The Big Bang is the most widely accepted scientific theory of how the universe developed into its present state, the actual cause of the Big Bang itself is still spectulated upon with inflation being the leading theory. According to one version of the theory, all of space and time (spacetime) "began" about 13.7 billion (± 200 million) years ago, and has been expanding ever since. Another version of the theory is that a "multiverse" existed before our universe began. While many details of the theory remain to be worked out, especially involving the first few instants of time (not to mention the larger issue of "pretime"), the Big Bang model is supported by many converging lines of evidence. Some of these are:
- The observed expansion of the universe
- The observed cosmic microwave background radiation and its an anisotropies
- The observed ratios of elements left over from the early universe
- Simulations involving galaxy formation
- The many measurements (most of which have nothing to do with cosmology) that show that dark matter is real and isn't a "fudge factor" as some creationists claim
It is important to note that the Big Bang theory does not attempt to describe the initial conditions or first cause of the universe. It is intended to describe the development of the universe from its extremely dense and hot early stages into its present form. It is instructive not to think of it as a localized explosion from which all matter moves away, but rather as a uniform expansion of space itself. An observer at any point in the universe sees the same thing: a homogeneous distribution of matter everywhere, with the more and more distant parts receding faster and faster.
 Origin of the Big Bang model
The basis of the Big Bang model, that the universe had a beginning, was speculated upon for hundreds of years with early astronomers, such as Johannes Kepler, arguing the universe was finite in age. Edgar Allen Poe in 1848 wrote that the universe was cyclic in nature, expanding and contracting from a single primordial state. Poe also believed that time and space were one, nearly 100 years before Albert Einstein would prove it so. In 1927 Catholic priest Georges LeMaitre proposed an expanding model of the universe to explain the observed redshifts of spiral nebulae with Edwin Hubble providing the observational evidence of redshifting galaxies in 1929. Einstein, having deliberately implied that there was a Big Bang in his theory of general relativity, proved that the mathematical evidence pointed towards a starting point of time and space. It was Georges LeMaitre who was intelligent enough to notice Einstein's implication, and so it was LeMaitre who officially announced the Big Bang model. At the time however, it was not called 'the Big Bang'. The term 'Big Bang' did not come about until years later, when it was coined by Fred Hoyle, who was a proponent of the continuous creation model and used the term "Big Bang" in a derogatory sense.
 Starting assumptions
There are two assumptions required to construct the Big Bang. There is empirical evidence for both of these assumptions, and are considered to be reasonable, defensible statements rather than postulates.
- The laws of physics are the same everywhere in the universe, and are the same throughout the history of the universe.
- On a sufficiently large scale, the universe is homogeneous and isotropic.
The first assumption is straightforward, because a.) There is no evidence to the contrary and b.) Without it you might as well give up on doing any astronomy, astrophysics, or cosmology at all, since if physical laws in the Andromeda galaxy are somehow different from where we live, but the differences are so subtle that we can't detect any from where we are -- well, it's pretty hard to go there and measure them. This assumption is necessary because when talking about how things interact on galactic, much less universal, scales, we need to use general relativity. It's much better if general relativity applies to other galaxies in the same way that it applies to ours.
The second assumption is known as the cosmological principle and has strong empirical support. It's essentially a stronger version of the Copernican principle, which says that the Earth has no special place in the cosmos.
 What the Big Bang theory actually says
In spite of its name, the Big Bang theory says nothing about how the Unvierse first came into being. In other words, it says nothing a about the Big Bang itself. All it says is "OK, we know the laws of physics at these energy scales, so we can extrapolate back to around 10-43 seconds, but beyond that we have no idea what happened; we'd need a quantum theory of gravity for that." This section will give a timeline of the big bang theory.
Inflation was developed by Alan Guth in the early 1980s to solve some problems with the standard big bang theory. These are:
- The horizon problem: The temperature of the photons of the cosmic microwave background from one direction are the same as from the opposite direction. These photons come from two points in the universe that were never in contact. Yet somehow, they are the same temperature! The only way to resolve this was if the universe expanded very rapidly in its early stages.
