Tiny, fuzzy blobs. I’ve spent a lot of time in the last few years looking at images of tiny, fuzzy blobs. They’re only ever a few pixels wide, like smudges on a photo, but they could be the key that unlocks the mystery of dark matter.
The blobs are galaxies: swirling pools of stars and planets suspended in space, millions of light-years away from Earth. The images were collected by an advanced camera with a 1m (3.3ft) lens mounted on the giant Victor M Blanco Telescope, 2,200m (7,200ft) up in the mountains of the Coquimbo Region of Chile.
Astronomers have spent years using it to scan the sky, gathering images of the cosmos. I, and my colleagues in the Dark Energy Survey, have pored over these images of millions of tiny fuzzy blobs in the hope that they reveal a vital missing piece in our understanding of the Universe.
We’re on the edge of our seats, because this piece fills the gaping hole in our understanding of the cosmos. It could even turn that understanding completely upside down.
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An invisible truth
You might think that cosmologists have the Universe sussed. And it’s true that we have learnt a lot about it and how it works.
But there’s an elephant in the room: our theory of the Universe hinges on the existence of dark matter, and we have no idea what dark matter is. In fact, less than one-fifth of the matter in the Universe is made up of particles whose physics we understand.
Do you feel confident admitting that you only understand 20 per cent of something?
Yes, we can be proud that we’ve honed a standard model of cosmology: a physical and mathematical description of the Universe. It’s a major achievement. But, thanks to this dark matter elephant, we can’t be absolutely sure that it makes any sense at all.
Perhaps we could just ignore the elephant. Maybe dark matter is simply something we’ve invented out of a misinterpretation of the theory. Maybe it’s not really out there at all.
Sadly, not. Because one thing we do know is that something is out there. We can’t see it and we don’t know what it is, but since Dr Vera Rubin first observed the effects it was having on stars in the late 1970s, there’s no denying it’s there. Rubin set out to study the motions of stars in spiral galaxies, but her measurements suggested that the stars weren’t moving as expected.
Her hypothesis was that stars at the edge of a galaxy would travel more slowly than those near the centre. But Rubin observed that the outer stars were moving just as fast as the inner stars, and that they were all orbiting more quickly than expected.
The only thing that could explain this finding would be if there is a tremendous amount of invisible matter in the outer regions of galaxies beyond the inner clump of visible stars.
This was important evidence for dark matter, which had been proposed by Prof Fritz Zwicky back in the 1930s. Zwicky coined the term after his observations suggested that the visible mass of a cluster of galaxies was too little to prevent the galaxies from escaping the hold they had on each other, and that therefore something else – something abundant and unseen – must be holding them together.
Dark matter, Zwicky was suggesting, was the missing mass of the Universe. Rubin’s observations seemed to confirm it. So, we know it’s there and that there’s a lot of it. But even though we can’t see it, we can’t ignore it, because without knowing what dark matter is, we can’t understand the Universe.
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Cosmology at a crossroads
The Standard Model is an undeniable success story. It describes the origin, content, evolution and date of the Universe using six numbers. These values have been used to investigate the forces that govern matter – and they have explained how we came to have a sky teeming with galaxies from a soup of particles 13.8 billion years ago.
Our experiments have cemented its success. We’ve launched missions into space, like the Planck satellite, to peer back to the very first light of the Universe and measure the differences in the temperature of the radiation in different directions of the sky, the so-called cosmic microwave background. This ‘baby picture’ of the Universe supports the evidence that the cosmos is composed predominantly of dark matter.
So what, if anything can we say for sure about it? Certainly, that dark matter is the key ingredient in the recipe for the Universe. Despite being invisible, it comprises more than three-quarters of all of the mass in galaxies, like our Milky Way. Dark matter dominates the mass in a galaxy and, through gravity, binds the stars together in an orbiting whirlpool.
If we zoom out for a better view of the cosmos, we see that the galaxies aren’t evenly distributed, but arranged in a web-like pattern. Our observations tell us that dark matter is the invisible scaffolding of the cosmos: it forms a cosmic web of clusters and filaments, with enormous voids in-between, that guide the location of galaxies.
Imagine a view from space of Earth at night, where the lights we see illuminate the towns and cities where humans live. Just like dark matter, we can’t see the humans, but we know they’re there because of the light.
The existence of dark matter is crucial for how galaxies are built and withstand time, and it’s crucial for how matter in the Universe clumped together in the first place. In other words, dark matter is crucial for our existence. But how do we uncover its nature, and put a face to the theory? That’s where the Dark Energy Survey and weak gravitational lensing come in.
Finding the cracks
Although we can’t see dark matter directly, we can detect its influence. Dark matter induces a distortion in the shapes of the galaxies we can see – those tiny fuzzy blobs I and my colleagues spend so long looking at. The way the shapes are distorted inscribes faint signatures of dark matter throughout the night sky.
Those signatures are what we’re looking for in the Dark Energy Survey. We measure the alignment of distant galaxy shapes to extreme precision and use those measurements to map the distribution of matter between those galaxies and our telescope.
The Dark Energy Survey is an international collaborative effort to map hundreds of millions of galaxies in order to find patterns in the cosmos and test the Standard Model. I’m immensely proud to play a leading role in it, as I co-lead the team searching for these patterns by using weak gravitational lensing.
