The universe began in a big bang about 15 billion years ago, but what is the long-term future prospect for our universe? Will it last forever?
There are two possibilities: the universe will either end in a "Big Crunch" - a cataclysmic fireball of unimaginable intensity - or else it will last forever, but in the eventual form of a frozen wasteland.
This may seem rather dismal news. Yet it is some comfort to learn that the frozen wasteland alternative seems much more probable, that neither scenario will arrive any time soon, and that it may be possible for human consciousness to persist indefinitely in the frozen wasteland. As Mr Spock of Star Trek fame often reported to Captain Kirk - "It's life Jim, but not as we know it."
The universe has been expanding since the Big Bang. Whether it will expand forever depends on how much matter it contains. All matter is attracted to all other matter by gravity. If there is enough matter in the universe gravity will gradually slow expansion to a halt and then reverse it, causing the universe to shrink back to a point source of unimaginable heat and density - the Big Crunch.
However, the most recent evidence indicates that expansion of the universe is speeding up rather than slowing down. Also the apparent density of matter in the universe is less than required to halt expansion. It seems that the universe will expand forever.
Stars do not last forever. In five billion years' time, our Sun will swell into a red giant as it exhausts its nuclear fuel, reaching Earth's orbit and vaporising all life on Earth. It will then shrink back to a cooling white dwarf star. The human race will end if it has not colonised a planet beyond our solar system before the red giant roasts Earth.
Also about five billion years from now the Andromeda galaxy, which is already approaching, will merge with our Milky Way galaxy. One might imagine this as a catastrophic collision, but most stars will remain unscathed (the chance of a head-on collision is less than 1 in 100 billion) although their motions will be severely perturbed. Just as with Milky Way and Andromeda, in time most galaxies will combine with others in the same group.
The universe will get darker and colder with time as galaxies disperse ever more thinly through the expanding space. The galaxies themselves will get intrinsically dimmer. Star brightness results from the energy released from nuclear fusion, principally when hydrogen is fused into helium and higher elements. When the hydrogen becomes depleted the nuclear furnace dims and the star dies, often in a violent explosion. Material released from dying stars is recycled through new stars being born.
However, as time goes on, it becomes increasingly difficult to create bright stars out of already spent fuel. More of the remaining hydrogen gas gets locked up in faint low-mass stars or in dead remnants of stars - neutron stars, white dwarfs or black holes.
As the galaxies combine, the black holes at the centres of individual galaxies will sink into the middle of a merged system surrounded by a swarm of dead stars. About 100 trillion years from now the last star will die.
One of the most amazing recent discoveries is that matter is destined to fade away as its atoms slowly decay - the protons in the nucleus of the atom are unstable. One would have to watch many tonnes of matter for at least a year to detect the decay of one atom. But, given enough time, all the galaxies that now blaze so conspicuously will evaporate.
When a proton decays it emits a positron, the antimatter equivalent of the electron. Each positron collides with an electron and the pair annihilate each other in a burst of gamma radiation. Some positrons and electrons will escape this deadly encounter and roam the dark chasm of space with only black holes for company.
Black holes are unaffected by proton decay, but even they do not last forever, shedding energy by "quantum evaporation". The larger the black hole, the longer it lasts. The massive black holes at the centres of galaxies, each as heavy as millions of stars, will disappear eventually in 10s9]s8] years, a shorthand form for a number written as 10 followed by 97 zeros.
Everything the black holes ever swallowed will be recycled back into radiation. At this stage the universe will contain nothing but radiation and the odd electron and positron.
The universe cools as it expands. In the beginning it was unimaginably hot. Today, the temperature of interstellar space is 2.7 degrees above absolute zero, the coldest theoretical temperature thought possible and equal to minus 273.15 degrees Celsius. One trillion years hence it will have fallen to less than a thousandth of a degree above absolute zero.
IF humanity survives long term it will live in a universe that is incredibly cold and dilute, but it will still have to gather enough energy from this void to keep itself going. In order to conserve energy, humans will have to reduce body temperature to ambient universe temperature.
The human body couldn't possibly adapt to this, so we will have to shed our bodies. There is no compelling reason to believe that conscious thought could not be performed by a sufficiently sophisticated computer or some other inorganic device. We still have many billions of years to design the physical device into which we must eventually transfer our consciousness.
The new conscious beings will operate at very low metabolic rates in this extreme cold, and will therefore only think slowly. Human consciousness could survive for a very long time under these conditions, provided it hibernated for much of the time. However, recent calculations indicate that eventually the task of gathering energy from the ever diluting universe will prove too much. Humanity will then go to sleep forever.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.