Euclid mission - investigating the dark universe
- 26th Jul 2023
- Author: Dhara Patel
Universal mysteries
In the twentieth and twenty-first centuries our understanding of the universe has grown in leaps and bounds. We’ve gone from Edwin Hubble proposing the relationship between the distance and recession of a galaxy to the James Webb Space telescope pushing the limits of observing the most distant and highly redshifted galactic systems. And we’ve also advanced from Einstein predicting the existence of black holes with his theory of general relativity to supermassive black holes actually being imaged with a global-scale network of radio telescopes. But for all we’ve discovered and explained, there remain some mysterious that still elude us.
Amongst questions like does life exist elsewhere in the Universe and what came before the Big Bang, one of the biggest questions that scientist hope to answer revolves around the nature of dark matter and dark energy along with its effects on the expansion of the universe and growth of the cosmic structures within it. But as the names suggest, we aren’t able to observe these ‘dark’ entities so how do we know they’re even there?
There must be some ‘dark matter’...
In the 1930’s, Fritz Zwicky (a Swiss astrophysicist) was studying the Coma Cluster – a large group of over 1000 galaxies bound together by their own gravity. He estimated the mass of the cluster using two methods. The first – looking at the brightness and number of galaxies, Zwicky determined the ‘visible mass’ of the Coma cluster.
He also looked at the galaxies at the edge of the cluster and measured their motions. In our Solar System we see planets close to the gravitational centre (the Sun) orbit faster and the planets further away orbit slower. If Neptune – the most distant planet orbited faster than it does, there wouldn’t be enough of a gravitational pull from the Sun to keep it from flying out the Solar System – a little like a playground carousel or merry-go-round, if you spin too fast you get flung off!
Zwicky found that there wasn’t enough visible mass to keep the galaxies bound to the cluster. With their fast motions, there wasn’t enough gravity in the cluster to keep those around the edge of the cluster from escaping the clutches of this galactic stronghold. He realised that the Coma cluster had to have extra matter within it, as the more matter there is, the more gravity there is. Unable to observe or identify this additional matter he named the substance dunkle Materie, or dark matter in German. In the 1970's, American astronomer Vera Rubin then went on to find compelling evidence for the existence of dark matter.
Because dark matter doesn’t interact with the electromagnetic force, (it doesn’t emit, absorb, or reflect light), pinning down exactly what it is, has proved challenging. For some time weakly interacting massive particles (WIMPs) were believed to make up dark matter, but they’ve never materialised at the Large Hadron Collider experiment near Geneva in Switzerland. Another idea is that dark matter is the super-light ‘axion’, a presumed subatomic particle. And more recently but with less support, primordial black holes left over from the Big Bang have been proposed as the source of dark matter.
Discovering dark energy
Space itself, and I mean the stuff in between the more exciting planets, stars and galaxies, is quite extraordinary. From his theory of gravity, Einstein realised that empty space isn’t nothing – it’s possible for space to come into existence and it can possess its own energy.
Since 1929 when Edwin Hubble determined the Universe is expanding by looking at the redshift of light from distant galaxies, scientists began wondering whether there was enough gravity in the Universe to decrease and perhaps halt the expansion of space. Like a ball rolling up a slope, it gently slows to a stop under the influence of gravity.
In 1998, two separate teams of researchers were able to present evidence to suggest that the expansion of the universe isn’t slowing but in fact accelerating in its expansion – expanding faster and faster every day!
They looked at distant type 1a supernovae (the collapse and subsequent explosion of a star exceeding a certain mass after sucking in material from a companion). Because this mass limit is fixed, the brightness of these supernovae is always the same, so there’s a predictability in their perceived brightness from Earth based on how far away they are.
It turns out that these supernovae appeared fainter than their redshifts would indicate for a universe decelerating under gravity, where we would instead expect them to look brighter.
The fainter supernova pointed to an accelerated expansion of the Universe, but what could be causing the Universe to expand ever faster?
One explanation is dark energy - an elusive anti-gravity agent that permeates space. And because this ‘energy’ is a property of space itself, it doesn’t thin out as more space comes into existence, rather more of it just appears as space expands.
Newer observations from the Hubble Space Telescope have shown the Universe began expanding at an accelerated rate roughly five to six billion years ago which is when astronomers think dark energy’s repulsive nature tipped the balance over gravity’s attractive pull.
The discovery of dark energy was such a monumental revelation that it was recognised for the Nobel Prize in Physics in 2011.
How “dark” is the Universe?
The universe is surprising dark when it comes to its composition. By observing the universe’s expansion, we can infer that there is a lot of dark energy – in fact, it turns out over two thirds of the universe is dark energy.
And by observing gravitational effects such as gravitational lensing where by light from a distant source is bent around a foreground source due to its immense gravity, we can determine how much dark matter there is compared to normal ‘atomic or visible’ matter. Dark matter outweighs the evident stars and galaxies by a factor of about six.
The normal matter in the universe accounts for just under 5% of the universal composition, whilst dark matter accounts for 27% and dark energy 68% - that’s a pretty ‘dark’ universe!
Euclid - shining light on the dark Universe
We know very little about the universe and so a mission dedicated to studying the dark universe may not only help reinforce our theories and provide more understanding, new findings could even make us rethink our ideas about gravity.
