Spiral galaxy NGC 628, situated 32 million light-years from Earth, is captured in an undated image from the James Webb Space Telescope. — Reuters file

New evidence supporting the baffling observation that the universe is expanding at a faster rate than anticipated has left scientists searching for answers—possibly involving the enigmatic cosmic components dark energy and dark matter. Data from NASA's James Webb Space Telescope, collected over two years, has confirmed the earlier finding by the Hubble Space Telescope that the universe's expansion rate is faster—by about 8%—than predicted based on current knowledge of the cosmos's initial conditions and its evolution over billions of years. This discrepancy is known as the Hubble Tension.

The observations made by Webb, the most advanced space telescope ever launched, seem to dismiss the idea that the Hubble data might have been flawed due to instrument error. 'This is the largest sample of Webb Telescope data—its first two years in space—and it confirms the puzzling finding from the Hubble Space Telescope that we have been grappling with for a decade: the universe is now expanding faster than our best theories can explain,' said astrophysicist Adam Riess of Johns Hopkins University in Maryland, the lead author of the study published on Monday in the Astrophysical Journal.

'Yes, it appears there is something missing in our understanding of the universe,' added Riess, a 2011 Nobel laureate in physics for the co-discovery of the universe's accelerating expansion. 'Our understanding of the universe contains a lot of ignorance about two elements—dark matter and dark energy—and these make up 96% of the universe, so this is no small matter.'

'The Webb results can be interpreted to suggest there may be a need to revise our model of the universe, although it is very difficult to pinpoint what this is at the moment,' said Siyang Li, a Johns Hopkins doctoral student in astronomy and astrophysics and a study co-author.

Dark matter, which is thought to constitute about 27% of the universe, is a hypothesized form of matter that is invisible but inferred to exist due to its gravitational effects on visible matter—stars, planets, moons, and everything on Earth—which accounts for roughly 5% of the universe. Dark energy, believed to make up approximately 69% of the universe, is a hypothesized form of energy that permeates vast regions of space, counteracting gravity and driving the universe's accelerating expansion.

What could explain the anomalous expansion rate? 'There are many hypotheses that involve dark matter, dark energy, dark radiation—for example, neutrinos (a type of ghostly subatomic particle)—or gravity itself having some exotic properties as possible explanations,' Riess said.

The researchers used three different methods to measure a specific metric—distances from Earth to galaxies where a type of pulsating star called Cepheids have been documented. The Webb and Hubble measurements were consistent. The universe's expansion rate, known as the Hubble constant, is measured in kilometers per second per megaparsec, a distance equivalent to 3.26 million light-years. A light-year is the distance light travels in a year, approximately 5.9 trillion miles (9.5 trillion km).

Under the standard model of cosmology—the conventional understanding of the universe—the value of the Hubble constant should be about 67-68. The Hubble and Webb data give an average value of about 73, with a range of about 70-76. The Big Bang event, occurring 13-14 billion years ago, initiated the universe, which has been expanding ever since. Scientists in 1998 revealed that this expansion was actually accelerating, with dark energy as the hypothesized cause.

The new study analyzed Webb data covering about a third of Hubble's full set of relevant galaxies. The researchers announced in 2023 that earlier interim Webb data had validated the Hubble findings. So, how might this Hubble Tension mystery be resolved? 'We need more data to better characterize this clue. Exactly what size is it (the discrepancy)? Is the mismatch at the lower end—4-5%—or the higher end—10-12%—of what the current data allows? Over what range of cosmic time is it present? These will further inform ideas,' Riess said.

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