arXiv subjects

Matias Zaldarriaga

Publications and source records attributed to Matias Zaldarriaga.

At least 181 records · Page 10Linked to original sources

arXiv2009

Cosmology from the Highly-Redshifted 21 cm Line

One of the next decade's most exciting prospects is to explore the cosmic "dark ages," during which the first stars in the Universe formed, with the 21 cm line of neutral hydrogen. At z>6, this light redshifts into the low-frequency radio regime and can be observed with new generations of wide-field arrays. These experiments have the potential to observe enormous volumes and to improve both cosmological and astrophysical constraints dramatically. In this white paper, we describe how the next decade will see the first steps toward unlocking this potential, allowing us to answer two key questions about the fundamental physics of our Universe and the intergalactic medium: Does the standard cosmological model describe the Universe during the "dark ages?" How did the intergalactic medium evolve during this important time, ending with the reionization of hydrogen?

astro-ph.CO↗

arXiv2009

Astrophysics from the Highly-Redshifted 21 cm Line

The cosmic dark ages and the epoch of reionization, during which the first generations of stars and galaxies formed, are among the most compelling frontiers of extragalactic astrophysics and cosmology. Here we describe an exciting new probe of these eras: the 21 cm line of neutral hydrogen, which will allow us to map the neutral intergalactic medium throughout the era of first galaxy formation. In this white paper, we describe how these studies can answer two key questions about galaxy formation: What were the properties of high-redshift galaxies? How did they affect the Universe around them?

astro-ph.CO↗

arXiv2009

Contributions to the Dark Matter 3-Pt Function from the Radiation Era

We consider the contribution to the three-point function of matter density fluctuations from nonlinear growth after modes re-enter the horizon, and discuss effects that must be included in order to predict the three-point function with an accuracy comparable to primordial nongaussianities with f_NL ~ few. In particular, we note that the shortest wavelength modes measured in galaxy surveys entered the horizon during the radiation era, and, as a result, the radiation era modifies their three-point function by a magnitude equivalent to f_NL ~ O(4). On longer wavelengths, where the radiation era is negligible, we find that the corrections to the nonlinear growth from relativistic effects become important at the level f_NL ~ few. We implement a simple method for numerically calculating the three-point function, by solving the second-order equations of motion for the perturbations with the first order perturbations providing a source.

astro-ph.CO↗

arXiv2009

Will point sources spoil 21 cm tomography?

21 cm tomography is emerging as a promising probe of the cosmological dark ages and the epoch of reionization, as well as a tool for observational cosmology in general. However, serious sources of foreground contamination must be subtracted for experimental efforts to be viable. In this paper, we focus on the removal of unresolved extragalactic point sources with smooth spectra, and evaluate how the residual foreground contamination after cleaning depends on instrumental and algorithmic parameters. A crucial but often ignored complication is that the synthesized beam of an interferometer array shrinks towards higher frequency, causing complicated frequency structure in each sky pixel as "frizz" far from the beam center contracts across unresolved radio sources. We find that current-generation experiments should nonetheless be able to clean out this points source contamination adequately, and quantify the instrumental and algorithmic design specifications required to meet this foreground challenge.

astro-ph↗

arXiv2009

Causality and Primordial Tensor Modes

We introduce the real space correlation function of $B$-mode polarization of the cosmic microwave background (CMB) as a probe of superhorizon tensor perturbations created by inflation. By causality, any non-inflationary mechanism for gravitational wave production after reheating, like global phase transitions or cosmic strings, must have vanishing correlations for angular separations greater than the angle subtended by the particle horizon at recombination, i.e. $θ\gtrsim 2^\circ$. Since ordinary $B$-modes are defined non-locally in terms of the Stokes parameters $Q$ and $U$ and therefore don't have to respect causality, special care is taken to define `causal $\tilde B$-modes' for the analysis. We compute the real space $\tilde B$-mode correlation function for inflation and discuss its detectability on superhorizon scales where it provides an unambiguous test of inflationary gravitational waves. The correct identification of inflationary tensor modes is crucial since it relates directly to the energy scale of inflation. Wrongly associating tensor modes from causal seeds with inflation would imply an incorrect inference of the energy scale of inflation. We find that the superhorizon $\tilde B$-mode signal is above cosmic variance for the angular range $2^\circ < θ< 4^\circ$ and is therefore in principle detectable. In practice, the signal will be challenging to measure since it requires accurately resolving the recombination peak of the $B$-mode power spectrum. However, a future CMB satellite (CMBPol), with noise level $Δ_P \simeq 1μ$K-arcmin and sufficient resolution to efficiently correct for lensing-induced $B$-modes, should be able to detect the signal at more than 3$σ$ if the tensor-to-scalar ratio isn't smaller than $r \simeq 0.01$.

