Over the last few years many of the fundamental properties of the universe have been accurately measured. We know better than ever when the universe began, what it is made of, and how it has evolved since the beginning. Moments after the Big Bang, 13.5 billion years ago, the universe was very hot, very dense and very simple – a mixture of energetic particles and radiation, or ‘plasma’. The universe expanded, becoming cooler and less dense, forming atoms, molecules, stars, planets, galaxies and eventually life. The history of our own Sun and planet, and indeed ourselves, has been determined by the history of the galaxy in which we live.

The transformation of the universe from a simple hot plasma into a complex hierarchy of stars and galaxies was a process spanning billions of years. Galaxies are massive systems containing mostly dark matter (mysterious particles that do not emit radiation), hydrogen gas and stars, bound together by the force of gravity. They are the basic building blocks of the universe.


Spitzer image of spiral galaxy M81. (NASA/JPL-Caltech)

Within individual galaxies, complex physical processes form stars out of hydrogen gas. Within the stars, nuclear fusion reactions turn the hydrogen into heavier elements (carbon, nitrogen and oxygen, for example). As the stars evolve with time these elements are released back into space, chemical and physical processes turning them into larger particles of complex molecules known as ‘dust’. Some of this dust is then incorporated into newly-forming stars in a continuous feedback cycle which is forever changing the physical and chemical composition of galaxies.

When new stars are formed, the surrounding gas and dust in a galaxy is heated, causing the dust to radiate in the infrared. Conversely, the dust absorbs most of the ultraviolet and visible light from new stars making them invisible at these shorter wavelengths. Thus the star formation that drives galaxy evolution can be most effectively studied at long wavelengths, principally in the near-infrared (wavelengths of 1-20 microns) and far-infrared (20-1000 microns) passbands. In fact, something like one-third of all the radiation in the universe is in the far-infrared passband, making it the second most important region of the electromagnetic spectrum in which to study the energy output of the universe (the first is the microwave region, a leftover of the Big Bang itself).


Distant galaxies from the Hubble Space Telescope's Ultra-Deep Field. (NASA/STScI)

Our work is to study the process of star formation in galaxies, in particular looking at how this process depends on the environment of a galaxy – whether there are lots of other galaxies nearby, or if it’s relatively isolated in space. Due to the constant speed of light, when we observe distant galaxies we actually see them as they were in the past, when the radiation (for example, visible light or infrared radiation) was emitted. Thus we can directly measure how galaxies have evolved over time by comparing distant objects (seen as they were in the past) with those nearby (seen as they are now).