AAS 195th Meeting, January 2000
Session 14. Early Universe and Cosmology
Display, Wednesday, January 12, 2000, 9:20am-6:30pm, Grand Hall

## [14.01] Measuring the Cosmic Microwave Background Temperature at High Redshift: Prospects for its Detection and Use in Calibrating Distance Indicators

G. J. Mathews, J. M. LoSecco, Yun Wang (U. Notre Dame, Center for Astrophysics)

We discuss how the local value of the cosmic microwave background (CMB) temperature at a given redshift z might be determined from the relative absorption line equivalent widths for excitation of the rotational modes of diatomic CN or CH molecules in cool HI regions. One interesting possibility might be to look at low dispersion Lyman \alpha clouds in which the appropriate molecular lines could be identified in the background continuum of a distant quasar. Another possibility might be the interstellar absorption from a resolved continuum source in a not too distant galaxy. We discuss the likelihood of such detections, which if made, would provide interesting tests of the expected scaling of the CMB temperature with the redshift of the intervening cloud. Another important application might be possible from a simultaneous measurement of the spectroscopic redshift of the cloud. This would yield a sum of the cosmic redshift plus radial peculiar-velocity-induced redshift. Thus, in principle, combining a measurement of the absorption-line CMB temperature and the spectroscopic redshift could enable a determination of the radial peculiar velocity of the cloud. We discuss the likelihood that sufficiently accurate measurements could be made and how such measurements might be used to reduce the current uncertainty in the cosmic distance ladder due to the unknown relative radial positions of host galaxies in which Cepheids have been detected.