arXiv subjects

Matias Zaldarriaga

Publications and source records attributed to Matias Zaldarriaga.

At least 235 records · Page 13Linked to original sources

arXiv2003

Probing the Friedmann equation during recombination with future CMB experiments

We show that by combining measurements of the temperature and polarization anisotropies of the Cosmic Microwave Background (CMB), future experiments will tightly constrain the expansion rate of the universe during recombination. A change in the expansion rate modifies the way in which the recombination of hydrogen proceeds, altering the shape of the acoustic peaks and the level of CMB polarization. The proposed test is similar in spirit to the examination of abundances of light elements produced during Big Bang Nucleosynthesis and it constitutes a way to study possible departures from standard recombination. For simplicity we parametrize the change in the Friedmann equation by changing the gravitational constant $G$. The main effect on the temperature power spectrum is a change in the degree of damping of the acoustic peaks on small angular scales. The effect can be compensated by a change in the shape of the primordial power spectrum. We show that this degeneracy between the expansion rate and the primordial spectrum can be broken by measuring CMB polarization. In particular we show that the MAP satellite could obtain a constraint for the expansion rate $H$ during recombination of $δH/H \simeq 0.09$ or $δG/G \simeq 0.18$ after observing for four years, whereas Planck could obtain $δH/H \leq 0.014$ or $δG/G \leq 0.028$ within two years, even after allowing for further freedom in the shape of the power spectrum of primordial fluctuations.

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arXiv2003

A new mechanism for generating density perturbations from inflation

We propose a new mechanism to generate density perturbations in inflationary models. Spatial fluctuations in the decay rate of the inflaton field to ordinary matter lead to fluctuations in the reheating temperature. We argue that in most realistic models of inflation the coupling of the inflaton to normal matter is determined by the vacuum expectation values of fields in the theory. If those fields are light during inflation (this is a generic situation in the minimal models of supersymmetric inflation) they will fluctuate leading to density perturbations through the proposed mechanism. We show that these fluctuations could easily dominate over the ones generated through the standard mechanism. The new scenario has several consequences for inflation model building and observations. The proposed mechanism allows to generate the observed level of density perturbations with a much lower scale of inflation and thus generically predicts a smaller level of gravitational waves. The relation between the slope of the spectrum of the produced density perturbations and the potential of the inflaton field is different from the standard relations obtained in the context of slow roll inflation. Because the field responsible for the fluctuations is not the inflaton, it can have significantly larger self couplings and thus density perturbations could be non-Gaussian. The non-Gaussianity can be large enough to be detectable by CMB and Large Scale Structure observations.

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arXiv2002

Inferring the Linear Power Spectrum from the Lyman-alpha Forest

We discuss the determination of the linear power spectrum of dark matter fluctuations from measurements of the transmitted flux Ly-alpha Forest power spectrum. We show that at most scales probed by current measurements, the flux power spectrum is significantly affected by non-linear corrections to the linear dark matter power spectrum due to gravitational clustering. Inferring the linear dark matter power spectrum shape is therefore difficult due to non-linear corrections driving the non-linear power spectrum to a k^{-1.4} shape nearly independent of initial conditions. We argue that some methods used in previous estimates underestimate the uncertainties in the shape of the linear dark matter power spectrum.

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arXiv2002

The last stand before MAP: cosmological parameters from lensing, CMB and galaxy clustering

Cosmic shear measurements have now improved to the point where they deserve to be treated on par with CMB and galaxy clustering data for cosmological parameter analysis, using the full measured aperture mass variance curve rather than a mere phenomenological parametrization thereof. We perform a detailed 9-parameter analysis of recent lensing (RCS), CMB (up to Archeops) and galaxy clustering (2dF) data, both separately and jointly. CMB and 2dF data are consistent with a simple flat adiabatic scale-invariant model with Omega_Lambda=0.72+/-0.09, omega_cdm=0.115+/- 0.013, omega_b=0.024+/-0.003, and a hint of reionization around z~8. Lensing helps further tighten these constraints, but reveals tension regarding the power spectrum normalization: including the RCS survey results raises sigma8 significantly and forces other parameters to uncomfortable values. Indeed, sigma8 is emerging as the currently most controversial cosmological parameter, and we discuss possible resolutions of this sigma8 problem. We also comment on the disturbing fact that many recent analyses (including this one) obtain error bars smaller than the Fisher matrix bound. We produce a CMB power spectrum combining all existing experiments, and using it for a "MAP versus world" comparison next month will provide a powerful test of how realistic the error estimates have been in the cosmology community.

