arXiv subjects

Yacine Ali-Haïmoud

Publications and source records attributed to Yacine Ali-Haïmoud.

53 records · Page 3Linked to original sources

arXiv2015

Heating of Baryons due to Scattering with Dark Matter During the Dark Ages

We explore the effects of elastic scattering between dark matter and baryons on the 21-cm signal during the dark ages. In particular, we consider a dark-matter---baryon interaction with a cross section of the form $σ= σ_0 v^{-4}$, in which case the effect of the drag force between the dark matter and baryon fluids grows with time. We show that, as opposed to what was previously thought, this effect heats up the baryons due to the relative velocity between dark matter and baryons. This creates an additional source of fluctuations, which can potentially make interactions easier to detect by 21-cm measurements than by using the cosmic microwave background and the Lyman-$α$ forest. Our forecasts show that the magnitude of the cross section can be probed to $σ_0\sim 3\times 10^{-42}$ cm$^2$ for $m_χ\ll 1$ GeV and $σ_0\sim 2 \times 10^{-41}\ (m_χ/10\, \rm GeV)$ cm$^2$ for $m_χ\gg 1$ GeV with next generation experiments, and improved to $σ_0\sim 4\times 10^{-44}$ cm$^2$ for $m_χ \ll 1$ GeV and $σ_0\sim 4 \times 10^{-43}\ (m_χ/10\, \rm GeV)$ cm$^2$ for $m_χ\gg 1$ GeV with futuristic experiments.

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arXiv2015

Primordial non-gaussianity from the bispectrum of 21-cm fluctuations in the dark ages

A measurement of primordial non-gaussianity will be of paramount importance to distinguish between different models of inflation. Cosmic microwave background (CMB) anisotropy observations have set unprecedented bounds on the non-gaussianity parameter f_NL but the interesting regime f_NL <~ 1 is beyond their reach. Brightness-temperature fluctuations in the 21-cm line during the dark ages (z ~ 30-100) are a promising successor to CMB studies, giving access to a much larger number of modes. They are, however, intrinsically non-linear, which results in secondary non-gaussianities orders of magnitude larger than the sought-after primordial signal. In this paper we carefully compute the primary and secondary bispectra of 21-cm fluctuations on small scales. We use the flat-sky formalism, which greatly simplifies the analysis, while still being very accurate on small angular scales. We show that the secondary bispectrum is highly degenerate with the primordial one, and argue that even percent-level uncertainties in the amplitude of the former lead to a bias of order Delta f_NL ~ 10. To tackle this problem we carry out a detailed Fisher analysis, marginalizing over the amplitudes of a few smooth redshift-dependent coefficients characterizing the secondary bispectrum. We find that the signal-to-noise ratio for a single redshift slice is reduced by a factor of ~5 in comparison to a case without secondary non-gaussianities. Setting aside foreground contamination, we forecast that a cosmic-variance-limited experiment observing 21-cm fluctuations over 30 < z < 100 with a 0.1-MHz bandwidth and 0.1-arcminute angular resolution could achieve a sensitivity of order f_NL[local] ~ 0.03, f_NL[equilateral] ~ 0.04, and f_NL[orthogonal] ~ 0.03.

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arXiv2015

Constraints on dark matter interactions with standard model particles from CMB spectral distortions

We propose a new method to constrain elastic scattering between dark matter (DM) and standard model particles in the early Universe. Direct or indirect thermal coupling of non-relativistic DM with photons leads to a heat sink for the latter. This results in spectral distortions of the cosmic microwave background (CMB), the amplitude of which can be as large as a few times the DM-to-photon number ratio. We compute CMB spectral distortions due to DM-proton, DM-electron and DM-photon scattering for generic energy-dependent cross sections and DM mass m_DM >~ 1 keV. Using FIRAS measurements we set constraints on the cross sections for m_DM <~ 0.1 MeV. In particular, for energy-independent scattering we obtain sigma[DM-proton] <~ 10^(-24) cm^2 (keV/m_DM)^(1/2), sigma[DM-electron] <~ 10^(-27) cm^2 (keV/m_DM)^(1/2) and sigma[DM-photon] <~ 10^(-39) cm^2 (m_DM/keV). An experiment with the characteristics of PIXIE would extend the regime of sensitivity up to masses m_DM ~ 1 GeV.

