Watching the Cosmic Web take shape

University of Groningen astronomer Karina Caputi has secured nearly 1800 hours of observing time on the Spitzer Space Telescope. It’s the biggest programme ever approved for a scientist outside the US. Caputi will use this time to study the formation of galaxies in the first billion years of cosmic history.

If you look at the sky on a clear night, you will see the stars in our own galaxy. If you use binoculars or a telescope, you may see more nebulous objects. These are other galaxies like our own, each with tens of billions of stars. ‘The galaxies are not distributed in a random way’, explains Caputi. ‘They seem to cluster in filaments and nodes, forming what are known as the cosmic structures.’ This ‘Cosmic Web’ is caused by gravity – regions in the universe with stronger gravitational forces attract more matter, leaving a void between the filaments.

‘I study how galaxies evolved in the first half of the life of the universe, the mass of these galaxies and what the distribution is of the different masses.’ Cosmological models have been developed to describe this evolution right from the origin of our universe. ‘They are quite accurate, but there are still some discrepancies between models and observations.’ And some observations are missing altogether.

‘We don’t know anything about the large-scale structures in the very early phase, the first billion years after the Big Bang’, Caputi explains. The models predict that the Cosmic Web started to form in this period. Caputi would therefore like to take a look.

Time travel may not be possible, but we can look into the past. All you need is a big telescope you can use for a long time, two requirements that are met by Caputi’s new observation programme, which was approved last December. The Spitzer Space Telescope is a less well-known relative of the Hubble Space Telescope. It collects light in the infrared part of the spectrum, an important wavelength range that cannot be observed with Hubble.

Spitzer can capture the light of objects that are very far away. This light travels through space for a long, long time, billions of years even. As the universe is expanding, the light from the very first galaxies is still arriving at Earth. Caputi will use the Spitzer telescope to study this light to see if there are already visible traces of the Cosmic Web in the early universe, as predicted by the models.

‘What we need to do is observe distant galaxies in a sizeable area of the sky. That is why we need so much observing time.’ To catch the very dim light of those far away galaxies, you need long ‘exposure times’, like you do to take a photo in a dark room. And as each ‘exposure’ covers a very small part of the sky, Caputi needs to make lots of them to cover enough space to see the large-scale structures in the Cosmic Web.

Competition for observing time on Spitzer is fierce: in this last round, the amount of time requested was five times what was available. So why did Caputi get such a generous amount? ‘First, I have worked with Spitzer data for over ten years, and second I have published many scientific papers about galaxy evolution in the young universe.’ She also assembled a ‘dream team’ of co-investigators. ‘For example, I have the developers of the Spitzer camera on board. They know all the ins and outs of the system and will help with the data analysis.’

There is another reason why the project was approved: ‘We need additional data from other telescopes that measure different wavelengths to complement the Spitzer data we’re going to collect. This data only recently became available.’

The first observations in her programme will be made in March. ‘This is just the first part of the programme. It will take about a year to get a significant amount of data, and two years before the programme is completed. The area that the telescope can study at one time is limited. You can’t just point it anywhere you want.' You have to wait until your chosen target becomes visible during the telescope’s orbit, as it trails the Earth around the Sun. ‘The patch of sky we want to study is available twice a year.’

It should be well worth the wait. ‘Once we have the data and can produce maps, we will be able to do the real science and study the birth of the Cosmic Web in the first billion years of the universe.’ And after that? The Spitzer measurements won’t reveal the first few hundred million years after the Big Bang, so it would be interesting to look back even further. ‘But for that, we will need a new and larger telescope, the James Webb Space Telescope, and that won’t be launched until 2018.’

Karina Caputi works at the Kapteyn Institute for Astronomy, University of Groningen. The University offer both bachelor and master programs in astronomy.