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An Introduction to Astronomy
Part XIV: Cosmology
Lambert E. Murray, Ph.D.
Professor of Physics
Cosmology
 Cosmology
is the study of the structure and
evolution of the Universe on its grandest
scale. In the study of cosmology we wish to
answer question like:
– What do our observations tell us, if anything,
about the size and geometry of the Universe?
– What do our observations tell us, if anything,
about how the Universe came into existence?
– What do our observations tell us, if anything,
about the future of the universe?
What Have we Learned about the
Nature of the Universe so far?
 There
seems to be an infinite number of
galaxies (of various types). No matter the
direction you look, as we build bigger and
bigger telescopes, you see more and more
galaxies.
They are Everywhere
Galactic Clusters and
Superclusters

There are enormous distances between galaxies,
but as we look deeper and deeper into our
Universe we find that galaxies are grouped
together by gravitational attraction.
 If we plot the location of galaxies, we begin to see
large-scale structure within the Universe.
 We find that although there are small scale
fluctuations within the universe – we can think of
the universe as rather homogeneous and isotropic
on the very largest scale.
Structure Within the Universe
Computer
Model
of
Universe
What Else Have we Learned about
the Nature of the Universe so far?

Hubble’s work indicates that the farther
away a galaxy is, the faster it moves away
from us. This is true in every direction we
look.
The Hubble Law
Are we in a “Special” Place?
The Cosmological Principle




Since we see a large number of galaxies in all directions,
and these are all moving away from us are we at the center
of the universe (a very special place)?
A fundamental assumption in the study of cosmology is
that we are not located in a unique region within the
universe – this is the “Copernican Principle”.
In addition, we assume that the universe at the largest scale
is isotropic and homogeneous, i.e., that the universe “looks
the same in all directions” and that this is true no matter
where you are. This is the “cosmological principle”.
This principle assumes that our location within the
universe is characteristic of any other location within the
universe and is the only assumption that will allow us to
make any progress in understanding our Universe as a
whole.
An Expanding Universe

If we are not located at a special spot in the
Universe, and
 If we would expect to obtain the same Hubble plot
of the recessional speeds of galaxies from any
arbitrary location within the Universe,
 This means that the distance between any two
galaxies (on average) is increasing all over the
Universe.
 Thus, the Universe is expanding – like a raisin
cake or an expanding balloon.
Is the Universe Infinite?
Olbers’ Paradox

Olbers’ paradox: If our universe is infinite in time
and space, and if we see galaxies wherever we
look, why is the sky dark?
– Although the light intensity drops off as 1/R2, the
number of stars you can see in an infinite universe
increases like R2 so these two effects cancel.
– Likewise, if the universe is filled with dark material
which would absorb the light, that material would heat
up and eventually glow – especially if there were an
infinite number of stars.

Conclusion: The universe must not be infinite in
space and time!
The Big Bang

Initially many astronomers believed that the
Universe was static and infinite. Olbers’ paradox
raised serious questions about this assumption.
 Hubble’s observations of an expanding Universe
seems to indicate that the Universe had a
beginning in time – and perhaps in space.
The “Hubble” Age of The Universe



Hubble’s Law can be written as velocity of recession =
(constant) x distance
– the “constant” is Hubble’s Constant, H0
– We believe that Hubble’s constant is “constant” over all
the universe at any instant of time, but it may actually
change over time.
Running the expansion backwards
– Since H0 = velocity/distance,
1/H0 = distance/velocity = time
– Thus, 1/H0 is a measure of the age of the universe.
Using a value for H0 of 50 km/sec/Mpc (kilometers per second
per megaparsec) leads to an age of about 19.7 x 1010 years, or
roughly 20 billion years, while using a value of 100
km/sec/Mpc leads to an age of only about 10 billion years.
A Correction to the “Hubble”
Age of the Universe



Most astronomers believe that Hubble’s “constant”
has changed with time. Gravity should be “slowing”
the expansion of the Universe, causing a deceleration.
Thus, the initial expansion should have been faster,
leading to a younger universe – one with an age of
about 2/3 of the age determined from the present value
of Hubble’s constant.
Astronomers are using several different techniques to
determine an accurate value of Hubble’s constant so
that we can get a handle of the actual age of the
Universe.
Measurements of Hubble’s
Constant
 Hubble’s
initial measurements of H0 was 550
km/sec/Mpc, but by the 1990’s the most frequently
quoted values were 50 – 70 km/sec/Mpc.
– Hubble’s measurements did not take into account
several effects now known to astronomers that would
have influenced his measurements.

