Abstract
We present measurements of absorption by the 21 cm hyperfine transition of neutral hydrogen toward radio sources at substantial look-back times. These data are used in combination with observations of rotational transitions of common interstellar molecules to set limits on the evolution of the fine structure constant: , to a look-back time of 4.8 Gyr. In the context of string theory, the limit on the secular evolution of the scale factor of the compact dimensions, , is . Including terrestrial and other astronomical measurements places limits on slow oscillations of from the present to the epoch of cosmic nucleosynthesis, just seconds after the big bang, of .
- Received 18 July 2000
DOI:https://doi.org/10.1103/PhysRevLett.85.5511
©2000 American Physical Society