Cosmological Imprint of an Energy Component with General Equation of State

Abstract

We examine the possibility that a significant component of the energy density of the universe has an equation-of-state different from that of matter, radiation or cosmological constant ($Λ$). An example is a cosmic scalar field evolving in a potential, but our treatment is more general. Including this component alters cosmic evolution in a way that fits current observations well. Unlike $Λ$, it evolves dynamically and develops fluctuations, leaving a distinctive imprint on the microwave background anisotropy and mass power spectrum.

Figures (2)

• Figure 1: CMB power spectrum, $\ell (\ell+1) C_{\ell}/2 \pi$ vs. $\ell$ where $C_{\ell}$ is the multipole moment, illustrating that the spectrum changes significantly as a function of $w$ and $\Omega_Q$. Thick solid lines represent SCDM; thick dashed lines correspond to $\Lambda$CDM. For thin solid lines, legends list the parameters in sequence according to the height of the curve beginning from the topmost. The variation with $\Omega_Q$ is shown in (a) and (b); (c) compares the predictions for a $Q$-component with fluctuations ($\delta Q \ne 0$) to a smooth component ($\delta Q$=0). The differences due to $\delta Q$ at large angular scales change COBE-normalization which is responsible for the substantial variation of the acoustic peaks with $w$, as shown in (d). Panel (e) shows the results for time-varying $w(\eta)$ as determined by specifying a potential $V(Q)$ and initial conditions, where $V(Q)$ is an exponential or cosine functional of $Q$ in the examples shown. Included also is the prediction for a constant $w=-1/6$, which differs from the results where $w(\eta)$ is time-varying and $w(\eta_0)=-1/6$. In (f) is shown the polarization (amplified 100-fold) for $w=-1/2$ compared to SCDM and $\Lambda$CDM.
• Figure 2: (a) Variation of mass power spectrum for some representative QCDM examples. (b) The variation of $\sigma_8$ with $\Omega_Q$. For $\Lambda$CDM, $\Omega_Q$ is $\Omega_{\Lambda}$. The suppression of $\sigma_8$ in QCDM compared to SCDM makes for a better fit with current observations. The grey swath illustrates constraints from x-ray cluster abundance.