Solving the problem of dark energy Founder of Euclid mission speaks on its goals and progress


The Euclid mission, whose aim is to “explore the evolution of the dark Universe,” is expected to launch by July 7. For UW professor Will Percival, founder of the mission, it’s been a long time in the making. 

“I helped to put together the requirements of the mission many years ago, which basically wrote the blueprint for what Euclid will be expected to do,” he said. 

The Euclid mission was formally adopted by the European Space Agency (ESA) in June 2012 and was fully built and operated by the ESA, with contributions from NASA. Along with being the founder of the mission, Percival is one of its four science coordinators who will be performing the primary experiments. 

Percival says the goal of the mission is to solve “the problem of dark energy.” ‘Dark energy’ is the name given to a currently unknown physical process that is driving the acceleration of the universe’s expansion, a baffling phenomenon considering how if the universe was behaving through gravity acting on regular material, that would theoretically lead to a deceleration of the universe’s expansion due to gravity being an attractive force.

Percival noted that dark energy is not to be confused with dark matter. “Dark energy is this name given to this physical process that is accelerating the expansion of the universe… Dark matter is material that behaves through gravity like baryonic material: protons, neutrons, electrons, the things that we are made of, but it does not interact with light.”

To solve the problem of dark energy, Euclid will conduct a large imaging survey of the universe, taking images in optical and near-infrared light. Euclid’s image quality will be at least four times better than that of ground-based sky surveys. Percival pointed out that despite comparisons of image quality between Euclid and NASA’s Hubble Space Telescope, “unlike Hubble, we are not pointing at a set small number of objects to get our images. We are just getting images of basically everything that’s out there. So the discovery space is huge.” 

Percival also noted the wide range of possible findings that may come from the Euclid mission, stating, “A lot of the science that will come from Euclid will not actually come from the experiment it’s been designed to do … it will come from the fact that the survey it is producing is an incredibly great resource for astronomy, for a large number of different other projects.”

In addition to answers to existing questions, the Euclid mission may incidentally further research on other topics. “When you do a survey like this and you have a new data set that pushes this far into the unknown, it’s actually the unknown objects that we’re not expecting, which will be really exciting. And finding those in the dataset will be another process that we will have to do,” Percival said. 

To track dark energy, Euclid will use two techniques: weak lensing and galaxy clustering. ‘Weak lensing’ is the name given to the slight ellipticity that occurs when we view images of galaxies, due to the light bending slightly as a result of material in its path to us. According to Percival, Euclid will cover enough of an area to allow scientists to accurately measure the distortion and use it to understand the universe’s expansion rate. ‘Galaxy clustering’ is a technique where scientists use the distribution of galaxies in the universe to study the expansion of space.

Euclid’s decade-long development can be attributed to the boundary-pushing nature of the mission in terms of its science, instrumentation, and developments which have never before been used in this particular configuration. “Consequently, there’s an incredible amount of very careful construction and testing of every aspect of the mission before it is deemed flight ready. And so, there are also many reviews, many meetings, many points during the development of the mission where ESA wants to check that everything will work [and] has worked so far,” Percival said. 

Percival described the role he played in identifying and resolving issues throughout development, citing one case where it was discovered that one of the four grisms, a part of the near-infrared camera that separates light into its separate components and allows scientists to measure things like an object’s redshift (the light from a retreating object), was inserted with the wrong orientation. This led to a review of whether or not the mission could be completed with the remaining three grisms, which then led to the discovery of a new way to measure observations, thereby mitigating the loss of the initial grism.

“I came in from the science side as … the person taking charge of the requirements and ultimately saying whether or not any of the changes that were made will impact on the overall goals of the mission,” Percival said. “So you can imagine this kind of process and trying to find the way of doing this compromise, this takes time.”

Percival expressed his feelings towards the launch as part stress, part excitement. “It will be the first launch I’ve actually got to see. And there’s a lot riding on it… I’ve done a lot of previous experiments, but they tend to have used ground based telescopes, not satellites.”