arXiv subjects

Wayne Hu

Publications and source records attributed to Wayne Hu.

At least 145 records · Page 8Linked to original sources

arXiv2009

Non-linear Evolution of f(R) Cosmologies III: Halo Statistics

The statistical properties of dark matter halos, the building blocks of cosmological observables associated with structure in the universe, offer many opportunities to test models for cosmic acceleration, especially those that seek to modify gravitational forces. We study the abundance, bias and profiles of halos in cosmological simulations for one such model: the modified action f(R) theory. In the large field regime that is accessible to current observations, enhanced gravitational forces raise the abundance of rare massive halos and decrease their bias but leave their (lensing) mass profiles largely unchanged. This regime is well described by scaling relations based on a modification of spherical collapse calculations. In the small field regime, enhanced forces are suppressed inside halos and the effects on halo properties are substantially reduced for the most massive halos. Nonetheless, the scaling relations still retain limited applicability for the purpose of establishing conservative upper limits on the modification to gravity.

astro-ph↗

arXiv2009

Observing the Evolution of the Universe

How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass. We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages.

astro-ph.CO↗

arXiv2009

Estimation of the Hubble Constant and Constraint on Descriptions of Dark Energy

Joint analysis of Cosmic Microwave Background, Baryon Acoustic Oscillation, and supernova data has enabled precision estimation of cosmological parameters. New programs will push to 1% uncertainty in the dark energy equation of state and tightened constraint on curvature, requiring close attention to systematics. Direct 1% measurement of the Hubble constant (H0) would provide a new constraint. It can be obtained without overlapping systematics directly from recessional velocities and geometric distance estimates for galaxies via the mapping of water maser emission that traces the accretion disks of nuclear black holes. We identify redshifts 0.02 0.02, out of ~100 known masers. A single-dish discovery survey of >10,000 nuclei (>2500 hours on the GBT) would build a sample of tens of potential distance anchors. Beyond 2020, a high-frequency SKA could provide larger maser samples, enabling estimation of H0 from individually less accurate distances, and possibly without the need for peculiar motion corrections.

astro-ph.CO↗

arXiv2008

Crossing the Phantom Divide with Parameterized Post-Friedmann Dark Energy

Dark energy models with a single scalar field cannot cross the equation of state divide set by a cosmological constant. More general models that allow crossing require additional degrees of freedom to ensure gravitational stability. We show that a parameterized post-Friedmann description of cosmic acceleration provides a simple but accurate description of multiple scalar field crossing models. Moreover the prescription provides a well controlled approximation for a wide range of "smooth" dark energy models. It conserves energy and momentum and is exact in the metric evolution on scales well above and below the transition scale to relative smoothness. Standard linear perturbation tools have been altered to include this description and made publicly available for studies of the dark energy involving cosmological structure out to the horizon scale.

astro-ph↗

arXiv2008

Challenges to the DGP Model from Horizon-Scale Growth and Geometry

We conduct a Markov Chain Monte Carlo study of the Dvali-Gabadadze-Porrati (DGP) self-accelerating braneworld scenario given the cosmic microwave background (CMB) anisotropy, supernovae and Hubble constant data by implementing an effective dark energy prescription for modified gravity into a standard Einstein-Boltzmann code. We find no way to alleviate the tension between distance measures and horizon scale growth in this model. Growth alterations due to perturbations propagating into the bulk appear as excess CMB anisotropy at the lowest multipoles. In a flat cosmology, the maximum likelihood DGP model is nominally a 5.3 sigma poorer fit than Lambda CDM. Curvature can reduce the tension between distance measures but only at the expense of exacerbating the problem with growth leading to a 4.8 sigma result that is dominated by the low multipole CMB temperature spectrum. While changing the initial conditions to reduce large scale power can flatten the temperature spectrum, this also suppresses the large angle polarization spectrum in violation of recent results from WMAP5. The failure of this model highlights the power of combining growth and distance measures in cosmology as a test of gravity on the largest scales.

