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 decaying MHD turbulence has significant impact on the thermal evolution and the ionization history of the post-recombination universe and on structure formation. The magnetic Jeans mass changes the typical mass scale of the star forming halos, and depending on the adopted stellar model we derive upper limits to the magnetic field strength between 0.7 and $5 $nG (comoving). For dark matter annihilation, we find an upper limit to the thermally averaged mass-weighted cross section of $10^{-33} \mathrm{cm}^3\mathrm{/s/eV}$. For dark matter decay, our calculations yield a lower limit to the lifetime of dark matter particles of $3\times10^{23}$ s. These limits are in agreement with constraints from recombination and the X-ray background and provide an independent confirmation at a much later epoch.
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.
We investigate the possibility that the Universe is reionized by the decay products of heavy particles. In particular we study under which circumstances this decay may produce a significant reionization at high redshift ($z \simeq 20$), as requested by the WMAP result.Extra sources of reionization at high redshifts increase the recombination rate. As a result, the number density of decaying particles required to produce a high level of ionization at $z \simeq 10-20$ is significantly higher ($\simeq 300$ times) than previously estimated. Moreover, the ionization produced by decay particles impl…
Full radiative transfer in 3D space has been solved to pursue the reionization history in an inhomogeneous universe. It has been shown that the reionization of an inhomogeneous universe is not a prompt event, but a fairly slow process. Also, QSO absorption line systems are simulated with using the results of radiative transfer calculations. Ly$α$ continuum depression implies that the metagalactic UV intensity decreases rapidly with $z$ at $z>5$, and the reionization must have taken place between $z=6$ and 10. Finally, it is stressed that H$α$ forest is a more powerful tool to probe the reioniz…
We consider a model for the reionization history of the Universe in which a significant fraction of the observed optical depth is a result of direct reionization by the decay products of a scaling cosmic defect network. We show that such network can make a significant contribution to the reionization history of the Universe even if its energy density is very small (the defect energy density has to be greater than about $10^{-11}$ of the background density). We compute the Cosmic Microwave Background temperature, polarization and temperature-polarization cross power spectrum and show that a con…
We investigate two sets of two-epoched reionization models and their manifestation in the CMB anisotropy and polarization data of the recent WMAP project and make some predictions for the future PLANCK missions. In the first set of models, the universe was reionized twice, first at z~15 by population III stars and then at z~6 by stars in large galaxies. In the second set of models, the extra peak-like reionization at high redshifts z > 100 is induced by the decay of unstable particles, followed by the standard picture of reionization at z~6. We examine the general properties of these two-epoch…
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.
Observational data on the early galactic abundances of the light elements lithium, beryllium and boron are combined with data related to the reionization of the intergalactic medium (IGM) in a search of processes happening in the early universe. Early massive metal-free stars (Pop III), largely held responsible for the reionization of the IGM, are proposed to have been also at the origin of the lithium-6 plateau, through their winds. In this sense the evolution of the Li-6/Be-9 ratio appears to be a key parameter for the history of nucleosynthesis, as a monitor of the early formation of metals…
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 quantum evaporation of primordial black holes may have a relevant effect on the reionization of the Universe at small redshifts ($z \leq 60$): in this paper, I studied the possibility of a reionization induced by the photons emitted during the last stages of the black holes life. I also took into account the quark and gluons jets production, that sensibly modifies the blackbody Hawking emission spectrum. In this way, one obtains a ``late and sudden'' reionization process, characterised by a very well controlled rise of the plasma temperature: the plasma heating is not so high to induce a s…
We present the star formation histories, luminosities, colors, mass to light ratios, and halo masses of "galaxies" formed in a simulation of cosmological reionization. We compare these galaxies with Lyman Break Galaxies observed at high redshift. While the simulation is severely limited by the small box size, the simulated galaxies do appear as fainter cousins of the observed ones. They have similar colors, and both simulated and observed galaxies can be fitted with the same luminosity function. This is significant to the process of reionization since in the simulation the stars alone are resp…
We examine the possible reionization of the intergalactic medium (IGM) by the source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep {\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al (2005) have identified HUDF-JD2 as a massive ($\sim6\times10^{11}M_\odot$) post-starburst galaxy at redshift z$\gtrsim6.5$. We find that HUDF-JD2 may be capable of reionizing its surrounding region of the Universe, starting the process at a redshift as high as z$\approx 15 \pm5$.
