Un «monde miroir» fantomatique invisible pourrait être la cause d’une controverse cosmique avec Hubble Constant

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Selon de nouvelles recherches, un «monde miroir» invisible de particules qui n’interagit avec notre monde que par la gravité pourrait être la clé pour résoudre le problème de la constante de Hubble.

Selon de nouvelles recherches, un «monde miroir» invisible de particules qui n’interagit avec notre monde que par la gravité pourrait être la clé pour résoudre un casse-tête majeur de la cosmologie aujourd’hui – le problème constant de Hubble.

La constante de Hubble est le taux actuel d’expansion de l’univers. Les prédictions pour ce taux – à partir du modèle standard de la cosmologie – sont nettement plus lentes que le taux trouvé par nos mesures locales les plus précises. Cette divergence est celle que de nombreux cosmologistes ont tenté de résoudre en modifiant notre modèle cosmologique actuel. Le défi consiste à le faire sans ruiner l’accord entre les prédictions des modèles standard et de nombreux autres phénomènes cosmologiques, tels que le fond diffus cosmologique. Déterminer si un tel scénario cosmologique existe est la question que se posent des chercheurs, dont Francis-Yan Cyr-Racine, professeur adjoint au Département de physique et d’astronomie de l’Université du Nouveau-Mexique, Fei Ge, et Lloyd Knox de l’Université de Californie, Davis. ont travaillé pour répondre.

Cosmologie est l’étude scientifique des propriétés à grande échelle de l’univers dans son ensemble. Il cherche à utiliser la méthode scientifique pour comprendre l’origine, l’évolution et le destin ultime de l’univers entier. Comme tout domaine scientifique, la cosmologie implique la formation de théories ou d’hypothèses sur l’univers qui font des prédictions spécifiques pour des phénomènes qui peuvent être testés avec des observations. Selon le résultat des observations, les théories devront être abandonnées, révisées ou étendues pour tenir compte des données. La théorie dite du Big Bang est la théorie dominante concernant l’origine et l’évolution de notre Univers.

Selon[{” attribute=””>NASA, cosmology is the scientific study of the large-scale properties of the universe as a whole. Cosmologists study concepts such as dark matter, and dark energy and whether there is one universe or many, sometimes called a multiverse. Cosmology entails the entire universe from birth to death with mysteries and intrigue at every turn.

Now, Cyr-Racine, Ge, and Knox have discovered a previously unnoticed mathematical property of cosmological models which could, in principle, allow for a faster expansion rate while hardly changing the most precisely tested other predictions of the standard cosmological model. They found that a uniform scaling of the gravitational free-fall rates and photon-electron scattering rate leaves most dimensionless cosmological observables nearly invariant.

“Basically, we point out that a lot of the observations we do in cosmology have an inherent symmetry under rescaling the universe as a whole. This might provide a way to understand why there appears to be a discrepancy between different measurements of the Universe’s expansion rate.”

The research, titled “Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant,” was published recently in the journal Physical Review Letters.

Cosmic Background Explorer (COBE) Miission

The COBE satellite was developed by NASA’s Goddard Space Flight Center to measure the diffuse infrared and microwave radiation from the early universe to the limits set by our astrophysical environment. Credit: NASA

This result opens a new approach to reconciling cosmic microwave background and large-scale structure observations with high values of the Hubble constant H0: Find a cosmological model in which the scaling transformation can be realized without violating any measurements of quantities not protected by the symmetry. This work has opened a new path toward resolving what has proved to be a challenging problem. Further model building might bring consistency with the two constraints not yet satisfied: the inferred primordial abundances of deuterium and helium.

If the universe is somehow exploiting this symmetry researchers are led to an extremely interesting conclusion: that there exists a mirror universe very similar to ours but invisible to us except through gravitational impact on our world. Such “mirror world” dark sector would allow for an effective scaling of the gravitational free-fall rates while respecting the precisely measured mean photon density today.

“In practice, this scaling symmetry could only be realized by including a mirror world in the model — a parallel universe with new particles that are all copies of known particles,” said Cyr-Racine. “The mirror world idea first arose in the 1990s but has not previously been recognized as a potential solution to the Hubble constant problem.

“This might seem crazy at face value, but such mirror worlds have a large physics literature in a completely different context since they can help solve important problem in particle physics,” explains Cyr-Racine. “Our work allows us to link, for the first time, this large literature to an important problem in cosmology.”

COBE Satellite

An artist’s rendition of the COBE Satellite. Credit: Matthew Verdolivo, UC, Davis

In addition to searching for missing ingredients in our current cosmological model, researchers are also wondering whether this Hubble constant discrepancy could be caused in part by measurement errors. While it remains a possibility, it is important to note that the discrepancy has become more and more significant as higher quality data have been included in the analyses, suggesting that the data might not be at fault.

“It went from two and a half Sigma, to three, and three and a half to four Sigma. By now, we are pretty much at the five-Sigma level,” said Cyr-Racine. “That’s the key number which makes this a real problem because you have two measurements of the same thing, which if you have a consistent picture of the universe should just be completely consistent with each other, but they differ by a very statistically significant amount.”

“That’s the premise here and we’ve been thinking about what could be causing that and why are these measurements discrepant? So that’s a big problem for cosmology. We just don’t seem to understand what the universe is doing today.”

Reference: “Symmetry of Cosmological Observables, a Mirror World Dark Sector, and the Hubble Constant” by Francis-Yan Cyr-Racine, Fei Ge and Lloyd Knox, 18 May 2022, Physical Review Letters.
DOI: 10.1103/PhysRevLett.128.201301

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