Effects of the sound speed of quintessence on the microwave background and large scale structure

Abstract

We consider how quintessence models in which the sound speed differs from the speed of light and varies with time affect the cosmic microwave background and the fluctuation power spectrum. Significant modifications occur on length scales related to the Hubble radius during epochs in which the sound speed is near zero and the quintessence contributes a non-negligible fraction of the total energy density. For the microwave background, we find that the usual enhancement of the lowest multipole moments by the integrated Sachs-Wolfe effect can be modified, resulting in suppression or bumps instead. Also, the sound speed can produce oscillations and other effects at wavenumbers k > 10 −2 h/Mpc in the fluctuation power spectrum.PACS numbers: PACS number(s): 98.80. 98.70.Vc, One of the greatest challenges in cosmology today is to identify the nature of the dark energy component that comprises most of the energy density of the universe and that is causing the expansion of the universe to accelerate.[1] Two candidates are a cosmological constant (or vacuum density) and quintessence,[2] a dynamical energy component with negative pressure. Distinguishing the two is important for cosmology in order to refine our knowledge of the composition of the universe and to trace more accurately its evolution. It is even more important for fundamental physics since it informs us how we must modify unified theories to incorporate dark energy.One way to distinguish whether the dark energy is due to a cosmological constant or quintessence is to measure the equation of state, w, the ratio of the pressure p to the energy density ρ. A cosmological constant always has w = −1 whereas a scalar field generally has a w(z) that differs from unity and varies with red shift z. Through measurements of supernovae, large-scale structure and the cosmic microwave background (cmb) anisotropy, the equation of state may be determined accurately enough in the next few years to find out whether w is different from −1 or not.A second way to distinguish the nature of dark energy is to measure its sound speed to determine if it is different from unity. The sound speed can be detected because it affects the perturbations in the quintessence energy distribution. This approach is less generic because the sound speed in many models of quintessence in the literature is equal to unity (the speed of light), e.g., models in which quintessence consists of a scalar field (φ) with canonical kinetic energy density (X ≡ 1 2 (∂ µ φ)2 ) and a positive potential energy density (V (φ)). However, in general, the sound speed can differ from unity and vary with time. Detecting these effects is an independent way of showing that dark energy does not consist of a cosmological constant.An important motivating example is k -essence. [3] In these models, the k -essence undergoes two transitions in its behavior, one beginning at the onset of matterdomination and a second when k -essence overtakes the matter density. During the radiation-dominated era, the k-essence energy tracks the rad...

First‐Year Wilkinson Microwave Anisotropy Probe ( WMAP ) Observations: Determination of Cosmological Parameters

“…This assumption is valid for the dark energy component made of a scalar field with canonical form of the kinetic energy density. Further analysis is needed for models where w and the quintessence sound speed are a function of time (Dedeo et al 2003). The addition of a new parameter introduces a new degeneracy between m , h, and w that cannot be broken by CMB data alone (Huey et al 1999;Verde et al 2003): models with the same values of m h 2 , b h 2 , and first peak position have nearly identical angular power spectra.…”

Observational probes of cosmic acceleration

“…More complicated models that have a sound speed (c 2 s = δp/δρ) much smaller than c allow fluctuations to grow on sub-horizon scales (e.g., Hu 1998;Erickson et al 2002;Weller and Lewis 2003;DeDeo et al 2003;Bean and Doré 2004). de Putter et al (2010) provide a clear discussion of the background physics and observable consequences of dark energy inhomogeneities.…”

Large-scale cosmic microwave background anisotropies and dark energy

“…This leads effectively to perturbations with a constant speed of sound of the fluctuations with . However, k‐essence models allow for an evolving sound speed (Armendariz‐Picon et al 2000; DeDeo, Caldwell & Steinhardt 2003). We therefore extend our analysis to models with constant w and a constant speed of sound as a free parameter.…”