[Paper Review] Towards Constraints on Dark Energy from Absorption Spectra of Close Quasar Pairs

Adam Lidz, Lam Hui+2 more|arXiv (Cornell University)|Sep 6, 2003

Adaptive optics and wavefront sensing3 citations

TL;DR

This paper proposes a novel, evolution-free method to constrain dark energy using the cross-power spectrum of Lyman-alpha forest absorption in close quasar pairs, comparing observed spectra with cosmological simulations that include redshift-space distortions and thermal broadening. Despite weak constraints from a small sample of five pairs, the method disfavors an Einstein-de-Sitter model at ~2σ, and future observations of ~50 moderate-resolution pairs could achieve 15% 2σ constraints on ΩΛ if systematics are controlled.

ABSTRACT

A comparison between the line of sight power spectrum of absorption in the Lyman-alpha forest and the cross power spectrum between the absorption in neighboring lines of sight offers an evolution-free means to constrain the cosmological constant, or dark energy. Using cosmological simulations, we consider a maximum likelihood method to obtain constraints from this comparison. In our method, measurements of the auto and cross spectra from observations are compared with those from a multi-parameter grid of simulated models of the intergalactic medium. We then marginalize over nuisance parameters to obtain constraints on the cosmological constant. Redshift space distortions due to peculiar velocities and thermal broadening, a potential difficulty in applying this test, are explicitly modeled in our simulations. To illustrate our method, we measure the cross spectrum from a new sample of five close quasar pairs, with separations of 0.5 to 3 arcmin. We attempt to obtain a constraint on Omega_Lambda, but find only weak constraints. An Einstein-de-Sitter cosmology is, however, disfavored by the data at a ~ 2 sigma confidence level. We consider the power of future observations, paying particular attention to the effects of spectral resolution and shot-noise. We find that ~ 50 moderate resolution, FWHM ~ 150 km/s, close separation (~ 30-120 arcsec) pairs should allow a (2 sigma) constraint on Omega_Lambda at the level of 15%, if other modeling parameters are determined through other means. We find that there is a sizeable gain from observing very close, ~ 30 arcsec, separation pairs provided they are observed with high spectral resolution. A sample of ~ 10 such pairs gives similar constraints to the 50 moderate resolution pairs mentioned above.

Motivation & Objective

• To develop a cosmological test based on the cross-correlation of Lyα forest absorption in close quasar pairs that is free from assumptions about cosmic distance ladder or source evolution.
• To model the effects of redshift-space distortions and thermal broadening on the three-dimensional flux power spectrum in cosmological simulations.
• To apply a maximum likelihood method comparing observed auto and cross spectra with a grid of simulated IGM models to constrain ΩΛ.
• To assess the feasibility and required sample size for future observations to achieve competitive constraints on dark energy.

Proposed method

• Use cosmological simulations with high dynamic range to model the intergalactic medium, including redshift-space distortions and thermal broadening effects on the flux power spectrum.
• Construct a multi-parameter grid of simulated models spanning different cosmological parameters, initial power spectrum amplitudes, and spectral indices to match observed flux statistics.
• Measure the auto and cross power spectra from simulations in Fourier space, using observed redshift and angular separations of quasar pairs.
• Apply a maximum likelihood framework to compare observed auto and cross spectra with simulated spectra, marginalizing over nuisance parameters to constrain ΩΛ.
• Use a 256³ particle, 20.0 Mpc/h box size simulation with resolution tests to validate results, excluding modes below the fundamental mode and high-k modes where resolution is insufficient.
• Assess the impact of spectral resolution and shot noise on future constraints by simulating different observational scenarios.

Experimental results

Research questions

• RQ1 Can the cross-spectrum of Lyα forest absorption in close quasar pairs provide a robust, evolution-free constraint on the cosmological constant?
• RQ2 How do redshift-space distortions and thermal broadening affect the three-dimensional flux power spectrum in the Lyα forest?
• RQ3 What level of statistical precision on ΩΛ can be achieved with current and future quasar pair samples?
• RQ4 How does spectral resolution and angular separation of quasar pairs influence the sensitivity of the cross-spectrum method?

Key findings

• The method disfavors an Einstein-de-Sitter cosmology at approximately 2σ confidence level, based on current data from five close quasar pairs.
• Current constraints on ΩΛ are weak due to limited sample size and large error bars in the cross-spectrum measurement.
• A sample of ~50 moderate-resolution quasar pairs (FWHM ~150 km/s, Δθ ~30′′–120′′) could yield a 15% 2σ constraint on ΩΛ if other modeling parameters are well determined.
• Observing ~10 very close pairs (Δθ ~30′′) with high spectral resolution offers a comparable constraint to the 50-pair moderate-resolution sample.
• High-resolution simulations confirm that the 256³ particle, 20.0 Mpc/h box is insufficient for small-scale power on k > 0.03 s/km, but this is acceptable given current data resolution.
• Sample variance and resolution limitations in simulations suggest that future constraints will require larger-volume, higher-resolution simulations to reduce uncertainty.