arXiv subjects

Cora Dvorkin

Publications and source records attributed to Cora Dvorkin.

At least 109 records · Page 6Linked to original sources

arXiv2016

Imprint of inflation on galaxy shape correlations

We show that intrinsic (not lensing-induced) correlations between galaxy shapes offer a new probe of primordial non-Gaussianity and inflationary physics which is complementary to galaxy number counts. Specifically, intrinsic alignment correlations are sensitive to an anisotropic squeezed limit bispectrum of the primordial perturbations. Such a feature arises in solid inflation, as well as more broadly in the presence of light higher spin fields during inflation (as pointed out recently by Arkani-Hamed and Maldacena). We present a derivation of the all-sky two-point correlations of intrinsic shapes and number counts in the presence of non-Gaussianity with general angular dependence, and show that a quadrupolar (spin-2) anisotropy leads to the analog in galaxy shapes of the well-known scale-dependent bias induced in number counts by isotropic (spin-0) non-Gaussianity. Moreover, in presence of non-zero anisotropic non-Gaussianity, the quadrupole of galaxy shapes becomes sensitive to far superhorizon modes. These effects come about because long-wavelength modes induce a local anisotropy in the initial power spectrum, with which galaxies will correlate. We forecast that future imaging surveys could provide constraints on the amplitude of anisotropic non-Gaussianity that are comparable to those from the Cosmic Microwave Background (CMB). These are complementary as they probe different physical scales. The constraints, however, depend on the sensitivity of galaxy shapes to the initial conditions which we only roughly estimate from observed tidal alignments.

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arXiv2015

On testing and extending the inflationary consistency relation for tensor modes

If observations confirm BICEP2's claim of a tensor-scalar ratio $r\approx 0.2$ on CMB scales, then the inflationary consistency relation $n_{t}=-r/8$ predicts a small negative value for the tensor spectral index $n_t$. We show that future CMB polarization experiments should be able to confirm this prediction at several sigma. We also show how to properly extend the consistency relation to solar system scales, where the primordial gravitational wave density $Ω_{gw}$ could be measured by proposed experiments such as the Big Bang Observer. This would provide a far more stringent test of the consistency relation and access much more detailed information about the early universe.

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arXiv2014

Polarization Predictions for Inflationary CMB Power Spectrum Features

We conduct a model-independent analysis of temporal features during inflation in the large-scale CMB temperature power spectrum allowing for the possibility of non-negligible tensor contributions. Of 20 principal components of the inflationary history, the suppression of power at low multipoles beginning with a glitch at multipoles $\ell \sim 20-40$ implies deviations in 2-3 of them with 2-3$σ$ deviations in each, with larger values reflecting cases where tensors are allowed.If tensors are absent, the corresponding $E$-mode polarization features follow a similar pattern but are predicted to be up to twice as large. They offer the opportunity to soon double the significance of inflationary features or eliminate them as an explanation of temperature features. The tensor degeneracy with features in the temperature power spectrum is broken not only by $B$ but also by $E$-polarization. A precision measurement of $E$-mode polarization at multipoles from $\ell\sim 20-60$ can potentially provide an independent constraint on tensors that is less subject to dust foreground uncertainties.

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arXiv2014

Neutrinos help reconcile Planck measurements with both Early and Local Universe

In light of the recent BICEP2 B-mode polarization detection, which implies a large inflationary tensor-to-scalar ratio r_{0.05}=0.2^{+0.07}_{-0.05}, we re-examine the evidence for an extra sterile massive neutrino, originally invoked to account for the tension between the cosmic microwave background (CMB) temperature power spectrum and local measurements of the expansion rate H0 and cosmological structure. With only the standard active neutrinos and power-law scalar spectra, this detection is in tension with the upper limit of r<0.11 (95% confidence) from the lack of a corresponding low multipole excess in the temperature anisotropy from gravitational waves. An extra sterile species with the same energy density as is needed to reconcile the CMB data with H0 measurements can also alleviate this new tension. By combining data from the Planck and ACT/SPT temperature spectra, WMAP9 polarization, H_0, baryon acoustic oscillation and local cluster abundance measurements with BICEP2 data, we find the joint evidence for a sterile massive neutrino increases to DeltaNeff=0.98\pm 0.26 for the effective number and ms= 0.52\pm 0.13 eV for the effective mass or 3.8 sigma and 4 sigma evidence respectively. We caution the reader that these results correspond to a joint statistical evidence and, in addition, astrophysical systematic errors in the clusters and H0 measurements, and small-scale CMB data could weaken our conclusions.

