This story is Part I of a two-part essay on the “Hubble tension.” Read Part II.
“I’ve called it ‘the biggest crisis in astronomy,’” says Adam Batten, an astronomer at Swinburne University, in Melbourne, Australia. Batten is referring to the so-called “Hubble tension”—a discrepancy between measurements of the Hubble constant, which describes how fast the universe is expanding and, in turn, the age of the cosmos. In recent years, this mismatch has grown, leading to a billion-year gap in estimates for the time that has passed since the Big Bang. As a result, cosmologists are wrestling with the possibility that our best understanding of the make-up and history of the universe may be wrong.
“The stakes are high,” says Daniel Scolnic, a cosmologist at Duke University in Durham, North Carolina. “This is the strongest evidence in the last 25 years that there’s something else needed in our standard model of cosmology.”
The tension centers on two different ways of calculating the Hubble constant, H0. The first sets H0 at around 68 km/s/Mpc and is based on analyses of the cosmic microwave background (CMB), the afterglow radiation from the Big Bang that serves as a snapshot of the infant universe. Crucially, this calculation uses the standard model of cosmology as a recipe and assumes that the cosmic ingredients are a dash of normal matter, a lot of invisible but unidentified ‘dark matter’ and an even greater helping of the mysterious ‘dark energy’ that is thought to be pushing space outwards, causing the universe’s expansion to speed up.
The second method fixes H0 at about 73 km/s/Mpc, and comes from ‘local’ measurements of the cosmic expansion from observations of stars and galaxies. The difference between 68 and 73 may seem pretty innocuous, and for the better part of a decade many cosmologists assumed it was. But as experiments got better and data more precise, the margin for error got smaller and it’s become increasingly apparent that they fundamentally disagree.
“It’s like when you’re a kid and the doctor measures how big you are and tells you you’re going to be this big when you’re a grown-up,” says Scolnic, “but then that doesn’t match your height as an adult.” Either one or other of the height measurements was off, or your doctor doesn’t understand how people grow.
Controversially then, either cosmologists don’t understand the beginning and evolution of the universe, the ingredients of the standard model are wrong, or astronomers don’t know how to measure what they can see.
“The Hubble Tension tells us something is wrong, but it does not precisely point out what,” says Adam Riess, an astrophysicist at Johns Hopkins University who won a share of the 2011 Nobel Prize in Physics for discovering that the expansion of the universe is accelerating. “Nature is making us work harder to figure out what is going on and why,” he says.
Cosmic Cycles
The history of cosmology has cycled through periods of confidence, complacency and then crisis. At the start of the 20th century, astronomers took for granted that the universe was a static thing that had always existed. But by the 1930s, observations by American astronomers Vesto Melvin Slipher, Edwin Hubble and others had established that light from all galaxies was shifted towards the red end of the spectrum, indicating they were moving away from us–and indeed each other. Cosmologists realized that the universe is expanding and had begun as a small, dense point in space and time, at the Big Bang.
The Hubble constant is a measure of the rate of its growth and, when inverted, tells physicists how long ago the universe was born.
As physicists were getting comfortable with this Big Bang picture, new observations told them the vast majority of matter is in locked in some invisible “dark” form, whose presence can only be inferred through its gravitational tug on visible matter. Things calmed again, until the late 1990s, when Riess and others threw more spice into the mix. While monitoring distant exploding stars, or supernovae, they realized cosmic expansion was speeding up, an effect now attributed to the unknown dark energy.
Looking back, Riess feels the implications of the new tension may be just as exciting and profound. “Comparing the 1998 discovery of cosmic acceleration and the present Hubble tension, I think the evidence for the tension is much stronger than what we had then,” Riess says.
While nobody yet knows the identity of dark matter or dark energy, for the past quarter of a century, cosmologists have at least felt reassured that they know how much of these weird entities are out there. A series of increasingly accurate experiments mapped tiny temperature bumps in the CMB and found that they fit beautifully with predictions of the standard model of cosmology, with its ingredients of roughly 70% dark energy, 25% dark matter and 5% ordinary matter. But the Hubble tension may now be calling this recipe into question.
The CMB data, in combination with the standard model, can also be used to calculate the Hubble constant, and for a little while, the numbers added up fine. Data from NASA’s WMAP probe, which mapped the CMB between 2001 and 2010, suggested H0 was a little over 70, in good agreement with local measurements, within margins of error. The trouble began with the arrival of the European Space Agency’s more precise Planck satellite, which found a significantly lower value for the Hubble constant. At first, astronomers weren’t ruffled; the assumption was that as instruments got better and mistakes were corrected, the numbers would realign. “Even five years ago, most people thought, ‘this thing’s going to go away,’” says Scolnic.
But it didn’t. When bugs were corrected, and equipment was improved and recalibrated, the discrepancy got sharper. Newer, higher resolution CMB observations by the Atacama Cosmology Telescope in Chile and the South Pole Telescope in Antarctica have found similar results to Planck, for instance. For many, 2021 was a turning point, when the SH0ES collaboration, led by Riess and including Scolnic, used measurements of stars and supernova made by the Hubble telescope to calculate a value of H0 that deviated from Planck’s by more than a golden amount: 5-standard deviations.
“This tells you there’s less than one in a few billion chance that statistically this discrepancy is a fluke,” says Batten “That’s what caused this crisis.”
This story is Part I of a two-part essay on the “Hubble tension.” Read Part II.
Zeeya Merali is a London-based science journalist and author of the popular physics book, A Big Bang in a Little Room.
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