Marshmallows, Hot Chocolate & Galaxies

It’s a cold winter night; you’re sitting by the campfire sipping on hot chocolate. You watch the marshmallows bob up and down and there is a blanket of stars overhead. You can see thousands of stars in the sky.

But what you can’t see is that surrounding the stars there is an envelope of invisible matter that astronomers call dark matter. “Dark matter” because astronomers are still in the dark about it. Visible matter, which is what makes up everything that we can see, is just 1/5 th the amount of dark matter in the universe, yet we know so little about it. What we do know is that it interacts only through gravity. It helps keep the stars in their orbits around the centre of the galaxy. If it wasn’t for dark matter, the stars would be flung out into space and life, as we know it, would not exist.

Researchers look at nearby galaxies to precisely calculate the amount of dark matter in them. The stars in these galaxies have a very small up and down wobble. Measuring this wobble is a direct estimate of the dark matter in the galaxy. This is similar to how your marshmallows bob up and down in hot chocolate. If it is in thick, delicious hot chocolate, it would move slower than if it was in diluted, watered down hot chocolate. Similarly, the slower the stars wobble, the denser the dark matter in the galaxy.

The up and down motions of these stars are as low as 10 km/s, which in the grand cosmic scale of the universe is very very tiny. Measuring this from Earth is as difficult as someone on Jupiter trying to see how fast the marshmallows in your hot chocolate are moving. Scientists use the most sensitive instruments from around the world for this work. The result from my thesis will give us the most accurate estimate of dark matter in nearby galaxies. That gets us one step closer in trying to understand this mysterious dark matter.

So as you sit by the campfire looking up at the night sky, you can’t help but wonder, are all the stars you see just giant marshmallows in a sea of invisible hot chocolate?

Guest post by NGC628 & HD173