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Register a new userConstraining Dark Energy with the Dark Energy Survey: Theoretical Challenges
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The Dark Energy Survey (DES) will use a new imaging camera on the Blanco 4-m telescope at CTIO to image 5000 square degrees of sky in the South Galactic Cap in four optical bands, and to carry out repeat imaging over a smaller area to identify and measure lightcurves of Type Ia supernovae. The main imaging area overlaps the planned Sunyaev-Zeldovich survey of the South Pole Telescope. The idea behind DES is to use four distinct and largely independent methods to probe the properties of dark energy: baryon oscillations of the power spectrum, abundance and spatial distribution of clusters, weak gravitational lensing, and Type Ia supernovae. This white paper outlines, in broad terms, some of the theoretical issues associated with the first three of these probes (the issues for supernovae are mostly different in character), and with the general task of characterizing dark energy and distinguishing it from alternative explanations for cosmic acceleration. A companion white paper discusses the kind of numerical simulations and other theoretical tools that will be needed to address the these issues and to create mock catalogs that allow end-to-end tests of analysis procedures. Although we have been thinking about these problems in the specific context of DES, many of them are also relevant to other planned dark energy studies.
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Low density regions are less affected by the nonlinear structure formation and baryonic physics. They are ideal places for probing the nature of dark energy, a possible explanation for the cosmic acceleration. Unlike void lensing, which requires identifications of individual voids, we study the stacked lensing signals around the low-density-positions (LDP), defined as places that are devoid of foreground bright galaxies in projection. The method allows a direct comparison with numerical results by drawing correspondence between the bright galaxies with halos. It leads to lensing signals that are significant enough for differentiating several dark energy models. In this work, we use the CFHTLenS catalogue to define LDPs, as well as measuring their background lensing signals. We consider several different definitions of the foreground bright galaxies (redshift range & magnitude cut). Regarding the cosmological model, we run six simulations: the first set of simulations have the same initial conditions, with $rm{w_{de}=-1,-0.5,-0.8,-1.2}$; the second set of simulations include a slightly different $Lambda$CDM model and a w(z) model from cite{2017NatAs...1..627Z}. The lensing results indicate that the models with $rm{w_{de}=-0.5,-0.8}$ are not favored, and the other four models all achieve comparable agreement with the data.
We investigate the possibility of using cosmological observations to probe and constrain an imperfect dark energy fluid. We consider a general parameterization of the dark energy component accounting for an equation of state, speed of sound and viscosity. We use present and future data from the cosmic microwave background radiation (CMB), large scale structures and supernovae type Ia. We find that both the speed of sound and viscosity parameters are difficult to nail down with the present cosmological data. Also, we argue that it will be hard to improve the constraints significantly with future CMB data sets. The implication is that a perfect fluid description might ultimately turn out to be a phenomenologically sufficient description of all the observational consequences of dark energy. The fundamental lesson is however that even then one cannot exclude, by appealing to observational evidence alone, the possibility of imperfectness in dark energy.
This overview article describes the legacy prospect and discovery potential of the Dark Energy Survey (DES) beyond cosmological studies, illustrating it with examples from the DES early data. DES is using a wide-field camera (DECam) on the 4m Blanco Telescope in Chile to image 5000 sq deg of the sky in five filters (grizY). By its completion the survey is expected to have generated a catalogue of 300 million galaxies with photometric redshifts and 100 million stars. In addition, a time-domain survey search over 27 sq deg is expected to yield a sample of thousands of Type Ia supernovae and other transients. The main goals of DES are to characterise dark energy and dark matter, and to test alternative models of gravity; these goals will be pursued by studying large scale structure, cluster counts, weak gravitational lensing and Type Ia supernovae. However, DES also provides a rich data set which allows us to study many other aspects of astrophysics. In this paper we focus on additional science with DES, emphasizing areas where the survey makes a difference with respect to other current surveys. The paper illustrates, using early data (from `Science Verification, and from the first, second and third seasons of observations), what DES can tell us about the solar system, the Milky Way, galaxy evolution, quasars, and other topics. In addition, we show that if the cosmological model is assumed to be Lambda+ Cold Dark Matter (LCDM) then important astrophysics can be deduced from the primary DES probes. Highlights from DES early data include the discovery of 34 Trans Neptunian Objects, 17 dwarf satellites of the Milky Way, one published z > 6 quasar (and more confirmed) and two published superluminous supernovae (and more confirmed).
