The transformation of atomic hydrogen to molecular hydrogen through three-body reactions is a crucial stage in the collapse of primordial, metal-free halos, where the first generation of stars (Population III stars) in the Universe are formed. However, in the published literature, the rate coefficient for this reaction is uncertain by nearly an order of magnitude. We report on the results of both adaptive mesh refinement (AMR) and smoothed particle hydrodynamics (SPH) simulations of the collapse of metal-free halos as a function of the value of this rate coefficient. For each simulation method, we have simulated a single halo three times, using three different values of the rate coefficient. We find that while variation between halo realizations may be greater than that caused by the three-body rate coefficient being used, both the accretion physics onto Population III protostars as well as the long-term stability of the disk and any potential fragmentation may depend strongly on this rate coefficient.
Astrophysical models of primordial star formation require rate constants for three-body recombination as input. The current status of these rates for H2 due to collisions with H is far from satisfactory, with published rate constants showing orders of magnitude disagreement at the temperatures relevant for H2 formation in primordial gas. This letter presents an independent calculation of this recombination rate constant as a function of temperature. An analytic expression is provided for the rate constant which should be more reliable than ones currently being used in astrophysical models.
Observational Manifestations of Primordial Objects in the Early Universe Through the Hydrogen Subordinate Lines
Special Astrophysical Observatory, Russian Academy of Sciences（俄罗斯科学院特别天文台）；Institute for Nuclear Research of the Russian Academy of Sciences（俄罗斯科学院核物理研究所）
We present a state-of-the-art primordial recombination code, HyRec, including all the physical effects that have been shown to significantly affect recombination. The computation of helium recombination includes simple analytic treatments of hydrogen continuum opacity in the He I 2 1P - 1 1S line, the He I] 2 3P - 1 1S line, and treats feedback between these lines within the on-the-spot approximation. Hydrogen recombination is computed using the effective multilevel atom method, virtually accounting for an infinite number of excited states. We account for two-photon transitions from 2s and hig…
Within the framework of a flat cosmological model a propagation of an instantaneous burst of isotropic radiation is considered from the moment of its beginning at some initial redshift z0 to the moment of its registration now (at z=0). We take into account Thomson scattering by free electrons and scattering in La and Lb lines of primordial hydrogen and in lines 1s2 - 1s2p, 1s3p (1S-1P*) of HeI. It is shown that relative amplitude of spectrum distortions caused by scattering in these lines may be from 1000 to 10000 times greater than maximum possible amplitude due to scattering in subordinate l…
Within the framework of a flat cosmological model a propagation of an instantaneous burst of isotropic radiation is considered from the moment of its beginning at some initial redshift z0 to the moment of its registration now (at z=0). Thomson scattering by free electrons and scattering in primordial hydrogen lines Ha, Hb, Pa and Pb are considered as the sources of opacity and when calculating an albedo of a single scattering in the lines we take into account deactivation of the upper levels of transitions by background blackbody radiation. Profiles for these lines in a burst spectrum are calc…
The large-scale structure of the Universe supplies crucial information about the physical processes at play at early times. Unresolved maps of the intensity of 21 cm emission from neutral hydrogen HI at redshifts z~1-5 are the best hope of accessing the ultralarge-scale information, directly related to the early Universe. A purpose-built HI intensity experiment may be used to detect the large scale effects of primordial non-Gaussianity, placing stringent bounds on different models of inflation. We argue that it may be possible to place tight constraints on the non-Gaussianity parameter f_NL, w…
We study the effects of sudden change in the sound velocity on primordial curvature perturbation spectrum in inflationary cosmology, assuming that the background evolution satisfies the slow-roll condition throughout. It is found that the power spectrum acquires oscillating features which are determined by the ratio of the sound speed before and after the transition and the wavenumeber which crosses the sound horizon at the transition, and their analytic expression is given. In some values of those parameters, the oscillating primordial power spectrum can better fit the observed Cosmic Microwa…