astro-ph↗

arXiv2008

Falsifying Paradigms for Cosmic Acceleration

Consistency relations between growth of structure and expansion history observables exist for any physical explanation of cosmic acceleration, be it a cosmological constant, scalar field quintessence, or a general component of dark energy that is smooth relative to dark matter on small scales. The high-quality supernova sample anticipated from an experiment like SNAP and CMB data expected from Planck thus make strong predictions for growth and expansion observables that additional observations can test and potentially falsify. We perform an MCMC likelihood exploration of the strength of these consistency relations based on a complete parametrization of dark energy behavior by principal components. For LCDM, future SN and CMB data make percent level predictions for growth and expansion observables. For quintessence, many of the predictions are still at a level of a few percent with most of the additional freedom coming from curvature and early dark energy. While such freedom is limited for quintessence where phantom equations of state are forbidden, it is larger in the smooth dark energy class. Nevertheless, even in this general class predictions relating growth measurements at different redshifts remain robust, although predictions for the instantaneous growth rate do not. Finally, if observations falsify the whole smooth dark energy class, new paradigms for cosmic acceleration such as modified gravity or interacting dark matter and dark energy would be required.

astro-ph↗

arXiv2008

Non-linear evolution of f(R) cosmologies II: power spectrum

We carry out a suite of cosmological simulations of modified action f(R) models where cosmic acceleration arises from an alteration of gravity instead of dark energy. These models introduce an extra scalar degree of freedom which enhances the force of gravity below the inverse mass or Compton scale of the scalar. The simulations exhibit the so-called chameleon mechanism, necessary for satisfying local constraints on gravity, where this scale depends on environment, in particular the depth of the local gravitational potential. We find that the chameleon mechanism can substantially suppress the enhancement of power spectrum in the non-linear regime if the background field value is comparable to or smaller than the depth of the gravitational potentials of typical structures. Nonetheless power spectrum enhancements at intermediate scales remain at a measurable level for models even when the expansion history is indistinguishable from a cosmological constant, cold dark matter model. Simple scaling relations that take the linear power spectrum into a non-linear spectrum fail to capture the modifications of f(R) due to the change in collapsed structures, the chameleon mechanism, and the time evolution of the modifications.

astro-ph↗

arXiv2008

Testable polarization predictions for models of CMB isotropy anomalies

Anomalies in the large-scale CMB temperature sky measured by WMAP have been suggested as possible evidence for a violation of statistical isotropy on large scales. In any physical model for broken isotropy, there are testable consequences for the CMB polarization field. We develop simulation tools for predicting the polarization field in models that break statistical isotropy locally through a modulation field. We study two different models: dipolar modulation, invoked to explain the asymmetry in power between northern and southern ecliptic hemispheres, and quadrupolar modulation, posited to explain the alignments between the quadrupole and octopole. For the dipolar case, we show that predictions for the correlation between the first 10 multipoles of the temperature and polarization fields can typically be tested at better than the 98% CL. For the quadrupolar case, we show that the polarization quadrupole and octopole should be moderately aligned. Such an alignment is a generic prediction of explanations which involve the temperature field at recombination and thus discriminate against explanations involving foregrounds or local secondary anisotropy. Predicted correlations between temperature and polarization multipoles out to l = 5 provide tests at the ~ 99% CL or stronger for quadrupolar models that make the temperature alignment more than a few percent likely. As predictions of anomaly models, polarization statistics move beyond the a posteriori inferences that currently dominate the field.

astro-ph↗

arXiv2008

Reionization constraints from five-year WMAP data

We study the constraints on reionization from five years of WMAP data, parametrizing the evolution of the average fraction of ionized hydrogen with principal components that provide a complete basis for describing the effects of reionization on large-scale E-mode polarization. Using Markov Chain Monte Carlo methods, we find that the resulting model-independent estimate of the total optical depth is nearly twice as well determined as the estimate from 3-year WMAP data, in agreement with simpler analyses that assume instantaneous reionization. The mean value of the optical depth from principal components is slightly larger than the instantaneous value; we find tau=0.097+/-0.017 using only large-scale polarization, and tau=0.101+/-0.019 when temperature data is included. Likewise, scale invariant n_s=1 spectra are no longer strongly disfavored by WMAP alone. Higher moments of the ionization history show less improvement in the 5-year data than the optical depth. By plotting the distribution of polarization power for models from the MCMC analysis, we show that extracting most of the remaining information about the shape of the reionization history from the CMB requires better measurements of E-mode polarization on scales of l~10-20. Conversely, the quadrupole and octopole polarization power is already predicted to better than cosmic variance given any allowed ionization history at z<30 so that more precise measurements will test the LambdaCDM paradigm.

