Horizon problem
A cosmological fine-tuning problem in the Big Bang model.
The horizon problem, also known as the homogeneity problem, is a cosmological fine-tuning issue within the Big Bang model. It arises from observations that widely separated regions of space appear homogeneous, yet normal physical processes require causal connection, and these regions are too far apart to have interacted since the Big Bang given the speed of light limit. This creates a difficulty in explaining the observed homogeneity without a mechanism that sets the same initial conditions everywhere.
- first_pointed_out_by
- Wolfgang Rindler
- field
- Cosmology
- key_observation
- Homogeneity of widely separated regions without causal contact
- most_accepted_hypothesis
- Cosmic inflation
Lore & Background
It stems from the observation that the cosmic microwave background (CMB) and galaxy surveys show the observable universe is nearly isotropic, implying homogeneity. CMB sky surveys show temperature differences of only about ΔT/T ≈ 10⁻⁵, indicating the entire sky must have been causally connected long enough for thermal equilibrium.
Reader's Guide
The horizon problem highlights a fundamental tension between the observed uniformity of the universe and the causal structure of the Big Bang model. Without a mechanism to set identical initial conditions across causally disconnected regions, the homogeneity of the CMB and large-scale structure is difficult to explain. The most commonly accepted hypothesis to resolve this is cosmic inflation, which posits a period of rapid expansion that would have brought distant regions into causal contact before they separated. Alternative solutions include a cyclic universe or a variable speed of light. The problem remains a key driver of theoretical cosmology, shaping models of the early universe and testing the limits of the standard Big Bang framework.
Did You Know?
- Observations of the cosmic microwave background show temperature differences of only about ΔT/T ≈ 10⁻⁵ across the sky.
- The most commonly accepted hypothesis to explain the horizon problem is cosmic inflation.
- Alternative solutions propose a cyclic universe or a variable speed of light.
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