The calculation of a highly accurate cosmological recombination history has been the object of particular attention recently, as it constitutes the major theoretical uncertainty when predicting the angular power spectrum of Cosmic Microwave Background anisotropies. Lyman transitions, in particular the Lyman-alpha line, have long been recognized as one of the bottlenecks of recombination, due to their very low escape probabilities. The Sobolev approximation does not describe radiative transfer in the vicinity of Lyman lines to a sufficient degree of accuracy, and several corrections have alread…
An absorption dip in the spectrum of the cosmic microwave background observed by the EDGES experiment suggests an unexplained reduction of the hydrogen spin temperature at cosmic redshift z ~ 17. The mass of dark-matter axions could correspond to the hyperfine splitting of 5.9 micro-eV, between the triplet (H1) and singlet (H0) state. We calculate the rate for a+ H0 <-> H1 in two ways, and find that it is orders of magnitude smaller than the CMB-mediated transition rate, so irrelevant. As a result, this process cannot be used to rule in or out dark matter axions of mass = hyperfine splitting. …
In massive primordial galaxies, the gas may directly collapse and form a single central massive object if cooling is suppressed. Line cooling by molecular hydrogen can be suppressed in the presence of a strong soft-ultraviolet radiation field, but the role played by other cooling mechanisms is less clear. In optically thin gas, Lyman-Alpha cooling can be very effective, maintaining the gas temperature below 10^4 K over many orders of magnitude in density. However, the large neutral hydrogen column densities present in primordial galaxies render them highly optically thick to Lyman-Alpha photon…
Cosmological constraints from the cosmic microwave background (CMB) anisotropies rely on accurate theoretical calculations of the cosmic recombination history. Recent work has emphasized the importance of radiative transfer calculations due to the high optical depth in the HI Lyman lines. Transfer in the Lyman-alpha line is dominated by true emission and absorption, Hubble expansion, and resonant scattering. Resonant scattering causes photons to diffuse in frequency due to random kicks from the thermal velocities of hydrogen atoms, and also to drift toward lower frequencies due to energy loss …
Cosmological hydrogen recombination has recently been the subject of renewed attention because of its importance for predicting the power spectrum of cosmic microwave background anisotropies. It has become clear that it is necessary to account for a large number n >~ 100 of energy shells of the hydrogen atom, separately following the angular momentum substates in order to obtain sufficiently accurate recombination histories. However, the multi-level atom codes that follow the populations of all these levels are computationally expensive, limiting recent analyses to only a few points in paramet…
The first billion years in the evolution of the Universe mark the formation of the first stars, black holes and galaxies. The radiation from the first galaxies plays an important role in determining the final state of primordial gas collapsing in a neighboring halo. This is due to the fact that the primary coolant for primordial gas is molecular hydrogen, which can be dissociated into atomic hydrogen by Lyman-Werner photons in the energy range $11.2 - 13.6$~eV. While cooling by molecular hydrogen leads to Pop. III star formation, cooling by atomic hydrogen can lead to the formation of a superm…
Numerous cosmological simulations have been performed to study the formation of the first objects. We present the results of high resolution 3-D cosmological simulations of primordial objects formation using the adaptive mesh refinement code FLASH by including in an approximate manner the radiative transfer effects of Lyman alpha photons. We compare the results of a Lyman alpha trapping case inside gas clouds with atomic and molecular hydrogen cooling cases.The principal objective of this research is to follow the collapse of a zero metallicity halo with an effective equation of state (that ac…
We explore the conditions prevailing in primordial planets in the framework of the HGD cosmologies as discussed by Gibson and Schild. The initial stages of condensation of planet-mass H-4He gas clouds in trillion-planet clumps is set at 300,000 yr (0.3My) following the onset of plasma instabilities when ambient temperatures were >1000K. Eventual collapse of the planet-cloud into a solid structure takes place against the background of an expanding universe with declining ambient temperatures. Stars form from planet mergers within the clumps and die by supernovae on overeating of planets. For pl…
Recent spacecraft observations exploring solar system properties impact standard paradigms of the formation of stars, planets and comets. We stress the unexpected cloud of microscopic dust resulting from the DEEP IMPACT mission, and the existence of molten nodules in STARDUST samples. And the theory of star formation does not explain the common occurrence of binary and multiple star systems in the standard gas fragmentation scenario. No current theory of planet formation can explain the iron core of the earth, under oceans of water. These difficulties are avoided in a scenario where the planet…
