At her8 May talk on “The Dark Side of the Universe”forming part of the BBVA Foundation’s Astrophysics and Cosmology lecture series

Catherine Heymans will explain the quest for a “new physics”able to solve the mysteries of the dark Universe

  • The British cosmologist co-leads the KiDs (Kilo-Degree Survey) program. This major international project aimed at detecting the mysterious dark energy and dark matter has studied 15 million galaxies and come up with some intriguing results
  • For Heymans, Professor of Astrophysics at the University of Edinburgh, this investigation is the chance to decipher the nature of the 95% of the Universe we still fail to understand, and find out why it is expanding at an accelerated rate
  • From its beginnings in 2011, the BBVA Foundation lecture series Science of the Cosmos, Science in the Cosmos has welcomed world authorities in the most active areas of astrophysics and cosmology

Madrid, May 5, 2017.- The Universe isgrowing or, as cosmologists would say, expanding. This has been a known fact for almost one hundred years. As if this expansion was not surprising enough, however, in the 1990s it was found to be gaining speed. The Universe, in other words, is getting larger at an ever faster rate. Catherine Heymans heard about this as a recent astrophysics graduate. She had asked her professor what he thought was “the most challenging problem in the world.” And he replied without hesitation: “To discover why the Universe’s expansion is accelerating.” Now, twenty years later, she co-leads one of the largest international observation programs attempting to do just that.

The answer is hiding in what has come to be known as “the dark side of the Universe,” also the title of the talkHeymans will give on Monday, May 8in the BBVA Foundation Madrid, as part of the Science of the Cosmos. Science in the Cosmos lecture series. The cosmologist, Professor of Astrophysics at the University of Edinburgh (United Kingdom), will explain the different waysscientists are probing this dark Universe, its link to the accelerated expansion conundrum, and why she is convinced that “we will need some new physics that will forever change our cosmic view.”

Almost all of the Universe is dark, or anyway 95%. In a dual sense, dark because it gives off no light and dark because its nature is unclear. A degree of ignorance that leaves Heymansfrankly astounded: “We do not understand what 95% of the Universe is made of!”This statistic comes from measurements of the total amount of matter and energy existing in the Universe. Numerous independent observations made over decades have generated an inventory that is now fully accepted by science, in which “normal matter”, the stuff making up the planets, bodies and stars and that we can see shining in other galaxies, represents barely 5%. The rest is in the form of dark matter and dark energy, which we cannot detect directly and whose nature is unknown. “A fact,” Heymans admits, “that makes you question the most fundamental tenets of physics.”

Despite having an adjective in common, dark matter and dark energy are two very different things. Dark matter was discovered in the 1970s. Measuring the movements of stars in other galaxies,scientists observed that shining or “normal” matter behaved as if it was surrounded by other, more abundant matter. It was undoubtedly there, although their telescopes were unable to capture it: dark matter, emitting no light, but exerting a gravitational pull.

Dark energy is “a different animal” as Heymanscalls it. Until it was discovered, astronomers believed that the Universe’s expansion had been kick-started by the Big Bang. It was taken as read that at some point this expansion would cease, halted by the opposing force of gravity, which pulls matter together. But from the painstaking measurements taken of the speeds of distant galaxies it was plain that expansion was not slowing. In fact the opposite was true. What was driving this unexpected acceleration? Cosmologists, stumped for an answer, postulated the existence of something that by some meansputsmore and more space between the galaxies, and named it dark energy.

For Heymans, all the models currently available to explain dark matter and energy are, to put it mildly, “fairly unsatisfactory.” Hunts for a new type of particle that could answer to dark matter have so far had no success, and the scientists working on them “must by now be a little concerned,” she reflects. She is referring here to the work of particle physicists at the LHC accelerator of the European Particle Physics Laboratory (CERN), and at the huge underground detectors built to spot the hypothetical trail that dark matter particles would leaveon their journey through Earth.

