Cosmologists can read the future of the universe - but it is a freezing, dark and lonely future, warns Dick Ahlstrom.
Some say the world will end in fire, Some say in ice.Robert Frost (1874-1963)
When Robert Frost wrote 'Fire and Ice' he considered two long-term options for the Earth. Cosmologists have considered similar options for the universe and it now seems certain that ice will be its ultimate end.
Yet more astronomical evidence has emerged for a perpetual expansion of the universe. The consequences of this will be a much colder, lonelier cosmos with stars gradually blinking out over billions of years and matter spreading out ever more thinly as space continues to expand.
The world of astronomy was rocked three years ago when two teams of scientists studying stellar explosions called supernovae presented evidence that the expansion of the universe that started at the Big Bang was accelerating rather than slowing down as had been expected.
Cosmologists had long assumed that the mass represented by all the stars and galaxies and planets sprinkled through space would produce enough gravity to halt the universe's gallop, turn it around, and reverse the Big Bang in an eventual Big Crunch.
Now a team of UK and Australian astronomers have come up with new, independent evidence of the continued acceleration of the universe, an expansion that has no hope of being reversed. Their work was published recently in the Monthly Notices of the Royal Astronomical Society.
Prof George Efstathiou of the University of Cambridge and colleagues used the Anglo-Australian Telescope in New South Wales to study the clustering pattern of 250,000 distant galaxies occupying a large chunk of the universe. They compared this structure to the way the universe looked when it was just 300,000 years old, a mere pup. They achieved this using what is known as the cosmic microwave background radiation.
Big Bang cosmology holds that everything we see in the universe today originated 12 to 14 billion years ago when a structure just a few millimetres across exploded to create space and time. It started impossibly hot and dense but over the aeons it has cooled and condensed to form the stars, galaxies and planets we see today.
The initial heat has dissipated but not entirely. Instruments that read radiation in microwave frequencies can detect the background radiation left behind by the Big Bang. Astronomers studying the radiation can use it to see backwards in time to when it first began bouncing off hydrogen gas that formed as the universe cooled, the radiation's "surface of last scattering". This happened just 300,000 after the Big Bang so reading the background radiation gives a view of how the universe looked then.
Prof Efstathiou's team took this microwave view and compared it to their galaxy study. It allowed them to prove the speed-up of the universe's expansion. And, like the supernova study, it also allowed them to show what might be driving this acceleration, a spooky, ill-defined substance known as "dark energy".
Albert Einstein first postulated the concept of dark energy, describing it as the cosmological constant, but later rejected it, explains Prof David Fegan of University College Dublin's Department of Experimental Physics. "Instead of allowing the acceleration of the universe to decrease with time, it allows the acceleration to increase in time," he says.
Scientists don't know what dark energy is but they have known for some time that something was missing, Prof Fegan says. Our galaxy spins on its axis once every 200 million years but when an inventory of its entire mass is calculated, there isn't enough visible matter to account for its rotational characteristics. "It appears as though there isn't enough mass to account for the movement," he says. Now the latest theories on dark energy and "dark matter" help explain this, also providing cosmologists with a handy "recipe" for making a universe, Prof Fegan says.
About 60 per cent is dark energy, a repulsive force the opposite of gravity that seems to drive the universe's continuing acceleration. It could be Einstein's cosmological constant or a quantum field known as "quintessence".
Most of the rest is dark matter, subatomic particles such as neutrinos. The remainder is the ordinary matter we can see plus non-luminous bodies such as stars that never caught fire and giant planets, so-called cold matter. The final ingredient is a trace of radiation that in another life could have been matter according to Einstein's formula E = mc2