- The flatness problem: kinematic tests and data from the CMB fluctuations suggest that the universe is flat. Moreover, the only way that large scale structure could form is if the universe was flat. If the universe were closed, with the density much greater than the critical density, ρ >> ρcrit, then the universe would have collapsed to a singularity a long time ago. (This scenario, by the way, is called the big crunch.) But if ρ << ρcrit then the universe would have started expanding very rapidly, and the galaxies and large scale structure that we see today could not have formed. The way inflation does this is by demanding that (classical) inhomogeneities be washed out. Or, more precisely, they be stretched to scales much larger than the observable universe. (Interestingly, inflation also predicts that various regions of the universe be causally disconnected (that is, the cannot communicate with each other), which allows for the possibility of a multiverse.)
- The monopole problem: Grand unification theories (GUTs) predict the existence of magnetic monopoles. However, we see very few of these in nature. Inflation solves the puzzle of magnetic monopoles.
- The initial fluctuations problem: As mentioned above, small over and under-densities in the early universe were the seeds for the formation of galaxies and large scale structure. The question remains, though: why are the fluctuations there, and what dictates their form? Inflation says that they are there from quantum (not classical; remember, those are wiped out by inflation!) fluctuations when the universe was Planck-sized. They were then amplified to galactic scales. Inflation also predicts the form of the fluctuations.
Of course, the natural question to ask about inflation is "What bizarre form of matter could cause that?" It turns out that if you have a scalar field with the right potential, then an inflationary epoch will take place, and it will satisfy the conditions to solve the problems listed above. Using the right potential, one can also arrange for a graceful exist. This means that there will be a smooth transition from the inflationary epoch to a Friedman expansion. There are many concrete inflationary scenarios (which of course result from choosing a potential) that have been proposed.
So, what is the inflaton? Physicists originally thought that the Higgs field was the field causing inflation (or, the Higgs boson is the inflaton). However, the potential (the Higgs, or Mexican hat potential) does not have the right properties, so some other scalar particle must be the inflaton. It is currently thought that the inflaton is a "beyond the Standard Model" particle.
Another important piece in this story is the phase transition known as reheating. Reheating is the process by which the inflaton field decays to the other particles, like quarks, electrons and photons.
 Thermal History and Nucleosynthesis
After reheating, there was a soup of Standard Model particles. At first, there was a quark-gluon plasma. The quarks and gluons are not bound to each other at these high energies. At first, this may seem puzzling; isn't the color supposed to be confined? Yes, but only at low energies. Quantum chromodynamics has a peculiar property called asymptotic freedom. That is, at high energies, the force actually becomes weaker. Anyway, when the plasma has cooled sufficiently, the quarks and gluons are bound together into baryons. Antibaryons (the antiparticles of baryons) are also present, and they annihilate with the baryons. It would seem that antibaryons and baryons would be produced in equal amounts. If this were the case, all antibaryons would annihilate with the baryons and there would be no baryons left. This is obviously not the case, so some process must have favored baryons over antibaryons. In other words, there were slightly more baryons than antibaryons. Some of these baryons eventually became nuclei of helium or heavier elements through nucleosynthesis. We will have more to say about this later, but first we will discuss the thermal history of the leptons.
We first need to talk about thermal decoupling. Consider two species of particles, A and B. They have some reaction that keeps them in thermal equilibrium. If the rate of the reaction γ is smaller than the rate of expansion (that is, the Hubble constant), then the particles are in thermal equilibrium and have the same temperature. When this condition fails to be met, the particles are no longer in thermal equilibrium and are said to have decoupled. In the early universe, the neutrinos were in thermal equilibrium with everything else. However, after a certain time, the reaction rate sustaining that equilibrium became greater than the Hubble constant and the neutrinos decouple. They should still be visible today, but because they would be swamped by high energy neutrinos from various astrophysical sources, they would be difficult to detect. Anyway, shortly after neutrino decoupling, electron-positron annihilation took place. As for baryon-antibaryon annihilation, there must have been a slight excess of electrons over positrons in the early universe.
