Horizon problem

It arises due to the difficulty in explaining the observed homogeneity of causally disconnected regions of space in the absence of a mechanism that sets the same initial conditions everywhere.

Different solutions propose a cyclic universe or a variable speed of light.

The distances of observable objects in the night sky correspond to times in the past.

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.

In accepted relativistic physical theories, no information can travel faster than the speed of light.

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.

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.

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 decoupling, or the last scattering, is thought to have occurred about 300,000 years after the Big Bang, or at a redshift of about

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.

If we assume a flat cosmology then, The epoch of recombination occurred during a matter dominated era of the universe, so we can approximate

Putting these together, we see that the angular diameter distance, or the size of the observable universe for a redshift

, 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.

In reality, the CMB has the same temperature in the entire sky, 2.726 ± 0.001 K.[3] The theory of cosmic inflation has attempted to address the problem by positing a 10−32-second period of exponential expansion in the first second of the history of the universe due to a scalar field interaction.

[4] According to the inflationary model, the universe increased in size by a factor of more than 1022, from a small and causally connected region in near equilibrium.

[5] Inflation then expanded the universe rapidly, isolating nearby regions of spacetime by growing them beyond the limits of causal contact, effectively "locking in" the uniformity at large distances.

It maintained thermal equilibrium to this large size because of the rapid expansion from inflation.

One consequence of cosmic inflation is that the anisotropies in the Big Bang due to quantum fluctuations are reduced but not eliminated.

The theory predicts a spectrum for the anisotropies in the microwave background which is mostly consistent with observations from WMAP and COBE.

[7] Cosmological models employing a variable speed of light have been proposed to resolve the horizon problem of and provide an alternative to cosmic inflation.

In the VSL models, the fundamental constant c, denoting the speed of light in vacuum, is greater in the early universe than its present value, effectively increasing the particle horizon at the time of decoupling sufficiently to account for the observed isotropy of the CMB.

When we look at the CMB it comes from 46 billion comoving light-years away. However, when the light was emitted the universe was much younger (300,000 years old). In that time light would have only reached as far as the smaller circles. The two points indicated on the diagram would not have been able to contact each other because their spheres of causality do not overlap.
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 .
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 .
This spacetime diagram shows how inflation changes the light cones for two light particles spaced some distance apart at the time of last scattering (ls) to allow them to intersect. In this scenario, they are in causal contact and can exchange information with one another. The horizontal axis is comoving distance, the vertical axis is conformal time, and the units have the speed of light as 1. For reference .