||The Big Bang theory is humankind\'s attempt to answer that most fundamental question How did the universe begin? We believe that the universe started as an unbelievably dense point, and has been expanding ever since. The origin of this point may have been simply a random fluctuation, requiring no intervention from another source, but theories disagree about this.
The most successful version of the very early universe, known as the inflationary model, tells us that immediately after the formation of the point universe, it began an astonishingly rapid expansion, much faster than the speed of light. As information cannot travel faster than this speed, few parts of this primeval explosion could be affected by other parts, so instead of having a perfectly smooth distribution of matter, the universe, even at this early stage, became slightly â€˜lumpyâ€™ or granular. These irregularities eventually caused the formation of galaxies, millions of years later.
By the time one microsecond had passed, fundamental particles called quarks had formed and become bound to each other. After one millisecond, an asymmetry had arisen between matter and antimatter, leading to the situation we have today, with far more matter than antimatter. By a few seconds, none of the more exotic particles had survived and only electrons, neutrinos, photons and their antiparticles were important. A few protons and neutrons were formed, some of which combined to form the nuclei of the lightest elements hydrogen and, after about 100 seconds, helium and a few others.
After approximately one million years had passed, the universe had cooled sufficiently for electrons to be bound to the nuclei that existed, and the first atoms (mostly hydrogen) were created. The formation of atoms was the beginning of our familiar universe. Much later, these atoms combined in vast clouds to form stars and galaxies.
There are two important pieces of evidence for the Big Bang. One is that the radiation that remains from the explosion is still around today, although it retains little of its ferocious energy. We see it as a constant energy background, equivalent to space having a temperature of about 3 degrees above absolute zero, or -270 degrees Celsius. The other piece of evidence is that the Universe is still expanding. We know this because the light from distant galaxies is of a lower frequency than we would expectâ€”it is said to be â€˜redshiftedâ€™. This means that those redshifted galaxies are travelling away from us at great speed.
One question that remains to be answered today is whether the universe will go on expanding, or whether it will eventually slow down and contract, due to the gravitational pull of all the matter inside it. If we knew how much matter the universe contained, we could answer this question, but we do not. It is believed that a lot of mass is located in so-called â€˜dark matterâ€™, which emits no radiation that we can detect. JJ
See also cosmology; particles.