astro-ph↗

arXiv2008

Parametrized Post-Friedmann Signatures of Acceleration in the CMB

We extend the covariant, parametrized post-Friedmann treatment of cosmic acceleration from modified gravity to an arbitrary admixture of matter, radiation, relativistic components and spatial curvature. Explicit expressions in the comoving, Newtonian and synchronous gauges facilitate the adaptation of Einstein-Boltzmann codes for solving CMB and matter perturbations in the linear regime. Using a comoving gauge code, we study the effect of metric evolution on the CMB through the integrated Sachs-Wolfe effect. Modified gravity can alter the low multipole spectrum, including lowering the power in the quadrupole. From a principal component description of the primary metric ratio parameter, we obtain general constraints from WMAP on modified gravity models of the acceleration.

astro-ph↗

arXiv2008

Lecture Notes on CMB Theory: From Nucleosynthesis to Recombination

These lecture notes comprise an informal but pedagogical introduction to the well established physics and phenomenology of the cosmic microwave background (CMB) between big bang nucleosynthesis and recombination. The dominant properties of the spectrum, temperature anisotropy and polarization anisotropy of the CMB all arise from this period. We review the physical processes involved and show how they are related to the observed phenomenology.

astro-ph↗

arXiv2007

Self Calibration of Tomographic Weak Lensing for the Physics of Baryons to Constrain Dark Energy

Numerical studies indicate that uncertainties in the treatment of baryonic physics can affect predictions for shear power spectra at a level that is significant for forthcoming surveys such as DES, SNAP, and LSST. Correspondingly, we show that baryonic effects can significantly bias dark energy parameter measurements. Eliminating such biases by neglecting information in multipoles beyond several hundred leads to weaker parameter constraints by a factor of approximately 2 to 3 compared with using information out to multipoles of several thousand. Fortunately, the same numerical studies that explore the influence of baryons indicate that they primarily affect power spectra by altering halo structure through the relation between halo mass and mean effective halo concentration. We explore the ability of future weak lensing surveys to constrain both the internal structures of halos and the properties of the dark energy simultaneously as a first step toward self calibrating for the physics of baryons. This greatly reduces parameter biases and no parameter constraint is degraded by more than 40% in the case of LSST or 30% in the cases of SNAP or DES. Modest prior knowledge of the halo concentration relation greatly improves even these forecasts. Additionally, we find that these surveys can constrain effective halo concentrations near m~10^14 Msun/h and z~0.2 to better than 10% with shear power spectra alone. These results suggest that inferring dark energy parameters with measurements of shear power spectra can be made robust to baryonic effects and may simultaneously be competitive with other methods to inform models of galaxy formation. (Abridged)

astro-ph↗

arXiv2007

Impact of reionization on CMB polarization tests of slow-roll inflation

Estimates of inflationary parameters from the CMB B-mode polarization spectrum on the largest scales depend on knowledge of the reionization history, especially at low tensor-to-scalar ratio. Assuming an incorrect reionization history in the analysis of such polarization data can strongly bias the inflationary parameters. One consequence is that the single-field slow-roll consistency relation between the tensor-to-scalar ratio and tensor tilt might be excluded with high significance even if this relation holds in reality. We explain the origin of the bias and present case studies with various tensor amplitudes and noise characteristics. A more model-independent approach can account for uncertainties about reionization, and we show that parametrizing the reionization history by a set of its principal components with respect to E-mode polarization removes the bias in inflationary parameter measurement with little degradation in precision.

astro-ph↗

arXiv2007

A Parameterized Post-Friedmann Framework for Modified Gravity

We develop a parameterized post-Friedmann (PPF) framework which describes three regimes of modified gravity models that accelerate the expansion without dark energy. On large scales, the evolution of scalar metric and density perturbations must be compatible with the expansion history defined by distance measures. On intermediate scales in the linear regime, they form a scalar-tensor theory with a modified Poisson equation. On small scales in dark matter halos such as our own galaxy, modifications must be suppressed in order to satisfy stringent local tests of general relativity. We describe these regimes with three free functions and two parameters: the relationship between the two metric fluctuations, the large and intermediate scale relationships to density fluctuations and the two scales of the transitions between the regimes. We also clarify the formal equivalence of modified gravity and generalized dark energy. The PPF description of linear fluctuation in f(R) modified action and the Dvali-Gabadadze-Porrati braneworld models show excellent agreement with explicit calculations. Lacking cosmological simulations of these models, our non-linear halo-model description remains an ansatz but one that enables well-motivated consistency tests of general relativity. The required suppression of modifications within dark matter halos suggests that the linear and weakly non-linear regimes are better suited for making complementary test of general relativity than the deeply non-linear regime.