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arXiv2002

The Accelerated Universe and the Moon

Cosmologically motivated theories that explain small acceleration rate of the Universe via modification of gravity at very large, horizon or super-horizon distances, can be tested by precision gravitational measurements at much shorter scales, such as the Earth-Moon distance. Contrary to the naive expectation the predicted corrections to the Einsteinian metric near gravitating sources are so significant that fall within sensitivity of the proposed Lunar Ranging experiments. The key reason for such corrections is the van Dam-Veltman-Zakharov discontinuity present in linearized versions of all such theories, and its subsequent absence at the non-linear level ala Vainshtein.

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arXiv2002

The end of unified dark matter?

Despite the interest in dark matter and dark energy, it has never been shown that they are in fact two separate substances. We provide the first strong evidence that they are separate by ruling out a broad class of so-called unified dark matter models that have attracted much recent interest. We find that they produce oscillations or exponential blowup of the matter power spectrum inconsistent with observation. For the particular case of generalized Chaplygin gas models, 99.999% of the previously allowed parameter space is excluded, leaving essentially only the standard Lambda-CDM limit allowed.

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arXiv2002

Higher-order moments of the lensing shear and other spin two fields

We present a method for defining higher-order moments of a spin-2 field on the sky using the transformation properties of these statistics under rotation and parity. For the three-point function of the cosmic shear we show that the eight logically possible combinations of the shear in three points can be divided into two classes, four combinations are even under parity transformations and four are odd. We compute the expected value of the even parity ones in the non-linear regime using the halo model and conclude that on small scales of the four combinations there is one that is expected to carry most of the signal for triangles close to isosceles. On the other hand, for collapsed triangles all four combinations are expected to have roughly the same level of signal although some of the combinations are negative and others positive. We estimate that a survey of a few square degrees area is enough to detect this signal above the noise at arc minute scales.

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arXiv2002

Benchmark Parameters for CMB Polarization Experiments

The recently detected polarization of the cosmic microwave background (CMB) holds the potential for revealing the physics of inflation and gravitationally mapping the large-scale structure of the universe, if so called B-mode signals below 10^{-7}, or tenths of a uK, can be reliably detected. We provide a language for describing systematic effects which distort the observed CMB temperature and polarization fields and so contaminate the B-modes. We identify 7 types of effects, described by 11 distortion fields, and show their association with known instrumental systematics such as common mode and differential gain fluctuations, line cross-coupling, pointing errors, and differential polarized beam effects. Because of aliasing from the small-scale structure in the CMB, even uncorrelated fluctuations in these effects can affect the large-scale B modes relevant to gravitational waves. Many of these problems are greatly reduced by having an instrumental beam that resolves the primary anisotropies (FWHM << 10'). To reach the ultimate goal of an inflationary energy scale of 3 \times 10^{15} GeV, polarization distortion fluctuations must be controlled at the 10^{-2}-10^{-3} level and temperature leakage to the 10^{-4}-10^{-3} level depending on effect. For example pointing errors must be controlled to 1.5'' rms for arcminute scale beams or a percent of the Gaussian beam width for larger beams; low spatial frequency differential gain fluctuations or line cross-coupling must be eliminated at the level of 10^{-4} rms.

