This proceeding concentrates on the BAO signature of dark energy, and how the SKA dark energy case has been complicated by the emergence of HI structure experiments modeled after the Epoch of Reionization observatories. The purpose of the conference talk was to review the current status of the Murchison Widefield Array (MWA), and show the applications of HI structure observations for both reionization and dark energy measurements. Since the status of the MWA is changing weekly, please see the website www.haystack.mit.edu/ast/arrays/mwa/ for the current status. This proceedings will instead concentrate on HI structure observations, their applicability to reionization and cosmography, and the implications for the SKA and future HI structure observations of dark energy.
The annihilation of dark matter particles releases energy, ionizing some of the gas in the Universe. We investigate the effect of dark matter halos on reionization. We show that the effect depends on the assumed density profile, the particle mass, and the assumed minimum halo mass. For NFW halos and typical WIMPs, we find the effect to be quite small. However, light dark matter candidates in the MeV range can contribute significantly to reionization and can make an important contribution to the measured optical depth. This effect may be used to constrain light dark matter models. We also study…
One of the possible methods to distinguish among various dark matter candidates is to study the effects of dark matter decays. We consider four different dark matter candidates (light dark matter, gravitinos, neutralinos and sterile neutrinos), for each of them deriving the decaying/annihilation rate, the influence on reionization, matter temperature and CMB spectra. We find that light dark matter particles (1-10 MeV) and sterile neutrinos (2-8 keV) can be sources of partial early reionization (z<~100). However, their integrated contribution to Thomson optical depth is small (<~0.01) with resp…
We examine the effect of inhomogeneous reionization on the galaxy power spectrum and the consequences for probing dark energy. To model feedback during reionization, we apply an ansatz setting the galaxy overdensity proportional to the underlying ionization field. Thus, inhomogeneous reionization may leave an imprint in the galaxy power spectrum. We evolve this imprint to low redshift and use the Fisher-matrix formalism to assess the effect on parameter estimation. We show that a combination of low- (z=0.3) and high- (z=3) redshift galaxy surveys can constrain the size of cosmological HII regi…
We derived the evolution of the energy deposition in the intergalactic medium (IGM) by different decaying (or annihilating) dark matter (DM) candidates. Heavy annihilating DM particles (with mass larger than a few GeV) have no influence on reionization and heating, even if we assume that all the energy emitted by annihilations is absorbed by the IGM. In the case of lighter particles, the impact on reionization and heating depends on the efficiency of energy absorption by the IGM. We calculated the fraction of energy produced by decays and annihilations which is effectively absorbed by the IGM.…
Limiting the number of model-dependent assumptions to a minimum, we discuss the detectability of the sources responsible for reionization with existing and planned telescopes. We conclude that if reionization sources are UV-efficient, minimum luminosity sources, then it may be difficult to detect them before the advent of the James Webb Space Telescope (JWST). The best approach before the launch of JWST may be either to exploit gravitational lensing by clusters of galaxies, or to search for strong Ly-alpha sources by means of narrow-band excess techniques or slitless grism spectroscopy.
The effects of reionization, occurring after standard recombination in cold dark matter-dominated models, on CMB anisotropies are investigated. Late-time reionization reduces the CMB anisotropies, in particular, on degree scales. It is found that constraints on cold dark matter-dominated models from the highest frequency channel of the 9-point South Pole data are significantly relaxed for models which are consistent with Big Bang nucleosynthesis if reionization is assumed to have occurred by redshift $\sim 20.$
A possible explanation of the present discrepancy between the abundance of galactic subhaloes predicted by N-Body simulations with those observed in the Local Group is presented. We study the impact of an early reionization on the baryonic component of the Universe using SPH simulations on group and galactic scales. We use a simplified model for reionization described as an instantaneous increment of the IGM temperature (with 1e5<T_{IGM}<1e6 K) at a given redshift (z_r > 8). We find that a key role is played by compton cooling (interaction between hot electrons and the CMB photons); at high re…
We use a semi-analytical approach to simulate absorption spectra of QSOs at high redshifts with the aim of constraining the cosmic reionization history. We consider two physically motivated and detailed reionization histories: (i) an Early Reionization Model (ERM) in which the intergalactic medium is reionized by PopIII stars at $z\approx 14$, and (ii) a more standard Late Reionization Model (LRM) in which overlapping, induced by QSOs and normal galaxies, occurs at $z\approx 6$. From the analysis of current Ly$α$ forest data at $z < 6$, we conclude that it is impossible to disentangle the two …
We calculate the reionization history in Cold Dark Matter (CDM) models. The epoch of the end of reionization and the Thomson scattering optical depth to the cosmic microwave background depend on the power spectrum amplitude on small scales and on the ionizing photon emissivity per unit mass in collapsed halos. We calibrate the emissivity to reproduce the measured ionizing background intensity at z=4. Models in which all CDM halos have either a constant emissivity or a constant energy emitted per Hubble time, per unit mass, predict that reionization ends near z~6 and the optical depth is in the…
