It’s become clear that something doesn’t add up in the cosmos. Every second, the universe grows larger. Actually, it expands much more quickly than it should.

For some time now, observations of the early universe made with the Planck Telescope of the European Space Agency and what astronomers see when they measure the nearer, more modern parts of space with NASA’s Hubble Telescope.

When scientists look at what happened 13 billion years ago, via Planck, and then extrapolate it to the present, the results do not match what Hubble sees today. For several years, it has been assumed that the disagreement is due to lack of precision in measurements. But, as scientists fine-tuned their tools, the discrepancy remained. On Thursday, researchers using Hubble said the mismatch is some sort of user error or fluke from 1 in 3,000 to 1 in 100,000.

“The Hubble tension between the early and late universe may be the most exciting development in cosmology in decades,” lead researcher and Nobel laureate Adam Riess of the Space Telescope Science Institute, which leads Hubble’s science mission, said in a statement. “This mismatch has been growing and has now reached a point that is really impossible to dismiss as a fluke. This disparity could not plausibly occur just by chance.”

The results of the Hubble team were accepted in The Astrophysical Journal.

Riess says the discrepancy strongly suggests that the puzzle that scientists have put together over the years to model the universe’s history is missing.

One possible explanation could long ago be the appearance of dark energy. It is now theorized that up to 70% of the universe may consist of mysterious stuff.

Another possibility is an undiscovered and speedy particle in the universe that affects its expansion, as is the idea that unseen dark matter might push the normal matter we can see more strongly than we thought.

The explanation remains a mystery. Riess and other scientists plan to continue to fine-tune their tools and measurements, but if the mismatch is not due to human error, the puzzle may need new physics.

“Previously, theorists would say to me, ‘it can’t be. It’s going to break everything.’ Now they are saying, ‘we actually could do this,'” Riess told CNET.

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