Today, we observe the universe spreading out and cooling down. Assuming this expansion has been going on for a long time, cosmologists imagine “rewinding” the universe to progressively earlier stages. As they trace the universe back to smaller sizes, it must get hotter and hotter — and, as it does so, the physics one needs to describe it changes. Today, the temperature of the universe, except for a few hospitable “hot spots” like galaxies, measures a few degrees above absolute zero, and the science is relatively well understood.
But 10 billion years ago, the universe glowed at many thousands of degrees — roughly the Sun’s surface temperature. Then, how light interacts with ions — atoms stripped of their electrons — played a leading role everywhere in the cosmos.
Looking farther back, when the universe as a whole burned hotter than the Sun’s core, interactions among atomic nuclei held paramount importance. The cosmos then was just a few cubic light-minutes in size — the smallest, earliest state for which scientists have any confidence in their ability to describe its properties. On Earth, only particle accelerators give us a glimpse of matter at energies before this time, when the universe was even hotter and nuclei gave way to quarks. This far back, we’ve reached the domain of conjecture and speculation.
In the 1930s, cosmologists hotly debated what we’d see by rewinding cosmic expansion. One view, the so-called Milne universe, pictured matter receding to a single point in space-time. But cosmologists today know that space-time itself is expanding uniformly and in all directions, according to the Friedman-Robertson-Walker model of the universe.
Finally, scientists think quantum fluctuations broke the early universe’s perfect uniformity. This ultimately allowed galaxies, stars, and planets to appear. Some researchers believe these inhomogeneities could have been large enough at early times to form tiny black holes. They’re working to predict how we might detect such primordial black holes today. — Simon Dedeo, University of Chicago