A gravitational singularity, spacetime singularity or just singularity is a "place or time" in which gravity is so intense that spacetime breaks.
A singularity in general relativity is weird:
- a geodesic (a "line" in curved spacetime) is incomplete and gets terminated, and spacetime curvature becomes infinite.
- stars collapsing beyond a certain point (the Schwarzschild radius) would form a black hole, inside which an infinite density and zero volume singularity (covered by an event horizon) would be formed. A black hole singularity is localized in space, meaning it exists at a single point within the black hole, deep inside its event horizon. However, this singularity spans through all time within the black hole’s framework. As you approach the singularity, spacetime curvature grows infinitely, and time for an infalling observer effectively “ends” at the singularity. This singularity sits in the future of everything that crosses the event horizon. It is a temporal destination, not just a spatial location.
- The initial state of the universe, at the beginning of the Big Bang, is a singularity when time and space was compressed into an infinitely dense and hot state.
Types of singularities[]
| 0d | Yang monopoles and point particles |
| 1d | Quantum gravity (strings and loops) |
| 2d | Domain wall |
| Naked | A singularity without an event horizon |
| BKL | The universe is chaotically oscillating around a gravitational singularity in which spacetime is zero |
Infinite density, or approaching infinity?[]
General relativity predicts infinite density. According to Einstein’s equations, when a massive star collapses into a black hole, all its mass is concentrated in zero volume, making the density (mass/volume) theoretically infinite. The gravitational force and space-time curvature at this point also become infinitely large.
However, quantum mechanics may prevent true infinity. At extremely small scales (near the Planck length at 10−35 m), quantum effects become crucial. Quantum gravity suggests that space and matter cannot be infinitely small but are instead "granular" at a fundamental level. This means that there might be an ultra-dense but finite exotic "core" inside black holes. Quantum effects could create a "Planck star" or "fuzzy core" instead of an infinite singularity, creating a rebound effect inside black holes that could prevent complete collapse.
Instead of an actual point of infinite density, the singularity could be a region where density approaches infinity but never truly reaches it.