The Hubble constant is one of the most important numbers in cosmology because it tells us how fast the universe is expanding, which can be used to determine the age of the universe and its history. It gets its name from UChicago alum Edwin Hubble, who was first to calculate the constant from his measurements of stars in 1929.
Despite nearly a hundred years of astronomical measurements and calculations, scientists still can’t agree on the exact value of the Hubble constant. The true number could reveal missing pieces in our understanding of physics, like new particles or a new form of dark energy.
What is the Hubble constant?
Figuring out the true value of the Hubble constant is one of the greatest challenges in modern astronomy and could revolutionize our understanding of the universe—so scientists at the University of Chicago and many other institutions around the world are trying to pin down the number using multiple methods.
For an astronomical object (e.g. a star or a galaxy) at a known distance from the Earth, the Hubble constant can be used to predict how fast it should be moving away from us.
However, the true value of the Hubble constant remains up for debate. Based on the fundamental physics that scientists believe has driven the evolution of the universe, the Hubble constant should be around 68 km/s/Mpc—but this doesn’t match up to observations of the actual stars and galaxies astronomers see around us. UChicago astronomer Wendy Freedman led a team that made a landmark measurement in 2001, which found a value of 72. The most recent precise measurements of the distances and movements of distant, exploding stars suggest a Hubble constant of 69.8 km/s/Mpc, but other reports have pushed the value as high as 74 km/s/Mpc.
Although these differences seem small, even a 2 km/s/Mpc discrepancy between predictions from physics and observations implies there could be something major missing from our current understanding of the universe.