Expansion of the Universe: the “Hubble tension” transforms into a cosmological crisis

Clearly, measuring the speed of expansion of the Universe is a mystery that astrophysicists do not know how to solve. This effectively grows in all directions and everywhere at the same time, even if, with gravity doing its work locally, this expansion is not remarkable. Historically, this discovery was attributed to the astronomer Edwin Hubble in 1929. But following how cosmologists (physicists specializing in the structure and history of the cosmos) measure this speed thanks to their observations, the results diverge, hence a “Hubble tension”. And scientists don’t like that… at all!

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It’s no longer a simple “tension”, it’s a crisis

A few weeks ago, we presented to you the measurements of the James-Webb space telescope which confirmed those of its Hubble counterpart, and thereby, the so-called “Hubble voltage”. A new study today drives home the point, its lead author Dan Scolnic announcing that “the tension is now turning into a crisis“. His co-author is not without bringing a certain credence to this assertion since it is Adam Riess, Nobel Prize in Physics 2011 for the discovery of the acceleration of expansion, precisely. Let's see why, and especially how, they came to conclude that cosmology is in crisis.

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The speed of expansion — which is actually more of a rate than a real physical speed — basically tells us that every “X” kilometers, space itself grows by “Z” meters. It is called the Hubble constant and denoted H0. However, to measure this expansion, we need benchmarks, a sort of cosmic scale. This is how the very first measurements were made using stars whose absolute luminosity is known, the “Cepheids”, but the results were not very precise. Then we used supernovae (type 1a), these perfectly calibrated explosions of stars, whose maximum luminosity is always strictly identical.

A cosmic scale at 320 million light years in the middle of 1000 galaxies

The team behind the new study wanted to adopt the most precise and neutral cosmic scale possible. She therefore used the work of an independent group, DESI, known to scan up to 100,000 galaxies in a single night. The cluster of galaxies within which the measurements were taken is called Coma, or Berenice's Hair.

Previously, measurements from space telescopes and those made on the oldest image we have of the Universe, called the Cosmic Microwave Background (CMB) diverged by 8% compared to the methods mentioned previously. It was already too much to consider that the problem is not serious.

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The FDC measurement indicates an expansion of 67 kilometers per second per megaparsec (km/s/Mpc), with one megaparsec being the equivalent of about 3.26 million light years. That carried out by space telescopes and based on supernovae gives around 73 km/s/Mpc, while the estimate from the latest DESI study gives 76.05, even 76.5 km/s/Mpc, i.e. a deviation of 13 to 14% compared to the FDC, far too significant to be rejected in the name of measurement uncertainties.

The dynamics of the Universe partly escapes us

Something wrong. Are our theories, the standard cosmological model in mind, that are dysfunctional? Or even more exciting, is there physics that completely escapes us? Because we must recognize that the entire dynamics of the Universe poses a problem today, that of matter with the mysterious dark matter and that of space-time with its expansion, even its acceleration attributed to the enigmatic dark energy. Many teams are trying to better understand it, including those from the Euclid collaboration (ESA) whose results should arrive in the coming months. There are also different approaches to what dark energy can be.

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