A broad area of research that includes theories of the structure and
development of the universe based upon Einstein’s theory of general
relativity, which implies an expanding universe. The expansion of the universe
was first confirmed in 1929 by Edwin Hubble’s observations of the retreating
motion of galaxies.
During the decade following the
publication of his special theory of relativity, Einstein worked on applying it
to a dynamical theory of gravity. His basic insight was to reconceptualize
gravity as the curvature of spacetime instead of as a (Newtonian) force in
space. Rather than being deflecting from their otherwise linear motion in a
Euclidean space with three dimensions, masses would move along geodesics
describing the shortest possible path in curved spacetime. Their motion, in
turn, would alter the curvature of spacetime, thus giving the field equations
General Theory of Relativity (GR) their highly nonlinear form aptly described
as: ‘spacetime tells mass how to move; mass tells spacetime how to curve’.
Shortly after the discovery of GR,
solutions to Einstein’s equations were developed for two distinct classes of
problems: i) point masses, which when applied to the solar system led to several
key tests of the theory and their eventual confirmation (including the
deflection of starlight by the sun and the precession in the perihelion of the
orbit of Mercury), and ii) dust, which when eventually applied to the
distribution of galaxies and galactic clusters described the universe as
expanding in time. During the 1920s, telescopic observations by Edwin Hubble
showed that galaxies were indeed receding from us and at a velocity proportional
to their distance. In essence, the expansion of the universe had been
discovered!
There are in fact three types of
expansion possible. i) Closed model: spherical. In one model the universe has
the shape of a 3dimensional sphere of finite size. It expands up to a maximum
size, approximately 100 billion years from now, then recontracts, eventually
recollapsing to a singularity that mirrors t=0 with infinite temperatures and
densities. ii) Open model 1: ‘flat’ and iii) open model 2: ‘saddleshaped’.
Both the ‘flat’ and ‘saddleshaped’ models are infinite in size and
expanding in time. In both cases the universe will expand forever and cool
indefinitely towards absolute zero. The future of these models is often used to
characterize them as ‘freeze’ (open, both cases) or ‘fry’ (closed). All
three came to be called “Big Bang” models because they describe the universe
as having a finite past life of 1020 billion years and beginning in an event of
infinite temperature and density, and zero volume. Since the age of the
universe, t, is calculated as starting here, it is convenient to label it “t=0";
technically this event is referred to as an “essential singularity.” In the
1960s, Stephen Hawking, Roger Penrose, and Robert Geroch proved key theorems
which showed that the existence of an essential singularity, t=0, given Einstein’s
GR, was unavoidable.
Related Topics:
Contributed
by: Robert Russell  CTNS
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