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arXiv2014

Can weak lensing surveys confirm BICEP2 ?

The detection of B-modes in the Cosmic Microwave Background (CMB) polarization by the BICEP2 experiment, if interpreted as evidence for a primordial gravitational wave background, has enormous ramifications for cosmology and physics. It is crucial to test this hypothesis with independent measurements. A gravitational wave background leads to B-modes in galaxy shape correlations (shear) both through lensing and tidal alignment effects. Since the systematics and foregrounds of galaxy shapes and CMB polarization are entirely different, a detection of a cross-correlation between the two observables would provide conclusive proof for the existence of a primordial gravitational wave background. We find that upcoming weak lensing surveys will be able to detect the cross-correlation between B-modes of the CMB and galaxy shapes. However, this detection is not sufficient to confirm or falsify the hypothesis of a primordial origin for CMB B-mode polarization.

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arXiv2014

On quantifying and resolving the BICEP2/Planck tension over gravitational waves

The recent BICEP2 measurement of primordial gravity waves (r = 0.2^{+0.07}_{-0.05}) appears to be in tension with the upper limit from WMAP (r<0.13 at 95% CL) and Planck (r<0.11 at 95% CL). We carefully quantify the level of tension and show that it is very significant (around 0.1% unlikely) when the observed deficit of large-scale temperature power is taken into account. We show that measurements of TE and EE power spectra in the near future will discriminate between the hypotheses that this tension is either a statistical fluke, or a sign of new physics. We also discuss extensions of the standard cosmological model that relieve the tension, and some novel ways to constrain them.

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arXiv2013

Cosmological Information in the Intrinsic Alignments of Luminous Red Galaxies

The intrinsic alignments of galaxies are usually regarded as a contaminant to weak gravitational lensing observables. The alignment of Luminous Red Galaxies, detected unambiguously in observations from the Sloan Digital Sky Survey, can be reproduced by the linear tidal alignment model of Catelan, Kamionkowski & Blandford (2001) on large scales. In this work, we explore the cosmological information encoded in the intrinsic alignments of red galaxies. We make forecasts for the ability of current and future spectroscopic surveys to constrain local primordial non-Gaussianity and Baryon Acoustic Oscillations (BAO) in the cross-correlation function of intrinsic alignments and the galaxy density field. For the Baryon Oscillation Spectroscopic Survey, we find that the BAO signal in the intrinsic alignments is marginally significant with a signal-to-noise ratio of 1.8 and 2.2 with the current LOWZ and CMASS samples of galaxies, respectively, and increasing to 2.3 and 2.7 once the survey is completed. For the Dark Energy Spectroscopic Instrument and for a spectroscopic survey following the EUCLID redshift selection function, we find signal-to-noise ratios of 12 and 15, respectively. Local type primordial non-Gaussianity, parametrized by fNL = 10, is only marginally significant in the intrinsic alignments signal with signal-to-noise ratios < 2 for the three surveys considered.

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arXiv2013

Constraining Dark Matter-Baryon Scattering with Linear Cosmology

We derive constraints on elastic scattering between baryons and dark matter using the cosmic microwave background (CMB) data from the Planck satellite and the Lyman-alpha forest data from the Sloan Digital Sky Survey. Elastic scattering allows baryons and dark matter to exchange momentum, affecting the dynamics of linear density perturbations in the early Universe. We derive constraints to scattering cross sections of the form sigma \propto v^n, allowing for a wide range of velocity dependencies with n between -4 and 2. We improve and correct previous estimates where they exist, including velocity-independent cross section as well as dark matter millicharge and electromagnetic dipole moments. Lyman-alpha forest data dominates the constraints for n>-3, where the improvement over CMB data alone can be several orders of magnitude. Dark matter-baryon scattering cannot affect the halo mass function on mass scales M>10^{12} M_{solar}. Our results imply, model-independently, that a baryon in the halo of a galaxy like our own Milky Way, does not scatter from dark matter particles during the age of the galaxy.

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arXiv2013

Probing patchy reionization through tau-21cm correlation statistics

We consider the cross-correlation between free electrons and neutral hydrogen during the epoch of reionization. The free electrons are traced by the optical depth to reionization tau, while the neutral hydrogen can be observed through 21 cm photon emission. As expected, this correlation is sensitive to the detailed physics of reionization. Foremost, if reionization occurs through the merger of relatively large halos hosting an ionizing source, the free electrons and neutral hydrogen are anti-correlated for most of the reionization history. A positive contribution to the correlation can occur when the halos that can form an ionizing source are small. A measurement of this sign change in the cross-correlation would directly measure the bubble bias and as, such, the halo mass. We estimate the signal-to-noise of the cross-correlation using the estimator for inhomogeneous reionization tau_{lm} proposed by Dvorkin and Smith (2009). We find that with upcoming radio interferometers and CMB experiments, the cross-correlation is measurable going up to multipoles l ~ 1000. We also derive parameter constraints and conclude that, despite the foregrounds, the cross-correlation proofs a complementary measurement of the EoR parameters to the 21 cm and CMB polarization auto-correlations expected to be observed in the coming decade.

