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Register a new userCosmology beyond BAO from the 3D distribution of the Lyman-$alpha$ forest
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We propose a new method for fitting the full-shape of the Lyman-$alpha$ (Ly$alpha$) forest three-dimensional (3D) correlation function in order to measure the Alcock-Paczynski (AP) effect. Our method preserves the robustness of baryon acoustic oscillations (BAO) analyses, while also providing extra cosmological information from a broader range of scales. We compute idealized forecasts for the Dark Energy Spectroscopic Instrument (DESI) using the Ly$alpha$ auto-correlation and its cross-correlation with quasars, and show how this type of analysis improves cosmological constraints. The DESI Ly$alpha$ BAO analysis is expected to measure $H(z_mathrm{eff})r_mathrm{d}$ and $D_mathrm{M}(z_mathrm{eff})/r_mathrm{d}$ with a precision of $sim0.9%$ each, where $H$ is the Hubble parameter, $r_mathrm{d}$ is the comoving BAO scale, $D_mathrm{M}$ is the comoving angular diameter distance and the effective redshift of the measurement is $z_mathrm{eff}simeq2.3$. By fitting the AP parameter from the full shape of the two correlations, we show that we can obtain a precision of $sim0.5-0.6%$ on each of $H(z_mathrm{eff})r_mathrm{d}$ and $D_mathrm{M}(z_mathrm{eff})/r_mathrm{d}$. Furthermore, we show that a joint full-shape analysis of the Ly$alpha$ auto-correlation and its cross-correlation with quasars can measure the linear growth rate times the amplitude of matter fluctuations in spheres of $8;h^{-1}$Mpc, $fsigma_8(z_mathrm{eff})$. Such an analysis could provide the first ever measurement of $fsigma_8(z_mathrm{eff})$ at redshift $z_mathrm{eff}>2$. By combining this with the quasar auto-correlation in a joint analysis of the three high-redshift two-point correlation functions, we show that DESI could be able to measure $fsigma_8(z_mathrm{eff}simeq2.3)$ with a precision of $5-12%$, depending on the smallest scale fitted.
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The statistical power of Lyman-${alpha}$ forest Baryon Acoustic Oscillation (BAO) measurements is set to increase significantly in the coming years as new instruments such as the Dark Energy Spectroscopic Instrument deliver progressively more constraining data. Generating mock datasets for such measurements will be important for validating analysis pipelines and evaluating the effects of systematics. With such studies in mind, we present LyaCoLoRe: a package for producing synthetic Lyman-${alpha}$ forest survey datasets for BAO analyses. LyaCoLoRe transforms initial Gaussian random field skewers into skewers of transmitted flux fraction via a number of fast approximations. In this work we explain the methods of producing mock datasets used in LyaCoLoRe, and then measure correlation functions on a suite of realisations of such data. We demonstrate that we are able to recover the correct BAO signal, as well as large-scale bias parameters similar to literature values. Finally, we briefly describe methods to add further astrophysical effects to our skewers - high column density systems and metal absorbers - which act as potential complications for BAO analyses.
We use the probability distribution function (PDF) of the lya forest flux at z=2-3, measured from high-resolution UVES/VLT data, and hydrodynamical simulations to obtain constraints on cosmological parameters and the thermal state of the intergalactic medium (IGM) at z 2-3. The observed flux PDF at z=3 alone results in constraints on cosmological parameters in good agreement with those obtained from the WMAP data, albeit with about a factor two larger errors. The observed flux PDF is best fit with simulations with a matter fluctuation amplitude of sigma_8=0.8-0.85 pm 0.07 and an inverted IGM temperature-density relation (gamma ~ 0.5-0.75), consistent with our previous results obtained using a simpler analysis. These results appear to be robust to uncertainties in the quasar (QSO) continuum placement. We further discuss constraints obtained by a combined analysis of the high-resolution flux PDF and the power spectrum measured from the Sloan Digital Sky Survey (SDSS) lya forest data. The joint analysis confirms the suggestion of an inverted temperature-density relation, but prefers somewhat higher values (sigma_8 ~ 0.9) of the matter fluctuation amplitude than the WMAP data and the best fit to the flux PDF alone. The joint analysis of the flux PDF and power spectrum (as well as an analysis of the power spectrum data alone) prefers rather large values for the temperature of the IGM, perhaps suggesting that we have identified a not yet accounted for systematic error in the SDSS flux power spectrum data or that the standard model describing the thermal state of the IGM at z ~ 2-3 is incomplete.