In cosmological models favored by current observations, the first astrophysical objects formed in dark matter halos at redshifts starting at z>20, and their properties were determined by primordial H_2 molecular chemistry. These protogalaxies were very abundant, but substantially less massive than typical galaxies in the local Universe. Extreme metal-poor stars, and massive black holes in their nuclei reionized the bulk of the hydrogen in the intergalactic medium. Reionization may have taken place over an extended redshift interval, ending around z=7. Observational probes of the process of rei…
A model of the density distribution in the intergalactic medium, motivated by that found in numerical simulations, is used to demonstrate the effect of a clumpy IGM and discrete sources on the reionization of the universe. In an inhomogeneous universe reionization occurs outside-in, starting in voids and gradually penetrating into overdense regions. Reionization should not be sudden but gradual, with a continuous rise of the photon mean free path over a fair fraction of the Hubble time as the emissivity increases. We show that a hydrogen Gunn-Peterson trough should be present at $z\simeq 6$ un…
We incorporate quasars into an analytic model to describe the reheating and reionization of the universe. In combination with a previous study of galaxies and Lyman-$α$ clouds, we are able to provide a unified description of structure formation, verified against a large variety of observations. We also take into account the clumping of the baryonic gas in addition to the presence of collapsed objects. We consider two cosmologies: a critical universe with a CDM power-spectrum and an open universe with $Ω_0=0.3$, $Λ=0$. The derived quasar luminosity function agrees reasonably well with observati…
The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at redshift 30 and reionized most of the hydrogen in the universe by redshift 7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade.
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 ea…
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.
In a universe reionized in patches, the Doppler effect from Thomson scattering off free electrons generates secondary cosmic microwave background (CMB) anisotropies. For a simple model with small patches and late reionization, we analytically calculate the anisotropy power spectrum. Patchy reionization can, in principle, be the main source of anisotropies on arcminute scales. On larger angular scales, its contribution to the CMB power spectrum is a small fraction of the primary signal and is only barely detectable in the power spectrum with even an ideal, i.e. cosmic variance limited, experime…
A remarkable result of the Wilkinson Microwave Anisotropy Probe (WMAP) observations is that the universe was significantly reionized at large redshifts. The standard explanation is that massive stars formed early and reionized the universe around redshift z=17. Here we explore an alternative possibility, in which the universe was reionized in two steps. An early boost of reionization is provided by a decaying sterile neutrino, whose decay products, relativistic electrons, result in partial ionization of the smooth gas. We demonstrate that a neutrino with a mass of m_nu ~ 200 MeV and a decay ti…
The recent discovery of a Gunn--Peterson (GP) trough in the spectrum of the redshift 6.28 SDSS quasar has raised the tantalizing possibility that we have detected the reionization of the universe. However, a neutral fraction (of hydrogen) as small as 0.1% is sufficient to cause the GP trough, hence its detection alone cannot rule out reionization at a much earlier epoch. The Cosmic Microwave Background (CMB) polarization anisotropy offers an alternative way to explore the dark age of the universe. We show that for most models constrained by the current CMB data and by the discovery of a GP tro…
The nature of Gamma-Ray Burst progenitors is still a debated issue, but consensus is growing on the association of GRBs with massive stars. Furthermore, current models for the reionization of the universe consider massive Pop--III stars as the sources of the ionizing photons. There could then be a natural link between GRBs and reionization. The reionization epoch can be measured through prompt IR spectral observations of high redshift GRBs. For this, GRBs are better than quasars: they produce a smaller HII region even if they are much brighter than quasars (but for a much shorter time) and the…
In this paper I will discuss the possibility of a reionization of the Universe due to the photons emitted by evaporating primordial black holes (PBHs); this process should happen during the last stages of the PBHs life, when the particle emission is very intense. I will study the time evolution of the ionization degree x, of the plasma temperature $T_{e}$ and of the photon number density $n_γ$ characterizing the Universe after the recombination epoch: a system of coupled differential equations for these variables is solved in an analytical way by assuming, as a photon source, PBHs having an in…
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.
