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With the discovery in the early 20th century that spiral-shaped nebula were, in fact, other galaxies
external to our own, our concept of a Universe became one of in a
Newtonian universe of infinite size and mass, galaxies spread out in
infinite space. However, there is a problem with a uniform, static
Universe, any density enhancements would become unstable to gravitational collapse. Thus,
the whole Universe should have collapsed (or be collapsing) into a giant
black hole.
In the 1930's, Edwin Hubble discovered that all galaxies have a positive
redshift. In other words, all
galaxies were receding from the Milky Way. By the Copernican principle
(we are not at a special place in the Universe), we deduce that all
galaxies are receding from each other, or we live in a dynamic, expanding Universe.
This solves the problem for gravitational collapse, only small regions
will collapse to form galaxies. The rest of space keeps expanding.
- an expanding Universe must have a Creation or Alpha point, and is a dynamic reality
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The expansion of the Universe is described by a very simple equation
called Hubble's law; the  velocity of the recession of a galaxy (determined
from its redshift, see below) is equal to a constant times its distance
(v=Hd). Where the constant is called Hubble's constant and
relates distance to velocity in units of megaparsecs (millions of
parsecs).
The velocity of a galaxy is measured by the Doppler effect, the fact
that light emitted from a source is shifted in wavelength by the motion of
the source. The change in wavelength, with respect to the source at rest, is called the
redshift (if moving away, blueshift if moving towards the observer) and is denoted by the
letter z. Redshift, z, is proportional to the velocity of the galaxy divided by the speed of
light. Since all galaxies display a redshift, i.e. moving away from us, this is referred to as
recession velocity. |
- the search for Hubble's constant consumed most of our technology in the late 20th century
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As a result, distance scale work uses a chain of distance indicators working outward from
nearby stars to star clusters in our own Galaxy to stars in nearby galaxies. Unusually bright
stars, such as variable stars and supernovae, complete the distance ladder out to cosmological
distances. The latest results from the Hubble Space Telescope are shown above, a plot of
recession velocity with distance (in megaparsecs, millions of light-years). The straight,
linear correlation indicates that the Universe is currently expanding at a rate of 72 km per
sec for every Mpc. The rate, known as Hubble's constant, may change with time (see next
lecture). |
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