  Neutral hydrogen is ubiquitous, absorbing and emitting 21 cm radiation
throughout much of the Universe's history. Active sources of perturbations,
such as cosmic strings, would generate simultaneous perturbations in the
distribution of neutral hydrogen and in the Cosmic Microwave Background (CMB)
radiation from recombination. Moving strings would create wakes leading to 21
cm brightness fluctuations, while also perturbing CMB light via the
Gott-Kaiser-Stebbins effect. This would lead to spatial correlations between
the 21 cm and CMB anisotropies. Passive sources, like inflationary
perturbations, predict no cross correlations prior to the onset of
reionization. Thus, observation of any cross correlation between CMB and 21 cm
radiation from dark ages would constitute evidence for new physics. We
calculate the cosmic string induced correlations between CMB and 21 cm and
evaluate their observability.
