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The expansion rate of the universe still has scientists baffled

The question of how fast the universe is expanding continues to confound scientists. Although it might seem like a fairly straightforward issue, the reality is that it has been perplexing the best minds in physics and astronomy for decades — and new research using the James Webb Space Telescope and the Hubble Space Telescope doesn’t make the answer any clearer.

Scientists know that the universe is expanding over time, but what they can’t agree on is the rate at which this is happening — called the Hubble constant. There are two main methods used to estimate this constant: one that looks at how fast distant galaxies are moving away from us, and one that looks at leftover energy from the Big Bang called the cosmic microwave background. The trouble is, these two methods give different results.

For a long time, scientists assumed that this was likely due to a measurement error. One or both of the measurements must be inaccurate, they thought, so it was expected that as technology progressed, the methods would get more accurate and come to an agreement. But that hasn’t happened. As the technology used in the estimates has improved, like the use of new and more powerful space telescopes, the discrepancy between the two results has stayed stubbornly in place.

This image of NGC 5468, a galaxy located about 130 million light-years from Earth, combines data from the Hubble and James Webb space telescopes. This is the most distant galaxy in which Hubble has identified Cepheid variable stars. These are important milepost markers for measuring the expansion rate of the Universe. The distance calculated from Cepheids has been cross-correlated with a Type Ia supernova in the galaxy. Type Ia supernovae are so bright they are used to measure cosmic distances far beyond the range of the Cepheids, extending measurements of the Universe’s expansion rate deeper into space.
This image of NGC 5468, a galaxy located about 130 million light-years from Earth, combines data from the Hubble and James Webb space telescopes. This is the most distant galaxy in which Hubble has identified Cepheid variable stars. These are important milepost markers for measuring the expansion rate of the universe. NASA, ESA, CSA, STScI, A. Riess (JHU/STScI)

Researchers recently used the latest measurements from James Webb and Hubble to check their calculations, and they didn’t find any inaccuracies in the previous measurements. In fact, they were able to narrow down these measurements to make them even more precise.

“With measurement errors negated, what remains is the real and exciting possibility that we have misunderstood the universe,” said lead researcher Adam Riess in a statement. “We’ve now spanned the whole range of what Hubble observed, and we can rule out a measurement error as the cause of the Hubble Tension with very high confidence.”

The researchers used both Webb and Hubble to cross-check the data, effectively checking each telescope’s measurements with the other. That was particularly important for calculations of very distant objects using Hubble, as Hubble wasn’t really designed to observe objects so far away, while Webb was. The two telescopes agreed, though, so it wasn’t Hubble data causing a problem.

“Combining Webb and Hubble gives us the best of both worlds. We find that the Hubble measurements remain reliable as we climb farther along the cosmic distance ladder,” said Riess.

Last year, early data from Webb supported the Hubble data, but there was still a question of whether the data was accurate for closer objects, but less accurate for more distant ones — as different distances are measured using different objects in the universe, which is referred to as the cosmic distant ladder. Now, the results show that the Hubble data is accurate not only for these closer objects, but also for the more distant ones.

So the big question is still as open and entrenched as ever. How fast is the universe expanding? We just don’t know, and we don’t know why the two types of measurement give contradictory results. The best explanation scientists have is that there is something different between the early universe and where we live now, called the late universe. “We need to find out if we are missing something on how to connect the beginning of the universe and the present day,” said Riess.

The research is published in The Astrophysical Journal Letters.

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Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
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