Sent November 10, 2022, 12:10 pmUpdated November 10, 2022, 12:11 pm
“Something is wrong with our understanding of the universe. These words of the American cosmologist Dillon Brout, lead author of a important study published on October 19 in “The Astrophysical Journal”, summarize the mood of all his peers on both sides of the Atlantic!
Since Alberto Einstein laid the foundations in 1915 with his theory of general relativity, modern cosmology, after a century of increasingly advanced observations, it has built a solid theoretical edifice, which describes the global evolution of the universe in the 13.8 billion years that separate us from the Big Bang: it is the “standard model of cosmology”, which plays infinitely large the same conceptual structure role of the “standard model of particle physics” for the infinitely small. But a major crack has appeared in recent years in this beautiful building – and the “Pantheon +” analysis (hence the title of Dillon Brout’s studio) only exacerbates it a little more. The name of this crack that threatens the structure? The “Hubble tension”.
Faster and faster expansion
Since 1998, cosmologists have known that the expansion of the universe, a phenomenon discovered jointly by the American astronomer Edwin Hubble and the Belgian physicist Georges Lemaître in the 1920s, is accelerating. Which is contrary to intuition, as it roughly amounts to saying that a ball thrown into the air, instead of slowing down and then falling to the ground, would fly faster and faster towards the sky – but it is firmly established by observations.
The cause of this acceleration in the expansion of the universe is a subject of bitter debate. For some it is due to a hypothetical form of energy, at negative pressure (which has no equivalent in known physics), called “dark energy”, whose nature remains to be defined. For the others it would be simply the cosmological constant (called lambda), introduced in 1917 by Einstein in his equations to make his theory compatible with the idea he then had of a static universe, which he later denied when the expansion of l ‘ universe was discovered, then, finally, rehabilitated and reintroduced from Georges Lemaitre.
What is certain is that this acceleration is there and that the expansion continues at ever faster rates. But at what pace, exactly? Well, this is where the problems begin …
The so-called “Hubble-Lemaître” constant.
The expansion rate depends on the value of the so-called “Hubble-Lemaître” constant (nothing to do with the cosmological constant lambda), denoted by H0 and expressed in kilometers per second per megaparsec (km / s / Mpc). You say that H0 worth some value Xthis means that two galaxies 1 Mpc apart (or 3.26 million light years) are moving away from each other at the speed of X km / s. “H0it is the most fundamental parameter of cosmology, he explains astrophysicist and cosmologist Marc Lachièze-Rey. Almost everything we can say about the universe depends on its value, starting with its age. “
There are two main methods, one direct, the other indirect, to measure H.0. “When I started cosmology, these two methods gave 50 and the other 100. Then, in the 1990s, the data from the Hubble Space Telescope, whose very precise observations allowed us to refine our calibrations, brought these two together. measurements around 70. We thought it was the end of the story, but we were wrong, “says Marc Lachièze-Rey.
As water has been flowing under bridges since the 1990s, and as, in both types of measurement, the models and calculation techniques have continued to improve, a divergence has reappeared, to the point of becoming unbearably screaming!
Researcher at the Harvard & Smithsonian Center for Astrophysics, Dillon Brout, the head of Pantheon + analysis, uses the direct method to measure H0. This method is based on the study of a particular category of supernovae, type Ia supernovae, which occur when a white dwarf disintegrates in a thermonuclear explosion.
These explosions have the double advantage of being extremely bright (they are detectable up to over 10 billion light years away) and, above all, of possessing a known intrinsic brightness, determined by the laws of physics. This point is crucial because, when we know the intrinsic brightness of an astronomical object, we can, from its apparent brightness (measured with a photometer), deduce the distance that separates us from this object – which makes it what astrophysicists call it ” standard candle “.
Furthermore, the analysis of the redshift of the light reaching us provides the speed with which this object, due to the expansion of the universe, is moving away from us. Once these two quantities are known – the distance from the supernova (or from its host galaxy) and its speed of recession -, the Hubble-Lemaître constant, which is nothing more than the ratio of these two quantities, s is deduced by itself .
The “Cosmic Microwave Fund” study
It is also using type Ia supernovae as standard candles that the American astrophysicist and cosmologist Adam Riess was able, in 1998, to highlight the phenomenon of expansion acceleration, which has earned (together with Brian Schmidt and Saul Perlmutter) the 2011 Nobel Prize in Physics.
Dillon Brout and his team then took back the baton, but based on the study of an unparalleled set of 1,550 type Ia supernovae (the most distant being is 10.7 billion light years). This allowed them to obtain, for H.0, at a value of 73.4 km / s / Mpc, with a record accuracy of 1.3%. However, this result, which in the words of Adam Riess constitutes “the culmination of more than two decades of effort by observers and theorists around the world”, is in total disagreement with that provided by the other indirect method of measurement.
This second method is based on the study of the “Cosmic Microwave Fund”, this omnipresent microwave radiation emitted by the universe when it was only 380,000 years old and still detectable today. Since 2009 it has been examined by the European Planck Space Observatory and it is mainly the data from this satellite that today allows cosmologists to work on their models of the universe. In particular, it is possible for them to deduce, from the measurement of some characteristics of the cosmic microwave background, the value of H.0.
Extremely low margins of error
This second way of determining the value of this constant, although less immediate than using type Ia supernovae as standard candles, is also very robust, since Planck’s data are corroborated by other sources and consistent with everything we know of. ‘universe. But it gives a value of H.0 much lower, about 67km / s / Mpc.
And it is there, in this incompatibility between two results that both claim extremely low margins of error, that the Hubble tension resides. Is it due to a systematic error of analysis, in one field or another? Or is it a sign that the standard model, or even the general relativity that forms its basis, is not the right theory to describe the universe and its evolution from the Big Bang to today? Nobody has the answer yet. But this, if it happens one day, will remove a pebble from the shoes of all the geometers of the cosmos …
Overly simplified universe models?
And if the controversy surrounding the precise value of the Hubble-Lemaître constant was due neither to an error of analysis nor to an inadequate theory, but simply to the fact that cosmology uses simplified models of it to describe the real universe, for example assuming it is homogeneous? This is the hypothesis advanced by Marc Lachièze-Rey, who makes an illuminating analogy with the radius of the Earth. To say that the radius of the Earth is 6,371,008 km is more of an abstraction than anything else. You can bet that nowhere on the globe would this mean value be found, simply because the Earth is not a perfect sphere: not only is it flattened at the poles, but here it is also bristling with mountains, there carved valleys … ‘the same thing in play with the value of H.0. The two calculation methods would give different values because they are part of extremely simplified universe models compared to the real universe.
The composition of the universe
According to the Panthéon + analysis data, the universe would be composed 66.2% fromdark energy.
The 33.8% remaining would be divided into black matter and in ordinary matter, also called baryon matter – the only one we see.