:How large was the original dense mass just before the cataclysmic explosion?

: For an incomprehensibly small fraction of a second (10^-35 seconds), : the universe is an infinitely dense, hot fireball. One millionth second after the : "Big Bang" the universe continued to expand, just not as quickly. As it : expanded, it became less dense and cooled (10^-6 seconds). With the : expansion, came the basic forces of nature. They were gravity, : electromagnetic forces, and the strong forces that held nuclei together. After : the first second passed, the universe developed basic particles, like quarks, : electrons, photons, neutrinos and less familiar types. Three seconds after the : explosion, protons and neutrons came together to form the nuclei of simple : elements: hydrogen, helium and lithium. It would take another 300,000 years : for electrons to be captured into orbits around these nuclei to form stable : atoms. The first major era in the history of the universe happened 10,000 : years after the explosion, in which most of the known energy was radiation. : A few different types of light waves created were X rays, radio waves and : ultraviolet rays. As the universe expanded, and the light waves were : stretched and diluted until today. As the waves continued to stretch : microwaves were formed. At 300,000 years, the energy in matter and the : energy in radiation were equal, but as the universe continued to expand, the : energy became less. At about this time, neutral Atoms formed as electrons : linked up with hydrogen and helium nuclei. The microwave background : radiation hailed from this moment and thus gives us a direct picture of how : matter was distributed at this early time. The creation of galaxies occurred : 300 million years after the "Big Bang". Gravity made small irregularities in : the density of the primordial gas. Even as the universe continued to expand : rapidly, pockets of gas became more and more dense. Stars ignited within : these pockets, and groups of stars become the earliest galaxies. This point is : between 12 and 15 billion years before today.

: Five billion years ago our sun was created. The sun was formed within : a cloud of gas in the spiral arm of the Milky Way Galaxy. (A huge circular : formation of gas and debris that swirled around this new star gave birth to the : planets, moons, and asteroids. Approximately 3.8 billion years ago the earth : cooled and an atmosphere developed. Microscopic living cells, neither plants : nor animals, begin to evolve and flourish in earth's many volcanic : environments.

: In 1917 Einstein and others applied General Relativity to the structure : and evolution of the universe. The theory called the "Big Bang" was : formulated in 1922 by the Russian mathematician and meteorologist : Alexander Friedmann. Friedmann started with Einstein's equations of : Realitivity. He found a solution to them, in which the universe began in a : state of extremely density and temperature. It expanded in time, thinning and : cooling. One of the most stunning successes of the Big Bang theory is the : prediction that the universe is approximately ten billion years old. This : approximation was obtained by the rate at which distant gallaxies are flying : away from each other.

: According to the "Big Bang" theory the universe may keep expanding : forever. Its inward gravity is not strong enough to counteract the outward : expantion, or maybe the universe will reach a maximum point of expansion : and start collapsing, growing denser and denser, eventually disrupting : galaxies, stars, planets, people, and eventually even individual atoms. The : fate of our universe can be determined by measuring the density of matter : versus the rate of expansion. Much of modern cosmology has been an : attempt to measure these two numbers with much better accuracy. The : numbers say that our universe will never stop expanding. It will continue to : grow thinner and cooler.