Saturday, June 10, 2023

The dark universe: can a scientist battling long Covid unlock the mysteries of the cosmos?

Last September, Catherine Heymans, one of the world’s leading cosmologists, was supposed to board a ferry for the northernmost island in the Orkney archipelago. The island, North Ronaldsay, is among the darkest inhabited places on earth. On a clear winter’s night, it is easy to be awed by the thousands upon thousands of stars visible to the naked eye, which spill their unpolluted light upon the Earth. Heymans, who is the first woman appointed astronomer royal for Scotland, was planning to explain to the island’s 60 or so residents that those stars, and the rest of the perceptible universe, represent a mere fraction of the stuff that makes up our cosmos. What she studies is everything we cannot see: the darkness.

Over the past two decades, Heymans, who is 45, has advanced our understanding of a vast, invisible cosmos that scientists are only beginning to comprehend. That “dark universe” is thought to constitute more than 95% of everything that exists. It is made up of entities more mysterious than the ordinary matter and energy – the light, atoms, molecules, lifeforms, stars, galaxies – that have been the subject of scientific inquiry throughout history. In the past 10 years, Heymans has learned that the dark universe shapes the visible cosmos in unexpected ways, and may not follow all the standard rules of physics. Her discoveries are unsettling a broad consensus on how our world works on its grandest scales. “I believe that, to truly understand the dark universe, we will need to invoke some new physics that will for ever change our cosmic view,” she has written.

Heymans is not alone in that belief. During the 20th century, scientists developed an extraordinarily precise account of almost 14bn years of the universe’s history. But an increasing number of scientists suspect that model may be profoundly limited, or even broken. Some leading astrophysicists have recently declared that we have entered an era of cosmological crisis, one that might lead to anything from the discovery of new fundamental particles to a new theory of gravity. “The proliferation of ideas is like nothing I’ve ever seen,” the Nobel prize winner Adam Riess, another key figure in cosmology’s current upheaval, recently told me.

Six months before she was scheduled to go to North Ronaldsay, Heymans was like many other world-class scientists: she worked a minimum of 12 hours a day, with an overwhelming schedule of international travel. In July alone, she was due to attend three international conferences, to appear on stage at an ideas festival with Sir Martin Rees, for a session entitled Astronomer Royal meets Astronomer Royal, and to perform Do You Matter?, one of the standup comedy shows that she had been doing with her fellow astrophysicist Joe Zuntz since 2017. (Scrutinising a pixelated image of distant galaxies is “like looking at your favourite Japanese pornography”, runs one of her edgier jokes.) She was also due to receive the Royal Astronomical Society’s Herschel medal for “investigations of outstanding merit in observational astrophysics”. In between all that, she was supposed to shuttle back and forth between the University of Edinburgh, where she is professor of observational cosmology, and Ruhr University in Bochum, Germany, where she uses a prestigious €1.5m-million-euro Max Planck-Humboldt award, which she won in 2018, to run a centre dedicated to exploring the dark universe.

Catherine Heymans at the Royal Observatory in Edinburgh in June 2021. Photograph: Katherine Anne Rose/The Observer

But then, in March 2022, Heymans and her family contracted Covid. While her partner and three children recovered relatively quickly, she continued to feel awful several weeks later. “Strange illness, but I’m trying to be patient,” she emailed me. We still planned to meet in Europe that summer, and in Scotland in the autumn. Two months later, though, she got in touch to say her health was getting worse – “a slow and steady decline”, wrote Heymans, who usually radiates a sunny optimism. “Unfortunately you find me battling long Covid,” she wrote. In July, I got a message from her saying, “Sadly on the long Covid front, it is fair to say that it has cancelled my life until further notice.” She had spent the previous few weeks housebound and barely able to speak.

