Why Euclid? /
QUESTIONS
EUCLID
The Name
The mission takes its name from the Greek mathematician and philosopher Euclid of Alexandria, who lived around 300 BC: “The Elements”, his best known work, represents one of the most influential works in the entire history of mathematics and contains those who were at the time the foundations of arithmetic and geometry.
Since, according to Einstein’s General Theory of Relativity, the geometry of the Universe is related to its matter-energy content, the mission was named in his honor.
Euclid is an ESA mission. It will performlead an observing campaign to verify how the acceleration mechanism
changes the expansion history and the three-dimensional distribution of matter
in the Universe.
Scientific questions
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Dark energy
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DARK MATTER
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SOLAR SYSTEM OBJECTS
DARK ENERGY
The universe is currently expanding: galaxies that are not gravitationally bound together are moving away from each other.
However, the gravity of matter tends to slow down the expansion, while in the late 20th century observations of a particular class of supernovae revealed that the expansion is not actually slowing down, rather it is accelerating.
In general relativity this is possible only if a constant term in its equations, called the cosmological constant, is not equal to zero but has a positive value. In fact, the cosmological constant has the effect of a repulsive force. If so, the universe is doomed to dilute and expand forever.
The cosmological constant is equivalent to a form of energy present in empty space, whose density remains constant over time and has the same value everywhere. This value currently represents more than two-thirds of the total density of the universe, and is more than double the density of matter.
On the other hand, quantum field theory predicts a vacuum energy that could be the physical cause of the acceleration. The problem is that, despite not being able to calculate the exact value of this density, the theoretical estimates are still enormously higher than the value deduced from observations.
It was hypothesized then that the acceleration could actually be due to a new form of energy that evolves over time and numerous proposals have been made on its nature.
There is also the possibility that general relativity no longer holds on cosmological scales and that it should be replaced by a modified theory of gravity.
The nature of dark energy remains one of the deepest mysteries of fundamental physics.
So far the observations are compatible with the cosmological constant, but more precise observations, such as those of the Euclid mission, aim to highlight discrepancies and establish which of the alternatives we have listed is the correct one.
DARK MATTER
Numerous observations indicate that, in galaxies, there is an amount of matter, called dark matter, much higher than the matter in the form of stars and gas, while, in clusters of galaxies, the quantity of dark matter is even a hundred times higher than the visible one. In the Universe, stars and gas represent only 5% of the total amount of mass and energy in the Universe, while dark matter constitutes 27% of the total (the remaining 68% is given by dark energy).
The observed abundances of light elements formed in the first few minutes after the Big Bang place strong limits on the density of matter in the forms we know, namely protons, neutrons and electrons. For this reason, black holes of stellar origin cannot contribute significantly to dark matter (primordial black holes are not excluded). The neutrino was for some time thought to be a possible candidate, but it has too little mass and, traveling with a speed close to that of light, it would cancel out small-scale fluctuations.
It is therefore believed that dark matter consists of a new type of particle, which does not fall within the standard model of particle physics and which moves at non-relativistic speeds: for this reason we speak of Cold Dark Matter (CDM). Among the favored candidates are WIMPs (Weakly Interacting Massive Particles), which interact only through the force of gravity and the weak force (the fundamental interaction responsible for the radioactive decay of atoms). The hypothetical lightest supersymmetric particle belongs to this class. Alternatives to WIMPs are axions, hypothetical much less massive particles.
Finally, we recall that in the Big Bang theory dark matter is a necessary component, since, by concentrating in halos during the first hundreds of thousands of years, when the radiation pressure prevented the ionized matter from aggregating, it allowed the subsequent formation of structures in the Universe.
SOLAR SYSTEM OBJECTS
The Euclid mission was designed to map the geometry of the Dark Universe. However, thanks to the characteristics of the survey, Euclid will allow studies to be carried out in secondary (but no less important) fields which are generally known as “legacy science”.
Although the survey will avoid observing the ecliptic, the programmed sequence of images acquired in different bands is well suited to the search and characterization of solar system objects (also known as SSOs).
These objects, observable in reflected light, appear to move against a background of motionless sources or, if their speed is particularly high, induce streaks detectable in Euclid’s images.
In order to maximize the number of objects observed, dedicated tools (some based on the use of AI) have been developed for the search for SSO.
We expect to observe and characterize more than 150,000 new objects (from near-Earth SSOs to sources in the Kuiper belt) and study their dynamic, physical and compositional characteristics and produce alerts for other observatories such as the Vera Rubin Observatory.
