The Big Bang

The Big Bang
theory developed
from observations
of the structure of
the Universe and
from theoretical
considerations. In
1912 Vesto Slipher
measured the first
Doppler shift of a
"spiral
nebula" (spiral
nebula is the
obsolete term for
spiral galaxies),
and soon
discovered that
almost all such
nebulae were
receding from
Earth. He did not
grasp the
cosmological
implications of this
fact, and indeed at
the time it was
highly
controversial
whether or not
these nebulae
were "island
universes" outside
our Milky Way.[14]
[15] Ten years
later, Alexander
Friedmann
, a Russian
cosmologist and
mathematician,
derived the
Friedmann
equations
from Albert
Einstein's
equations of
general relativity,
showing that the
Universe might be
expanding in
contrast to the
static Universe
model advocated
by Einstein at that
time.[16] In 1924,
Edwin Hubble's
measurement of
the great distance
to the nearest
spiral nebulae
showed that
these systems
were indeed other
galaxies.
Independently
deriving
Friedmann's
equations in 1927,
Georges Lemaître,
a Belgian physicist
and Roman
Catholic priest,
proposed that the
inferred recession
of the nebulae
was due to the
expansion of the
Universe.[17]
In 1931 Lemaître
went further and
suggested that
the evident
expansion in
forward time
required that the
Universe
contracted
backwards in time,
and would
continue to do so
until it could
contract no
further, bringing all
the mass of the
Universe into a
single point, a
"primeval atom"
where and when
the fabric of time
and space comes
into existence.[18]
Starting in 1924,
Hubble
painstakingly
developed a series
of distance
indicators, the
forerunner of the
cosmic distance
ladder
, using the 100-
inch (2,500 mm)
Hooker telescope
at Mount Wilson
Observatory
. This allowed him
to estimate
distances to
galaxies whose
redshifts had
already been
measured, mostly
by Slipher. In 1929,
Hubble discovered
a correlation
between distance
and recession
velocity —now
known as Hubble's
law.[7][19]
Lemaître had
already shown
that this was
expected, given
the Cosmological
Principle

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