arXiv subjects

Matias Zaldarriaga

Publications and source records attributed to Matias Zaldarriaga.

At least 217 records · Page 12Linked to original sources

arXiv2004

Perturbations in bouncing cosmologies: dynamical attractor vs scale invariance

For bouncing cosmologies such as the ekpyrotic/cyclic scenarios we show that it is possible to make predictions for density perturbations which are independent of the details of the bouncing phase. This can be achieved, as in inflationary cosmology, thanks to the existence of a dynamical attractor, which makes local observables equal to the unperturbed solution up to exponentially small terms. Assuming that the physics of the bounce is not extremely sensitive to these corrections, perturbations can be evolved even at non-linear level. The resulting spectrum is not scale invariant and thus incompatible with experimental data. This can be explicitly shown in synchronous gauge where, contrary to what happens in the commonly used Newtonian gauge, all perturbations remain small going towards the bounce and the existence of the attractor is manifest.

hep-th↗

arXiv2004

Single field consistency relation for the 3-point function

We point out the existence of a consistency relation involving the 3-point function of scalar perturbations which is valid in any inflationary model, independently of the inflaton Lagrangian under the assumption that the inflaton is the only dynamical field. The 3-point function in the limit in which one of the momenta is much smaller than the other two is fixed in terms of the power spectrum and its tilt. This relation, although very hard to verify experimentally, could be easily proved wrong by forecoming data, thus ruling out any scenario with a single dynamical field in a model independent way.

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arXiv2004

21 Centimeter Fluctuations from Cosmic Gas at High Redshifts

The relatively large Thomson optical depth, tau_e, inferred recently from the WMAP observations suggests that the Universe was reionized in a more complex manner than previously believed. However, the value of tau_e provides only an integral constraint on the history of reionization and, by itself, cannot be used to determine the nature of the sources responsible for this transition. Here, we show that the evolution of the ionization state of the intergalactic medium at high redshifts can be measured statistically using fluctuations in 21 centimeter radiation from neutral hydrogen. By analogy with the mathematical description of anisotropies in the cosmic microwave background, we develop a formalism to quantify the variations in 21 cm emission as a function of both frequency and angular scale. Prior to and following reionization, fluctuations in the 21 cm signal are mediated by density perturbations in the distribution of matter. Between these epochs, pockets of gas surrounding luminous objects become ionized, producing large HII regions. These ``bubbles'' of ionized material imprint features into the 21 cm power spectrum that make it possible to distinguish them from fluctuations produced by the density perturbations. The variation of the power spectrum with frequency can be used to infer the evolution of this process. As has been emphasized previously by others, the absolute 21 cm signal from neutral gas at high redshifts is undetectable owing to contamination by foreground sources. However, we argue that this source of noise can be suppressed by comparing maps closely spaced in frequency, i.e. redshift, so that 21 cm fluctuations from the IGM can be measured against a much brighter, but smoothly varying (in frequency) background.

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arXiv2004

CMB 3-point functions generated by non-linearities at recombination

We study the 3-point functions generated at recombination in the squeezed triangle limit, when one mode has a wavelength much larger than the other two and is outside the horizon. The presence of the long wavelength mode cannot change the physics inside the horizon but modifies how a late time observer sees the anisotropies. The effect of the long wavelength mode can be divided into a redefinition of time and spatial scales, a Shapiro time delay and gravitational lensing. The separation is gauge dependent but helps develop intuition. We show that the resulting 3-point function corresponds to an f_NL < 1 and that its shape is different from that created by the f_NL (or local) model.

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arXiv2004

Primordial Bispectrum Information from CMB Polarization

After the precise observations of the Cosmic Microwave Background (CMB) anisotropy power spectrum, attention is now being focused on the higher order statistics of the CMB anisotropies. Since linear evolution preserves the statistical properties of the initial conditions, observed non-Gaussianity of the CMB will mirror primordial non-Gaussianity. Single field slow-roll inflation robustly predicts negligible non-Gaussianity so an indication of non-Gaussianity will suggest alternative scenarios need to be considered. In this paper we calculate the information on primordial non-Gaussianity encoded in the polarization of the CMB. After deriving the optimal weights for a cubic estimator we evaluate the Signal-to-Noise ratio of the estimator for WMAP, Planck and an ideal cosmic variance limited experiment. We find that when the experiment can observe CMB polarization with good sensitivity, the sensitivity to primordial non-Gaussianity increases by roughly a factor of two. We also test the weakly non-Gaussian assumption used to derive the optimal weight factor by calculating the degradation factor produced by the gravitational lensing induced connected four-point function. The physical scales in the radiative transfer functions are largely irrelevant for the constraints on the primordial non-Gaussianity. We show that the total (S/N)^2 is simply proportional to the number of observed pixels on the sky.

