53)], $D$ [Eq. (49)], and $f$ [Eq. (52)]. For definiteness, we have taken ${\mathrm{\Omega }}_m=0.3$ and ${\mathrm{\Omega }}_k=0$.