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The universe began with a cataclysm that created space and time, as well as all the matter and energy the universe will ever hold.  At the instant of the Big Bang, the universe was infinitely dense and unimaginably hot.  All forms of matter and energy, as well as space and time itself, were formed at this instant.  Since "before" is a temporal concept, one cannot ask what came before the Big Bang and therefore "caused" it, at least not within the context of any known physics.

The early universe was extremely small, dense, and hot. For the first fraction of a second, only energy existed.  As the universe expanded and cooled, the four fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces) became distinct. Quarks, then atomic particles and their antimatter partners, appeared. As matter and antimatter met, they annihilated each other, leaving behind energy and a slight excess of ordinary matter - almost exclusively the lightest elements, hydrogen and helium. The faint residual heat from the Big Bang can be observed coming from everywhere in the sky.

Force Separation

In the so-called "Standard Model" of matter, physicists postulate that just four fundamental forces govern the behavior the cosmos. The most familar to us is, gravity, the long-range force that dominates the interactions of matter across astronomic al distances. Then there's the electromagnetic force, also long-range, but it manifests in the inner realm of the atom, keeping electrons inside their nuclear orbits. The other two forces, the strong nuclear force that binds the particles th at make up the nuclei of atoms, and the weak nuclear force underlying some types of radioactive decay -- both these forces operate only over very short, subatomic distances.
Physicists also theorize that all four forces are carried by particles named bosonswhich interact at sufficient energies. At high enough temperatures, these particles become indistinguishable, as do the forces they carry.

In the Grand Unification Theory (GUT) proposed in the 1970's, the strong, weak and electromagnetic forces merge at the mind-boggling temperatures that existed 10^-35 seconds after the Big Bang. Yet gravity remains separate. Looking still further ba ckwards in time, at 10^-43 seconds, gravity too merges with the other forces into one force. Scientists are still struggling to work out the underlying physics of such a Theory of Everything (TOE).

The diagram depicts the separation forces as a function of energy measured billions of electron volts (Gev).

An electron volt is a unit of energy that is related to the wavelength of radiation, which can range from light to X-rays or even a beam of electrons, as employed by an electron microscope. The higher the temperature, the shorter the wavelength of the rad iation, and the larger the number of electron volts of energy carried by a single particle or photon of that radiation.

 Movie (.mov) of early cooling and Separation of Forces 

The young universe did not have a perfectly even distribution of energy and particles. These irregularities allowed forces to start to collect and concentrate matter. Accumulations started to develop ever more complicated structures. Concentrations of matter formed into clouds, then condensed into stars and the collections of stars we call galaxies. The way in which galaxies spin indicates that their visible portions of stars and diffuse gas and dust clouds known as nebulae constitute only one tenth of the total mass. The so-called "missing mass" could hold the key to the ultimate fate of the universe — that is whether it expands forever or is pulled back together by the combined gravitational attraction of all of its mass. Hubble Deep Field From the standpoint of the development of life, what matters is that each galaxy is a stellar factory, producing stars out of giant gas clouds; each star is a chemical factory, transmuting simple elements into heavier, more complex ones; and life is a collection of some of these complex molecules. Visible matter comes in a wonderful variety of galaxy forms, characterized by their distributions of stars and glowing or dark nebulae.

The largest inhabitants of galaxies are giant clouds of molecules that contain the raw material for stars and planets. A cloud with a diameter of 300 light years (1 light year is equal to about 10 trillion kilometers) contains enough mass to manufacture 10,000 to a million stars, each with the mass of our Sun. However, only about 10 percent of the cloud will be in clumps dense enough for stars to form -- enough to produce a few hundred to a few thousand new stars. Giant molecular clouds last for 10 to 100 million years before they dissipate.

Gravity acts on individual particles to form collections that attract still more particles. Under the right conditions, gravity can overcome the disruptive forces of heat and turbulence to create spheres of gas that are hot enough and dense enough at their centers so that hydrogen can fuse into helium -- creating a star. But this new star will probably not yet be apparent in visible light. The young star is surrounded by a dense, opaque shroud of dust. As the star heats the dust, the star becomes detectable by infrared telescopes as a "hot spot" within a large, dense molecular cloud. Winds from the star will eventually blow away residual gas and dust and the star will become visible in optical telescopes.

Young stars grow and shrink as they try to strike an evolving balance between gravity, which tries to compress the star, and the pressure from the fusion reactions that try to make the star expand. Mature stars have achieved that delicate balance and spend almost their entire lives that way.
A star's size, color, brightness, and lifespan are the consequence of the total amount of its mass. Stars with only small amounts of material (a few tenths the mass of our Sun) become cool "red dwarfs" that live for many billions of years. Stars with the mass of our Sun last for about 10 billion years. Giant stars, with a few tens of the mass of our Sun, consume their fuel furiously and burn, white-hot, for only a few million years.

Over its entire lifetime, a star's hydrogen is being fused into helium. Late in the star's life, its helium mass becomes great enough to reach the necessary pressure and temperature, and the helium begins to fuse into still heavier elements. Shells of fusion, each requiring higher and higher pressures and temperatures, form from the ashes of the previous reaction and create new elements in the process known as nucleosynthesis. The additional heat produced in the core causes the star to swell.