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Weighing heavenly bodies based on bending light

Many astronomers suspect that most of the matter in the universe is invisible. So how can you weigh dark matter if you can't actually see it? Professor Henk Hoekstra is looking for a solution. Inaugural lecture 25 June.

It is a question that has fascinated astronomers for a long time. Stars and gas in the outer parts of galaxies move faster than we might expect on the basis of visible matter. That could mean one of two things: either there is something wrong with the theory of gravity or of there is more matter in the vast universe than we can see. The majority of astronomers tend towards the latter idea and point the finger at invisible particles, otherwise referred to as dark matter. 

Space-time distortion

Professor of Observational Cosmology Henk Hoekstra tests the theory of gravity. He does this by charting the mass of galaxies and even larger cosmic structures. In his research he makes use of what is known as space-time distortion. ‘Light rays in space are slightly deflected when they skim past solid objects of this kind. The gravity of a galaxy always attracts the ray of light. You can calculate the weight of the galaxy based on the angle of refraction.' 

Bowl of water

Something similar happens if you put coins in a bowl of water. If the water isn't moving, the coins remain perfectly round, and you can barely distinguish between an empty bowl and one filled with water. Hoekstra: ‘The situation changes if you make some waves in the water: the coins seem to move. If you make a photo, you will see that the images of the coins are distorted. The variation in the column of water influences the rays of light. This tells us that the bowl is filled with water. And if there are enough coins on the bottom of the bowl, we can even reconstruct the shape of the surface of the water.' 

Better measurements

Hoekstra hopes this will help him learn more about how the universe works. Even though space in some ways seems to work amazingly simply following the laws of physics, appearances can be deceptive. Hoekstra: ‘Our present knowledge of physics is incomplete. The more we find out, the less we understand. But we now have the rather exceptional situation where better measurements may help us further.’

European Space Agency

Hoekstra is currently working on the Euclid satellite which is expected to be launched by the European Space Agency (ESA) in 2022. Hoekstra is responsible for the research on developing the technique of weak gravitational lensing that allows astronomers to map dark matter through the distortion of light.

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