We argue that the global signal of neutral hydrogen 21cm line can be a powerful probe of primordial power spectrum on small scales. Since the amplitude of small scale primordial fluctuations is important to determine the early structure formation and the timing when the sources of Lyman $α$ photons are produced, they in turn affect the neutral hydrogen 21cm line signal. We show that the information of the position of the absorption trough can severely constrain the small scale amplitude of primordial fluctuations once astrophysical parameters relevant to the 21cm line signal are fixed. We also…
In low-energy effective string theory and modified gravity theories, the propagating speed $c_T$ of primordial gravitational waves may deviate from unity. We find that the step-like variation of $c_T$ during slow-roll inflation may result in an oscillating modulation to the B-mode polarization spectrum, which can hardly be imitated by adjusting other cosmological parameters, and the intensity of the modulation is determined by the dynamics of $c_T$. Thus provided that the foreground contribution is under control, high-precision CMB polarization observations will be able to put tight constraint…
Population III stars are the first stars in the Universe to form at z=20-30 out of a pure hydrogen and helium gas in minihalos of 10^5-10^6 M$_\odot$ . Cooling and fragmentation is thus regulated via molecular hydrogen. At densities above 10^8 cm$^{-3}$, the three-body H2 formation rates are particularly important for making the gas fully molecular. These rates were considered to be uncertain by at least a few orders of magnitude. We explore the impact of new accurate three-body H2 formation rates derived by Forrey (2013) for three different minihalos, and compare to the results obtained with …
Thermal inflation, a brief low energy inflation after the primordial inflation, resolves the moduli problem in the context of supersymmetric cosmology. In the thermal inflation scenario, the primordial power spectrum is modestly redshifted on large scales, but suppressed by a factor of 1/50 on scales smaller than the horizon size at the beginning of thermal inflation. We compare the thermal inflation model with the warm dark matter and $Λ$CDM scenarios by studying CMB spectral distortions, halo abundances, and 21cm hydrogen lines.
Current theory and measurements establish the age of the universe as ca. 13.8 billion years. For the first several hundred million years of its existence, it was a dark, opaque void. After that, the hydrogen atoms comprising most of the "ordinary" matter began to condense and ionize, eventually forming the first stars that would illuminate the sky. Details of how these "primordial" stars formed have been widely debated, but remain elusive. A central issue in this process is the mechanism by which the primordial gas (mainly hydrogen and helium atoms) collected via the action of dark matter cool…
The success of primordial nucleosynthesis has been limited by the long-standing Lithium problem. We use a self-consistent perturbative analysis of the effects of relevant theoretical parameters on primordial nucleosynthesis, including variations of nature's fundamental constants, to explore the problem and its possible solutions, in the context of the latest observations and theoretical modeling. We quantify the amount of depletion needed to solve the Lithium problem, and show that transport processes of chemical elements in stars are able to account for it. Specifically, the combination of at…
Primordial nucleosynthesis is an observational cornerstone of the Hot Big Bang model and a sensitive probe of physics beyond the standard model. Its success has been limited by the so-called Lithium problem, for which many solutions have been proposed. We report on a self-consistent perturbative analysis of the effects of variations in nature's fundamental constants, which are unavoidable in most extensions of the standard model, on primordial nucleosynthesis, focusing on a broad class of Grand Unified Theory models. A statistical comparison between theoretical predictions and observational me…
The success of primordial nucleosynthesis as a cornerstone of the hot Big Bang model has been limited by the long-standing lithium problem. Recent work presented a self-consistent perturbative analysis of the effects of variations in nature's fundamental constants on primordial nucleosynthesis for a broad class of grand unified theory models, showing that such models provide a possible solution to the lithium problem, provided the value of the fine-structure constant $α$ at the nucleosynthesis epoch is larger than the current laboratory one by a few parts per million of relative variation. Her…
In the Simulations and Constructions of the Reionization of Cosmic Hydrogen (SCORCH) project, we compare analytical models of the hydrogen ionization fraction with radiation-hydrodynamic simulations. We derive analytical models of the mass-weighted hydrogen ionization fraction from the local ionization balance equations as a more accurate alternative to the widely adopted model based on the volume filling factor. In particular, our model has a recombination term quadratic in the ionization fraction, which is consistent with the two-body interaction nature of recombination. Then, we use the rad…
