Euclid: The Secrets of the Dark Universe

On Europe's newest space telescope and the mysteries it will investigate

A sky full of galaxies. Euclid’s view of the Perseus cluster, containing over one thousand galaxies. Image credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi, CC BY-SA 3.0 IGO

The first images from a new telescope are often spectacular. By and large they show the things we’ve come to expect from space: the stars and the galaxies; the nebulae and the clusters. Sometimes, thanks to better optics and sensors, their visions seem a little brighter or sharper than before, or reveal things until now unseen by human eyes.

In this, the Euclid space telescope was no exception. The images scientists showed off at the start of November counted three galaxies between them, a new view of the famous Horsehead Nebula, and one of the Perseus cluster, itself holding at least a thousand galaxies. All, as expected, are images of stunning cosmic beauty.

Look more carefully, however, and Euclid’s images show something rather more mysterious. Scattered across them are the ghostly fingerprints of dark matter and dark energy, two elusive forces that bind galaxies together and drive the expansion of the universe. Like a good detective, Euclid will seek out their presence in the cosmos; tracking them in the curves of galaxies, in the shapes of clusters and in the very structure of the universe.

Euclid, unlike general purpose telescopes like Hubble or the James Webb, will dedicate itself to this mission for the next six years. Over that time the telescope will photograph a third of the celestial sphere, map out the position of two billion galaxies and capture light at least ten billion years old. 

The end result will be a cosmic map of unprecedented scale and detail, covering not just the physical location of galaxies, but also their ages. Euclid, project scientists like to say, will map out the universe in both time and space. In doing so it will reveal how dark matter has positioned galaxies in space and shaped them over the eons.

It will also cast light of one of the greatest battles taking place on the cosmic stage; waged between the familiar force of gravity and the profoundly mysterious dark energy. The victor will determine nothing less than the fate of the universe, leaving it to either fly apart or crash back together, to be consumed by either ice or fire.

But first, before we turn our minds to such ominious thoughts, we must return to a more familiar world of falling apples and changing seasons. The story of the dark universe begins in the 17th Century, with the work of the great physicist Isaac Newton.

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The Dark Universe

Four centuries ago Newton wrote down the laws of gravity. That was easy enough to do. Others before him had already worked out many of the ideas of gravity, and derived some of the mathematics behind them. Newton’s true act of genius, indeed, was not to do that, but instead to extend the realm of gravity beyond the Earth - where its action was obvious - to that of the Moon and Sun.

The Moon, Newton realised, was held in orbit around the Earth by the same force that made apples fall from trees. It also held the planets in orbit around the Sun, he said; and so the passing of the years, the phases of the Moon, and the falling of autumn fruit were all found to spring from the same fundamental law of nature.

Newton’s law thus became known as the universal law of gravitation: “universal”, because it held sway across heaven and Earth, and applied as equally to cosmic bodies as it did to Earthly matters. It was a revolutionary idea, one which challenged old ideas of the Earth’s place in the heavens. But the idea of universal laws was one that stuck, and that in time would become a founding stone of modern science.

For most of the following centuries Newton’s universal law held. True, a few problems did crop up with the orbit of Mercury - but Einstein, when he made a few corrections to the law of gravity in 1915, managed to resolve them. Gravity, despite the modification, remained an iron-cast law of nature, as applicable in the Andromeda Galaxy as it was on Earth.

And then, in the 1930s, a new problem emerged. Astronomers had started to find clusters of galaxies in space, apparently held together by the force of gravity. Yet when they added up all the mass they could see in those clusters, they fell far short of the amount needed to stop them flying apart.

Soon afterwards the Andromeda Galaxy was found to be spinning faster than Newton’s Law predicted, at least based on the visible mass of the galaxy. Astronomers were thus faced with a stark choice. Either the law of gravity was not as universal as Newton and Einstein had said, or there was more mass out there than we could see.

Given the sanctity of universal laws of physics, the second option was considered far more palatable. The catch, however, is how much extra matter was required - roughly five times more than we could see. True, some of it is probably made up of black holes and planets that give off little light, but getting that to add up to such an enormous amount is hard.

Over the intervening decades physicists have come up with a myriad of ideas about the nature of this “dark” matter. They have covered everything from microscopic black holes to hordes of neutrinos. Yet no hard evidence to support any of these ideas has ever emerged, and the true nature of dark matter - or whether it even exists at all - remains uncertain.