Introducing ESA’s cosmology survey mission - Euclid
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![A dark matter halo mapped in galaxy Abell 1689.]()
NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), and T. Broadhurst (Tel Aviv University) Mission objective
Designed to explore the composition and evolution of the dark Universe, the Euclid mission will accurately measure the universe's acceleration. To achieve this, the telescope will make a 3D-map of the universe by measuring the shapes of galaxies at varying distances from Earth (out to 10 billion light-years, across more than a third of the sky). It will then investigate the relationship between distance and redshift to more accurately determine the increasing rate of the acceleration of the expanding universe.
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![Euclid launch and journey to L2.]()
ESA Launch and final destination
Euclid launched on 1 July 2023 from Cape Canaveral, Florida on a SpaceX Falcon 9 rocket. It spent roughly a month travelling out to its operational orbit – a halo orbit around a point known as the Sun-Earth Lagrange point 2 (L2). This lies at an average distance of 1.5 million kilometres from Earth. In this special location which is ideal for astronomy missions, it will keep the Sun, Earth and Moon behind it at all times, as not to interfere with observations. The mission is designed to last six years and any extension will be dependent on the cold gas reserves that are used for propulsion.
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![A replica of the Euclid spacecraft at Thales Alenia Space’s premises in Cannes, France, in 2019.]()
Stephane Corvaja/ESA Size and instruments
The Euclid spacecraft is approximately 4.7 m tall and 3.7 m in diameter – about the size of a big garden shed or outhouse.
It has a service module containing all the satellite systems and a payload module made up of a 1.2-metre-diameter telescope and two other scientific instruments: a visible-wavelength camera (the VISible instrument, VIS) and a near-infrared camera/spectrometer (the Near-Infrared Spectrometer and Photometer, NISP).
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![General relativity links gravity to the geometry of spacetime itself, and particularly to its curvature.]()
ESA–C.Carreau Key scientific questions
Euclid is designed to help scientists answer five primary questions:
- What is the structure and history of the cosmic web?
- What is the nature of dark matter?
- How has the expansion of the Universe changed over time?
- What is the nature of dark energy?
- Is our understanding of gravity complete?
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![The VIS instrument (delivered by a team of astronomers and engineers from the UK) is one of the largest camera instruments ever sent into space.]()
CEA Collaboration and construction
Euclid is an ESA (European Space Agency) mission. It is built and operated by ESA with contributions from NASA – namely the near-infrared detectors of the NISP instrument. More than 2000 scientists from 300 institutes in 13 European countries, the US, Canada and Japan – provided the scientific instruments and data analysis. The VIS instrument in particular was delivered by a team of astronomers and engineers from the UK.
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![Euclid of Alexandria - the 'father of geometry'.]()
public domian Naming the mission
Euclid is named after the Greek mathematician Euclid of Alexandria. He lived around 300 BC and is known as the ‘father of geometry’. As the density of matter and energy in the universe is linked its geometry, it was a fitting name for the mission.
Euclid is his anglicized name, his Greek name was actually Eukleides!
What next?
As the spacecraft reaches its home in space at L2 around the end of July 2023, the Euclid mission will spend about three month testing out its instruments before beginning its operational phase. We eagerly await what it may find, but we’re going to have to wait a little - the first data release containing findings from it first year surveying the sky, is due to be publicly released in 2025.
Full references / credits:
(Banner) An artist's impression of ESA's Euclid spacecraft. Credit: ESA/ATG medialab (spacecraft); NASA, ESA, CXC, C. Ma, H. Ebeling and E. Barrett (University of Hawaii/IfA), et al. and STScI (background)
(1 – video) Each bright knot is an entire galaxy, while the purple filaments show where material exists between the galaxies. This visualisation allows us to see the strands of material connecting the galaxies and forming the cosmic web. Credit: NASA/NCSA University of Illinois Visulization by Frank Summers, Space Telescope Science Institute, Simulation by Martin White and Lars Hernquist, Harvard University
(2a) Artist’s impression of dark matter surrounding the Milky Way. Credit: ESO/L. Calçada
(2b – gif) Left: expected rotation curve of a galaxy. Right: A galaxy with a flat rotation curve and high velocity stars on the outer edges bound to the galaxy by the effects of dark matter. Credit: Ingo Berg CC BY-SA 3.0 DEED https://en.wikipedia.org/wiki/File:Galaxy_rotation_under_the_influence_of_dark_matter.ogv
(3a) A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. Credit: NASA/CXC/M. Weiss
(3b) A timeline of our Universe extending from an unknown origin on the left to a darkening future on the right. Credit: NASA and the WMAP consortium
(4) The composition of the Universe. Credit: ESA
(5a) A dark matter halo mapped in galaxy Abell 1689. Credit: NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d'Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), and T. Broadhurst (Tel Aviv University)
(5b) Euclid launch and journey to L2. Credit: ESA
(5c) A replica of the Euclid spacecraft at Thales Alenia Space’s premises in Cannes, France, in 2019. Credit: Stephane Corvaja/ESA
(5d) General relativity links gravity to the geometry of spacetime itself, and particularly to its curvature. Credit: ESA–C.Carreau
(5e) The VIS instrument (delivered by a team of astronomers and engineers from the UK) is one of the largest camera instruments ever sent into space. Credit: CEA
(5f) Euclid of Alexandria - the 'father of geometry'. Credit: public domain