We will now, as promised, discuss nucleosynthesis. Neutrons and protons are kept in chemical equilibrium by certain reactions. Once the rate of these reactions is greater than the rate of expansion, they are no longer in equilibrium and the ratio of protons to neutrons "freezes-out." This means that the neutron-to-proton ration is constant. Now, one neutron and one proton will sometimes be fused together into a deuterium nucleus. These could be fused together into a helium-4 nucleus, but the reactions are not efficient enough for this to happen. Once the temperature cools enough, nucleosynthesis begins. Helium is fused extremely rapidly, far from equilibrium. However, in practice, we can use a quasi-equilibrium approximation for nucleosynthesis calculations. Once the reaction rate is again bigger than the rate of expansion, the helium abundance freezes out. Similar things happen for lithium and a few other metals. (Astronomers use metals to mean anything other than hydrogen or helium.) One can calculate the abundance after big bang nucleosynthesis and it is roughly 75% hydrogen, 25% helium, and trace amounts of metals. This is exactly what is seen in interstellar medium.
So now one has a plasma of ionized hydrogen and helium nuclei. Eventually the temperature will cool enough for electrons to be bound to nuclei. This is known as the epoch of recombination.
 Galaxy and large scale structure formation
As mentioned, the quantum fluctuations from inflation became the seeds for galaxy formation--they grew by gravitational instability and became the galaxies and large scale structure we see today.
 Evidence for the Big Bang
There are four primary pieces of evidence for the Big Bang that are so well-established that they are referred to as the "four pillars" of the Big Bang. While other pieces of evidence exist, these four are the most compelling.
 Pillar 1: The universe is expanding
Up until the early 20th century, the universe was thought by most scientists to be static and unchanging. However, Edwin Hubble's observations and analysis in the late 1920s showed that assumption to be mistaken. Furthermore, he found the farther away a galaxy is from our own, the faster it is receding from us. Unsurprisingly, the relationship between a galaxy's distance and recessional velocity is known as Hubble's Law. There are two possible explanations for these observations.
- The Earth is at the center of a massive explosion of galaxies.
- The universe is uniformly expanding.
Explanation 1 is untenable because it is in conflict with the cosmological principle (see above starting assumptions). That leaves explanation 2.
If the consequences of Explanation 2 are extrapolated into the past, all the matter in the observable universe would have been at a single point approximately 13.7 billion years ago.
 Pillar 2: Cosmic microwave background radiation
If the matter in the early universe was highly compressed, it would have been extremely hot and dense -- so much so that baryons couldn't form, much less atoms, and there was simply a sea of electrons, quarks, and photons. The photons would constantly interact with the electron-quark plasma, constantly forming and annihilating without going very far. Over time, the universe cooled enough that the quarks could combine into baryons (mostly protons and neutrons). After further cooling, about 3-20 minutes in, the protons and neutrons could combine into small atomic nuclei (although most protons did not). After even more cooling, about 370000 years in, the nuclei could combine with electrons to form neutral atoms.
Once the universe cooled enough to allow electrons and nuclei to combine into neutral atoms, the remaining photons were "released," meaning they could travel large distances as radiation without interacting with a charged particle. Thus, if the Big Bang occurred, we should see vestiges of this radiation permeating all space, and it should look the same in all directions. Since it was emitted by a universe entirely at thermal equilibrium, this radiation should also display a black body spectral pattern.
Furthermore, the radiation would have been very highly energetic, with a very short wavelength, at the time of the universe becoming transparent to light. However, the universe's expansion since that time would have lengthened the wavelength of that radiation, or, equivalently, cooled it considerably. Over time, the radiation would transition from X-ray levels, to ultraviolet, to visible (yikes, good thing our eyes didn't exist then), to infrared, to microwave.
Today, anyone can point a radio telescope at the sky and find an isotropic, black body spectrum of radiation peaked in the microwave region of the spectrum, with a temperature corresponding to 2.726 Kelvin on average. If you don't own a telescope just try tuning your TV reception into a nonexistent channel. Some of the static you see is the left over radiation from the Big Bang. Cool eh?
 Pillar 3: Abundance of light chemical elements
Starting at about three minutes after the Big Bang, and ending at about twenty minutes after, the temperature of the universe was low enough that protons and neutrons could form, but still hot enough that nuclear fusion reactions could occur. During this period, the bulk of the universe's Helium was formed (the amount of Helium added by stellar fusion since is small compared to the primordial amount). Additionally certain light elements, such as Deuterium and certain isotopes of Lithium and Beryllium, can not be formed in significant amounts in stellar fusion reactions since any stellar core hot enough to create them is also hot enough to continue to fuse them into heavier elements given enough time. These elements can only be created in a fusion epoch much shorter than the lifespan of a star.
