Shedding light on the Dark Universe
It would be easy to imagine that the Dark Universe was a malevolent force in the latest Star Wars movie, it’s leaders the enemy of the Federation, or that dark energy had some kind of demonic origin. However sinister it may sound, the dark side is entirely innocent and, in fact, it comprises 95% of our Universe.
To give this perspective, Earth is an almost infinitesimal speck in the cosmos. It orbits the Sun, one of billions of stars, swirling around and bound together to form our galaxy, the Milky Way. Moreover, there are billions of galaxies in our Universe, each boasting their own hoard of stars and planets! Observational cosmology tells us that these structures, that are made of particles whose physics we understand, only constitute about 5% of everything in the Universe. The rest is dark matter and dark energy.
Dark matter is a special type of matter that neither emits nor interacts with light, but plays an important role in the story of our Universe. More than three quarters of the mass in our Milky Way galaxy (and other galaxies) is the invisible dark matter, rather than the stars and the planets. Therefore, the dark matter creates a large gravitational effect and acts as the glue holding our galaxies together.
Dark energy is even more mysterious. It is a form of energy that drives the accelerated expansion of our Universe. That is, our observations reveal that while stars stay tightly bound in galaxies, as cosmic time marches on the galaxies themselves are moving further away from each other, and our best theory holds dark energy responsible. While we can’t see these entities, we infer that they exist from their effect on things we can see.
It may sound like cosmologists have the Universe sussed, but there are cracks in our Standard Cosmological Model. While we understand the effect of dark matter in the universe, particle physicists are yet to detect its particle in their giant dark matter net experiments. On the other hand the best theory for dark energy, as predicted by quantum physics, is starkly wrong. To put it politely, there is much work to be done! It is possible that we are missing something in our theory of gravity- Einstein’s General Relativity- and may need to invoke some new physics in order to solve the dark energy phenomenon. That is, just as Newtonian gravity, which satisfies experiments on Earth, was revolutionised by Einstein’s theory in order to explain measurements in the solar system, perhaps we need another upgrade to explain even larger-scale observations. We focus on observing how dark matter changes over cosmic time, which sheds light on how dark energy evolves and allows us to test gravity on cosmological scales.
Cosmology has a vast toolbox of independent methods to understand the nature of the Dark Universe and to test the laws of gravity. Techniques include measurements of the brightness of supernovae- the explosive ends of binary pairs of unequal mass stars; exquisite observations of the Cosmic Microwave Background-temperature fluctuations across the sky from the light emitted in very early universe, just 380 000 years after the Big Bang; charting the distant Universe by obtaining precise velocities of and distances to galaxies; and meticulously measuring the shapes of distant galaxies. The latter is called weak gravitational lensing.
Weak gravitational lensing
As we observe a distant galaxy, we collect its light in our telescopes after it has journeyed across the Universe. According to General Relativity, dark matter, like any massive structure, warps the very fabric of the Universe, space-time, as depicted by the grid in the image below. The path that the light travels along, indicated by an arrow, also gets bent with the space-time and as such, the image of the galaxy that we capture appears distorted. The presence of dark matter or massive structures along the line of sight has the effect of lensing the galaxy- making it appear more elliptical in our images and inducing a coherent alignment among nearby galaxies.
The stronger the average galaxy ellipticity is in a patch of sky, the more dark matter there is in that region of the Universe, assuming galaxies are in reality, randomly oriented. Therefore, the induced ellipticity of the galaxies is a faint signature of dark matter inscribed across the Universe. If we can measure this alignment to extreme precision, and combine with the equations of General Relativity, we can infer the location and properties of the matter- both visible and dark- between us and the galaxies. By mapping the evolution of the dark-matter structures with cosmic history and documenting the accelerating expansion of space and time, we learn about dark energy.
I work as part of a European team, called the Kilo-Degree Survey, imaging a 5% chunk of the sky a few hundred times the size of the full moon. We have measured the positions and shapes of tens of millions galaxies, as the universe was when (at most) half its current age. While this sounds wildly impressive, we are only now seeing the tip of the iceberg for what is required to truly understand our Universe. That is because while gravitational lensing is a powerful cosmological technique, it is extremely technologically challenging. The typical distortion induced by dark matter as a galaxy’s light travels through the universe, is only enough to alter the shape of that galaxy by less than 1%. As the lensing effect is weak, in order to detect it we need to analyse the images of millions of galaxies. This entails a data challenge, necessitating rapid processing of petabytes of data. A scientific hurdle arises as the weak lensing distortions are significantly smaller than the distortions that arise in the last moments of the the light’s journey. Due to the effect of the Earth’s atmosphere and our imperfect telescopes and detectors, instead of measuring the shapes of galaxies in images that are beautifully resolved like the Hubble Space Telescope image below, in large lensing surveys, galaxies can appear as fuzzy blobs that only span a few pixels. Just to up the ante, the terrestrial effects change between and throughout the night’s observations as the wind, temperature and weather vary, even in the exquisite conditions of the mountaintops of the Atacama Desert, Chile, where lensing data is often collected. In order to isolate the dark matter signature, the nuisance distortions are modelled to extremely high precision and then inverted, allowing an accurate recovery of the cosmological signal. Further complications arise from the physics of the galaxies. They have an intrinsic ellipticity and dynamical processes that we do not perfectly understand, but must also factor into our calculations.
The Kilo-Degree Survey, as well as similar American and Japanese experiments, act as stepping stones and a training ground for an epic coming decade for observational cosmologists. We are at the dawn of several major international projects that will survey the sky to greater depths and resolution than ever before. The Large Synoptic Survey Telescope will image the entire Southern sky every few nights, building the deepest and largest map of our cosmos, the Euclid satellite will survey the sky from space, eradicating the worry of Earth’s atmosphere and the the Dark Energy Spectroscopic Instrument will delivery extremely precise locations and velocities of over 30 million galaxies. I look forward to helping these projects to map the distant Universe, trace the evolution of the dark matter and dark energy from 10 billion years ago to the present day and in doing so, bringing us closer to fathoming the other 95% of our Universe: the dark side.
It is a humbling field that asks what the Universe is made of and how its structure evolved for the formation of galaxies and our existence. In our insignificant snippet in the grand story of the Universe, it is remarkable that technology allows us to observe objects at distances beyond our comprehension and that our diverse range of measurements even vaguely fit a consistent model.