A recent study published in The Astrophysical Journal Letters has provided the most precise distance to the Coma Cluster of galaxies yet — and also has deepened a crisis in cosmology.
The problem is that when we look around the local universe, the universe seems to be expanding faster than predicted by the current models that govern physics. This gap between what models predict and what is actually observed has become known as the Hubble tension.
For years, the Hubble tension has been a vexing reminder to scientists that something is not quite right in their models. But with the new observations of the Coma Cluster — which lies practically in our own backyard by cosmological standards — scientists say the discrepancy stands out more glaringly than ever. “The tension now turns into a crisis,” said the paper’s first author, Daniel Scolnic of Duke University, in a press release.
So, how fast is the universe expanding?
Scolnic’s new works expanded on a previous study released in 2024 that used data collected by the Dark Energy Spectroscopic Instrument (DESI) mounted to the Mayall Telescope in Kitt Peak, Arizona. The instrument’s goal is to study the nature of the expansion of the universe by observing distant galaxies that are being carried away from us.
Scientists characterize a galaxy’s distance based on how much of its light is shifted toward redder wavelengths due to the Doppler effect, called its redshift. Generally speaking, the faster a galaxy appears to be moving away from us, the farther from us it is. But to properly measure how quickly the universe is expanding, scientists need independent measurements of the distances to galaxies with alternative methods.
One such method is called the Fundamental Plane method. It applies to elliptical galaxies — older galaxies whose spiral structures have been erased by mergers. The method infers a galaxy’s true size by looking at how bright it is and how broad the range of speeds is at which its stars move. When the true size is known, it’s easy to calculate the true distance based on how large or small it appears.
The Fundamental Plane method is one of a variety of methods to calculate distances to galaxies. Together, they make up what scientists call the cosmic distance ladder; as astronomers look farther out, each method is anchored to a reference point established by the previous method.
The 2024 study from the DESI collaboration released a version of the Fundamental Plane method that was anchored to the known distance to the massive Coma Cluster — a nearby cluster filled with elliptical galaxies. From it, they calculated the universe’s current expansion rate — known as the Hubble constant, or H0. They found a value of 76 kilometers per second per megaparsec — but with an uncertainty of nearly 5 km/s/Mpc due to the imprecision of the distance to the Coma Cluster.
Thus, the Coma Cluster served as an important reference: If astronomers could measure a more precise distance to the cluster, they could greatly improve their estimate of the expansion rate of the universe.
“When the DESI collaboration released their … paper, and I saw in the abstract how good their measurement could be if they just had one more piece, I got extremely excited, cause I knew I could help provide that piece,” Scolnic tells Astronomy.
A cosmological crisis
That piece that Scolnic could provide was a better distance to the Coma Cluster using another independent method. In addition to being rich in elliptical galaxies, the Coma Cluster also has an abundance of Type Ia supernovae. These stellar explosions are referred to as “standard candles” because physics dictates that they explode when white dwarf stars reach a critical mass, meaning their absolute brightnesses are well known. This means they can be used to reliably measure distances: The dimmer it appears, the farther away it is. Scolnic’s team observed the light curves of 13 supernovae in the cluster and found the distance to the Coma Cluster to be around 320 million light-years, with an uncertainty of just 7 million light-years.
With that anchor point, astronomers refined the Fundamental Plane relation and derived a value for the Hubble constant of 76.5 kilometers per second per megaparsec with an uncertainty of just 2.2 km/s/Mpc — twice as precise as the DESI Collaboration’s original value.
This value is an excellent match to other independent measurements of the Hubble constant that are based on looking at objects in the nearby universe.
But it only exacerbates the tension with the expansion rate predicted by the standard model of cosmology, known as the Lambda Cold Dark Model (ΛCDM). To calculate the model’s predictions, scientists begin with observations of the light radiated from after the Big Bang, known as the cosmic microwave background (CMB). Then, they use ΛCDM to extrapolate forward through time. But this approach yields a present-day Hubble constant of only 67.4 km/s/Mpc.
The new Coma Cluster distance measurement’s solidification of the much higher value suggests that the root of the Hubble tension lies within ΛCDM itself — not a error in measurement. “We’re at a point where we’re pressing really hard against the models we’ve been using for two and a half decades, and we’re seeing that things aren’t matching up,” Scolnic said in a press release.
Can this tension be relieved?
The Hubble tension has prompted some cosmologists to turn to models besides the standard ΛCDM. A study published in Physical Review D on Feb. 18 explored one such model called the Interacting Dark Energy (IDE) model in which dark matter can transfer its energy to dark energy or vice versa.
The IDE model has the potential to alleviate not only the Hubble tension but also the so-called S8 tension — a disagreement between predictions and observations of the degree to which matter in the universe is clustered together — essentially, how “clumpy” the universe is.
In doing so, the IDE scenario reconciles both the CMB and Type Ia supernova data with the Fundamental Plane method. While in ΛCDM, the only way dark matter and dark energy can interact is via gravity, in IDE, they can also interact outside of gravity and exchange energy and momentum.
The paper proposes that for most of the lifespan of the universe, this flow of energy tended to go in one direction. But around 3 billion years ago, when the amount of dark matter and dark energy in the universe equalized, that flow of energy reversed.
First author of the study, Miguel Sabogal at the Universidade Federal do Rio Grande do Sul in Brazil, explains that this transition then results in the universe expanding faster than in ΛCDM, which could explain why Scolnic’s measured Hubble constant value — and other measurements rooted in the recent universe — is higher than ΛCDM predicts.
Sabogal and his team have a long road ahead to fully validate the IDE model, but they say work like this is needed when other well-known methods haven’t fully explained the observational data.
“History has repeatedly shown that when our best theoretical frameworks come into tension with observations, it is only a matter of time before a more comprehensive and refined paradigm emerges,” says first author Miguel Sabogal at the Universidade Federal do Rio Grande do Sul in Brazil. “Our role as scientists is to seek and develop promising models, such as our IDE framework.”
