Euclid: mapping the dark universe
The spacecraft’s instruments will observe billions of faint galaxies to reveal the signature of dark energy
“If we can be exact with Euclid, we’ll be able to tell if dark energy is constant or if it’s dynamic”
a 3,200-megapixel camera – the world’s largest. Currently under construction in Chile, its 8.4-metre (27.5-foot) diameter primary mirror will capture an image of the whole sky every three nights over a ten-year period.
And then there’s the European Space Agency’s Euclid telescope, which is expected to be launched into space in 2021. It will cover 15,000 square degrees of sky – that’s ten-times the area of the Kilo-Degree Survey – observing a billion galaxies and peering 10 billion years into the past. The images it sends back to Earth will be spectacular – similar in resolution to those taken by the Hubble Space Telescope.
In preparation for the launch, Paniez Paykari is modelling Euclid to ensure the instrument performs as intended. A research associate at the Mullard Space Science Laboratory, she explains that while all measurements come with statistical uncertainty, Euclid will give cosmologists more precise data to work with. “In the past 20 years evidence from all probes has pointed to the existence of dark energy – that it’s probably a cosmological constant but nothing more. If we can be exact with Euclid, we’ll be able to tell if dark energy is constant or if it’s dynamic.”
What if the treasure trove of data from new observations, expected in the mid-2020s, sounds the death knell for the cosmological constant? One possibility could be that, despite passing every test thrown at it for decades, Einstein’s general theory of relativity is wrong. Or at least that gravity may, in some circumstances, work differently than the theory describes.
“It might be gravity is not attractive on all scales. If you put two things close to each other they will attract because of gravity, but it may be that if you put them far enough apart they will repel. It could be that gravity is repulsive or somehow needs changing on cosmic scales,” says Kitching.
Many modified-gravity theories have been cooked up by theoretical physicists, and only more data will determine whether or not they’re correct. By a quirk of fate, many of these models were ruled out last year thanks to a gravitational wave signal named GW170817.
“It came out of the blue,” says Kitching. “A merger of two neutron stars produced a gravitational wave signal and a flash of light at the same time. These arrived [at Earth] at the same time too, which means that light and gravitational waves travel at the same speed.”
At a stroke, all the modified-gravity theories that also predicted different speeds for light and gravitational waves bit the dust. But there are plenty left, and it’s likely to be quite some years before the dark energy riddle is solved once and for all.
Solar arrayThe solar array consists of three panels which together supply up to 2,430 watts of electricity depending on the spacecraft’s orientation to the Sun. Visible imager (inside)VIS will take high-quality images equivalent in resolution to those captured by the Hubble Space Telescope. It’s designed to measure the shapes of galaxies. ThrustersThe thrusters are powered by cold nitrogen gas so as not to disturb any measurements. This is supplied by four highpressure tanks, holding a seven-year supply. Dichroic plate (inside)The dichroic plate splits incoming light, sending visible light to the VIS instrument and near-infrared light to NISP, allowing observations of both simultaneously. SunshieldThe sunshield protects the payload module, which contains the instruments, from the Sun, and supports the solar panels on the other side. TelescopeThe telescope has three mirrors. Its primary mirror is 1.2 metres (4 foot) in diameter and made of silicon carbide with a silver coating.Near-Infrared Spectrometer and Photometer (inside)The NISP instrument’s photometric measurements will be in the near-infrared to obtain the redshifts of millions of galaxies. Star trackersThree star trackers measure the telescope’s attitude (the direction it’s pointing) by comparing what it sees to a built-in star catalogue.
The Abell 370 cluster consists of hundreds of galaxies held together by gravity
Sunset over the Large Synoptic Survey Telescope, under construction in Cerro Pachón, Chile