Euclid Telescope Poised to Investigate Mysteries of Cosmic Expansion

On Saturday (July 1), the European Space Agency’s Euclid space telescope is set to take its first voyage into orbit, where it will help researchers tackle a fundamental scientific mystery: why the universe is expanding at an accelerating rate.

Over the next six years, Euclid will conduct a survey of the positions, shapes, and distances of more than one billion galaxies across about one-third of the sky, in search of traces of dark matter and dark energy, which thus far have eluded every attempt to observe or measure them. The observations will provide clues as to why the expansion of the universe is speeding up, allowing scientists to make predictions about the eventual fate of the cosmos. The satellite will begin its mission aboard a SpaceX Falcon 9 rocket, which will launch from Cape Canaveral, Florida.

Cosmic acceleration is one of the “biggest mysteries in all of physics,” Eric Huff, a staff scientist at NASA’s Jet Propulsion Laboratory (JPL) who worked on the Euclid project, told SpaceRef. Dark energy is thought to fuel the universe’s growing expansion, working in opposition to gravity to push objects in space apart over time. Dark matter, defined as matter that does not interact with light, counteracts dark energy by exerting a gravitational force that pulls objects together. Although more than 95 percent of the universe is made up of dark energy and dark matter, neither are directly measurable, and scientists have no clue what they are.

Still, both phenomena have observable effects on the 5 percent of the universe we can see. Indeed, the galaxies of luminous matter that light up the night sky are thought to be held together by dark matter’s invisible tug, and its gravitational pull can bend light as it travels through the cosmos. By studying how dark energy and matter influence distant galaxies, scientists can begin to understand the push and pull of cosmic acceleration.

Chasing Dark Energy

The Euclid project is more than two decades in the making, explained Huff. In the early 1990s, scientists first discovered evidence that the universe was expanding at an increasing rate, sprouting one of the biggest puzzles in cosmology.

“The way we understand the evolution of the universe is that there was the Big Bang, and the universe started expanding by inertia. And people thought that this expansion was going to be decelerating at some point, given that matter pulls things together due to gravity,” Judit Prat, a cosmologist at the University of Chicago, told SpaceRef.

“What we found is that it’s speeding up,” said Huff.

With Euclid, researchers can improve and challenge current models of how the universe behaves. Ever since Einstein developed the theory of relativity, models of the universe have included a cosmological constant, a term Einstein initially used to mathematically account for the expansion of the universe and negate the effect of gravity. For years, the prevailing view has been that dark energy is the source of the cosmological constant.

Since the 1990s, after the discovery that the universe is expanding at an accelerating rate, scientists have predicted that the cosmological constant has a positive value, greater than the negative value of gravity’s pull. Were it any lower, the mathematical models would instead predict or correspond to a contracting, shrinking universe. The cosmological constant has been the field’s “null hypothesis,” said Huff. “All of the measurements we made seemed pretty consistent with that story.”

But in recent years, experiments using instruments on the ground have provided hints that the cosmological constant may not actually be a constant after all, said Prat.

“The kind of universe where dark energy remains the same forever looks very different from one where it changes in time,” said Huff. If the cosmological constant was increasing over time, explained Prat. For example, the universe may eventually rip apart in a cosmological catastrophe that would leave nothing left, not even space itself.

The effects of dark energy and dark matter on the observable universe are very slight, meaning that to test these theories, scientists need to make extremely precise measurements—which Euclid can provide. Unlike earlier grounded telescopes, Euclid’s position in space will allow the satellite to make better measurements than ever before. On the ground, telescopes need to filter out the effects of the atmosphere, which can distort their measurements. Euclid will also have a much larger field of view than other instruments. It carries two instruments on board: The Visible Instrument (VIS) and the near-infrared spectrometer. VIS allows Euclid to measure the shapes and positions of galaxies, but not their distances. The spectrometer will allow Euclid to measure the distance and position of galaxies.

Euclid will study the accumulation of matter using weak gravitational lensing. Gravity influences light in the same way it does matter, pulling it towards any sufficiently massive object. Whenever the gravitational field around a massive celestial body, such as a galaxy, causes a sufficient curvature of spacetime for the path of light around it to visibly bend. “It’s a lot like looking through a thick piece of glass at a distant scene,” said Huff. This technique will allow the telescope to measure clumps of dark matter, as the mass of dark matter will correspond to the extent of this warping or bending.

The telescope will also look at galaxies that are farther away in order to study how cosmic expansion may have changed over time. Since light moves at a constant speed, faraway galaxies provide a glimpse into what the early universe was like. Scientists will study the comparative distance between galaxies that formed in different eras to answer questions about how the universe developed and how fast it expanded.

“The ultimate goal is to understand our universe better,” said Prat. “There are some indications that the standard cosmological model may or may not be completely right. And on top of this, we didn’t know what that energy would even be. We really have no clue. That’s what makes us think that there’s something that we’re not really grasping about the universe.”

“I think it’s a good bet that we’re usually better off understanding how nature works,” said Huff. “Whatever the future of the universe looks like, it’s mostly going to be a result of whatever the properties of dark energy are.”

Euclid is just one of the many projects seeking to map dark matter and dark energy. NASA, which supported the Euclid mission, is set to launch its own Nancy Grace Roman Space Telescope, in 2027. Roman will cover a smaller field of view than Euclid but will take even more sensitive and precise data. The Vera Rubin Telescope, in Chile, will perform similar experiments on the ground. “It’s an exciting time to be involved,” said Huff.

Editor’s note (6/30/23): This article was updated shortly after publication to include the following audio embed.

On 30 June 2023 SpaceRef.com editor Keith Cowing spoke with Deutsche Welle about the upcoming launch of ESA’s Euclid mission and how it will search for answers regarding dark energy and dark matter in the universe.