page_title: Horizon problem
section_title: Horizon problem
hierarchical_section_title: Horizon problem / Background / Horizon problem
caption_attribution_description: English: 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: &lt;http://courses.las.illinois.edu/spring2020/astr507/&gt;.
context_page_description: The horizon problem is a cosmological fine-tuning problem within the Big Bang model of the universe. It arises due to the difficulty in explaining the observed homogeneity of causality disconnected regions of space in the absence of a mechanism that sets the same initial conditions everywhere. It was first pointed out by Wolfgang Rindler in 1956.
  The most commonly accepted solution is cosmic inflation. An explanation in terms of variable speed of light has also been proposed.
context_section_description: 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  where  is the difference between the observed temperature in a region of the sky and the average temperature of the sky . 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. They are now observed as the CMB. 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).
  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. 
  The angular diameter distance, in terms of redshift z, is described by . If we assume a flat cosmology then, 
  The epoch of recombination occurred during a matter dominated era of the universe, so we can approximate H(z) as  Putting it together, we see that the angular diameter distance, or the size of the observable universe, for a redshift  is, 
  .
  Since , we can approximate ,
  
  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,
  To get the physical size of the particle horizon , 
  We would expect any region of the CMB within 2 degrees of angular separation to have been in casual 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, 2.726 ± 0.001 K.
caption: 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.
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