In 1998, two teams of cosmologists observed dozens of distant supernovas and inferred that they’re racing away from Earth faster and faster all the time. This meant that—contrary to expectations—the expansion of the universe is accelerating, and thus the fabric of space must be infused with a repulsive “dark energy” that comprises more than two-thirds of everything. For this discovery, the team leaders, Saul Perlmutter of the Supernova Cosmology Project and Brian Schmidt and Adam Riess of the High-Z Supernova Search Team, won the 2011 Nobel Prize in Physics.
Fast-forward to July of this year.
On a Monday morning three weeks ago, many of the world’s leading cosmologists gathered in Santa Barbara, California, to discuss a major predicament. Riess, now 49, strolled to the front of a seminar room to give the opening talk. A bulldog of a man in a short-sleeved box-check shirt, Riess laid out the evidence, gathered by himself and others, that the universe is currently expanding too fast—faster than theorists predict when they extrapolate from the early universe to the present day. “If the late and early universe don’t agree, we have to be open to the possibility of new physics,” he said.
At stake is the standard theory of the cosmos that has reigned since the discovery of dark energy. The theory, called ΛCDM, describes all the visible matter and energy in the universe, along with dark energy (represented by the Greek letter Λ, or lambda) and cold dark matter (CDM), showing how they evolve according to Albert Einstein’s theory of gravity. ΛCDM perfectly captures features of the early universe—patterns best seen in ancient microwaves coming from a critical moment when the cosmos was 380,000 years old. Since the Planck Space Telescope’s first map of this “cosmic microwave background” was released in 2013, scientists have been able to precisely infer a distance scale in the young universe and use ΛCDM to fast-forward from the 380,000-year mark to now, to predict the current rate of cosmic expansion—known as the Hubble constant, or H0.