Publications (131)585.09 Total impact

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ABSTRACT: We present Southern African Large Telescope (SALT) followup observations of seven massive clusters detected by the Atacama Cosmology Telescope (ACT) on the celestial equator using the SunyaevZel'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) x 10$^{14}$ solar masses (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. To our knowledge, this is only the third such system discovered, and therefore may be a rare example of a very massive halo in which quasarmode feedback is actively taking place.Monthly Notices of the Royal Astronomical Society 06/2015; 449(4):40104026. DOI:10.1093/mnras/stv595 · 5.23 Impact Factor 
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ABSTRACT: We present a statistical detection of the gravitational lensing of the cosmic microwave background by $10^{13}$ solar mass dark matter halos. Lensing convergence maps from the Atacama Cosmology Telescope Polarimeter (ACTPol) are stacked at the positions of around 12,000 opticallyselected CMASS galaxies from the SDSSIII/BOSS survey. The mean lensing signal is consistent with simulated dark matter halo profiles, and is favored over a null signal at 3.2 sigma significance. This result demonstrates the potential of microwave background lensing to probe the dark matter distribution in galaxy group and galaxy cluster halos.Physical Review Letters 04/2015; 114(15):151302. DOI:10.1103/PhysRevLett.114.151302 · 7.73 Impact Factor 
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ABSTRACT: Twopoint correlation functions of cosmic microwave background polarization provide a physically independent probe of the surprising suppression of correlations in the cosmic microwave background temperature anisotropies at large angular scales. We investigate correlation functions constructed from both the Q and U Stokes parameters and from the E and B polarization components. The dominant contribution to these correlation functions comes from local physical effects at the last scattering surface or from the epoch of reionization at high redshift, so all should be suppressed if the temperature suppression is due to an underlying lack of correlations in the cosmological metric perturbations larger than a given scale. We evaluate the correlation functions for the standard ΛCDM cosmology constrained by the observed temperature correlation function, and compute statistics characterizing their suppression on large angular scales. Future fullsky polarization maps with minimal systematic errors on large angular scales will provide strong tests of whether the observed temperature correlation function is a statistical fluke or reflects a fundamental shortcoming of the standard cosmological model. 
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ABSTRACT: Twopoint correlation functions of cosmic microwave background polarization provide a physically independent probe of the surprising suppression of correlations in the cosmic microwave background temperature anisotropies at large angular scales. We investigate correlation functions constructed from both the Q and U Stokes parameters and from the E and B polarization components. The dominant contribution to these correlation functions comes from local physical effects at the last scattering surface or from the epoch of reionization at high redshift, so all should be suppressed if the temperature suppression is due to an underlying lack of correlations in the cosmological metric perturbations larger than a given scale. We evaluate the correlation functions for the standard $\Lambda$CDM cosmology constrained by the observed temperature correlation function, and compute statistics characterizing their suppression on large angular scales. Future fullsky polarization maps with minimal systematic errors on large angular scales will provide strong tests of whether the observed temperature correlation function is a statistical fluke or reflects a fundamental shortcoming of the standard cosmological model. 
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ABSTRACT: We measure the crosscorrelation of cosmic microwave background lensing convergence maps derived from Atacama Cosmology Telescope data with galaxy lensing convergence maps as measured by the CanadaFranceHawaii Telescope Stripe 82 Survey. The CMBgalaxy lensing cross power spectrum is measured for the first time with a significance of 3.2{\sigma}, which corresponds to a 16% constraint on the amplitude of density fluctuations at redshifts ~ 0.9. With upcoming improved lensing data, this novel type of measurement will become a powerful cosmological probe, providing a precise measurement of the mass distribution at intermediate redshifts and serving as a calibrator for systematic biases in weak lensing measurements.Physical Review D 03/2015; 91:062001. DOI:10.1103/PhysRevD.91.062001 · 4.86 Impact Factor 
Article: The Atacama Cosmology Telescope: measuring radio galaxy bias through crosscorrelation with lensing
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ABSTRACT: We correlate the positions of radio galaxies in the FIRST survey with the CMB lensing convergence estimated from the Atacama Cosmology Telescope over 470 square degrees to determine the bias of these galaxies. We remove optically crossmatched sources below redshift $z=0.2$ to preferentially select Active Galactic Nuclei (AGN). We measure the angular crosspower spectrum $C_l^{\kappa g}$ at $4.4\sigma$ significance in the multipole range $100<l<3000$, corresponding to physical scales between $\approx$ 260 Mpc at an effective redshift $z_{\rm eff}= 1.5$. Modelling the AGN population with a redshiftdependent bias, the crossspectrum is well fit by the Planck bestfit $\Lambda$CDM cosmological model. Fixing the cosmology we fit for the overall bias model normalization, finding $b(z_{\rm eff}) = 3.5 \pm 0.8$ for the full galaxy sample, and $b(z_{\rm eff})=4.0\pm1.1 (3.0\pm1.1)$ for sources brighter (fainter) than 2.5 mJy. This measurement characterizes the typical halo mass of radioloud AGN: we find $\log(M_{\rm halo} / M_\odot) = 13.6^{+0.3}_{0.4}$. 
