Cosmology

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Cosmology
Understanding the origin and evolution of the
Universe is the ultimate goal of astronomy
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Red Shifts of Galaxies The Expanding Universe
Using the 100-inch telescope on Mt. Wilson,
California, Edwin Hubble took spectra of galaxies
in order to measure their radial velocities using
the Doppler Effect. In 1929, Hubble announced the
discovery that nearly all galaxies are moving
away from us with speeds that increase with the
distance of a galaxy. This discovery showed that
the Universe is expanding. Because the galaxies
are receding from us, their spectra are shifted
to longer wavelengths, hence the term red shift.
X
X
X
X
Recessional Velocity
X
Hubbles Law Recessional Velocity Ho x
distance
X
X
Distance to galaxy
The rate of expansion, which is given by the
slope of the curve shown at right, has been very
challenging to measure, but observations with the
Hubble Space Telescope now indicate that the
Hubble Constant, Ho, is 72 km/s/Mpc.
Hubble showing Sir James Jeans the Mt. Wilson
100-inch telescope
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The Raisin Bread Analogy
C
Now, let the dough rise for a time Dt so that
all dimensions expand by a factor of two
E
A
F
C
E
F
A
B
D
B
D
D
2xD
Distances between raisin pairs Pair
Separation/D AB 1 AC
1/6 AD 1/2 AF
1/3 AE 2/3
New Separation/D 2 2/6
2/2 2/3 4/3
Speed (DD/Dt) 1 1/6
1/2 1/3 2/3
1
AB
AE
Speed
AD
AF
The speed vs. separation curve is a straight
line, no matter which raisin is labeled as
A. This means there is no center of the
Universe.
AC
0
0
1
Separation
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Discovering the Cosmic Microwave Background
In 1964, physicists Arno Penzias and Robert
Wilson used a horn antenna constructed by Bell
Labs in Holmdel, NJ for the purpose of exploring
sources of noise in microwave transmissions to
and from satellites. They discovered a source of
noise that appeared to come from the background
sky. At the same time, and less than 50 miles
away, Princeton University scientists were
deliberately looking for a background noise
predicted from the Big Bang model for the
formation of the Universe. It was quickly
realized that Penzias and Wilson had discovered
just what the Princeton group was looking for.
Penzias and Wilson won the 1978 Nobel Prize in
physics for this discovery.
Pigeon trap
A. Penzias
R. Wilson
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The Spectrum of the Cosmic Microwave Background
Penzias and Wilson found that the spectrum of
this CMB radiation was precisely fit by a
blackbody with a temperature of 3 K (3o above
absolute zero temperature). This very cold
spectrum is exactly what is expected from the
effect the expanding Universe has on redshifting
light from the instant that matter and radiation
decoupled when the Universe had a temperature of
about 3,000 K. As the Universe continues to
expand, the temperature corresponding to the CMB
spectrum will continue to decrease. The spectrum
at right shows how well data from a variety of
sources fit a blackbody curve. The discovery of
the CMB radiation is accepted as proof of the Big
Bang.
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Fluctuations in the Cosmic Microwave Background
Two NASA spacecraft, the Cosmic Background
Explorer (COBE 1992) and the Wilkinson Microwave
Anisotropy Probe (WMAP 2002) were designed to
look for fluctuations in the CMB that are
predicted by cosmology. The results from WMAP are
strongly supportive of a period of inflation in
the early Universe and a Universe with
accelerating expansion, i.e. a Universe that will
expand forever. The WMAP data originate from when
the Universe was 380,000 years old.
WMAP heads out for the L2 point
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The Expansion of the Universe
Raisin bread analogy shows that every location in
the Universe sees the same expansion law, i.e.
there is no center to the Universe. Taken alone,
expansion is evidence in support of but not proof
of the Big Bang. The age of the Universe can be
estimated by calculating Age 1/Ho, which gives
Age 13.6 billion years. Modern observations of
supernova in very distant galaxies show that the
expansion at great distances, i.e. looking back
at the early Universe, shows that the expansion
is accelerating. From a variety of observations,
the Universe appears to be composed of 3
visible matter (stars, gas, etc.) 30 dark
matter (from galaxy rotation curves, clusters,
etc.) 67 dark energy (which Einstein called the
Cosmological Constant) The Universe will expand
forever.
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The Timescale of the Universe
T 0 Big Bang Occurs T 10-33 seconds
Universal Inflation Causes Universe to expand by
a factor of 1030 T 100 seconds Light
elements (Hydrogen through Boron) formed T
3x105 years Radiation and matter decouple
(origin of CMBR) T 108 years First quasars
form T 109 years First galaxies form T
14x109 years The Present Time T 20x109 years
Sun will evolve to a white dwarf T 1012
years All stars are burned out T 1020
years all stars will have collided with other
stars disrupting all galaxies T 1040 years
All protons will have decayed into radiation and
smaller particles T 10100 years All black
holes will have evaporated
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The Hubble Deep Field
This is a small patch of the sky, about 1/50th
the size of the full moon, showing the faintest
objects ever detected. This image was produced by
the Hubble Space Telescope in 1995 from 276
separate exposures obtained over a 10-day period.
Only one or two stars are shown here all the
rest are very distant galaxies and protogalaxies.
(Hubble Space Telescope Image)