Wednesday, November 29, 2023

The universe is growing faster than expected

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December 3 humanity suddenly had at their fingertips information that people have always dreamed of: precise distances from the stars.

“You type in the name of a star or its position, and in less than a second you’ll have the answer,” Barry Madore, cosmologist at the University of Chicago and Carnegie Observatories, said on a Zoom call. last week. “I mean…” He paused.

“We’re drinking from a fire hose right now,” said Wendy Freedman, also a cosmologist in Chicago and the wife and collaborator of Carnegie and Madore.

“I can’t overstate how excited I am,” Adam Riess of Johns Hopkins University, who won the 2011 Nobel Prize in Physics for the co-discovery of dark energy, said during a call telephone. “Can I show you visually what excites me so much?” We switched to Zoom so it can share nice graphics of the new star data on screen.

The data comes from the European Space Agency’s Gaia spacecraft, which has spent the past six years observing the stars from a perch 1 million kilometers high. The telescope measured the “parallaxes” of 1.3 billion stars – tiny changes in the apparent positions of stars in the sky that reveal their distances. “Gaia parallaxes are by far the most precise and precise distance determinations available,” said Jo Bovy, astrophysicist at the University of Toronto.

Better yet for cosmologists, Gaia’s new catalog includes special stars whose distances serve as criteria for measuring all further cosmological distances. Because of this, the new data quickly sharpened the biggest conundrum in modern cosmology: the surprisingly rapid expansion of the universe, known as the Hubble Tension.

The tension is this: the known ingredients of the cosmos and the governing equations predict that it is currently expected to expand at a rate of 67 kilometers per second per megaparsec – which means that we should see galaxies flying away from us 67 kilometers per second faster for each additional megaparsec of distance. Yet the actual measurements consistently exceed the mark. Galaxies are retreating too quickly. The gap excitingly suggests that an unknown invigorating agent may be going on in the cosmos.

“It would be incredibly exciting if there was a new physique,” ​​Freedman said. “I have a secret in my heart that I hope there is, that there is a discovery to be made. But we want to make sure we’re right. There is work to be done before we can say it unequivocally.

This work consists in reducing the possible sources of error in the measurements of the cosmic expansion rate. One of the biggest sources of this uncertainty has been distances from nearby stars – distances that the new parallax data seems to almost nail down.

In one article posted on December 15 and subject to The astrophysical journalThe Riess team used the new data to set the expansion rate at 73.2 kilometers per second per megaparsec, in line with their previous value, but now with a margin of error of just 1.8%. This apparently cements the gap with the much lower expected rate of 67.

Freedman and Madore plan to release their group’s new improved measure of the rate of cosmic expansion in January. They too expect the new data to strengthen, rather than change, their measurement, which has tended to land lower than Riess and those of the other groups but still higher than the prediction.

Since Gaia launched in December 2013, it has released two more massive datasets that have revolutionized our understanding of our cosmic neighborhood. Yet Gaia’s first parallax measurements were a bummer. “When we looked at the first data release” in 2016, Freedman said, “we wanted to cry.”

An unforeseen problem

If parallaxes were easier to measure, the Copernican revolution could have happened sooner.

Copernicus proposed in the 16th century that the Earth revolves around the sun. But even then, astronomers knew about parallax. If the Earth was moving, as Copernicus held it, then they expected to see nearby stars moving across the sky as it did, just as a lamppost appears to move relative to the background hills when you cross the street. Astronomer Tycho Brahe did not detect such a stellar parallax and thus concluded that the Earth is not moving.


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