At the instant of the Big Bang, all the matter in the Universe was condensed into a single point. Other then that, we know nothing about what went on in the first instants of the Universe's existence. But by looking far out into today's Universe and peering deeply into the world of fundamental particles, scientists have managed to piece together the evolution of the universe from the inconceivably short time of just 10-43 seconds (that's a decimal point followed by 43 zeros and then a 1) after the Big Bang.
At that point in time, things were happening very fast. When the Universe was 10-43 seconds old nature's forces were indistinguishable. Particles of matter and antimatter (the white circles in the picture) existed in equal portions. They were constantly annihilating to produce radiation, represented by red spirals, and being recreated from that radiation. Matter was compressed so densely that even light could not travel far and the Universe was opaque.
Just before this time physicists think that Universe expanded at a dizzying rate. This period of so-called Cosmic Inflation is necessary in the Big Bang theory to explain the large scale uniformity of the Universe today.
During the next phase of the Universe's existence, up to around 10-34 seconds after the Big Bang, the strong force that binds particles called quarks together into protons and neutrons became distinct from the electromagnetic and weak forces which remained indistinguishable. Protons and neutrons didn’t start to form, however, because any groupings of quarks were rapidly broken up by the high energy radiation that still pervaded the Universe. Matter was a sort of high density cosmic soup called Quark Gluon Plasma, QGP.
To understand fully this phase of the Universe's existence, physicists try to recreate QGP in the laboratory. CERN and Brookhaven are the leaders in the field, with a complementary research programme that will culminate at CERN's Large Hadron Collider, LHC.
The carriers of the weak force, W and Z particles were as abundant as photons, the carriers of the electromagnetic force, and they behaved in exactly the same way.
Also around this time a tiny excess of matter over antimatter, just one matter particle surviving for every thousand million particles to annihilate with antimatter, began to develop. It is these survivors that make up our Universe today. The precise mechanism that has allowed some matter to survive is poorly measured up to now, but it is another phenomenon that will be studied in depth at the LHC.
Between 10-34 seconds to 10-10 seconds the electromagnetic and weak forces separated. There was no longer enough energy to produce W and Z particles and those that had already been made decayed away. The energy of the radiation had also fallen sufficiently to allow protons (red) and neutrons (green) to form as well as short-lived particles, called mesons, made of a quark and an antiquark (blue).
Antimatter started to disappear because when quarks annihilated with antiquarks there was no longer enough energy in the radiation to recreate them.
Particle physics experiments have already begun to probe back in time as far as this by crashing particles together with enough energy to recreate the conditions of the early Universe at laboratories like CERN.
Up to about 10-5 seconds proton and neutron building continued. The remaining antimatter, in the form of positrons disappeared as the radiation energy density dropped below that necessary to create electron-positron pairs. With no antimatter left in the Universe other than a few particles locked up inside mesons, all that is left is the one in a thousand million matter particles resulting from nature's apparent preference for matter.
After that, things really started to slow down. Up to around three minutes protons and neutrons started to combine to produce light atomic nuclei. Only Deuterium (heavy hydrogen), Helium and a tiny amount of Lithium were made. The Universe was like a giant thermonuclear reactor until, at around three minutes, the reactions stopped leaving a Universe composed of hydrogen, deuterium, helium, and a little lithium. Even today, the universe is made up of about 75% hydrogen and 25% helium, with just traces of heavier elements cooked up in stars to make everything else that we consider to be "ordinary" matter.
During the next 300,000 years the Universe became transparent as photons no longer interacted as soon as they were made. Electrons became captured by the hydrogen, deuterium, helium and lithium nuclei to form the first atoms.
Galaxy formation took 1000 million years as matter began to clump together in a way that is still not clearly understood. Gravity pulled the light elements together to form stars which ignited, cooking up elements as heavy as iron. Some stars, at the end of their lives, exploded in spectacular supernovae, rapidly generating even heavier elements like gold and scattering them around the Universe.
Slowly, these elements began to clump together into planets. Molecules formed and chemical processes began.
Finally, after 15,000 million years, people emerged on one of those planets and began to contemplate the Universe around them, trying to piece together the story which led to us being here.