The visible universe has grown at least 1060 times larger since the Big Bang. Half of these would be accounted for by the hypothetical inflationary phase, a period of accelerated growth that happens in a eye-blink just after the Big Bang itself. The next 15 factors of 10 takes a trillionth of a second to accomplish, while the last 15 factors of 10 require the nearly 14 billion years that elapse between that moment and the present day.

Overdensities in the post-inflationary universe; white regions show locations where the density is 100 times greater than average.

Inflation gives way to a phase in which the Universe is filled with a mirror-smooth, cold, ultra-dense quantum condensate, a novel state of matter. This condensate must fragment into familiar particles and radiation, “reheating” the Universe to produce a viable cosmology. Immediately after inflation, interactions occur at energies far beyond the validity of known particle physics but typical energies decrease as the Universe expands  and a trillionth of a second after the Big Bang they reach the level where the so-called Standard Model is applicable. This initial era is sometimes called the “primordial dark age”; the Universe contains no light or radiation as it begins, and it is governed by rules we do not understand, but it can account for half of the total growth of the post-inflationary universe (in terms of the number of times the Universe doubles in size). This phase is critical to understanding the full history of the Universe, and may hold clues to the origin of the dark matter and excess of matter over antimatter that characterises the present-day Universe.

The idea of inflation is now forty years old, and many of its key predictions have been successfully verified. However, the detailed physics of this phase remains the subject of conjecture, and the reheating phase – which must somehow connect inflationary physics to the familiar particles we observe – is doubly mysterious. In some cases, reheating can happen explosively, but in others the condensate is long-lived. In these scenarios initially tiny irregularities in the condensate grow slowly but eventually reach the point where their own self-gravity causes overdense regions to undergo gravitational collapse. In 2019, Musoke, Hotchkiss and Easther showed (now published in Physical Review Letters) that this process is governed by the Schrödinger-Poisson equation, which describes the interaction between quantum matter and its own gravitational field.

Using this insight, Musoke, Hotchkiss and Easther  performed the first numerical simulations of the collapse of the quantum condensate, showing that the peak density quickly grew to be at least 200 times larger than the average density once gravitationally-driven collapse begins. This marks a key step forward in understanding the primordial dark age. Possible implications of this work include a better understanding of predictions for key cosmological observables, insight into the production of dark matter and the origin of the asymmetry between matter and anti-matter in the early universe which ensures that our present-day cosmos is built from regular matter alone.

The picture this work reveals is that the primordial universe can contain a phase filled with minute, gravitationally bound objects. These objects will be short-lived on human timescales; they survive for a trillionth of a second at most, but very long-lived relative to the time it takes the early Universe to double in size.  Mathematically, this era resembles the formation of galaxies and clusters of galaxies in the present universe. This parallel has allowed Niemeyer and Easther to develop a formal analogy between the mathematical description of the growth of galaxies and the primordial dark age, showing that collapsed objects could eventually have a density a million times larger than the average density of the universe.