Lyman alpha absorbers in motion: consequences of gravitational lensing for the cosmological redshift drift experiment

TL;DR

This paper investigates how the transverse motion of Lyman alpha absorbers affects the measurement of cosmological redshift drift, concluding that such motions are unlikely to hinder future detection efforts with advanced telescopes.

Contribution

It provides a detailed analysis of the impact of Lyman alpha absorber motions on redshift drift measurements, including the roles of velocities and accelerations, which had not been fully assessed before.

Findings

Peculiar velocities and accelerations influence redshift measurements.

Absorber motions are unlikely to significantly impede redshift drift detection.

The study offers estimates reassuring the feasibility of future measurements.

Abstract

The evolution of the expansion rate of the Universe results in a drift in the redshift of distant sources over time. A measurement of this drift would provide us with a direct probe of expansion history. The Lyman alpha forest has been recognized as the best candidate for this experiment, but the signal would be weak and it will take next generation large telescopes coupled with ultra-stable high resolution spectrographs to reach the cm/s resolution required. One source of noise that has not yet been assessed is the transverse motion of Lyman alpha absorbers, which varies the gravitational potential in the line of sight and subsequently shifts the positions of background absorption lines. We examine the relationship between the pure cosmic signal and the observed redshift drift in the presence of moving Lyman alpha clouds, particularly the collapsed structures associated with Lyman limit systems (LLSs) and damped Lyman alpha systems (DLAs). Surprisingly, the peculiar velocities and peculiar accelerations both enter the expression, although the acceleration term stands alone as an absolute error, whilst the velocity term appears as a fractional noise component. An estimate of the magnitude of the noise reassures us that the motion of the Lyman alpha absorbers will not pose a threat to the detection of the signal.