In 1929, Edwin Hubble made an important discovery about the universe: it is expanding. Galaxies are, in general, moving away from each other, with space itself expanding between them. So the question arises: Will the expansion continue forever? And if so, how fast will it go? Scientist have determined that the universe consists primarily of three substances: “dark matter,” “normal matter,” and an unknown energy, “dark energy.” The first substance, dark matter, contributes to the force of gravity that would cause the universe to collapse in on itself. The second one, normal matter, consists of atoms that make up human beings, stars, planets, and other visible objects. The third substance,
dark energy, is proposed to be a repulsive force that would cause the universe to expand. The expansion rate is related to the sum of the densities of each substance. According to the data of the WMAP satellite, 72% of the universe is composed of dark energy, 24% of the universe is dark matter, and 4% is normal matter. The proportions of dark matter, normal matter, and dark energy are such that the universe’s expansion is currently accelerating; however, we do not know for sure what the expansion rate will be in the future.
With the discovery of the expanding universe, astronomers realized that the universe is a dynamic, evolving environment. While the universe has been expanding ever since the Big Bang, a natural question is, will that expansion continue forever? In other words, what will be the final fate of the universe?
There have been four proposed ways in which the universe might comes to an end. These are “The Big Rip,” “The Big Freeze,” “The Big Crunch” and “The Big Bounce.” Today, I will talk about the Big Freeze. In this scenario, the universe expands forever. Since the expansion rate of the universe at a particular distance is currently accelerating and will eventually exceed the speed of light, after billion or maybe trillion years, light emitted from currently observed clusters of galaxies will no longer reach us. Thus, neighboring clusters will disappear, and we will end up as an isolated community. Stars will have used up their nuclear fuel and become dark, and will perhaps become white dwarfs, neutron stars or black holes. After 10
23 years, the temperature all across the universe will reach to nearly absolute Zero. Eventually, it will be cold enough for black holes to shrink away and evaporate. At that point, even consciousness or thought cannot exist. The entire universe will become dark, cold, and lifeless.
What is the ultimate fate of our universe? A Big Crunch? A Big Freeze? A Big Rip? or a Big Bounce? Current observations have led cosmologists to favor “The Big Freeze.” However, the other three theories still cannot be totally ignored until the dark energy is fully understood.
See:
https://cosmology.carnegiescience.edu/timeline/1929
http://www.aip.org/history/cosmology/ideas/expanding.htm
http://wmap.gsfc.nasa.gov/media/080998/
http://www.universetoday.com/36917/big-freeze/
https://www.youtube.com/watch?v=NLSkZ4Z2ttk
