Publications (18)82.42 Total impact
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ABSTRACT: We present Southern African Large Telescope followup observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the Sunyaev–Zel'dovich (SZ) effect. We conducted multiobject spectroscopic observations with the Robert Stobie Spectrograph in order to measure galaxy redshifts in each cluster field, determine the cluster lineofsight velocity dispersions, and infer the cluster dynamical masses. We find that the clusters, which span the redshift range 0.3 < z < 0.55, range in mass from (5–20) × 1014 M⊙ (M200c). Their masses, given their SZ signals, are similar to those of Southern hemisphere ACT clusters previously observed using Gemini and the VLT. We note that the brightest cluster galaxy in one of the systems studied, ACTCL J0320.4+0032 at z = 0.38, hosts a type II quasar. Only a handful of such systems are currently known, and therefore ACTCL J0320.4+0032 may be a rare example of a very massive halo in which quasarmode feedback is actively taking place.  [Show abstract] [Hide abstract]
ABSTRACT: We present constraints on cosmological and astrophysical parameters from highresolution microwave background maps at 148 GHz and 218 GHz made by the Atacama Cosmology Telescope (ACT) in three seasons of observations from 2008 to 2010. A model of primary cosmological and secondary foreground parameters is fit to the map power spectra and lensing deflection power spectrum, including contributions from both the thermal SunyaevZeldovich (tSZ) effect and the kinematic SunyaevZeldovich (kSZ) effect, Poisson and correlated anisotropy from unresolved infrared sources, radio sources, and the correlation between the tSZ effect and infrared sources. The power ell^2 C_ell/2pi of the thermal SZ power spectrum at 148 GHz is measured to be 3.4 +\ 1.4 muK^2 at ell=3000, while the corresponding amplitude of the kinematic SZ power spectrum has a 95% confidence level upper limit of 8.6 muK^2. Combining ACT power spectra with the WMAP 7year temperature and polarization power spectra, we find excellent consistency with the LCDM model. We constrain the number of effective relativistic degrees of freedom in the early universe to be Neff=2.79 +\ 0.56, in agreement with the canonical value of Neff=3.046 for three massless neutrinos. We constrain the sum of the neutrino masses to be Sigma m_nu < 0.39 eV at 95% confidence when combining ACT and WMAP 7year data with BAO and Hubble constant measurements. We constrain the amount of primordial helium to be Yp = 0.225 +\ 0.034, and measure no variation in the fine structure constant alpha since recombination, with alpha/alpha0 = 1.004 +/ 0.005. We also find no evidence for any running of the scalar spectral index, dns/dlnk = 0.004 +\ 0.012.  [Show abstract] [Hide abstract]
ABSTRACT: Using highresolution microwave sky maps made by the Atacama Cosmology Telescope, we for the first time present strong evidence for motions of galaxy clusters and groups via microwave background temperature distortions due to the kinematic SunyaevZel'dovich effect. Galaxy clusters are identified by their constituent luminous galaxies observed by the Baryon Oscillation Spectroscopic Survey, part of the Sloan Digital Sky Survey III. We measure the mean pairwise momentum of clusters, with a probability of the signal being due to random errors of 0.002, and the signal is consistent with the growth of cosmic structure in the standard model of cosmology.  [Show abstract] [Hide abstract]
ABSTRACT: Baryon Acoustic Oscillation (BAO) surveys will be a leading method for addressing the dark energy challenge in the next decade. We explore in detail the effect of allowing for small amplitude admixtures of general isocurvature perturbations in addition to the dominant adiabatic mode. We find that nonadiabatic initial conditions leave the sound speed unchanged but instead excite different harmonics. These harmonics couple differently to Silk damping, altering the form and evolution of acoustic waves in the baryonphoton fluid prior to decoupling. This modifies not only the scale on which the sound waves imprint onto the baryon distribution, which is used as the standard ruler in BAO surveys, but also the shape, width and height of the BAO peak. We discuss these effects in detail and show how more general initial conditions impact our interpretation of cosmological data in dark energy studies. We find that the inclusion of these additional isocurvature modes leads to an increase in the Dark Energy Task Force Figure of merit by 140% and 60% for the BOSS and ADEPT experiments respectively when considered in conjunction with Planck data. We also show that the incorrect assumption of adiabaticity has the potential to bias our estimates of the dark energy parameters by $3\sigma$ ($1\sigma$) for a single correlated isocurvature mode, and up to $8\sigma$ ($3\sigma$) for three correlated isocurvature modes in the case of the BOSS (ADEPT) experiment. We find that the use of the large scale structure data in conjunction with CMB data improves our ability to measure the contributions of different modes to the initial conditions by as much as 100% for certain modes in the fully correlated case.  [Show abstract] [Hide abstract]
