I am constant as the northern star” makes for great Shakespeare but, as any astronomer will tell you, the bard's words are factually inaccurate. Although the stars appear fixed in position for the duration of our lives, they are moving in their great orbits and slowly creep across the sky, changing the shape of the constellations over the course of tens of thousands of years. Neither are stars immortal.
The energy-generation models pioneered by Eddington can be used to predict their lifespans. Depending upon its mass, a star can live for between 10 million and 100 billion years. The most massive stars live the shortest lives because they use their fuel the fastest. The result is that the stellar population will change with time, as new generations are born and older generations die.
Stars of the future will not be the same as stars of the present. They are likely to be dimmer and longer-lived because they will contain a greater proportion of heavy chemical elements produced by the fusion of hydrogen and helium in the previous generations of stars. At the end of a star's life, these chemicals are returned to space, either in titanic explosions known as supernovae, or in more genteel stellar cataclysms where the dying star gracefully wafts its outer layers into space. The heavier elements have a cooling effect on the interstellar gas clouds, making them more susceptible to gravitational collapse and this results in a larger number of smaller stars, which are naturally dimmer and longer-lived.
With a greater quantity of heavier elements, it should also be easier to build planets. This could mean that the Universe has not yet entered its most habitable phase. In the future there could be more small stars, each with a wealth of planets in orbit around it.
On the larger scale, the most important factor governing the Universe's future is its rate of expansion. As lemaitre showed, this can be calculated from general relativity and is related to the Universe's overall gravitational field. It is possible that the Universe will expand forever, but this is not guaranteed. If there is enough matter, the expansion will slow down and then reverse, speeding everything back together in a cataclysmic 'Big Crunch'.
In the first scenario, known as an open universe, everything will appear more or less business as usual to begin with. Stars will continue to live and die much as they have done for the last 13 billion years, but the Universe itself will change around them.
Space will expand and the galaxy clusters will recede ever farther away from each other. Eventually, the galaxies that make up Herschel's luxuriant garden will be lost from sight because their light will be redshifted away from visible wavelengths into the infrared. As the aeons pass, the redshift will transform the starlight into weak radio signals.
These faint whispers will be the only clue to some future civilisation of the existence of other galaxies, but as time continues to pass, the already weak signal will be redshifted even more, rendering it so feeble as to be unobservable. The cosmic microwave background radiation that gave us our first definitive evidence of the Big Bang will also be lost in the same manner. Future civilisations may thus lose the ability to perform cosmology because the Universe beyond our immediate cosmic neighbourhood will be unutterably dark and empty.
The 50 or so nearest galaxies in our own cluster will not recede because they are bound to us by gravity, and that means they must be in some kind of orbit. Eventually these epic circulations will result in the merger of the galaxies with each other.
As the galaxies collide, some stars will be flung out into intergalactic space to wander the Universe alone. Abraham leob, of the Center for Astrophysics at Harvard University, has suggested that these could be seen with extraordinarily powerful telescopes and used in the same way that receding galaxies are used today to tell us about the expansion of space. Others think the faintness of the stars makes this a very long shot indeed, and besides, none of us is likely to be here in 100 billion years to see if it's possible anyway.
Many stars that are not catapulted out will be tipped into the supermassive black hole at the centre of their galaxy, temporarily reigniting the activity around the lurking behemoths. Eventually, the supermassive black hole at the centre of each merging galaxy will collide, releasing a cataract of gamma rays strong enough to sterilise all remaining planets in the galaxies.
After some 100,000 billion years all the cosmic gas will have been either pulled into existing stars or sucked into black holes. One by one this last generation of stars will run the course of their lives and die.
With no gas clouds left to collapse into new stars, stellar activity will begin to come to an end. The lights will be going out across the cosmos, leaving space to contain nothing but isolated collections of dead stars. These stellar corpses will occasionally collide, releasing a sudden burst of radiation, but other than that, there will be no light shining through the Universe.
Protons are composed of three quark particles, and in some extensions of our current particle physics theories, this makes them liable to decay into other particles.
If protons do eventually decay, then all the atoms in the Universe will disintegrate, leaving nothing but a sea of subatomic particles. Chemical reactions and nuclear reactions will become impossible because there will no longer be any particles capable of forming an atomic nucleus, let alone a chemical element or molecule. All these particles will be capable of doing is falling into existing black holes, or clumping together to form new ones of their own. But not even the black holes may last forever.
The Unknown Universe written by Stuart Clark is published by Speaking Tiger