Lyα absorbers in motion: consequences of gravitational lensing for the cosmological redshift drift experiment★

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 α (Lyα) 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 ultrastable high‐resolution spectrographs to reach the cm s−1 resolution required. One source of noise that has not yet been assessed is the transverse motion of Lyα 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 Lyα clouds, particularly the collapsed structures associated with Lyman limit systems and damped Lyα systems. 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 Lyα absorbers will not pose a threat to the detection of the signal.

Real-time cosmology

“…Such new prospects lately prompted renewed interest in the theoretical predictions of the redshift variation in different scenarios (see e.g. [20][21][22][23][24][25][26][27][28][29]).…”

Velocity effects on second order contributions to gravitational frequency shift by a moving Kerr–Newman black hole

“…The translational motion of a gravitational system makes a difference on the propagation of electromagnetic signals, which is the so-called velocity effect [1,2]. The issue of velocity effects on the gravitational frequency shift of light was discussed in many papers [2][3][4][5][6][7][8][9][10][11]. Birkinshaw and Gull [3] studied the transversal velocity effect of cluster of galaxies and showed that the gravitational frequency shift of light is proportional to the low transverse lens velocity and the deflection angle of light.…”

Forecast and analysis of the cosmological redshift drift

“…However, even with future precision radio telescopes, the measurement of the SL effect represents a difficult enterprise [27] as it demands several years of observation (usually some decades) to register enough signal-to-noise ratio so as to yield a possible reliable detection of the cosmological redshift drift signal. Best candidate objects for a feasible detection of this faint signal are good Hubble flow tracers as far as possible [28].…”