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ABSTRACT: Quasidilaton massive gravity offers a physically welldefined gravitational theory with nonzero graviton mass. We present the full set of dynamical equations governing the expansion history of the universe, valid during radiation domination, matter domination, and a latetime selfaccelerating epoch related to the graviton mass. The existence of selfconsistent solutions constrains the amplitude of the quasidilaton field and the graviton mass, as well as other model parameters. We point out that the effective mass of gravitational waves can be significantly larger than the graviton mass, opening the possibility that a single theory can explain both the latetime acceleration of the cosmic expansion and modifications of structure growth leading to the suppression of largeangle correlations observed in the cosmic microwave background.Physical Review D 12/2014; 91(4). DOI:10.1103/PhysRevD.91.041301 · 4.86 Impact Factor 
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ABSTRACT: We present a measurement of the gravitational lensing of the Cosmic Microwave Background (CMB) temperature and polarization fields obtained by crosscorrelating the reconstructed convergence signal from the first season of ACTPol data at 146 GHz with Cosmic Infrared Background (CIB) fluctuations measured using the Planck satellite. Using an overlap area of 206 square degrees, we detect gravitational lensing of the CMB polarization by largescale structure at a statistical significance of 4.5 sigma. Combining both CMB temperature and polarization data gives a lensing detection at 9.1 sigma significance. A Bmode polarization lensing signal is present with a significance of 3.2 sigma. We also present the first measurement of CMB lensingCIB correlation at small scales corresponding to l > 2000. Null tests and systematic checks show that our results are not significantly biased by astrophysical or instrumental systematic effects, including Galactic dust. Fitting our measurements to the bestfit lensingCIB cross power spectrum measured in Planck data, scaled by an amplitude A, gives A=1.02 +0.12/0.18 (stat.) +/0.06(syst.), consistent with the Planck results. 
Article: The Atacama Cosmology Telescope: A Measurement of the Thermal SunyaevZel'dovich OnePoint PDF
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ABSTRACT: We present a measurement of the onepoint probability distribution function (PDF) of the thermal SunyaevZel'dovich (tSZ) decrement in the pixel temperature histogram of filtered 148 GHz sky maps from the Atacama Cosmology Telescope (ACT). The PDF includes the signal from all galaxy clusters in the map, including objects below the signaltonoise threshold for individual detection, making it a particularly sensitive probe of the amplitude of matter density perturbations, $\sigma_8$. We use a combination of analytic halo model calculations and numerical simulations to compute the theoretical tSZ PDF and its covariance matrix, accounting for all noise sources and including relativistic corrections. From the measured ACT 148 GHz PDF alone, we find $\sigma_8 = 0.793 \pm 0.018$, with additional systematic errors of $\pm 0.017$ due to uncertainty in intracluster medium gas physics and $\pm 0.006$ due to uncertainty in infrared point source contamination. Using effectively the same data set, the statistical error here is a factor of two lower than that found in ACT's previous $\sigma_8$ determination based solely on the skewness of the tSZ signal. In future temperature maps with higher sensitivity, the tSZ PDF will break the degeneracy between intracluster medium gas physics and cosmological parameters. 
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ABSTRACT: The accelerating expansion of the universe at recent epochs is encoded in the cosmic microwave background: a few percent of the total temperature fluctuations are generated by evolving gravitational potentials which trace the largescale structures in the universe. This signature of dark energy, the Integrated SachsWolfe Effect, has been detected by averaging temperatures in the WMAP sky maps corresponding to the directions of superstructures in the Sloan Digital Sky Survey data release 6. We model the maximum average peak signal expected in the standard $\Lambda$CDM cosmological model, using Gaussian random realizations of the microwave sky, including correlations between different physical contributions to the temperature fluctuations and between different redshift ranges of the evolving gravitational potentials. We find good agreement with the mean temperature peak amplitude from previous theoretical estimates based on largescale structure simulations, but with larger statistical uncertainties. We apply our simulation pipeline to four different foregroundcleaned microwave temperature maps from Planck and WMAP data, finding a mean temperature peak signal at previously identified sky locations which exceeds our theoretical mean signal at a statistical significance of about $2.5\sigma$ and which differs from a null signal at $3.5\sigma$.Physical Review D 10/2014; 91(4). DOI:10.1103/PhysRevD.91.043510 · 4.86 Impact Factor 
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ABSTRACT: A period of inflation in the early universe produces a nearly scaleinvariant spectrum of gravitational waves over a huge range in wavelength. If the amplitude of this gravitational wave background is large enough to be detectable with microwave background polarization measurements, it will also be detectable directly with a spacebased laser interferometer. Using a Monte Carlo sampling of inflation models, we demonstrate that the combination of these two measurements will strongly constrain the expansion history during inflation and the physical mechanism driving it. 
Article: Dynamics of Gauge Field Inflation
