Science › Creation theories
The horizon and flatness puzzles
1969· Open question
ProvenTested many times; part of working physics.
Two coincidences the plain Big Bang could not explain: why never-connected regions share one temperature, and why space is so flat.
See it on the Science map →How it works
Horizon (Charles Misner, 1969): by 380,000 years, light could have crossed only regions that now look one to two degrees wide. The sky holds thousands of such patches that never touched, yet all match to 1 part in 100,000. Flatness (Robert Dicke, 1969): the universe's total density sits almost exactly at the knife-edge value that makes space flat. Any early deviation grows, so at one second the match must have been tuned to about 1 part in 10¹⁵. A third puzzle, missing magnetic monopoles, first drew Guth in.
Why it matters
They showed the Big Bang theory, however successful, needed a precisely arranged start it could not explain. Solving them was inflation's original purpose.
Think of it like
Like thousands of separate buildings, never wired together and never visited, all found at exactly the same indoor temperature.
How we know
Both rest on measurement, not theory: the afterglow's evenness and space's flatness are observed directly. Neither can be explained within the plain Big Bang; only how to explain them is debated.
Still a mystery
Are they real puzzles, or just starting conditions to accept? Roger Penrose argues inflation needs its own special starting conditions, so it may move the problem rather than solve it.
Connections (2)
▸Cosmic microwave background: the Big Bang's afterglow exposed and sharpened The horizon and flatness puzzlesProven
The relationship
Its sameness exposed the horizon puzzle: points on opposite sides of the sky share one temperature to 1 part in 100,000, yet had never exchanged light. Its spots measure flatness: the main spot size, about one degree, acts as a ruler of known length at a known distance, and curved space would magnify or shrink it. The observed size matches flat space.
How the link was found
Charles Misner framed the horizon puzzle in 1969 using the afterglow's evenness, known since 1965. The BOOMERanG balloon and MAXIMA measured the one-degree spot size in 2000, showing space is flat.
Why it's proven
COBE (1992), WMAP and Planck (2013–2018) mapped the full sky in increasing detail. Combined with galaxy surveys, Planck finds space flat within about 0.2%.
Open questions and mysteries
A few large-scale oddities, such as one half of the sky showing slightly stronger ripples than the other, may be flukes or hints that the evenness is not perfect.
▸The horizon and flatness puzzles found their leading answer in Cosmic inflationWell supported
The relationship
Inflation treats all three puzzles by stretching. Any curvature is flattened, so flatness becomes the expected outcome rather than a coincidence. Regions now far apart were in contact before inflation pulled them out of reach. Any magnetic monopoles made earlier were diluted until essentially none remain in view. Fine-tuning is traded for one requirement: enough inflation.
How the link was found
Guth set out in 1979, with Henry Tye, to solve the monopole puzzle; John Preskill had shown in 1979 that grand unified theories make far too many. Guth then saw the same mechanism fixed flatness and horizons.
Is it proven?
Inflation predicted more than it was built for: slightly tilted, random-looking ripples, and correlations on scales larger than the early horizon (WMAP, 2003). Those successes suggest the answer is real, not just possible.
Open questions and mysteries
Bouncing and cyclic universes, and theories in which light was once faster, also claim to solve the puzzles. Whether any can match inflation's full record is debated.