arXiv subjects

Lloyd Knox

Publications and source records attributed to Lloyd Knox.

At least 73 records · Page 4Linked to original sources

arXiv2009

Exploring Dark Energy with Next-Generation Photometric Redshift Surveys

The coming decade will be an exciting period for dark energy research, during which astronomers will address the question of what drives the accelerated cosmic expansion as first revealed by type Ia supernova (SN) distances, and confirmed by later observations. The mystery of dark energy poses a challenge of such magnitude that, as stated by the Dark Energy Task Force (DETF), "nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration." The lack of multiple complementary precision observations is a major obstacle in developing lines of attack for dark energy theory. This lack is precisely what next-generation surveys will address via the powerful techniques of weak lensing (WL) and baryon acoustic oscillations (BAO) -- galaxy correlations more generally -- in addition to SNe, cluster counts, and other probes of geometry and growth of structure. Because of their unprecedented statistical power, these surveys demand an accurate understanding of the observables and tight control of systematics. This white paper highlights the opportunities, approaches, prospects, and challenges relevant to dark energy studies with wide-deep multiwavelength photometric redshift surveys. Quantitative predictions are presented for a 20000 sq. deg. ground-based 6-band (ugrizy) survey with 5-sigma depth of r~27.5, i.e., a Stage 4 survey as defined by the DETF.

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arXiv2008

Simulations and cosmological inference: A statistical model for power spectra means and covariances

We describe an approximate statistical model for the sample variance distribution of the non-linear matter power spectrum that can be calibrated from limited numbers of simulations. Our model retains the common assumption of a multivariate Normal distribution for the power spectrum band powers, but takes full account of the (parameter dependent) power spectrum covariance. The model is calibrated using an extension of the framework in Habib et al. (2007) to train Gaussian processes for the power spectrum mean and covariance given a set of simulation runs over a hypercube in parameter space. We demonstrate the performance of this machinery by estimating the parameters of a power-law model for the power spectrum. Within this framework, our calibrated sample variance distribution is robust to errors in the estimated covariance and shows rapid convergence of the posterior parameter constraints with the number of training simulations.

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arXiv2008

Distance, Growth Factor, and Dark Energy Constraints from Photometric Baryon Acoustic Oscillation and Weak Lensing Measurements

Baryon acoustic oscillations (BAOs) and weak lensing (WL) are complementary probes of cosmology. We explore the distance and growth factor measurements from photometric BAO and WL techniques and investigate the roles of the distance and growth factor in constraining dark energy. We find for WL that the growth factor has a great impact on dark energy constraints but is much less powerful than the distance. Dark energy constraints from WL are concentrated in considerably fewer distance eigenmodes than those from BAO, with the largest contributions from modes that are sensitive to the absolute distance. Both techniques have some well determined distance eigenmodes that are not very sensitive to the dark energy equation of state parameters w_0 and w_a, suggesting that they can accommodate additional parameters for dark energy and for the control of systematic uncertainties. A joint analysis of BAO and WL is far more powerful than either technique alone, and the resulting constraints on the distance and growth factor will be useful for distinguishing dark energy and modified gravity models. The Large Synoptic Survey Telescope (LSST) will yield both WL and angular BAO over a sample of several billion galaxies. Joint LSST BAO and WL can yield 0.5% level precision on ten comoving distances evenly spaced in log(1+z) between redshift 0.3 and 3 with cosmic microwave background priors from Planck. In addition, since the angular diameter distance, which directly affects the observables, is linked to the comoving distance solely by the curvature radius in the Friedmann-Robertson-Walker metric solution, LSST can achieve a pure metric constraint of 0.017 on the mean curvature parameter Omega_k of the universe simultaneously with the constraints on the comoving distances.