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ABSTRACT: We analyze the existence and stability of dynamical attractor solutions for cosmological inflation driven by the coupling between fermions and a gauge field. Assuming a spatially homogeneous and isotropic gauge field and fermion current, the interacting fermion equation of motion reduces to that of a free fermion up to a phase shift. Consistency of the model is ensured via the Stuckelberg mechanism. We prove the existence of exactly one stable solution, and demonstrate the stability numerically. Inflation arises without fine tuning, and does not require postulating any effective potential or nonstandard coupling. 
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ABSTRACT: If a primordial magnetic field in the universe has nonzero helicity, the violation of parity symmetry results in nonzero correlations between cosmic microwave background temperature and Bmode polarization. In this paper we derive approximations to the relevant microwave background power spectra arising from a helical magnetic field. Using the crosspower spectrum between temperature and Bmode polarization from the WMAP nineyear data, we set a 95\% confidence level upper limit on the helicity amplitude to be 10 nG$^2$ Gpc for helicity spectral index $n_H = 1.9$, for a cosmological magnetic field with effective field strength of 3 nG and a powerlaw index $n_B = 2.9$ near the scaleinvariant value. Future microwave background polarization maps with greater sensitivity will be able to detect the helicity of an inflationary magnetic field well below the maximum value allowed by microwave background constraints on the magnetic field amplitude.Physical Review D 08/2014; 90(8). DOI:10.1103/PhysRevD.90.083004 · 4.86 Impact Factor 
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ABSTRACT: Future arcminute resolution polarization data from groundbased Cosmic Microwave Background (CMB) observations can be used to estimate the contribution to the temperature power spectrum from the primary anisotropies and to uncover the signature of reionization near $\ell=1500$ in the small angularscale temperature measurements. Our projections are based on combining expected smallscale Emode polarization measurements from Advanced ACTPol in the range $300<\ell<3000$ with simulated temperature data from the full Planck mission in the low and intermediate $\ell$ region, $2<\ell<2000$. We show that the six basic cosmological parameters determined from this combination of data will predict the underlying primordial temperature spectrum at high multipoles to better than $1\%$ accuracy. Assuming an efficient cleaning from multifrequency channels of most foregrounds in the temperature data, we investigate the sensitivity to the only residual secondary component, the kinematic SunyaevZel'dovich (kSZ) term. The CMB polarization is used to break degeneracies between primordial and secondary terms present in temperature and, in effect, to remove from the temperature data all but the residual kSZ term. We estimate a $15 \sigma$ detection of the diffuse homogeneous kSZ signal from expected AdvACT temperature data at $\ell>1500$, leading to a measurement of the amplitude of matter density fluctuations, $\sigma_8$, at $1\%$ precision. Alternatively, by exploring the reionization signal encoded in the patchy kSZ measurements, we bound the time and duration of the reionization with $\sigma(z_{\rm re})=1.1$ and $\sigma(\Delta z_{\rm re})=0.2$. We find that these constraints degrade rapidly with large beam sizes, which highlights the importance of arcminutescale resolution for future CMB surveys.Journal of Cosmology and Astroparticle Physics 06/2014; JCAP08(2014)010. DOI:10.1088/14757516/2014/08/010 · 6.04 Impact Factor 
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ABSTRACT: We report on measurements of the cosmic microwave background (CMB) and celestial polarization at 146 GHz made with the Atacama Cosmology Telescope Polarimeter (ACTPol) in its first three months of observing. Four regions of sky covering a total of 270 square degrees were mapped with an angular resolution of $1.3'$. The map noise levels in the four regions are between 11 and 17 $\mu$Karcmin. We present TT, TE, EE, TB, EB, and BB power spectra from three of these regions. The observed Emode polarization power spectrum, displaying six acoustic peaks in the range $200<\ell<3000$, is an excellent fit to the prediction of the bestfit cosmological models from WMAP9+ACT and Planck data. The polarization power spectrum, which mainly reflects primordial plasma velocity perturbations, provides an independent determination of cosmological parameters consistent with those based on the temperature power spectrum, which results mostly from primordial density perturbations. We find that without masking any point sources in the EE data at $\ell<9000$, the Poisson tail of the EE power spectrum due to polarized point sources has an amplitude less than $2.4$ $\mu$K$^2$ at $\ell = 3000$ at 95\% confidence. Finally, we report that the Crab Nebula, an important polarization calibration source at microwave frequencies, has 8.7\% polarization with an angle of $150.9^\circ \pm 0.5^\circ$ when smoothed with a $5'$ Gaussian beam.Journal of Cosmology and Astroparticle Physics 05/2014; JCAP10(2014)007. DOI:10.1088/14757516/2014/10/007 · 5.88 Impact Factor 
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ABSTRACT: We present the temperature power spectra of the cosmic microwave background (CMB) derived from the three seasons of data from the Atacama Cosmology Telescope (ACT) at 148 GHz and 218 GHz, as well as the crossfrequency spectrum between the two channels. We detect and correct for contamination due to the Galactic cirrus in our equatorial maps. We present the results of a number of tests for possible systematic error and conclude that any effects are not significant compared to the statistical errors we quote. Where they overlap, we crosscorrelate the ACT and the South Pole Telescope (SPT) maps and show they are consistent. The measurements of higherorder peaks in the CMB power spectrum provide an additional test of the Lambda CDM cosmological model, and help constrain extensions beyond the standard model. The small angular scale power spectrum also provides constraining power on the SunyaevZel'dovich effects and extragalactic foregrounds. We also present a measurement of the CMB gravitational lensing convergence power spectrum at 4.6sigma detection significance.Journal of Cosmology and Astroparticle Physics 04/2014; 2014(04):014. DOI:10.1088/14757516/2014/04/014 · 5.88 Impact Factor 
