Estimating the power spectrum covariance matrix with fewer mock samples

The covariance matrices of power-spectrum (P(k)) measurements from galaxy surveys are difficult to compute theoretically. The current best practice is to estimate covariance matrices by computing a sample covariance of a large number of mock catalogues. The next generation of galaxy surveys will require thousands of large volume mocks to determine the covariance matrices to desired accuracy. The errors in the inverse covariance matrix are larger and scale with the number of P(k) bins, making the problem even more acute. We develop a method of estimating covariance matrices using a theoretically justified, few-parameter model, calibrated with mock catalogues. Using a set of 600 BOSS DR11 mock catalogues, we show that a seven parameter model is sufficient to fit the covariance matrix of BOSS DR11 P(k) measurements. The covariance computed with this method is better than the sample covariance at any number of mocks and only ~100 mocks are required for it to fully converge and the inverse covariance matrix converges at the same rate. This method should work equally well for the next generation of galaxy surveys, although a demand for higher accuracy may require adding extra parameters to the fitting function.Comment: 7 pages, 7 figure

Constraining dark energy with gamma-ray bursts

We use the measurement of gamma-ray burst (GRB) distances to constrain dark energy cosmological model parameters. We employ two methods for analyzing GRB data - fitting luminosity relation of GRBs in each cosmology and using distance measures computed from binned GRB data. Current GRB data alone cannot tightly constrain cosmological parameters and allow for a wide range of dark energy models.Comment: 27 pages, 12 figures, two methods of analysing GRB data, updated to match published version

Simulating redshift-space distortions for galaxy pairs with wide angular separation

The analysis of redshift-space distortions (RSD) within galaxy surveys provides constraints on the amplitude of peculiar velocities induced by structure growth, thereby allowing tests of General Relativity on extremely large scales. The next generation of galaxy redshift surveys, such as the Baryon Oscillation Spectroscopic Survey and the Euclid experiment, will survey galaxies out to z= 2, over 10-000-20-000-deg2. In such surveys, galaxy pairs with large comoving separation will preferentially have a wide angular separation. In standard plane-parallel theory the displacements of galaxy positions due to RSD are assumed to be parallel for all galaxies, but this assumption will break down for wide-angle pairs. Szalay, Matsubara & Landy, Szapudi, and Papai & Szapudi provided a methodology, based on tripolar spherical harmonics expansion, for computing the redshift-space correlation function for all angular galaxy pair separations. In this paper, we introduce a new procedure for analysing wide-angle effects in numerical simulations. We are able to separate, demonstrate and fit each of the effects described by the wide-angle RSD theory. Our analysis highlights some of the nuances of dealing with wide-angle pairs and shows that the effects are not negligible even for relatively small angles. This analysis will help to ensure the full exploitation of future surveys for RSD measurements, which are currently confined to pair separations less than -80-h-1-Mpc out to z- 0.5. \ua9 2010 The Authors. Journal compilation \ua9 2010 RAS

Forecasting cosmological parameter constraints from near-future space-based galaxy surveys

The next generation of space-based galaxy surveys are expected to measure the growth rate of structure to about a percent level over a range of redshifts. The rate of growth of structure as a function of redshift depends on the behaviour of dark energy and so can be used to constrain parameters of dark energy models. In this work we investigate how well these future data will be able to constrain the time dependence of the dark energy density. We consider parameterizations of the dark energy equation of state, such as XCDM and wCDM, as well as a consistent physical model of time-evolving scalar field dark energy, \phi CDM. We show that if the standard, specially-flat cosmological model is taken as a fiducial model of the Universe, these near-future measurements of structure growth will be able to constrain the time-dependence of scalar field dark energy density to a precision of about 10%, which is almost an order of magnitude better than what can be achieved from a compilation of currently available data sets.Comment: 27 pages, 7 figures and 3 table

The observational constraints on the flat ϕ\phiCDM models

Most dark energy models have the $\Lambda$CDM as their limit, and if future observations constrain our universe to be close to $\Lambda$CDM Bayesian arguments about the evidence and the fine-tuning will have to be employed to discriminate between the models. Assuming a baseline $\Lambda$CDM model we investigate a number of quintessence and phantom dark energy models, and we study how they would perform when compared to observational data, such as the expansion rate, the angular distance, and the growth rate measurements, from the upcoming Dark Energy Spectroscopic Instrument (DESI) survey. We sample posterior likelihood surfaces of these dark energy models with Monte Carlo Markov Chains while using central values consistent with the Planck $\Lambda$CDM universe and covariance matrices estimated with Fisher information matrix techniques. We find that for this setup the Bayes factor provides a substantial evidence in favor of the $\Lambda$CDM model over most of the alternatives. We also investigated how well the CPL parametrization approximates various scalar field dark energy models, and identified the location for each dark energy model in the CPL parameter space.Comment: 11 pages, 5 figures; 4 tables; published in European Journal of Physics

A Detection of the Baryon Acoustic Oscillation Features in the SDSS BOSS DR12 Galaxy Bispectrum

We present the first high significance detection ($4.1\sigma$) of the Baryon Acoustic Oscillations (BAO) feature in the galaxy bispectrum of the twelfth data release (DR12) of the Baryon Oscillation Spectroscopic Survey (BOSS) CMASS sample ($0.43 \leq z \leq 0.7$). We measured the scale dilation parameter, $\alpha$, using the power spectrum, bispectrum, and both simultaneously for DR12, plus 2048 MultiDark-PATCHY mocks in the North and South Galactic Caps (NGC and SGC, respectively), and the volume weighted averages of those two samples (N+SGC). The fitting to the mocks validated our analysis pipeline, yielding values consistent with the mock cosmology. By fitting to the power spectrum and bispectrum separately, we tested the robustness of our results, finding consistent values from the NGC, SGC and N+SGC in all cases. We found $D_{\mathrm{V}} = 2032 \pm 24 (\mathrm{stat.}) \pm 15 (\mathrm{sys.})$ Mpc, $D_{\mathrm{V}} = 2038 \pm 55 (\mathrm{stat.}) \pm 15 (\mathrm{sys.})$ Mpc, and $D_{\mathrm{V}} = 2031 \pm 22 (\mathrm{stat.}) \pm 10 (\mathrm{sys.})$ Mpc from the N+SGC power spectrum, bispectrum and simultaneous fitting, respectively.Comment: Submitted to Monthly Notices of the Royal Astronomical Society. 13 pages, 11 figure

Constraints on dark energy models from radial baryon acoustic scale measurements

We use the radial baryon acoustic oscillation (BAO) measurements of Gaztanaga et al. (2008) to constrain parameters of dark energy models. These constraints are comparable with constraints from other "non-radial" BAO data. The radial BAO data are consistent with the time-independent cosmological constant model but do not rule out time-varying dark energy. When we combine radial BAO and the Kowalski et al. (2008) Union type Ia supernova data we get very tight constraints on dark energy.Comment: 24 pages, 8 figures, 1 table. Minor changes to match the published versio