ally equal to one another, implying that there may have been just one unified force. The Planck era was extremely unstable, with the four forces quickly evolving into their current forms, starting with gravity and then the strong nuclear force (what binds protons and neutrons together in the nucleus of an atom), the weak nuclear force (associated with radioactive decay, it is some 100 times weaker than the strong force), and finally electromagnetic energy. This process is known as symmetry breaking and led to a longer period in the universe's history - though, at one millionth of a second, still extremely brief in ordinary time - known as the "inflation era." Physicists, however, are not certain of the energy force that led to this inflation. At one second in age, the universe now consisted of fundamental energy and sub-atomic particles such as quarks, electrons, photons, and other less familiar particles.
The next stage in the Big Bang-lasting for roughly 100,000 years and beginning about three seconds after the Planck era-consisted of the process of nucleosynthesis, as protons and neutrons came into being and began to the form the nuclei of various elements, predominantly hydrogen and helium, the two lightest elements in the periodic table and the two most common elements in the universe. Yet matter as we know it still did not exist and for those hundred thousand or so years, the universe essentially consisted of radiation in the form of light, radio waves, and X-rays. This period, known as the "radiation era," came to a gradual end as free floating atomic nuclei bonded with free-floating electrons to produce the matter with which the universe would subsequently consist. While time was critical to the process so was temperature and density, with the various changes corresponding to a gradual cooling of the universe and the gradual dispersing of matter.
It took some 200 million years for gravity to begin coalescing these free-floating atoms into the primordial gas out of which the first stars and galaxies would emerge. Over billions of years, such early stars and galaxies phased through their lifecycle, using up their nuclear fuel and collapsing in on themselves, spewing out vast new clouds of matter and energy that would eventually form new generations of stars and galaxies. The sun around which the earth and the solar system rotate is one of these later generation stars, formed roughly five billion years ago.
Fate of the Universe
The Big Bang theory concerns not just the origins of the universe but its ultimate fate. The critical question, of course, is whether the universe will continue expanding forever or eventually fall back into itself, creating, perhaps, the conditions for the next Big Bang. Gravity is the critical factor here, with three outcomes possible. The first, and most widely accepted by physicists, is that there is not the critical density, kno...
