The signature of subgalactic dark matter clumping in the global 21-cm signal of hydrogen

It is thought that the Universe went through an early period known as the Dark Ages, during which primeval density fluctuations grew to form the first luminous objects, marking the beginning of cosmic dawn around 100 million years after the Big Bang. The 21-cm line of hydrogen atoms is the most promising probe of these epochs, with extensive observational efforts underway. Here we combine hydrodynamical simulations with a large-scale grid to precisely calculate the effect of nonlinear structure formation on the global (sky-averaged) 21-cm radio intensity. We show that it presents a potential opportunity to probe the properties of dark matter in a new regime, corresponding to a length-scale of only 150,000 light years and a mass scale of 20 million solar masses. This effect can in principle be detected unambiguously during the Dark Ages, where the weak signal requires an array of global signal antennae. During cosmic dawn, when stellar radiation boosts the signal, a single global antenna suffices, but the clumping effect must then be separated from the effect of the stars. Our findings open new avenues for testing the nature of dark matter as well as non-standard cosmological models.