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arXiv2006

How Tomographic Cosmic Shear Maps Lead to Constraints on Dark Energy Properties

Using a number of numerical tests and analytic arguments we investigate how measurements of cosmic shear lead to constraints on dark energy. We find that, in contrast to the case with galaxy number density correlation functions, standard rulers in the matter power spectrum play no significant role. Sensitivity to distance ratios is provided by the ratios in the lensing kernel. An absolute distance scale can only be established by breaking a potential degeneracy between growth and distance which can be done if the growth-redshift relation and distance-redshift relations are parameterized with sufficiently few parameters. For the quality of dark energy determination, growth determination is primarily important because it improves the distance reconstructions. The information about dark energy in the growth-redshift relation is always of secondary importance though the amount it contributes is highly dependent on what priors are taken in the cosmological parameter space. We also explain the dependence of dark energy constraints from cosmic shear, relative distance measures (supernovae) and absolute distance measures (baryon acoustic oscillations) on assumptions about the mean curvature.

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arXiv2006

Report of the Dark Energy Task Force

Dark energy appears to be the dominant component of the physical Universe, yet there is no persuasive theoretical explanation for its existence or magnitude. The acceleration of the Universe is, along with dark matter, the observed phenomenon that most directly demonstrates that our theories of fundamental particles and gravity are either incorrect or incomplete. Most experts believe that nothing short of a revolution in our understanding of fundamental physics will be required to achieve a full understanding of the cosmic acceleration. For these reasons, the nature of dark energy ranks among the very most compelling of all outstanding problems in physical science. These circumstances demand an ambitious observational program to determine the dark energy properties as well as possible.

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arXiv2006

Reduction of Cosmological Data for the Detection of Time-varying Dark Energy Density

We present a method for reducing cosmological data to constraints on the amplitudes of modes of the dark energy density as a function of redshift. The modes are chosen so that (1) one of them has constant density and (2) the others are non-zero only if there is time-variation in the dark energy density and (3) the amplitude errors for the time-varying modes are uncorrelated with each other. We apply our method to various combinations of three-year WMAP data, baryon acoustic oscillation data, the 'Gold' supernova data set, and the Supernova Legacy Survey data set. We find no significant evidence for a time-varying dark energy density or for non-zero mean curvature. Although by some measure the limits on four of the time-varying mode amplitudes are quite tight, they are consistent with the expectation that the dark energy density does not vary on timescales shorter than a Hubble time. Since we do not expect detectable time variation in these modes, our results should be viewed as a systematic error test which the data have passed. We discuss a procedure to identify modes with maximal signal-to-noise ratio.

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arXiv2006

Twenty-one centimeter tomography with foregrounds

Twenty-one centimeter tomography is emerging as a powerful tool to explore the end of the cosmic dark ages and the reionization epoch, but it will only be as good as our ability to accurately model and remove astrophysical foreground contamination. Previous treatments of this problem have focused on the angular structure of the signal and foregrounds and what can be achieved with limited spectral resolution (bandwidths in the 1 MHz range). In this paper we introduce and evaluate a ``blind'' method to extract the multifrequency 21cm signal by taking advantage of the smooth frequency structure of the Galactic and extragalactic foregrounds. We find that 21 cm tomography is typically limited by foregrounds on scales $k\ll 1h/$Mpc and limited by noise on scales $k\gg 1h/$Mpc, provided that the experimental bandwidth can be made substantially smaller than 0.1 MHz. Our results show that this approach is quite promising even for scenarios with rather extreme contamination from point sources and diffuse Galactic emission, which bodes well for upcoming experiments such as LOFAR, MWA, PAST, and SKA.

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arXiv2006

Weighing the Universe with Photometric Redshift Surveys and the Impact on Dark Energy Forecasts

With a wariness of Occam's razor awakened by the discovery of cosmic acceleration, we abandon the usual assumption of zero mean curvature and ask how well it can be determined by planned surveys. We also explore the impact of uncertain mean curvature on forecasts for the performance of planned dark energy probes. We find that weak lensing and photometric baryon acoustic oscillation data, in combination with CMB data, can determine the mean curvature well enough that the residual uncertainty does not degrade constraints on dark energy. We also find that determinations of curvature are highly tolerant of photometric redshift errors.