But she was still working. In a dark room, lit only by the glare of her laptop screen, Heymans would reply to emails and review the computer code for an international collaboration investigating the dark universe. After half an hour, an alarm would go off, and she would type a note to remind her future self what she was working on. Then she would shut her laptop and lie in silence or go to sleep. (Attempting to do too much physically or mentally often causes long Covid sufferers to crash for extended periods.) As soon as she felt able, she would get up, open her laptop, read the note and continue working. She repeated that cycle multiple times a day. Then she ate a meagre supper with her family while lying on the sofa – Covid had taken away her appetite – and slept fitfully for the rest of the night, frequently waking in pain or panic.

The next morning, Heymans would begin work again. From the narrowing confines of her life, she was still trying to help humanity peer deeper into the universe. “I’ve waited 20 years for science to advance this far,” she told me recently. “I’m not about to give up now.”

Perhaps the most profound insight in all of cosmology is that our universe has a history. It has not existed for ever and unchanging; it was born and it evolves. Cosmology’s ambition is to tell the story of that evolution and to explain the physical processes that govern it. Scientists do that in the language of mathematics, with sets of equations that describe how the universe changes. Those equations, and the stories we tell to make intuitive sense of them, are called “models” of the universe. The better the model – the more accurately it describes the world as we observe it – the deeper we presume our understanding of the universe to be. What makes Heymans’s work so striking is that it conflicts with the most accurate model of the universe we have ever had.

The cornerstone of that model is the big bang theory, which holds that everything in the universe began in an unfathomably hot, compact state – possibly in an infinitely dense point known as a singularity – and then expanded. In addition to the big bang, the model includes the two enigmatic constituents of the dark universe that Heymans studies. One is “dark matter”, which doesn’t emit, reflect, or absorb light, but exerts the attractive force of gravity. The existence of dark matter helps to explain why galaxies aren’t torn apart as they whirl through the void. “If all that was there was the stuff that we see, the stars would simply fly out into space,” Heymans explains in one of her public performances. The other constituent is “dark energy”, which causes the universe to expand at faster and faster speeds. Dark energy and dark matter “play out an epic battle of cosmic proportions”, Heymans likes to say – the former ripping the cosmos apart, and the latter trying to bind it closer together. Dark matter and dark energy are now thought to constitute more than 95% of the universe, ordinary matter less than 5%.

Together, the big bang, dark matter and dark energy – along with equations derived from Einstein’s theory of general relativity – make up what is now known as the “standard cosmological model” of the universe. We can test this model by gazing into the past to survey the universe in its infancy. Because light takes time to travel to us, when we look out into space, we are seeing the universe as it appears not at any one moment, but rather at many different ages. It’s as if we are looking at a composite image of someone’s face made up of billions of fragmentary photographs – a freckle from their toddler years, a wrinkle from their 60s – taken over the course of their entire life. If you have the right cosmological model, then you should be able to take the parts of the universe you see at one point in time, run them through the model’s equations, and have it spit out the universe you see at another point in time – thus demonstrating that your understanding of the universe is correct.

A image of the Abell 901/902 supercluster of galaxies, with the magenta patches representing dark matter.
A image of the Abell 901/902 supercluster of galaxies, with the magenta patches representing dark matter. Photograph: Alamy

The furthest back in time we can see is nearly 14bn years, to a moment just 380,000 years after the big bang, when light shot through the universe in every direction. That early burst of light still hums throughout space in the form of microwaves that fall well outside the visible spectrum. Scientists call it the “cosmic microwave background”. Mapping subtle differences in the temperature of those microwaves gives us an image of where the matter and energy were distributed across the entire early universe. Admiring the first detailed maps of the cosmic microwave background in 1992, the astrophysicist George Smoot, who later won the Nobel prize, famously remarked: “It’s like seeing God.”

By the mid-2010s, maps of the background had become so refined that physicists declared that human beings had finally entered the era of “precision cosmology”. “More has been discovered about the large-scale structure and history of the visible cosmos in the last 20 years than in the whole of prior human history,” said the philosopher of science Tim Maudlin in 2014. Meanwhile, the standard cosmological model seemed to be doing an impressive job of connecting those ever-more detailed maps of the infant universe to the universe we saw around us in the recent past. Some physicists thought that all we had to do was figure out exactly what dark matter and dark energy consisted of, and our knowledge of the workings of the universe at the largest levels would be complete.

