The European Space Agency’s Euclid space telescope will study dark matter and dark energy, two of astronomy’s great mysteries.
As of this writing, the telescope is on its way to a solar orbit at the L2 Lagrange point, one of four spots in interplanetary space where the Earth’s and sun’s gravitational influences balance each other. The L2 location is about a million miles more distant from the sun than Earth’s orbit. It will join the Webb Space Telescope there.
The telescope’s orbit is thus outside and parallel to Earth’s path around the sun. It will face away from both the Earth and the sun. As a result, Euclid will be able to photograph about 35% of the sky and collect data on two billion galaxies over its six-year run if everything goes well.
Euclid will chart the universe’s history back to a period of universal development 10 billion years ago, about 3.7 billion years after the Big Bang.
The telescope’s primary light-gathering mirror is 1.2 meters (47.2 inches) in diameter. By comparison, the Hubble Space Telescope’s mirror is 2.5 meters wide.
Key to its mission is its Korsch optical system, a three-mirror design that allows the telescope to image a one-degree field of sky, about twice the full moon’s diameter. The Korsch design also limits distortion issues that plague many telescopes — spherical aberration, coma, astigmatism, and field curvature. Consequently, despite its relatively small light-gathering mirror, Euclid should be able to produce images of galaxies that rival the Hubble Space Telescope.
On board are two instruments. A visible-light camera will collect images of galaxies. A near-infrared spectrometer and photometer will attempt accurate measurements of the distance to those galaxies.
Euclid’s dual mission is to discover the nature and distribution of dark matter in the universe and help astronomers understand the “dark energy” that causes the universe to expand at a faster-than-expected rate.
When astronomers measured the mass of the universe, they discovered that they could see only 15% of it. That leaves 85% of all the universe’s matter that neither absorbs, reflects, nor emits light.
As a result, that “dark” matter cannot be studied with telescopes, which depend on gathering light in one form or another. However, dark matter does produce gravity and therefore interacts with visible gravitational matter.
Euclid will help astronomers understand dark matter by using two fundamental principles of astronomy — weak gravitational lensing and the redshift of receding galaxies.
Weak gravitational lensing occurs when intervening matter distorts the images of galaxies behind it. That warpage is called cosmic shearing. By precisely measuring the cosmic shearing of a given galaxy, astronomers can calculate the total mass of the material between that galaxy and the telescope.
When astronomers combine the shearing data of two billion galaxies, they will better understand the distribution of dark matter throughout the universe.
But there’s more. As galaxies fly away from us, their visible light stretches into longer wavelengths toward the infrared portion of the electromagnetic spectrum, a phenomenon called redshift. The farther away a galaxy is, the faster it is moving and the closer it is in time to the Big Bang.
At 13.7 years after the initial moment of the Big Bang, astronomers expected the expansion of the universe to be slowing down because of its collective gravity. Instead, the expansion is accelerating because of a mysterious force dubbed “dark energy.”
Using the redshift at various distances, astronomers can peel back the layers of time as if they were peeling the thin layers of an onion. As a result, they can discover the distribution of matter and its changing distribution over time.
They may thus begin to understand whether dark energy is a constant in the universe or whether it changes over time.
But the effects of Euclid on astronomy will go far beyond its mission objectives. Because each of Euclid’s images covers such a wide field of view, the telescope will gather clear images of an enormous patch of the sky over the next six years.
Consequently, the Euclid astronomical team hopes to image and catalog 12 billion galaxies. Their galactic survey will be helpful to a wide range of astronomers for many years to come.
Tom Burns is the former director of the Perkins Observatory in Delaware.
