Toward a direct measurement of the cosmic acceleration: The pilot observation of H I 21cm absorption line at FAST

This study presents results on detecting neutral atomic hydrogen (HI) 21cm absorption in the spectrum of PKS1413+135 at redshift $z=0.24670041$. The observation was conducted by FAST, with a spectral resolution of 10 Hz, using 10 minutes of observing time. The global spectral profile is examined by modeling the absorption line using a single Gaussian function with a resolution of 10 kHz within a 2 MHz bandwidth. The goal is to determine the rate of the latest cosmic acceleration by directly measuring redshift evolution of H I 21 cm absorption line with Hubble flow towards a same background Quasar over a decade or longer time span. This will serve as a detectable signal generated by the accelerated expansion of the Universe at redshift $z < 1$, referred to as redshift drift $\dot{z}$ or the SL effect. The measured HI gas column density in this DLA system is approximately equivalent to the initial observation value, considering uncertainties of the spin temperature of a spiral host galaxy. The high signal-to-noise ratio of 57, obtained at a 10 kHz resolution, strongly supports the feasibility of using the H I 21 cm absorption line in DLA systems to accurately measure the redshift drift rate at a precision level of around $10^{-10}$ per decade.