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=Problems with the Big Bang model=

=Problems with the Big Bang model=

==The Flatness problem==

==The Flatness problem==

The [[critical density]] of the universe is the density at which the universe's rate of expansion will slow down, and eventually reach a rate of zero, as the age of the universe becomes very, very large. Densities above or below this would lead to the universe eventually collapsing back on itself during to gravity, or expanding at an every increasing rate, respectively. <ref>http://www.daviddarling.info/encyclopedia/C/critical_density.html</ref>Today's best estimates of the universe's density place it staggeringly close to the critical density - the universe is described as being almost "flat", since the shape of the universe's geometry is decided by its density - hence the name of ''the flatness problem''. If the universe's density is extrapolated back until just after the [[Big Bang]], it is found to be exceedingly close to 1 - given that it can take any value, this value leading to a perfectly "flat" universe seems unlikely to have been chosen at random. <ref>http://www.daviddarling.info/encyclopedia/C/critical_density.html</ref>

The [[critical density]] of the universe is the density at which the universe's rate of expansion will slow down, and eventually reach a rate of zero, as the age of the universe becomes very, very large. Densities above or below this would lead to the universe eventually collapsing back on itself during to gravity, or expanding at an every increasing rate, respectively. <ref name="darling">http://www.daviddarling.info/encyclopedia/C/critical_density.html</ref>Today's best estimates of the universe's density place it staggeringly close to the critical density - the universe is described as being almost "flat", since the shape of the universe's geometry is decided by its density - hence the name of ''the flatness problem''. If the universe's density is extrapolated back until just after the [[Big Bang]], it is found to be exceedingly close to 1 - given that it can take any value, this value leading to a perfectly "flat" universe seems unlikely to have been chosen at random. <ref name="darling" />

The explanation offered by cosmic inflation is that the period of exponential expansion which occurred essentially "ironed out" any small creases in the early universe, leading to the visible universe's shape being flat.

The explanation offered by cosmic inflation is that the period of exponential expansion which occurred essentially "ironed out" any small creases in the early universe, leading to the visible universe's shape being flat.