The cosmic reionization era, which includes formation of the first stars, galaxies, and AGN, is now one of the most active frontiers of cosmological research. We review briefly our current understanding of the early structure formation, and use the ideas about a joint formation of massive black holes (which power the early QSOs) and their host galaxies to employ high-redshift QSOs as probes of the early galaxy formation and primordial large-scale structure. There is a growing evidence for a strong biasing in the formation of the first luminous sources, which would lead to a clumpy reionization…
We explore a mechanism for the formation of the first globular clusters, operating during the assembly of dwarf galaxies at high redshifts, z > 10. The substructure in the dark matter and the corresponding potential wells are responsible for setting the cluster scale of ~10^5 M_sun. The second mass scale in the formation problem, the stellar scale of ~1 M_sun, is determined in turn by the processes that cool the gas. We address the origin of the first, cluster scale by means of three-dimensional numerical simulations of the collapsing dark matter and gaseous components. We find that the gas fa…
Many lines of evidence suggest that nonbaryonic dark matter constitutes roughly 30% of the critical closure density, but the composition of this dark matter is unknown. One class of candidates for the dark matter is compact objects formed in the early universe, with typical masses M between 0.1 and 1 solar masses to correspond to the mass scale of objects found with microlensing observing projects. Specific candidates of this type include black holes formed at the epoch of the QCD phase transition, quark stars, and boson stars. Here we show that accretion onto these objects produces substantia…
The first sources of ionizing radiation to condense out of the dark and neutral IGM sent ionization fronts sweeping outward through their surroundings, overtaking other condensed objects and photoevaporating them. This feedback of universal reionization on cosmic structure formation is demonstrated here by gas dynamical simulations, including radiative transfer, for a cosmological minihalo of dark matter and baryons exposed to an external source of ionizing radiation, either starlight or quasar light, just after the passage of the global ionization front created by the source.
The first sources of ionizing radiation to condense out of the dark and neutral IGM sent ionization fronts sweeping outward through their surroundings, overtaking other condensed objects and photoevaporating them. This feedback of universal reionization on cosmic structure formation is demonstrated here by gas dynamical simulations, including radiative transfer, for the case of a cosmological minihalo of dark matter and baryons exposed to an external source of ionizing starlight, just after the passage of the global ionization front created by the source.
Dark stars powered by dark matter annihilation have been proposed as the first luminous sources in the universe. These stars are believed to form in the central dark matter cusp of low-mass minihalos. Recent calculations indicate stellar masses up to \sim1000 solar masses and/or have very long lifetimes. The UV photons from these objects could therefore contribute significantly to cosmic reionization. Here we show that such dark star models would require a somewhat artificial reionization history, based on a double-reionization phase and a late star-burst near redshift $z\sim6$, in order to fu…
Direct detection of the Dark Ages and the Epoch of Reionization (EOR) is among the main scientific objectives of all current and future low-frequency radio facilities. In this paper we summarize and discuss recent results, based on state-of-the-art numerical simulations, regarding the fundamental EOR properties and its observability with current and future radio arrays, like the Giant Metrewave Radio Telescope (GMRT), the Low Frequency Array (LOFAR), the 21-CM Array (21CMA), the Murchison Widefield Array (MWA) and the Square Kilometre Array (SKA). Results show that the optimal observational fr…
The early reionization of the intergalactic medium, which is favored from the WMAP temperature-polarization cross-correlations, contests the validity of the standard scenario of structure formation in the cold dark matter cosmogony. It is difficult to achieve early enough star formation without rather extreme assumptions such as very high escape fraction of ionizing photons from proto-galaxies or a top-heavy initial mass function. Here we propose an alternative scenario that is additional fluctuations on small scales induced by primordial magnetic fields trigger the early structure formation. …
The H atoms inside minihalos (i.e. halos with virial temperatures T_vir < 10^4 K, in the mass range roughly from 10^4 M_sun to 10^8 M_sun) during the cosmic dark ages in a LambdaCDM universe produce a redshifted background of collisionally-pumped 21-cm line radiation which can be seen in emission relative to the cosmic microwave background (CMB). Previously, we used semi-analytical calculations of the 21-cm signal from individual halos of different mass and redshift and the evolving mass function of minihalos to predict the mean brightness temperature of this 21-cm background and its angular f…
Sterile neutrinos with masses in the keV range can be the dark matter, and their emission from a supernova can explain the observed velocities of pulsars. The sterile neutrino decays could produce the x-ray radiation in the early universe, which could have an important effect on the formation of the first stars. X-rays could ionize gas and could catalyze the production of molecular hydrogen during the ``dark ages''. The increased fraction of molecular hydrogen could facilitate the cooling and collapse of the primordial gas clouds in which the first stars were formed.
