• Do your course evaluations.
• http://www.pa.uky.edu
• Only 10% of the class has completed the
evaluations so far.
• I will add 5 points on your final exam if you
complete the evaluation.
Cosmology: A brief history
• Ancient peoples thought the universe was much much
smaller than it really is.
• Many ancient cultures considered mountain tops to be
sacred ground. This was because the top of the
mountain is closer to the heavens.
• Ancient Egyptians considered the sky to be like a giant
tent suspended over head and held up by mountains at
the four corners of the Earth.
• The ancient Greeks considered the celestial sphere on
which the stars resided, to be anywhere from tens of
miles to thousands of miles overhead.
Water in ancient cosmologies
• The Mesopotamian civilizations of Sumer, Babylon,
Cannan, and Judea all had a common concept of the
cosmos. First, it was based on water as the
fundamental primordial substance.
• The water surrounded the Earth and was separated
from the Earth by the firmament.
Firmament
Water
Earth
• In this world view, the Earth was a disk and the
firmament rested at the edges of the Earth. It held
the waters of the universe out.
• The universe, of the Sun, Moon, planets and stars
was tiny and inside the firmament.
• This is the same firmament that God opened in order
to flood the Earth in the Noah story.
Educated guesses for the distance to the stars
• Ptolemy (90 – 168 AD) developed a mathematical model to
predict the locations of the planets in the sky. He assumed
the Earth was at the center of the universe. He estimated
from the speeds that the stars need to move to orbit the Earth
that the stars were about 50 million miles away.
• Archimedes (287 – 212 BC) uses the measurements of
Aristarchus to estimate the distance to the stars. He arrived
at 6 trillion miles. (1 light year)
Galileo (1564-1642 AD)
• There was little progress on cosmology until the
invention of the telescope. Up until this point in time
it was assumed that the stars were all equidistant
and resided on the celestial sphere. When Galileo
turned his telescope to the Milky Way he realized
this was wrong.
• “I have observed the nature and the material of the
Milky Way… The galaxy is, in fact, nothing but
congeries of innumerable stars grouped together in
clusters. Upon whatever part of it the telescope is
directed, a vast crowd of stars is immediately
presented to view. Many of them are rather large
and quite bright, while the number of smaller ones is
quite beyond calculation.”
New technologies
• Friedrich Bessel in 1838 made the first parallax
measurement of a star, 61 Cygni. This was the
first measurement showing the distance to
the stars is enormous.
• William Herschel (1738-1822) observed, by
eye through his telescope, many star clusters,
star forming regions, spiral nebula (galaxies)
and did star counts through his telescope to
map the Milky Way.
Herschel’s
40 foot
telescope
Herschel’s sketch of the Milky Way
Lord Rosse spiral nebula sketch in 1845
The spiral nebula debate
• Through the 1800s and into the 1900s, a
debate raged as to whether the spiral nebula
where star forming regions in the Milky Way
or if they were individual galaxies beyond the
Milky Way.
• The question was finally settled with data
from the new 100” Mt Wilson telescope
(Completed in 1919) and observations by
Edwin Hubble in 1925.
The Mount Wilson 100” Telescope
• Hubble’s observation of cepheid variable stars
(which have a known luminosity) in M31 and
several other spiral nebula, allowed him to
calculate the distance to these spiral nebula.
He found them to be millions of light years
away, well beyond the established size of the
Milky Way.
• This landmark discovery meant that all the
spiral nebula were individual galaxies and that
some must be hundreds of millions to billions
of light years away.
Relatively close by NGC 3370 and other more
distant galaxies.
NGC 7331 (50 million light years away) and other
background galaxies that are farther away.
Estimate the distance of the other galaxies in
the image
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1. About 100 million
light years
2. About 500 million
light years
3. About 5000 million
light years
4. There is no way to
tell
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What assumption did you make to estimate the
distance?
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1. I didn’t assume
anything, it is
impossible to tell.
2. I assumed that the
smaller galaxies were
dwarf galaxies
3. I assumed all the
galaxies were about
the same size.
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Starting in the 1980s large surveys of galaxies in the
universe began to reveal that the universe has
structure.
Filaments
Voids
Model of filaments and voids.
• Filaments are long strands of galaxy clusters
which are loosely bound by gravity and are
being stretched out by the expansion of the
universe.
• Voids are volumes of space where there are
very few galaxies. These regions have opened
up about 5 billion years ago as the universe
expanded. They continue to grow in size with
the expansion
Scaling the universe as we know it today.
• When we scaled the Milky Way galaxy, it was
necessary to shrink the entire solar system to the
width of a hair. Then the distance to the next
nearest star system, Alpha Centauri, was one meter.
The size of the Milky Way on this scale is the size of
Lexington, with a thickness of about 700 feet. M31,
the Andromeda galaxy on this scale is at about, St.
Louis.