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arXiv2013

Perturbed Recombination from Dark Matter Annihilation

We show that dark matter annihilation around the time of recombination can lead to growing ionization fraction perturbations, that track the linear collapse of matter over-densities. This amplifies small scale cosmological perturbations to the free electron density by a significant amount compared to the usual acoustic oscillations. Electron density perturbations distort the CMB, inducing secondary non-gaussianity. We calculate the CMB bispectrum from recombination, that is marginally observable by Planck. Even though electron perturbations can be markedly boosted compared with the Standard Model prediction, the dark matter effect in the CMB bispectrum turns out to be small and will be difficult to disentangle from the standard model in the foreseeable future.

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arXiv2012

Low-mass suppression of the satellite luminosity function due to the supersonic baryon--cold-dark-matter relative velocity

We study the effect of the supersonic baryon--CDM flow, which has recently been shown to have a large effect on structure formation during the dark ages 10 <~ z <~ 1000, on the abundance of luminous, low-mass satellite galaxies around galaxies like the Milky Way. As the supersonic baryon--CDM flow significantly suppresses both the number of halos formed and the amount of baryons accreted onto such halos of masses 10^6 < M_{halo} / M_solar < 10^8 at z >~ 10, a large effect results on the stellar luminosity function before reionization. As halos of these masses are believed to have very little star formation after reionization due to the effects of photo-heating by the ultraviolet background, this effect persists to the present day. We calculate that the number of low-mass 10^6 < M_{halo} / M_solar < 10^8 halos that host luminous satellite galaxies today is typically suppressed by 50 percent, with values ranging up to 90 percent in regions where the initial supersonic velocity is high. We show that this previously-ignored cosmological effect resolves most of the tension between the observed and predicted number of low-mass satellites in the Milky Way, obviating the need for any other mass-dependent star-formation suppression before reionization.

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arXiv2012

Fast Computation of Bispectrum Features with Generalized Slow Roll

We develop a fast technique based on the generalized slow roll (GSR) approach for computing the curvature bispectrum of inflationary models with features. We show that all triangle configurations can be expressed in terms of three simple integrals over the inflationary background with typical accuracy of better than ~20%. With a first order GSR approach the typical accuracy can be improved to better than the 5% level. We illustrate this technique with the step potential model that has been invoked to explain the WMAP temperature power spectrum glitches at ell ~ 20-40 and show that the maximum likelihood model falls short of observability by more than a factor of 100 in amplitude. We also explicitly demonstrate that the bispectrum consistency relation with the local slope of the power spectrum is satisfied for these models. In the GSR approach, the bispectrum arises from integrals of nearly the same function of the background slow-roll parameters as the power spectrum but with a stronger weight to the epoch before horizon crossing. Hence this technique enables reverse engineering of models with large bispectrum but small power spectrum features.

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arXiv2012

Non-Gaussianity from Step Features in the Inflationary Potential

We provide analytic solutions for the power spectrum and bispectrum of curvature fluctuations produced by a step feature in the inflaton potential, valid in the limit that the step is short and sharp. In this limit, the bispectrum is strongly scale dependent and its effective non-linearity attains a large oscillatory amplitude. The perturbations to the curvature power spectrum, on the other hand, remain a small component on top of the usual spectrum of fluctuations generated by slow roll. We utilize our analytic solutions to assess the observability of the predicted non-Gaussian signatures and show that, if present, only very sharp steps on scales larger than ~ 2 Gpc are likely to be able to be detected by Planck. Such features are not only consistent with WMAP7 data, but can also improve its likelihood by 2 Delta ln L ~ 12 for two extra parameters, the step location and height. If this improvement were due to a slow roll violating step as considered here, a bispectrum or corresponding polarization power spectrum detection would provide definitive checks as to its primordial origin.