If light enough primordial black holes (PBH) account for dark matter, then its density decreases with time as they lose mass via Hawking radiation. We show that this time-dependence of the matter density can be formulated as an equivalent $w(z)$ dark energy model and we study its implications on the expansion history. Using our approach and comparing with the latest cosmological data, including the supernovae type Ia, Baryon Acoustic Oscillations, Cosmic Microwave Background and the Hubble expansion H(z) data, we place observational constraints on the PBH model. We find that it is statisticall…
Rovibronic molecular hydrogen (H$_2$) transitions at redshift $z_{\rm abs} \simeq 2.659$ towards the background quasar B0642$-$5038 are examined for a possible cosmological variation in the proton-to-electron mass ratio, $μ$. We utilise an archival spectrum from the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of $\sim$35 per 2.5-km$\,$s$^{-1}$ pixel at the observed H$_2$ wavelengths (335--410 nm). Some 111 H$_2$ transitions in the Lyman and Werner bands have been identified in the damped Lyman $α$ system for which a kinetic gas temperature of $…
Hydrogravitional-dynamics (HGD) cosmology of Gibson/Schild 1996 predicts that the primordial H-He^4 gas of big bang nucleosynthesis became proto-globular-star-cluster clumps of Earth-mass planets at 300 Kyr. The first stars formed from mergers of these 3000 K gas planets. Chemicals C, N, O, Fe etc. created by stars and supernovae then seeded many of the reducing hydrogen gas planets with oxides to give them hot water oceans with metallic iron-nickel cores. Water oceans at critical temperature 647 K then hosted the first organic chemistry and the first life, distributed to the 10^80 planets of …
We present a new determination of the primordial helium mass fraction Yp, based on 93 spectra of 86 low-metallicity extragalactic HII regions, and taking into account the latest developments concerning systematic effects. These include collisional and fluorescent enhancements of HeI recombination lines, underlying HeI stellar absorption lines, collisional and fluorescent excitation of hydrogen lines and temperature and ionization structure of the HII region. Using Monte Carlo methods to solve simultaneously for the above systematic effects, we find the best value to be Yp=0.2565+/-0.0010(stat.…
We study synergies between HI 21cm and H$α$ intensity map observations, focusing on SKA1-like and SPHEREx-like surveys. We forecast how well such a combination can measure features in the angular power spectrum on the largest scales, that arise from primordial non-Gaussianity and from general relativistic effects. For the first time we consider Doppler, Sachs-Wolfe and integrated SW effects separately. We confirm that the single-tracer surveys on their own cannot detect general relativistic effects and can constrain the non-Gaussianity parameter $f_{\rm NL}$ only slightly better than Planck. U…
Molecular hydrogen is now understood to be the main coolant of the primordial gas clouds leading to the formation of the very first stars and galaxies. The line emissions associated with molecular hydrogen should then be a good tracer of the matter distribution at the onset of reionization of the universe. Here we propose intensity mapping of H2 line emission in rest-frame mid-infrared wavelengths to map out the spatial distribution of gas at redshifts z > 10. We calculate the expected mean intensity and clustering power spectrum for several H2 lines. We find that the 0-0S(3) rotational line a…
We perform a set of cosmological simulations of early structure formation with incorporating baryonic streaming motions. We present a case where a significantly elongated gas cloud with $\sim\!10^4\,$solar masses (${\rm M_\odot}$) is formed in a pre-galactic ($\sim\!10^7\,{\rm M_\odot}$) dark halo. The gas streaming into the halo compresses and heats the massive filamentary cloud to a temperature of $\sim\!10,000\,$Kelvin. The gas cloud cools rapidly by atomic hydrogen cooling, and then by molecular hydrogen cooling down to $\sim\!400\,$Kelvin. The rapid decrease of the temperature and hence o…
Mapping Cosmic Dawn with 21-cm tomography offers an exciting new window into the era of primordial star formation. However, self-consistent implementation of both the process of star formation and the related 21-cm signal is challenging, due to the multi-scale nature of the problem. In this study, we develop a flexible semi-analytical model to follow the formation of the first stars and the process of gradual transition from primordial to metal-enriched star formation. For this transition we use different in scenarios with varying time-delays (or recovery times) between the first supernovae an…
