Dark matter and the far future

Theories of the big bang and the existence of the ripples in the back ground radiation suggest that the matter in the universe is greatly the dark matter. With in the dark matter the universe may be only a thin froth on the ocean of the matter.

If the amount of matter in the universe consists only of what we can observe place the dark matter, then the gravitational force is not enough to reverse the expansion of the universe. The clusters of galaxies will recede ever further apart and fade from each other's view. Black holes at the centers of the galaxies will grow, swallowing some of the matter in each galaxy, while the rest will disperse into intergalactic space. The last stars will be born a trillion year from now.  After 100 trillion years, even the longest lived stars, the low mass red dwarfs, will have faded. After 10 ¹⁹   years, most dead stars and other matters in galaxies will have dispersed while the remainder will have been swallowed up by black holes. After 10 ¹¹⁷ years, even black holes will have evaporated into radiation, and the universe will consists of a featureless enormously diluted sea of light weight particles and low energy radiation

Impacts of Heavenly bodies

In 1994 the comet Shoemaker Levy-9 demonstrated the power of a cometary impact with a planet. The ‘target’ was Jupiter, but there have undoubtedly been many collisions between the Earth and comets or asteroids in the past. Traces of some craters survive; one off the Yucatan peninsula in Mexico seems to mark the impact of a body 10Km wide 65 million years ago. Although the theory is contested, many scientists believe that the dust cloud generated by this impact resulted in significant climatic change. The closest known approach to the Earth by any asteroid in historical times was within 170,000 Km less than half the distance of Moon, in 1991.

The asteroids

Most of the thousands of known asteroids, or minor planets, move in a belt on the large gap between Mars and Jupiter. The largest, Ceres, the first to be discovered, is only 930 Kms across, and only 200 are more than 100Kms across. They are too small for their own gravity to pull them into a spherical shape. They fall into types that match the meteorite types.

 The asteroids are probably debris left over from the birth of the Solar System, debris that failed to coalesce into a full sixed planet because of the disturbing gravitational pull of Jupiter. Some of them were deflected by collisions or close encounters into orbits outside the main asteroid belt. The so called Earth grazers come within Earth’s orbit. Other asteroids have been observed moving beyond the orbit of Saturn, and there may be more. Two groups of asteroids share the orbit of Jupiter, one 60^. Ahead of the planet and another 60^. Behind.