Euclid - Challenge the Machines

Physics concepts in the project

What is a galaxy?

A galaxy is a collection of many billions of stars, dark matter and clouds of gas and dust, all held together by the force of gravity. Our sun is just one of the hundred billion stars in the galaxy we live in, called the Milky Way. The Milky Way is a disc galaxy, meaning it has a flattened disc shape. Because we live inside it, it looks to us like a band of bright stars across the night sky on a dark night.

The image below shows some examples of other galaxies. From left to right they are: (a) an elliptical galaxy, (b) a spiral galaxy, and (c) an irregular galaxy.

(Image from The Open University.)

What is gravitational lensing?

Einstein's theory of gravity, known as general relativity, predicts that every object in the Universe curves the space and time around it. Right now, you are curving the space around you. This curvature is too tiny to measure directly, but for giant objects in astronomy like galaxies or clusters of galaxies, this curvature becomes measurable.

The way it is measured is to deflect the paths that light takes. This becomes visible to us when we have a background object and a foreground object lined up almost exactly along our line of sight. In this situation, the background object is being seen through the warped space of the foreground one. The larger the mass of the foreground massive object, the more the light's path is deflected.

The image below shows a foreground cluster of galaxies, in an image captured by the Hubble Space Telescope. The foreground galaxies are the orange-yellow elliptical galaxies. The background galaxies are being warped into long thin arcs, because we are seeing them through the warped space of the foreground galaxy cluster. This warping of background galaxies by foreground matter is called gravitational lensing.

(Image credit: Andrew Fruchter (STScI) et al., WFPC2, HST, NASA. Image from Astronomy Picture of the Day.)

Our place in the Universe

Our planet, Earth, orbits the Sun once a year. Also orbiting the Sun are seven other major planets, some with one or more moons and ring systems, and other minor bodies (dwarf planets, asteroids and comets). This is shown in the picture below.

(Image courtesy of The Open University.)

Our star, the Sun, is one of a hundred billion in our galaxy, the Milky Way. There are also more planets than stars in our galaxy! In turn, the Milky Way is only one of about a hundred billion galaxies in the observable Universe.

What is the Universe made of?

The composition of the Universe is shown in the picture below. Some of it is in the form of elements like hydrogen and helium, but most of the matter in the universe is in the form of dark matter. More on this mysterious stuff below. Because most of the matter in the Universe is dark matter, it's the dark matter that does most of the work in gravitational lensing.

(Image courtesy of The Open University.)

Why do astronomers think the Universe is so full of dark matter?

You can tell how much matter is in a galaxy by how fast it's rotating. The faster the stars are moving, the stronger the gravitational force has to be to keep the galaxy together.
Some edge-on spiral galaxies have had their masses estimated by measuring how fast their stars are orbiting. But this runs into a problem. If you add up all the masses of the stars, and add the mass of the galaxy’s gas, it's not enough to account for all the gravitational force. If the only matter were just the stars and the gas that we can see, then the stars would be flung out of the galaxy because they're moving so fast. So what's keeping the galaxy together? It has to be something that doesn't emit light, or we'd see it. Astronomers call this extra matter "dark matter".

What is dark matter?

The short answer is, no-one knows, at least not yet.

Want to learn more?

If you enjoyed this short introduction to the physics concepts in the project and would like to take your study further, here is a general introduction to galaxies, stars and planets. And if you want to take your learning even further the Open University is open to people, places, methods and ideas so you could study for an entire degree in Natural Sciences (Astronomy and Planetary Science) at a distance with no formal entry requirements.