Searcharxiv⌕ Search

arXiv subjects

Denis Puy

Publications and source records attributed to Denis Puy.

10 recordsLinked to original sources
arXiv2024

21cm signal from Dark Ages collapsing halos with detailed molecular cooling treatment

Context. To understand the formation of the first stars, a detailed description of the thermal and chemical processes in collapsing gas clouds is essential. Molecular cooling, particularly via H2, plays a significant role in triggering thermal instabilities that lead to star formation. The 21 cm hydrogen line serves as a potential probe of the first collapsing structures during the dark ages of the early Universe, and it is affected by the gas temperature evolution. Aims. We aim to investigate the molecular cooling in the gas halos prior to the formation of the first stars, with a particular focus on how the H2 cooling affects the gas temperature. Additionally, we explore the sensitivity of the 21 cm hydrogen line to these cooling processes during the collapse of the first overdense regions. Results. We introduce the \texttt{CHEMFAST} code, which tracks the evolution of chemical abundances and computes the 21cm neutral hydrogen signal in collapsing halos. Our results show that molecular cooling significantly affects the gas temperature inside collapsing clouds of mass ranging from $10^6$ to $10^9$ M$_\odot$, influencing the 21cm signal. The signal exhibits an emission feature that is distinct from the one predicted in simpler expansion models. Conclusions. The 21cm brightness temperature inside collapsing clouds displays an emission feature driven by molecular cooling, closely mirroring the gas temperature evolution. This makes the dark-age 21cm signal a promising probe for studying the thermal processes and structure formation in the early Universe.

astro-ph.GA↗
arXiv2014

A critical assessment of some inhomogeneous pressure Stephani models

We consider spherically symmetric inhomogeneous pressure Stephani universes, the center of symmetry being our location. The main feature of these models is that comoving observers do not follow geodesics. In particular, comoving perfect fluids have necessarily a radially dependent pressure. We consider a subclass of these models characterized by some inhomogeneity parameter $\beta$. We show that also the velocity of sound, like the (effective) equation of state parameter, of comoving perfect fluids acquire away from the origin a time and radial dependent change proportional to $\beta$. In order to produce a realistic universe accelerating at late times without dark energy component one must take $\beta < 0$. The redshift gets a modified dependence on the scale factor $a(t)$ with a relative modification of $-9\%$ peaking at $z\sim 4$ and vanishing at the big-bang and today on our past lightcone. The equation of state parameter and the speed of sound of dustlike matter (corresponding to a vanishing pressure at the center of symmetry $r=0$) behave in a similar way and away from the center of symmetry they become negative -- a property usually encountered for the dark energy component only. In order to mimic the observed late-time accelerated expansion, the matter component must significantly depart from standard dust, presumably ruling this subclass of Stephani models out as a realistic cosmology. The only way to accept these models is to keep all standard matter components of the universe including dark energy and take an inhomogeneity parameter $\beta$ small enough.

gr-qc↗
arXiv2010

Limits on decaying dark energy density models from the CMB temperature-redshift relation

The nature of the dark energy is still a mystery and several models have been proposed to explain it. Here we consider a phenomenological model for dark energy decay into photons and particles as proposed by Lima (J. Lima, Phys. Rev. D 54, 2571 (1996)). He studied the thermodynamic aspects of decaying dark energy models in particular in the case of a continuous photon creation and/or disruption. Following his approach, we derive a temperature redshift relation for the CMB which depends on the effective equation of state $w_{eff}$ and on the "adiabatic index" $\gamma$. Comparing our relation with the data on the CMB temperature as a function of the redshift obtained from Sunyaev-Zel'dovich observations and at higher redshift from quasar absorption line spectra, we find $w_{eff}=-0.97 \pm 0.034$, adopting for the adiabatic index $\gamma=4/3$, in good agreement with current estimates and still compatible with $w_{eff}=-1$, implying that the dark energy content being constant in time.

astro-ph.CO↗
arXiv2007

Molecular fluorine chemistry in the early Universe

Some models of Big Bang nucleosynthesis suggest that very high baryon density regions were formed in the early Universe, and generated the production of heavy elements other than lithium such as fluorine F. We present a comprehensive chemistry of fluorine in the post-recombination epoch. Calculation of F, F- and HF abundances, as a function of redshift z, are carried out. The main result is that the chemical conditions in the early Universe can lead to the formation of HF. The final abundance of the diatomic molecule HF is predicted to be close to 3.75 10(-17) when the initial abundance of neutral fluorine F is 10(-15). These results indicate that molecules of fluorine HF were already present during the dark age. This could have implications on the evolution of proto-objects and on the anisotropies of cosmic microwave background radiation. Hydride of fluorine HF may affect enhancement of the emission line intensity from the proto-objects and could produce spectral-spatial fluctuations.

astro-ph↗
arXiv2002

Key observational tests of the modern cosmology

Over the last decade, cosmological observations have attained a level of precision which allows for detailed comparison with theoretical predictions. In this paper, we briefly review some studies of the current and prospected constraints imposed by measurements of the cosmic microwave background radiation, of the mass and of the expansion of the Universe.

physics.pop-ph↗
arXiv2001

The Dark Universe: The First Molecules

The dark Universe corresponds to the period between the thermal decoupling matter-radiation and the formation of the first objects. During this epoq, molecules can appeared. In this talk, I will recall the nucleosynthesis processes, then the chemistry of the primordial Universe. I will discuss the influence of the primordial molecules on the protocloud evolution .

physics.pop-ph↗
arXiv2000

Cosmological thermal decoupling and primordial molecules

Primordial chemistry began, at the recombination epoch, when the adiabatic expansion caused the temperature of the radiation to fall below 4000K. The chemistry of the early Universe involves the elements hydrogen, its isotope deuterium, helium with its isotopic forms and lithium. In this contribution I will discuss the influence of the primordial molecules on the cosmological decoupling. In the framework of the gravitational instability theory, each protostructure started as a tiny local overdensity. As long as these inhomogeneities are small, their evolution can be studied by the classical linear perturbation theory. Once the deviations become large, the linear theory is no more valid. we present the role played by these molecules on the transition between the linear regime and the non-linear regime, and show that the molecules can lead to a thermal change at the turn-around point between these two regimes.

astro-ph↗
arXiv2000

Cosmochemistry in the early Universe

At early times the Universe was filled up with an extremely dense and hot gas. Due to the expansion it cooled below the binding energies of atoms which led to the formation of the first nuclei. In the physical environment of the post-recombination period of hydrogen, molecules such as $H_2$, $HD$ and $LiH$ can be formed. The proto-object formation, resulting from the growth of linear density fluctuations in the early Universe, can have an important impact on the chemical state of the Universe. Hence it can be enriched with metals, and thus lead to the formation of the first pre-biotic molecules. In this contribution, I will present some scheme for the formation of primordial molecules and discuss the consequence of the formation of first stars on the existence of possible primordial pre-biotic.

astro-ph↗
arXiv1999

Evolution of primordial H2 for different cosmological models

Primordial chemistry began in the recombination epoch when the adiabatic expansion caused the temperature of the radiation to fall below 4000 K. The chemistry of the early Universe involves the elements hydrogen, its isotope deuterium, helium with its isotopic forms and lithium. In this communication I will present results on the evolution of the primordial $H_2$ abundance for different cosmological models and the influence on the thermal decoupling.

astro-ph↗