Download Before people could understand the history of the universe, they had

4,000 Years of Cosmology
(The Abbreviated Version)
What is Cosmology?
The study of the
Universe as a whole
From the start of
time until now...
And until the end
of time.
Before people could understand the history of the Universe,
they had to know its geography...
Thales
• 6th century BC, philosopher and
traveler
• brought back to Greece
knowledge and records of the
Babylonians and Egyptians
• first ideas that lay between the
mythologies and true science
• Note: he believed the Earth was flat and
floated on water like a log
Aristotle
 lived from 384 to 322 BC
 3 experimental proofs
that the Earth was round
 Believed the Earth was
the center of the
Universe and the Sun,
Moon, planets, and a
sphere containing all the
"fixed" stars revolved
around it
What’s Popular is Not Always Right,
and What’s Right is Not Always Popular
 Most Greeks believed
Aristotle and his ideas
 The exception (a century
later) ... Aristarchus, who
thought the Earth moved
around the Sun
- he found very few
supporters for his
ideas
- often overlooked in the
history of heliocentric
ideas
What’s Popular is Not Always Right, and
What’s Right is Not Always Popular
Hipparchus
 100s BC, compiled a star catalog
 giving a magnitude or comparative brightness
scale
 calculated the Moon's distance from the Earth
Claudius Ptolemaeus
(better known as Ptolemy of Alexandria)
 came along 300 years later
 first astronomer to make truly scientific
maps of the heavens
The Ideas of Ptolemy
• Believed that the Sun, Moon, and planets circled the Earth
• Planets sometimes appear to travel backward?
-each moved in a small circle (epicycle)
that was orbiting Earth
• Summarized the theory in
the Almagest
-written in the 140 AD
-theory was believed for
about 1500 years
Copernican Theory

Polish monk Nicolaus
Copernicus says “Sun is the
center.”

