Directory ยท Cosmology
Frontiers, theories and ideas on the edge
The arguments astronomers are still having, the models that compete with the standard Big Bang and inflation, and the concepts that remain speculation. Every entry below has its own page.
The frontiers
The Hubble tension
Two ways of measuring cosmic expansion disagree, and the gap keeps widening.
Open questionDark energy
Expansion is accelerating, and no one knows what drives it.
Open questionEarly galaxies
JWST finds galaxies that look too mature for their age.
Open questionGravitational-wave hum
Pulsar timing arrays hear a low rumble across the whole sky.
Alternative theories
String theory / M-theory
Particles as vibrating strings, with extra dimensions folded into every point.
Alternative theoryLoop quantum gravity
Space-time built from tiny loops, with a bounce instead of a singularity.
Alternative theoryCyclic universe
The Big Bang as one turn of an endless cycle.
Alternative theoryInflation's variants
The leading model for the universe's smoothness, still with many competing versions.
Ideas on the edge
Multiverse
Our universe as one member of a vast ensemble.
Idea on the edgeExtra dimensions
Hidden directions curled up inside every point of space.
Idea on the edgeWhite holes
The time-reversed twin of a black hole, from which nothing can enter.
Idea on the edgeWormholes
Tunnels through space-time that could link distant places.
Idea on the edgeVacuum decay
A rare quantum event that could rewrite physics as it spreads.
Return to the Universe timeline, or compare these ideas against the standard chronology.