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arXiv2015

Perturbative interaction approach to cosmological structure formation

A new approach to cosmological perturbation theory has been recently introduced by Bartelmann et al., relying on nonequilibrium statistical theory of classical particles, and treating the gravitational interaction perturbatively. They compute analytic expressions for the nonlinear matter power spectrum, to first order in the interaction, and at one-loop order in the linear power spectrum. The resulting power spectrum is well behaved even at large wavenumbers and seems in good agreement with results from numerical simulations. In this paper, we rederive their results concisely with a different approach, starting from the implicit integral solution to particle trajectories. We derive the matter power spectrum to first order in the interaction, but to arbitrary order in the linear power spectrum, from which the one-loop result follows. We also show that standard linear perturbation theory can only be recovered at infinite order in the gravitational interaction. At finite order in the interaction, we find that the linear power spectrum is systematically and significantly underestimated. A comprehensive study of the convergence of the theory with the order of the interaction for nonlinear scales will be the subject of future work.

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arXiv2014

Search for PAHs in the Perseus molecular cloud with the Green Bank Telescope

Polycyclic Aromatic Hydrocarbons (PAHs) are believed to be the small-size tail of the interstellar carbonaceous dust grain population. Their vibrational emission is the most widely accepted source of the aromatic near-infrared features, and their rotational radiation is a likely explanation for the dust-correlated anomalous microwave emission (AME). Yet, no individual interstellar PAH molecule has been identified to date. It was recently recognized that quasi-symmetric planar PAHs ought to have a well identifiable comb-like rotational spectrum, and suggested to search for them in spectroscopic data with matched-filtering techniques. We report the results of the first such search, carried out with the Green Bank Telescope, and targeting the star-forming region IC348 in the Perseus molecular cloud, a known source of AME. Our observations amounted to 16.75 hours and spanned a 3 GHz-wide band extending from 23.3 to 26.3 GHz. Using frequency switching, we achieved a sensitivity of 0.4 mJy per 0.4 MHz channel (1-sigma). The non-detection of comb-like spectra allowed us to set upper bounds on the abundance of specific quasi-symmetric PAH molecules (specified uniquely by their moments of inertia) of approximately 0.1% of the total PAH abundance. This bound generically applies to PAHs with approximately 15 to 100 carbon atoms.

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arXiv2013

A new light on 21 cm intensity fluctuations from the dark ages

Fluctuations of the 21 cm brightness temperature before the formation of the first stars hold the promise of becoming a high-precision cosmological probe in the future. The growth of over densities is very well described by perturbation theory at that epoch and the signal can in principle be predicted to arbitrary accuracy for given cosmological parameters. Recently, Tseliakhovich and Hirata pointed out a previously neglected and important physical effect, due to the fact that baryons and cold dark matter (CDM) have supersonic relative velocities after recombination. This relative velocity suppresses the growth of matter fluctuations on scales $k \sim 10-10^3$ Mpc$^{-1}$. In addition, the amplitude of the small-scale power spectrum is modulated on the large scales over which the relative velocity varies, corresponding to $k \sim 0.005-1$ Mpc$^{-1}$. In this paper, the effect of the relative velocity on 21 cm brightness temperature fluctuations from redshifts $z \geq 30$ is computed. We show that the 21 cm power spectrum is affected on ${\it most}$ scales. On small scales, the signal is typically suppressed several tens of percent, except for extremely small scales ($k \gtrsim 2000$ Mpc$^{-1}$) for which the fluctuations are boosted by resonant excitation of acoustic waves. On large scales, 21 cm fluctuations are enhanced due to the non-linear dependence of the brightness temperature on the underlying gas density and temperature. The enhancement of the 21 cm power spectrum is of a few percent at $k \sim 0.1$ Mpc$^{-1}$ and up to tens of percent at $k \lesssim 0.005$ Mpc$^{-1}$, for standard $Λ$CDM cosmology. In principle this effect allows to probe the small-scale matter power spectrum not only through a measurement of small angular scales but also through its effect on large angular scales.