As observed, the composition of the matter in the universe is basically 75% hydrogen and 25% helium with trace amounts of the light elements created in the nucleosynthesis epoch. Even more cool, it's possible to predict relative abundances of this matter using a single parameter, the photon to baryon ratio. The correct photon to baryon ratio can be determined by measuring tiny fluctuations in the cosmic microwave background radiation. Using the value of the photon to baryon ratio derived from the cosmic microwave background to calculate the predicted elemental ratios yields numbers extremely close to those observed spectroscopically.
 Pillar 4: Galactic morphology and distribution
The distant galaxies from us are many light years away, so when we observe them, we are seeing them as they were long ago due to the light travel time. Consequently, we can get pretty good ideas about star formation, galaxy formation, galaxy cluster formation, and supercluster formation because we can see snapshots of these things happening at different eras. It turns out that galaxies that formed long ago are quite different from the nearby ones that we see today, as measured by star and quasar formation.
These observations suggest that the universe was different in the past than it is now, which is evidence against the "steady state model" of the universe that was an alternative to the Big Bang before the cosmic microwave background radiation was discovered. These days pretty much all scientists acknowledge that the Big Bang is the way to think about the early formation and growth of the universe.
One of the biggest problems with the original Big Bang theory is the extreme homogeneity of matter in the universe; the density of galaxies and gas clouds is the same no matter which direction we look. Scientists like Professor Alan Guth solved this problem, known as the horizon problem, by introducing the concept of "inflation" - shortly after the initial moment, the universe underwent a period of rapid expansion which smoothed out the density fluctuations. While it explains a few things, inflation leaves even more questions - most prominently, "what force of nature could cause that?".
Chaotic inflation theory, or bubble universe theory, is an alternative model of inflation. Developed by physicist Andrei Linde and others in 1986, it solves a problem of the inflation theory, namely how to end the inflationary period.
Bears no relation to the macroeconomic phenomenon of inflation, except insofar as books about it cost slightly more every year.
Of course, any time two scientists disagree about a minor aspect of the theory, the conversation is mined for any quotes which could be misrepresented to support creationism. However, any discussion about the development of the universe is bound to be severely limited - the whole of our observations are made from one tiny corner of space, in the blink of an eye. We are not done learning yet.
 How to create a universe
Dr. Guth and others hope to figure out how to create a universe in the laboratory. Guth once stated in an interview:
I in fact have worked with several other people for some period of time on the question of whether or not it's in principle possible to create a new universe in the laboratory. Whether or not it really works we don't know for sure. It looks like it probably would work. It's actually safe to create a universe in your basement. It would not displace the universe around it even though it would grow tremendously. It would actually create its own space as it grows and in fact in a very short fraction of a second it would splice itself off completely from our Universe and evolve as an isolated closed universe growing to cosmic proportions without displacing any of the territory that we currently lay claim to.
- "What was there before the Big Bang?"
One possible answer is:
- "The question is nonsensical, because there was no time (or space) for anything to exist in, so the word before is meaningless." This meaninglessness was pointed out in an excellent analogy by Stephen Hawking, who described the question as "like asking what is north of the North Pole."
Another possible answer is:
- "The multiverse existed before the Big Bang.”
As both of the above are non-intuitive and go counter to all everyday human experience, it is the major point upon which popular understanding fails.
The limit for knowledge about the Big Bang is Plank Time. The Planck Time is the shortest meaningful length of time. It is somewhere around 10−43 seconds, which is extremely short, but not zero. It is not possible to know what happened less than one Planck Time after the Big Bang. Indeed, it is not just not possible to know what happened, it is actually meaningless to even ask the question. That being the case the question of what happened before the Big Bang is also meaningless. We just have to lump it and get on with asking questions which are meaningful. As Brian Greene put it, "A common misconception is that the Big Bang provides a theory of cosmic origins. It doesn't. The Big Bang is a theory ... that delineates cosmic evolution from a split second after whatever happened to bring the universe into existence, but it says nothing at all about time zero itself. And since, according to the Big Bang theory, the bang is what is supposed to have happened at the beginning, the Big Bang leaves out the bang. It tells us nothing about what banged, why it banged, how it banged, or, frankly, whether it really banged at all."