The Church, however, says
Earth was central
– Copernicus' theory was
heresy!
– He did not publish it until
the day he died
(On the Revolution of the
Celestial Spheres)
Two events made the "Copernican revolution" inevitable:
• Brahe's precise observations of the sky
• Galileo's use of the telescope.
Tycho Brahe
• Danish astronomer
• In 1572, saw brilliant “new
star” in the sky in the
constellation Cassiopeia;
supernova!
• In 1604, a second supernova was observed
• These discoveries undermined the cornerstone
of the Ptolemaic system, i.e., outermost sphere
contained all the stars and it was unchanging
New “Eyes” on the Skies
• In 1609, Italian scientist Galileo Galilei
heard about the spyglass, made one for
himself, and turned it on the heavens
• One of the first discoveries was four moons
circling Jupiter
• He revealed a miniature version of
Copernicus' solar system, with the
moons moving around the planet in simple,
circular orbits
• The Catholic Church later forced Galileo to
recant his ideas and live out his life
under house arrest, but his discoveries had
already changed the face of astronomy
Saving Face
• British genius Isaac Newton unwittingly gave the Church a "way
out”
• Newton put mathematics to observations
- the equations to describe the relationships
observed for motion, and by gravity
- applied his equations to Kepler's
observations (it fit almost perfectly)
• The conclusions were indisputable: Copernicus had been right
all along (actually Aristarchus had been!)
Saving Face
• It is the force of gravity which keeps all the planets
in their elliptical orbits around the Sun
• Newton's Laws say that nothing is naturally at rest;
all heavenly bodies should be constantly moving, with
no limits on space and time
• Newton believed that the Universe was eternal and
infinite; this made the Church happy 
• It was an uneasy reconciliation, but one that allowed
science to move forward unimpeded
Big Bang
• In 1917, Albert Einstein proposed a description of the
Universe based on his Theory of General Relativity
• In Belgium, Georges Lemaitre came up with his own
model when he heard that the Universe was expanding
• Universe began as a “primeval atom”
• Hot and dense
• Explosion caused expansion to start
• Lemaitre can be called the “father of the big bang”
Note: He was an ordained priest before
turning to cosmology.
Big Bang; As We Now Know It
• George Gamow, a Russian-American physicist (late
1940s), conceived of Big Bang we know today
• He and his colleagues proposed
- elements created in very brief period early in
expansion of Universe
- predicted an afterglow, still should be present
arising from hot, early phase
- largely forgotten until afterglow was
discovered in the 1960s
Note: Gamow was also a popularizer of science,
wrote poetry, and did research in genetics
George Gamow
1904 - 1968
Hey Look at Me… I’m Expanding!
• Edwin Hubble in 1920’s; major breakthrough in understanding of
Universe
• For centuries, astronomers believed Milky Way entire Universe
• In 1923, Hubble was among the first to
show that the fuzzy patches they saw in
the sky through their telescopes were
whole other galaxies, not parts of the
Milky Way
What Hubble Saw
In 1929, by looking at the spectra of these galaxies, he
concluded that they were speeding away from us - the
Universe was expanding!
QuickTime™ and a
GIF dec ompres sor
are needed to s ee this pic ture.
Edwin Hubble
1889 - 1953
Note: Before turning to astronomy, Hubble was a lawyer.
Why Do We Believe the Big
Bang Theory?
Observations, Observations, Observations!
• Expansion of the Universe
• Origin of the cosmic background radiation
• Nucleosynthesis of the light elements
• Formation of galaxies and large-scale structure
Other Theories
• Steady State Theory
• Chronometric Theory incompatible with CMB observations,
incorrect source counts, predicts
redshift is quadratic function of distance
• Tired Light Theory no known mechanism to degrade photon
energy, contradicts high redshift supernova
observations
Steady State Theory
• Universe had a beginning? Not appealing to all
scientists
• In 1948, Fred Hoyle, Hermann Bondi, and Tommy Gold,
came up with Steady State Theory
- Universe has no beginning and no end
-Although expanding, it stays in perfect balance like a pool kept full to overflowing by a trickle
from a faucet
-The "faucet" is the continuous creation of matter
from energy
Steady State Theory (cont)
• They
showed that only one atom per cubic mile of
space per hour was needed
• Even this amount proved to be the theory's undoing
- It defied the laws of physics; It did not match
observations
Evidence Against the Steady State Theory
• Background cosmic heat radiation discovered
• Amount of helium observed in the Universe exactly fit
what was predicted by the big bang
• Radio astronomers found that galaxies were more
crowded together in the past
- meaning the idea of an unchanging universe
was incorrect
Basking in The Afterglow
• In 1963/1965, physicists Arno Penzias and Robert Wilson started
to search for faint radio signals (actually microwaves) from the
outskirts of the Milky Way Galaxy
- famous antenna located in Holmdel, New Jersey
Penzias and Wilson
•Bedeviled by a persistent interference (a constant signal from
all over the sky) equivalent to temperature of - 454 °F (-270 °C
or 3 Kelvin)
• They thought the signal was from pigeon droppings in the
telescope, but colleagues soon realized that they had found
the afterglow predicted by Gamow
Inflation Solves Some
Big Bang Problems
• In 1979, particle physicist Alan Guth
- Stanford Linear Accelerator Center
- interested in the Grand Unification Theory of Forces
- calculations led to the idea of "cosmic inflation” (brief
period of rapid expansion in the early Universe)
• It explains why the Universe is so big and so smooth, why (at
least) four different forces act in it today, and where the vast
amounts of matter came from.
Accelerating Expansion?
•
In 1998, Science magazine says "breakthrough of year"
Universe is expanding at an
accelerating rate!
• For over ten years astronomers studying expansion of
Universe by measuring:
- Redshifts
- Brightness of Type Ia supernovae
Accelerating Expansion
• Note: Type Ia supernovae are stars that explode in
thermonuclear cataclysms brighter than entire galaxies
• Ideal "standard candles" to survey the Universe
- Their light curves and spectra are all nearly alike and
they are bright enough to be seen across billions of light-years
• By 1998, many of these supernovae analyzed in detail, leading
scientists to conclude that the expansion of the Universe is not
slowing, but accelerating!
HST
Observations
of Distant
Supernova
What Drives the Acceleration?
• The supernova data strongly suggest
that Universe is filled with an unidentified
form of energy which is the cause of the
accelerating expansion
• This is called "Dark Energy"