astro-ph.CO↗

arXiv2009

Impact of Instrumental Systematic Contamination on the Lensing Mass Reconstruction using the CMB Polarization

In this paper, we study the effects of instrumental systematics on the reconstruction of the deflection angle power spectrum from weak lensing of Cosmic Microwave Background (CMB) temperature and polarization observations. We consider seven types of effects which are related to known instrumental systematics: calibration, rotation, pointing, spin-flip, monopole leakage, dipole leakage and quadrupole leakage. These effects can be characterized by 11 distortion fields. Each of these systematic effects can mimic the effective projected matter power spectrum and hence contaminate the lensing reconstruction. To demonstrate the effect of these instrumental systematics, we consider two types of experiments, one with a detector noise level for polarization of 9.6 uK-arcmin and FWHM of 8.0', typical of upcoming ground and balloon-based CMB experiments, and a CMBPol-like instrument with a detector noise level for polarization of 2.0 uK-arcmin and FWHM of 4.0', typical of future space-based CMB experiments. For each systematics, we consider various choices of coherence scale. Among all the 11 systematic parameters, rotation and monopole leakage place the most stringent requirements, while quadrupole leakage, pointing error, and calibration are among the least demanding. The requirements from lensing extraction are about 1-2 orders of magnitude less stringent than the requirements to measure the primordial B-modes with inflationary energy scale of 1.0*10^{16} GeV. On the other hand the requirements for lensing reconstruction are comparable or even more stringent for some systematic parameters than the requirements to detect primordial B-modes with inflationary scale E_i = 3.0*10^{16} GeV.

astro-ph.CO↗

arXiv2008

HeII Reionization and its Effect on the IGM

Observations of the intergalactic medium (IGM) suggest that quasars reionize HeII in the IGM at z ~ 3. We have run a set of 190 and 430 comoving Mpc simulations of HeII being reionized by quasars to develop an understanding of the nature of HeII reionization and its potential impact on observables. We find that HeII reionization heats regions in the IGM by as much as 25,000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ~ 12,000 K and with large temperature fluctuations on ~ 50 Mpc scales. Much of the heating occurs far from QSOs by hard photons. We find a temperature-density equation of state of gamma -1 ~ 0.3 during HeII reionization, but with a wide dispersion in this relation having sigma ~ 10^4 K. HeII reionization by the observed population of quasars cannot produce an inverted relation (gamma - 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the HeII Ly-alpha forest. We argue that the heat input due to HeII reionization is unable to cause the observed depression at z = 3.2 in the HI Ly-alpha forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of HeII Lyman-limit systems influence our predictions.

astro-ph↗

arXiv2008

Cosmological Perturbations at Second Order and Recombination Perturbed

We derive the full set of second-order equations governing the evolution of cosmological perturbations, including the effects of the first-order electron number density perturbations, δ_e. We provide a detailed analysis of the perturbations to the recombination history of the universe and show that a perturbed version of the Peebles effective 3-level atom is sufficient for obtaining the evolution of δ_e for comoving wavenumbers smaller than 1Mpc^{-1}. We calculate rigorously the perturbations to the Lyαescape probability and show that to a good approximation it is governed by the local baryon velocity divergence. For modes shorter than the photon diffusion scale, we find that δ_e is enhanced during recombination by a factor of roughly 5 relative to other first-order quantities sourcing the CMB anisotropies at second order. Using these results, in a companion paper we calculate the CMB bispectrum generated during recombination.

astro-ph↗

arXiv2008

CMBPol Mission Concept Study: Reionization Science with the Cosmic Microwave Background

We summarize existing constraints on the epoch of reionization and discuss the observational probes that are sensitive to the process. We focus on the role large scale polarization can play. Polarization probes the integrated optical depth across the entire epoch of reionization. Future missions such as Planck and CMBPol will greatly enhance our knowledge of the reionization history, allowing us to measure the time evolution of the ionization fraction. As large scale polarization probes high redshift activity, it can best constrain models where the Universe was fully or partially ionized at early times. In fact, large scale polarization could be our only probe of the highest redshifts.

astro-ph↗

arXiv2008

CMBPol Mission Concept Study: A Mission to Map our Origins

Quantum mechanical metric fluctuations during an early inflationary phase of the universe leave a characteristic imprint in the polarization of the cosmic microwave background (CMB). The amplitude of this signal depends on the energy scale at which inflation occurred. Detailed observations by a dedicated satellite mission (CMBPol) therefore provide information about energy scales as high as $10^{15}$ GeV, twelve orders of magnitude greater than the highest energies accessible to particle accelerators, and probe the earliest moments in the history of the universe. This summary provides an overview of a set of studies exploring the scientific payoff of CMBPol in diverse areas of modern cosmology, such as the physics of inflation, gravitational lensing and cosmic reionization, as well as foreground science and removal .