Heymans was among the first people to detect a crack in the standard model. When the first ultra-precise maps of the cosmic microwave background were released by the European Space Agency’s Planck observatory, in March 2013, they fit the predictions of the standard model with exquisite precision. But Heymans quickly realised those maps no longer agreed with one of the measurements she was making of the more recent universe. The recent universe, she found, had less large-scale structure – less clumping and clustering of galaxies – than our maps of the early universe implied. The standard model’s universe was like rice pudding, Heymans likes to say, but hers was more like custard. Either Heymans had committed a major error, or there was something awry with the standard model.

At first, few people took Heymans’s findings seriously. “Nobody believed me because I wasn’t a professor then, and I certainly wasn’t astronomer royal,” Heymans told me. “I was just a lowly lecturer with a newborn baby, and people were just like, ‘No, you’re doing something wrong.’”

Heymans is at her best in the teeth of a challenge. “She is very creative, and she understands what the important problems are,” said the astrophysicist Alan Heavens, who taught Heymans as a student and has written many papers with her over the years. She is also prodigiously hardworking and inexhaustibly enthusiastic. She used to wake up at 4.30am, prepare supper for her family, and then take the earliest bus from her home in Portobello, a suburb of Edinburgh, to the base of Blackford Hill. She says she would literally skip up the hill to the Royal Edinburgh Observatory, arriving at work before anyone else. “Sleep is overrated,” she told me recently, during one of her half-hour working blocks. “Well, now I have a lot of it – but I didn’t use to sleep very much.”

Before developing long Covid, Heymans was, by her own description, “efficient, effervescent, and unstoppable” (also, “stupidly tall” – she’s 6ft 1in). Now, she calls herself “resilient”. Even in the grip of her illness, she completes almost every sentence with a laugh. She hasn’t owned a mobile phone since university, in part because she has “quite an addictive personality” and fears it would distract her from work and family. She’s almost never worn makeup, because “there’s simply not enough time to put it on”, she said. “What’s kind of annoying about Catherine is that she’s more productive when she’s asleep for two-thirds of the day than I am at my best,” said Zuntz, her comedy partner and colleague at the University of Edinburgh.

Heymans was born in 1978, and family lore has it that at the age of six she decided to become either an astrophysicist or a brain surgeon, after asking her primary school teacher what the hardest job in the world was. Her parents were baffled by her, she said: “They were very proud, but they did keep trying to divert me into more suitable careers.” The family lived in Hitchin, Hertfordshire, in the home-counties commuter belt.

Like nerds the world over, Heymans had a small circle of smart but socially ostracised friends. “If you were academically bright, then you were considered a bit sad, really,” her friend since childhood Esther Gamble, told me. “But we didn’t mind – we had each other and our little group of geeks.” (Gamble is now a barrister.) At Hitchin girls’ school, which is now one of the few remaining single-sex state schools in the country, Heymans felt she had the opportunity to be a leader in her science and maths classes. “If you do a random poll of female physicists, you’ll find the majority of them went to single-sex schools,” she said. Her interest in astronomy was nurtured by a teacher who was fanatical about space and organised an international field trip to Nasa’s Kennedy Space Center in Florida. “We didn’t have families with enough money to send us, and we were very jealous of everyone who got to go,” Gamble recalled.

In a precursor to her ordeal with long Covid, Heymans contracted Epstein-Barr virus just before her GCSEs, and spent the next year in bed with glandular fever. “I’d kind of filed that little part of my life away in a part of my brain that I didn’t like to think about,” she told me. “But partly because my daughter’s now that age” – Heymans has two boys and a girl – “I’m sort of looking at her and thinking, ‘Oh, yeah, no, we never did the things that you’re doing right now, because I was asleep.’” When she wasn’t sleeping that year, she alternated between watching daytime television and teaching herself A-level physics in bed. “It’s pretty similar to how I’m working now – except no daytime TV, because that is soul-destroying,” she said.