Current data of high-redshift absorption-line systems imply that hydrogen reionization occurred before redshifts of about 5. Previous works on reionization by the first stars or quasars have shown that such scenarios are described by a large number of cosmological and astrophysical parameters. Here, we adopt a semi-analytic model of stellar reionization in order to quantify how the optical depth to reionization depends on such parameters, and combine this with constraints from the cosmic microwave background (CMB). We find this approach to be particularly useful in alleviating the well-known 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…
The earliest generation of stars, far from being a mere novelty, transformed the universe from darkness to light. The first atoms to form after the Big Bang filled the universe with atomic hydrogen and a few light elements. As gravity pulled gas clouds together, the first stars ignited and their radiation turned the surrounding atoms into ions. By looking at gas between us and distant galaxies, we know that this ionization eventually pervaded all space, so that few hydrogen atoms remain today between galaxies. Knowing exactly when and how it did so is a primary goal of cosmologists, because th…
The process of reionization is now believed to have proceeded in an orchestrated manner beginning with UV photons emitted by high redshift galaxies containing a large fraction of Population III stars carving out ionised regions around them. The physics during this era can be studied with a combination of redshifted 21-cm spin-flip transition tracing neutral hydrogen gas, IR emission from massive primordial stars that trace the global star-formation rate during reionization, and the imprint of hot-electrons in first supernovae remnants Compton-cooling off of cosmic microwave background (CMB) ra…
The high ionization level and universal metallicity (1% solar) of the intergalactic gas at redshifts z<5 implies that nonlinear structure had started to form in the universe at earlier times than we currently probe. In Cold Dark Matter (CDM) cosmologies, the first generation of baryonic objects emerges at redshifts z=10-50. Here we examine the observable consequences of the possibility that an early generation of stars reionized the universe and resulted in the observed metallicity of the Lyman-alpha forest. Forthcoming microwave anisotropy experiments will be sensitive to the damping of aniso…
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…
For a source of Ly alpha radiation embedded in a neutral intergalactic medium (IGM) prior to the reionization epoch, the emission line is strongly suppressed by the intervening IGM. The damping wing of the so-called Gunn-Peterson trough can extend to the red side of the emission line, and erase a significant fraction of the total line flux. However, the transmitted fraction increases with the size of the local cosmological HII region surrounding the source, and therefore with the ionizing luminosity and age of the source. Motivated by the recent discovery of a Ly alpha emitting galaxy at a red…
We present results of combined N-body and three-dimensional reionization calculations to determine the relationship between reionization history and local environment in a volume 1 Gpc/h across and a resolution of about 1 Mpc. We resolve the formation of about 2x10^6 halos of mass greater than ~10^12 Msun at z=0, allowing us to determine the relationship between halo mass and reionization epoch for galaxies and clusters. For our fiducial reionization model, in which reionization begins at z~15 and ends by z~6, we find a strong bias for cluster-size halos to be in the regions which reionized fi…
The observational tests for the period of reionization of the universe are discussed. We show that this period can be observed as {\it negative} intensity patches of the CMB radiation with the amplitude $δT/T\sim 10^{-5}$ and the angular sizes $θ_T\sim$10 angular seconds in range of the wavelength 0.1 cm$\leqλ\leq$1 cm. The expected number density and frequency dependence of the amplitude permit to recognize this effect and to discriminate it from the noise. This method applied to the small scale variations of CMB temperature complements well the traditional spectral approach. The number densi…
A sterile neutrino with mass of several keV can account for cosmological dark matter, as well as explain the observed velocities of pulsars. We show that X-rays produced by the decays of these relic sterile neutrinos can boost the production of molecular hydrogen, which can speed up the cooling of gas and the early star formation, which can, in turn, lead to a reionization of the universe at a high enough redshift to be consistent with the WMAP results.
