Big Bang

In physical cosmology, the Big Bang is the scientific theory that the universe emerged from an enormously dense and hot state nearly 14 billion years ago. The Big Bang is a consequence of the observed Hubble's law velocities of distant galaxies that when taken together with the cosmological principle implies that space is expanding according to the Friedmann model of general relativity. Extrapolated into the past, these observations show that the universe has expanded from a primeval state, in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity.

Theoretical underpinnings

As it stands today, the Big Bang is dependent on three assumptions:

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• The universality of physical laws
• The cosmological principle
• The Copernican principle

When first developed, these ideas were simply taken as postulates, but today there are efforts underway to test each of them. Tests of the universality of physical laws have found that the largest possible deviation of the fine structure constant over the age of the universe is of order 10-5. The isotropy of the universe that defines the Cosmological Principle has been tested to a level of 10-5 and the universe has been measured to be homogenous on the largest scales to the 10% level. There are efforts underway to test the Copernican Principle by means of looking at the interaction of galaxy clusters with the CMB through the Sunyaev-Zeldovich effect to a level of 1% accuracy.

Related Topics:
Fine structure constant - Isotropy - Galaxy clusters - Sunyaev-Zeldovich effect

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The Big Bang theory uses Weyl's postulate to unambiguously measure time at any point as the "time since the Planck epoch". Measurements in this system rely on conformal coordinates in which so-called comoving distances and conformal times remove the expansion of the universe, parameterized by the cosmological scale factor, from consideration of spacetime measurements. The comoving distances and conformal times are defined so that objects moving with the cosmological flow are always the same comoving distance apart and the particle horizon or observational limit of the local universe is set by the conformal time.

Related Topics:
Weyl's postulate - Time - Planck epoch - Conformal - Comoving distance - Scale factor - Spacetime - Particle horizon

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As the universe can be described by such coordinates, the Big Bang is not an explosion of matter moving outward to fill an empty universe; what is expanding is spacetime itself. It is this expansion that causes the physical distance between any two fixed points in our universe to increase. Objects that are bound together (for example, by gravity) do not expand with spacetime's expansion because the physical laws that govern them are assumed to be uniform and independent of the metric expansion. Moreover, the expansion of the universe on today's local scales is so small that any dependence of physical laws on the expansion is unmeasurable by current techniques.

Related Topics:
Gravity - Metric expansion

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