# Toward a direct measurement of the cosmic acceleration: roadmap and forecast on FAST

## TL;DR

This paper proposes a method to directly measure cosmic acceleration using HI absorption systems observed by the FAST telescope, forecasting significant improvements in the precision of redshift drift measurements over a decade.

## Contribution

It introduces a combined observation strategy with FAST to enhance the accuracy of the Sandage-Loeb effect measurement for cosmological model testing.

## Key findings

- FAST can discover about 800 HI absorption systems in one year.
- A decade of targeted observations can measure the redshift drift with high precision.
- The proposed method could detect the first-order derivative of cosmological redshift.

## Abstract

HI absorption systems are great targets for direct measurement of the Sandage-Loeb (SL) effect throughout a wide range of redshift for ground-based radio telescopes. We demonstrate the significance of improving the accuracy of SL effect measurement in cosmological model selection. With its wide sky coverage and high sensitivity, we forecast that for 1 year of the upcoming commensal survey (CRAFTS) the FAST telescope is capable of discovering about 800 HI absorption systems thereby improving the SL measurement accuracy. Aiming to measurement the cosmic redshift drift rate at the precision of $\dot{z} \sim 10^{-10} \mathrm{decade^{-1}}$, we propose combined observation mode with blind-searching and targeted observation. For a decade of consecutive targeted spectroscopic observation with the frequency resolution at a level of sub-$0.1\ \rm Hz$, we could detect the first-order derivative of the cosmological redshift with the expected precision.