Jonathan R. Pritchard

21-cm fluctuations from inhomogeneous X-ray heating before reionization

Many models of early structure formation predict a period of heating immediately preceding reionization, when X-rays raise the gas temperature above that of the cosmic microwave background. These X-rays are often assumed to heat the intergalactic medium (IGM) uniformly, but in reality will heat the gas more strongly closer to the sources. We develop a framework for calculating fluctuations in the 21-cm brightness temperature that originate from this spatial variation in the heating rate. High-redshift sources are highly clustered, leading to significant gas temperature fluctuations (with fractional variations ∼40 per cent, peaking on k∼0.1 Mpc^−1 scales). This induces a distinctive peak-trough structure in the angle-averaged 21-cm power spectrum, which may be accessible to the proposed Square Kilometre Array. This signal reaches the ∼10 mK level, and is stronger than that induced by Lyα flux fluctuations. As well as probing the thermal evolution of the IGM before reionization, this 21-cm signal contains information about the spectra of the first X-ray sources. Finally, we consider disentangling temperature, density and Lyα flux fluctuations as functions of redshift.

Reionization and the Cosmic Dawn with the Square Kilometre Array

The Square Kilometre Array (SKA) will have a low frequency component (SKA-low) which has as one of its main science goals the study of the redshifted 21 cm line from the earliest phases of star and galaxy formation in the Universe. This 21 cm signal provides a new and unique window both on the time of the formation of the first stars and accreting black holes and the subsequent period of substantial ionization of the intergalactic medium. The signal will teach us fundamental new things about the earliest phases of structure formation, cosmology and even has the potential to lead to the discovery of new physical phenomena. Here we present a white paper with an overview of the science questions that SKA-low can address, how we plan to tackle these questions and what this implies for the basic design of the telescope.

Cosmological and Astrophysical Neutrino Mass Measurements

Cosmological and astrophysical measurements provide powerful constraints on neutrino masses complementary to those from accelerators and reactors. Here we provide a guide to these different probes, for each explaining its physical basis, underlying assumptions, current and future reach.

Evolution of the 21 cm signal throughout cosmic history

The potential use of the redshifted 21 cm line from neutral hydrogen for probing the epoch of reionization is motivating the construction of several low-frequency interferometers. There is also much interest in the possibility of constraining the initial conditions from inflation and the nature of the dark matter and dark energy by probing the power spectrum of density perturbations in three dimensions and on smaller scales than probed by the microwave background anisotropies. Theoretical understanding of the 21 cm signal has been fragmented into different regimes of physical interest. In this paper, we make the first attempt to describe the full redshift evolution of the 21 cm signal between 0 < z < 300. We include contributions to the 21 cm signal from fluctuations in the gas density, temperature, and neutral fraction, as well as the Lya flux, and allow for a post-reionization signal from damped Lya systems. Our comprehensive analysis provides a useful foundation for optimizing the design of future arrays whose goal is to separate the particle physics from the astrophysics, either by probing the peculiar velocity distortion of the 21 cm power spectrum, or by extending the 21 cm horizon to z ≥ 25 before the first galaxies had formed, or to z ≤ 6 when the residual pockets of hydrogen trace large-scale structure.

Probing the first stars and black holes in the early Universe with the Dark Ages Radio Explorer (DARE)

Above redshift 6, the dominant source of neutral hydrogen in the Universe shifts from localized clumps in and around galaxies and filaments to a per vasive, diffuse component of the intergalactic medium (IGM). This transition tracks the glo bal neutral fraction of hydrogen in the IGM and can be studied, in principle, through the redshifted 21 cm hyperfine transition line. During the last half of the reionization epoch, the mean (global) br ightness temperature of the redshifted 21 cm emission is proportional to the neutral fraction, but a t e rlier times (10< z < 25), the mean brightness temperature should probe the spin temperat ur of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature o f the redshifted 21 cm line as a function of frequency (and hence redshift) would chart the e arly evolution of galaxies through the heating and ionizing of the IGM by their stellar populations . Experiments are already underway to accomplish this task or, at least, provide basic constraint s o the evolution of the mean brightness temperature. We provide a brief overview of one of these proj ects, the Experiment to the Detect the Global EOR Signature (EDGES), and discuss prospects for fut u e results.

Cosmic microwave background fluctuations from gravitational waves: An analytic approach☆

Abstract We develop an analytic approach to calculation of the temperature and polarisation power spectra of the cosmic microwave background due to inflationary gravitational waves. This approach complements the more precise numerical results by providing insight into the physical origins of the features in the power spectra. We explore the use of analytic approximations for the gravitational-wave evolution, making use of the WKB approach to handle the radiation-matter transition. In the process, we describe scaling relations for the temperature and polarisation power spectra. We illustrate the dependence of the amplitude, shape, and peak locations on the details of recombination, the gravitational-wave power spectrum, and the cosmological parameters, and explain the origin of the peak locations in the temperature and polarisation power spectra. The decline in power on small scales in the polarisation power spectra is discussed in terms of phase-damping. In an appendix we detail numerical techniques for integrating the gravitational-wave evolution in the presence of anisotropic stress from free-streaming neutrinos.

