An international team of astronomers generated the largest map yet of the most abundant (and most mysterious) form of matter in our universe — dark matter. Results support the view that this invisible substance is the cosmic glue which holds galaxies and clusters of galaxies together.
Unseen Matter and the Motion of Galaxies
In 1933, Swiss/American astronomer Fritz Zwicky proposed the theory that galaxies harbor a huge quantity of unseen matter. Based on the motion of outlying galaxies in the Coma galaxy cluster, Zwicky concluded there was not enough visible matter to hold these fast-moving galaxies together. He insisted something invisible was producing additional gravity out there in the heavens.
Astronomers at the time thought that this was just another wacky idea from Zwicky. As British/American physicist Freeman Dyson put it:
Fritz Zwicky’s radical ideas and pugnacious personality brought him into frequent conflict with his colleagues at Caltech. They considered him crazy and he considered them stupid.
Some forty years later, American astronomer Vera Rubin (along with W.K. Ford and others) confirmed Zwicky’s wild idea. Since then, numerous astronomical surveys show stars moving within galaxies and galaxies moving within clusters of galaxies are going faster than they should. Some huge amount of invisible matter must be pulling on stars and galaxies — or they would fly apart. This unknown substance has been dubbed dark matter.
A number of independent observations now tell us that dark matter is the most prevalent form of matter in the universe — there is some five times more dark matter in the cosmos than ordinary matter.
Dark Matter Lensing
How do astrophysicists map dark matter? How can they see something which is invisible? They detect this mystery mass by inference; by indirect observation.
Like ordinary matter, dark matter is a source of gravity. And, as Einstein’s general relativity predicts, gravity bends light. So the presence of dark matter bends light from distant galaxy clusters, making them appear distorted.
Think of a spoon in a glass of water. To us, it appears bent. Why? Because — acting like a lens — the water in the glass distorts the light from the spoon. What if the water were invisible? We would still know it is there because the spoon appears bent.
Similarly, dark matter reveals its presence by acting as a gravitational lens, bending the light emitted by galaxies. So, as water makes the spoon appear distorted to our eye, dark matter makes galaxies appear distorted as seen in our astronomical telescopes.
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