KEY POINTS:
Where did we come from? It's a question as old as the universe and, says Garth Illingworth, Professor of Cosmology at the University of Santa Cruz, if you want to understand how life formed on this planet, you have to step back and ask how the planet formed, then step back again to ask how the star around which it orbits formed, how the galaxy formed and then all the way back to the beginning of the universe to ask how the first galaxies came into being.
This is exactly what he and more than 100 international astrophysicists have been doing for the past week at the "New Zeal for Old Galaxies" cosmology conference in Rotorua.
Though astronomers are used to exotic locations - the world's best observatories sit atop mountains, in deserts, across whole continents, in ice, underground and in space - this is the first time such a conference has been held in this country. The choice of New Zealand was, in part, to honour Beatrice Tinsley, the young New Zealander who pioneered early models of galactic and stellar evolution.
She died in 1980, aged only 40, but Illingworth remembers her "jolly, outgoing nature" well - though others, such as Professor Elaine Sadler, of Sydney University, remember her more for her courage in standing up to one of the world's leading astronomers of the day, Allan Sandage, telling him he was wrong. "It was tough on all of us when she died so young," says Illingworth. "The techniques she pioneered at looking at mixtures in stars were very important and original contributions to the study of galaxies."
By "mixtures" he means not only the mass and distribution of the extraordinary variety of stars from red dwarfs to blue giants, but also concentrations of hot and cold gas and interstellar dust found in varying proportions in different types of galaxies.
It's the subtle interplay between these components that determines the evolution of galaxies over billions of years and the puzzle for astronomers is to tease out where they come from and how they interact.
Hydrogen gas, the simplest element, is what stars are made of and it was pretty much the only element around in the early universe. But thermonuclear reactions in the cores of stars produced the wide range of elements that we see in the universe today: the gold in your jewellery, silicon in computers and fluoride in toothpaste were all forged in stars billions of years ago, as were elements like carbon and oxygen, essential to most living things, including you.
So how, over 13.7 billion years, did we get from a simple, hydrogen-filled universe to one filled with stars and galaxies and us?
"All the stars in the universe are in galaxies, so by tracing their evolution we can trace our own and there is a sequence," says Illingworth. "When we look at the current universe we see that most stars are in 'red dead' galaxies - they've run out of gas and not much is going on. But when we look back in time, as we are able to do with our telescopes, we see young blue galaxies - the colour indicates that there is a lot of star formation and that occurs when galaxies collide and there is an injection of new gas."
Those early galaxies had more gas than stars but through mergers and "cannibalism" of other galaxies, many quickly grew from small spiral-shaped galaxies into large spherical ones called ellipticals. "But about two billion years into the life of universe they appeared to run out of gas - or neighbours to feed off," says Illingworth. "The stars are still there, but new ones aren't being formed."
Our own galaxy, the Milky Way, is one of the largest spiral galaxies and it still produces new stars at a low rate, probably because it is still accreting nearby dwarf galaxies, such as the Magellanic Clouds.
But although we understand the broad hierarchical picture that gas turns into stars, successive generations of stars enrich the gas with more elements, big galaxies evolve from mergers between little ones, and eventually the gas runs out, there are still many questions to be answered.
One astronomer put it in terms of television crime show CSI: "We've got the bodies and can give an approximate time of death, but we're still not sure of the cause."
Astronomers are the ultimate forensic scientists, reconstructing events from snapshots of "crime scenes" using X-ray, infrared, and other astronomical cameras to reveal information hidden at optical wavelengths. They fingerprint stars, galaxies and gas through their spectra to identify their "DNA" (composition), velocities and redshifts (distances); and run it all through computers for analysis.
And as detectives might have to confront the dark forces of evil, cosmologists regularly grapple with dark matter and the mysterious dark energy that make up about 95 per cent of the universe. Dark matter is exactly that - ordinary matter which we cannot see but know is there because of the way galaxies rotate.
Stars in the outer arms move at much the same speed as stars in the inner arms, which means the galaxies would have disintegrated long before now were it not for huge haloes of dark matter that hold them together.
Professor Rosie Wyse of Johns Hopkins University says, "The real fundamental issue in astronomy today is what is the nature of dark matter and one of the good ways of getting to that is studying the galaxies in the Local Group [of about 30 nearby galaxies, including the Milky Way and Andromeda] and trying to understand the merging history."
One of the "hot" issues about dark matter is whether it is cold dark matter (CDM) or merely warm, as a group led by New Zealand astronomer Gerry Gilmore at Cambridge University suggests.
"CDM predicts a special place for the Milky Way because there should be a lot more dwarf galaxies than what we currently see," says Wyse. "Warm dark matter does away with the need for more dwarf galaxies, so it solves the problem at a broad level. But you need to look in detail because you want to make sure you don't wipe them all out, and that they form early enough in the universe to gives us the old stars in galaxies that we see."
And there are others, such as Bonn University's Professor Pavel Kroupa, who argue against the need to invoke any sort of dark matter to explain the very dim dwarf galaxies which are generally thought of as a few stars in a dark matter halo. Kroupa's controversial explanation, that they're simply clumps in tidal streams produced by the Milky Way's early interactions with other galaxies, which are aligned in a particular way, caused a few ripples at the conference.
But, warm or cold, there are at least some contenders for dark matter in the form of exotic particles such as sterile neutrinos. For dark energy, the mysterious "anti-gravity" force accelerating the rate of expansion of the universe, there are none.
Professor Warwick Couch, from Swinburne University's Centre for Astrophysics and Supercomputing, explains how observations of supernovae established there is a repulsive force which is driving everything apart faster and faster.
"These supernovae have a fixed brightness so at different distances you can infer how far away they are - we call them 'standard candles'. At cosmological distances you can infer the rate of expansion and using them, we discovered that about 5 to 7 billion years ago the universe started to expand more rapidly."
Couch is about to embark on a major project, to measure the effect of dark energy over time. "Instead of a standard candle, we'll be using a 'standard ruler', imaging thousands of galaxies and measuring where they lie in space and how they cluster on different scales at different times. That will give us an indication of when this force started to dominate and what effect it is having."
Though the prognosis for the universe looks decidedly bleak and lonely, with galaxies hurtling away from each other at ever increasing rates, astronomers don't seem deterred. "This is a most exciting time," says Wyse. "There have been so many breakthroughs, we've got wonderful new instruments and we're really moving forward in our understanding of how we fit into the universe."