Prof Catherine Heymans (in mask) talking at the official opening of the Aberdeen Science Centre (ASC) in October 2021
Heymans at the official opening of the Aberdeen Science Centre in October 2021. Photograph: Aberdeen Science Centre

Although her health slowly improved, after a year spent mostly alone, Heymans felt depressed. But then she got a little “kick” that helped her to fully recover. “He’ll probably kill me for saying this, but I met my partner,” she said. The local schools had organised a trip to Oxford to check out the university, and she had summoned the strength to get on the bus. Rory MacLeod’s parents had put him on it, too. “We got chatting and then – yeah,” she continued. “We’ve been together since we were 17.” Heymans and MacLeod, who was born in Scotland, decided to go to the University of Edinburgh together.

When she arrived at Edinburgh in 1996, Heymans was one of only a handful of women in a cohort of nearly 100 physics students. All her professors were men. To help fund her studies, she worked in bars, as a nanny, and as a tour guide at the Royal Observatory, where she now has her office. At the observatory, the awestruck reaction of a boy upon seeing the rings of Saturn for the first time convinced her to dedicate her life to studying space.

In 1998, she became fascinated by cosmology, the study of the universe as a whole. That May, Adam Riess published the first draft of a seminal article that seemed to establish that, contrary to prevailing theories, the expansion of the universe was accelerating. If the universe was expanding faster and faster, there had to be some extra form of energy stretching it more and more. “I was like, ‘Whoa, so cool,’” Heymans recounted thinking, with the self-aware geeky awe she often uses when discussing the riddles of the cosmos. (Even today, she responds to the universe the way that other people might react to meeting their favourite movie star.)

That force came to be known as dark energy. When Heymans heard about it in 1998 she thought, “I want to solve this in my PhD.” Attempting to discover the nature of dark energy in the span of three years turned out to be a ludicrous ambition – “so laughable”, Heymans said. But it was also an early sign of her eagerness to take on ferocious problems that require exceptional creativity to solve. “That’s something that people don’t get about science,” she said. “They think that to be creative, you need to be an artist, or a writer. But science is probably the most creative job there is, because you ask questions that nobody knows the answer to.”

“For the past 20 years, I have been catching the light in remote mountain-top observatories all the way around the world,” Heymans told an audience in 2018. In the Australian outback and the Chilean highlands, on the snowy slopes of a 4,000-metre-high Hawaiian volcano, and with the Hubble space telescope, she has captured light from over a hundred more than 100m galaxies. Her work has been facilitated by the growing power of telescopes, cameras and computers, which have allowed cosmologists to poke into corners of the universe they could never see before.

Heymans’s specialty is known as weak gravitational lensing, a powerful but diabolically complex method for mapping the distribution of dark matter in the universe. The method involves studying how the path of light bends as it travels to Earth from galaxies as far as 10bn light years away. That bending is caused by gravity, which curves the space through which light travels. The amount of gravity in a given region of space, and therefore the amount of curving, depends on how much mass there is in that region. (If you have ever been encouraged to picture space as a sheet drooping under the weight of a bowling ball, you know roughly what I’m talking about.) Since dark matter accounts for most of the mass and gravity in the universe, the more light has bent on its journey, the more dark matter it has passed. Charting the passage of light through large swathes of the sky can therefore allow scientists to map how much dark matter there is in the universe, and where. That, in turn, can tell us important things about how dark energy, too, has shaped the cosmos.