Absorption between the rest-frame wavelengths of 973 and 1026 Angstroms in quasar spectra arises from two sources (apart from occasional metals): one is due to Lyman-alpha (Lya) absorption by materials at a low redshift, and the other from Lyman-beta (Lyb) at a higher redshift. These two sources of absorption are to a good approximation uncorrelated because of their wide physical separation. Therefore, the two-point correlation of absorption in this region of quasar spectra neatly factorizes into two pieces: the Lyb correlation at high z, and the Lya correlation at low z. The latter can be independently measured from quasar spectra at lower redshifts using current techniques. A simple division then offers a way to statistically separate out the Lyb two-point correlation from the Lya correlation. Several applications of this technique are discussed. First, since the Lyb absorption cross-section is lower than Lya by about a factor of 5, the Lyb forest is a better probe of the intergalactic medium (IGM) at higher redshifts where Lya absorption is often saturated. Second, for the same reason, the Lyb forest allows a better measurement of the equation of state of the IGM at higher overdensities, yielding stronger constraints on its slope when used in conjunction with the Lya forest. Third, models of the Lya forest based on gravitational instability make unique predictions for the Lyb forest, which can be tested against observations. We briefly point out that feedback processes that affect higher density regions but leave low density structure intact may be better constrained by the Lyb forest.
We present constraints on neutrino masses, the primordial fluctuation spectrum from inflation, and other parameters of the $Lambda$CDM model, using the one-dimensional Ly$alpha$-forest power spectrum measured by Palanque-Delabrouille et al. (2013) from SDSS-III/BOSS, complemented by Planck 2015 cosmic microwave background (CMB) data and other cosmological probes. This paper improves on the previous analysis by Palanque-Delabrouille et al. (2015) by using a more powerful set of calibrating hydrodynamical simulations that reduces uncertainties associated with resolution and box size, by adopting a more flexible set of nuisance parameters for describing the evolution of the intergalactic medium, by including additional freedom to account for systematic uncertainties, and by using Planck 2015 constraints in place of Planck 2013. Fitting Ly$alpha$ data alone leads to cosmological parameters in excellent agreement with the values derived independently from CMB data, except for a weak tension on the scalar index $n_s$. Combining BOSS Ly$alpha$ with Planck CMB constrains the sum of neutrino masses to $sum m_ u < 0.12$ eV (95% C.L.) including all identified systematic uncertainties, tighter than our previous limit (0.15 eV) and more robust. Adding Ly$alpha$ data to CMB data reduces the uncertainties on the optical depth to reionization $tau$, through the correlation of $tau$ with $sigma_8$. Similarly, correlations between cosmological parameters help in constraining the tensor-to-scalar ratio of primordial fluctuations $r$. The tension on $n_s$ can be accommodated by allowing for a running ${mathrm d}n_s/{mathrm d}ln k$. Allowing running as a free parameter in the fits does not change the limit on $sum m_ u$. We discuss possible interpretations of these results in the context of slow-roll inflation.
The angular positions of quasars are deflected by the gravitational lensing effect of foreground matter. The Lyman-alpha forest seen in the spectra of these quasars is therefore also lensed. We propose that the signature of weak gravitational lensing of the forest could be measured using similar techniques that have been applied to the lensed Cosmic Microwave Background, and which have also been proposed for application to spectral data from 21cm radio telescopes. As with 21cm data, the forest has the advantage of spectral information, potentially yielding many lensed slices at different redshifts. We perform an illustrative idealized test, generating a high resolution angular grid of quasars (of order arcminute separation), and lensing the Lyman-alphaforest spectra at redshifts z=2-3 using a foreground density field. We find that standard quadratic estimators can be used to reconstruct images of the foreground mass distribution at z~1. There currently exists a wealth of Lya forest data from quasar and galaxy spectral surveys, with smaller sightline separations expected in the future. Lyman-alpha forest lensing is sensitive to the foreground mass distribution at redshifts intermediate between CMB lensing and galaxy shear, and avoids the difficulties of shape measurement associated with the latter. With further refinement and application of mass reconstruction techniques, weak gravitational lensing of the high redshift Lya forest may become a useful new cosmological probe.
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