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arXiv2002

Separating the Early Universe from the Late Universe: cosmological parameter estimation beyond the black box

We present a method for measuring the cosmic matter budget without assumptions about speculative Early Universe physics, and for measuring the primordial power spectrum P*(k) non-parametrically, either by combining CMB and LSS information or by using CMB polarization. Our method complements currently fashionable ``black box'' cosmological parameter analysis, constraining cosmological models in a more physically intuitive fashion by mapping measurements of CMB, weak lensing and cluster abundance into k-space, where they can be directly compared with each other and with galaxy and Lyman alpha forest clustering. Including the new CBI results, we find that CMB measurements of P(k) overlap with those from 2dF galaxy clustering by over an order of magnitude in scale, and even overlap with weak lensing measurements. We describe how our approach can be used to raise the ambition level beyond cosmological parameter fitting as data improves, testing rather than assuming the underlying physics.

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arXiv2002

The Far-Infrared Background Correlation with CMB Lensing

The intervening large--scale structure distorts cosmic microwave background (CMB) anisotropies via gravitational lensing. The same large--scale structure, traced by dusty star--forming galaxies, also induces anisotropies in the far--infrared background (FIRB). We investigate the resulting inter--dependence of the FIRB and CMB with a halo model for the FIRB. In particular, we calculate the cross--correlation between the lensing potential and the FIRB. The lensing potential can be quadratically estimated from CMB temperature and/or polarization maps. We show that the cross--correlation can be measured with high signal--to--noise with data from the Planck Surveyor. We discuss how such a measurement can be used to understand the nature of FIRB sources and their relation to the distribution of dark matter.

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arXiv2002

How Neutral is the Intergalactic Medium at z ~ 6?

Recent observations of high redshift quasar spectra reveal long gaps with little flux. A small or no detectable flux does not by itself imply the intergalactic medium (IGM) is neutral. Inferring the average neutral fraction from the observed absorption requires assumptions about clustering of the IGM, which the gravitational instability model supplies. Our most stringent constraint on the neutral fraction at z ~ 6 is derived from the mean Lyman-beta transmission measured from the z=6.28 SDSS quasar of Becker et al. -- the neutral hydrogen fraction at mean density has to be larger than 4.7 times 10^{-4}. This is substantially higher than the neutral fraction of ~ 3-5 times 10^{-5} at z = 4.5 - 5.7, suggesting that dramatic changes take place around or just before z ~ 6, even though current constraints are still consistent with a fairly ionized IGM at z ~ 6. An interesting alternative method to constrain the neutral fraction is to consider the probability of having many consecutive pixels with little flux, which is small unless the neutral fraction is high. This constraint is slightly weaker than the one obtained from the mean transmission. We show that while the derived neutral fraction at a given redshift is sensitive to the power spectrum normalization, the size of the jump around z ~ 6 is not. We caution that systematic uncertainties include spatial fluctuations in the ionizing background, and the continuum placement. Tests are proposed. In particular, the sightline to sightline dispersion in mean transmission might provide a useful diagnostic. We express the dispersion in terms of the transmission power spectrum, and develop a method to calculate the dispersion for spectra that are longer than the typical simulation box.

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arXiv2002

E/B decomposition of finite pixelized CMB maps

Separation of the E and B components of a microwave background polarization map or a weak lensing map is an essential step in extracting science from it, but when the map covers only part of the sky and/or is pixelized, this decomposition cannot be done perfectly. We present a method for decomposing an arbitrary sky map into a sum of three orthogonal components that we term ``pure E'', ``pure B'' and ``ambiguous''. This method is useful both for providing intuition for experimental design and for analyzing data sets in practice. We show how to find orthonormal bases for all three components in terms of bilaplacian eigenfunctions. The number of ambiguous modes is proportional to the length of the map boundary so fairly round maps are preferred. For real-world data analysis, we present a simple matrix eigenvalue method for calculating nearly pure E and B modes in pixelized maps. We find that the dominant source of leakage between E and B is aliasing of small-scale power caused by the pixelization. This problem can be eliminated by heavily oversampling the map, but is exacerbated by the fact that the E power spectrum is expected to be much larger than the B power spectrum and extremely blue. We found that a factor of 2 to 3 more pixels are needed in a polarization map to achieve the same level of contamination by aliased power than in a temperature map.

