Constraining the Dark Energy Equation of State with Cosmic Voids

Our universe is observed to be accelerating due to the dominant dark energy with negative pressure. The dark energy equation of state (w) holds a key to understanding the ultimate fate of the universe. The cosmic voids behave like bubbles in the universe so that their shapes must be quite sensitive to the background cosmology. Assuming a flat universe and using the priors on the matter density parameter (Ωm) and the dimensionless Hubble parameter (h), we demonstrate analytically that the ellipticity evolution of cosmic voids may be a sensitive probe of the dark energy equation of state. We also discuss the parameter degeneracy between w and Ωm. Subject headings: cosmology:theory — large-scale structure of universe Recent observations have revealed that our universe is flat and in a phase of acceleration (Riess et al. 1998; Perlmutter et al. 1999; Spergel et al. 2003). It implies that some mysterious dark energy fills dominantly the universe at present epoch, exerting anti-gravity. The nature of this mysterious dark energy which holds a key to understanding the ultimate fate of the universe is often specified by its equation of state, i.e., the ratio of its pressure to density: w ≡ Pde/ρde. The anti-gravity of the dark energy corresponds to the negative value of w. The simplest candidate for the dark energy is the vacuum energy (Λ) with w = −1 that is constant at all times (Einstein 1917). Although all current data are consistent with the vacuum energy model (e.g., Wang & Tegmark 2004; Jassal et al. 2004; Percival 2005; Guzzo et al. 2008), the notorious failure of the theoretical estimate of the vacuum energy density (see Caroll et al. 1992, for a review) has led a dynamic dark energy model to emerge as an alternative. In this dynamic dark energy models which is often collectively called quintessence, the dark energy is described as a slowly rolling scalar field with time-varying equation of state in the range of −1 < w < 0 (Caldwell et al. 1998).