James-Webb confirms the cosmological problem of “Hubble tension”

The Hubble tension problem is very pernicious. For some, this is a major crisis in cosmology which, let us remember, is the science which studies the structure, origin and evolution of the Universe as a whole. For others, there may not even be an enigma, but few cosmologists specializing in the subject think that.

To fully understand, we must remember that we know that the Universe is expanding thanks to studies and observations ofEdwin Hubble and Georges Lemaître. It grows every second and in all directions, like a balloon being inflated or a raisin cake being baked. We noticed this phenomenon 100 years ago by measuring the speeds of distant galaxies: on average they are all moving away from us, like the grapes in the cake in the dough swelling in the oven!

The Universe “inflates”, but to a point!

Of which act. But a problem arose when Nobel Prize-winning physicist Adam Riess and his team decided to use two—robust—different observational methods to measure this rate of expansion of the Universe. And there, unfortunately, a disagreement appeared, albeit slight, but a disagreement between the measures. On the one hand, the team measured the expansion by studying certain distant stars whose absolute luminosity is well known, which makes it possible to calculate their distance from us. On the other hand, she analyzed tiny variations in the Cosmic microwave backgroundthis first light of the Universe emitted 380,000 years after the Big Bang.

Oddly, these two measurements differed by more than 10%. The Universe therefore seemed to say two different things about itself when asked the same question at different ages. Many felt that these measurements were not precise enough to cause people to tear their hair out. So, in 2019, the Hubble space telescope scanned space and its nooks and crannies as closely as possible. And there, almost everyone had to admit the problem: the 10% difference remains, and in astrophysics, 10% is a lot. This is how the term “Hubble tension” emerged, even going so far as to call into question our standard cosmological model. Nevertheless, there was still hope for harmony at the time thanks to the future James-Webb telescope, more precise than Hubble.

The tension rises, James-Webb drives the point home

And falter again. While it was hoped to reduce the divide between the two measures of expansion, quite the opposite happened. Here are the figures… For Hubble and James-Webb, the Universe is growing at 74 km per second per megaparsec (astronomical unit of distance equal to approximately 3 million light years). And when scientists study the Cosmic Microwave Background, the expansion is then “only” 67 km per second per megaparsec.

What little difference, you might say, when other more “down to earth” problems prevent you from sleeping? For cosmologists, this is huge! Above all, they fail to understand where this 10% comes from. There are obviously hypotheses, particularly depending on the type ofdark energy believed to be responsible for the expansion, but nothing conclusive yet.

Let us remember that another space telescope is dealing with this immense mystery, and that it is European. This is Euclid, responsible for photographing galaxies in the hope of better understanding the two enigmas of dark matter and dark energy.

Rejoice in not being an astrophysicist or cosmologist, “Hubble tension” will probably not spoil your end-of-year celebration meals!