'Very accurate' understanding of Universe's expansion will take scientists 5-10 years to calculate

Are pinning down the exact rate of expansion of the Hubble constant

Scientists from University of Chicago have claimed they will have a very accurate measurement of how fast our universe is expanding within five to ten years.

Until now, pinning down the exact rate of expansion, called the Hubble constant, after famed astronomer and University of Chicago alumnus Edwin Hubble, has been a difficult task.

Scientists have used two methods to calculate the value, but they provide very different results.

However, last year's surprising capture of gravitational waves radiating from a neutron star collision offered a third way to calculate the Hubble constant.

"The Hubble constant tells you the size and the age of the universe; it's been a holy grail since the birth of cosmology," said study author Daniel Holz, a University of Chicago professor in physics who co-authored the first such calculation from the 2017 discovery.

"Calculating this with gravitational waves could give us an entirely new perspective on the universe," he added. "The question is: When does it become game-changing for cosmology?"

In 1929, Edwin Hubble announced that, based on his observations of galaxies beyond the Milky Way, they seemed to be moving away from us - and the further away the galaxy, the faster it appeared to be receding. This is a cornerstone of the Big Bang theory, and it kicked off almost a century-long search for the exact rate at which this is occurring.

To calculate the rate at which the Universe is expanding, scientists need two numbers. One is the distance to a faraway object; the other is how fast the object is moving away from us because of the expansion of the Universe.

If you can see it with a telescope, the second quantity is relatively easy to determine, because the light you see when you look at a distant star gets shifted into the red as it recedes. Astronomers have been using that trick to see how fast an object is moving for more than a century - it's like the Doppler effect, in which (for example) a siren changes pitch as an ambulance passes.

The fresh paper predicts that once scientists have detected 25 readings from neutron star collisions, they'll measure the expansion of the universe within an accuracy of three per cent. With 200 readings, that number narrows to one per cent.

"It was quite a surprise for me when we got into the simulations," Chen said. "It was clear we could reach precision, and we could reach it fast."

A precise new number for the Hubble constant would be fascinating no matter the answer.

"With the collision we saw last year, we got lucky - it was close to us, so it was relatively easy to find and analyse," added Maya Fishbach, University of Chicago graduate student and another author on the paper.

She continued: "Future detections will be much farther away, but once we get the next generation of telescopes, we should be able to find counterparts for these distant detections as well."

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