You can't see it, you can't touch it, nor is it something you could hear, taste or smell. Dark matter is all around us, and its mass produces measurable gravitational effects. Other than that, it interacts with the visible universe even less than previously thought, according to the results of recent EU-funded research.
© Richard Massey
An EU-funded astronomer has generated new knowledge about dark matter. Together with his team, he has shown that its rate of interaction with the type of particles that compose the visible world is even lower than scientists had assumed. His project, named DarkMatterDarkEnergy, is also involved in the development of new soft- and hardware that is making its mark on current and upcoming observation campaigns.
Snapshots of an invisible world
DarkMatterDarkEnergy was funded by a reintegration grant as part of a programme set up by the EU to enable European researchers working abroad to re-establish their careers in one of the EU Member States. Coordinated by Durham University, the project was the brainchild of Dr Richard Massey, a British scientist returning to the UK after several highly productive years at the California Institute of Technology.
Massey used the grant to build on a major achievement completed in 2007: the publication of a map of the dark matter in a section of the universe, based on imagery provided by the Hubble space telescope. He studies dark matter by observing a phenomenon called gravitational lensing, i.e. the bending of light around large chunks of matter in the observable universe.
Gravitational lensing is a well-understood phenomenon, but conventional matter can’t account for its full extent, says Massey. The rest, he explains, is caused by dark matter, which makes up 85% of the mass of the universe.
This ubiquity obviously gives this elusive substance a certain pull — dark matter plays a key role in shaping the trajectory and velocity of astronomical objects. If it weren’t for this, you’d hardly know it’s there. It’s invisible, and it passes right through conventional matter.
Enlightened by colliding galaxy clusters
“The dark matter map showed the distribution of dark matter over a large area,” Massey explains. “So we know where it is. Now, we want to know what it is and how it behaves. So rather than looking at the big picture again, I am looking at small areas of the sky where dark matter is doing interesting things.”
Interesting things such as collide. On a very large scale. Of the magnitude of galaxy clusters crashing into each other. One such event — named the Bullet Cluster collision in reference to the shape of one of the galaxy clusters involved — is widely regarded as having confirmed the existence of dark matter and revealed its surprising characteristics.
Based on the analysis of this collision, Massey formulated a hypothesis about the behaviour of this puzzling substance and started to look for similar collisions to see if they would prove him right. By the end of the funding period, he had identified two more collisions that both bore out his theory, he reports, adding that he has since been able to validate it for many more. One of his key observations is that dark matter interacts with itself and with conventional matter even less than astronomers had known.
Massey’s work on Hubble has also given him a unique view of the requirements for such telescopes, which he is putting to good use for the development of innovative soft- and hardware components. He has notably found a way to correct imagery from Hubble for the growing damage caused by the telescope’s prolonged exposure to radiation from space. He is also involved in the design of a European Space Agency mission to map the geometry of the dark universe and leads a team developing an innovative balloon-borne telescope.
In describing the strange world of dark matter, Massey focuses on the facts. But it’s hard not to speculate. Are there scientists in the dark universe who are wondering why 15% of its mass seems to be missing? They may not believe in us either.