AcademiaNet: Professor Caputi, what are your research interests?
Prof Caputi: I research very distant galaxies. Galaxies that are so far away that the light we see emitted from them today is millions of years old. It is from the time the universe was only a fraction of what it is today.
You are looking at the beginning of time?
We are indeed. It sounds like magic. But really, it is only possible because the speed of light is finite. This is the key of our research because light takes time to reach two different points. Especially when it's so far away. By the time it reaches earth it has been travelling for millions of years. That way we can reconstruct the past of the universe by looking at very distant objects such as galaxies.
Galaxies – that sounds interesting. What exactly is a galaxy?
A galaxy is a big collection of many, many stars. Millions of stars with gas and dust in between. The first galaxies appeared about 500 to 600 million years after the big bang. But we don't know exactly when they first formed. Only that they were already there one billion years after the big bang. The oldest ones we ever found so far appeared a bit before the first billion years of the universe.
How can you spot them when they are that far away?
To find distant galaxies you need to point your telescope at the black regions of the sky. A region where you think is nothing. That is the magic: With the naked eye you think there is nothing. But when you look with the telescope you realize that your empty sky is not empty at all. There are many objects in the black regions of the sky. Some of these objects are very distant galaxies. Often ancient distant galaxies.
Do all galaxies look the same as ours?
In principle they do have the same kind of pattern, we believe. But for the distant galaxies we cannot really say because we cannot see individual stars. They are too far away. We don't have the power to dissolve for individual stars. What we study instead is integrated light. That is light we see as the product of all these stars together. The physics of these galaxies is the same as we find in ours. Most likely they therefore do look similar to the Milky Way and have solar systems.
How can you tell if these galaxies still exist when you see them with the telescope?
In fact you can't. We cannot say what happens to the galaxies right now or what happened in the past thousands of years. The light that we detect now is very ancient. These galaxies might not even exist anymore. We simply don't know. If we want to know what happens to galaxies right now, we'd have to look at the ones nearby in our universe.
Why then look at distant galaxies? What do you try to find out by looking at them?
We try to reconstruct how they developed by taking various snapshots from galaxies at different distances. The light they emit give us the different cosmic times we are looking at. Then we try to connect these populations and link them in an evolutionary time frame. It doesn't tell us what might happen to one single galaxy. But as we see many of them we can statistically reconstruct how galaxies in general evolve throughout time according to their properties. Particularly how the big ones evolve and why they are different to small ones.
What role do these galaxies play in the universe? What is their function?
Galaxies don't have a certain function. They just form when you have a given amount of matter in the universe. In the universe itself large matter is distributed uniformly. But at smaller scales matter is not distributed homogeneously. It prefers to be concentrated in some regions. In these regions the gravitational potential will act to produce conglomerates of matter which then form galaxies. The matter will collapse into stars and the different stars will coalesce among themselves or will aggregate to form big structures that then form galaxies. Just gravitational potential! But you need to start with small fluctuations in the uniform distribution of matter. Only when these fluctuations are big enough matter will congregate and form galaxies. Much of the universe, however, is empty.
So, you have for example two particles. They vibrate and that attracts other particles?
You start that way. It particularly is the action of gravity, not at the level of elementary particles. That happened millions of years before. When galaxies form we are already past the initial soup of elementary particles. The elementary particles have already aggregated into atoms and ions. Mostly hydrogen ions because they are the lightest ones.
Last autumn you published a paper on galaxy formation...
We did indeed. We found that more galaxies than previously thought are star bursting, meaning they produce many, many stars in a short period of time. Our Milky Way for instance is forming one or two new stars per year. This is a very slow rate of star formation but normal for our surrounding universe. There are, however, galaxies that form new stars very suddenly. Very strange cases! So far we believed them to be very rare. We even thought they did not significantly contribute to the total amount of stars that is being formed in the universe.
Now with this study we proofed different. For the first time we were able to go deeper. To be precise: go much deeper into the universe and cover a relatively large area of the sky. The combination of both is very powerful. It allowed us to go deep enough to see the galaxies from 1.5 billion years after the big bang. At that time, galaxies were still very small, but some of them produced stars at an enhanced rate. These galaxies with enhanced star formation made up for just 15% of all galaxies present in that epoch, but their star formation rates were so high that they were making 50% of all new stars at that time.
What does that tell you about the history of the universe?
It's telling us the way in which galaxies form their stars. This is in fact one of the big questions in modern astronomy. In the history of galaxy evolution the galaxies had millions of stars. One question we face is how they formed those stars. Did they form little by little through cosmic time? Or was there a sudden burst and then no further star formation? Until now we thought galaxies formed stars little by little. Episodes of sudden star formation we thought were not common. But now we have to rethink our models.
What will you research next?
We will try to go even deeper into the universe and explore what happened at the very beginning. The next telescope time we have will be in 2019 where we will look at galaxies from a few hundred years after the big bang.
One last question: If you could go anywhere in the universe, where would you go?
To the very beginning of time. I'd love to see what happened back then. Although, the conditions were probably very harsh for a human being back then.
Questions were asked by Sonja Klein for AcademiaNet and Spektrum.de.(© Sonja Klein / AcademiaNet / Spektrum.de)