The origin, evolution, and future of the universe are modelled by several cosmological theories, notably The Big Bang, according to which the universe arose from an "explosion" of space-time some 15 billion years ago. The Universe is thought to be 15 billion years old. It started with a huge explosion called the Big Bang The Universe is expanding - all the galaxies are flying away from each other, like spots on a balloon being blown up.

The Expanding Universe

The Universe is expanding, at an apparently uniform rate - the galaxies are receding from us and each other. Mentally running the expansion backwards, we see that there must have been a point in time when all the matter of the Universe was together in an arbitrarily small volume; the 'event' that created our Universe is known as the Big Bang (a term coined by the English astrophysicist Fred Hoyle in 1950. Hoyle, who championed a rival cosmological theory (steady state), meant the "Big Bang" to be a term of derision, but the name was so catchy that it stuck.). Though the Big Bang suggests a colossal explosion, it wasn't really an "explosion" in the sense that we understand it. Space itself exploded.

Space and time came into existence and all the matter in the cosmos started to expand. This 'event' created space-time and so there was no time 'before' the Big Bang, and no space outside it. The popular image of the Big Bang as an explosion 'into' the void - from an era of nothingness - is really just a poor approximation. It was not an explosion like those familiar on earth, starting from a definite center and spreading out to engulf more and more, but an explosion which occurred simultaneously everywhere, filling all space from the beginning with energy.

At the instant of the Big Bang, the universe was infinitely dense and unimaginably hot. Cosmologists believe that all forms of matter and energy, as well as space and time itself, were formed at this instant. One cannot ask what came before the Big Bang and therefore "caused" it, at least not within the context of any known physics. At time t=0 a singularity is predicted, where all laws as we know them break down. Space and time are infinitely distorted. The uncertainty principle of quantum mechanics prevents any accurate predictions to be made for times less than 10^-45 s.

History of The Universe

Theory has to guide our understanding of the first fraction of a second, since we can�t recreate the extremely high temperatures that existed during the earliest history of the universe in any earthly laboratory. Theorists have deduced the history of the universe dating back to just 10^-43 second (10 million trillion trillion trillionths of a second) after the Big Bang. Before this time the four fundamental forces�gravity, electromagnetism, and the strong and weak nuclear forces�were one, but physicists have yet to develop a workable theory that can describe these conditions.

Immediately after the big bang, the universe consisted primarily of radiation. From an initial state in which matter and radiation are both in an extremely hot and dense form, the universe expands and the matter cools. As it expanded matter came to dominate--roughly 1,000 years after the explosive beginning.

The evolution of the earliest universe is not well understood because it is not clear exactly what laws were at work. During the first second or so, the building blocks of matter had formed: protons, neutrons, and electrons�the building blocks of atoms� formed when photons collided and converted their energy into mass, and the four forces split into their separate identities. The universe cooled during this time, from about 10³² (100 million trillion trillion) degrees to 10 billion degrees.

Approximately three minutes after the Big Bang, when the temperature fell to one billion degrees, protons and neutrons combined to form the nuclei of a few heavier elements, most notably helium and other light nuclei (like deuterium). For a long time, temperatures remained too high for the formation of most atoms.

300,000 years after the Big Bang, the universe had cooled to 3000 degrees. At this temperature, electrons could combine with atomic nuclei to form neutral atoms. With no free electrons left to scatter photons of light, the universe became transparent to radiation. (It is this light that we see today as the cosmic background radiation.)

At around one million years following the Big Bang, nuclei and electrons were at low enough temperatures to coalesce to form atoms.

But the universe didn�t start to look like it does today until small perturbations in the matter distribution were able to condense to form the Stars and galaxies, about one billion years after the Big Bang.

Since then the universe has continued to grow larger and cooler, creating conditions conducive to life. Hubble's discovery and other more recent findings seem to suggest that the universe is still expanding and that it will continue to do so forever.

Analysis of the Theory

Four excellent reasons exist for believing in the big-bang theory.

1. The universe is expanding. The fact that galaxies are receding from us in all directions is a consequence of this initial explosion and was first discovered by Hubble. There is now excellent evidence for Hubble's law which states that the recessional velocity of a galaxy is proportional to its distance away from us. Projecting galaxy trajectories backwards in time means that they converge to a high density state - the initial fireball.
2. The theory predicts that 25 percent of the total mass of the universe should be the helium that formed during the first few minutes, an amount that agrees with observations. Prior to about one second after the Big Bang, matter - in the form of free neutrons and protons (nucleons) - was very hot and dense. As the Universe expanded, the temperature fell and some of these nucleons were synthesised into the light elements: deuterium, helium-3, and helium-4. Theoretical calculations predict that about a quarter of the Universe consists of helium-4, a result which is in good agreement with current observations.
3. The cosmic background radiation, which now glows at a temperature just 3 degrees above absolute zero (270 centigrade degrees below freezing). About 100,000 years after the Big Bang, the temperature of the Universe had dropped sufficiently for electrons and protons to combine into hydrogen atoms. From this time onwards, radiation was effectively unable to interact with the background gas; it has propagated freely ever since, while constantly losing energy because its wavelength is stretched by the expansion of the Universe. Originally, the radiation temperature was about 3000 degrees Kelvin; today it has fallen to only 3K - a ghostly remnant of the unimaginably intense heat of the primeval fireball of the Big Bang.
4. The collapse of matter to form galaxies and other large-scale structures observed in the Universe today. At about 10,000 years after the Big Bang, the temperature had fallen to such an extent that the energy density of the Universe began to be dominated by matter, rather than the light and other radiation which predominated earlier. Gravitational forces between the particles began to take effect, and small perturbations in their density grew; 15 billion years later we see the results of this collapse.

At least one cosmological theory predicts that our universe's Big Bang is part of a chain reaction in which the demise of one universe spawns the birth of many, parallel, universes. According to this scenario, our universe may simply be part of a huge, infinitely growing fractal.

Certain questions about the Big Bang still remain only in the province of philosophical speculation; e.g. why the Big Bang 'happened'; why the laws of physics are as they are; why there is something instead of nothing; etc. Quantum physics, Grand Unified Theories (GUTs) and new 'theories of everything' (TOEs) seem to be providing tantalising glimpses at possible answers. They attempt to explain the physical behaviour of particles and forces during the very early stages of the Universe, in a single set of equations. Recently there has been strong interest in String Theories. These have had some success in providing a desription of gravity, but they are intensely mathematical, sometimes defying physical intuition.