When were the first galaxies born?
What can you see with ALMA?

When were the first galaxies born?

Read time: 3 minutes

The Earth is a planet, orbiting the Sun. The Sun is a star – a giant ball of hot gas, emitting light and heat. The Sun is one of the few hundred billion stars in our Milky Way galaxy. And the Milky Way is one of the hundred billion or so galaxies in the Universe.

Astronomers have discovered that the Universe has not existed forever. It all started some 13.8 billion years ago in the big bang – a giant explosion of energy, matter, time and space. Shortly after the big bang, the expanding Universe was filled with a glowing hot gas of simple atoms. So where did all the galaxies, stars and planets come from? ALMA will help to answer that question.

First, the hot gas in the Universe cooled down and became dark. Then, the cold gas clumped together because of its own gravity. These clumps would later become the first galaxies. Smaller clumps of gas in these dark clouds became hot again, and started to shine. Thus, the very first stars in the Universe were born.

Illustration of the expansion of the Universe. Credit: Frannerd, A. Peredo - ALMA (ESO/NAOJ/NRAO)
Illustration of the expansion of the Universe. Credit: Frannerd, A. Peredo - ALMA (ESO/NAOJ/NRAO)

At least, that’s the theory. ALMA will check if the theory is right. Like every other telescope, ALMA is able to look back into time. That’s because light from a distant star or galaxy takes a long time to travel all the way to Earth. So at very large distances, we see galaxies whose light has been underway for billions of years. We see them as they were billions of years ago.

To look far back into time, you have to look very deep into space. Large telescopes on the ground, and space telescopes like Hubble, have detected galaxies at more than twelve billion light years distance. That means their light took more than twelve billion years to reach us. So we are seeing them as they were when the Universe was still very young.

But current telescopes cannot see the first individual stars in these distant galaxies. So how do we know when the very first stars appeared on the scene? That’s where ALMA comes in. At the end of their lives, stars blow large amounts of gas and dust into space. The largest stars even explode completely. Such supernova explosions also produce lots of dust.

ALMA can detect the millimeter waves emitted by this dust. If dust is detected in a distant galaxy, you know for sure that there must have been stars in the galaxy. ALMA will determine when the first stellar dust was created. Thus, it will answer the question about when the first stars were formed.

Of course, ALMA will also discover a lot of new things about cold dust clouds in other galaxies that are not so far away. It will help to unravel the full life history of our Universe!