GRBs are expected to occur at redshifts far higher than the highest quasar redshifts so far detected. And unlike quasars, GRB afterglows may provide "clean" probes of the epoch of reionization because no complications (such as the presence of a strong Ly-alpha emission line) or "proximity effects" (such as the Stromgren sphere produced by ionizing photons from the quasar) are expected. Thus NIR and optical observations of GRB afterglows may provide unique information about the epoch of reionization. In particular, the flux at wavelengths shortward of Ly-alpha provides a direct measure of the d…
Observations of QSOs at z ~ 5.7 - 6.4 show the appearance of Gunn-Peterson troughs around z ~ 6, and a change in the slope of the IGM optical depth tau(z) near z ~ 5.5. These results are interpreted as a signature of the end of the reionization era, which probably started at considerably higher redshifts. However, there also appears to be a substantial cosmic variance in the transmission of the IGM, both along some lines of sight, and among different lines of sight, in this intriguing redshift regime. We suggest that this is indicative of a spatially uneven reionization, possibly caused by the…
The universe was reionized by redshift z ~ 6 by a small fraction of the baryons in the universe, which released energy following their condensation out of a cold, dark, and neutral IGM into the earliest galaxies. The theory of this reionization is a critical missing link in the theory of galaxy formation. Its numerous observable consequences include effects on the spectrum, anisotropy and polarization of the cosmic microwave background and signatures of high-redshift star and quasar formation. This energy release also created feedback on galaxy formation which left its imprint on the mass spec…
In this paper we show how the rescattering of CMB photons after cosmic reionization can give a significant linear contribution to the temperature-matter cross-correlation measurements. These anisotropies, which arise via a late time Doppler effect, are on scales much larger than the typical scale of non-linear effects at reionization; they can contribute to degree scale cross-correlations and could affect the interpretation of similar correlations resulting from the integrated Sachs-Wolfe effect. While expected to be small at low redshifts, these correlations can be large given a probe of the …
We study the reionization scenario in which ionizing UV photons emitted from decaying particle, in addition to usual contributions from stars and quasars, ionize the universe. It is found that the scenario is consistent with both the first year data of the Wilkinson Microwave Anisotropy Probe and the fact that the universe is not fully ionized until z \sim 6 as observed by Sloan Digital Sky Survey. Likelihood analysis revealed that rather broad parameter space can be chosen. This scenario will be discriminated by future observations, especially by the EE polarization power spectrum of cosmic m…
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…
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 effect of universal reionization on cosmic structure formation is demonstrated here for the case of a cosmological minihalo of dark matter and baryons exposed to an external source of ionizing radiation with a quasar-like spectrum, just after the passage of the global ionization front created by the source. We model the pre-ionization minihalo as a truncated, nonsingula…
I use simulations of cosmological reionization to quantify the effect of photoionization on the gas fraction in low mass objects, in particular the characteristic mass scale below which the gas fraction is reduced compared to the universal value. I show that this characteristic scale can be up to an order of magnitude lower than the linear theory Jeans mass, and that even if one defines the Jeans mass at a higher overdensity, it does not track the evolution of this characteristic suppression mass. Instead, the filtering mass, which corresponds directly to the scale over which baryonic perturba…
We generalize previous work on early photoionization to CDM models with Omega<1. Such models have received recent interest because the excess power in the large-scale galaxy distribution is phenomenologically fit if the ``shape parameter" Gamma=h Omega_0 is about 0.25. It has been argued that such models may require early reionization to suppress small-scale anisotropies in order to be consistent with experimental data. We find that if the cosmological constant lambda=0, the extent of this suppression is quite insensitive to Omega_0. Given a sigma_8-normalization today, the loss of small-scale…
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…
We simulate a plausible cosmological model in considerable physical and numerical detail through the successive phases of reheating (at 10<z<20), formation of Pop III stars at z=15 (due to molecular hydrogen cooling), with subsequent reionization at z=7. We assume an efficiency of high mass star formation appropriate to leave the universe, after it becomes transparent, with an ionizing background J_21=0.4 (at z=4), near (and perhaps slightly below) the observed value. Since the same stars produce the ionizing radiation and the first generation of heavy elements, a mean metallicity of Z/Z_\sun=…
The development of primordial inhomogeneities into the non-linear regime and the formation of the first bound objects mark the transition from a simple cooling universe -- described by just a few parameters -- to a very messy hot one -- the realm of radiative, hydrodynamic and star formation processes. In popular cold dark matter cosmological scenarios, stars and quasars may have first appeared in significant numbers around a redshift of 10 or so, as the gas within subgalactic halos cooled rapidly due to atomic processes and fragmented. This early generation of sources generated the ultraviole…
Recent years have seen major advances in understanding the state of the intergalactic medium (IGM) at high redshift. Some aspects of this understanding are reviewed here. In particular, we discuss: (1) Different probes of IGM like Gunn-Peterson test, CMBR anisotropies, and neutral hydrogen emission from reionization, and (2) some models of reionization of the universe.
