Big Bang Theory

(essay)

A cosmological model to explain the origins of matter, energy, space, and time, the Big Bang theory asserts that the universe began at a certain point in the distant past—current estimates put it at roughly 13.7 billion years ago—expanding from a primordial state of tremendous heat and density. The term is also used more generally to describe the vast explosion that erupted at the beginning of space and time, bringing the universe into being. First conceived by astronomers and physicists in the early twentieth century, the Big Bang was effectively confirmed in the middle and latter years of the century, once new telescopes and computers made it possible to peer further into the universe and process the enormous amounts of data those observations generated. The term “big bang” comes from its underlying hypothesis, that the universe has not been eternal but emerged out of a sudden, almost incomprehensibly vast explosion.

Scientists’ understanding of the Big Bang theory emerges out of two separate fields of inquiry: theoretical physics and observational astronomy. According to what are called the Friedmann models, a set of complex metrics named for Alexander Friedmann, an early twentieth century Soviet physicist who first developed them, the Big Bang theory fits in with two of the most important theories of twentieth century physics: the cosmological principle (which says that basic physical properties are the same throughout the universe) and Albert Einstein’s General Theory of Relativity of 1915-1916, which conceives of gravity as a curvature in space and time. That convergence of ideas, say physicists, provides the theoretical underpinning of the Big Bang theory.

Astronomers have made their own confirmations of the Big Bang theory. Analyzing the light coming from other galaxies, they have noted shorter and longer wavelengths proportional to the distances of the galaxies from Earth, indicating that they are moving away from the Earth and thus that space itself is expanding. The existence of cosmic microwave radiation, a remnant of hot ionized plasma of the early universe offers more proof of the Big Bang, as does the distribution of heavier and lighter elements through the universe.

Timeline of the Big Bang

 

The Big Bang theory hypothesizes that there were time-based stages in the origins of the universe. The first stage—or, at least, the first stage that cosmologists can theorize about given current understanding of physics—is known as the Planck era, after the German scientist of the late nineteenth and early twentieth centuries who studied the physics that explain it. The Planck era was extremely brief—just 10-43 seconds (also known as one Planck time). During this period, all four forces of the universe—gravity, electromagnetic energy, and the weak and strong nuclear forces—were theoretically 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, known as omega and estimated at roughly six hydrogen atoms per cubic meter, necessary to pull the universe back in on itself. ............