With the imminent promise of constraints on the epoch of reionization from observations of microwave background anisotropies, the question of whether or not the standard Cold Dark Matter (CDM) model permits early reionization has been subjected to detailed investigation by various authors, with the conclusion that reionization may occur at quite high redshift. However, it is widely accepted that this model is excluded, as when normalised to the COBE observations it possesses excessive galaxy clustering on scales below tens of megaparsecs. We examine the trends of observations, first in a fairl…
The H atoms inside minihalos (i.e. halos with virial temperatures T_vir < 10^4 K, in the mass range roughly from 10^4 M_sun to 10^8 M_sun) during the cosmic dark ages in a LCDM universe produce a redshifted background of collisionally-pumped 21-cm line radiation which can be seen in emission relative to the cosmic microwave background (CMB). Previously, we used semi-analytical calculations of the 21-cm signal from individual halos of different mass and redshift and the evolving mass function of minihalos to predict the mean brightness temperature of this 21-cm background and its angular fluctu…
We study first structure formation in Lambda-dominated universes using large cosmological N-body/SPH simulations. We consider a standard LCDM model and a LWDM model in which the mass of the dark matter particles is taken to be m_X=10 keV. The linear power spectrum for the LWDM model has a characteristic cut-off at a wavenumber k=200 /Mpc, suppressing the formation of low mass (< 10^6 Msun) nonlinear objects early on. The absence of low mass halos in the WDM model makes the formation of primordial gas clouds with molecular hydrogen very inefficient at high redshifts. The first star-forming gas …
In this article, we explore the possibility of using the properties of gamma ray bursts (GRBs) to probe the physical conditions in the epochs prior to reionization. The redshift distribution of GRBs is modelled using the Press-Schechter formalism with an assumption that they follow the cosmic star formation history. We reproduce the observed star formation rate obtained from galaxies in the redshift range 0 < z < 5, as well as the redshift distribution of the GRBs inferred from the luminosity-variability correlation of the burst light curve. We show that the fraction of GRBs at high redshifts,…
We study the constraints that high-redshift structure formation in the universe places on warm dark matter (WDM) dominated cosmological models. We modify the extended Press-Schechter formalism to derive the halo mass function in WDM models. We show that our predictions agree with recent numerical simulations at low redshift over the halo masses of interest. Applying our model to galaxy formation at high redshift, we find that the loss of power on small scales, together with the delayed collapse of low-mass objects, results in strong limits on the root-mean-square velocity dispersion v_rms of t…
Proposed 21 cm cosmology observatories for studying the epoch of reionization (z ~6-15) and dark energy (z ~0-6) envision compact arrays with tens of thousands of antenna elements. Fully correlating this many elements is computationally expensive using traditional XF or FX correlators, and has led some groups to reconsider direct imaging/FFT correlators. In this paper we develop a variation of the direct imaging correlator we call the MOFF correlator. The MOFF correlator shares the computational advantages of a direct imaging correlator, while avoiding a number of its shortcomings. In particul…
The Dark Age is the period between the time when the cosmic microwave background was emitted and the time when the evolution of structure in the universe led to the gravitational collapse of objects in which the first stars were formed. The period of reionization started with the ionizing light from the first stars, and it ended when all the atoms in the intergalactic medium had been reionized. The most distant sources of light known at present are galaxies and quasars at redshift z ~ 6, and their spectra indicate that the end of reionization was occurring just at that time. The Cold Dark Matt…
We present a new derivation of the cosmic microwave background anisotropy spectrum from the Ostriker-Vishniac effect for an open, flat, or closed Universe, and calculate the anisotropy expected in cold dark-matter (CDM) models. We provide simple semi-analytic fitting formulas for the Vishniac power spectrum that can be used to evaluate the expected anisotropy in CDM models for any arbitrary ionization history. In a flat Universe, CDM models normalized to cluster abundances produce rms temperature anisotropies of 0.8--2.4 $μ$K on arcminute angular scales for a constant ionization fraction of un…
We calculate the reionization history for different models of the stellar population and explore the effects of primordial magnetic fields, dark matter decay and dark matter annihilation on reionization. We find that stellar populations based on a Scalo-type initial mass function for Population II stars can be ruled out as sole sources for reionization, unless star formation efficiencies of more than 10% or very high photon escape fractions from the parental halo are adopted. When considering primordial magnetic fields, we find that the additional heat injection from ambipolar diffusion and de…
We study the evolution of the ionization state of the IGM at the end of the reionization epoch using spectra of a sample of nineteen quasars at 5.74<z<6.42 discovered in the SDSS. Three methods are used to trace IGM properties: (a) the evolution of the Gunn-Peterson optical depth in the Ly alpha,beta, and gamma; (b) the distribution of dark absorption gaps, and (c) the size of HII regions around quasars. We find that the evolution of the IGM accelerated at z>5.7: the optical depth evolution changes from tau ~ (1+z)^{4.3} to (1+z)^{>11}, and the average length of dark gaps with tau>3.5 increase…
The first light from stars and quasars ended the ``dark ages'' of the universe and led to the reionization of hydrogen by redshift 7. Current observations are at the threshold of probing this epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.
