Giuseppe Racca, Director of the Euclid Program at the UniversityEuropean Space AgencyESA). He talks about this scientific mission in an interview with Sciences et Avenir.
Sciences et Avenir: Following a launch scheduled for July 2023, the space telescope will return to its orbit around the L2 Lagrange point one month later, 1.5 million kilometers from Earth opposite the Sun. What will be its mission?
Giuseppe Racca: the scientific mission of Euclid, which should start in October 2023, cosmological mission dedicated to the evolution of the Universe over the past ten billion years. However, something intriguing happened during this period. If in 1929 Edwin Hubble demonstrated that the Universe is expanding, then in 1998 two international groups showed that this expansion of the Universe began to accelerate five billion years ago.
How to explain it?
One of the hypotheses put forward returns the cosmological constant, a term added to the equations of general relativity in 1917 by Einstein, so that his theory describes the universe that does not change in time, according to the idea that prevailed at that time. This cosmological constant could be a property of empty space rather than matter, which would act as a repulsive force that has a constant density in time and space.
So in the early epochs of the universe, when space was small, this energy was little felt. But once the universe got big enough, that vacuum energy became more powerful than matter. To understand this, the Euclid satellite will allow us to measure the constancy or variability of vacuum energy (or dark energy) over the past ten billion years.
What are the other assumptions?
The existence of a scalar field as described by quantum mechanics is also brought forward. But we have not yet managed to combine general relativity and quantum mechanics, so it is difficult to imagine the solutions offered by the latter. Moreover, this field has a magnitude of about 130 times different from what we observe!
What is the second part of the mission?
Euclid will also study the distribution and evolution of matter over the past ten billion years. Moreover, by matter we mean not only visible matter, i.e. 4.6% of the composition of the Universe, but also dark matter, the gravitational influence of which is measured, but whose nature remains unknown.
It does not emit light and probably cannot be described by the Standard Model of particle physics. All attempts to detect neutrinos and other exotic particles have so far been unsuccessful. Euclid will not allow us to find out the nature of this dark matter, but will allow us to describe its distribution. It seems that dark matter is distributed in halos around galaxies. It appears to have played a significant role in the formation of stars and large structures in the universe. Without it, there wouldn’t be enough materials to allow gravity to light stars. Also, some people call dark matter the stellar uterus …
How will this mapping of dark matter differ from previous ones?
Existing maps focus on parts of the universe. Euclid will do the same, but in even more detail and throughout the observable vault of heaven. With wide-angle lenses,Telescope with a diameter of 1.2 meters Thus, we will observe for six years 15,000 square degrees of the 40,000 square degrees of the sky, or about 36% of the universe, and the rest is somehow hidden by the stars and clouds of the Milky Way and the bodies of the solar system. It would take the Hubble Space Telescope 900 years to provide that kind of sky coverage! This will allow us to test Einstein’s general theory of relativity on a cosmological scale. And we are not immune from surprises…