An artist’s impression of the Euclid space telescope. Image: ESA.
The European Space Agency’s (ESA) Euclid space telescope will blast off from Cape Canaveral on 1 July on a mission to shed light on the ‘dark universe’ dominated by dark matter and dark energy.
Only 4.9 per cent of the universe is made from visible matter – i.e. the stuff that we can see and taste and touch and is made from atoms. This includes all the stars, gas, dust, planets, asteroids, comets and so on in the cosmos. The remainder is composed of dark matter (26.8 per cent) and dark energy (68.3 per cent). The fact that the nature of both, and hence the nature of the vast majority of the universe, remains mysterious is “the biggest embarrassment in cosmology”, ESA’s Guadalupe Cañas-Herrera told reporters during a pre-launch press conference on 23 June.
“We know that 95 per cent of our universe is something that is totally unknown to us,” she added.
We cannot see dark matter because it does not interact with light, but we can infer its existence because of its gravitational effect. We find dark matter filling the haloes of galaxies and making up the bulk of the mass of galaxy clusters.
Dark energy, meanwhile, is some kind of repulsive energy field that permeates the universe and has an anti-gravity effect. Whereas gravity tries to slow the expansion of the universe, dark energy is working against gravity and over the past 10 billion years has caused the expansion of the universe to accelerate.
“Euclid will observe the past 10 billion years, from when most of the stars and galaxies had formed and when dark energy started to be dominant,” said the mission’s Project Manager, ESA’s Giuseppe Racca.
There are various models put forward to explain both dark matter and dark energy. Is dark matter a new type of particle such as an axion or a WIMP (weakly interacting massive particle), or can it be explained by a modified version of the theory of gravity? And is dark energy the cosmological constant, or a dynamic, changeable energy field called quintessence?
“Euclid will measure dark energy and dark matter with unprecedented high precision and accuracy,” said the mission’s
Project Scientist, René Laureijs of ESA. In doing so, the intention is to be able to distinguish which of these models can be ruled out, and which are still possible. This will be accomplished by Euclid’s quite impressive technological capabilities.
Euclid’s 1.2-metre mirror, as well as components of its instruments, is made from silicon carbide, a material that combines the properties of ceramics, which provides Euclid with rigidity, and metal, which has good heat conductivity, so that the telescope can handle the thermal stresses that it will experience at the L2 Lagrange point, 1.5 million kilometres from Earth, which it will reach a month after being launched on a SpaceX Falcon 9 rocket. There are already numerous missions at L2, including the JWST. It’s a good place for missions to venture to, with the Sun, Earth and Moon all behind the spacecraft stationed there.
Euclid’s view is widescreen – its optics show an area of sky two-and-a-half times greater than the angular diameter of the full Moon in the night sky (which is about half-a-degree). Euclid needs to be able to cover a lot of ground fast because it has the entirety of the extragalactic sky to cover. This leaves out both the dusty Zodiacal plane and the plane of the Milky Way because both obscure the galaxies beyond, meaning Euclid only has to cover 36 per cent of the entire sky – in the region of 15,000 square degrees. Still, this will take Euclid a total of six years to cover but compared to the Hubble Space Telescope, that’s fast.
Euclid will survey the areas outside of the blue lines, away from the Milky Way and the Zodiacal light. The yellow areas represent the deep fields, which Euclid will observe to help maintain collimation of its instruments as well as obtain exceptionally deep views going to high redshift. Image: ESA/Gaia/DPAC; Euclid Consortium. Acknowledgment: Euclid Consortium Survey Group.
“For the Hubble Space Telescope it would take about 1,000 years to do the same job,” said Racca. Indeed, add up all the sky that Hubble has observed since it launched in 1990 and it amounts to about 100 square degrees. Euclid can survey an equivalent area in 10 days.
During the course of its six-year mission Euclid will observe more than 12 billion galaxies, going back 10 billion years in cosmic time. Euclid will perform its observations with two instruments, its Visual Imager (VIS) and its Near-Infrared Imaging Spectrometer and Photometer (NISP). Together they will build a three-dimensional map showing the distribution of all these galaxies. In particular, both will determine the shapes of galaxies, looking for the effects of ‘weak lensing’. This is a type of gravitational lensing, as gravity bends space and therefore the path of light through that space. The presence of diffuse intergalactic dark matter will subtly distort the light coming from these galaxies, and by looking for these weak distortions imprinted on galaxy shapes, somewhat akin to looking at something distorted that lies under water, Euclid will be able to map the presence of dark matter to a much higher fidelity than ever before.
The other tactic that scientists are taking with Euclid is to measure the degree by which galaxies tend to cluster. During the first 280,000 years of cosmic history, the universe was a sea of plasma, through which photons of light could not escape. The plasma was so dense that acoustic waves rippled through it. When the universe finally cooled enough for the plasma to dissolve, light was finally released, and we see it today as the cosmic microwave background (CMB) radiation. However, those acoustic waves had swept up many of those atoms, forming denser regions of matter that were able to gravitationally attract more matter to them over time. It’s these regions where galaxy clusters formed. The acoustic waves, which cosmologists call baryonic acoustic oscillations, have a characteristic size. By comparing their size based on the degree by which galaxies cluster during different epochs in the universe’s history with their angular size on the CMB, it’s possible to measure how much the universe has expanded, and how much that expansion has accelerated because of dark energy.
Of the 12 billion objects that will comprise Euclid’s dataset, the best 1.5 billion will have very accurate shape information as well as photometric redshifts (i.e. a good estimate of their redshift based on how red they appear). Of these 1.5 billion galaxies, Euclid will also measure spectroscopic redshifts for 30 million of them. Spectroscopically measuring redshift is more accurate than photometric measurements because it can precisely show how much spectral lines have shifted in wavelength in a galaxy’s spectrum of light as a result of the expansion of the universe stretching the wavelength of that light. The degree by which the light has been redshifted indicates how strong dark energy must be at different epochs.
“Euclid is more than a space telescope, it’s really a dark energy detector,” said Laureijs of the 1.4 billion euro mission.
Yannick Mellier, of the Institut d’Astrophysique de Paris IAP Paris and the Euclid Consortium Lead, went even further in his assessment of what the mission can achieve.
“It will reconstruct the cosmic history of the Universe over the last 10 billion years [and] in principle, Euclid should provide a decisive response on the nature of dark energy,” he said.
The 170 million gigabytes of data that Euclid will accrue over its nominal six-year mission won’t just tell us more about dark matter and dark energy. It will be used to extrapolate the future history of the universe. Will dark energy continue the expansion of space until it tears the cosmos apart in a ‘Big Rip’? Or can dark energy weaken, allowing gravity to get a grip and either maintain a status quo or cause the universe to collapse in a ‘Big Crunch’? In conjunction with ground-based surveys conducted by the likes of the Dark Energy Spectroscopic Instrument at the Kitt Peak National Observatory in Arizona, Euclid will help us close in on what fate holds for the Universe.