astro-ph↗

arXiv2007

Photometric Redshift Requirements for Self-Calibration of Cluster Dark Energy Studies

The ability to constrain dark energy from the evolution of galaxy cluster counts is limited by the imperfect knowledge of cluster redshifts. Ongoing and upcoming surveys will mostly rely on redshifts estimated from broad-band photometry (photo-z's). For a Gaussian distribution for the cluster photo-z errors and a high cluster yield cosmology defined by the WMAP 1 year results, the photo-z bias and scatter needs to be known better than 0.003 and 0.03, respectively, in order not to degrade dark energy constrains by more than 10% for a survey with specifications similar to the South Pole Telescope. Smaller surveys and cosmologies with lower cluster yields produce weaker photo-z requirements, though relative to worse baseline constraints. Comparable photo-z requirements are necessary in order to employ self-calibration techniques when solving for dark energy and observable-mass parameters simultaneously. On the other hand, self-calibration in combination with external mass inferences helps reduce photo-z requirements and provides important consistency checks for future cluster surveys. In our fiducial model, training sets with spectroscopic redshifts for ~5%-15% of the detected clusters are required in order to keep degradations in the dark energy equation of state lower than 20%.

astro-ph↗

arXiv2007

CMB Cluster Lensing: Cosmography with the Longest Lever Arm

We discuss combining gravitational lensing of galaxies and the cosmic microwave background (CMB) by clusters to measure cosmographic distance ratios, and hence dark energy parameters. Advantages to using the CMB as the second source plane, instead of galaxies, include: a well-determined source distance, a longer lever arm for distance ratios, typically up to an order of magnitude higher sensitivity to dark energy parameters, and a decreased sensitivity to photometric redshift accuracy of the lens and galaxy sources. Disadvantages include: higher statistical errors, potential systematic errors, and the need for disparate surveys that overlap on the sky. Ongoing and planned surveys, such as the South Pol Telescope in conjunction with the Dark Energy Survey, can potentially reach the statistical sensitivity to make interesting consistency tests of the standard cosmological constant model. Future measurements that reach 1% or better precision in the convergences can provide sharp tests for future supernovae distance measurements.

astro-ph↗

arXiv2007

Cosmological Constraints on f(R) Acceleration Models

Models which accelerate the expansion of the universe through the addition of a function of the Ricci scalar f(R) leave a characteristic signature in the large-scale structure of the universe at the Compton wavelength scale of the extra scalar degree of freedom. We search for such a signature in current cosmological data sets: the WMAP cosmic microwave background (CMB) power spectrum, SNLS supernovae distance measures, the SDSS luminous red galaxy power spectrum, and galaxy-CMB angular correlations. Due to theoretical uncertainties in the nonlinear evolution of f(R) models, the galaxy power spectrum conservatively yields only weak constraints on the models despite the strong predicted signature in the linear matter power spectrum. Currently the tightest constraints involve the modification to the integrated Sachs-Wolfe effect from growth of gravitational potentials during the acceleration epoch. This effect is manifest for large Compton wavelengths in enhanced low multipole power in the CMB and anti-correlation between the CMB and tracers of the potential. They place a bound on the Compton wavelength of the field be less than of order the Hubble scale.

astro-ph↗

arXiv2007

Stability of Cosmological Solution in f(R) Models of Gravity

We reconcile seemingly conflicting statements in the literature about the behavior of cosmological solutions in modified theories of gravity where the Einstein-Hilbert Lagrangian for gravity is modified by the addition of a function of the Ricci scalar, f(R). Using the example of f(R) = +/-μ^4/R we show that only such choices of f(R) where d^2f/dR^2>0 have stable high-curvature limits and well-behaved cosmological solutions with a proper era of matter domination. The remaining models enter a phase dominated by both matter and scalar kinetic energy where the scalar curvature remains low.

astro-ph↗