Dark matter and dark energy are paradoxical; we see them in our theories of everything, yet they remain invisible to direct observation. Resolving this paradox is what the European Space Agency's (ESA's) future telescope — called Euclid — will attempt to help us do, mapping the structure of the universe to help us see some of the most mysterious phenomena in existence.
The story of dark matter
It's worth asking why anyone assumes dark matter exists if no one has seen or measured it. "We are orbiting the centre of our galaxy at 220 kilometers (roughly 137 miles) per second," said Henk Hoekstra, scientific coordinator of Euclid and Leiden astronomer, to phys.org. We don't notice this intangible speed, yet here we are. "Based on the number of stars in our Milky Way, the stars at the edge of the Milky Way should have a much lower speed, but they move as fast as the Sun. Yet these stars are not being slung into the universe. Something is holding them together," he added.
This is where the inference happens: there's something invisible to humans, and our equipment, but that nonetheless exerts a lot of gravitational force. Hence: dark matter. "Or the theory of gravity is wrong," warned Hoekstra. "But everything indicates that dark matter exists, only we still don't know what it is. What we do know is that it does not absorb light or interact with it. So that literally makes it invisible."
And the story of dark matter gets even more bizarre: we've known the universe is accelerating since 1998. To learn why, astronomers needed an even more fantastic ingredient: Dark Energy, a catch-all term encompassing all ideas astronomers and physicists offer in the study of this expansion.
The need for Euclid to study cosmic speed
"We have some knowledge-gaps and these cannot be filled with existing observations. So the only way forward is to take better measurements," said Hoekstra. Enter Euclid; an ESA satellite set to launch in the year 2022. Once it reaches its nominal distance of 1.5 million kilometers (roughly 932 thousand miles) from Earth, Euclid will map a third of the sky. This will help us answer crucial questions about how the structure in the universe formed as a result of gravity, how all the matter in the universe is distributed, and how it all changes over time.
"An answer to the last question enables us to test models for dark energy directly," Hoekstra said to phys.org.
As one of four cosmology coordinators and leader of the project called "weak lensing," Hoekstra and his team "are going to investigate how dark matter distorts space," he said. No one can say whether this will yield more answers than questions about either dark matter or dark energy, but — during its mission — Euclid could help us come to know the things we don't know better than ever before.