Twenty years ago, scientists were shocked to realize that our universe is not only expanding, but that it’s expanding faster over time.
Pinning down the exact rate of expansion, called the Hubble constant after famed astronomer and UChicago alumnus Edwin Hubble, has been surprisingly difficult. Since then scientists have used two methods to calculate the value, and they spit out distressingly different results. But last year’s surprising capture of gravitational waves radiating from a neutron star collision offered a third way to calculate the Hubble constant.
That was only a single data point from one collision, but in a new paper published Oct. 17 in Nature, three University of Chicago scientists estimate that given how quickly researchers saw the first neutron star collision, they could have a very accurate measurement of the Hubble constant within five to ten years.
“The Hubble constant tells you the size and the age of the universe; it’s been a holy grail since the birth of cosmology. Calculating this with gravitational waves could give us an entirely new perspective on the universe,” said study author Daniel Holz, a UChicago professor in physics who co-authored the first such calculation from the 2017 discovery. “The question is: When does it become game-changing for cosmology?”
In 1929, Edwin Hubble announced that based on his observations of galaxies beyond the Milky Way, they seemed to be moving away from us—and the farther away the galaxy, the faster it was receding. This is a cornerstone of the Big Bang theory, and it kicked off a nearly century-long search for the exact rate at which this is occurring.
To calculate the rate at which the universe is expanding, scientists need two numbers. One is the distance to a faraway object; the other is how fast the object is moving away from us because of the expansion of the universe. If you can see it with a telescope, the second quantity is relatively easy to determine, because the light you see when you look at a distant star gets shifted into the red as it recedes. Astronomers have been using that trick to see how fast an object is moving for more than a century—it’s like the Doppler effect, in which a siren changes pitch as an ambulance passes.