Once again, the universe reminds us how much we still have to understand about its deepest mysteries. In a new twist that is shaking the foundations of modern cosmology, recent measurements not only confirm earlier controversial results about the rapid expansion of the cosmos, but deepen the enigma: the universe is expanding faster than our theoretical models can explain. The discrepancy is so significant that experts have stopped referring to it as a mere “strain” and are now openly talking about a “Hubble crisis,” signaling a possible turning point in our understanding of the cosmos.
The dilemma dates back to 1929, when Edwin Hubble discovered that the universe is constantly expanding. Since then, scientists have attempted to precisely determine the speed of this expansion, known as the Hubble constant, using two different methods.
Measuring expansion: two divergent paths
On the one hand, direct measurements use “standard candles”: objects with known luminosity, such as type Ia supernovae or Cepheid variables. By measuring their apparent brightness, their distance is determined and compared to the speed at which they are receding, deduced from their redshift. According to Space.com, These observations suggest that the Hubble constant is around 73 kilometers per second per megaparsec (73 km/s/Mpc).
The second method looks into the universe’s most remote past, studying the oldest light we can detect: the cosmic microwave background (CMB), emitted when the universe was just 379,000 years old. The European Space Agency’s Planck satellite analyzed this primordial radiation and, applying the standard model of cosmology (dominated by cold dark matter and dark energy, operating under the province of Albert Einstein’s General Theory of Relativity), predicted a value of 67.4 km/s/Mpc. Both measurements are very precise, but incompatible with each other, and from this disparity the so-called “Hubble tension” is born.
A new discovery exacerbates the crisis
Now, a new discovery has further stirred the waters of this cosmological debate. A team led by Dan Scolnic of Duke University and Adam Riess of Johns Hopkins University has found evidence that deepens the crisis in our own cosmic “neighborhood.”
According to their study, published in The Astrophysical Journal Letters, The Coma galaxy cluster – one of the closest galaxy groups to Earth – is significantly closer than it should be: Space.com It is reported to be 321 million light-years away, while the Standard Model predicts it should be 359 million light-years away. This 38 million light-year discrepancy is not a simple measurement error, but a new indication that something fundamental could be going wrong in our understanding of the cosmos.
“I like to think of Hubble’s tension as when you were a child at the doctor’s office,” explains Scolnic in statements collected by Space.com. “The doctor might say, well, if you’re this big when you’re young, you’ll be this big when you’re an adult… In astronomy we can do the same thing with images of the cosmic microwave background, which is when the universe was a baby, to predict how big and fast the universe would be expanding today. And then astronomers like me can go and measure the expansion of the universe today. And it doesn’t match the prediction.”
The Scolnic team’s results are especially significant because the Coma cluster has been studied for decades, long before the debate about the Hubble strain existed. As the researcher points out in a statement from Duke University, “This cluster is in our backyard, it was measured long before anyone knew how important it was going to be.”
The cosmic ladder: precise measurements
For Scolnic, this new measurement serves as the first rung of a “cosmic ladder” that calibrates all other distances. Not only does it coincide with previous studies that also placed Coma about 320 million light years away, but it intensifies the discrepancy between the observed reality and the prediction of the cosmological model. In other words, as Duke University points out, the measurement matches the expansion rate of the universe as other teams have recently measured it, but not as our current understanding of physics predicts it. “The tension now becomes a crisis,” warns Scolnic himself.
To reach this conclusion, the researchers employed an innovative “cosmic ladder” based on data from the Dark Energy Spectroscopic Instrument (DESI). As a fundamental step in their measurement, they studied 12 type Ia supernovae in the Coma cluster. These stellar explosions act as perfect “standard candles”: their brightness is so predictable that they allow cosmic distances to be calculated with great precision. Using this new calibration, the team calculated that the Hubble constant is 76.5 kilometers per second per megaparsec, meaning that each 3.26 million light-year segment of the universe expands by 76.5 kilometers every second, a speed significantly higher than that predicted by the standard model.
From its location at the Kitt Peak National Observatory, DESI currently observes more than 100,000 galaxies. “The DESI collaboration made the really difficult part,” Scolnic said, although he added that “their ladder was missing the first rung.”
In the past there are those who have aimed to bury this tension. In the summer of 2024, a team led by Wendy Freedman (University of Chicago) measured the distance to ten galaxies using the James Webb Space Telescope and obtained a value that matched the Standard Model. But, as Scolnic points out, basing it on just ten galaxies is risky, and the new Coma results bring the enigma back to the forefront.
“We should not be too quick to throw away what has served us so well until now,” warns Scolnic, acknowledging that the Coma cluster data leaves little room to deny the existence of the Hubble strain.
What does all this mean?
In that sense, scientists are faced with two possibilities: either there is some error in our measurements (something increasingly less likely as different teams obtain similar results), or our model of the universe needs a serious revision.
Some tentative explanations suggest that there could have been an additional burst of dark energy in the early universe, or that exotic particles called axions could have injected extra energy into the early cosmos. But as you point out Space.comeverything remains speculative.
What is clear is that this discrepancy can no longer be ignored. As Scolnic says: “We’re pushing very hard against the models we’ve been using for two and a half decades, and we’re seeing that things don’t add up. This may be changing the way we think about the universe, and it’s exciting! There are still surprises in cosmology and who knows what discoveries will come next.
Edited by Felipe Espinosa Wang with information from Space.com, Duke University and Cosmos.