The Canada-France-Hawaii Observatory on Mauna Kea in Hawaii.
The Canada-France-Hawaii Observatory on Mauna Kea in Hawaii. Photograph: Tetra Images/Alamy

When Heymans started her DPhil at Oxford, in 2000, no one yet understood how to do weak lensing properly. But she was thrilled by the possibilities and challenges it presented. “I was like, ‘That’s new, that’s exciting, and that can tell me what dark energy is!’” She spent part of her DPhil at an observatory on the rim of a volcano in the Canary Islands, but found the camera there at the time wasn’t powerful enough to do any real weak lensing science. Instead, she “learned Spanish, drank a lot of whiskey for breakfast, learned how to make ice-cream with liquid nitrogen”, which was available on tap to keep the instruments cool.

Weak lensing requires the collection and analysis of a huge amount of data, which is a demanding computational task. During her DPhil, Heymans co-wrote a piece of computer software to measure the shapes of distant galaxies in order to understand how gravity, the atmosphere and telescopes distorted the path of their light. “It was a brilliant idea,” she recalled, with self-mocking pride, “but there was absolutely no way we had the computer power to use it.” Five years later, though, machines had become powerful enough to make her code viable. Cosmologists are still using it today.

After her DPhil, Heymans won fellowships in Germany, Canada and France, and continued to break new ground in mapping the universe using weak lensing. As she moved around the world, MacLeod was “kind enough to follow me”, she said. He had studied biology, but retrained as an English teacher, because it was an easy job to travel with. Then, in their late 20s, he decided he wanted to have kids. “I said, ‘You can’t have kids and be a high-flying astrophysicist at the same time,’” Heymans recalled. (Vera Rubin, who had four children, was a notable exception.) But MacLeod said he would do the bulk of the childcare. They had their first child in 2006. (MacLeod declined to be interviewed for this article; Heymans told me he wanted to “retain his invisibility” in her professional life.)

By the time their second child came along, in 2009, Heymans had significantly refined her gravitational lensing techniques, and was leading a team of researchers exploring the dark universe. Another revolution in cosmology was afoot. The cliche of the lone genius was receding into history. The new science was an increasingly vast social undertaking. In her public lectures, the Sri Lankan-born British cosmologist Hiranya Peiris often displays a series of photographs of the teams who helped discover and map the cosmic microwave background. The first shows the four white men, two at Princeton and two at Bell Labs, who detected the microwave background in the 1960s. The second shows a few of the 18 or so people who mapped it with George Smoot in the 1990s, almost all of whom were white men. The next photo is of Peiris and some of the other members of a 30-member team that made even more detailed measurements of the microwave background in the early 2000s; only a handful were people of colour or women. By 2010, the team for the European Space Agency’s Planck Space Observatory, which has made the most precise measurements of the microwave background to date, had more than 300 members from around the world, many of them women.

“That’s the name of the game in science now, isn’t it?” Heymans said. “Looking back in history, it was all about competition. You read stories about Newton and Hooke who had these fights over the nature of light, and Hubble and Sandage arguing about how fast the universe was expanding. I really think science shouldn’t be like that now. We’ve seen all of the Covid developments that have happened because people are sharing data and working together. So that’s where cosmology’s heading – big projects working together.”

When the era of precision cosmology dawned, and Heymans and her team’s results no longer fit with the standard cosmological model, she was worried. To posit a universe different from the one entailed by the results of the European Space Agency’s Planck mission, which was led by some of the most towering scientists in the field, was a heresy. “My first thought was: ‘Oh, God, I’ve done something wrong,’” Heymans recalled. “I always think I’ve done something wrong, that’s just inherent in me. And Planck was the ultimate cosmology survey, the holy grail. So for us not to agree with it was – it was a problem.”

In scientific terms, it was a “tension”. Many scientific measurements have a degree of imprecision, and good science involves rigorously estimating both the range within which an answer is likely to fall, and the probability that the truth lies elsewhere. A common example is when meteorologists say there’s a 95% chance of one to two inches of rain the next day – an estimate that leaves open the possibility of a downpour, or that you might not need your umbrella at all.

