The Big Bang
The Big Bang is a beautiful theory which is an effort to understand where the universe came from. Some of the most fundamental questions concerning our origins, such as that of the elements, can be explained with the Big Bang theory. But just where did everything come from? What existed before the Big Bang? Where did space come from? And what caused the Big Bang? Well, the simple answer is: We don’t know.
We know the universe is expanding; it’s accelerating, actually. This means that yesterday, the universe was a little bit smaller than it is today. A month ago, it was even smaller. A year ago, smaller still. Turning the clock backwards, the universe seems to be getting smaller, the galaxies closer together. If we go further enough back in time, the universe was so small that everything was contained in a point of space and time. Everything that exists today; you, me, the Earth, our Galaxy, everything came from this point.
Approximately 14.6 billion years ago, the Universe was created and it was very hot. Radiation (simply photons) dominated the early universe which cooled down as it expanded. Analysis of the CMB data suggests that the universe is a perfect blackbody; a higher blackbody temperature means typical photons have higher energies. In the early universe, these photons were so energetic that they produced matter-antimatter particles copiously seemingly out of “nothing” which can be explained using Einstein’s E=mc² formula (see this post.) The early universe was constantly creating matter and antimatter which quickly annihilated; this is the Particle Era. The universe was bubbling with matter, the prerequisite for everything in our Universe. Since our Universe is made of matter, and not antimatter, a baryonic asymmetry is proposed to be the origin of our matter dominated Universe.
Once the mean photon energy drops below ~1MeV, nuclei may be formed. This is the nuclear binding energy and thus, the Nucleosynthesis Era. During the Nucleosynthesis Era, the universe is one big nuclear reactor. This era sets the primordial chemical composition of the universe: 76% Hydrogen and 24% Helium.
The Nucleosynthesis Era is followed by the Era of Nuclei. Photon energies are at this point beyond the electron binding energy (~1eV). This era of the universe is foggy since photons are continuously being scattered by nuclei. At the very special moment during which photon energies drop below the electron binding energy, electrons may then bind to nuclei to form the first atoms - the fog is lifted. The Universe, during the era of atoms, becomes transparent. Photons are no longer being continuously scattered and they are suddenly released. This release of photons during the Era of Atoms is the origin of the Cosmic Microwave Background and is a significant use of study. Recall that beyond the CMB, before stable atoms are made, the universe is still foggy. It is for this reason that we cannot see beyond this point in the universe.
Not only can we not see past this point in the universe, but we cannot (yet) study what is happening at the moment of the Big Bang. There are no mathematical tools that can be used at the moment of the Big Bang, and thus, we cannot study what happened before the Big Bang. The current laws of physics seem to break down at the singularity in the beginning of the Universe, similar to what happens when we attempt to understand what happens inside a black hole. What caused the Big Bang is still a mystery, and there is still a lot left to discover, but we have achieved a lot in our understanding. The origin of our species, of the stars in the sky, of the elements that compose our Universe, can all be explained with this elegant theory.