We show how the morphological analysis of the maps of the secondary CMB anisotropies can detect an extended period of ``smoldering'' reionization, during which the universe remains partially ionized. Neither radio observations of the redshifted 21cm line nor IR observations of the redshifted Lyman-alpha forest will be able to detect such a period. The most sensitive to this kind of non-gaussianity parameters are the number of regions in the excursion set, the perimeter of the excursion set, and the genus of the largest (by area) region. For example, if the universe reionized fully at z=6, but …
We study the anisotropy of the cosmic microwave background (CMB) in cold and mixed dark matter (CDM and MDM) models, with non scale-invariant primordial power spectra (i.e. $n \neq 1$) and a late, sudden reionization of the intergalactic medium at redshift $z_{rh}$. We test these models against recent detections of CMB anisotropy at large and intermediate angular scales. We find that current CMB anisotropy measurements cannot discriminate between CDM and MDM models. Our likelihood analysis indicates that models with blue power spectra ($n \simeq 1.2$) and a reionization at $z_{rh} \sim 20$ are…
The first galaxies to appear in the universe at redshifts z>20 created ionized bubbles in the intergalactic medium of neutral hydrogen left over from the Big-Bang. It is thought that the ionized bubbles grew with time, surrounded clusters of dwarf galaxies and eventually overlapped quickly throughout the universe over a narrow redshift interval near z~6. This event signaled the end of the reionization epoch when the universe was a billion years old. Measuring the hitherto unknown size distribution of the bubbles at their final overlap phase is a focus of forthcoming observational programs aime…
The epoch of reionization (EoR) corresponds to a 'cosmic phase transition', when the neutral intergalactic medium (IGM) becomes ionized by the first stars and/or AGN. While the discoveries of Gunn-Peterson (GP) absorption troughs in the spectra of the highest redshift QSOs, and large scale polarization of the CMB, have set the first hard constraints on the EoR, the redshift and process of reionization, and the nature of the first luminous objects, remain two of the paramount questions in cosmic structure formation. Moreover, the GP effect is such that observations of objects during this epoch …
We show the universe was reionized twice, first at z~15-16 and second at z~6. Such an outcome appears inevitable, when normalizing to two well determined observational measurements, namely, the epoch of the final cosmological reionization at z~6 and the density fluctuations at z~6, which in turn are tight ly constrained by Lyman alpha forest observations at z~3. These two observations most importantly fix the product of star formation efficiency and ionizing photon escape fraction from galaxies at high redshift. To the extent that the relative star formation efficiencies in gaseous minihalos w…
We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semi-analytic methods, we compute the number density of observable features caused by both ``minihalos'' (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided t…
In this paper we investigate the effects that a dynamic dark energy component dominant in the universe at late epochs has on reionization. We follow the evolution of HII regions with the analytic approach of Furlanetto and Oh (2005) in two different universes for which we assume the Peebles and Ratra (2003) and Brax and Martin (2000) quintessence models and we compare our results to the LCDM scenario. We show that, for a fixed ionization efficiency, at the same cosmological epoch the topology of bubbles is dominated by high-mass objects and the characteristic size of the ionized regions is sli…
We show that the delay of structure formation can not fully account for the reduction of electron optical depth from WMAP1 to WMAP3 when the radiative transfer effects and feedback mechanisms are took into account in computing the reionization history of the Universe. We also show that a PopIII stellar cluster with a mass of 80Mo and a heavy Larson initial mass function has an ionizing efficiency high enough to account for WMAP3 results, while in the case of WMAP1, a higher stellar mass of 1000Mo was required.
We show that the epoch(s) of reionization when the ionization fraction of the universe is about half can be determined by correlating Cosmic Microwave Background (CMB) temperature maps with 21-cm line maps at degree scales ($l\sim 100$). During reionization peculiar motion of free electrons induces the Doppler anisotropy of the CMB, while density fluctuations of neutral hydrogen induce the 21-cm line anisotropy. In our simplified model of inhomogeneous reionization, a positive correlation arises as the universe reionizes whereas a negative correlation arises as it recombines; thus, the sign of…
We study the reionization histories where ionizing UV photons are emitted from decaying particles, in addition to usual contributions from stars and quasars, taking account of the fact that the universe is not fully ionized until z = 6 as observed by Sloan Digital Sky Survey. Likelihood analysis of the three-year data from the WMAP (Wilkinson Microwave Anisotropy Probe) severely constrains the decaying particle scenario.In particular, the decaying particle with relatively short lifetime is not favored by the polarization data.
Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, tau_e ~ 0.17. The reionization of the universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results. Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously …
The first-year Wilkinson Microwave Anisotropy Probe data suggest a high optical depth for Thomson scattering of 0.17 +/- 0.04, implying that the universe was reionized at an early epoch, z ~ 20. Such early reionization is likely to be caused by UV photons from first stars, but it appears that the observed high optical depth can be reconciled within the standard structure formation model only if star-formation in the early universe was extremely efficient. With normal star-formation efficiencies, cosmological models with non-Gaussian density fluctuations may circumvent this conflict as high den…
Structure formation in the early universe is expected to produce a cosmological background of free-free emission. The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) will measure the spectrum of the cosmic microwave background at centimeter wavelengths to detect this signature of structure formation. I describe the ARCADE instrument and the cryogenic engineering required to achieve mK accuracy at such long wavelengths.