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arXiv2012

Cosmic String constraints from WMAP and the South Pole Telescope

The predictions of the inflationary LCDM paradigm match today's high-precision measurements of the cosmic microwave background anisotropy extremely well. The same data put tight limits on other sources of anisotropy. Cosmic strings are a particularly interesting alternate source to constrain. Strings are topological defects, remnants of inflationary-era physics that persist after the big bang. They are formed in a variety of models of inflation, including string theory models such as brane inflation. We assume a "Nambu-Goto" model for strings, approximated by a collection of unconnected segments with zero width, and show that measurements of temperature anisotropy by the South Pole Telescope break a parameter degeneracy in the WMAP data, permitting us to place a strong upper limit on the possible string contribution to the CMB anisotropy: the power sourced by zero-width strings must be <1.75% (95% CL) of the total or the string tension Gmu <1.7x10^{-7}. These limits imply that the best hope for detecting strings in the CMB will come from B-mode polarization measurements at arcminute scales rather than the degree scale measurements pursued for gravitational wave detection.

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arXiv2011

Supersonic baryon-CDM velocities and CMB B-mode polarization

It has recently been shown that supersonic relative velocities between dark matter and baryonic matter can have a significant effect on formation of the first structures in the universe. If this effect is still non-negligible during the epoch of hydrogen reionization, it generates large-scale anisotropy in the free electron density, which gives rise to a CMB B-mode. We compute the B-mode power spectrum and find a characteristic shape with acoustic peaks at l ~ 200, 400, ... The amplitude of this signal is a free parameter which is related to the dependence of the ionization fraction on the relative baryon-CDM velocity during the epoch of reionization. However, we find that the B-mode signal is undetectably small for currently favored reionization models in which hydrogen is reionized promptly at z ~ 10, although constraints on this signal by future experiments may help constrain models in which partial reionization occurs at higher redshift, e.g. by accretion onto primordial black holes.

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arXiv2011

Complete WMAP Constraints on Bandlimited Inflationary Features

Using a principal component (PC) basis that accommodates order unity features in the slow roll parameters as fine as 1/10 of a decade across more than 2 decades of the inflationary expansion, we test slow roll and single field inflation with the WMAP7 data. Detection of any non-zero component would represent a violation of ordinary slow roll and indicate a feature in the inflaton potential or sound speed. Although one component shows a deviation at the 98% CL, it cannot be considered statistically significant given the 20 components tested. The maximum likelihood PC parameters only improves 2DeltalnL by 17 for the 20 parameters associated with known glitches in the WMAP power spectrum at multipoles l<60. We make model-independent predictions for the matching glitches in the polarization spectrum that would test their inflationary origin. This complete analysis for bandlimited features in the source function of generalized slow roll can be used to constrain parameters of specific models of the inflaton potential without requiring a separate likelihood analysis for each choice. We illustrate its use by placing bounds on the height and width of a step-like feature in the potential proposed to explain the glitch at 20<l<40. Even allowing for the presence of features in the temperature spectrum, single field inflation makes sharp falsifiable predictions for the acoustic peaks in the polarization whose violation would require extra degrees of freedom.

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arXiv2010

CMB Constraints on Principal Components of the Inflaton Potential

We place functional constraints on the shape of the inflaton potential from the cosmic microwave background through a variant of the generalized slow roll approximation that allows large amplitude, rapidly changing deviations from scale-free conditions. Employing a principal component decomposition of the source function G'~3(V'/V)^2 - 2V''/V and keeping only those measured to better than 10% results in 5 nearly independent Gaussian constraints that maybe used to test any single-field inflationary model where such deviations are expected. The first component implies < 3% variations at the 100 Mpc scale. One component shows a 95% CL preference for deviations around the 300 Mpc scale at the ~10% level but the global significance is reduced considering the 5 components examined. This deviation also requires a change in the cold dark matter density which in a flat LCDM model is disfavored by current supernova and Hubble constant data and can be tested with future polarization or high multipole temperature data. Its impact resembles a local running of the tilt from multipoles 30-800 but is only marginally consistent with a constant running beyond this range. For this analysis, we have implemented a ~40x faster WMAP7 likelihood method which we have made publicly available.

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arXiv2009

Generalized Slow Roll for Large Power Spectrum Features

We develop a variant of the generalized slow roll approach for calculating the curvature power spectrum that is well-suited for order unity deviations in power caused by sharp features in the inflaton potential. As an example, we show that predictions for a step function potential, which has been proposed to explain order unity glitches in the CMB temperature power spectrum at multipoles l=20-40, are accurate at the percent level. Our analysis shows that to good approximation there is a single source function that is responsible for observable features and that this function is simply related to the local slope and curvature of the inflaton potential. These properties should make the generalized slow roll approximation useful for inflation-model independent studies of features, both large and small, in the observable power spectra.

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