According to the standard theory, our universe sprang into existence as a “singularity” around 13.7 billion years ago. What is a “singularity” and where does it come from? To be honest, we don’t know for sure. Singularities are zones – not even places in the geographical sense – which defy our current understanding of physics. They are thought to exist at the core of “black holes”, areas of intense gravitational pressure. The pressure is thought to be so intense that finite matter is actually squashed into mind-bogglingly infinite density so not even light, which can be bent gravitationally, can get out. These zones of infinite density are called “singularities.”
Our universe is thought to have begun as an infinitesimally small, infinitely hot, infinitely dense ‘something’ – a singularity. Where did it come from? We don’t know. Why did it appear? We don’t know. The first evidence was when in the late 1960’s Stephen Hawking and Roger Penrose reworked Einstein’s General Theory of Relativity and arrived at the conclusion that there must have been a ‘beginning’. The singularity didn’t appear in space, there was no space before it. Secondly, Hubble’s expanding Universe is a matter of fact, redshifted galaxies are moving away from us, evidenced by spectral fingerprints and the greater their distance, the greater their recession velocity, as the fabric of spacetime expands. Thirdly, if the universe was once very, very hot as the Big Bang suggests, we should be able to find some remnant of this heat. In 1965, radio astronomers Arno Penzias and Robert Wilson discovered that the temperature of the Universe isn’t absolute zero, instead a 2.725K Cosmic Microwave Background radiation (CMB) pervades the observable universe and is thought to be the heat signature everyone was looking for. Their discovery brought Penzias and Wilson the 1978 Nobel Prize for Physics. Fourthly, H and He are abundant, instead of heavier elements requiring more energy to create them.
In the first 10-43 seconds, time was not. This is the so-called ‘quantum of time’, or the Planck time – the time it would take a photon travelling at the speed of light to across a distance equal to the Planck length l(p). Derived, interestingly from three fundamental constants.
The Planck length is the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate. This is the ‘quantum of length’, the smallest measurement of length with any meaning, roughly equal to about 10-20 times the size of a proton.
The ‘quantum of time is the smallest measurement of meaningful time. Within the framework of the laws of physics as we understand them today, we can say only that the universe came into existence when it already had an age of 10-43 seconds.
This timeline summarises the rest…
Click on the image to enlarge it.
Within a ridiculously small fraction of a second, the universe expanded enough, and thereby cooled sufficiently, for the fundamental interactions (or ‘forces’) to evolve into the ones we know about today. Very soon after that, but still only a fraction of a second old, the fundamental particles in the universe were cool enough to condense into protons and neutrons. After about 100s, the protons and neutrons were cool enough to begin to form nuclei. The energy density of the universe at this point was still dominated by radiation, however. This intense and hot radiation continuously bombarded the protons, neutrons and electrons in the universe. After about 50,000 years, the universe was cool enough that the matter overcame the radiation domination in the total energy density. At this point, called the epoch of matter domination, gravitational effects began to become important. After about 400,000 years, the universe was sufficiently cool and the density sufficiently low for electrons and protons to form hydrogen atoms. This epoch is called recombination.
At that point, the radiation stopped interacting constantly with the matter in the universe. This is called decoupling. The temperature of the universe at that point is thought to have been about 47000C. Think of the radiation as having been emitted from every point (in space) in the universe, with an intensity proportional to the matter density at that point in space. It is expected that this radiation should be observed today, but red-shifted by an amount due to the expansion of the universe since the time the radiation was released. This radiation from decoupling is the CMB.
But the big bang model alone cannot explain all of the observations. In particular, it cannot account for the inhomogeneities seen in the CMB, meaning the structure of the Universe isn’t perfectly smooth, neglecting little gravitational blips because of galaxies, or indeed the large scale structure of galaxies. Also, the big bang does not predict the fate of the universe. Will it expand like this for ever? Will it collapse back on itself?