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arXiv2004

Constraining the Topology of Reionization Through Lyman-alpha Absorption

The reionization of hydrogen in the intergalactic medium (IGM) is a crucial landmark in the history of the universe, but the processes through which it occurs remain mysterious. In particular, recent numerical and analytic work suggest that reionization by stellar sources is driven by large-scale density fluctuations and must be inhomogeneous on scales of many comoving Mpc. We examine the prospects for constraining the topology of neutral and ionized gas through Lyman-alpha absorption of high-redshift sources. One method is to search for gaps in the Gunn-Peterson absorption troughs of luminous sources. These could occur if the line of sight passes sufficiently close to the center of a large HII region. In contrast to previous work, we find a non-negligible (though still small) probability of observing such a gap before reionization is complete. In our model the transmission spike at z=6.08 in the spectrum of SDSS J1148+5251 does not necessarily require overlap to have been completed at an earlier epoch. We also examine the IGM damping wing absorption of the Lyman-alpha emission lines of star-forming galaxies. Because most galaxies sit inside of large HII regions, we find that the severity of absorption is significantly smaller than previously thought and decoupled from the properties of the observed galaxy. While this limits our ability to constrain the mean neutral fraction of the IGM from observations of individual galaxies, it presents the exciting possibility of measuring the size distribution and evolution of the ionized bubbles by examining the distribution of damping wing optical depths in a large sample of galaxies.

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arXiv2004

The shape of non-Gaussianities

We study the dependence on configuration in momentum space of the primordial 3-point function of density perturbations in several different scenarios: standard slow-roll inflation, curvaton and variable decay models, ghost inflation, models with higher derivative operators and the DBI model of inflation. We define a cosine between the distributions using a measure based on the ability of experiments to distinguish between them. We find that models fall into two broad categories with fairly orthogonal distributions. Models where non-Gaussianity is created at horizon-crossing during inflation and models in which the evolution outside the horizon dominates. In the first case the 3-point function is largest for equilateral triangles, while in the second the dominant contribution to the signal comes from the influence of long wavelength modes on small wavelength ones. We show that, because the distributions in these two cases are so different, translating constraints on parameters of one model to those of another based on the normalization of the 3-point function for equilateral triangles can be very misleading.

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arXiv2004

Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation

The thermal evolution of the cosmic gas decoupled from that of the cosmic microwave background (CMB) at a redshift z~200. Afterwards and before the first stars had formed, the cosmic neutral hydrogen absorbed the CMB flux at its resonant 21cm spin-flip transition. We calculate the evolution of the spin temperature for this transition and the resulting anisotropies that are imprinted on the CMB sky due to linear density fluctuations during this epoch. These anisotropies at an observed wavelength of 10.56[(1+z)/50] meters, contain an amount of information that is orders of magnitude larger than any other cosmological probe. Their detection, although challenging, could tightly constrain any possible running of the spectral index from inflation (as suggested by WMAP), small deviations from Gaussianity, or any significant contribution from neutrinos or warm dark matter to the cosmic mass budget.

astro-ph↗

arXiv2004

Statistical Probes of Reionization With 21 cm Tomography

We consider the degree to which "21 cm tomography" of the high-redshift Universe can distinguish different ionization histories. Using a new analytic model for the size distribution of HII regions that associates these ionized bubbles with large-scale galaxy overdensities, we compute the angular power spectrum and other statistical properties of the 21 cm brightness temperature during reionization. We show that the HII regions imprint features on the power spectrum that allow us to separate histories with discrete bubbles from those with partial uniform ionization (by, for example, X-rays). We also show that "double reionization" scenarios will modify the morphology of the bubbles in ways that depend on the mechanism through which the first generation of sources shuts off. If, for example, the transition occurs globally at a fixed redshift, the first generation imprints a persistent feature on the 21 cm power spectrum. Finally, we compare our model to one in which voids are ionized first. While the power spectra of these two models are qualitatively similar, we show that the underlying distributions of neutral hydrogen differ dramatically and suggest that other statistical tests can distinguish them. The next generation of low-frequency radio telescopes will have the sensitivity to distinguish all of these models and strongly constrain the history and morphology of reionization.