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arXiv2002

Supernovae, CMB, and Gravitational Leakage into Extra Dimensions

We discuss observational constraints coming from CMB and type Ia supernovae, for the model of accelerated universe produced by gravitational leakage into extra dimensions. Our fits indicate that the model is currently in agreement with the data. We also give the equations governing the evolution of cosmological perturbations. Future observations will be able to severely constrain the model.

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arXiv2002

First attempt at measuring the CMB cross-polarization

We compute upper limits on CMB cross-polarization by cross-correlating the PIQUE and Saskatoon experiments. We also discuss theoretical and practical issues relevant to measuring cross-polarization and illustrate them with simulations of the upcoming BOOMERanG 2002 experiment. We present a method that separates all six polarization power spectra (TT, EE, BB, TE, TB, EB) without any other "leakage" than the familiar EE-BB mixing caused by incomplete sky coverage. Since E and B get mixed, one might expect leakage between TE and TB, between EE and EB and between BB and EB - our method eliminates this by preserving the parity symmetry under which TB and EB are odd and the other four power spectra are even.

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arXiv2002

Is cosmology consistent?

We perform a detailed analysis of the latest CMB measurements (including BOOMERaNG, DASI, Maxima and CBI), both alone and jointly with other cosmological data sets involving, e.g., galaxy clustering and the Lyman Alpha Forest. We first address the question of whether the CMB data are internally consistent once calibration and beam uncertainties are taken into account, performing a series of statistical tests. With a few minor caveats, our answer is yes, and we compress all data into a single set of 24 bandpowers with associated covariance matrix and window functions. We then compute joint constraints on the 11 parameters of the ``standard'' adiabatic inflationary cosmological model. Out best fit model passes a series of physical consistency checks and agrees with essentially all currently available cosmological data. In addition to sharp constraints on the cosmic matter budget in good agreement with those of the BOOMERaNG, DASI and Maxima teams, we obtain a heaviest neutrino mass range 0.04-4.2 eV and the sharpest constraints to date on gravity waves which (together with preference for a slight red-tilt) favors ``small-field'' inflation models.

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arXiv2001

Dimming of Supernovae by Photon-Pseudoscalar Conversion and the Intergalactic Plasma

Recently Csaki, Kaloper and Terning (hep-ph/0111311) suggested that the observed dimming of distant type Ia supernovae may be a consequence of mixing of the photons with very light axions. We point out that the effect of the plasma, in which the photons are propagating, must be taken into account. This effect changes the oscillation probability and renders the dimming frequency-dependent, contrary to observations. One may hope to accommodate the data by averaging the oscillations over many different coherence domains. We estimate the effect of coherence loss, either due to the inhomogeneities of the magnetic field or of the intergalactic plasma. These estimates indicate that the achromaticity problem can be resolved only with very specific, and probably unrealistic, properties of the intergalactic medium.

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arXiv2001

Changing alpha With Time: Implications For Fifth-Force-Type Experiments And Quintessence

If the recent observations suggesting a time variation of the fine structure constant are correct, they imply the existence of an ultra light scalar particle. This particle inevitably couples to nucleons through the α-dependence of their masses and thus mediates an isotope-dependent long-range force. The strength of the coupling is within a couple of orders of magnitude from the existing experimental bounds for such forces. The new force can be potentially measured in the precision experimental tests of the equivalence principle. Due to an interesting coincidence of the required time-scales, the scalar filed in question can at the same time play the role of a quintessence field.

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arXiv2001

The nature of the E-B decomposition of CMB polarization

We present a derivation of the transformation between the Q and U Stokes parameters and the E and B scalar and pseudo-scalar fields. We emphasize the geometrical properties that such transformation must satisfy. We present the E and B decomposition of some simple maps and of a model for a supernova remnant. We discuss the relative amplitudes of the E and B components and argue that for generic random maps E and B should have roughly the same amplitudes.

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