The Universe appears to be expanding at an astonishing rate—perhaps too astonishing. Recent measurements have validated earlier, contentious findings: the Universe is indeed expanding at a pace beyond what theoretical models predicted and beyond what our current understanding of physics can clarify. This gap between observations and predictions is referred to as the Hubble tension. Recent results have provided even clearer evidence supporting this accelerated cosmic expansion.
The Universe appears to be expanding at an astonishing rate—perhaps too astonishing.
Recent measurements have validated earlier, contentious findings: the Universe is indeed expanding more swiftly than theoretical models have anticipated and surpassing what our existing knowledge of physics can clarify.
This gap between observations and predictions is commonly known as the Hubble tension. New findings published in the Astrophysical Journal Letters further reinforce the notion of this increased rate of expansion.
“The tension now turns into a crisis,” remarked Dan Scolnic, the lead of the research team.
Since Edwin Hubble’s groundbreaking discovery in 1929 that the Universe was expanding, determining its expansion rate—referred to as the Hubble constant—has been a key focus of scientific inquiry.
Scolnic, an associate professor of physics at Duke University, likens this endeavor to creating a timeline of the Universe’s growth: while we understand its size at the moment of the Big Bang, the challenge lies in charting its development to the current size. In his analogy, the Universe’s infant photograph symbolizes the far-off Universe, the initial seeds that formed galaxies, whereas its present image represents the local Universe, which includes the Milky Way and its neighboring galaxies. The standard cosmological model acts as the growth trajectory linking these two moments. The issue is that this connection seems broken.
“What this indicates, in some way, is that our cosmological model may not be accurate,” Scolnic noted.
Quantifying the Universe’s expansion necessitates a cosmic ladder—an arrangement of methods for gauging distances to celestial bodies, where each level or “rung” builds on the previous one for calibration.
The ladder employed by Scolnic’s team stemmed from another group utilizing data from the Dark Energy Spectroscopic Instrument (DESI), which observes over 100,000 galaxies each night from its position at Kitt Peak National Observatory.
Scolnic recognized that a more accurate measurement of a nearby galaxy cluster, known as the Coma Cluster, could improve the initial rung of the ladder.
“The DESI collaboration did the challenging part, but their ladder lacked the first rung,” Scolnic explained. “I was confident that I could supply that, leading to one of the most precise measurements of the Hubble constant we might achieve, so when their findings were released, I dedicated all my efforts to this without pause.”
To obtain an accurate distance to the Coma Cluster, Scolnic and his team, with support from the Templeton Foundation, utilized light curves from 12 Type Ia supernovae in the cluster. These supernovae serve as beacon-like sources with predictable brightness that relates to their distance, making them trustworthy for distance measurements.
The team determined the distance to be around 320 million light-years, which aligns well with distances reported in the past 40 years, reinforcing the accuracy of their measurements.
“This measurement isn’t influenced by preconceived notions about how the Hubble tension will resolve,” Scolnic remarked. “This cluster is close by; it had been measured long before we understood its significance.”
Using this precise measurement as the initial rung, the team calibrated the subsequent steps of the cosmic distance ladder, arriving at a Hubble constant value of 76.5 kilometers per second for every megaparsec. This implies that the local Universe is expanding at a rate of 76.5 kilometers per second for every 3.26 million light-years.
This value corresponds with existing measurements of local Universe expansion rates. However, like other prior measurements, it contradicts those derived from predictions about the distant Universe. In essence, while it aligns with the expansion rate as observed by other researchers, it diverges from what current physics theories suggest. The ongoing dilemma is whether the discrepancies arise from measurement errors or flawed models.
New results from Scolnic’s team substantially bolster the assertion that the root of the Hubble tension may lie within the models themselves.
“Over the past decade, there has been considerable re-evaluation within the community to assess the accuracy of my team’s original conclusions,” Scolnic noted, whose work often challenges traditional predictions of the Hubble constant based on established physics models. “Ultimately, despite changing many variables, we consistently obtain a similar outcome. Thus, for me, this serves as one of the strongest confirmations we’ve seen.”
“We’re at a critical juncture where we’re pushing back against the models we’ve been utilizing for over twenty-five years, noticing that things aren’t lining up,” Scolnic said. “This may radically alter our understanding of the Universe, and that’s exhilarating! There are still surprises awaiting discovery within cosmology, and who knows what will come next?”
This research was funded by the Templeton Foundation, the Department of Energy, the David and Lucile Packard Foundation, the Sloan Foundation, the National Science Foundation, and NASA.
