Already mindbogglingly large, the universe is actually getting bigger all the time. Some of the nearest galaxies to ours are receding at a rate surpassing 240,000 kilometers per hour (150,000 miles per hour). And those are the slow-pokes; the most distant galaxies actually zoom away from us faster than the speed of light. This high-speed galactic exodus breaks no laws of physics, however, for it is the universe itself that is expanding—the very space-time fabric upon which all of existence is stitched.
In cosmology, no number is as important as this rate of recession in understanding the origin, evolution, and fate of our universe. Ever since famed astronomer Edwin Hubble discovered the universe's expansion in the 1920s, scientists have sought to nail down the universe's growth rate, aptly named the Hubble constant. One might expect convergence, as new and better techniques are brought to bear in gauging the Hubble constant. In sharp distinction, a profound and ever-more-perplexing gap has instead emerged between the most powerful techniques. The measuremental chasm has split so wide that researchers are now strongly, albeit reluctantly, questioning our basic grasp of cosmic history. Whispers of resorting to "new physics"—essentially, introducing speculative "fudge factors" to provisionally constrain the problem and outline potential solutions—are growing louder.
Among the most central players in this unfolding scientific drama is Wendy Freedman. The John and Marion Sullivan University Professor in Astronomy and Astrophysics at the University of Chicago, as well as a member of its Kavli Institute for Cosmological Physics (KICP), Freedman has studied the Hubble constant for three decades.
She has been a pioneer in the direct measurement of the Hubble constant here in the present-day universe. Freedman and colleagues rely on stars called Cepheid variables, whose brightnesses change in a regular cycle. (Hubble himself made his groundbreaking discovery relying on these same sorts of stars.) Scientists can compare these star's apparent brightnesses, which diminish with distance, to their already-known inherent brightnesses. The method works just as if the exact same sort of candle were placed at varying distances down a road from an observer here on Earth. Coupling this brightness comparison to a shift in light from receding objects known as redshift, which reveals just how fast a galaxy is receding, lets the researchers build a robust "cosmic distance ladder," as they call it.
Over the years, researchers have continued whittling down the error bars inherent to the Cepheid technique, arriving at ever-firmer estimates of how fast our universe is expanding. "That is the beauty of really accurate measurements in cosmology," says Freedman. "Locally, we can measure the Hubble constant—the expansion rate—directly."
The direct measurements—along with those taken of exploding, more distant stars called supernovae—have yielded a Hubble constant value of about 73 kilometres per second (45 miles per second) per megaparsec. Translating that from astronomer-speak: for every unit of distance from us called a megaparsec, which is equal to about 3.3 million light-years, with a single light-year being how far light travels over the course of a year (a gobsmacking 9.5 trillion kilometers, or 5.9 trillion miles), a galaxy is moving away from us at that 74 kilometer-per-second rate, due to the universe's expansion.
A recent study, led by Adam Riess of the Space Telescope Science Institute (STScI) and Johns Hopkins University, further locked in that value of the local Hubble constant. Using the Hubble Space Telescope—again named for the father of modern cosmology—Riess and colleagues observed a large sample of Cepheid variable stars in a neighboring galaxy, carefully building on the evidence that has accumulated to date. Their work has reduced remaining uncertainty in the accuracy of the Cepheid technique down to a measly 1.9%.
Another, vying technique for measuring the Hubble constant has settled on a value of 67.4 kilometres per second per megaparsec. This value comes from observing the earliest light in the universe than can reach our telescopes, known as the cosmic microwave background. This light dates back to when the universe was only 380,000 years old, and is often called the relic radiation of the Big Bang, the moment when our cosmos began.
Overall, the odds of the values arrived at by the two Hubble constant techniques being just a statistical fluke are quite small—about 1 in 100,000. The discrepancy appears to be very real.
So what gives?
"Either there are errors that we haven't uncovered, in one or both methods," says Freedman, "or perhaps there is new, fundamental physics that is missing from our current standard model."
Before upsetting the apple cart, Freedman and her fellows in the field are developing new techniques that can get a bead on the Hubble constant. These methods are independent of the seemingly tried-and-true Cepheids and cosmic background radiation. "Cepheids are a great method—I have spent a good deal of my career working on them!" says Freedman. "With a given technique, however, one worries about the 'unknowns.' The only way to test for those is to have independent measurements."
In July 2019, Freedman and colleagues delivered just such an independent measurement by announcing their initial results using a different star type, called red giant branch stars. Thickening the plot further, the method arrived at a Hubble constant figure of about 70—smack-dab in the middle of the dueling, predominant methods.
Another promising new method involves gravitational waves—the highly publicized "ripples" in the spacetime fabric of the universe first definitively detected only in 2015 by the LIGO experiment. (The cofounders of LIGO won the 2016 Kavli Prize in Astrophysics, and one of the winners was Rainer Weiss, of the Massachusetts Institute of Technology's Kavli Institute for Astrophysics and Space Research, initialized as MKI.) Unleashed by the cataclysmic mergers of black holes, neutron stars, or both, these gravitational waves travel at the speed of light through the cosmos. In cases where light also reaches Earth from such mergers, allowing for a recessional velocity measurement, the gravitational waves can serve as an independent index of the inherent distances to the colliding objects. Two Kavli Institute-affiliated researchers—Daniel Holz of KICP and Scott Hughes of MKI—came up with this technique in 2005. They recently applied it to the first neutron star merger caught via gravitational waves on record. Lo and behold, the Hubble constant value it spit out was also 70, like Freedman's red giant star approach.
Ultimately, then, there is still hope that the nearly 10% gap between the dug-in Hubble constant values can yet be bridged. Maybe new physics will not be necessary. Maybe the universe is expanding in a straightforward manner, no tricks up its sleeve.
But if some cosmological shenanigans are indeed afoot, Wendy Freedman and her many Kavli-affiliated colleagues will let us know. As the saying goes, "watch this space."