Detecting Early Galaxies Through Their 21-cm Signature

New observations over the next few years of the emission of distant objects will help unfold the chapter in cosmic history around the era of the first galaxies. These observations will use the neutral hydrogen emission or absorption at a wavelength of 21-cm as a detector of the hydrogen abundance. We predict the signature on the 21-cm signal of the early generations of galaxies. We calculate the 21-cm power spectrum including two physical effects that were neglected in previous calculations. The first is the redistribution of the UV photons from the first galaxies due to their scattering off of the neutral hydrogen, which results in an enhancement of the 21-cm signal. The second is the presence of an ionized hydrogen bubble near each source, which produces a cutoff at observable scales. We show that the resulting clear signature in the 21-cm power spectrum can be used to detect and study the population of galaxies that formed just 200 million years after the Big Bang.Comment: 5 pages, 3 figures, submitted to MNRAS Let

The Sensitivity of First Generation Epoch of Reionization Observatories and Their Potential for Differentiating Theoretical Power Spectra

Statistical observations of the epoch of reionization (EOR) power spectrum provide a rich data set for understanding the transition from the cosmic "dark ages" to the ionized universe we see today. EOR observations have become an active area of experimental cosmology, and three first generation observatories--MWA, PAST, and LOFAR--are currently under development. In this paper we provide the first quantitative calculation of the three dimensional power spectrum sensitivity, incorporating the design parameters of a planned array. This calculation is then used to explore the constraints these first generation observations can place on the EOR power spectrum. The results demonstrate the potential of upcoming power spectrum observations to constrain theories of structure formation and reionization.Comment: 7 pages with 5 figures. Submitted to Ap

Space missions to detect the cosmic gravitational-wave background

It is thought that a stochastic background of gravitational waves was produced during the formation of the universe. A great deal could be learned by measuring this Cosmic Gravitational-wave Background (CGB), but detecting the CGB presents a significant technological challenge. The signal strength is expected to be extremely weak, and there will be competition from unresolved astrophysical foregrounds such as white dwarf binaries. Our goal is to identify the most promising approach to detect the CGB. We study the sensitivities that can be reached using both individual, and cross-correlated pairs of space based interferometers. Our main result is a general, coordinate free formalism for calculating the detector response that applies to arbitrary detector configurations. We use this general formalism to identify some promising designs for a GrAvitational Background Interferometer (GABI) mission. Our conclusion is that detecting the CGB is not out of reach.Comment: 22 pages, 7 figures, IOP style, References Adde

Sermons by Hogan

https://digitalcommons.acu.edu/crs_books/1339/thumbnail.jp

A Characterisation of Strong Wave Tails in Curved Space-Times

A characterisation of when wave tails are strong is proposed. The existence of a curvature induced tail (i.e. a Green's function term whose support includes the interior of the light-cone) is commonly understood to cause backscattering of the field governed by the relevant wave equation. Strong tails are characterised as those for which the purely radiative part of the field is backscattered. With this definition, it is shown that electromagnetic waves in asymptotically flat space-times and fields governed by tail-free propagation have weak tails, but minimally coupled scalar fields in a cosmological scenario have strong tails.Comment: 17 pages, Revtex, to appear in Classical and Quantum Gravit

Redshift space 21 cm power spectra from reionization

We construct a simple but self-consistent analytic ionization model for rapid exploration of 21cm power spectrum observables in redshift space. It is fully described by the average ionization fraction $x_e(z)$ and HII patch size $R(z)$ and has the flexibility to accommodate various reionization scenarios. The model associates ionization regions with dark matter halos of the number density required to recover $x_e$ and treats redshift space distortions self-consistently with the virial velocity of such halos. Based on this model, we study the line-of-sight structures in the brightness fluctuations since they are the most immune to foreground contamination. We explore the degeneracy between the HII patch size and nonlinear redshift space distortion in the one dimensional power spectrum. We also discuss the limitations experimental frequency and angular resolutions place on their distinguishability. Angular resolution dilutes even the radial signal and will be a serious limitation for resolving small bubbles before the end of reionization. Nonlinear redshift space distortions suggest that a resolution of order 1 -- 10\arcsec and a frequency resolution of 10kHz will ultimately be desirable to extract the full information in the radial field at $z\sim 10$. First generation instruments such as LOFAR and MWA can potentially measure radial HII patches of a few comoving Mpc and larger at the end of reionization and are unlikely to be affected by nonlinear redshift space distortions.Comment: 13 pages, 10 figures. Revised version. Includes minor changes. Adds appendix on accomodating a distribution of radii for the HII regions. Accepted for publication in Ap

What initiated planetesimal formation?

The physical structure of primitive (chondritic) meteorites, even after some geological processing and modification, is thought by most to contain clues as to the first stage of accretion of solid matter into objects that might be called planetesimals. However, theoretical understanding of the processes responsible for this important stage is shaky. We note what we believe are fundamental obstacles for the Goldreich-Ward version of rapid and direct planetesimal formation via gravitational instability in a settled particle layer, and describe an alternative scenario which might lead from grainy nebula gas to primitive planetesimals in a way that has intriguing connections to the meteorite evidence

Gravitational Waves from Mesoscopic Dynamics of the Extra Dimensions

Recent models which describe our world as a brane embedded in a higher dimensional space introduce new geometrical degrees of freedom: the shape and/or size of the extra dimensions, and the position of the brane. These modes can be coherently excited by symmetry breaking in the early universe even on ``mesoscopic'' scales as large as 1 mm, leading to detectable gravitational radiation. Two sources are described: relativistic turbulence caused by a first-order transition of a radion potential, and Kibble excitation of Nambu-Goldstone modes of brane displacement. Characteristic scales and spectral properties are estimated and the prospects for observation by LISA are discussed. Extra dimensions with scale between 10 \AA and 1 mm, which enter the 3+1-D era at cosmic temperatures between 1 and 1000 TeV, produce backgrounds with energy peaked at observed frequencies in the LISA band, between $10^{-1}$ and $10^{-4}$ Hz. The background is detectable above instrument and astrophysical foregrounds if initial metric perturbations are excited to a fractional amplitude of $10^{-3}$ or more, a likely outcome for the Nambu-Goldstone excitations.Comment: Latex, 5 pages, plus one figure, final version to appear in Phys. Rev. Let

Mapping the gravitational wave background

The gravitational wave sky is expected to have isolated bright sources superimposed on a diffuse gravitational wave background. The background radiation has two components: a confusion limited background from unresolved astrophysical sources; and a cosmological component formed during the birth of the universe. A map of the gravitational wave background can be made by sweeping a gravitational wave detector across the sky. The detector output is a complicated convolution of the sky luminosity distribution, the detector response function and the scan pattern. Here we study the general de-convolution problem, and show how LIGO (Laser Interferometric Gravitational Observatory) and LISA (Laser Interferometer Space Antenna) can be used to detect anisotropies in the gravitational wave background.Comment: 16 pages, 6 figures. Submitted to CQ