astro-ph↗

arXiv2008

Improved estimation of cluster mass profiles from the cosmic microwave background

We develop a new method for reconstructing cluster mass profiles and large-scale structure from the cosmic microwave background (CMB). By analyzing the likelihood of CMB lensing, we analytically prove that standard quadratic estimators for CMB lensing are unbiased and achieve the optimal condition only in the limit of no lensing; they become progressively biased and sub-optimal, when the lensing effect is large, especially for clusters that can be found by ongoing Sunyaev-Zel'dovich surveys. Adopting an alternative approach to the CMB likelihood, we construct a new maximum likelihood estimator that utilizes delensed CMB temperature fields based on an assumed model. We analytically show that this estimator asymptotically approaches the optimal condition as our assumed model is refined, and we numerically show that our estimator quickly converges to the true model as we iteratively apply it to CMB maps. For realistic CMB experiments, we demonstrate the applicability of the maximum likelihood estimator with tests against numerical simulations in the presence of CMB secondary contaminants. With significant improvement on the signal-to-noise ratio, our new maximum likelihood estimator can be used to measure the cluster-mass cross-correlation functions at different redshifts, probing the evolution of dark energy.

astro-ph↗

arXiv2008

The Phase Transition to Eternal Inflation

For slow-roll inflation we study the phase transition to the eternal regime. Starting from a finite inflationary volume, we consider the volume of the universe at reheating as order parameter. We show that there exists a critical value for the classical inflaton speed, \dotϕ^2/H^4 = 3/(2 π^2), where the probability distribution for the reheating volume undergoes a sharp transition. In particular, for sub-critical inflaton speeds all distribution moments become infinite. We show that at the same transition point the system develops a non-vanishing probability of having a strictly infinite reheating volume, while retaining a finite probability for finite values. Our analysis represents the exact quantum treatment of the system at lowest order in the slow-roll parameters and H^2/M_Pl^2.

hep-th↗

arXiv2008

How accurately can 21 cm tomography constrain cosmology?

There is growing interest in using 3-dimensional neutral hydrogen mapping with the redshifted 21 cm line as a cosmological probe, as it has been argued to have a greater long-term potential than the cosmic microwave background. However, its utility depends on many assumptions. To aid experimental planning and design, we quantify how the precision with which cosmological parameters can be measured depends on a broad range of assumptions. We cover assumptions related to modeling of the ionization power spectrum and associated nonlinearity, experimental specifications like array layout and noise, cosmological assumptions about reionization history and inter-galactic medium (IGM) evolution, and assumptions about astrophysical foregrounds. We derive simple analytic approximations for how various assumptions affect the results, and find that ionization power modeling is most important, followed by array layout (crudely, the more compact, the better). We also present an accurate yet robust method for measuring cosmological parameters in practice, separating the physics from the astrophysics by exploiting both gravitationally induced clustering anisotropy and the fact that the ionization power spectra are rather smooth functions that can be accurately fit by 7 phenomenological parameters. For example, a future square kilometer array optimized for 21 cm tomography could improve the sensitivity of the Planck CMB satellite to spatial curvature and neutrino masses by up to two orders of magnitude, to Delta-Omega_k ~ 0.0002 and Delta m_nu ~ 0.007 eV, and give a 4 sigma detection of the spectral index running predicted by the simpliest inflation models.

astro-ph↗

arXiv2008

The morphology of HII regions during reionization

It is possible that the properties of HII regions during reionization depend sensitively on many poorly constrained quantities (the nature of the ionizing sources, the clumpiness of the gas in the IGM, the degree to which photo-ionizing feedback suppresses the abundance of low mass galaxies, etc.), making it extremely difficult to interpret upcoming observations of this epoch. We demonstrate that the actual situation is more encouraging, using a suite of radiative transfer simulations, post-processed on outputs from a 1024^3, 94 Mpc N-body simulation. Analytic prescriptions are used to incorporate small-scale structures that affect reionization, yet remain unresolved in the N-body simulation. We show that the morphology of the HII regions is most dependent on the global ionization fraction x_i. This is not to say that the bubble morphology is completely independent of all parameters besides x_i. The next most important dependence is that of the nature of the ionizing sources. The rarer the sources, the larger and more spherical the HII regions become. The typical bubble size can vary by as much as a factor of 4 at fixed x_i between different possible source prescriptions. The final relevant factor is the abundance of minihalos or of Lyman-limit systems. These systems suppress the largest bubbles from growing, and the magnitude of this suppression depends on the thermal history of the gas as well as the rate at which minihalos are photo-evaporated. We find that neither source suppression owing to photo-heating nor gas clumping significantly affect the large-scale structure of the HII regions. We discuss how observations of the 21cm line with MWA and LOFAR can constrain properties of reionization, and we study the effect patchy reionization has on the statistics of Lyman-alpha emitting galaxies. [abridged]

