James' World 2

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The vast majority of the cosmos is made of different kinds of stuff we cannot see. We know it’s there, but it emits no light and may not even interact directly with normal matter (so we cannot touch it, either). How do you even begin to understand such things?

The key is to study them through the ways they do affect the universe. Dark matter has gravity, so it influences the way normal matter moves around and clumps up in galaxies. (Given how there’s five or more times as much dark matter as normal matter in the universe, I often wonder which one we should call “normal,” though.) It even dictates how light flows through the space it warps by its gravity. And dark energy is causing the expansion of the universe to accelerate, which changes how we measure distances to far-flung galaxies and alters the distribution of galaxies and galaxy clusters in larger-scale cosmic structures.

These are measurable effects, if you’re clever and have access to advanced technology. We humans are, and we do, and now the European Space Agency (or ESA) has put an exceptionally powerful new tool into our kit: Euclid, a space-based observatory that is designed to reveal the secrets of the dark universe—and quite a bit about the visible universe, too.

As telescopes go, in some ways, Euclid is modest. It sports a 1.2-meter-wide mirror, only half the width of Hubble’s and one-fifth the size of the one for the James Webb Space Telescope (JWST). But despite its smaller size, Euclid’s mirror is in one important respect superior to those of these two huge observatories: Unlike the narrow field of view provided by Hubble’s and JWST’s mirrors, Euclid’s offers a panoramic vista. Each of the telescope’s celestial snapshots can capture a staggering half a square degree of sky, more than twice the apparent area of the full moon. It scans a long, wide strip of the sky every day, and its lifetime goal is to observe an incredible 15,000 square degrees in total, more than one-third of the entire sky. And it will do so in high resolution—the ability to see small details.

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