Simulating the Cosmos by Romeel Davé

Author:Romeel Davé
Language: eng
Format: epub
Publisher: Reaktion Books

A GALAXY IS BORN!

A cosmologist’s definition of a galaxy is a collection of baryonic matter at the centre of a dark matter halo. But the same question arises here as in gravitational collapse: Why does the baryonic matter not continue cooling and losing energy ad infinitum, until it collapses into a black hole? What halts the collapse and causes the formation of a disc galaxy like the Milky Way?

Think of ice skaters. You’ve all seen this trick: an ice skater begins spinning with their arms outstretched, and as they pull their arms in, they start spinning faster and faster. This is a manifestation of the law of conservation of angular momentum. Since angular momentum is the product of the physical extent and the spin velocity, when the skater lowers their physical extent by drawing their arms in, the rate of spinning must increase in order to keep the same angular momentum.

What does this have to do with forming a galaxy? In a sense, the halo is like a slowly spinning ice skater whose arms are fully extended. As radiative cooling kicks in, the visible matter breaks away from the dark matter and starts falling towards the centre. As it does, its physical extent gets smaller – like a skater when they pull in their arms, the baryonic matter starts spinning faster and faster.

Now let’s think about merry-go-rounds. You probably remember from childhood that it is difficult to move towards the centre of a spinning merry-go-round. This is because of centrifugal force – the idea that your body would prefer to move in a straight line but you are forcing your body to turn by hanging on to the merry-go-round. You feel this as a force that flings you outwards, relative to the centre of the merry-go-round. In contrast, standing up versus sitting down on a merry-go-round is comparatively easy (assuming you aren’t dizzy), because the centrifugal force only applies in the plane of the spin.

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