Dragan Huterer

Dark Energy and the Accelerating Universe

Ten years ago, the discovery that the expansion of the universe is accelerating put in place the last major building block of the present cosmological model, in which the universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy—for example, the quantum energy of the vacuum—or it may signal that general relativity (GR) breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years.

Imprints of primordial non-Gaussianities on large-scale structure: Scale-dependent bias and abundance of virialized objects

We study the effect of primordial non-Gaussianity on large-scale structure, focusing upon the most massive virialized objects. Using analytic arguments and N-body simulations, we calculate the mass function and clustering of dark matter halos across a range of redshifts and levels of non-Gaussianity. We propose a simple fitting function for the mass function valid across the entire range of our simulations. We find pronounced effects of non-Gaussianity on the clustering of dark matter halos, leading to strongly scale-dependent bias. This suggests that the large-scale clustering of rare objects may provide a sensitive probe of primordial non-Gaussianity. We very roughly estimate that upcoming surveys can constrain non-Gaussianity at the level of |f{sub NL}| < or approx. 10, which is competitive with forecasted constraints from the microwave background.

Is the low-l microwave background cosmic?

The large-angle (low-l) correlations of the cosmic microwave background exhibit several statistically significant anomalies compared to the standard inflationary cosmology. We show that the quadrupole plane and the three octopole planes are far more aligned than previously thought (99.9% C.L.). Three of these planes are orthogonal to the ecliptic at 99.1% C.L., and the normals to these planes are aligned at 99.6% C.L. with the direction of the cosmological dipole and with the equinoxes. The remaining octopole plane is orthogonal to the supergalactic plane at 99.6% C.L.

PROBING DARK ENERGY: METHODS AND STRATEGIES

The presence of dark energy in the Universe is inferred directly from the accelerated expansion of the Universe, and indirectly, from measurements of cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3 of the critical density, is very smoothly distributed, and has large negative pressure. Its nature is very much unknown. Most of its discernible consequences follow from its effect on evolution of the expansion rate of the Universe, which in turn affects the growth of density perturbations and the age of the Universe, and can be probed by the classical kinematic cosmological tests. Absent a compelling theoretical model (or even a class of models), we describe the dark energy by an effective equation-of-state w=p_X/\rho_X which is allowed to vary with time. We describe and compare different approaches for determining w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies and clusters, and CMB anisotropy, focusing particular attention on the use of a sample of several thousand type Ia supernova with redshifts z\lesssim 1.7, as might be gathered by the proposed SNAP satellite. Among other things, we derive optimal strategies for constraining cosmological parameters using type Ia supernovae. While in the near term CMB anisotropy will provide the first measurements of w, supernovae and number counts appear to have the most potential to probe dark energy.

Systematic errors in future weak-lensing surveys: requirements and prospects for self-calibration

We study the impact of systematic errors on planned weak-lensing surveys and compute the requirements on their contributions so that they are not a dominant source of the cosmological parameter error budget. The generic types of error we consider are multiplicative and additive errors in measurements of shear, as well as photometric redshift errors. In general, more powerful surveys have stronger systematic requirements. For example, for a SuperNova/Acceleration Probe (SNAP)-type survey the multiplicative error in shear needs to be smaller than 1 per cent of the mean shear in any given redshift bin, while the centroids of photometric redshift bins need to be known to be better than 0.003. With about a factor of 2 degradation in cosmological parameter errors, future surveys can enter a self-calibration regime, where the mean systematic biases are self-consistently determined from the survey and only higher order moments of the systematics contribute. Interestingly, once the power-spectrum measurements are combined with the bispectrum, the self-calibration regime in the variation of the equation of state of dark energy w a is attained with only a 20‐30 per cent error degradation. Ke yw ords: cosmological parameters ‐ large-scale structure of Universe.

Uncorrelated estimates of dark energy evolution

Type Ia supernova data have recently become strong enough to enable, for the first time, constraints on the time variation of the dark energy density and its equation of state. Most analyses, however, are using simple two or three-parameter descriptions of the dark energy evolution, since it is well known that allowing more degrees of freedom introduces serious degeneracies. Here we present a method to produce uncorrelated and nearly model-independent band power estimates of the equation of state of dark energy and its density as a function of redshift. We apply the method to recently compiled supernova data. Our results are consistent with the cosmological constant scenario, in agreement with other analyses that use traditional parametrizations, though we find marginal (2-sigma) evidence for w(z)<-1 at z<0.2. In addition to easy interpretation, uncorrelated, localized band powers allow intuitive and powerful testing of the constancy of either the energy density or equation of state. While we have used relatively coarse redshift binning suitable for the current set of ~150 supernovae, this approach should reach its full potential in the future, when applied to thousands of supernovae found from ground and space, combined with complementary information from other cosmological probes.

