Dark energy: investigation and modeling

Abstract

Constantly accumulating observational data continue to confirm that about 70% of the energy density today consists of dark energy responsible for the accelerated expansion of the Universe. We present recent observational bounds on dark energy constrained by the type Ia supernovae, cosmic microwave background, and baryon acoustic oscillations. We review a number of theoretical approaches that have been adopted so far to explain the origin of dark energy. This includes the cosmological constant, modified matter models (such as quintessence, k-essence, coupled dark energy, unified models of dark energy and dark matter), modified gravity models (such as f(R) gravity, scalar-tensor theories, braneworlds), and inhomogeneous models. We also discuss observational and experimental constraints on those models and clarify which models are favored or ruled out in current observations.

Figures (6)

• Figure 1: 68.3 %, 95.4 %, and 99.7 % confidence level contours on $w_{\rm DE}$ and $\Omega_m^{(0)}$ (denoted as $w$ and $\Omega_m$ in the figure) constrained by the Union08 SN Ia datasets. The equation of state $w_{\rm DE}$ is assumed to be constant. From Ref. Kowalski.
• Figure 2: Observational constraints on the parameters $w_0$ and $w_1$ (denoted as $w_a$ in the figure) for the parametrization (\ref{['Polapara']}). The contours show the 68 % and 95 % confidence level from WMAP+$H_0$+SN (red), WMAP+BAO+$H_0$+SN (blue), and WMAP+BAO+$H_0$+$D_{\Delta t}$+SN (black), for a flat universe. From Ref. WMAP7.
• Figure 3: 68.3 %, 95.4 % and 99.7 % confidence level contours on $w_{{\rm DE}}$ and $\Omega_m^{(0)}$ (denoted as $w$ and $\Omega_m$ in the figure, respectively) for a flat Universe. The left panel illustrates the individual constraints from SN Ia, CMB, and BAO, as well as the combined constraints (filled gray contours, statistical errors only). The upper right panel shows the effect of including systematic errors. The lower right panel illustrates the impact of the Supernova Cosmology Project (SCP) Nearby 1999 data. From Ref. Kowalski.
• Figure 4: The allowed region in the $(w_{\phi},w_{\phi}')$ plane for thawing and freezing models of quintessence ($w_{\phi}$ is denoted as $w$ in the figure). The thawing models correspond to the region between two curves: (a) $w_{\phi}'=3(1+w_{\phi})$ and (b) $w_{\phi}'=1+w_{\phi}$, whereas the freezing models are characterized by the region between two curves: (c) $w_{\phi}'=0.2w_{\phi}(1+w_{\phi})$ and (d) $w_{\phi}'=3w_{\phi}(1+w_{\phi})$. The dotted line shows the border between the acceleration and deceleration of the field ($\ddot{\phi}=0$), which corresponds to $w_{\phi}'=3(1+w_{\phi})^{2}$. From Ref. Caldwell.
• Figure 5: Evolution of the dark energy equation of state $w_{\rm DE}$ for the model (B) with $A=1$ in Eqs. (\ref{['mofR1']}) and (\ref{['mofR2']}). The phantom equation of state and the cosmological constant boundary crossing are realized. From Ref. Motohashi10.