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ABSTRACT: The measurement of Bmode polarization of the cosmic microwave background at large angular scales by the BICEP experiment suggests a stochastic gravitational wave background from earlyuniverse inflation with a surprisingly large amplitude. The power spectrum of these tensor perturbations can be probed both with further measurements of the microwave background polarization at smaller scales, and also directly via interferometry in space. We show that sufficiently sensitive highresolution Bmode measurements will ultimately have the ability to test the inflationary consistency relation between the amplitude and spectrum of the tensor perturbations, confirming their inflationary origin. Additionally, a precise Bmode measurement of the tensor spectrum will predict the tensor amplitude on solar system scales to 20% accuracy for an exact power law tensor spectrum, so a direct detection will then measure the running of the tensor spectral index to high precision.Physical Review Letters 03/2014; 112(19). DOI:10.1103/PhysRevLett.112.191302 · 7.73 Impact Factor 
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ABSTRACT: We present the temperature power spectrum of the Cosmic Microwave Background obtained by crosscorrelating maps from the Atacama Cosmology Telescope (ACT) at 148 and 218 GHz with maps from the Planck satellite at 143 and 217 GHz, in two overlapping regions covering 592 square degrees. We find excellent agreement between the two datasets at both frequencies, quantified using the variance of the residuals between the ACT power spectra and the ACTxPlanck crossspectra. We use these crosscorrelations to calibrate the ACT data at 148 and 218 GHz, to 0.7% and 2% precision respectively. We find no evidence for anisotropy in the calibration parameter. We compare the Planck 353 GHz power spectrum with the measured amplitudes of dust and cosmic infrared background (CIB) of ACT data at 148 and 218 GHz. We also compare planet and point source measurements from the two experiments.Journal of Cosmology and Astroparticle Physics 03/2014; DOI:10.1088/14757516/2014/07/016 · 5.88 Impact Factor 
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ABSTRACT: We present a statistical analysis of the millimeterwavelength properties of 1.4 GHzselected sources and a detection of the SunyaevZel'dovich (SZ) effect associated with the halos that host them. The Atacama Cosmology Telescope (ACT) has conducted a survey at 148 GHz, 218 GHz and 277 GHz along the celestial equator. Using samples of radio sources selected at 1.4 GHz from FIRST and NVSS, we measure the stacked 148, 218 and 277 GHz flux densities for sources with 1.4 GHz flux densities ranging from 5 to 200 mJy. At these flux densities, the radio source population is dominated by active galactic nuclei (AGN), with both steep and flat spectrum populations, which have combined radiotomillimeter spectral indices ranging from 0.5 to 0.95, reflecting the prevalence of steep spectrum sources at high flux densities and the presence of flat spectrum sources at lower flux densities. The thermal SZ effect associated with the halos that host the AGN is detected at the 5$\sigma$ level through its spectral signature. When we compare the SZ effect with weak lensing measurements of radio galaxies, we find that the relation between the two is consistent with that measured by Planck for local bright galaxies. We present a detection of the SZ effect in some of the lowest mass halos (average $M_{200}\approx10^{13}$M$_{\odot}h_{70}^{1}$) studied to date. This detection is particularly important in the context of galaxy evolution models, as it confirms that galaxies with radio AGN also typically support hot gaseous halos. With Herschel observations, we show that the SZ detection is not significantly contaminated by dust. We show that 5 mJy$<S_{1.4}<$200 mJy radio sources contribute $\ell(\ell+1)C_{\ell}/(2\pi)=0.37\pm0.03\mu$K$^2$ to the angular power spectrum at $\ell=3000$ at 148 GHz, after accounting for the SZ effect associated with their host halos.Monthly Notices of the Royal Astronomical Society 10/2013; 445(1):460478. DOI:10.1093/mnras/stu1592 · 4.90 Impact Factor 
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ABSTRACT: A lack of correlations in the microwave background temperature between sky directions separated by angles larger than 60 degrees has recently been confirmed by data from the Planck satellite. This feature arises as a random occurrence within the standard LCDM cosmological model less than 0.3 per cent of the time, but so far no other compelling theory to explain this observation has been proposed. Here we investigate the theoretical crosscorrelation function between microwave background temperature and the gravitational lensing potential of the microwave background, which in contrast to the temperature correlation function depends strongly on gravitational potential fluctuations interior to our Hubble volume. For standard LCDM cosmology, we generate random sky realizations of the microwave temperature and gravitational lensing, subject to the constraint that the temperature correlation function matches observations, and compare with random skies lacking this constraint. The distribution of largeangle temperaturelensing correlation functions in these two cases is different, and the two cases can be clearly distinguished in around 40 per cent of model realizations. We present an a priori procedure for using similar largeangle correlations between other types of data, to determine whether the lack of largeangle correlations is a statistical fluke or points to a shortcoming of the standard cosmological model.Monthly Notices of the Royal Astronomical Society 10/2013; 442(3). DOI:10.1093/mnras/stu942 · 5.23 Impact Factor
Publication Stats
5k  Citations  
585.09  Total Impact Points  
Top Journals
Institutions

