## Definition A **Planck Star** is the hypothetical end-state of a collapsing stellar remnant in which quantum gravitational pressure (arising from the granularity of space predicted by [[Loop Quantum Gravity]]) halts the collapse before a classical singularity is reached. Matter is compressed to Planck density — the highest physically meaningful density — and then *bounces* outward, triggering the eventual explosion of the black hole. The **Big Bounce** is the cosmological application of the same idea: the Big Bang was not an origin from nothing but the rebound of a prior contracting universe that passed through a Planck-density phase. ## The Collapse Problem Classical General Relativity predicts that a sufficiently massive star collapses without limit — all its mass compressing toward a mathematical point (a singularity) of infinite density. This prediction is widely regarded as a signal that GR breaks down at extreme densities, precisely where quantum effects on spacetime must be important. Loop Quantum Gravity resolves the singularity by positing that no region of space can be compressed below the Planck volume. ## The Planck Star in Detail When a star's nuclear fuel is exhausted, it collapses under gravity. If the remnant is massive enough (beyond the Tolman–Oppenheimer–Volkoff limit), it forms a black hole. In classical GR, the in-falling matter continues to collapse indefinitely. In LQG, as density approaches the Planck scale, quantum pressure — analogous to electron degeneracy pressure in white dwarfs but arising from the discreteness of space itself — grows strong enough to halt the collapse. The result is a *Planck Star*: stellar mass compressed to roughly atomic size, held in a state of extreme quantum pressure. A Planck Star is not stable. Once maximally compressed, it rebounds and begins to re-expand. From the perspective of a hypothetical observer inside the black hole, this bounce is extremely fast. From the outside, however, the enormous gravitational time dilation means that what is brief inside corresponds to an astronomically long time outside. A black hole is, in this picture, a Planck Star seen in extreme slow motion. ## The Big Bounce Applied cosmologically, the same LQG equations that prevent singularity inside black holes also modify the behaviour of the universe at extreme compression. They introduce a quantum repulsive pressure that grows at Planck densities. The equations allow the history of the universe to be traced back *through* the apparent Big Bang singularity to a prior contracting phase. The Big Bang may have been a *Big Bounce*: a universe contracting under gravity reached Planck density, generated quantum repulsion, and rebounded into the expanding universe we observe today. During the bounce, space and time effectively dissolved into a cloud of quantum probabilities — a phase beyond the reach of current equations. ## Possible Observational Signatures Because the bounce of ancient primordial black holes (formed in the early universe) would eventually produce an explosion, such events might be observable today as high-energy cosmic rays arriving from the sky. Searches for these signatures are ongoing. ## Related - [[Loop Quantum Gravity]] - [[General Relativity]] - [[The Problem of Quantum Gravity]] - [[Black Holes and Hawking Radiation]] - [[Architecture of the Cosmos]] ## Sources - [[Seven Brief Lessons on Physics (Rovelli 2014)]]