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Cosmology
• The study of the universe,
its current nature, its origin,
and evolution
Origins of the
Universe
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The Big Bang Theory
Big Bang Theory
• Expansion indicates a denser, hotter past –
uniform, hot gas that cools as it expands
• NOT an explosion from a point, but a
simultaneous expansion from all places
• 13.7 billion years ago
Big Bang
• EVIDENCE –
1. Cosmic microwave
background radiation – “noise,”
same in all directions
• Scientists sense energy from the
explosion, and it is everywhere in
space.
• 1965 Arno Penzias and Robert
Wilson
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Doppler Effect:
Big Bang
• EVIDENCE –
• Red Shift:
2. Expansion measured by Hubble
constant
– slower, moving away
• Distant galaxies are redshifted
(based on electromagnetic
spectrum)
– Wavelength is longer thus
the color appears red.
–This is the doppler effect being
applied to light
• 1929 Edwin Hubble
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Doppler Effect:
Big Bang
• EVIDENCE –
• Blue Shift:
3. most elements in universe are
H and He (light elements)
– faster, moving toward
• Created during first moments of
Big Bang
–Wavelength is shorter,
thus the color appears
blue.
• when stars reach the end of their
life cycle, they distribute C, O, and
Fe into the universe
• if the universe was older, there
would be more C, O, and Fe
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Age of the materials
Expanding Universe
• All distant galaxies are redshifted. What
does this mean about their distance from
us?
• Hubble’s Law – further away a galaxy is,
the faster it is speeding away
• If we can determine how fast the universe
is expanding (aka Hubble constant), we
can determine the AGE OF THE
UNIVERSE!
• Oldest material should be
furthest away.
• Quasars at the edge of the
universe are 12 billion lightyears away.
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Beginnings
http://www.youtube.com/watch?v=hVApTLE7Csc
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Steady State Theory
Types of
Universes
• The universe looks the same
to all observers, everywhere,
and it has always looked that
way.
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Closed Universe
Open Universe
• Universe will expand
forever
• At some point the
universe will stop
expanding and collapse
inward (continuous)
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Visible Light
• Telescopes, spectroscopes
• (ROY G BIV)
How we study the universe
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Invisible Light
• Radio,
• UV,
• Infrared,
• X-Ray,
• and Space telescopes.
• These measure energy waves beyond
the Visible Light Spectrum
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A little drama
Astronomy
• Characters in the great drama
A Historical
Perspective
– Claudius Ptolemy (140)
– Aristotle (350 BC)
– Nicolaus Copernicus (1473-1543)
– Tycho Brahe (1546-1601)
– Johannes Kepler (1571-1630)
– Galileo Galilei (1564-1642)
– Isaac Newton (1642-1727)
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• The Earth is spherical.
• It seems natural that our
first hypothesis regarding
the “structure” of the
Solar System would be
geocentric.
Aristotle’s shoes
Then came
Ptolemy
• A good theory should explain what is
observed and be able to make
predictions.
• Planets move in circles called
epicycles.
• The center of the epicycle moves in
a circle called a deferent.
• To make theory match prediction,
Earth isn’t exactly at the center of
the deferent.
• Being more philosophical
and less empirical, we
would hope to see
harmony and perfection in
the heavens that fit our
philosophy--thus, the
motions of bodies are
perfect circles.
• The test of all knowledge is
measurement
– Ptolemy’s theory explained the
retrograde motion of the planets
– Predicted future locations of the
planets
Geocentric Model
• C. 100-170: Ptolemy places Earth
at the center of the universe with
everything (including the sun and
the seven planets) revolving around
Earth.
Observed throughout a year, the stars always
go in one direction, but not so for the planets.
• He believed the heavens were
perfect and that everything moved
in a perfect circle and at a constant
speed.
• His biggest challenge was
explaining the planet’s retrograde
motion and apparent changing
speeds.
1400 years later -heliocentric idea
• Copernicus, for philosophical reasons,
sought to explain the retrograde
motion of planets using a heliocentric
solar system. He still assumed perfect
circles for the orbits of planets (with
the Sun at the center of the orbits).
• He could calculate
– the relative distances to the planets
– the orbital periods of the planets
• Predictions of the future positions of
planets were not much better than
those from the Ptolemaic model
We need better
data!
• Tycho Brahe had
ideas for a new
model but
recognized the need
for more precise
measurements.
• He devoted his life
to making more
precise
measurements of
the positions of stars
and planets.
• He built the first
modern observatory
• He amassed records
of planetary
positions from 1576
to 1591
• His observations
were 2.5 times more
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Finding a needle in a haystack
Heliocentric vs Geocentric
Model
• Kepler believed the Copernican model and
sought to prove that it was correct using
Brahe’s data for the positions of the planets.
• He found that
– Planets orbit in elliptical paths (not circles!) with the Sun
at one focus of the ellipse.
– A line from the Sun to a planet will sweep out the same
area in a certain time interval, regardless of where the
planet is in its path.
– The ratio of the (period)2 to (semi-major axis)3 was the
same for every planet.
Finding a needle in a haystack
• He described the planets’ orbits, but could they be
explained? Kepler answered the “What?” , but didn’t
know “Why?”
Kepler’s Laws
1st Law: planet’s orbit the Sun in ellipses, with the Sun at
one focus.
2nd Law: The line joining the Sun and a planet sweeps out
equal areas in equal time intervals. As a result, planets
move fastest when they are near the Sun (perihelion) and
slowest when they are far from the Sun (aphelion).
3rd Law: The larger the orbit of a planet, the slower its
speed in orbit and the longer its orbital period .
Standing On The Shoulders Of Giants
• Isaac Newton formulated three laws of
motion and a law of gravitation.
• This model for understanding motion
(how motion is related to forces) and
gravitation explained Kepler’s three laws.
• When “Why?” matches “What?” (theory
matches observation), we must
reexamine our dearly held beliefs.
Standing On The Shoulders Of Giants
• This happened again in 1911 with
Einstein’s publication of the General
Theory of Relativity
– an entirely different explanation of
gravity
– explained phenomena that Newton’s
law of gravitation could not explain.
– has been verified by experiment to this
day
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