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arXiv2013

Rotational spectroscopy of interstellar PAHs

Polycyclic aromatic hydrocarbons (PAHs) are believed to be ubiquitous in the interstellar medium. Yet, to date no specific PAH molecule has been identified. In this paper, a new observational avenue is suggested to detect individual PAHs, using their rotational line emission at radio frequencies. Previous PAH searches based on rotational spectroscopy have only targeted the bowl-shaped corannulene molecule, with the underlying assumption that other polar PAHs are triaxial and have a complex and diluted spectrum unusable for identification purposes. In this paper the rotational spectrum of quasi-symmetric PAHs is computed analytically. It is shown that the asymmetry of planar, nitrogen-substituted symmetric PAHs is small enough that their rotational spectrum, when observed with a resolution of about a MHz, has the appearance of a "comb" of evenly spaced stacks of lines. The simple pattern of these "comb" spectra allows for the use of matched-filtering techniques, which can result in a significantly enhanced signal-to-noise ratio. Detection forecasts are discussed for regions harbouring "anomalous microwave emission", believed to originate from the collective PAH rotational emission. A systematic search for PAH lines in various environments is advocated. If detected, PAH "combs" would allow to the conclusive and unambiguous identification of specific, free-floating interstellar PAHs.

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arXiv2012

CMB photons shedding light on dark matter

The annihilation or decay of Dark Matter (DM) particles could affect the thermal history of the universe and leave an observable signature in Cosmic Microwave Background (CMB) anisotropies. We update constraints on the annihilation rate of DM particles in the smooth cosmological background, using WMAP7 and recent small-scale CMB data. With a systematic analysis based on the Press-Schechter formalism, we also show that DM annihilation in halos at small redshift may explain entirely the reionization patterns observed in the CMB, under reasonable assumptions concerning the concentration and formation redshift of halos. We find that a mixed reionization model based on DM annihilation in halos as well as star formation at a redshift z~6.5 could simultaneously account for CMB observations and satisfy constraints inferred from the Gunn-Peterson effect. However, these models tend to reheat the inter-galactic medium (IGM) well above observational bounds: by including a realistic prior on the IGM temperature at low redshift, we find stronger cosmological bounds on the annihilation cross-section than with the CMB alone.

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arXiv2012

An efficient implementation of massive neutrinos in non-linear structure formation simulations

Massive neutrinos make up a fraction of the dark matter, but due to their large thermal velocities, cluster significantly less than cold dark matter (CDM) on small scales. An accurate theoretical modelling of their effect during the non-linear regime of structure formation is required in order to properly analyse current and upcoming high-precision large-scale structure data, and constrain the neutrino mass. Taking advantage of the fact that massive neutrinos remain linearly clustered up to late times, this paper treats the linear growth of neutrino overdensities in a non-linear CDM background. The evolution of the CDM component is obtained via N-body computations. The smooth neutrino component is evaluated from that background by solving the Boltzmann equation linearised with respect to the neutrino overdensity. CDM and neutrinos are simultaneously evolved in time, consistently accounting for their mutual gravitational influence. This method avoids the issue of shot-noise inherent to particle-based neutrino simulations, and, in contrast with standard Fourier-space methods, properly accounts for the non-linear potential wells in which the neutrinos evolve. Inside the most massive late-time clusters, where the escape velocity is larger than the neutrino thermal velocity, neutrinos can clump non-linearly, causing the method to formally break down. It is shown that this does not affect the total matter power spectrum, which can be very accurately computed on all relevant scales up to the present time.

