The speed of expansion of the universe exceeds all predictions
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Located nearly 200,000 light-years from Earth, the Large Magellanic Cloud is a satellite galaxy of the Milky Way, ie it orbits around the latter. As the Milky Way pulls on its clouds of gas, they collapse to give birth to new stars that illuminate the Large Magellanic Cloud in glorious color like a kaleidoscope.
A new study has heightened the mystery surrounding the Hubble constant, one of the most important values in astronomy.
New evidence suggests that the current rate of expansion of the universe may be greater than that of its younger years, a difference that is prompting scientists to conduct research into the cosmic forces that may be at play. , the rate of change (9% faster than expected) would force us to rethink some fundamental aspects of the cosmos.

The results published in the Astrophysical Journal are the latest in the long-running controversy over the Hubble constant, which describes the rate of expansion of the universe at any given time.
In recent years, many studies have shown that measurements of the Hubble constant from the cosmic microwave background (the oldest light in the universe, emitted 380,000 years after the big bang) are at odds with estimates made from younger stars, for example those in our Milky Way , even after accounting for mysterious cosmic forces like dark energy that is accelerating the expansion of the universe.
"The universe is outpacing all of our expansion predictions, and that's very disturbing," says study lead author Adam Riess , an award-winning Johns Hopkins University astronomer. in 2011 by the Nobel Prize in Physics for helping to highlight dark energy alongside Saul Perlmutter and Brian P. Schmidt.
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For some, this discrepancy is due to the incomplete nature of the data, or undetected errors that skew the estimates . However, based on recent measurements of our cosmic surroundings made by the Hubble Space Telescope, Riess and his colleagues argue that the discrepancy is not only real, but is even greater than it appears. never been.
In the new study, Riess' team puts the Hubble constant at a value of 74.03 km/s/Mpc (kilometers per second per megaparsec), plus or minus 1.42. A figure inconsistent with the best estimates of Planck , the space observatory developed by the European Space Agency and author of the most precise measurements to date of cosmic microwave radiation. Planck's data pegs the Hubble constant at around 67.4 km/s/Mpc, plus or minus 0.5. In statistical terms, the difference between these two results is approximately 4.4 sigma, or a 1 in 100,000 chance that the discrepancy is simply due to chance.
“To illustrate, take, for example, a two-year-old child and measure his height and then try to estimate how tall he will be when he grows up. We can then wait until he has grown to measure him again,” explains Riess. “If it goes far beyond this extrapolation, we are faced with a real mystery. There is something wrong with our understanding of how he grew up. »
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TIME THE UNIVERSE
Calculating the Hubble constant and therefore the rate of expansion of the universe based on the motion of the stars requires two types of data: how far away a given star is and how fast it is moving away from us.
To measure the speed of a star, astronomers track variations in the light emitted by that star. To measure the distance, they use different tools ranging from pure and simple geometry to the careful observation of variable stars called Cepheids. The luminosity of these stars increases and decreases at regular intervals, the rhythm of these pulsations is closely linked to the general luminosity of the star: the brighter it is, the slower its pulsations.
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In February 1997, astronauts aboard the space shuttle Discovery took this photograph of the Hubble Space Telescope after a servicing mission to outfit the telescope with new, more capable devices.
Astronomers can use this relationship as a rule. By measuring the pulsation rate of the Cepheids, they can deduce the luminosity of the star and by comparing this absolute luminosity to that which they observe, they can deduce the distance which separates us from this star. It is also possible to combine the Cepheids with the observation of certain stellar explosions to measure further distances in the universe.
The development of this "scale of cosmic distances" required several years of work for astronomers and they are still trying to make it more precise. For this study, Riess's team used the Hubble Space Telescope to scan 70 Cepheids from the Large Magellanic Cloud, an irregularly shaped satellite galaxy of ours. This new data allowed them to more accurately estimate the distances between Earth and objects in the Large Magellanic Cloud, thanks to which they were then able to deduce the Hubble constant with very high precision.
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BALANCING THE ACCOUNTS
If the speed of expansion of the universe is really higher than our expectations, then it will be necessary to establish new physical theories to explain this divergence. Is dark energy more exotic or more energetic than we thought? Is dark matter more complex than we predict? Is there any other type of non-visible particle in the universe like sterile neutrinos that interact with other type of matter only by gravity?
And if our ledger of cosmic accounts was really in the red, then we would have to go to an outside accountant and he might be coming soon. In 2017, scientists detected gravitational waves, space-time vibrations and light projection following the collision of two neutron stars. This historic survey allowed astronomers to derive an estimate of the independent Hubble constant. For the moment, this value is inserted right between the estimates of Planck and those obtained thanks to the scale of cosmic distances.
The effectiveness of using such events as a "standard siren" to measure the expansion of the universe, however, rests on the number of neutron star collisions detected by gravitational wave observatories like LIGO. So far, only one of these events has been detected by astronomers, but LIGO may well have detected a second on the morning of April 25. That said, locating where the waves are coming from in the sky has proven difficult, complicating follow-up measurements using telescopes.

At the same time, Riess and astronomers around the world are working to improve the accuracy of their measurement of the Hubble constant in the hope that even a small deviation could yield new clues about how the universe works.
“A 9% discrepancy is already very worrying when the margin of error is 1 or 2%,” says Riess. “We have the impression that the universe is teaching us again and again. »


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