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arXiv2006

On Precision Measurement of the Mean Curvature

Very small mean curvature is a robust prediction of inflation worth rigorous checking. Since current constraints are derived from determinations of the angular-diameter distance to the CMB last-scattering surface, which is also affected by dark energy, they are limited by our understanding of the dark energy. Measurements of luminosity or angular-diameter distances to redshifts in the matter-dominated era can greatly reduce this uncertainty. With a one percent measurement of the distance to z=3, combined with the CMB data expected from Planck, one can achieve sigma(Ω_k h^2) \sim 10^{-3}. A non-zero detection at this level would be evidence against inflation or for unusually large curvature fluctuations on super-Hubble scales.

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arXiv2005

Exploring Large-scale Structure with Billions of Galaxies

We consider cosmological applications of galaxy number density correlations to be inferred from future deep and wide multi-band optical surveys. We mostly focus on very large scales as a probe of possible features in the primordial power spectrum. We find the proposed survey of the Large Synoptic Survey Telescope may be competitive with future all-sky CMB experiments over a broad range of scales. On very large scales the inferred power spectrum is robust to photometric redshift errors, and, given a sufficient number density of galaxies, to angular variations in dust extinction and photometric calibration errors. We also consider other applications, such as constraining dark energy with the two CMB-calibrated standard rulers in the matter power spectrum, and controlling the effect of photometric redshift errors to facilitate the interpretation of cosmic shear data. We find that deep photometric surveys over wide area can provide constraints that are competitive with spectroscopic surveys in small volumes.

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arXiv2005

Baryon Oscillations and Consistency Tests for Photometrically-Determined Redshifts of Very Faint Galaxies

Weak lensing surveys that can potentially place strong constraints on dark energy parameters can only do so if the source redshift means and error distributions are very well known. We investigate prospects for controlling errors in these quantities by exploiting their influence on the power spectra of the galaxies. Although, from the galaxy power spectra alone, sufficiently precise and simultaneous determination of redshift biases and variances is not possible, a strong consistency test is. Given the redshift error rms, galaxy power spectra can be used to determine the mean redshift of a group of galaxies to subpercent accuracy. Although galaxy power spectra cannot be used to determine the redshift error rms, they can be used to determine this rms divided by the Hubble parameter, a quantity that may be even more valuable for interpretation of cosmic shear data than the rms itself. We also show that galaxy power spectra, due to the baryonic acoustic oscillations, can potentially lead to constraints on dark energy that are competitive with those due to the cosmic shear power spectra from the same survey.

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arXiv2004

Effect of Hot Baryons on the Weak-Lensing Shear Power Spectrum

We investigate the impact of the intracluster medium on the weak-lensing shear power spectrum (PS). Using a halo model we find that, compared to the dark matter only case, baryonic pressure leads to a suppression of the shear PS on the order of a few percent or more for $l \gtrsim 1000$. Cooling/cooled baryons and the intergalactic medium can further alter the shear PS. Therefore, the interpretation of future precision weak lensing data at high multipoles must take into account the effects of baryons.

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arXiv2004

Testing cosmological models and understanding cosmological parameter determinations with metaparameters

Cosmological parameters affect observables in physically distinct ways. For example, the baryon density, omega_b, affects the ionization history and also the pressure of the pre-recombination fluid. To investigate the relative importance of different physical effects to the determination of omega_b, and to test the cosmological model, we artificially split omega_b into two `metaparameters': omega_{be} which controls the ionization history and omega_{bp} which plays the role of omega_b for everything else. In our demonstration of the technique we find omega_b = .0229 +/- .0012 (with no parameter splitting), omega_{bp} = .0238 +/- .0021, omega_{be}= .0150 +/- .0034 and omega_{bp}-omega_{be} = .0088 +/- .0039.

