Big Bang
From SkepticWiki
The "Big Bang" refers to a time approximately 13.7 billions years in the past when, according to the current standard cosmological model, the universe was extremely hot, extremely dense, and expanding very rapidly. The known laws of physics break down or become highly uncertain when applied to these extreme conditions. If one attempts to extrapolate back a fraction of a second more, the density becomes infinite and the equations break down entirely. This moment is known as the "Big Bang singularity".
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[edit] Evidence for the Big Bang
If the stars and galaxies that make up the universe are currently moving away from each other, when one reverses the direction of time and "runs the clock backwards" they are moving towards each other. Since the laws of physics are essentially time-reversal invariant, they apply equally well going backwards in time. Matter gravitationally attracts other matter with a strength proportional to its density, and therefore running back in time the contraction will become ever more rapid as the density increases (so long as gravity is the only relevant force). At a finite time in the past the density will become infinite, unless some repulsive force first becomes stronger than gravity.
Therefore the existence of the Big Bang follows from two essential ingredients: that the objects in the universe are currently receding from each other, and that gravity is an attractive force that dominated any repulsive forces relevant to the large-scale evolution of the universe in the past.
[edit] Observational evidence
The most direct evidence that objects are receding is the redshift-distance relation observed in cosmology. A light source that is moving away from the detector will appear red-shifted; that is, the frequency of electromagnetic radiation it emits will be Doppler shifted down. The greater the velocity, the greater the Doppler shift. Therefore by observing the frequency of known atomic and molecular transition lines in the spectra of electromagnetic radiation emitted by stars and galaxies, astronomers can determine their velocity with respect to the earth. Measuring their apparent brightness, angular size, and position with respect to other astronomical objects determines their distance from the earth. Combining these gives a velocity-distance relation which turns out to be roughly linear: the relative velocity of a distant astronomical increases in proportionally to its distance from earth. The constant of proportionality is known as the Hubble constant in honor of the astronomer who first observed it; its current central value is roughly 70 (km/s)/Mpc. In other words, a typical galaxy one mega-parsec away will be receding directly away from the earth with a velocity of 70 km/s. While this may make it sound as though the earth is the center of the expansion, this does not follow---even without spacetime curvature, an explosion in which objects follow this law exactly looks precisely the same to any observer who is moving along with the expansion; alien astronomers on a distance planet would see the earth (and all other bodies) receding directly away from them according to exactly the same law observed on earth.
See also the Observational Evidence section of the main Wikipedia article on the Big Bang.
[edit] Theoretical evidence
Einstein's General Theory of Relativity (GTR) is the theory of gravity that best fits the currently available scientific data, and is nearly universally accepted among physicists. According to the GTR space and time themselves are dynamical, and their warped geometry accounts for the gravitational forces we observe. But in the GTR the universe must be expanding or contracting---there do not exist any (stable) static solutions. All expanding solutions have Big Bang singularities in their past, and all contracting solutions "Big Crunch" singularities to their future (and some have both phases and both singularities). Therefore, all evidence for the GTR is evidence for the Big Bang, and there are multiple independent lines of evidence for the GTR (which will not be reviewed here). The GTR allows for more general expansion laws than a strictly linear relation between distance and velocity, while still maintaining the essential feature that all co-moving observers see the same expansion law (so that the earth is no more the center of the universe than any other point is). Modern observational data indicate that the expansion is not completely linear, and in fact that the rate of its expansion is currently increasing.
[edit] Possible alternatives
As for the potential existence of repulsive forces, none are known that would significantly affect the evolution of the average density of the universe in the past. The reason for this is that the other three fundamental forces of nature can be either attractive or repulsive depending on the sign of the charges they act on. Since the universe is on average neutral with respect to these charges, the attraction nearly balances the repulsion on average. As the density increases going back in time gravity grows stronger, while the other forces remain weak due to neutrality. Hence there are no known forces which could prevent the density from becoming infinite. However, physicists do not expect the known laws of physice to apply under such extreme conditions, and therefore it remains logically possible that something happens to prevent the singularity.
The only other known source for a repulsion is a that of a positive cosmological constant, which is a vacuum energy which can be added to the GTR. However, while it could explain the current acceleration of the expansion, vacuum energy becomes less and less important as the universe contracts and becomes more dense (since the total effect of vacuum energy on a region is proportional to its volume, which shrinks during a contraction).
[edit] The beginning of time?
It is sometimes claimed that the the Big Bang was the beginning of time, and therefore that questions regarding what came before it are misguided. This is incorrect---physics simply cannot definitively answer the question of what happened in the very early stages of the universe or at the putative singularity. We can only be confident that the known laws of physics apply to energy densities and temperatures that we have experience with, either from particle physics accelerators, nuclear reactors and explosions, or from observing stars. Of these, particle accelerators probe by far the highest energies and densities, but are (of course) still limited in reach. At sufficiently early times the density of the universe was greater than those tested by current accelerators, and no fully reliable predictions can be made then. Even if one extrapolates the known laws without modification further back, they certainly break down entirely at a very high density (the Planck density). So the nature of the Big Bang singularity is currently unknown.
Many speculative theories have been proposed to elucidate the nature of the singularity. In some, the Big Bang was actually a "bounce", where an earlier stage of contraction reversed itself due to the action of some extremely powerful repulsive force and then began to expand. In some such models the contracting phase began infinitely far in the past with an infinitely big universe, in others it was finite and the universe has undergone many cycles of such bounces. All the theories in this class rely on extremely speculative and (in nearly all cases) require theoretically problematic assumptions about the required repulsive force.
In others, our universe is a bubble inside a much larger structure that is sometimes called a multiverse. In these theories the Big Bang was not singular, because the very special conditions involved in the quantum formation of such a bubble mean that the matter in the universe did not exist prior to a moment shortly after the would-be singularity. Instead, going back in time all the matter and radiation in the universe coherently dissolves into vacuum energy just before it contracts to infinite density. Since the amount of vacuum energy in a region is proportional to the volume of that region, its density is constant and never singular. However, while these theories successfully resolve the Big Bang singularity of our bubble, they do not account for the origin of the larger multiverse around it.
In still other theories the Big Bang really is the beginning of time. In this class of theories asking what came before is akin to asking what is south of the south pole---a seemingly reasonable question with a false premise and no answer. It is sometimes claimed that the existence of such a universe requires an act by a creator, but---to paraphrase the famous words of the Marquis de Laplace---we have no need for such a hypothesis. These models are logically self-consistent in that any question one can ask within them has an answer. The non-existence of time before the Big Bang is an unproblematic fact about the model (as in the above example of the south pole). Just as the set of positive real numbers has no smallest element, such universes need not have an earliest time, in which case all events have an infinite chain of prior events that can causally determine them.
Moreover the statement that "time began" at a finite point in the past is itself not very well-defined, because periods of time may appear of different lengths when measured by different observers or with different sorts of clocks. This kind of ambiguity is particularly acute under the extreme conditions near the Big Bang, when interaction times become arbitrarily short, the characteristic velocities of all particles become ultra-relativistic, and the curvature of space-time becomes very large.