astro-ph↗

arXiv2004

The Growth of HII Regions During Reionization

Recently, there has been a great deal of interest in understanding the reionization of hydrogen in the intergalactic medium (IGM). One of the major outstanding questions is how this event proceeds on large scales. Motivated by numerical simulations, we develop a model for the growth of HII regions during the reionization era. We associate ionized regions with large-scale density fluctuations and use the excursion set formalism to model the resulting size distribution. We then consider ways in which to characterize the morphology of ionized regions. We show how to construct the power spectrum of fluctuations in the neutral hydrogen field. The power spectrum contains definite features from the HII regions that should be observable with the next generation of low-frequency radio telescopes through surveys of redshifted 21 cm emission from the reionization era. Finally, we also consider statistical descriptions beyond the power spectrum and show that our model of reionization qualitatively changes the distribution of neutral gas in the IGM.

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arXiv2004

Ghost Inflation

We propose a new scenario for early cosmology, where an inflationary de Sitter phase is obtained with a ghost condensate. The transition to radiation dominance is triggered by the ghost itself, without any slow-roll potential. Density perturbations are generated by fluctuations around the ghost condensate and can be reliably computed in the effective field theory. The fluctuations are scale invariant as a consequence of the de Sitter symmetries, however, the size of the perturbations are parametrically different from conventional slow-roll inflation, and the inflation happens at far lower energy scales. The model makes definite predictions that distinguish it from standard inflation, and can be sharply excluded or confirmed by experiments in the near future. The tilt in the scalar spectrum is predicted to vanish (n_s=1), and the gravity wave signal is negligible. The non-Gaussianities in the spectrum are predicted to be observable: the 3-point function is determined up to an overall order 1 constant, and its magnitude is much bigger than in conventional inflation, with an equivalent f_NL ~ 100, not far from the present WMAP bounds.

hep-th↗

arXiv2003

Probing Primordial Non-Gaussianity with Large-Scale Structure

We consider primordial non-Gaussianity due to quadratic corrections in the gravitational potential parametrized by a non-linear coupling parameter fnl. We study constraints on fnl from measurements of the galaxy bispectrum in redshift surveys. Using estimates for idealized survey geometries of the 2dF and SDSS surveys and realistic ones from SDSS mock catalogs, we show that it is possible to probe |fnl|~100, after marginalization over bias parameters. We apply our methods to the galaxy bispectrum measured from the PSCz survey, and obtain a 2sigma-constraint |fnl|< 1800. We estimate that an all sky redshift survey up to z~1 can probe |fnl|~1. We also consider the use of cluster abundance to constrain fnl and find that in order to be sensitive to |fnl|~100, cluster masses need to be determined with an accuracy of a few percent, assuming perfect knowledge of the mass function and cosmological parameters.

astro-ph↗

arXiv2003

CMBfit: Rapid WMAP likelihood calculations with normal parameters

We present a method for ultra-fast confrontation of the WMAP cosmic microwave background observations with theoretical models, implemented as a publicly available software package called CMBfit, useful for anyone wishing to measure cosmological parameters by combining WMAP with other observations. The method takes advantage of the underlying physics by transforming into a set of parameters where the WMAP likelihood surface is accurately fit by the exponential of a quartic or sextic polynomial. Building on previous physics based approximations by Hu et.al., Kosowsky et.al. and Chu et.al., it combines their speed with precision cosmology grade accuracy. A Fortran code for computing the WMAP likelihood for a given set of parameters is provided, pre-calibrated against CMBfast, accurate to Delta lnL ~ 0.05 over the entire 2sigma region of the parameter space for 6 parameter ``vanilla'' Lambda CDM models. We also provide 7-parameter fits including spatial curvature, gravitational waves and a running spectral index.