When we observe a distant galaxy, we collect its light in our telescopes after it has journeyed for billions of years across the cosmos. According to our theory of gravity, any massive structure, whether it’s visible or not, will warp the space-time fabric of the Universe.
That warping alters the path that a galaxy’s light journeys along and, as a result, the galaxy image that we capture appears slightly distorted, or ‘lensed’.
For galaxies that are near to each other, their light travels past similar structures in the dark matter web. We observe their images to be skewed along the same direction due to gravitational lensing. Assuming those galaxies are, in reality, randomly oriented, stronger distortions in the images indicate a region that’s densely packed with dark matter. Similarly, weaker or no distortions indicate a region with relatively little or no dark matter – a gap in the scaffolding.
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In a way, we’re hoping that our results don’t agree with the prediction from the Standard Model, because it might serve as a clue for the next breakthrough.
But, in reality, it’s not as simple as that. For one thing, the distortions caused by weak gravitational lensing are significantly smaller than the distortions that arise as a galaxy’s light passes through Earth’s atmosphere.
It’s for this reason the Dark Energy Survey’s Victor M Blanco Telescope sits high atop the Cerro Tololo mountain in Chile, where the light from distant galaxies has less atmosphere to pass through before it reaches the telescope. Still, we have to carefully model these effects, and account for the imperfections of the telescope and detector.
Unwanted distortions due to Earth’s atmosphere and our instruments aren’t the only issue. Further complications arise due to the simple fact that galaxies are complicated structures. Their complex processes must be modelled as precisely as possible, if we’re to discern the distortions dark matter is causing.
Tiny distortions
While gravitational lensing is a powerful technique on paper, it’s extremely technologically challenging. As the term ‘weak’ suggests, the typical distortion induced by dark matter is less than a one per cent alteration to the observed shape – smaller than a strand of hair wrapped around a tennis ball.
In order to detect such a tiny signal, we need extensive samples of galaxies. This data challenge necessitates rapid processing of petabytes of data (one petabyte is roughly equivalent to the information held in 20 million four-drawer filing cabinets filled with files).
The Dark Energy Survey has confronted these challenges. We’ve spent years examining the images and have made precise measurements of the positions, shapes and distances between over 100 million galaxies. With this data, we’ve conducted the most statistically powerful weak lensing analysis.
But given the high stakes concerning our current understanding of the Universe, even that might not be enough. Humans are fallible and we need to be certain, so we can’t risk that any bias, conscious or unconscious, might influence the process.
So, to tease dark matter secrets from these exceptionally tricky measurements with minimal human bias, we do the experiment blindly. While calibrating our measurements and turning the dials in our analysis, we use data that’s been intentionally tampered with.
The blind data could agree with the Standard Model, or it could not. The analysis is complex and multi-faceted, and we do every step without looking at the final answer. We focus on doing the step in the best way possible, without thinking about how it influences the outcome. Once we’re absolutely convinced of our experimental choices, we reveal what the Universe is really like.
To set the scene: there are several teams performing weak lensing measurements independently. We each use vast datasets and we’re desperate to see whether what we find is consistent with the Standard Model or if it highlights new cracks in the grand picture.
And the results that we’re getting are intriguing.
A hint of discrepancy
What we’re seeing in all those images of tiny, fuzzy blobs are hints of something funny. Like our colleagues at the ESO Kilo-Degree Survey and Japanese Hyper Supreme Camera, the Dark Energy Survey found that weak lensing predicts a Universe that’s slightly less clumpy than would be expected based on the prevailing model of cosmology.
This hint of a discrepancy with the Standard Model isn’t the only one – other types of measurements point to cracks too, like the expansion rate of the Universe.
What could these hints mean? Either they herald a call for new physics, or they point to challenges in the analysis that are unsolved. With more data, we can scrutinise the findings with better statistics. It’s compelling to think that the time has come for a new landmark moment – a call for some modification to our peculiar model of a dark matter-dominated Universe.
In the scenario that we’re able to say convincingly that there is a real discrepancy with a measurement compared to the prediction from our Standard Model, we would overturn decades of thinking.
Revolutionising our idea of the Universe is certainly an exciting prospect, but the case for doing so must be watertight. We must have independent teams confirm the results and be able to build a consistent model. This piece of the puzzle must be able to fit the wider picture.
Dark matter’s bright future
There’s an exceedingly bright future ahead for the Dark Universe. Experiments like the Dark Energy Survey are epic, but they’re mere training grounds for a coming decade that will totally knock cosmology’s socks off.
We’re at the dawn of several major international projects that will observe the sky more deeply and in more detail than ever before: our tiny, fuzzy blobs are on the brink of bringing great clarity. It’s impossible to imagine what our understanding of the Universe will look like in a decade.
With these epic experiments opening new, more-powerful eyes onto the sky, we aren’t about to be spoiled with data – the truth is, we’re about to be fire-hosed with it. We are on the brink of making big discoveries.
It’s humbling to work in a field that asks what the Universe is made of and how its structure evolved to form galaxies and planets like Earth. In years to come, scientists who are young students now will look at the data delivered by these experiments and put more pieces of the puzzle together.
My only hope is that our views will be turned upside-down once again to reveal that our cosmos is even more beautiful than we had imagined – and unleash yet more questions that keep us looking up.
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