THE STANDARD COSMOLOGICAL MODEL
The standard cosmological model describes the evolution of the Universe starting from a primordial state of very high density and temperature, in which it was 13.8 billion years ago.
It is based on the solution of the field equations of general relativity, assuming that matter and energy are uniformly distributed in space, and depends on the matter and energy content of the Universe, in particular on the densities of matter, vacuum energy, radiation, neutrinos.
Observations have long indicated the presence of large amounts of dark matter in galaxies and galaxy clusters. At the same time, in the framework of the Big Bang theory, dark matter is necessary to explain the formation of structures starting from small initial density fluctuations, but it cannot be in forms known to us and cannot interact with electromagnetic radiation: it is hypothesized that it is a new type of particle, moving at non-relativistic speeds. This new type of matter has been called Cold Dark Matter or CDM.
The observations of a particular type of supernovae (the Ia), of which the maximum luminosity is known and the distance can be estimated, show that the expansion of the Universe is accelerating: an effect which in general relativity is due to the presence of a cosmological constant (denoted by the capital Greek letter lambda Λ) greater than zero.
Finally, the analysis of the observations of the cosmic microwave background has shown that the density of the Universe has a value corresponding to what is called the critical density, which implies that space on large scales is not curved and that the Universe is infinite. Since there is always an uncertainty in the measurements, the density could be slightly higher than the critical density: in this case the space would have an imperceptible curvature and the Universe would be finite.
At the end of the 20th century, thanks to several independent observations (supernovae Ia, cosmic microwave background, mass of galaxy clusters and their abundance, statistical properties of the large-scale structure) which gave consistent results, it was possible to determine with good precision the current densities of the main components that define this model.
Thus we know that luminous matter in the form of stars and gas, made up of particles known to us, such as protons and neutrons, represents only 5% of the total amount of mass and energy in the Universe, while dark matter constitutes 27% of the total. The dominant component, more than two-thirds of the total, is the density of dark energy, associated in the standard model with the vacuum energy corresponding to the cosmological constant.
Based on its two principal components, the standard cosmological model is called the ΛCDM model. Although it is in good agreement with all observations made so far, there are some discrepancies: it is not clear if they are due to observational problems or if they represent symptoms of real problems with the model.
GALAXY EVOLUTION
The main purpose of Euclid is to build a map of the distant Universe. Only a small fraction of existing matter emits light, mostly in the form of stars, collected in galaxies. These are therefore the main tracers of what we call the large-scale structure of the Universe. Euclid will observe a billion and a half galaxies: of these Euclid will precisely measure the distance, with spectroscopic methods, of about 20 million of them.
We know that galaxies are not distributed randomly in the sky but follow a pattern dictated by the evolution of matter under the action of its own gravity. This scheme retains memory of primordial density fluctuations, interwoven with information about how gravity caused them to grow. A precise measurement of the large-scale structure therefore makes it possible to recover information on matter, mostly dark matter, on the expansion history of the Universe, accelerated by dark energy, and on the functioning of gravity, which could deviate from general relativity.
Galaxies are objects of great interest in themselves: they are the first step in the transition from the “simplicity” of the large-scale Universe to complexity. They can be seen as primordial ecosystems, governed by the birth and death of stars that evolve by receiving primordial matter from the outside, expelling it by galactic winds fueled by supernova explosions, and colliding with each other in merger events. This complexity is very difficult to decipher: the immense amount of high quality data that will be produced by Euclid will be invaluable for understanding the process of galaxy formation and its relationship with the large-scale structure of the Universe.
THE LAGRANGIAN POINT L2
Euclid’s observations require very high pointing stability.
The satellite was built to minimize perturbations, including those induced by temperature variations. To obtain a thermal stability of a few thousandths of a degree, it is necessary to place the satellite as far as possible from the Sun, while keeping it in a stable orbit that does not cause it to escape.
Physics tells us that the Sun, the Earth and a satellite can remain linked together in their reciprocal motion if the satellite is located in specific positions, the so-called ‘Lagrangian’ points. For Euclid, the Lagrangian point L2 was chosen, 1.5 million kilometers from the Earth: the farthest.
By orbiting this point, the satellite will follow the revolution of our planet, always keeping the telescope in the shaded part, opposite the Sun. It will be in good company with the James Webb Space Telescope!
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