What to Know About Euclid, Europe’s Space Telescope Launched to Study the ‘Dark Universe’

The telescope will image more than one billion galaxies and peer ten billion years into the past

The gold and white Euclid telescope sits in a lab with scientists observing it.
Euclid will travel almost a million miles from Earth to image distant galaxies. Valery Hache / AFP via Getty Images

At its orbit roughly one million miles away from Earth, the James Webb Space Telescope is about to get a companion: At 11:12 a.m. Eastern time on Saturday, another high-tech observatory—the European Space Agency’s (ESA) Euclid Space Telescope—launched from Cape Canaveral, Florida.

Euclid will image light that has traveled for ten billion years, build the most accurate and largest map of the cosmos and—scientists hope—provide insights about the “dark universe.”

By looking at how space has evolved over time, astronomers want to learn about dark energy, the mysterious cause of the universe’s increasing rate of expansion. The mission will also study the invisible, hypothetical form of matter in the universe, dubbed dark matter.

“If we want to understand the universe we live in, we need to uncover the nature of dark matter and dark energy and understand the role they played in shaping our cosmos,” Carole Mundell, the ESA’s director of science, says in a statement. “To address these fundamental questions, Euclid will deliver the most detailed map of the extra-galactic sky.”

Until the 1990s, scientists thought that the rate of the universe’s expansion must be slowing, since the gravity of all the matter in the cosmos should be pulling objects together, fighting against the universe’s expansion. But the Hubble telescope discovered that the universe is actually expanding faster now than it used to be.

Scientists don’t know exactly what’s causing this acceleration, but they call it dark energy. About 68 percent of the universe is dark energy, and an additional 27 percent is dark matter, which holds galaxies together. Galaxies don’t have enough “regular” matter to do the trick, so scientists say dark matter must be binding the cosmos together with its gravity.

The Euclid telescope aims to deepen our understanding of these cosmic mysteries. It’s equipped with two instruments for measuring visible light and infrared light. The visible light instrument will image the shape of more than a billion galaxies, while the infrared instrument will measure how far away those galaxies are.

Light traveling vast distances takes a long time to reach us, so it can provide scientists with a view of what a galaxy looked like long ago. Euclid’s instruments will scan more than one-third of the sky, peering deep into the universe’s past to capture galaxies as they appeared up to ten billion years ago.

The light from far-off galaxies will be bent a little bit by dark matter, allowing scientists to map where this mysterious matter lies. Then, scientists will trace the distribution of dark matter in the cosmos, revealing how the expansion of space has changed over time and how dark energy has influenced the universe’s structure, writes the Wall Street Journal’s Aylin Woodward.

“Euclid will be able to give us information on whether the universe is accelerating in its expansion in the particular way that we’ve already got in our models, or whether it’s changing and whether that has impacts on the ultimate fate of the universe,” Alina Kiessling, a cosmologist at NASA’s Jet Propulsion Laboratory, tells the Wall Street Journal.

Euclid successfully launched atop a SpaceX Falcon 9 rocket, though that was not the original plan. It was intended to launch on a Russian Soyuz rocket, but Russia stopped Soyuz launches with the ESA in the aftermath of the 2022 invasion of Ukraine, according to Sky & Telescope’s David Dickinson.

The telescope will spend approximately one month traveling to its destination—a point almost one million miles away from Earth in the opposite direction from the sun. Euclid will enter orbit around this spot, called the second Lagrange point (L2), joining the Webb telescope.

From L2, Euclid can block the light from the sun, moon and Earth so it can better see into deep space. After running some tests for about three months, the telescope will start collecting data.

“Ever since we could see stars we’ve wondered, is the universe infinite? What is it made out of? How does it work?” Michael Seiffert, a Euclid project scientist for NASA, tells the Agence France-Presse. “It’s just absolutely amazing that we can take data and actually start to make even a little bit of progress on some of these questions.”

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