We investigate the effect of non-evaporating primordial black holes (PBHs) on the ionization and thermal history of the universe. X-rays emitted by gas accretion onto PBHs modify the cosmic recombination history, producing measurable effects on the spectrum and anisotropies of the Cosmic Microwave Background (CMB). Using the third-year WMAP data and FIRAS data we improve existing upper limits on the abundance of PBHs with masses > 0.1 M ⊙ by several orders of magnitude.Fitting WMAP3 data with cosmological models that do not allow for non-standard recombination histories, as produced by PBHs or other early energy sources, may lead to an underestimate of the best-fit values of the amplitude of linear density fluctuations (σ 8 ) and the scalar spectral index (n s ). Cosmological parameter estimates are affected because models with PBHs allow for larger values of the Thomson scattering optical depth, whose correlation with other parameters may not be correctly taken into account when PBHs are ignored. Values of τ e ∼ 0.2, n s ∼ 1 and σ 8 ∼ 0.9 are allowed at 95% CF. This result that may relieve recent tension between WMAP3 data and clusters data on the value of σ 8 .PBHs may increase the primordial molecular hydrogen abundance by up to two orders of magnitude, this promoting cooling and star formation. The suppression of galaxy formation due to X-ray heating is negligible for models consistent with the CMB data. Thus, the formation rate of the first galaxies and stars would be enhanced by a population of PBHs. Subject headings: cosmology: theory -cosmology: observations -early universe -cosmic microwave background -cosmological parameters -black hole physics

We develop a method to extract the "effective equation of state" of the intergalactic medium from the Doppler (b) parameter distribution of the low-density Lyα forest. We test the method on numerical simulations and then apply it to published observations of the Lyα forest at redshifts z ≃ 0 to 4. We find that the effective equation of state is close to isothermal at redshift z ∼ 3, indicating that a second reheating of the IGM took place at z ∼ 3. This reheating can plausibly be identified with the reionization of He ii observed to occur at z ∼ 3.

We use three-dimensional cosmological simulations with radiative transfer to study the formation and evolution of the first galaxies in a ÃCDM cosmology. The simulations include continuum radiative transfer using the optically thin variable Eddington tensor (OTVET) approximation and line radiative transfer in the H 2 Lyman-Werner bands of the UV background radiation. Chemical and thermal processes are treated in detail, particularly the ones relevant for H 2 formation and destruction. We find that the first luminous objects ('' small-halo objects '') are characterized by bursting star formation (SF) that is self-regulated by a feedback process acting on cosmological instead of galactic scales. The global SF history is regulated by the mean number of ionizing photons that escape from each source, UV h f esc i. It is almost independent of the assumed SF efficiency parameter, * , and the intensity of the dissociating background. The main feedback process that regulates the SF is the reformation of H 2 in front of H ii regions and inside relic H ii regions. The H ii regions remain confined inside filaments, maximizing the production of H 2 in overdense regions through cyclic destruction/reformation of H 2 . If UV h f esc i > 10 À7 = Ã , the SF is self-regulated, photoevaporation of smallhalo objects dominates the metal pollution of the low-density intergalactic medium, and the mass of produced metals depends only on h f esc i. If UV h f esc id10 À7 = Ã , positive feedback dominates, and small-halo objects constitute the bulk of the mass in stars and metals until at least redshift z $ 10. Small-halo objects cannot reionize the universe because the feedback mechanism confines the H ii regions inside the large-scale structure filaments. In contrast to massive objects ('' large halos ''), which can reionize voids, small-halo objects partially ionize only the dense filaments while leaving the voids mostly neutral.

We compare the properties of dwarf galaxies in the Local Group with the simulated galaxies formed before reionization in a cosmological simulation of unprecedented spatial and mass resolution, including radiative feedback effects. We find that a subset of the Local Group dwarfs are already remarkably similar to the simulated dwarf galaxies in all their properties before reionization. On the basis of this similarity, we propose the hypothesis that Local Group dwarfs form in a variety of ways: some of them are ''true fossils'' of the pre-reionization era, some of them form most of their stars later, after reionization (we call them ''survivors'' of the reionization era), and the rest of them form an intermediate group of ''polluted fossils.'' We also identify a simple observational test that is able to test our hypothesis.

We propose the existence of ultracompact minihalos as a new type of massive compact halo object (MACHO) and suggest an observational test to discover them. These new MACHOs are a powerful probe into the nature of dark matter and physics in the high energy Universe.Non-Gaussian energy-density fluctuations produced at phase transitions (e.g., QCD) or by features in the inflation potential can trigger primordial black hole (PBH) formation if their amplitudes are δ > ∼ 30%. We show that a PBH accumulates over time a sufficiently massive and compact minihalo to be able to modify or dominate its microlensing magnification light curve. Perturbations of amplitude 0.03% < ∼ δ < ∼ 30% are too small to form PBHs, but can nonetheless seed the growth of ultracompact minihalos. Thus, the likelihood of ultracompact minihalos as MACHOs is greater than that of PBHs. In addition, depending on their mass, they may be sites of formation of the first Population III stars.Ultracompact minihalos and PBHs produce a microlensing light curve that can be distinguished from that of a "point-like" object if high-quality photometric data are taken for a sufficiently long time after the peak of the magnification event. This enables them to be detected below the stellar-lensing "background" toward both the Magellanic Clouds and the Galactic bulge.