• Now we scale the observable universe. To do this we
need to shrink the distance from the Earth to Alpha
Centauri to 1 millimeter.
• On this scale the Milky Way is about the size of
Yankee Stadium in New York.
• On this scale, the most distant galaxies ever observed
are at the distance of the Rose Bowl in Pasadena,
California.
• Remember, the distance from the Sun to Alpha
Centauri is the size of the head of a pin.
• The solar system is 10,000 times smaller than the
head of a bin, and the Earth is 30 billion times
smaller than the head of a pin.
• In about 2500 years human understanding of the size
of the universe went from a little bigger than the
Earth to 120 quintillion times the size of the Earth.
(1.2 x 1020) that’s 120 billion billion times size of the
Earth.
Back to Einstein and his biggest blunder
• Einstein published his theory of general relativity in 1915.
This theory not only described how gravity worked it
made prediction about phenomena, such as gravitational
lensing, which had never been observed. The theory also
allowed the calculation of the space-time curvature of
the entire universe.
• When Einstein attempted to do this, he found that his
theory predicted that the universe could not be static. It
had to be either expanding or contracting.
• In 1915 it was accepted by the scientific community that
the universe was infinite in extent and infinitely old. It
was also accepted that the universe was static, and
unchanging.
• Einstein had the ability to predict, just from
calculations using his general theory of relativity, that
the universe was either expanding or contracting.
• For the only time in Einstein’s career he was unable
to trust his own theory to this extent.
• He postulated an additional force, which he called
the cosmological constant, which he added to his
equations in order to make his theory predict a
static, unchanging universe.
• Einstein was wrong.
Edwin Hubble’s next project.
• Having established in 1925 that galaxies were not in
the Milky Way, Hubble used the Mt Wilson telescope
to obtain spectra of galaxies.
• When he analyzed the spectra he found that, with
the exception of the galaxies in our local group, all
other galaxies where moving away from us. He
found this using the Doppler shift of absorption lines
in the galaxy spectra.
• Not only were all the galaxies found to be moving
away, the farther away a galaxy was, the faster it was
moving away.
• In order to establish the relation between distance
and recessional velocity, Hubble needed to
determine the distance to the galaxies. He did this
by assuming all the galaxies he observed were of
similar size, and figured out their relative distance
from their apparent size.
• He had already calculated the actual distance to
several nearby galaxies, so this allowed him to find
the true distance to the galaxies that were farther
away.
• Here is a plot from his 1929 publication.
Recessional velocity verses Distance.
Modern Day Hubble plots
What is the equation for a straight line?
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1. y = ax2
2. y = 1/x + b
3. y = mx + b
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• So a straight line has the equation of
• y = mx + b
• In the Hubble plot the y-axis is velocity (v) and
the x-axis is distance (d)
• So we can write the equation as
• v = md + b
• What are m and b?
What are m and b?
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1. The slope and yintercept
2. The slope and the
rise over run
3. The x-intercept and
the y-intercept
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Modern Day Hubble plots
The Hubble Constant – The slope of the line
• The Hubble constant is a very fundamental quantity,
which tells us the age of the universe.
• Today we see that the universe is expanding, and it is
growing larger every day. This means that the
distance between galaxies are growing in size.
• What would happen if we could run the clock in
reverse? Move backward in time.
• The galaxies would get closer together, with the most
distant galaxies moving toward us at the greatest
speed. The result is that all galaxies would arrive at
the same point, at the same time.
• The equation of a straight line for the Hubble plot
looks like this, with b = 0
•
v = Hod
velocity = Ho times distance
• What does our normal velocity equation say?
• v = d/t
velocity = distance divided by time
• Or
• v = (1/t)d compared to v = (Ho)d
• So Ho tells us the time it takes for all the galaxies in the
universe to collapse to the same point. This is the age of
the universe.
• Ho = 1/tage
• Or
• tage = 1/Ho
Notice: Ho has units of km/s/Mpc
Today the best value is Ho = 71 km/s/Mpc
• We can use this value to compute the age of the
universe. We first need consistent distance units.
• 1 Mpc = 3.1 x 1019 km
• So Ho = (71km/s/Mpc)(1 Mpc/3.1 x 1019 km)
• Ho = 2.3 x 10-18 (1/seconds)
• tage = 1/Ho = 1/2.3 x 10-18 (1/seconds)
• tage = 4.37 x 1017 seconds
• There are 3.14 x 107 seconds in a year. So dividing
we get:
• tage = 13.7 x 109 years
• tage = 13.7 billion years
• The age of 13.7 billion years is consistent with
the age of the oldest known stars, which are
found to be between 12 and 14 billion years
old.
• Note that star age is based off of stellar
physics and has nothing to do, whatsoever
with the calculation of the age of the universe
using the Hubble constant.
Quiz 11
• Explain why we see many high luminosity
quasars at extremely large distances but none
in our local area of the universe.