In the 1990s things got even worse. Astronomers had already known the universe, set in motion by the Big Bang, was expanding over time. Theory and logic suggested this expansion should be slowing. Eventually, many speculated, this could even go into reverse, and the universe collapse in on itself in a kind of backwards big bang.

Studies of supernovae, however, put this idea to rest. Not only was the universe expanding, it was somehow accelerating. Nothing in the known laws of physics could explain this, but to paper over the cracks, physicists introduced the idea of dark energy. Somehow, they said, this dark energy was pushing the universe apart, forcing it to expand at an ever increasing rate.

How and why it was doing this remains unknown. Dark energy, indeed, is still even more mysterious than dark matter. This - since it must be the dominant force in the cosmos, even more so than gravity - is deeply disturbing, and points to a major missing piece in modern physics.

This, then, is the situation today. Cosmologists reckon only five percent of the universe, as measured by mass-energy, is made of the kind of visible matter we see and interact with. The rest - the remaining ninety-five percent - is dark, made of dark matter and dark energy. However you cut it, it is a deeply embarrassing fact that modern physics has so little to say on so much of creation.

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The Balance of Things

Since the discovery of dark matter in the 1930s, astronomers have found its fingerprints across the universe. They see it at work in galaxy clusters, like the one Euclid photographed in Perseus. It is invoked to explain gravitational lensing, a curvature of light created by the presence of large masses. It has even been blamed for the very existence of galaxies - and therefore stars, planets and, ultimately, us.

The clearest evidence for this comes from the Cosmic Microwave Background, a buzz of radiation left over from the Big Bang. Analysis of this radiation shows the early universe must have been almost perfectly uniform. Matter, in other words, was once evenly and finely spread through the cosmos.

Over time, researchers reasoned, gravity had changed that. Matter had clumped together, forming stars and galaxies. Yet when they ran simulations, taking the starting point implied by the cosmic microwave background, they hit a problem. Matter and gravity alone did not seem to be enough to recreate the universe we see today. Only with the addition of dark matter - and later dark energy - could this be done.

The success of one model in particular - known as Lambda-CDM - is hailed as strong proof of the existence of dark matter and energy. It assumes a certain mixture of both were created in the Big Bang, along with a dose of ordinary matter; and that their movements were dictated by the laws of gravity.

When researchers run simulations using Lambda-CDM, they can create model universes that look much like our own. Crucially, they end up with the right balance of atomic elements and the right spread of galaxies and clusters throughout space. That, many physicists argue, is good proof that the universe really is dominated by its dark side.

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The Euclid Space Telescope

Since both dark matter and dark energy are hidden from view, no telescope can ever hope to see them directly. Instead Euclid will look for subtle effects betraying their presence. It will do this by scanning billions of galaxies, mapping them out and looking for distortions created by dark matter and energy.

Euclid’s instruments are designed to image those galaxies in two forms of light. The first is visible light, recording the galaxies much as our eyes would see them. The other is infrared, which sits some way beyond the red light we can see.

With its visible light camera Euclid will record the shapes of over a billion galaxies. This - since the presence of matter (both dark and not) distorts the light coming from more distant objects - will allow researchers to map out the distribution of matter and dark matter across the celestial sphere.

Thanks to the expansion of the universe, almost all galaxies are moving away from us, with those furthest away receding at the fastest speeds. This results in a slight stretching, and therefore reddening, of the waves of light coming from them. The infrared camera will measure this "redshift" effect.

Armed with redshift data, scientists can calculate two things. On a broad scale, by looking at the redshift of millions of galaxies, they can infer how the expansion of the universe has changed over time. That will show the effects of dark energy and how prevalent it really is. On a smaller scale, they can also work out how far away any given galaxy is.

By combining the data from the two cameras they can thus position galaxies in the night sky. Euclid will do this for tens of millions of galaxies, creating the largest ever map of the cosmos. This will give them a measure of how galaxies cluster in the sky - a parameter that should show how dark matter is spread across it.

All in all, Euclid should give us a better idea of where dark matter is in the night sky, and how it has influenced the motion of galaxies. Researchers will compare its findings with the results of simulations, and thus validate - or not - models like Lambda-CDM. That, in turn, should improve our knowledge of how both dark matter and dark energy have shaped the universe.

Euclid, then, is an effort to shed light on the two biggest mysteries of modern cosmology; to reveal - and to map out - the hidden dark side of the universe. If it succeeds it should create a map of unprecedented detail and scale. If it fails - if dark matter turns out to be an illusion - then it will, at the very least, challenge us to rethink the sacred laws of gravity.