A Phantom Menace? Cosmological consequences of a dark energy component with super-negative equation of state

Abstract

It is extraordinary that a number of observations indicate that we live in a spatially flat, low matter density Universe, which is currently undergoing a period of accelerating expansion. The effort to explain this current state has focused attention on cosmological models in which the dominant component of the cosmic energy density has negative pressure, with an equation of state $w \ge -1$. Remarking that most observations are consistent with models right up to the $w=-1$ or cosmological constant ($Λ$) limit, it is natural to ask what lies on the other side, at $w<-1$. In this regard, we construct a toy model of a ``phantom'' energy component which possesses an equation of state $w<-1$. Such a component is found to be compatible with most classical tests of cosmology based on current data, including the recent type 1a SNe data as well as the cosmic microwave background anisotropy and mass power spectrum. If the future observations continue to allow $w<-1$, then barring unanticipated systematic effects, the dominant component of the cosmic energy density may be stranger than anything expected.

Figures (7)

• Figure 1: The age in units of the Hubble time is plotted versus $\Omega_m$ for a series of cosmological models containing dark energy with different values of $w$.
• Figure 2: The volume - red shift relationship is shown for phantom energy models with $w = -3,\, -3/2$, $\Lambda$CDM with $w=-1$, QCDM with $w =-1/2$, and CDM. All the dark energy models have $\Omega_m = 0.3$.
• Figure 3: The magnitude - red shift relationship is shown for the redshift-binned type 1a SNe data (HZT-- mlcsref; SCP-- scpref; binning-- Riess2001), alongside the predictions of various cosmological models. Both the phantoms ($w=-3/2, \Omega_m=0.4$; $-3/2,\, 0.3$) and the $\Lambda$ model ($-1,\,0.3$) provide good fits to the data (low $\chi^2$/d.o.f.). The magnitudes are calculated relative to an empty, open Universe. The light, dashed lines are for pure phantom, deSitter, Milne, and Einstein-deSitter, from top to bottom.
• Figure 4: The contraint on the allowed values of $\Omega_m$ in phantom and quintessence dark energy models is shown as a function of $w$, or alternatively the red shift at which matter-domination ends, when $\Omega_m = 0.9$. We have traversed the $w=-1$ boundary, finding that there are phantom energy models in accord with the observations.
• Figure 5: The CMB anisotropy spectra are shown for various cosmological models with equal input power, to demonstrate the effect of $w< -1$. On large angular scales, the strength of the late-time ISW effect diminishes as $w$ becomes more and more negative, because the expansion is CDM-dominated until later and later. The locations of the acoustic peaks shift to smaller angular scales as $w$ becomes more negative, due to the increased distance to the last scattering surface. The models shown all have $\Omega_m = 0.35$, $\Omega_b h^2 = 0.02$, and $h=0.70$. The horizontal axis is logarithmic for $2 < l < 200$ in order to give enough space to both large and small angular scale features. Comparing the curves with a compilation of current CMB data Tegmark2001 suggest a more negative dark energy equation-of-state may allow for a better fit to the small angular scale data.