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arXiv2004

Effects of sub-mm and radio point sources on the recovery of Sunyaev-Zeldovich galaxy cluster parameters

Observations of clusters in the 30 to 350 GHz range can in principle be used to determine a galaxy cluster's Comptonization parameter, y, peculiar velocity, v and gas temperature, T_e via the dependence of the kinetic and thermal Sunyaev-Zeldovich (SZ) effects on these parameters. Despite the significant contamination expected from thermal emission by dust in high-redshift galaxies, we find that the simultaneous determination of optical depth (tau), v and T_e is possible from observations with sensitivity of a few uK in three or more bands with arc minute resolution. After allowing for realistic levels of contamination by dusty galaxies and primary CMB anisotropy, we find that simultaneous determinations of velocities to an accuracy of better than 200 km/s and temperatures to roughly keV accuracy should be possible in the near future. We study how errors change as a function of cluster properties (angular core radius and gas temperature) and experimental parameters (observing time, angular resolution and observing frequencies). Contaminating synchrotron emission from cluster galaxies will probably not be a major contaminant of peculiar velocity measurements.

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arXiv2003

Dark Energy Tomography

We study how parameter error forecasts for tomographic cosmic shear observations are affected by sky coverage, density of source galaxies, inclusion of CMB experiments, simultaneou fitting of non--dark energy parameters, and the parametrization of the history of the dark energy equation-of-state parameter w(z). We find tomographic shear-shear power spectra on large angular scales (l<1000) inferred from all-sky observations, in combination with Planck, can achieve sigma(w0)=0.06 and sigma(wa)=0.09 assuming the equation-of-state parameter is given by w(z)=w0+wa(1-a(z)) and that nine other matter content and primordial power spectrum parameters are simultaneously fit. Taking parameters other than w0, wa and Omegam to be completely fixed by the CMB we find errors on w0 and wa that are only 10% and 30% better respectively, justifying this common simplifying assumption. We also study `dark energy tomography': reconstuction of w(z) assumed to be constant within each of five independent redshift bins. With smaller-scale information included by use of the Jain & Taylor ratio statistic we find sigma(wi)<0.1 for all five redshift bins and sigma(wi)<0.02 for both bins at z<0.8. Finally, addition of cosmic shear can also reduce errors on quantities already determined well by the CMB. We find the sum of neutrino masses can be determined to +-0.013eV and that the primordial power specrum power-law index, ns, as well as dns/dlnk, can be determined more than a factor of two better than by Planck alone. These improvements may be highly valuable since the lower bound on the sum of neutrino masses is 0.06eV as inferred from atmospheric neutrino oscillations, and slow-roll models of inflation predict non-zero dns/dlnk at the forecasted error levels when |n_S-1|>0.04.

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arXiv2003

Determining Neutrino Mass from the CMB Alone

Distortions of Cosmic Microwave Background (CMB) temperature and polarization maps caused by gravitational lensing, observable with high angular resolution and high sensitivity, can be used to measure the neutrino mass. Assuming two massless species and one with mass m_nu we forecast sigma(m_nu) = 0.15 eV from the Planck satellite and sigma(m_nu)=0.04 eV from observations with twice the angular resolution and about 20 times the sensitivity. A detection is likely at this higher sensitivity since the observation of atmospheric neutrino oscillations require mass-squared differences greater than about (0.04 eV)^2.

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arXiv2003

Normal Parameters for an Analytic Description of the CMB Cosmological Parameter Likelihood

The normal parameters are a non--linear transformation of the cosmological parameters whose likelihood function is very well--approximated by a normal distribution. This transformation serves as an extreme form of data compression allowing for practically instantaneous calculation of the likelihood of any given model, as long as the model is in the parameter space originally considered. The compression makes all the information about cosmological parameter constraints from a given set of experiments available in a useable manner. Here we explicitly define the normal parameters that work for the current CMB data, and give their mean and covariance matrix which best fit the likelihood function calculated by the Monte Carlo Markov Chain method. Along with standard parameter estimation results, we propose that future CMB parameter analyses define normal parameters and quote their mean and covariance matrix.

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arXiv2003

COMPASS: An Upper Limit on CMB Polarization at an Angular Scale of 20 arc minutes

COMPASS is an on-axis 2.6 meter telescope coupled to a correlation polarimeter operating at a wavelength of 1 cm. The entire instrument was built specifically for CMB polarization studies. We report here on observations of February 2001 - April 2001 using this system. We set an upper limit on E-mode polarized anisotropies of 33.5 uK (95% confidence limit) in the l-range 200-600.

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