2013–2015

University of Oxford
Oxford, England, United Kingdom


2005–2015

University of Pittsburgh
 Physics and Astronomy
Pittsburgh, Pennsylvania, United States


2012–2013

University of Toronto
 Canadian Institute for Theoretical Astrophysics
Toronto, Ontario, Canada


2010–2013

Princeton University
 • Department of Astrophysical Sciences
 • Department of Physics
Princeton, New Jersey, United States 
Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE)
Cholula de Riva dabia, Puebla, Mexico


2010–2012

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


2011

University of British Columbia  Vancouver
 Department of Physics and Astronomy
Vancouver, British Columbia, Canada


1999–2005

Rutgers, The State University of New Jersey
 Department Physics and Astronomy
New Brunswick, New Jersey, United States


1992–2002

University of Chicago
 • Department of Astronomy and Astrophysics
 • Department of Physics
Chicago, Illinois, United States


1998

Tufts University
 Department of Physics and Astronomy
Georgia, United States


1995–1997

Harvard University
 Department of Physics
Cambridge, Massachusetts, United States


1996

HarvardSmithsonian Center for Astrophysics
Cambridge, Massachusetts, United States


1992–1995

NASA
Вашингтон, West Virginia, United States


1994

Vascular and Interventional Radiology
Chicago, Illinois, United States
