 The Soviet physicist Yakov Zel’dovich memorably called the early universe the “poor man’s accelerator”, because relatively inexpensive observations of the early universe may reveal features of high-energy physics well beyond the reach of even the most lavishly funded earth-bound accelerators. For many aspects of fundamental physics, including quantum gravity in particular, cosmology provides the only feasible way to assess competing ideas. This ambitious conception of cosmology as the sole testing ground for new physics extends beyond the standard model of particle physics (which is generally thought to be incomplete, even though there are no observations that contradict it). Big bang nucleosynthesis, for example, is an application of well-tested nuclear physics to the early universe, with scattering cross-sections and other relevant features of the physics fixed by terrestrial experiments. While working out how nuclear physics applied in detail required substantial effort, there was little uncertainty regarding the underlying physics. By contrast, in some domains cosmologists now aim to explain the universe’s history while at the same time evaluating new physics used in constructing it.