The presence of inhomogeneities modifies the cosmic distances through the gravitational lensing effect, and, indirectly, must affect the main cosmological tests. Assuming that the dark energy is a smooth component, the simplest way to account for the influence of clustering is to suppose that the average evolution of the expanding Universe is governed by the total matter-energy density whereas the focusing of light is only affected by a fraction of the total matter density quantified by the $alpha$ Dyer-Roeder parameter. By using two different samples of SNe type Ia data, the $Omega_m$ and $alpha$ parameters are constrained by applying the Zeldovich-Kantowski-Dyer-Roeder (ZKDR) luminosity distance redshift relation for a flat ($Lambda$CDM) model. A $chi^{2}$-analysis using the 115 SNe Ia data of Astier {it et al.} sample (2006) constrains the density parameter to be $Omega_m=0.26_{-0.07}^{+0.17}$($2sigma$) while the $alpha$ parameter is weakly limited (all the values $in [0,1]$ are allowed even at 1$sigma$). However, a similar analysis based the 182 SNe Ia data of Riess {it et al.} (2007) constrains the pair of parameters to be $Omega_m= 0.33^{+0.09}_{-0.07}$ and $alphageq 0.42$ ($2sigma$). Basically, this occurs because the Riess {it et al.} sample extends to appreciably higher redshifts. As a general result, even considering the existence of inhomogeneities as described by the smoothness $alpha$ parameter, the Einstein-de Sitter model is ruled out by the two samples with a high degree of statistical confidence ($11.5sigma$ and $9.9sigma$, respectively). The inhomogeneous Hubble-Sandage diagram discussed here highlight the necessity of the dark energy, and a transition deceleration/accelerating phase at $zsim 0.5$ is also required.
In this work, we study a class of early dark energy (EDE) models, in which, unlike in standard DE models, a substantial amount of DE exists in the matter-dominated era, self-consistently including DE perturbations. Our analysis shows that, marginalizing over the non DE parameters such as $Omega_m, H_0, n_s$, current CMB observations alone can constrain the scale factor of transition from early DE to late time DE to $a_t geq 0.44$ and width of transition to $Delta_t leq 0.37$. The equation of state at present is somewhat weakly constrained to $w_0 leq -0.6$, if we allow $H_0 < 60$ km/s/Mpc. Taken together with other observations, such as supernovae, HST, and SDSS LRGs, the constraints are tighter-- $w_0 leq -0.9, a_t leq 0.19, Delta_t leq 0.21$. The evolution of the equation of state for EDE models is thus close to $Lambda$CDM at low redshifts. Incorrectly assuming DE perturbations to be negligible leads to different constraints on the equation of state parameters, thus highlighting the necessity of self-consistently including DE perturbations in the analysis. If we allow the spatial curvature to be a free parameter, then the constraints are relaxed to $w_0 leq -0.77, a_t leq 0.35, Delta_t leq 0.35$ with $-0.014 < Omega_{kappa} < 0.031$ for CMB+other observations. For perturbed EDE models, the $2sigma$ lower limit on $sigma_8$ ($sigma_8 geq 0.59$) is much lower than that in $Lambda$CDM ($sigma_8 geq 0.72$), thus raising the interesting possibility of discriminating EDE from $Lambda$CDM using future observations such as halo mass functions or the Sunyaev-Zeldovich power spectrum.
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