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arXiv2012

Effective conductance method for the primordial recombination spectrum

As atoms formed for the first time during primordial recombination, they emitted bound-bound and free-bound radiation leading to spectral distortions to the cosmic microwave background. These distortions might become observable in the future with high-sensitivity spectrometers, and provide a new window into physical conditions in the early universe. The standard multilevel atom method habitually used to compute the recombination spectrum is computationally expensive, impeding a detailed quantitative exploration of the information contained in spectral distortions thus far. In this work it is shown that the emissivity in optically thin allowed transitions can be factored into a computationally expensive but cosmology-independent part and a computationally cheap, cosmology-dependent part. The slow part of the computation consists in pre-computing temperature-dependent effective "conductances", linearly relating line or continuum intensity to departures from Saha equilibrium of the lowest-order excited states (2s and 2p), that can be seen as "voltages". The computation of these departures from equilibrium as a function of redshift is itself very fast, thanks to the effective multilevel atom method introduced in an earlier work. With this factorization, the recurring cost of a single computation of the recombination spectrum is only a fraction of a second on a standard laptop, more than four orders of magnitude shorter than standard computations. The spectrum from helium recombination can be efficiently computed in an identical way, and a fast code computing the full primordial recombination spectrum with this method will be made publicly available soon.

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arXiv2012

Spinning dust radiation: a review of the theory

This article reviews the current status of theoretical modeling of electric dipole radiation from spinning dust grains. The fundamentally simple problem of dust grain rotation appeals to a rich set of concepts of classical and quantum physics, owing the the diversity of processes involved. Rotational excitation and damping rates through various mechanisms are discussed, as well as methods of computing the grain angular momentum distribution function. Assumptions on grain properties are reviewed. The robustness of theoretical predictions now seems mostly limited by the uncertainties regarding the grains themselves, namely their abundance, dipole moments, size and shape distribution.

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arXiv2011

Slowly-rotating stars and black holes in dynamical Chern-Simons gravity

Chern-Simons (CS) modified gravity is an extension to general relativity (GR) in which the metric is coupled to a scalar field, resulting in modified Einstein field equations. In the dynamical theory, the scalar field is itself sourced by the Pontryagin density of the space-time. In this paper, the coupled system of equations for the metric and the scalar field is solved numerically for slowly-rotating neutron stars described with realistic equations of state and for slowly-rotating black holes. An analytic solution for a constant-density nonrelativistic object is also presented. It is shown that the black hole solution cannot be used to describe the exterior spacetime of a star as was previously assumed. In addition, whereas previous analysis were limited to the small-coupling regime, this paper considers arbitrarily large coupling strengths. It is found that the CS modification leads to two effects on the gravitomagnetic sector of the metric: (i) Near the surface of a star or the horizon of a black hole, the magnitude of the gravitomagnetic potential is decreased and frame-dragging effects are reduced in comparison to GR. (ii) In the case of a star, the angular momentum J, as measured by distant observers, is enhanced in CS gravity as compared to standard GR. For a large coupling strength, the near-zone frame-dragging effects become significantly screened, whereas the far-zone enhancement saturate at a maximum value max(Delta J) ~ (M/R) J. Using measurements of frame-dragging effects around the Earth by Gravity Probe B and the LAGEOS satellites, a weak but robust constraint is set to the characteristic CS lengthscale, xi^{1/4} <~ 10^8 km.

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arXiv2011

Revisiting the double-binary-pulsar probe of non-dynamical Chern-Simons gravity

One of the popular modifications to the theory of general relativity is non-dynamical Chern-Simons (CS) gravity, in which the metric is coupled to an externally prescribed scalar field. Setting accurate constraints to the parameters of the theory is important owing to their implications for the scalar field and/or the underlying fundamental theory. The current best constraints rely on measurements of the periastron precession rate in the double-binary-pulsar system and place a very tight bound on the characteristic CS lengthscale k_cs^{-1} <~ 3*10^{-9} km. This paper considers several effects that were not accounted for when deriving this bound and lead to a substantial suppression of the predicted rate of periastron precession. It is shown, in particular, that the point mass approximation for extended test bodies does not apply in this case. The constraint to the characteristic CS lengthscale is revised to k_cs^{-1} <~ 0.4 km, eight orders of magnitude weaker than what was previously found.