Hawking's book A Brief History of Time gives a reasoned explanation of the Big Bang and subsequent events, but is popularly reckoned to be intensely dense to the point of unreadability. Another book A Briefer History of Time has since been published.
Julian Barbour suggests that reality simply terminates on nothing at the alpha point, as a brute fact, in the same way that England abuts the sea at Land's End without requiring an explanation.
 Theory of everything?
 Addendum: God?
Theists of all stripes have attempted to use the theory as a "proof" of the existence of God. Well Goddidit:
- "How was the Big Bang initiated, if not by a supernatural being?" they ask.
- To which there is only one reply necessary:
- "How was the supernatural being created if not by a supernatural being?"
- And so on ad infinitum.
- "How was the supernatural being created if not by a supernatural being?"
- To which there is only one reply necessary:
This paradox is unsolvable, so in the end, it comes down to a question of faith, or lack thereof.
In A Brief History of Time, Stephen Hawking outlines the mathematical use of imaginary time which results in the description of the universe as being of a hyperspherical nature without start or end - these being merely points on a "surface" undistinguished from others. The upshot is that the requirement for "start" and "cause" are removed, as is the need for faith (a concept which has no place in science).
Interestingly enough, the Big Bang theory was first proposed by Catholic priest and professor of physics Georges Lemaître. He first brought the theory to public attention after the discovery of redshift of nearby nebulae, although it was Fred Hoyle who coined the actual name as a derisory term. Compared with current Big Bang theory, which incorporates aspects such as inflation, LeMaitre hypothesized that all matter for the universe came forth from a "primeval atom", today more commonly described as a singularity.
 So what happens next?
The universe, from this point, could:
- Keep expanding, until it expands so far it cannot collapse back upon itself. Eventually matter and energy would be so spread out that no particles would interact again. This is called "the heat death of the universe".
- Keep expanding, but reduce the rate of expansion, asymtopically reaching 0.
- Keep expanding, but at slower and slower rates, until gravity takes over, compressing all the mass back into a singularity, perhaps kicking off another Big Bang.
A fourth option, which was only recently discovered, is that the expansion will keep accelerating until the universe is torn apart at the atomic level. Recent studies into the cosmic microwave background radiation, gravitational lensing, and, most importantly, improved measurements of supernovas have led to the discovery that expansion really is accelerating. A possible explanation for this acceleration is the fact that, as the universe expands, the density of dark matter decreases while the density of dark energy remains constant, thus leading to an eventual predomination of dark energy which in turn drives the expansion.
 See also
- ↑  "The age of the Universe can also be estimated from a cosmological model based on the Hubble constant and the densities of matter and dark energy. This model-based age is currently 13.7 +/- 0.2 Gyr. But this Web page will only deal with actual age measurements, not estimates from cosmological models. The actual age measurements are consistent with the model-based age which increases our confidence in the Big Bang model."
- ↑ See Wikipedia on speculative physics beyond the Big Bang
- ↑ Scientific American Special Edition - The Cosmic Life Cycle, 1994-2007: "The Evolution of the Universe", P. James E. Peebles, David N. Schramm, Edwin L. Turner and Richard G. Kron.
- ↑ Cosmos, Carl Sagan. 1985.
- ↑ http://www.nytimes.com/2002/11/02/books/think-tank-what-did-poe-know-about-cosmology-nothing-but-he-was-right.html
- ↑ Goodman, J. "Geocentrism reexamined," Phys. Rev. D 51 (4), 1821-1827 (1995) 
- ↑ Nevertheless, creationists would have everyone believe it's true; see quantized redshift for one of their thoroughly debunked claims.
- ↑ http://www.nasa.gov/vision/universe/starsgalaxies/cobe_background.html
- ↑ Scientific American Special Edition - The Cosmic Life Cycle, 1994-2007: "Inflation in a Low-Density Universe", Martin A. Bucher and David N. Spergel.
- ↑ Physicist Aims to Create a Universe, Literally, Malcolm W. Browne, The New York Times, 1987.
- ↑ Parallel Universes (transcript), BBC, 2002.
- ↑ Big Bang, Wikipedia
- ↑ Greene, Brian. The Fabric of the Cosmos Vintage paperback ed., 2005, p. 272. Emphasis in original.
- ↑ A Brief History of Time, ISBN 0-553-34614-8 A Briefer History of Time, ISBN 0-553-80436-7