General relativity and cosmological models

Newton described gravity as an invisible force reaching instantly across space between any two masses. General relativity replaces that with geometry: mass and energy curve the fabric of spacetime itself, and every object moving under gravity alone — planets, light, a falling apple — is simply following the straightest possible path available to it (a geodesic) through that curved spacetime. An astronaut in free fall feels weightless not because gravity has switched off, but because there is, in fact, no force acting on them at all; what we call 'gravity' on the ground is really the ground pushing up on us, preventing us from following our natural geodesic through the Earth. The same curvature that bends the paths of falling objects also bends the path of light passing a massive object, and slows clocks that sit deeper in a gravitational well relative to clocks farther out — two effects that read as unrelated only until you see that both come from the same curved geometry.

Applied to the universe as a whole, general relativity's field equations relate the overall curvature of spacetime to the total distribution of mass and energy within it. Solved for a universe that is, on the largest scales, roughly the same everywhere and in every direction, those equations naturally allow — and, combined with the observation that galaxies are receding from us in every direction with speed proportional to distance (Hubble's law) — predict an expanding universe. Running that expansion backward in time implies a much hotter, denser early universe, a conclusion independently confirmed by the cosmic microwave background: a near-uniform bath of radiation left over from when the universe first became transparent, about 380,000 years after the initial hot, dense state usually called the Big Bang.

It is important to be precise about what is expanding: not galaxies flying apart through pre-existing space from some central point, but space itself stretching everywhere at once, carrying galaxies apart with it and stretching the wavelength of light in transit — which is also why there is no centre to the expansion and no location any observer could point to and call the origin. The modern cosmological model refines this picture further with an extremely brief early phase of exponential expansion (inflation), thought to explain why the universe looks so uniform on the largest scales, and with the discovery that the present-day expansion is accelerating rather than merely continuing or slowing — attributed to dark energy, a repulsive effect built into spacetime itself that is, in current models, distinct from gravitating dark matter.