Timeline of the Big Bang
From Wikinfo
Contents |
Overview
According to the Big Bang theory, the following sequence of events is believed to have occurred. The starting point for this timeline, 13.7 billion years ago, is the time at which in general relativity there exists a gravitational singularity. At this time, general relativity is unable to make statements about what the Universe is like because the theory gives infinite values for the temperature and density of the universe.
It is believed that general relativity is insufficient to make predictions about the very beginning of the universe and that a theory of quantum gravity will be needed to do so. Nevertheless the time at which general relativity predicts a singularity makes a convenient starting point to begin the timeline, despite the fact that this singularity may or may not actually have existed.
One concept which is important to understand this table is the concept of decoupling or freezeout. Imagine a block of ice and an aluminum Coca-Cola can. If you increase the temperature to an extremely high value, then both objects will vaporize and one will have a mixture of water and aluminum vapor which can be considered a single entity. Now if one decreases the temperature, then below a certain value the aluminium will condense and freeze and stop interacting with the water vapor. The exact temperature at which this occurs can be estimated.
A similar process occurs during the course of the Big Bang as entities freeze out and decouple from the rest of the soup that makes up the universe. The temperature at which freezeout occurs can be estimated and the temperature corresponds to the time after the Big Bang.
One final note is that this timeline will refer to the diameter of the universe. This is not the total size of the universe, which may be infinite. Rather one starts with the current size of the observable universe which is about thirteen billion light years because thirteen billion years is the estimated length of time since the beginning of the universe and anything outside that sphere cannot be observed. One then calculates how large that sphere is at a particular time.
Stephen Hawking has theorized that the events of the Big Bang (the expansion of a singularity into the current space time continuum) can be seen as a reversal of the events that occur in a black hole, where space-time condenses into a singularity.
Science tells us nothing about what happened from the time of the Big Bang until 10-43 seconds, a concept known as Planck time. After this, the time is grouped into epochs.
This timeline of the Big Bang describes the events according to the scientific theory of the Big Bang, using the cosmological time parameter of comoving coordinates.
The very early universe
All ideas concerning the very early universe (cosmogony) are necessarily speculative. As of today no accelerator experiments probe energies of sufficient magnitude to provide any insight into the period. All proposed scenarios differ radically, some examples being: the Hartle-Hawking initial state, string landscape, brane inflation, string gas cosmology, and the ekpyrotic universe. Some of these are mutually compatible, while others are not.
Augustinian era
- Before the Big Bang
In 1952, George Gamow, one of the founding fathers of Big Bang cosmology, proposed that the period before the Big Bang be called the Augustinian era,[1] after the philosopher Saint Augustine, who believed time was solely a property of the God-created Universe, so that there was no time prior to the creation of the universe. The phrase "Augustinian Era" is meant to convey the idea that the known laws of physics break down in a gravitational singularity of infinite density at the time zero of the Big Bang, so that according to Albert Einstein's general theory of relativity there were no times prior to that point. However, physicists believe that general relativity becomes incompatible with quantum mechanics at the Planck scale, so that the predictions of general relativity cannot be trusted before the Planck era when energies and temperatures reached the Planck scale, and that we need a theory of quantum gravitation before we can say anything about times before the Planck era.[2]
The Planck epoch
- Up to 10-43 seconds after the Big Bang
If supersymmetry is correct, then during this time the four fundamental forces — electromagnetism, weak nuclear force, strong nuclear force and gravitation — all have the same strength, so they are possibly unified into one fundamental force. Little is known about this epoch, although different theories propose different scenarios. General relativity proposes a gravitational singularity before this time, but under these conditions the theory is expected to break down due to quantum effects. Physicists hope that proposed theories of quantum gravitation, such as string theory and loop quantum gravity, will eventually lead to a better understanding of this epoch.
The grand unification epoch
- Between 10-43 seconds and 10-36 seconds after the Big Bang [3]
As the universe expands and cools from the Planck epoch, gravitation begins to separate from the fundamental gauge interactions: electromagnetism and the strong and weak nuclear forces. Physics at this scale may be described by a grand unified theory in which the gauge group of the Standard Model is embedded in a much larger group, which is broken to produce the observed forces of nature. Eventually, the grand unification is broken as the strong nuclear force separates from the electroweak force. This occurs as soon as inflation does. According to some theories, this should produce magnetic monopoles. Unification of the strong and electroweak forces, means that the only particle expected at this time is the Higgs boson.
The electroweak epoch
- Between 10-36 seconds and 10-12 seconds after the Big Bang[4]
The temperature of the universe is low enough (1028K) to separate the strong force from the electroweak force (the name for the unified forces of electromagnetism and the weak interaction). This phase transition triggers a period of exponential expansion known as cosmic inflation. After inflation ends, particle interactions are still energetic enough to create large numbers of exotic particles, including W and Z bosons and Higgs bosons.
The inflationary epoch
- Between 10-36 seconds and 10-32 seconds after the Big Bang
The temperature, and therefore the time, at which cosmic inflation occurs is not known for certain. During inflation, the universe is flattened (its spatial curvature is critical) and the universe enters a homogeneous and isotropic rapidly expanding phase in which the seeds of structure formation are laid down in the form of a primordial spectrum of nearly-scale-invariant fluctuations. Some energy from photons becomes virtual quarks and hyperons, but these particles decay quickly. One scenario suggests that prior to cosmic inflation, the universe was cold and empty, and the immense heat and energy associated with the early stages of the big bang was created through the phase change associated with the end of inflation.
Reheating
During reheating, the exponential expansion that occurred during inflation ceases and the potential energy of the inflaton field decays into a hot, relativistic plasma of particles. If grand unification is a feature of our universe, then cosmic inflation must occur during or after the grand unification symmetry is broken, otherwise magnetic monopoles would be seen in the visible universe. At this point, the universe is dominated by radiation; quarks, electrons and neutrinos form.
Baryogenesis
No known physics can explain the fact that there are so many more baryons in the universe than antibaryons. In order for this to be explained, the Sakharov conditions must be met at some time after inflation. There are hints that this is possible in known physics and from studying grand unified theories, but the full picture is not known.3 minutes after the Big Bang
- Three minutes after the Big Bang, the universe is too cool for nuclear activity to occur, and these reactions stop. At this point the universe consists of about 75% hydrogen, 25% helium and trace amounts of deuterium, lithium, beryllium, and boron. Elements heavier than this do not have time to form before nuclear reactions stop. By looking at conditions between 1 second and 3 minutes after the Big Bang, one can predict the elemental abundance of the Universe. These predictions are broadly in agreement with observations.
300,000 years after the Big Bang
- The temperature of the Universe is approximately 10,000 Kelvin. At this temperature hydrogen nuclei capture electrons to form stable atoms. This is particularly significant because free electrons are effective at scattering light, which is why fire is not transparent, while hydrogen atoms will allow light to pass through.
- This implies that this is the time at which space becomes transparent to light, since photons no longer interact strongly with atoms. This means that what we normally think of as matter and what we normally think of as energy become separate.
- The light from the moment at which the universe became transparent has been redshifted to radio waves and makes up the cosmic microwave background.
For later events, see Timeline of the Universe.
References
- Adapted from the Wikipedia article, "Timeline_of_the_Big_Bang" http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang, used under the GNU Free Documentation License