In this letter we revisit the idea of reionization feedback on dwarf galaxy formation. We show that primordial minihalos with v_cir<20 km/s stop accreting gas after reionization, as it is usually assumed, but in virtue of their increasing concentration and the decreasing temperature of the intergalactic medium as redshift decreases below z=3, they have a late phase of gas accretion and possibly star formation. We expect that pre-reionization fossils that evolved on the outskirts of the Milky Way or in isolation show a bimodal star formation history with 12 Gyr old and <10 Gyr old population of…
I present first principle model calculations for the reionization of the Universe, which provide simple diagnostics to identify the reionization sources. Within this frame, I also discuss the results derived from the Hubble Ultra-Deep Field observations, and the possible need for future observations with JWST to provide the final answer.
The emission from neutral hydrogen (HI) clouds in the post-reionization era (z < 6), too faint to be individually detected, is present as a diffuse background in all low frequency radio observations below 1420 MHz. The angular and frequency fluctuations of this radiation (~ 1 mK) is an important future probe of the large scale structures in the Universe. We show that such observations are a very effective probe of the background cosmological model and the perturbed Universe. In our study we focus on the possibility of determining the redshift space distortion parameter, coordinate distance and…
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 fi…
We study the evolution of baryonic gas before the reionization in the lognormal (LN) model of cosmic clustering. We show that the thermal history of the universe around the reionization can roughly be divided into three epochs: 1) cold dark age $z>z_r$, in which baryon gas is neutral, and opaque to Ly$α$ photons; 2) hot dark age $z_r > z> z_{gp}$, in which a predominant part of baryon gas is ionized and hot, but it is still opaque to Ly$α$ photons; 3) bright age $z<z_{gp}$, in which the universe is ionized highly enough to be transparent to Ly$α$ photons. In the flat cold dark matter cosmologi…
Measurements of the CMB temperature anisotropy and the temperature-polarization cross correlation by WMAP suggest a reionization redshift of $z \sim 17 \pm 5$. On the other hand, observations of high redshift galaxies indicate a presence of a significant fraction of neutral hydrogen at redshift $z\sim 6-7$. We show that cosmic strings with tensions well within, but not far from, current observation bounds could cause early star formation at a level sufficient to explain the high reionization redshift.
I review the potential for observing cosmic reionization using the HI 21cm line of neutral hydrogren. Studies include observations of the evolution of large scale structure of the IGM (density, excitation temperature, and neutral fraction), through HI 21cm emission, as well as observations of small to intermediate scale structure through absorption toward the first discrete radio sources. I summarize predictions for the HI signals, then consider capabilities of facilities being built, or planned, to detect these signals. I also discuss the significant observational challenges.
Statistical observations of the Epoch of Reionization using the 21 cm line of neutral hydrogen have the potential to revolutionize our understanding of structure formation and the first luminous objects. However, these observations are complicated by a host of strong foreground sources. Several foreground removal techniques have been proposed in the literature, and it has been assumed that these would be used in combination to reveal the Epoch of Reionization (EOR) signal. By studying the characteristic subtraction errors of the proposed foreground removal techniques, we identify an additional…
The exploration of the end of the Dark Ages will be one of the most exciting field of the next decade. While most of the proposed observations must await the next-generation telescopes, the observational window of the redshifted 21cm line offers the possibility to investigate the physics of reheating and reionization on a short term. Here we describe several possible signatures detectable in the wavelength range 100-200 MHz. Among the physics that can be investigated: the epoch of reheating and reionization; topology and timescales of reheating; the nature of the ionizing sources; the baryon d…
The cosmic microwave background provides an image of the Universe 0.4 million years after the big bang, when atomic hydrogen formed out of free electrons and protons. One of the primary goals of observational cosmology is to obtain follow-up images of the Universe during the epoch of reionization, hundreds of millions of years later, when cosmic hydrogen was ionized once again by the UV photons emitted from the first galaxies. To achieve this goal, new observatories are being constructed, including low-frequency radio arrays capable of mapping cosmic hydrogen through its redshifted 21cm emissi…
