Editing Particle horizon (section)

==Horizon problem==
{{main|Horizon problem}}
The concept of a particle horizon can be used to illustrate the famous horizon problem, which is an unresolved issue associated with the [[Big Bang]] model. Extrapolating back to the time of [[recombination (cosmology)|recombination]] when the [[cosmic microwave background]] (CMB) was emitted, we obtain a particle horizon of about

{{block indent|<math> H_p(t_\text{CMB}) = c\eta_\text{CMB} = 284\text{ Mpc} = 8.9 \times 10^{-3} H_p(t_0) </math>}}

which corresponds to a proper size at that time of:

{{block indent|<math> a_\text{CMB}H_p(t_\text{CMB}) = 261\text{ kpc} </math>}}

Since we observe the CMB to be emitted essentially from our particle horizon (<math>284\text{ Mpc} \ll 14.4\text{ Gpc}</math>), our expectation is that parts of the [[cosmic microwave background]] (CMB) that are separated by about a fraction of a [[great circle]] across the sky of

{{block indent|<math>f = \frac{H_p(t_\text{CMB})}{H_p(t_0)}</math>}}

(an [[angular size]] of <math>\theta \sim 1.7^\circ</math>)<ref>{{Cite web |last=Tojero |first=Rita |date=March 16, 2006 |title=Understanding the Cosmic Microwave Background Temperature Power Spectrum |url=http://www.roe.ac.uk/ifa/postgrad/pedagogy/2006_tojeiro.pdf |access-date=5 November 2015 |website=[[Royal Observatory, Edinburgh]]}}</ref> should be out of [[causal contact]] with each other. That the entire CMB is in [[thermal equilibrium]] and approximates a [[blackbody]] so well is therefore not explained by the standard explanations about the way the [[expansion of the universe]] proceeds. The most popular resolution to this problem is [[cosmic inflation]].