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December 19, 2018, 4:56 pm
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Cosmology from Space – Euclid

The Euclid mission is an M-class mission of ESA whose main goal is to explain the accelerated expansion of the Universe and the nature of dark-energy and dark matter. The mission concept was approved in the framework of the ESA’s Cosmic Vision programme on Oct. 4th, 2011. Euclid is currently in its implementation phase. The expected year of launch is 2020.

General information about Euclid

Euclid will consist of a 1.2 m telescope which will be able to collect images of the sky covering a broad range of wavelengths. It will have two channels. The first is equipped with an imager capable to observing the sky in the wavelength range 550-900nm (VIS channel). The second will be sensitive in the Near Infra-Red (NIR) part of the electromagnetic spectrum (900-2000nm) and will possess both imaging and spectroscopy capabilities.

With these instruments, Euclid will observe the extra-galactic sky, performing (at least) two surveys:

  • a “wide” survey, covering an area of 15000 sq. degrees at high and low galactic latitudes (excluding the galactic and the ecliptic planes), will reach a depth of 24.5 AB mag in the VIS band (riz) and 24 AB mag in the NIR bands (YJH);
  • a “deep” survey of 40 sq. degrees, which will be 2 mags deeper than the “wide”.

Over its lifetime of 6.25 years, Euclid will provide high quality images of almost 2 billions of galaxies. Additionally, it will deliver spectra for several millions of galaxies. All this will enable research on a wide range of topics.

What questions will Euclid answer?

First of all, Euclid will address the following questions, which represent the primary goals of the mission:

  • Is dark-energy a cosmological constant or is it a new kind of time evolving field?
  • Is dark-energy a manifestation of the break-down of the Theory of General-Relativity, meaning that we need a new Theory of Gravity?
  • What is the nature of dark matter?
  • What were the origin and what will be the faith of the Universe?
This will be achieved combining several probes.  Very shortly, Euclid aims at detecting the imprint of dark energy and gravity on the growth of the cosmic structures. There are several techniques to work this out. Euclid has been designed to achieve its goals through the measurement of the  weak lensing signal (a.k.a “Cosmic Shear”) generated by the Large-Scale-Structure of the Universe and through the detection of the so called “Baryonic-Acoustic-Oscillations” and redshift space distortions.

What is the cosmic shear?

Any mass in the Universe is a gravitational lens. The Theory of General Relativity explains the effects of gravity as a space-time distortion. The light traveling through a perturbed space-time bends around the massive bodies, similarly to the light passing through an optical lens. If the emitting source is extended, like a galaxy, its image is also distorted, being elongated around the gravitational lens or perpendicularly to it.  It is intuitive that if this effect can be measured, important information can be drawn about the mass which is causing it. This is not an easy task, though, because the light emitted by very distant galaxies travels through the space-time being deflected by any cosmic structure along its path. At each deflection the shapes of these galaxies are distorted. Moreover, unless the photons pass through the center of galaxies or galaxy clusters, the deflection  is tiny and can be revealed only in a statistical sense. This is the “weak gravitational lensing” effect.
The great resolution and sensitivity of Euclid coupled with it large field of view will allow to measure  the shapes of  almost two billions galaxies. Combining the observations of Euclid with other observations taken with ground based facilities, the distance of these galaxies will be determined with good accuracy using the “photometric redshift” approach. This means that Euclid will allow to measure the lensing effects produced by the large scale structure of the Universe as a function of distance from us, i.e. as a function of  time.  In other words, Euclid will use the weak lensing measurements to carry out a sort of “tomography” of the Universe which will allow to study how the cosmic structures evolve during time.
What are the Baryon Acoustic Oscillations?
In its early stage, the Universe was filled with an hot plasma of electrons, protons and neutrons.  Photons could not travel freely in this medium as  they were continuously interacting with the free electrons via Thompson scattering. When the Universe cooled down, the electrons started to be trapped into hydrogen atoms. Being these atoms much heavier than the free electrons, the interaction between the matter and the radiation became much less efficient and the photons could escape. Until this happened, the fluid of baryons and radiation was subject to two kind of competing forces. Imagine that the primordial universe is dominated by dark matter. Dark matter particles are collision-less and interact only via gravity. If, at some point, a small dark matter density fluctuation is originated, gravity pulls the baryons to collapse into the dark matter potential well, but, at the same time, the interaction with radiation generates a sort of pressure counter-acting the effect of gravity. As a result, acoustic waves propagate through the fluid outward of the over density.  When photons stop interacting with the baryons, they diffuse away and  the photon-baryon pressure driving the wave outwards ceases. At this stage, the matter distribution is characterized by a central over-density (generated by the dark matter fluctuation) which attract gravitationally the baryons and by a ripple of baryons. The radius of this ripple is a well defined distance: it is the distance traveled by the wave between the time when the over-density was originated and the time when the baryons and the photons stopped interacting. Since we know what this distance is (~100 Mpc) we can use the size of the ripple as a standard ruler. By measuring the angle it subtends at different epochs we can measure how the Universe is expanding.
How can we see the ripple? The baryons in the ripple eventually cool down and form stars and galaxies. The ripple can then be revealed as an excess of galaxies around the center of the over-density. In fact, many of such over-densities are originated in the primordial universe, meaning that the galaxy distribution will be characterized by many overlapping ripples. Nevertheless, the angular size of the ripples can be detected again using a statistical approach, i.e. by finding if there is a characteristic correlation length in the galaxy spatial distribution. This is what Euclid will do using the positions and distances of all the galaxies it will observe.