A new space telescope called Euclid, an ESA (European Space Agency) mission with a major contribution from NASA, is set to launch in July to investigate why the expansion of the universe is accelerating. Scientists call the unknown cause of this cosmic acceleration “dark energy.” By May 2027, NASA’s Nancy Grace Roman Space Telescope will join Euclid’s mission in exploring this puzzle in ways never before possible.

“Twenty-five years after its discovery, the accelerating expansion of the universe remains one of the most pressing mysteries in astrophysics,” said Jason Rhodes, senior scientist at NASA’s Jet Propulsion Laboratory in Southern California. Rhodes is deputy project scientist for Roman and the US chief scientist for Euclid. “With these telescopes, we will measure dark energy in different ways and with much greater precision than was previously possible, opening a new era of exploration into this mystery.”

Scientists aren’t sure if the accelerated expansion of the universe is caused by an additional energy component or if it signals that our understanding of gravity needs to change in some way. Astronomers will use Roman and Euclid to test both theories simultaneously, and scientists expect both missions to reveal important information about the underlying workings of the universe.

Euclid and Roman are both designed to study cosmic acceleration, but using different and complementary strategies. Both missions will make 3D maps of the universe to answer fundamental questions about the history and structure of the universe. Together, they will be much more powerful than either of them individually.

Euclid will observe a much larger area of ​​the sky — about 15,000 square degrees, or about a third of the sky — in both infrared and optical wavelengths of light, but in less detail than Roman. It will look back 10 billion years when the universe was about 3 billion years old.

Roman’s larger probe will be able to probe the universe in much greater depth and precision, but in a smaller area – about 2,000 square degrees, or one-twentieth of the sky. Its infrared vision will reveal the universe when it was 2 billion years old, revealing a greater number of fainter galaxies. While Euclid will focus exclusively on cosmology, Roman will also investigate nearby galaxies, find and investigate planets throughout our galaxy, study objects on the outskirts of our solar system, and more.
The Hunt for Dark Energy

The universe has been expanding since its birth – a fact discovered by Belgian astronomer Georges Lemaître in 1927 and Edwin Hubble in 1929. But scientists expected that the gravity of the universe’s matter would gradually slow this expansion. In the 1990s, looking at a particular type of supernova, scientists discovered that about 6 billion years ago, dark energy began to increase its influence on the universe, and no one knows how or why.
The fact that it is accelerating means that there is something fundamental missing from our picture of the Universe. Roman and Euclid will separately provide exciting new data to fill gaps in our understanding. They will try to identify the cause of cosmic acceleration in a few different ways. First, both Romeus and Euclid would study the accretion of matter using a technique called weak gravitational lensing.