Approaches to Understanding Cosmic Acceleration

Abstract

Theoretical approaches to explaining the observed acceleration of the universe are reviewed. We briefly discuss the evidence for cosmic acceleration, and the implications for standard General Relativity coupled to conventional sources of energy-momentum. We then address three broad methods of addressing an accelerating universe: the introduction of a cosmological constant, its problems and origins; the possibility of dark energy, and the associated challenges for fundamental physics; and the option that an infrared modification of general relativity may be responsible for the large-scale behavior of the universe.

Figures (7)

• Figure 1: Hubble diagram for Type Ia Supernovae, plotting the effective magnitude $m_B$ versus redshift $z$ (Knop et al. Knop:2003iy). The solid line is the best-fit cosmology and corresponds to a universe with $\Omega_{\rm M}^0=0.25$ and $\Omega_{\Lambda}^0=0.75$.
• Figure 2: The Cosmic Microwave Background angular power spectrum, representing the two-point correlation function of temperature fluctuations (in momentum space) on the sky as a function of angular separation, [courtesy of NASA/WMAP Science team]. The solid line is the best fit $\Lambda$CDM model. On larger scales, i.e. at lower multipoles l, we notice the Integrated Sachs-Wolfe plateau, at $l\sim220$ we see the first acoustic peak and on smaller angular scales the subsequent acoustic peaks
• Figure 3: Observational constraints in the $\Omega^0_{\rm M}-\Omega^0_{\Lambda}$ plane Kowalski:2008ez. The contours represent the $68.3\%$, $95.4\%$ and $99.7\%$ confidence level from CMB, BAO and SNeIa and their combination, with different colors corresponding to different data sets. The vertical green contours, centered around $\Omega^0_{\rm M}\simeq 0.3$, represent BAO constraints from the SDSS galaxy survey Eisenstein:2005su; the orange narrow diagonal contours correspond to constraints from WMAP-5 year observations of the CMB anisotropies (including a prior on the Hubble parameter) Dunkley:2008ie. Finally the wide blue diagonal contours represent constraints from supernovae Union Set Kowalski:2008ez and the gray-scale contours the combined constraints from BAO, SNeIa and CMB.
• Figure 4: Schematic representation of the Integrated Sachs-Wolfe effect (ISW) on CMB photons. On the left side of the figure, a photon travels through a constant potential, gaining and losing the same amount of energy while passing through the potential well. In this case, the outgoing wavelength corresponds to the incoming one, and there is no net gain (or loss) of energy. On the right side of the figure, a photon travels through a shallowing potential. As the potential well from which the photon exits is shallower than the one it entered, there is an overall gain in energy and the outgoing wavelength is shorter than the incoming one.
• Figure 5: The large scale redshift-space correlation function of the SDSS LRG (luminous red galaxies) sample, from Eisenstein:2005su. The magenta line shows a pure CDM model ($\Omega_{\rm M}^0h^2=0.105$) which lacks the acoustic peak. The green, red and blue lines represent models with a baryon fractional density of $\Omega_b^0h^2=0.024$ and a CDM density of, respectively, $\Omega_{\rm M}^0h^2=0.12,0.13,0.14$. The bump at $100\,h^{-1}{\textrm{M}pc}$ corresponds to the acoustic peak. Details about the data set and error bars can be found in Eisenstein:2005su.