Euclid and the origin of the accelerating universe

Credits: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler

The largest NASA Hubble Space Telescope image ever assembled, this sweeping bird’s-eye view of a portion of the Andromeda galaxy (M31) is the sharpest large composite image ever taken of our galactic next-door neighbor. Though the galaxy is over 2 million light-years away, The Hubble Space Telescope is powerful enough to resolve individual stars in a 61,000-light-year-long stretch of the galaxy’s pancake-shaped disk. It’s like photographing a beach and resolving individual grains of sand. And there are lots of stars in this sweeping view — over 100 million, with some of them in thousands of star clusters seen embedded in the disk. (Jan 2015)


The Euclid mission aims at understanding why the expansion of the Universe is accelerating and what is the nature of the source responsible for this acceleration which physicists refer to as dark energy. Dark energy represents around 75% of the energy content of the Universe today, and together with dark matter it dominates the Universes’ matter-energy content. Both are mysterious and of unknown nature but control the past, present and future evolution of Universe.

Euclid will explore how the Universe evolved over the past 10 billion years to address questions related to fundamental physics and cosmology on the nature and properties of dark energy, dark matter and gravity, as well as on the physics of the early universe and the initial conditions which seed the formation of cosmic structure.

The imprints of dark energy and gravity will be tracked by using two complementary cosmological probes to capture signatures of the expansion rate of the Universe and the growth of cosmic structures: Weak gravitational Lensing and Galaxy Clustering (Baryonic Acoustic Oscillations and Redshift Space Distortion).

Launch is planned for 2020.

Primary Science

Euclid will then address to the following questions:

  • is dark energy merely a cosmological constant, as first discussed by Einstein, or
  • is it a new kind of field that evolves dynamically with the expansion of the universe?
  • alternatively, is dark energy instead a manifestation of a breakdown of General Relativity and deviations from the law of gravity?
  • what are the nature and properties of dark matter?
  • what are the initial conditions which seed the formation of cosmic structure?
  • what will be the future of the Universe over the next ten billion years?

The VIS Instrument


The visible (VIS) instrument in under the responsibility of the Euclid Consortium. VIS at imaging all galaxies of the Euclid survey with very high image quality. It will be used to measure the shapes of galaxies and derive the gravitational lensing effects induced by large scale structures of the universe on galaxies. It will probe how the dark matter is distributed and how this distribution changed over the last 10 billion years.

The different subsystems of the VIS instruments are shown on the image below. The VIS focal plane is composed of a matrix of 6×6 4096×4096 e2v CCDs (Charge Coupled Devices), specially optimised for the Euclid mission (CCD273 ), covering a field of view of 0.57 deg2 (twice the angular size of the full Moon and about 450 times the field of view of the Hubble Space Telescope ACS camera) with 0.1 arc-second pixels. It will be equipped with one single very broad band filter covering the wavelength range from 550 nm to 900 mn with a mean image quality of about 0.23 arc-second.

The overall throughput of the VIS instrument will enable scientists to measure shapes of galaxies with sufficient accuracy. VIS will be able to get a signal-to-noise ratio of at least 10 for 1.5 billion galaxies down to magnitude 24.5 in 4000 seconds.

Gigapixels of Andromeda [4K] 



h/t Horizon and the BBC, click images to enlarge.