What is a theory?
In casual conversation a "theory" tends to be no more than a guess. But in science a theory is a good explanation of what we observe, and can make testable predictions about things we haven't yet observed. For a theory to become the standard one - the one most accepted by scientists - it has to be more convincing than the others. When it's tested it passes the tests.
Big Bang theory - what it is and what it isn't
"Big bang" is catchy, but misleading. Fred Hoyle, who opposed the theory, used it as an insult, yet somehow it stuck. But the "big bang" wasn't an explosion with bits flying into space. It was a rapid expansion of the universe - more like blowing up a balloon than a bomb.
The theory doesn't try to explain the origin of the Universe. It starts with a Universe that has just begun to expand, and follows its evolution. The theory doesn't suggest what the Universe was, how it came to exist or what started the expansion.
And for anyone who's already confused, I'm sorry to have to add that this teensy Universe didn't expand into space. It may have been small, but it was still the Universe, and we can't say there was anything outside it. Space and time are within the Universe.
The Universe was a singularity, with no size, yet an enormous density. It contained what would become space and time, energy and matter, and the physical laws of our Universe. If the idea of a singularity bothers you, you're not alone. Scientists don't like singularities either, as the known laws of physics break down there.
So how did the Big Bang theory get such an unlikely start?
The Universe used to be smaller
Until the twentieth century, most people assumed that our Galaxy was all there was of the Universe. Then things began to change.
American astronomer Vesto Slipher observed the spectra of what were called spiral nebulae. He found that 21 out of 25 of them had redshifted spectra. This meant that they were moving away from us. Slipher argued in his 1917 paper that the spiral nebulae were actually other galaxies far away from us. (Click for a brief explanation of the Doppler effect for light.)
Six years later Edwin Hubble showed that the Andromeda spiral nebula was a galaxy far beyond the Milky Way. He's also credited with showing that galaxies are moving apart and that the farther away they are, the faster they're moving. This led to the conclusion that the Universe was expanding. If we could accurately measure the velocities and the distances, we would know the rate of expansion, and therefore could work out the size and age of the Universe.
If the Universe is getting bigger now, it must have been smaller in the past. It would have had an origin. Even Einstein accepted the expanding Universe. He had adjusted his equations of general relatively to describe a static Universe, but admitted that it was a mistake after he saw Hubble's results.
The Big Bang story
The Universe that had started to expand was a fireball. But as it expanded, it cooled. Within a tiny fraction of a second, there appeared quarks and gluons, elementary particles of matter. And within another fraction of a second it had cooled enough for these particles to join together to make protons and neutrons.
Three minutes later nucleosynthesis began - protons and neutrons fused to make atomic nuclei. In another 17 minutes it was no longer hot enough for nuclear fusion. Nucleosynthesis was over, leaving atomic nuclei of the light elements hydrogen, helium and lithium.
Plasma - a sort of particle soup - resulted. It was a jumble of positively-charged nuclei, negatively-charged electrons, and photons, the particles of light. Although it wasn't hot enough for nuclear fusion, it was too hot for the nuclei and electrons to form neutral atoms. And although there were plenty of photons, they couldn't get anywhere without colliding with the other particles. If we had a telescope that could show that time, we'd see nothing but a fuzzy glow.
Seeing the light
Fast-forward about 400 thousand years and the temperature has dropped enough for stable neutral atoms to form. The photons were at last free to travel.
In 1948 George Gamov and two colleagues calculated that these photons were still around, completely surrounding us, but no longer hot. They would have been redshifted by the expansion of the Universe, and therefore in the form of microwave radiation at a temperature of 5K (not much above absolute zero). No one paid much attention to this prediction, because the Big Bang theory wasn't highly regarded and there were no telescopes that could detect such radiation.
In 1963 Arno Penzias and Robert Wilson discovered the microwave background radiation, though its temperature turned out to be 3K, not 5K.
Passed the tests, but not there yet
The Big Bang theory predicts that galaxies will appear to be rushing apart. The greater the distance, the greater the velocity. This has been observed with precision since Hubble's day.
Big Bang nucleosynthesis predicts abundances of hydrogen and helium that matches what astronomers have observed.
Not surprisingly, the discovery of the cosmic microwave background radiation was the key to getting the theory accepted. It couldn't be explained in a static Universe.
No one thinks that the theory explains everything. Some scientists are working to improve it and others are working on other theories. Someday someone will either fit the Big Bang into a more complete theory, or come up with something entirely new to explain the Universe we see. Science tends to be a work in progress.
References:
(1) Cambridge Cosmology, http://www.damtp.cam.ac.uk/research/gr/public/bb_pillars.html#CMBR
(2) http://www.periodictable.com/Properties/A/UniverseAbundance.html
