Astronomical distances and particle horizons The distances of observable objects in the night sky correspond to times in the past. We use the light-year (the distance light can travel in the time of one Earth year) to describe these cosmological distances. A galaxy measured at ten billion
light-years appears to us as it was ten billion years ago, because the light has taken that long to travel to the observer. If one were to look at a galaxy ten billion light-years away in one direction and another in the opposite direction, the total distance between them is twenty billion light-years. This means that the light from the first has not yet reached the second because the universe is only about 13.8 billion years old. In a more general sense, there are portions of the universe that are visible to us, but invisible to each other, outside each other's respective
particle horizons.
Causal information propagation In accepted relativistic physical theories, no information can travel faster than the
speed of light. In this context, "information" means "any sort of physical interaction". For instance, heat will naturally flow from a hotter area to a cooler one, and in physics terms, this is one example of information exchange. Given the example above, the two galaxies in question cannot have shared any sort of information; they are not in
causal contact. In the absence of common initial conditions, one would expect, then, that their physical properties would be different, and more generally, that the universe as a whole would have varying properties in causally disconnected regions.
Horizon problem Contrary to this expectation, the observations of the
cosmic microwave background (CMB) and
galaxy surveys show that the observable universe is nearly
isotropic, which, through the
Copernican principle, also implies
homogeneity. CMB sky surveys show that the temperatures of the CMB are coordinated to a level of \Delta T/T \approx 10^{-5}, where \Delta T is the difference between the observed temperature in a region of the sky and the average temperature of the sky T. This coordination implies that the entire sky, and thus the entire
observable universe, must have been causally connected long enough for the universe to come into thermal equilibrium. According to the Big Bang model, as the density of the
expanding universe dropped, it eventually reached a temperature where photons fell out of
thermal equilibrium with matter; they
decoupled from the electron-proton
plasma and began
free-streaming across the universe. This moment in time is referred to as the epoch of
Recombination, when electrons and protons became bound to form electrically neutral hydrogen; without free electrons to scatter the photons, the photons began free-streaming. This epoch is observed through the CMB. Since we observe the CMB as a background to objects at a smaller redshift, we describe this epoch as the transition of the universe from opaque to transparent. The CMB physically describes the 'surface of last scattering' as it appears to us as a surface, or a background, as shown in the figure below. Note we use
conformal time in the following diagrams. Conformal time describes the amount of time it would take a photon to travel from the location of the observer to the farthest observable distance (if the universe stopped expanding right now). File:Surface of Last Scattering.png|thumb|The blue circle is the CMB surface which we observe at the time of last scattering. The yellow lines describe how photons were scattered before the epoch of recombination and were free-streaming after. The observer sits at the center at present time. For reference. The decoupling, or the last scattering, is thought to have occurred about 300,000 years after the Big Bang, or at a redshift of about z_{rec} \approx 1100. We can determine both the approximate angular diameter of the universe and the physical size of the particle horizon that had existed at this time. The
angular diameter distance, in terms of redshift z, is described by d_{A}(z)=r(z) / (1+z). If we assume a
flat cosmology then, :r(z) = \int_{t_{em}}^{t_0} \frac{dt}{a(t)} = \int_{a_{em}}^{1} \frac{da}{a^2 H(a)} = \int_{0}^{z} \frac{dz}{H(z)}. The epoch of recombination occurred during a matter dominated era of the universe, so we can approximate H(z) as H^2(z) \approx \Omega_m H_0^2 (1+z)^3. Putting these together, we see that the angular diameter distance, or the size of the observable universe for a redshift z_{rec} \approx 1100 is :r(z)=\int_{0}^{z} \frac{dz}{H(z)} = \frac{1}{\sqrt{\Omega_m} H_0} \int_{0}^{z} \frac{dz}{(1+z)^{3/2}} = \frac{2}{\sqrt{\Omega_m} H_0}\left(1-\frac{1}{\sqrt{1+z}}\right). Since z \gg 1, we can approximate the above equation as :r(z) \approx \frac{2}{\sqrt{\Omega_m}H_0}. Substituting this into our definition of angular diameter distance, we obtain :d_A(z) \approx \frac{2}{\sqrt{\Omega_m}H_0}\frac{1}{1+z}. From this formula, we obtain the angular diameter distance of the cosmic microwave background as d_A(1100) \approx 14\ \mathrm{Mpc}. The
particle horizon describes the maximum distance light particles could have traveled to the observer given the age of the universe. We can determine the comoving distance for the age of the universe at the time of recombination using r(z) from earlier, :d_\text{hor,rec}(z)=\int_{0}^{t(z)} \frac{dt}{a(t)}= \int_{z}^{\infin} \frac{dz}{H(z)} \approx \frac{2}{\sqrt{\Omega_m}H_0}\left [ \frac{1}{\sqrt{1+z}} \right ]_z^\infin \approx \frac{2}{\sqrt{\Omega_m}H_0}\frac{1}{\sqrt{1+z}} File:Spacetime Diagram without Inflation.png|thumb|This spacetime diagram shows how the light cones for two light particles spaced some distance apart at the time of last scattering (ls) do not intersect (i.e. they are causally disconnected). The horizontal axis is comoving distance, the vertical axis is conformal time, and the units have the speed of light as 1. For reference. To get the physical size of the particle horizon D, :D(z)=a(z)d_\text{hor,rec}= \frac{d_\text{hor,rec}(z)}{1+z} :D(1100) \approx 0.03~\text{radians} \approx 2^\circ We would expect any region of the CMB within 2 degrees of angular separation to have been in causal contact, but at any scale larger than 2° there should have been no exchange of information. CMB regions that are separated by more than 2° lie outside one another's particle horizons and are causally disconnected. The horizon problem describes the fact that we see isotropy in the CMB temperature across the entire sky, despite the entire sky not being in causal contact to establish thermal equilibrium. Refer to the timespace diagram to the right for a visualization of this problem. If the universe started with even slightly different temperatures in different places, the CMB should not be isotropic unless there is a mechanism that evens out the temperature by the time of decoupling. In reality, the CMB has the same temperature in the entire sky, . ==Inflationary model==