New data shows that the Coma Cluster of galaxies appears 38 million light-years closer than it ought to be to us.
Scientists struggle today to precisely measure how fast the universe expands following Hubble-Lemaître's rule. Based on cosmic expansion models our observations show that objects speed away from us at a rate which equals the Hubble constant times their distance. The farther a galaxy is from Earth the faster it moves away from our planet.
Researchers Dan Scolnic of Duke University and Adam Riess of Johns Hopkins University located all type Ia supernovas in the Coma Cluster of galaxies as seen by the Hubble Space Telescope. What we learned from studying these astronomical events tells us the Coma Cluster lies near us than its expected spot according to current models.
Research groups use two standard ways to determine the Hubble constant. Scientists estimate galactic distances using standard candles which have known brightness amounts like Cepheid variables supernovae and red giant stars. With our knowledge of how fast galaxies recede from Earth measured through redshift we can estimate the Hubble constant from the Hubble–Lemaître law.
We can return our travel to the earliest period of existence. At 379,000 years following Big Bang formation the cosmos emitted its CMB light. The early universe contained plasma as plasma gas but before that moment optimal temperature conditions were absent for electrons and atomic nuclei to combine. Light could not pass through the plasma which sent out density wave patterns.
Then, as the cosmic clock rolled over to 379,000 years, the cosmos had cooled sufficiently for the electrons and nuclei to join and create complete atoms. The plasma disintegrated into a fog of neutral hydrogen and helium gas, but those density waves were fixed into the distribution of matter, a distribution that exists to this day, albeit on far bigger scales than back then. The CMB preserves what those density waves, known to as 'baryonic acoustic oscillations', or BAOs, looked like 379,000 years after the Big Bang.
Using CMB measurement results scientists apply the standard cosmology model to create predictions about our cosmos. According to our base view the universe contains primarily cold dark matter and dark energy plus the forces defined in Albert Einstein's theory of relativity.
According to CMB radiation measurements from Planck space probe studies the standard model shows Hubble constant should be 67.4 km/s/Mpc. According to Planck and the standard model a space area one million parsecs wide should expand at a speed of 67.4 kilometers per second.
Yet most measurements of standard candles in the here and now show a different value, in the area of 73.2 km/s/Mpc. Both measurements are performed to a great degree of accuracy and, according to our understanding of astrophysics and cosmology, both should be true - yet both can't be correct. It needs to be one or the other. This inexplicable disparity is what we call the Hubble tension.
"I like thinking of the Hubble tension as like when you were a kid at the doctor's office, and you got measured when you were a baby, and the doctor might say, well if you're this big when you're young, you'll be this big when you're an adult," Scolnic said at a press conference at the 245th meeting of the American Astronomical Society this January in Maryland.
"In astronomy we can do the same thing using photos of the cosmic microwave background, which is when the universe was a newborn, to estimate how big and how rapidly the cosmos would be growing today. And then scientists like me can go and test the expansion of the cosmos today, and it doesn't fit the forecast," Solnic added."
Nobody understands the Hubble tension. When it comes to mysteries of the cosmos, it's virtually usually toward the top of the list. Some scientists think there's some kind of hidden yet persistent inaccuracy in how we measure standard candles — there's an understandable reluctance to toss aside the standard model that has served us so well till now. Yet astronomers who perform the standard candle readings point their finger at the standard model, or at least some unknown phenomena that the standard model doesn't anticipate.
Either way, we need to know more, which is why a lot relies on the shoulders of the Dark Energy Spectroscopic Instrument (DESI), situated at the Mayall 4-meter telescope at Kitt Peak National Observatory in Arizona.
Beginning its five-year quest to study the expanding cosmos in 2021, DESI comprises 5,000 small robotic devices that can place 5,000 optical fibers to acquire redshift data from 5,000 objects at a time. Over the course of its five-year study, it will measure the redshifts of around 30 million galaxies distributed over cosmic time to determine how dark energy is speeding the expansion of the universe.
Uniquely, DESI is able to derive the Hubble constant based on predictions from both the early and late universe. By seeing the newly scaled-up BAOs in the distribution of galaxies throughout the cosmic web and comparing them to the angular scale of the BAOs in the CMB, DESI estimates the Hubble constant to be 68.5 km/s/Mpc, extremely near to the Planck/standard model prediction.
However, DESI's measurements of the Hubble constant in the current universe, using the brightness of type Ia supernova explosions in galaxies, produces a value of 76 km/s/Mpc — still indicating the tension, but with a huge error in the results.
Astronomers would wish to minimize such uncertainty. "The question is, can we help DESI make a better measurement?" asked Scolnic.
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So that its conclusions are completely impartial, DESI develops its own cosmic distance ladder from beginning, rather than relying on past measurements, with one exception. This is why its observations have a larger uncertainty than earlier measurements. That distance ladder has to be anchored somehow, calibrated with a credible distance measurement to a galaxy or group of galaxies nearby.
To that goal, Scolnic and Riess resorted to Hubble Space Telescope studies of type Ia supernova explosions in the Coma Cluster of galaxies. "It's a nice, rich cluster with lots of galaxies, and lots of galaxies mean lots of supernovae, and they're all at the same distance," said Scolnic.
This allowed them to compute a distance of 321 million light-years, give or take an uncertainty of 7 million light-years, to anchor the DESI observations with. This nicely agrees with prior measurements of the distance of the Coma Cluster, but wait on — what does the mainstream model have to say? It suggests that the Coma Cluster should be 359 million light-years distant, but it clearly is not that far.
"You can see in all those previous measurements, many coming before we even knew there was a Hubble tension, none of them ever came close to what the prediction would be if the standard model was correct," said Scolnic. "They all show that the standard model with the Planck measurement isn't right."
Given that the Coma Cluster is one of the nearest galaxy clusters to us, Scolnic characterizes this result as being the Hubble tension in our backyard. It should also put to rest the idea that the Hubble tension isn't real, which is an idea that gained traction in the summer of 2024 when a team led by Wendy Freedman of the University of Chicago used James Webb Space Telescope observations of Cepheid variables and red giant stars to measure the distance to 10 galaxies. Freedman's observations of the Hubble constant based on these 10 galaxies were in line with the conventional model, suggesting that the Hubble tension may have been a mistake. However, it's a strong conclusion from just 10 galaxies, and Scolnic argues that Freedman's discovery has "now been understood in better context, and the Hubble tension is not gone."
So what may be producing the Hubble tension? Cynics wanting to overturn the scientific establishment could say that we need to dismiss the standard model, but it's too early to be tossing the baby out with the bathwater. It's important recalling that the standard model has had multiple achievements, from explaining the origin, development and evolution of galaxies, and the presence of large-scale structure in the universe, to forecasting the properties of the CMB including the magnitude of the BAOs.
So what may be producing the Hubble tension? Attention is now focused on the early cosmos and if there was something there not expected by the standard model that may have influenced the observations. Perhaps there was an additional burst of dark energy in the early universe, or maybe energy was pumped into the early cosmos by radiation from axions, which are theorized particles and one possibility for the identification of dark matter. It's all still pretty hypothetical.
In the meanwhile the Coma cluster result really hammers home how worrying the Hubble tension is. Indeed, Scolnic argues that the Coma Cluster revelations have irreparably exacerbated the enigma, in conclusion adding ominously that "the Hubble tension is now a Hubble crisis."
The Coma Cluster discoveries have been approved for publication in The Astrophysical Journal, and a pre-print is available.
