Report of the Dark Energy Task Force

Abstract

Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible.

Figures (8)

• Figure 1: Fig. VI-1: Fluctuations in the temperature of the early Universe, as measured by the WMAP experiment.
• Figure 2: Fig. VI-2: The primary observables for dark-energy - the distance-redshift relation $D(z)$ and the growth-redshift relation $g(\mathrm{z})$ - are plotted vs. redshift for three cosmological models. The green curve is an open-Universe model with no dark energy at all. The black curve is the "concordance" $\Lambda$ CDM model, which is flat and has a cosmological constant, i.e., $w=-1$. This model is consistent with all reliable present-day data. The red curve is a dark-energy model with $w=-0.9$, for which other parameters have been adjusted to match WMAP data. At left one sees that dark-energy models are easily distinguished from non-dark-energy models. At right, we plot the ratios of each model to the $\Lambda C D M$ model, and it is apparent that distinguishing the $w=-0.9$ model from $\Lambda C D M$ requires percent-level precision on the diagnostic quantities.
• Figure 3: Fig VI-3: Left: High-redshift supernovae observed from HST by Riess et al (2004). Right: Cosmological results from the GOODS SNe (Riess et al. 2004). Upper panel: distance ( $\mu=5 \log _{10} d_{L}+$ const.) vs. redshift; lower: constraints on present-day acceleration.
• Figure 4: Fig. VI-4: The baryon acoustic oscillations are seen as wiggles in the power spectrum of the CMB (left, Hinshaw et al. 2003), and have now been detected as a feature in the correlation function of nearby galaxies using the Sloan Digital Sky Survey (right, Eisenstein et al 2005).
• Figure 5: Fig. VI-5: Galaxy clusters as viewed in three different spectral regimes: top left, an optical view showing the concentration of yellowish member galaxies (SDSS); top right, Sunyaev, Zel'dovich flux decrements at 30 GHz (Carlstrom, et al.2001); bottom, $x$-ray emission (Chandra Science Center). These images are not at a common scale.