But if one meteorologist says there’s a 95% chance of eight inches of rain, and another says there’s a 95% chance of pure sunshine, then you have a tension or, worse, a crisis. You can be reasonably sure that either one of the scientists made a mistake, or there’s something very wrong with the model they’re using to make predictions. What happened in 2013 is that an improbably large gap opened up between the range the standard model predicted for the clumpiness of the universe and the range Heymans and her team found.

The Planck map of the cosmic microwave background released in 2013.
The Planck map of the cosmic microwave background released in 2013. Photograph: Science History Images/Alamy

Heymans desperately wanted her measurements to agree with Planck and the standard model. Among other things, she worried she might lose funding for her future projects if her findings were wrong or continued to fall outside the mainstream. But when her next major results came out, in 2017, they were in even greater disagreement with the predictions of the standard model. The anomaly she was detecting was coming to seem less like an error, and more like a potential discovery. “I felt less bad – like I wasn’t wasting everybody’s time any more,” Heymans said.

The response of her colleagues was not universally positive. In particular, the Cambridge professor George Efstathiou, one of the “fathers” of the Planck mission, was convinced that Heymans had blundered. “We do genuinely like each other now,” Heymans said. “But when I was a young academic, he kind of led the pack of, ‘You’re doing something wrong, you don’t know what you’re doing.’” Efstathiou, in the Q&A after her public presentations, liked to ask her to tell the audience what she had done wrong. “I didn’t have the balls to say, ‘George, can you tell the audience what your team’s done wrong?’” she told the magazine New Scientist in 2017.

Heymans’s findings were not the only ones now in tension with the standard model. When the first Planck results came out, Adam Riess soon realised that the recent universe was growing faster than Planck and the standard model predicted. “Luckily, he’s got a Nobel prize, so there’s no way people are gonna say to him, ‘You don’t know what you’re doing,’” Heymans joked.

“We went through the stages of grieving – denial, anger, and so on,” Riess told me recently, reflecting on the period in which he and his team realised they were no longer in agreement with the standard model. Accusations were thrown around about who had got their analyses wrong, and how. “But a lot of work has been done since then, and we have a much stronger result now,” Riess continued. His teams have used more than 1,000 orbits of the Hubble space telescope to make their observations – a massive investment of scientific resources. Something similar has been happening with Heymans’s results. “There’s been loads of improvement in our weak lensing measurements,” Heymans said. “There are three different teams now, and they’re all finding the same thing that we were finding 10 years ago.”

Not everyone agrees that there is a crisis in the field, or, if there is a crisis, exactly where it lies. Efstathiou takes seriously Heymans’s findings about the structure of the universe. But he isn’t convinced they will entail an overhaul of physics as we know it; instead, he has proposed, along with one of Heymans’s former graduate students, Alexandra Amon, that Heymans’s results may be due to something more mundane, such as the messy ways that galaxies evolve. He recently argued that it’s still entirely plausible that, in 50 years’ time, physicists will be saying that the standard cosmological model describes the universe “all the way down”. (There are also distant outliers in the current cosmological landscape, such as Pavel Kroupa at the University of Bonn, who reject the standard model, and the dark universe, altogether.)

Riess hopes that the community as a whole is moving towards the final stage of grief: acceptance that there might really be something wrong with the standard model. Yes, it’s surprising to find cracks in such a solid theoretical edifice. But, he asked, “What does surprising mean in a universe we don’t understand?” When Heymans reflected on the state of the field, she adopted a tone of theatrical despair mixed with wonderment: “We know so much, and yet … so little!”

After Heymans became ill in the spring of 2022, she drafted an automatic reply message, which she still uses today. “Dear Sender,” it reads, “Please accept my apologies as I’m unlikely to be able to respond to your email at this time. I’ve unfortunately joined the several million across the UK who are currently suffering from long Covid.”

Heymans faced her predicament like a scientist. Someone who studies billions of galaxies and countless variables must necessarily revel in data. Soon after she became ill, she opened a spreadsheet in which she began to track, in two-hour blocks, all her symptoms and activities. She was trying to understand how her symptoms related to her activity levels, so that she could find a more scientific basis for managing her wellbeing. She found that her chest pain correlated with talking and laughing, so she tried to cut down on both, which in her case is sort of like trying to extinguish the sun. Stress made her symptoms worse, which made her more stressed, so she had become “a convert to mindfulness”.

