What kind of death will claim the universe?
Modern science cannot say with certainty what eventually will become of the universe. The greatest uncertainty concerns the ongoing expansion of the universe. Will the clusters of galaxies making up the universe continued to fly apart from each other, or will they add some stage start to come together again under the force of their mutual gravity?
This depends on a simple question, to which no scientist knows the answer: how much matter is there are in the universe? The matter we can observe stars, gas, dust and so on amounts to only a hundredth of what would be required to hold the expansion in that has been going on since the Big Bang created the universe and some 15 thousand million years ago.
But the motions of some of galaxies show that the gravitational force of matter we cannot see is pulling on them, this invisible matter, known as dark matter, exists within and between the galaxies, and may comprise most of the matter in the universe. But even the combination of these unseen substances with the stars and other visible matter still adds up to only a 10th of the mass that would be needed to turn the expansion into a contraction.
Therefore, cosmologists think that the universe will continue to expand, and look ahead to developments at truly staggering distances in the future. In 100 million million years, the longest lived stars will have used their fuel and faded out. On a time scale that is much longer yet, the galaxies will disappear. Even in the vastness of space, stars sometimes come close enough to be affected by each other’s gravitational fields and, if the timescale is long enough, chance near misses among dead stars will cause at least 90% of them to be ejected into intergalactic space. The rest will settle at the galaxies centers, forming enormous black hole.
While the clusters of galaxies continue to move apart, each cluster will begin to collapse upon itself. The black holes, the remnants of the galaxies, and the stray dead stars in intergalactic space will spiral in towards each other. As they emerge even greater black holes will grow. This will take place over period of 10 million million million years.
The sea of particles
Far beyond even this horizon lies the death of matter. Most physicists believe that the seemingly stable matter around us today will it eventually decay into a sea of lighter particles and radiation. Even a body the size of a star will vanish in this way, but taking a number of years written as one followed by 32 zeros.
Although for all practical purposes neither matter nor radiation can escape from a blackhole, on the truly cosmic timescale black holes will slowly evaporate giving out a slow stream of radiation. The number of years required for the decay of the blackhole formed from the collapse of a galaxy cluster is written as a one followed by 117 zeros.
This is the furthest horizon that physicists can currently discern. Many believe that the universe at this distant epoch will be a feature lists mass of subatomic particles, bathed in a sea of radiation, close to the absolute zero of temperature.