astro-ph↗

arXiv2003

The significance of the largest scale CMB fluctuations in WMAP

We investigate anomalies reported in the Cosmic Microwave Background maps from the Wilkinson Microwave Anisotropy Probe (WMAP) satellite on very large angular scales and discuss possible interpretations. Three independent anomalies involve the quadrupole and octopole: 1. The cosmic quadrupole on its own is anomalous at the 1-in-20 level by being low (the cut-sky quadrupole measured by the WMAP team is more strikingly low, apparently due to a coincidence in the orientation of our Galaxy of no cosmological significance); 2. The cosmic octopole on its own is anomalous at the 1-in-20 level by being very planar; 3. The alignment between the quadrupole and octopole is anomalous at the 1-in-60 level. Although the a priori chance of all three occurring is 1 in 24000, the multitude of alternative anomalies one could have looked for dilutes the significance of such a posteriori statistics. The simplest small universe model where the universe has toroidal topology with one small dimension of order half the horizon scale, in the direction towards Virgo, could explain the three items above. However, we rule this model out using two topological tests: the S-statistic and the matched circle test.

astro-ph↗

arXiv2003

Towards Constraints on Dark Energy from Absorption Spectra of Close Quasar Pairs

A comparison between the line of sight power spectrum of absorption in the Lyman-alpha forest and the cross power spectrum between the absorption in neighboring lines of sight offers an evolution-free means to constrain the cosmological constant, or dark energy. Using cosmological simulations, we consider a maximum likelihood method to obtain constraints from this comparison. In our method, measurements of the auto and cross spectra from observations are compared with those from a multi-parameter grid of simulated models of the intergalactic medium. We then marginalize over nuisance parameters to obtain constraints on the cosmological constant. Redshift space distortions due to peculiar velocities and thermal broadening, a potential difficulty in applying this test, are explicitly modeled in our simulations. To illustrate our method, we measure the cross spectrum from a new sample of five close quasar pairs, with separations of 0.5 to 3 arcmin. We attempt to obtain a constraint on Omega_Lambda, but find only weak constraints. An Einstein-de-Sitter cosmology is, however, disfavored by the data at a ~ 2 sigma confidence level. We consider the power of future observations, paying particular attention to the effects of spectral resolution and shot-noise. We find that ~ 50 moderate resolution, FWHM ~ 150 km/s, close separation (~ 30-120 arcsec) pairs should allow a (2 sigma) constraint on Omega_Lambda at the level of 15%, if other modeling parameters are determined through other means. We find that there is a sizeable gain from observing very close, ~ 30 arcsec, separation pairs provided they are observed with high spectral resolution. A sample of ~ 10 such pairs gives similar constraints to the 50 moderate resolution pairs mentioned above.

astro-ph↗

arXiv2003

Non-Gaussianities in models with a varying inflaton decay rate

We consider the expected level of primordial non-Gaussianities in models in which density perturbations are produced by spatial fluctuations in the decay rate of the inflaton. We consider both the non-Gaussianities resulting from the self-couplings of the field that controls the decay rate as well as from the non-linear relation between field and curvature perturbations. We show that in these scenario non-Gaussianities are of the "local" form, that is well described be the ansatz R =Rg + f_{NL} (Rg^2 - ). This is a consequence of the fact that they were created when modes were already outside the horizon. We show that f_{NL} is naturally of order a few in these models, much larger than what is expected in the standard one field models of inflation (f_{NL}\sim 10^{-2}) and possibly accessible to observations.

astro-ph↗

arXiv2003

Cosmological Perturbations From Inhomogeneous Reheating, Freeze-Out, and Mass Domination

We generalize a recently proposed mechanism for the origin of primordial metric perturbations in inflationary models. Quantum fluctuations of light scalar fields during inflation give rise to super-horizon fluctuations of masses and reaction rates of various particles. Reheating, freeze-out, and matter-domination processes become inhomogeneous and generate super-horizon metric perturbations. We also calculate the degree of non-Gaussianity $f_{nl}$ for this new model of cosmological perturbations. The precise value of $f_{nl}$ depends on the specific models, but $|f_{nl}|\sim$few is a natural lower bound for our mechanisms. This is much larger than the currently assumed theoretical value $f_{nl}\sim tilt \lesssim 0.05$, and is thought to be observable. In a particularly attractive model of inhomogeneous mass-domination, the non-Gaussianity of perturbations generated by our mechanism is simply $f_{nl}=5$, irrespective of the detailed structure of the underlying field theory.

astro-ph↗

arXiv2003

The Polarization of the Cosmic Microwave Background

We summarize the physical mechanism by which the Cosmic Microwave Background acquires a small degree of polarization. We discuss the imprint left by gravitational waves and the use of polarization as a test of the inflationary paradigm. We discuss some physical processes that affect the CMB polarization after recombination such as gravitational lensing and the reionization of the universe.

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