Her recovery was slow, and faltering, but six months after developing long Covid, Heymans was starting to feel glimmers of improvement. In early September, though, she contracted Covid for the second time. “A bit of an almighty setback,” she wrote to me shortly afterward. “Even though I don’t go anywhere, or see anyone, I have 3 kids who go to school and are therefore biohazards.” The good news was her kids hadn’t developed long-term symptoms, she continued. “The bad news is that I’ve moved from housebound mode to bedbound mode – who would have thought things could get worse!! Amazingly and happily I’m still quite sane and positive though.”

In late September, when the infection passed, Heymans began a five-week course of an experimental treatment to alleviate some of her symptoms. Hyperbaric oxygen therapy, or HBOT, involves sitting in a high-pressure chamber for 90 minutes while having oxygen forced into your lungs. (HBOT has been used for years by people suffering from myalgic encephalomyelitis, commonly known as chronic fatigue syndrome, to help keep symptoms at bay.) “Picture the scene from Star Wars where Darth Vader is inside a pressurised tank,” she told me. “That is more or less what I’m putting myself through – complete with the black face mask and tubes.” Because the pressure is equivalent to being 10 metres under water, patients call the sessions “dives”.

A chart Catherine Heymans made to track her long Covid symptoms.
A chart Heymans made to track her long Covid symptoms. Photograph: Catherine Heymans

Heymans began keeping a record of her weeks in the chamber to share with other long Covid sufferers, which she called The HBOT Diaries. “I’m petrified,” she wrote before a trial dive on 26 September 2022. “This is the first time I’ve left the house in over a month, and the outside world is overwhelming.” There were no beds in the chamber, so she was forced to sit. “My head starts to throb, probably because this is the first time I’ve sat upright in a long time.” After the session, climbing the stairs back to her flat at the top of an Edinburgh tenement was “like climbing Mount Everest”. Two days later, she was suffering from insomnia, accompanied by “the usual spiral of despair and pain”. Heymans was told the next three weeks of treatment – four dives per week – would be gruelling. “Based on where I am at the moment I think that is an understatement,” she told me after the trial dive.

The following week, she began the full protocol feeling “tired, anxious and horrible”. In the waiting room, she met a “spiritual healer” who came over and silently held her hands. “Even though we are complete strangers” – one an astronomer, the other an astrologer – “it is so comforting to not be alone.” Later that week, the strange dreams began: “I spent hours feeding large French baguettes to giant ducks … Will the giant duck eat me?”

Long Covid is a highly unpredictable and frustrating condition, and it offers no clear trajectory or narrative. A string of good days can easily precede a collapse. By the end of her second week, in mid October, Heymans was able to bake her 10-year-old son a cake for his birthday, but it took her the entire day. She was able to join the family for dinner on a couple of nights, which she hadn’t been able to do for more than six weeks. “It is so lovely to hear their chatter and join in with the jokes,” she reported. But three days later, her diary entry read: “Glum crash”. “Maybe I’ve done too much or maybe it’s just the usual menstrual cycle dip” – she had previously had at least two major setbacks on day 21 of her cycle, when progesterone levels peak – “but I feel like death today.”

One week later, though, there was a hopeful note. Heymans wrote, “I can see the old me emerging.” She had spent the entirety of her 15th session writing computer code, and afterward she was able to go for a brief dip with a friend in the North Sea. A week after that, at the beginning of November, she was beaming: “What a transformation!” There was no change in her lung pain and sore throat, her heart palpitations, or her chronic runny nose, and her tinnitus was getting worse. But by the end of five weeks of treatment and 20 dives, there were improvements, though not total relief, from her fatigue, ear ache and the blurred vision in her right eye. Her brain fog and insomnia had dissipated, and her anxiety had gone away. She had built her own air purifier, out of air filters, a box fan and duct tape, in the hope of reducing the chances of reinfection.