ABSTRACT: Statistically anomalous signals in the microwave background have been extensively studied in general in multipole space, and in real space mainly for circular and other simple patterns. In this paper we search for a range of nontrivial patterns in the temperature data from WMAP 7year observations. We find a very significant detection of a number of such features and discuss their consequences for the essential character of the cosmos.  [Show abstract] [Hide abstract]
ABSTRACT: A fundamental assumption in cosmology is that of statistical isotropy  that the universe, on average, looks the same in every direction in the sky. Statistical isotropy has recently been tested stringently using Cosmic Microwave Background (CMB) data, leading to intriguing results on large angular scales. Here we apply some of the same techniques used in the CMB to the distribution of galaxies on the sky. Using the multipole vector approach, where each multipole in the harmonic decomposition of galaxy density field is described by unit vectors and an amplitude, we lay out the basic formalism of how to reconstruct the multipole vectors and their statistics out of galaxy survey catalogs. We apply the algorithm to synthetic galaxy maps, and study the sensitivity of the multipole vector reconstruction accuracy to the density, depth, sky coverage, and pixelization of galaxy catalog maps.  [Show abstract] [Hide abstract]
ABSTRACT: Small fractions of isocurvature perturbations correlated with the dominant adiabatic mode are shown to be a significant primordial systematic for Baryon Acoustic Oscillation (BAO) surveys which must be accounted for in future surveys. Isocurvature modes distort the standard ruler distance by broadening and shifting the peak in the galaxy correlation function. While a single isocurvature mode does not significantly degrade dark energy constraints, the general case with multiple isocurvature modes leads to biases that exceed 7σ on average in the dark energy parameters even for isocurvature amplitudes undetectable by Planck. Accounting for all isocurvature modes corrects for this bias but degrades the dark energy figure of merit by at least 50% in the case of the Boss experiment. However the BAO data in turn provides significantly stronger constraints on the nature of the primordial perturbations. Future large galaxy surveys will thus be powerful probes of exotic physics in the early Universe in addition to helping pin down the nature of dark energy. 
Article: Direct Reconstruction of Dark Energy
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ABSTRACT: An important issue in cosmology is reconstructing the effective dark energy equation of state directly from observations. With so few physically motivated models, future dark energy studies cannot only be based on constraining a dark energy parameter space. We present a new nonparametric method which can accurately reconstruct a wide variety of dark energy behavior with no prior assumptions about it. It is simple, quick and relatively accurate, and involves no expensive explorations of parameter space. The technique uses principal component analysis and a combination of information criteria to identify real features in the data, and tailors the fitting functions to pick up trends and smooth over noise. We find that we can constrain a large variety of w(z) models to within 10%20% at redshifts z≲1 using just SNAPquality data.  [Show abstract] [Hide abstract]
ABSTRACT: The use of standard rulers, such as the scale of the baryonic acoustic oscillations (BAOs), has become one of the more powerful techniques employed in Cosmology to probe the entity driving the accelerating expansion of the Universe. In this paper, the topology of largescale structure (LSS) is used as one such standard ruler to study mysterious ‘dark energy’. By following the redshift evolution of the clustering of luminous red galaxies as measured by their 3D topology (counting structures in the cosmic web), we can chart the expansion rate and extract information about the equation of state of dark energy. Using the technique first introduced in Park & Kim, we evaluate the constraints that can be achieved using 3D topology measurements from nextgeneration LSS surveys, such as the Baryonic Oscillation Spectroscopic Survey. In conjunction with the information that will be available from the Planck satellite, we find a single topology measurement on three different scales is capable of constraining a single dark energy parameter to within 5–10 per cent when dynamics are permitted. This offers an alternative use of the data available from redshift surveys and serves as a crosscheck for BAO studies.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze correlations between the first letter of the name of an author and the number of citations their papers receive. We look at simple mean counts, numbers of highlycited papers, and normalized hindices, by letter. To our surprise, we conclude that orthographically senior authors produce a better body of work than their colleagues, despite some evidence of discrimination against them. Comment: 4 pages, 4 figures, 2 tables  [Show abstract] [Hide abstract]