The scattering of Lyman-series photons in the intergalactic medium

We re-examine scattering of photons near the Lyα resonance in the intergalactic medium (IGM). We first derive a general integral solution for the radiation field around resonance within the usual Fokker–Planck approximation. Our solution shows explicitly that recoil and spin diffusivity source an absorption feature, whose magnitude increases with the relative importance of recoil compared to Doppler broadening. This spectrum depends on the Lyα line profile, but approximating it with the absorption profile appropriate to the Lorentzian wings of natural broadening accurately reproduces the results for a full Voigt profile so long as the IGM temperature is less than ~1000 K. This approximation allows us to obtain simple analytic formulae for the total scattering rate of Lyα photons and the accompanying energy exchange rate. Our power series solutions converge rapidly for photons that redshift into the Lyα resonance as well as for photons injected at line centre. We confirm previous calculations showing that heating through this mechanism is quite slow and probably negligible compared to other sources. We then show that energy exchange during the scattering of higher-order Lyman-series photons can be much more important than naively predicted by recoil arguments. However, the resulting heating is still completely negligible.

Constraining massive neutrinos using cosmological 21 cm observations

Observations of neutrino oscillations show that neutrinos have mass. However, the best constraints on this mass currently come from cosmology, via measurements of the cosmic microwave background and large-scale structure. In this paper, we explore the prospects for using low-frequency radio observations of the redshifted 21 cm signal from the epoch of reionization to further constrain neutrino masses. We use the Fisher matrix formalism to compare future galaxy surveys and 21 cm experiments. We show that by pushing to smaller scales and probing a considerably larger volume, 21 cm experiments can provide stronger constraints on neutrino masses than even very large galaxy surveys. Finally, we consider the possibility of going beyond measurements of the total neutrino mass to constraining the mass hierarchies. For a futuristic, 21 cm experiment we show that individual neutrino masses could be measured separately from the total neutrino mass.

Constraining reionization using 21-cm observations in combination with CMB and Lyα forest data

In this paper, we explore the constraints on the reionization history that are provided by current observations of the Lyα forest and the cosmic microwave background. Rather than using a particular semi-analytic model, we take the novel approach of parametrizing the ionizing sources with arbitrary functions and perform likelihood analyses to constrain possible reionization histories. We find model-independent conclusions that reionization is likely to be mostly complete by z = 8 and that the intergalactic medium was 50 per cent ionized at z = 9-10. Upcoming low-frequency observations of the redshifted 21-cm line of neutral hydrogen are expected to place significantly better constraints on the hydrogen neutral fraction at 6 ≲ z ≲ 12. We use our constraints on the reionization history to predict the likely amplitude of the 21-cm power spectrum and show that observations with the highest signal-to-noise ratio will most likely be made at frequencies corresponding to z = 9-10. This result provides an important guide to the upcoming 21-cm observations. Finally, we assess the impact that measurement of the neutral fraction will have on our knowledge of reionization and the early source population. Our results show that a single measurement of the neutral fraction mid-way through the reionization era will significantly enhance our knowledge of the entire reionization history.

Cosmological parameters after WMAP5: forecasts for Planck and future galaxy surveys

With its increased sensitivity and resolution, the Planck s atellite is expected to improve the measurement of most cosmological parameters by several factors with respect to current WMAP results. The actual performance however, may depend upon various aspects of the data analysis. In this paper we analyse the impact of specific s of the data analysis on the actual final results. We also explore the synergies in combinin g Planck results with future galaxy surveys. We find that Planck will improve constraints on most cosmological parameters by a factor 3‐4 and on the tensor‐to‐scalar ratio r by a factor 9. Also inflationary parameters, like r, ns and nrun , are practically not degenerate any longer. The tensor spec tral index, however, is little constrained. A combination of the 70 to 143 GHz channels will contain about 90% of all possible information, with 143 GHz polarisation information carrying about half of the constraining power on r. Also, the error on r degrades by a factor 2 if no B modes are considered in the analysis. High‐l temperature information is essential for determination of ns andb, while improving noise properties increase the l ‐range where Planck would be cosmic variance limited in polarisation, implying a significant im provement on the determination of r, � and As. However, a sub-percent difference in the FWHM used in the data analysis with respect to the one in the map will result in a bias for several p arameters. Finally, Planck will greatly help future missions like LSST and CIP reach their potentials by providing tight constraints on parameters like ns and nrun . Considering Planck together with these probes will