Euclid: Peering into dark matter 10 billion light years away

At the Lagrange L2 point (hereinafter referred to as "L2 point"), 1.5 million kilometers away from the Earth, the James Webb Space Telescope will welcome a "new friend".

At 11:12 a.m. Eastern Time on July 1, the European Space Agency's Euclid Space Telescope (hereinafter referred to as "Euclid") was launched from Cape Canaveral Air Force Base in Florida on a Falcon 9 rocket, starting its mission to explore dark matter and dark energy in the universe.

Unlike the Webb Space Telescope, which focuses on only a portion of the sky at a time, Euclid will cover a large swath of the sky outside the Milky Way at the same time, observing billions of galaxies within 10 billion light-years to create the largest and most accurate 3D map of the universe to date.

Why is it still L2?

There have always been two "dark clouds" in the sky of physics, namely dark matter and dark energy.

The 3D map of the universe that Euclid wants to "draw" contains information such as the shape, position and movement of galaxies. This will reveal the distribution of cosmic matter and the evolution of the universe, help astronomers infer the properties of dark energy and dark matter in the universe, and enhance human understanding of the nature of the universe.

The mission is so arduous that Euclid has to be given more "special functions". "Euclid's survey method is to take pictures and observe seamless spectra of a selected 15,000 square degree area of ​​the sky, which can completely cover the sky." Li Ran, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences, told the Popular Science Times reporter that after arriving at the L2 point, Euclid will undergo a two-month debugging period before carrying out scientific missions, including turning on two major devices, calibrating data, and confirming the thermal stability of the system.

"Euclid's near-infrared detector is very important for the completion of its sky survey mission. Launching to the L2 point will help it obtain lower observation noise. At the same time, it needs to make precise measurements of celestial body images, and the environment at the L2 point is more stable," Li Ran analyzed.

What's the difference with Webb?

There is already a Webb Space Telescope at the L2 point, so why launch Euclid?

Since a large number of galaxies need to be observed in a short period of time, telescopes such as Euclid that carry out sky surveys usually have a very large field of view, which means that a large area of ​​the sky can be exposed at one time.

"From a design perspective, Euclid and Webb are completely different." Li Ran explained that Webb is a general-purpose precision telescope with a small field of view, which can only see a smaller area of ​​the sky, but its observation accuracy for individual galaxies is higher; while Euclid is a survey telescope that needs to conduct census-style observations over a large area, and the requirements for very detailed observations of individual galaxies are lower.

Specifically, the primary mirror of Euclid is only 1.2 meters in diameter, much smaller than the 6.5 meters of the Webb Space Telescope, so its ability to observe faint celestial bodies is smaller. However, the range of Euclid's observation is about 0.57 square degrees at a time, while the Webb Space Telescope can only observe about 0.002 square degrees at a time. This large field of view determines that we will see a lot of galaxies in one photo. Ultimately, Euclid will study the nature of the universe itself by observing a large area of ​​galaxies.

Results are constantly changing

According to the official website of the Euclid Space Telescope, by comparing the observed distribution structure of matter in the universe over a large area with the physical model, researchers can infer the process of cosmic expansion and predict the future fate of the universe.

During the next six-year mission cycle, Euclid will use two instruments for observations, namely the Visible Light Camera (VIS) and the Near Infrared Imaging Spectrometer and Photometer (NISP). The Visible Light Camera contains 600 million pixels, which can observe the tiny deformation of the shape of galaxies and the change of luminosity in the visible light band, and infer the strength of the gravitational field that causes the deformation, so as to obtain the distribution information of dark matter. The Near Infrared Imaging Spectrometer and Photometer consists of 4×4 infrared detectors, containing 65 million pixels, which can perform high-precision spectral redshift determination of millions of galaxies.

Li Ran believes that Euclid is expected to measure several important dark energy parameters with an accuracy error of less than 10%, helping astronomers to see the evolution of dark energy at such a precision.

"Of course, it is possible that we still cannot see this evolution at this precision." Li Ran added that this will further confirm the current standard cosmological model and may rule out some theoretical models that allow dark energy to evolve significantly.

How has the universe expanded? What is the nature of dark matter and dark energy? In the future, Euclid is expected to collect more than 100PB of data, which will be supplemented and enhanced by observations from multiple ground-based telescopes. The scientific data obtained by the detector will be publicly released in 2025, 2027 and 2030.