astro-ph↗

arXiv2008

Probing Reionization with the 21 cm-Galaxy Cross Power Spectrum

The cross-correlation between high redshift galaxies and 21 cm emission from the high redshift intergalactic medium (IGM) promises to be an excellent probe of the Epoch of Reionization (EoR). On large scales, the 21 cm and galaxy fields are anti-correlated during most of the reionization epoch. However, on scales smaller than the size of the H II regions around detectable galaxies, the two fields become roughly uncorrelated. Consequently, the 21 cm-galaxy cross power spectrum provides a tracer of bubble growth during reionization, with the signal turning over on progressively larger scales as reionization proceeds. The precise turnover scale depends on the minimum host mass of the detectable galaxies, and the galaxy selection technique. Measuring the turnover scale as a function of galaxy luminosity constrains the characteristic bubble size around galaxies of different luminosities. The cross spectrum becomes positive on small scales if ionizing photons fail to escape from low mass galaxies, and these galaxies are detectable longward of the hydrogen ionization edge, because in this case some identifiable galaxies lie outside of ionized regions. LOFAR can potentially measure the 21 cm-galaxy cross spectrum in conjunction with mild extensions to the existing Subaru survey for $z=6.6$ Lyman-alpha emitters, while the MWA is slightly less sensitive for detecting the cross spectrum. A futuristic galaxy survey covering a sizable fraction of the MWA field of view ($\sim 800$ deg$^2$) can probe the scale dependence of the cross spectrum, constraining the filling factor of H II regions at different redshifts during reionization, and providing other valuable constraints on reionization models.

astro-ph↗

arXiv2008

Probing the Neutral Fraction of the IGM with GRBs during the Epoch of Reionization

We show that near-infrared observations of the red side of the Ly-alpha line from a single gamma ray burst (GRB) afterglow cannot be used to constrain the global neutral fraction of the intergalactic medium (IGM), x_H, at the GRB's redshift to better than ~0.3. Some GRB sight-lines will encounter more neutral hydrogen than others at fixed x_H owing to the patchiness of reionisation. GRBs during the epoch of reionization will often bear no discernible signature of a neutral IGM in their afterglow spectra. We discuss the constraints on x_H from the z = 6.3 burst, GRB050904, and quantify the probability of detecting a neutral IGM using future spectroscopic observations of high-redshift, near-infrared GRB afterglows. Assuming an observation with signal-to-noise similar to the Subaru FOCAS spectrum of GRB050904 and that the column density distribution of damped Ly-alpha absorbers is the same as measured at lower redshifts, a GRB from an epoch when x_H = 0.5 can be used to detect a partly neutral IGM at 98% confidence level 10% of the time (and, for an observation with three times the sensitivity, 30% of the time).

astro-ph↗

arXiv2007

Bumpy black holes from spontaneous Lorentz violation

We consider black holes in Lorentz violating theories of massive gravity. We argue that in these theories black hole solutions are no longer universal and exhibit a large number of hairs. If they exist, these hairs probe the singularity inside the black hole providing a window into quantum gravity. The existence of these hairs can be tested by future gravitational wave observatories. We generically expect that the effects we discuss will be larger for the more massive black holes. In the simplest models the strength of the hairs is controlled by the same parameter that sets the mass of the graviton (tensor modes). Then the upper limit on this mass coming from the inferred gravitational radiation emitted by binary pulsars implies that hairs are likely to be suppressed for almost the entire mass range of the super-massive black holes in the centers of galaxies.

hep-th↗

arXiv2007

Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array

We forecast the sensitivity with which the Murchison Widefield Array (MWA) can measure the 21 cm power spectrum of cosmic hydrogen, using radiative transfer simulations to model reionization and the 21 cm signal. The MWA is sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}), with foreground contamination precluding measurements on larger scales, and thermal detector noise limiting the small scale sensitivity. This amounts primarily to constraints on two numbers: the amplitude and slope of the 21 cm power spectrum on the scales probed. We find, however, that the redshift evolution in these quantities can yield important information about reionization. Although the power spectrum differs substantially across plausible models, a generic prediction is that the amplitude of the 21 cm power spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM) is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales flattens as the ionization fraction increases and the sizes of the HII regions grow. Considering detection sensitivity, we show that the optimal MWA antenna configuration for power spectrum measurements would pack all 500 antenna tiles as close as possible in a compact core. The MWA is sensitive enough in its optimal configuration to measure redshift evolution in the slope and amplitude of the 21 cm power spectrum. Detecting the characteristic redshift evolution of our models will confirm that observed 21 cm fluctuations originate from the IGM, and not from foregrounds, and provide an indirect constraint on the volume-filling factor of HII regions during reionization. After two years of observations under favorable conditions, the MWA can constrain the filling factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.

astro-ph↗