ABSTRACT: We propose consistency tests for the cosmological constant which provide a direct observational signal if Lambda is wrong, regardless of the densities of matter and curvature. As an example of its utility, our flat case test can warn of a small transition of the equation of state w(z) from w(z)=1 of 20% from SNAP (Supernova Acceleration Probe) quality data at 4sigma, even when direct reconstruction techniques see virtually no evidence for deviation from Lambda. It is shown to successfully rule out a wide range of nonLambda dark energy models with no reliance on knowledge of Omega_{m} using SNAP quality data and a large range for using 10;{5} supernovae as forecasted for the Large Synoptic Survey Telescope. 
Article: A litmus test for Lambda
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ABSTRACT: The critical issue in cosmology today lies in determining if the cosmological constant is the underlying ingredient of dark energy. Our profound lack of understanding of the physics of dark energy places severe constrains on our ability to say anything about its possible dynamical nature. Quoted errors on the equation of state, w(z), are so heavily dependent on necessarily oversimplified parameterisations they are at risk of being rendered meaningless. Moreover, the existence of degeneracies between the reconstructed w(z) and the matter and curvature densities weakens any conclusions still further. We propose consistency tests for the cosmological constant which provide a direct observational signal if Lambda is wrong, regardless of the densities of matter and curvature. As an example of its utility, our flat case test can warn of a small transition from w(z)=1 of 20% from SNAP quality data at 4sigma, even when direct reconstruction techniques see virtually no evidence for deviation from Lambda. It is shown to successfully rule out a wide range of nonLambda dark energy models with no reliance on knowledge of Omega_m using SNAPquality data and a large range for using 10^5 supernovae as forecasted for LSST.  [Show abstract] [Hide abstract]
ABSTRACT: It has been suggested that Einstein's theory of General Relativity can be modified to accomodate mismatches between the gravitational field and luminous matter on a wide range of scales. Covariant theories of modified gravity generically predict the existence of extra degrees of freedom which may be interpreted as dark matter. We study a subclass of these theories where the overall energy density in these extra degrees of freedom is subdominant relative to the baryon density and show that they favour the presence of massive neutrinos. For some specific cases (such as a flat Universes with a cosmological constant) one finds a conservative lower bound on the neutrinos mass of $m_\nu>0.31$ eV.  [Show abstract] [Hide abstract]
ABSTRACT: We present a Bayesian technique based on a maximumentropy method to reconstruct the dark energy equation of state (EOS) w(z) in a nonparametric way. This Maximum Entropy (MaxEnt) technique allows to incorporate relevant prior information while adjusting the degree of smoothing of the reconstruction in response to the structure present in the data. After demonstrating the method on synthetic data, we apply it to current cosmological data, separately analysing Type Ia supernova measurement from the HST/GOODS programme and the firstyear Supernovae Legacy Survey (SNLS), complemented by cosmic microwave background and baryonic acoustic oscillation data. We find that the SNLS data are compatible with w(z) =−1 at all redshifts 0 ≤z≲ 1100, with error bars of the order of 20 per cent for the mostconstraining choice of priors. The HST/GOODS data exhibit a slight (about 1σ significance) preference for w > −1 at z∼ 0.5 and a drift towards w > −1 at larger redshifts which, however, is not robust with respect to changes in our prior specifications. We employ both a constant EOS prior model and a slowly varying w(z) and find that our conclusions are only mildly dependent on this choice at high redshifts. Our method highlights the danger of employing parametric fits for the unknown EOS, that can potentially miss or underestimate real structure in the data.  [Show abstract] [Hide abstract]
