A brief History of Time: From Big Bang to Black Holes

Friedmann models: Descriptions of the evolution and fate of the universe, based on Einstein's
general relativity. For sufficiently high densities of matter, these models predict that the universe will
eventually collapse in on itself in a ‘big crunch’.
Gamma rays: Electromagnetic rays of very short wavelength, produced in radioactive decay or by
collisions of elementary particles.
General relativity: Einstein’s theory based on the idea that the laws of science should be the same
for all observers, no matter how they are moving. It explains the force of gravity in terms of the
curvature of a four-dimensional space-time.
Geodesic: The shortest (or longest) path between two points.
Grand unification energy: The energy above which, it is believed, the electromagnetic force, weak
force, and strong force become indistinguishable from each other.
Grand unified theory (GUT): A theory which unifies the electromagnetic, strong, and weak forces.
Gravitational waves: Ripples in the curvature of space, predicted in 1916 by Albert Einstein and
first directly detected in 2016 by the LIGO consortium.
Imaginary time: Time measured using imaginary numbers.
Inflation: The theory that space expanded rapidly in the early universe, giving rise to an almost but
not perfectly uniform universe today, in agreement with observations.
Information paradox: The information content of objects falling into a black hole at first sight
seems to be lost, but this contradicts the laws of quantum mechanics. It is now believed that the
information is returned in an altered form in the Hawking radiation escaping the black hole.
Large Hadron Collider: The world’s largest and most powerful particle collider. Built by the
European Organization for Nuclear Research (CERN), it is situated deep in the earth near Geneva.
Light cone: A surface in space-time that marks out the possible directions for light rays passing
through a given event.