Molecular hydrogen transitions in the sub-damped Lyman alpha absorber at redshift z = 2.69, toward the background quasar SDSS J123714.60+064759.5, were analyzed in order to search for a possible variation of the proton-to-electron mass ratio mu over a cosmological time-scale. The system is composed of three absorbing clouds where 137 H2 and HD absorption features were detected. The observations were taken with the Very Large Telescope/Ultraviolet and Visual Echelle Spectrograph with a signal-to-noise ratio of 32 per 2.5 km/s pixel, covering the wavelengths from 356.6 to 409.5 nm. A comprehensi…
Dark matter may consist, at least partially, of primordial black holes formed during the radiation-dominated era. The radiation produced by accretion onto primordial black holes leaves characteristic signatures on the properties of the medium at high redshift, before and after Hydrogen recombination. Therefore, reliable modelling of accretion onto these objects is required to obtain robust constraints on their abundance. We investigate the effect of mechanical feedback, i.e. the impact of outflows (winds and/or jets) on the medium, on primordial black hole accretion, and thereby on the associa…
We study the thermal evolution of primordial star-forming gas clouds using three-dimensional cosmological simulations. We critically examine how assumptions and approximations made in calculating radiative cooling rates affect the dynamics of the collapsing gas clouds. We consider two important molecular hydrogen cooling processes that operate in a dense primordial gas; H_2 line cooling and continuum cooling by H_2 collision-induced emission. To calculate the optically thick cooling rates, we follow the Sobolev method for the former, whereas we perform ray-tracing for the latter. We also run t…
Globular clusters are dense stellar systems that have typical ages of ~13 billion years, implying that they formed at redshifts of z>~6. Massive stars in newly formed or primordial globular clusters could have played an important role during the epoch of cosmological reionisation (z>~6) as sources of energetic, neutral hydrogen ionising UV photons. We investigate whether or not these stars could have been as important in death as sources of energetic X-ray photons as they were during their main sequence lives. Most massive stars are expected to form in binaries, and an appreciable fraction of …
Observational measures of the primordial helium mass fraction, $Y_{\rm P}$, are of interest for cosmology and fundamental particle physics. Current measures obtained from H II regions agree with the Standard Model prediction to approximately 1% precision, although these determinations may be affected by systematic uncertainties. This possibility can only be tested by independently measuring the helium abundance in new ways. Here, we propose a novel method to obtain a measurement of $Y_{\rm P}$ using hydrogen and helium recombination line emission from RELHICs: pristine, gas-rich but star-free …
The radiative cooling of shocked gas with primordial chemical composition is an important process relevant to the formation of the first stars and structures, as well as taking place also in high velocity cloud collisions and supernovae explosions. Among the different processes that need to be considered, the formation kinetics and cooling of molecular hydrogen are of prime interest, since they provide the only way to lower the gas temperature to values well below $\sim$10$^4$~K. In previous works, the internal energy level structure of H$_2$ and its cation has been treated in the approximatio…
The collapse of the primordial gas in the density regime $\sim 10^{8}\hbox{--}10^{10}$ cm$^{-3}$ is controlled by the three-body $\rm H_2$ formation process, in which the gas can cool faster than free-fall time $\hbox{--}$ a condition proposed as the chemothermal instability. We investigate how the heating and cooling rates are affected during the rapid transformation of atomic to molecular hydrogen. With a detailed study of the heating and cooling balance in a 3D simulation of Pop~III collapse, we follow the chemical and thermal evolution of the primordial gas in two dark matter minihaloes. T…
This work investigates the formation of primordial black holes within a radiation fluid with an anisotropic pressure. We focus our attention on the initial conditions describing cosmological perturbations in the super horizon regime, using a covariant form of the equation of state in terms of pressure and energy density gradients. The effect of the anisotropy is to modify the initial shape of the cosmological perturbations with respect to the isotropic case. Using the dependence of the threshold $δ_\mathrm{c}$ for primordial black holes with respect to the shape of cosmological perturbations, …
The most up to date femto- and micro-lensing constraints indicate that primordial black holes of $\sim 10^{-16} M_\odot$ and $\sim 10^{-12} M_\odot$, respectively, may constitute a large fraction of the dark matter. We describe analytically and numerically the dynamics by which inflationary fluctuations featuring a time-varying propagation speed or an effective Planck mass can lead to abundant primordial black hole production. As an example, we provide an ad hoc DBI-like model. A very large primordial spectrum originating from a small speed of sound typically leads to strong coupling within th…