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arXiv2011

Metals at the surface of last scatter

Standard big-bang nucleosynthesis (BBN) predicts only a trace abundance of lithium and no heavier elements, but some alternatives predict a nonzero primordial metallicity. Here we explore whether CMB measurements may set useful constraints to the primordial metallicity and/or whether the standard CMB calculations are robust, within the tolerance of forthcoming CMB maps, to the possibility of primordial metals. Metals would affect the recombination history (and thus CMB power spectra) in three ways: (1) Lyman-alpha photons can be removed (and recombination thus accelerated) by photoionizing metals. (2) The Bowen resonance-fluorescence mechanism may degrade Lyman-beta photons and thus enhance the Lyman-beta escape probability and speed up recombination. (3) Metals could affect the low-redshift tail of the CMB visibility function by providing additional free electrons. The last two of these provide the strongest CMB signal. However, the effects are detectable in the Planck satellite only if the primordial metal abundance is at least a few hundredths of solar for (2) and a few tenths of solar for (3). We thus conclude that Planck will not be able to improve upon current constraints to primordial metallicity, at the level of a thousandth of solar, from the Lyman-alpha forest and ultra-metal-poor halo stars, and that the CMB power-spectrum predictions for Planck suffer no uncertainty arising from the possibility that there may be primordial metals.

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arXiv2011

HyRec: A fast and highly accurate primordial hydrogen and helium recombination code

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 higher levels as well as frequency diffusion in Lyman-alpha with a full radiative transfer calculation. We present a new method to evolve the radiation field simultaneously with the level populations and the free electron fraction. These computations are sped up by taking advantage of the particular sparseness pattern of the equations describing the radiative transfer. The computation time for a full recombination history is ~2 seconds. This makes our code well suited for inclusion in Monte Carlo Markov chains for cosmological parameter estimation from upcoming high-precision cosmic microwave background anisotropy measurements.

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arXiv2010

Radiative transfer effects in primordial hydrogen recombination

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 already been computed in other works. In this paper, the impact of some previously ignored radiative transfer effects is calculated. First, the effect of Thomson scattering in the vicinity of the Lyman-alpha line is evaluated, using a full redistribution kernel incorporated into a radiative transfer code. The effect of feedback of distortions generated by the optically thick deuterium Lyman-alpha line blueward of the hydrogen line is investigated with an analytic approximation. It is shown that both effects are negligible during cosmological hydrogen recombination. Secondly, the importance of high-lying, non overlapping Lyman transitions is assessed. It is shown that escape from lines above Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected without loss of accuracy. Thirdly, a formalism generalizing the Sobolev approximation is developed to account for the overlap of the high-lying Lyman lines, which is shown to lead to negligible changes to the recombination history. Finally, the possibility of a cosmological hydrogen recombination maser is investigated. It is shown that there is no such maser in the purely radiative treatment presented here.

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arXiv2010

Ultrafast effective multi-level atom method for primordial hydrogen recombination

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 parameter space. In this paper, we present a new method for solving the multi-level atom recombination problem, which splits the problem into a computationally expensive atomic physics component that is independent of the cosmology, and an ultrafast cosmological evolution component. The atomic physics component follows the network of bound-bound and bound-free transitions among excited states and computes the resulting effective transition rates for the small set of "interface" states radiatively connected to the ground state. The cosmological evolution component only follows the populations of the interface states. By pre-tabulating the effective rates, we can reduce the recurring cost of multi-level atom calculations by more than 5 orders of magnitude. The resulting code is fast enough for inclusion in Markov Chain Monte Carlo parameter estimation algorithms. It does not yet include the radiative transfer or high-n two-photon processes considered in some recent papers. Further work on analytic treatments for these effects will be required in order to produce a recombination code usable for Planck data analysis.

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