Since the Universe was filled with hydrogen atoms at early times, the most promising method for observing the epoch of the first stars is to measure the hyperfine line of hydrogen at a wavelength of 21 cm. Such an observation of the "cosmic dawn" era was considered speculative when I helped lay out the theoretical predictions a decade ago, but there is now an enormous, promising observational effort (in which I am involved within the Square Kilometre Array). There is also an important aspect of early cosmic history that can potentially be probed today. The difference between the early evolution of the baryons and the dark matter should imprint a specific signature that can be probed via galaxy surveys. We have recently attempted the first such measurement; detecting this would provide a novel confirmation of the standard cosmological model.