The reprieve from her symptoms turned out to be painfully brief. At the start of 2023 she wrote to me, “Alas – two bars on the lateral flow test on Christmas Eve – what a kind festive family present of Covid from my daughter’s school. I’m back to mush-brain.” She was frustrated that people weren’t still required to mask, despite the the prevalence of long Covid – a study last June found that there were 2 million people living with it in the UK – but she felt she couldn’t force her children to be the only ones to cover up. Around that time she tweeted one of the lowest, angriest sentiments I’d ever heard her express: “Vetoing ‘get well soon’s as it feels that I will never be allowed to get well in our ‘let it rip’ society.”

“It’s a very lovely day, and the sun is shining on my bed, so that’s wonderful,” Heymans told me on a recent afternoon. She was “still very up and down on the long Covid rollercoaster”, and had sent me a copy of a cartoon that was circulating in the long Covid community. It was a drawing of a board game called Long Covid: All Snakes, No Ladders. One of the snakes, which took players back 21 squares, was labelled “Cried Too Much”. But not all days were bad. One evening last month, more than a year into her illness, her husband had given her a piggyback ride down to Portobello beach, where Mercury and Venus could be glimpsed together in the sky. Mercury is usually blotted out by the sun, and it was the first time Heymans had ever seen it.

Before she developed long Covid, a thousand opportunities seemed to lie at Heymans’s feet. The Queen had appointed her astronomer royal for Scotland in 2021, and she had since been approached as a potential director of some major scientific institutes. Now she worried that too much of the latest research would pass her by. “Maybe my new role, when this finally is all over, is to make things better for people coming up behind me?” she wondered aloud. “To show them that they can do this work and be normal.” Lying in her bed, she began to laugh her dam-bursting laugh and added, “You’re gonna say I’m not normal now.”

She, too, had been through the stages of grieving, not for some scientific ideal, but for her past self. She had been making her way towards acceptance that her life had changed, possibly permanently. “You can’t battle this, and you’ve only got a fixed amount of energy per day, so you can’t waste it on getting really upset,” she told me in April. “You’ve got to spend your energy doing stuff that fulfils you. For me, that’s astronomy.”

The Milky Way visible over Yesnaby sea stack in the Orkney Islands.
The Milky Way visible over Yesnaby sea stack in the Orkney Islands. Photograph: Mark Ferguson/Alamy

There’s no dearth of work for Heymans to do. The scientific questions she and her collaborators have opened up remain unanswered. In astronomy, “whenever you have a big controversy that you just can’t figure out, the answer is always to build a bigger telescope”, she likes to joke. She is currently waiting for the domes to open at the Vera Rubin Observatory in Chile’s Atacama desert, so she and her colleagues can begin making the most detailed images of space ever. Many cosmologists hope that the observatory, which boasts the world’s largest lens and camera, will provide enough new information about the universe to resolve the tensions between the standard cosmological model and the findings of Heymans, Riess and their respective collaborators. It may also help scientists to discover the nature of dark energy and dark matter.

And there are always bigger telescopes to be built. Among Heymans’s many ambitions is to build one on the dark side of the moon. It would be a kilometre wide – 30 times bigger than its biggest rival on Earth – with a perfectly smooth mirror of liquid mercury. It would be immune to artificial light, hazy atmosphere, and all the other contaminations here on the planet. She once dreamed of travelling to the moon herself to peer through it, in order to see the universe anew.

For now, Heymans is still largely confined to her bed or the chair in the HBOT chamber. She knows there is a chance that her disease may remain fundamentally mysterious, as may the universe she studies. But even if she never fully recovers, she will continue to wake up each morning and begin working again, gazing hopefully into darkness.

Reporting for this project was supported by a Silvers Grant for Work in Progress from the Robert B Silvers Foundation

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