Editing Big Bang (section)

===Horizon problem===
{{Main|Horizon problem}}
The horizon problem results from the premise that information cannot travel [[Faster-than-light|faster than light]]. In a universe of finite age this sets a limit—the particle horizon—on the separation of any two regions of space that are in [[causality (physics)|causal]] contact.<ref name="kolb_c8">{{harvnb|Kolb|Turner|1988|loc=chpt. 8}}</ref> The observed isotropy of the CMB is problematic in this regard: if the universe had been dominated by radiation or matter at all times up to the epoch of last scattering, the particle horizon at that time would correspond to about 2 degrees on the sky. There would then be no mechanism to cause wider regions to have the same temperature.<ref name="Ryden2003">{{harvnb|Ryden|2003}}</ref>{{rp|191–202}}

A resolution to this apparent inconsistency is offered by inflation theory in which a homogeneous and isotropic [[Scalar field|scalar energy field]] dominates the universe at some very early period (before baryogenesis). During inflation, the universe undergoes [[Exponential growth|exponential]] expansion, and the particle horizon expands much more rapidly than previously assumed, so that regions presently on opposite sides of the observable universe are well inside each other's particle horizon. The observed isotropy of the CMB then follows from the fact that this larger region was in causal contact before the beginning of inflation.<ref name="Guth1998" />{{rp|180–186}}

Heisenberg's uncertainty principle predicts that during the inflationary phase there would be [[primordial fluctuations|quantum thermal fluctuations]], which would be magnified to a cosmic scale. These fluctuations served as the seeds for all the current structures in the universe.<ref name="Ryden2003"/>{{rp|207}} Inflation predicts that the primordial fluctuations are nearly [[Scale invariance|scale invariant]] and [[Normal distribution|Gaussian]], which has been confirmed by measurements of the CMB.<ref name="wmap1year" />{{rp|sec 6}}

A related issue to the classic horizon problem arises because in most standard cosmological inflation models, inflation ceases well before [[Higgs mechanism|electroweak symmetry breaking]] occurs, so inflation should not be able to prevent large-scale discontinuities in the [[False vacuum|electroweak vacuum]] since distant parts of the observable universe were causally separate when the [[electroweak epoch]] ended.<ref>{{harvnb|Penrose|2007}}</ref>