ABSTRACT: A range of experimental results point to the existence of a massive neutrino. The recent high precision measurements of the cosmic microwave background and the large scale surveys of galaxies can be used to place an upper bound on this mass. In this paper we perform a thorough analysis of all assumptions that go into obtaining a credible limit on $\sum m_{\nu}$. In particular we explore the impact of extending parameter space beyond the current standard cosmological model, the importance of priors and the uncertainties due to biasing in large scale structure. We find that the mass constraints are independent of the choice of parameterization as well as the inclusion of spatial curvature. The results of including the possibility of dark energy and tensors perturbations are shown to depend critically on the data sets used. The difference between an upper bound of 2.2 eV, assuming generic initial conditions, and an upper bound of 0.63 eV, assuming adiabaticity and a galaxy bias of 1, demonstrate the dependence of such a constraint on the assumptions in the analysis. Comment: 8 pages, 4 figures, submitted to JCAP, includes tensors and spectral running  [Show abstract] [Hide abstract]
ABSTRACT: We show, using a covariant and gauge– invariant approach to cosmological perturbation theory, that velocity and gravitational wave perturbations of the Friedmann– Lematre– Robertson– Walker (FLRW) model can lead to the generation and amplification of cosmic magnetic fields. It is argued that under certain conditions these fields can reach strengths capable of supporting the galactic dynamo mechanism.  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we apply secondorder gaugeinvariant perturbation theory to investigate the possibility that the coupling between gravitational waves (GWs) and a largescale inhomogeneous magnetic field acts as an amplification mechanism in an ``almost'' FriedmannLemaîtreRobertsonWalker Universe. The spatial inhomogeneities in the magnetic field are consistently implemented using the magnetohydrodynamic (MHD) approximation, which yields an additional source term due to the interaction of the magnetic field with velocity perturbations in the plasma. Comparing the solutions with the corresponding results in our previous work indicates that, on superhorizon scales, the interaction with the spatially inhomogeneous field in the dust regime induces the same boost as the case of a homogeneous field, at least in the ideal MHD approximation. This is attributed to the observation that the MHD induced part of the generated field effectively only contributes on scales where the coherence length of the initial field is less than the Hubble scale. At subhorizon scales, the GW induced magnetic field is completely negligible in relation to the MHD induced field. Moreover, there is no amplification found in the longwavelength limit. 
Article: Reply to "Comment on 'Primordial magnetic seed field amplification by gravitational waves'"
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ABSTRACT: Using secondorder gaugeinvariant perturbation theory, a selfconsistent framework describing the nonlinear coupling between gravitational waves and a largescale homogeneous magnetic field is presented. It is shown how this coupling may be used to amplify seed magnetic fields to strengths needed to support the galactic dynamo. In situations where the gravitational wave background is described by an `almost' FriedmannLema{\^i}treRobertsonWalker (FLRW) cosmology we find that the magnitude of the original magnetic field is amplified by an amount proportional to the magnitude of the gravitational wave induced shear anisotropy and the square of the field's initial comoving scale. We apply this mechanism to the case where the seed field and gravitational wave background are produced during inflation and find that the magnitude of the gravitational boost depends significantly on the manner in which the estimate of the shear anisotropy at the end of inflation is calculated. Assuming a seed field of $10^{34}$ $\rm{G}$ spanning a comoving scale of about $10 \rm{kpc}$ today, the shear anisotropy at the end of inflation must be at least as large as $10^{40}$ in order to obtain a generated magnetic field of the same order of magnitude as the original seed. Moreover, contrasting the weak field approximation to our gaugeinvariant approach, we find that while both methods agree in the limit of high conductivity, their corresponding solutions are otherwise only compatible in the limit of infinitely longwavelength gravitational waves. Comment: Matches published version
Publication Stats
354  Citations  
82.42  Total Impact Points  
Top Journals
Institutions

20122015

University of KwaZuluNatal
 • School of Chemistry and Physics
 • School of Mathematics, Statistics and Computer Science
Port Natal, KwaZuluNatal, South Africa


20102011

Princeton University
Princeton, New Jersey, United States


20052008

University of Oxford
 Department of Physics
Oxford, England, United Kingdom


2006

University of Cape Town
 Department of Mathematics and Applied Mathematics
Cape Town, Province of the Western Cape, South Africa
