How fine are the laces of the universe?
Or to put it more prosaically, how does the content of the universe focus on the threads and knots of the great cosmic web that constitutes its fabric?
This parameter, its “granularity”, is fundamental in cosmology because it results in a certain way from the balance between the two most important and mysterious “substances” which make up the universe: dark matter (around 25% of its content). ), which is only felt by its gravitational effects and attracts things to it;
and dark energy (nearly 70% of its current content), whose nature and origin are even more uncertain, which on the contrary “repels” all things and would be responsible for the accelerated expansion of the universe.
However, there is currently “tension” on the value of this granularity (through a parameter called S8 by specialists).
On the one hand, the Planck satellite measured with stunning finesse the very slight fluctuations in fossil radiation, the first light emitted by the universe in which we are still immersed today.
Also called “cosmological diffuse background”, the latter carries within itself the trace of small primordial “lumps”, these small heterogeneities which have evolved to give rise to the large current structures.
Had the primordial soup been perfectly homogeneous, the universe would still be so today.
On the other side of the ring, “weak gravitational lensing” measurements also provide access to the granularity of the universe.
This time, scientists are looking at how the image of galaxies is slightly distorted by all the dark matter on the line of sight.
In particular dark matter which, contrary to the name given to it, is perfectly transparent.
By multiplying observations on a large number of galaxies, scientists are able to constrain the value of the S8 parameter.
Problem is, this then differs from that established by Planck's teams (it is slightly weaker).
When the same physics gives two different results, there are only two options: either at least one of the results is wrong, or the physics is not good.
Scientists would like it to be the second option: it would open a breach in the current structure, with the possibility of glimpsing new physics.
But to reach this extreme conclusion, solid arguments will be required.
It is in this context that the new work made possible by the Russian-German eRosita space telescope must be considered.
Launched in 2019, the latter provides a complete map of the sky in the X-ray field. A first catalog containing nearly a million sources, most of them entirely new, was made public at the end of January.
It covers the entire western hemisphere of the sky.
Unfortunately, the “est” catalog is missing, which remains the property of the Russians, and with which the Germans interrupted their collaboration after the start of the conflict in Ukraine.
Most of the objects that emit X-rays in the universe are active galactic nuclei (around 700,000 sources).
But they are not what interests us here.
There is in fact another more “cosmological” source: the very hot gas in which clusters of galaxies are bathed, which are, with superclusters (clusters of clusters), the largest structures in the universe.
This gas is very tenuous.
It contains only about one atom per liter.
This is the density that would be obtained by diluting a liter of air in a volume equivalent to twice that of the Moon.
For comparison, it is at least 10 to 100 times less dense than the void between stars.
But it is very hot: around ten million degrees - this means that the atoms move very quickly there.
It was by collapsing at the moment of formation of the cluster that the gas heated up, to the point that the atoms ended up tearing each other's electrons off to form a plasma.
However, these electrons emit X-ray photons when they slow down (this is called continuous braking radiation).
The eRosita catalog thus contains 12,000 of these “cosmological” sources of X-rays, the most distant of which are located between 7 and 8 billion light years from us (i.e. more than half the age of the universe ).
“We only used a little more than 5000 here, 5259 to be exact
,” explains Emmanuel Artis, astrophysicist at the Max Planck Institute for extraterrestrial physics in Garching, Germany, co-author of this first interpretation work. cosmological data from eRosita to be published in the journal
Astronomy & Astrophysics
.
“We wanted the cleanest catalog possible, we were very demanding on the selection criteria.
»
Although X-rays are a very powerful tool for detecting clusters, they do not directly give their distance and mass, although essential parameters.
“We rely on optical surveys to complete our data
,” recalls Nicolas Clerc, CNRS researcher at the Institute for Research in Astrophysics and Planetology (IRAP), specialist in galaxy clusters and their X-ray observations.
The game then consists of finding which parameters best explain the observations, based in particular on the results of large numerical simulations which retrace the evolution of the universe.
Results: eRosita's observations lead to a measurement of granularity compatible with that of Planck.
“We must remain very careful, but for the moment this rather suggests that the measurements by weak lensing effect are affected by a sort of still poorly understood systematics which gives a slightly different result
,” analyzes Emmanuel Artis.
“Theorists may be a little disappointed, but that’s not the end of the story.”
There are indeed other cosmological tensions which could also lead us to review our models of evolution of the universe, in particular on the Hubble constant which quantifies its speed of expansion.
The different calculation methods do not lead to the same result for reasons that are still unclear.
On the other hand, large-scale astronomical surveys are currently increasing.
In visible light and near infrared, the European Euclid space telescope is producing the most complete catalog of galaxies ever produced.
It will be duplicated in the southern sky by the LSST, currently under construction in Chile.
Large microwave and submillimeter radio telescopes, installed in particular at the South Pole and in Chile, should allow us to deepen our understanding of the cosmic microwave background (CMB-S4 project).
“All these data sets will give us increasingly strong constraints on the granularity and growth rate of the large structures of the universe
,” estimates Emmanuel Artis.
“Our theoretical model of evolution will collapse at one point or another, that’s for sure.
We are a few steps from the precipice, it’s very exciting.”