On the large-angle anomalies of the microwave sky

We apply the multipole vector framework to full-sky maps derived from the first-year Wilkinson Microwave Anisotropy Probe (WMAP) data. We significantly extend our earlier work showing that the two lowest cosmologically interesting multipoles, � = 2 and 3, are not statistically isotropic. These results are compared to the findings obtained using related methods. In particular, we show that the planes of the quadrupole and the octopole are unexpectedly aligned. Moreover, the combined quadrupole plus octopole is surprisingly aligned with the geometry and direction of motion of the Solar system: the plane they define is perpendicular to the ecliptic plane and to the plane defined by the dipole direction, and the ecliptic plane carefully separates stronger from weaker extrema, running within a couple of degrees of the null-contour between a maximum and a minimum over more than 120 ◦ of the sky. Even given the alignment of the quadrupole and octopole with each other, we find that their alignment with the ecliptic is unlikely at >98 per cent confidence level (CL), and argue that it is in fact unlikely at >99.9 per cent CL. Most of the � = 2 and 3 multipole vectors of the known Galactic foregrounds are located far from those of the observed sky, strongly suggesting that residual contamination by such foregrounds is unlikely to be the cause of the observed correlations. Multipole vectors,

Prospects for probing the dark energy via supernova distance measurements

Distance measurements to type Ia supernovae (SNe Ia) indicate that the Universe is accelerating and that two-thirds of the critical energy density exists in a dark-energy component with negative pressure. Distance measurements to SNe Ia can be used to distinguish between different possibilities for the dark energy, and if it is an evolving scalar field, to reconstruct the scalar-field potential. We derive the reconstruction equations and address the feasibility of this approach by Monte Carlo simulation. {copyright} {ital 1999} {ital The American Physical Society}

Uncorrelated universe: Statistical anisotropy and the vanishing angular correlation function in WMAP years 1-3

The large-angle (low-l) correlations of the cosmic microwave background (CMB) as reported by the Wilkinson Microwave Anisotropy Probe (WMAP) after their first year of observations exhibited statistically significant anomalies compared to the predictions of the standard inflationary big-bang model. We suggested then that these implied the presence of a solar system foreground, a systematic correlated with solar system geometry, or both. We reexamine these anomalies for the data from the first three years of WMAPs operation. We show that, despite the identification by the WMAP team of a systematic correlated with the equinoxes and the ecliptic, the anomalies in the first-year internal linear combination (ILC) map persist in the three-year ILC map, in all-but-one case at similar statistical significance. The three-year ILC quadrupole and octopole therefore remain inconsistent with statistical isotropy - they are correlated with each other (99.6% C.L.), and there are statistically significant correlations with local geometry, especially that of the solar system. The angular two-point correlation function at scales > 60 deg in the regions outside the (kp0) galactic cut, where it is most reliably determined, is approximately zero in all wavebands and is even more discrepant with the best-fit Lambda CDM inflationary model than in the first-year data - 99.97% C.L. for the new ILC map. The full-sky ILC map, on the other hand, has a nonvanishing angular two-point correlation function, apparently driven by the region inside the cut, but which does not agree better with Lambda CDM. The role of the newly-identified low-l systematics is more puzzling than reassuring.

Effects of photometric redshift uncertainties on weak-lensing tomography

We perform a systematic analysis of the effects of photometric redshift uncertainties on weak-lensing tomography. We describe the photo-z distribution with a bias and Gaussian scatter that are allowed to vary arbitrarily between intervals of δz = 0.1 in redshift. While the mere presence of bias and scatter does not substantially degrade dark energy information, uncertainties in both parameters do. For a fiducial next-generation survey each would need to be known to better than about 0.003-0.01 in redshift for each interval in order to lead to less than a factor of 1.5 increase in the dark energy parameter errors. The more stringent requirement corresponds to a larger dark energy parameter space, when redshift variation in the equation of state of dark energy is allowed. Of order 104-105 galaxies with spectroscopic redshifts fairly sampled from the source galaxy distribution will be needed to achieve this level of calibration. If the sample is composed of multiple galaxy types, a fair sample would be required for each. These requirements increase in stringency for more ambitious surveys; we quantify such scalings with a convenient fitting formula. No single aspect of a photometrically binned selection of galaxies such as their mean or median suffices, indicating that dark energy parameter determinations are sensitive to the shape and nature of outliers in the photo-z redshift distribution.