The situation with dark energy is, if anything, even less encouraging, because “we would like to at least have a good model explaining why dark energy has to exist, and we don’t,” affirms Heymans. She is unconvinced – “I don’t get it”– by the idea of a fifth force, the quintessence, that would join the four fundamental physical forces known to science. Which takes her to another of the hypotheses wielded: that the force of gravity changes with time.

“It’s a bit of a heretical idea, “ she admits, “because the force of gravity has passed every test in our own Solar System, and worksto perfection, accurately predicting the orbits of the planets, eclipses, etc.But then the Universe is very large, and gravity has not been put to the test at truly vast distances. Maybe it’s not a constant, but changes with time.”

How to find out? According to Heymans, there is just one way forward: to collect more data. For instance, cosmologists can ascertain whether thevalueof dark energy is constant or not by measuring the speed of millions of galaxies at different distances;theequivalent in astrophysics of exploring distant ages–the further we look, the further back we go in time. And something similar occurs with the force of gravity. But the observations needed to answer these questions are not easy to obtain, explains Heymans: “The effects we are looking for are so very small that we need extremely sensitive observation programs. The exciting thing is that for the first time we have the technology to achieve that: large ground- andspace-based telescopes, and the computing power to analyze the hugequantities of data they provide. This is undoubtedly a thrilling moment for cosmology.”

“Good news” from the KiDs project

Heymans co-leads one of these large-scale observation programs, launched in 2012 under the name of KiDs (Kilo-Degree Survey). Its preliminary results, published a few months back, were “good news,” to Heymans’ way of thinking, precisely because they do not fit with the current state of knowledge, and “could be a pointer to something new.” For years, KiDS has been observing an area of sky the size of 2,200 full moons and containing 15 million galaxies, from the European Southern Observatory’s telescopes in Chile. The goal is to search for dark matter using gravitational lenses;a phenomenon predicted by Albert Einstein,who believed nonetheless that they could never be detected.

As stated in the general theory of relativity, large agglomerations of matter – dark or visible – warp space-time, and the light travelling through it also bends, meaning the receiver of that light sees a distorted image. Astrophysics have exploited this phenomenon by learning how to measure the distortion, in the processrevealing the presence of dark matter.

So far KiDS has observed that dark matter appears to be less dense than anticipated, and to be distributed more smoothly throughout space. This also provides important clues for our understanding of dark energy, because it is this energy’s repellent force that determines how easily matter can clump together. The most precise data previously available (from the year 2014) were gathered by Europe’s Planck satellite. The discrepancy between KiDS and Planck “is significant,” says Heymans, and could mark the first breach in the wall of the dark Universe. The cosmologist is convinced that “we need some new physics” to spark a revolution comparable to that unleashed a century ago by general relativity and quantum mechanics. “We need a lot of good, new ideas, and also more observations. A mixture of genius and data.”

Heymans is also involved with the Euclid mission of the European Space Agency (ESA), likewise devoted to investigating the dark Universe. But right now she has an added worry: Brexit. “My whole career has been funded through European programs. What will happen in future? These are very uncertain times.”

Bio notes

Catherine Heymans is Professor of Astrophysics at the University of Edinburgh, and a European Research Council Fellow. Since completing her PhD at Oxford University in 2003, she has held fellowships from the Max Planck Institute and the Canadian Institute of Theoretical Astrophysics. She is an active communicator of cosmology, through lectures – including a TED talk – articles and books. Her latest book,The Dark Universe, has just been published by the Institute of Physics (IOP), and is available free of charge from

About Science of the Cosmos, Science in the Cosmos

Sinceit began in March 2011, the lecture series Science of the Cosmos, Science in the Cosmos has explored some of the main open questions in modern astrophysics. Experts from the top ranks of the world scientific community have shared their vision of the origins of the Universe, the search for life on other planets, how chemical elements are forged in the heart of stars, or the nature of dark matter and energy. The whole of the current series will be available for viewing, along with videos of past editions, on and our YouTube channel

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