2/3 of the Hubble age calculated from these values
would give 9 – 13 billion years for the
approximate age of the Universe.
An Alternate Age Measurement
 An
alternate method of determining the age of
the Universe is to determine the age of objects
within the Universe, since the Universe cannot
be younger than the oldest objects in the
Universe.
 We can get an estimate of the age of globular
clusters based upon their HR diagrams –
observing their turning points.
HR Diagram
for M13
Additional Evidence for the Big Bang?

Cosmic Microwave Background
– In the 1940’s Russian physicist George Gamov worked
out a theory for the creation of elements (a
nucleosynthesis) in a big-bang type event.
– He predicted that copious amounts of radiation would
be emitted from the hot gas of this Big Bang with a
black-body spectrum of about 3000K when the
Universe became transparent to photons. (Prior to this
the photons were “trapped” by the non-transparent
universe.)
– Later it was realized that this radiation would have been
red-shifted over time and would look like emissions
from a very cool gas.
– In the mid-60’s Robert Dicke at Princeton began
building a receiver to detect this radiation.
Penzias’ & Wilson’s Microwave
Experiment


1964-65 Bell Labs scientists Arno Penzias & Robert
Wilson were working on a microwave horn antenna
for satellite communication.
They were troubled by a persistent background noise.
After trying everything to eliminate the background:
– recalibrated antenna
–
–
–
–
cooled their detectors
removed nesting pigeons from horn
cleaned antenna
could not eliminate the static
they found that the background persisted.
A Cosmic Background?

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
The peak in the background radiation from Penzias’ and
Wilson’s experiment occurred at 7.35 cm (4080 MHz)
They found that the background was constant regardless
of time of day, season of year, direction in the sky, etc.
When they learned of Dicke’s work, they realized that
they might be seeing this residual radiation remnant
from the Big-Bang now called the Cosmic Microwave
Background Radiation.
If a blackbody curve is assumed, the 7.35 cm peak
corresponds to a background temperature of roughly 3.5
Kelvin.
1976 Penzias & Wilson received the Nobel Prize for
their work.
Experimental Measurements of the
Cosmic Background Radiation

Very little of the 3 K background radiation can
penetrate the atmosphere:
– This made it difficult to measure radiation curve
– Early attempts used balloons, rockets, aircraft, etc.

In 1989 NASA launched the Cosmic Background
Explorer (COBE) which orbited the Earth.
 Within two months it was determined that this
background followed (within 1%) a blackbody
radiation curve corresponding to a temperature of
2.735 Kelvin (within 0.06K).
Blackbody Spectrum of COBE Satellite and fit to a 2.73 K Curve
Variations in the Background
Radiation Pattern
 COBE data is accurate enough to measure the
motion of the earth relative to the background
radiation by analyzing variations in the data to 1
part in 10,000. This is shown in the next image.
 A more detailed analysis, with the dipole effect of
the Earth’s motion subtracted out, shows small
scale variations in the background radiation data.
COBE Dipole: Speeding Through the Universe
Credit: NASA, COBE, DMR, Four-Year Sky Map
Astronomy Picture of the Day, February 5, 1996
A map of the brightness of the cosmic microwave background made by the
cosmic background explorer (COBE) satellite.
Notice the patchiness of the brightness. Each pink patch may represent a
"lump" of matter from which groups of galaxies ultimately grew. The patches
were approximately one half billion light years across when they emitted the
radiation. (NASA GSFC and the COBE Science Working Group.)
Confidence in the Big Bang
 There
seems little doubt that the Universe as
we know it is expanding, and that it was the
result of a Big Bang.
 Small fluctuations in the cosmic
background radiation are consistent with
our observations of the grouping of galaxies
within the Universe.
Recent Confirmation
 The
excellent agreement between the
current theories of cosmology and the most
recent data from Cosmic Background
Radiation (WMAP) indicate that:
– The age of the Universe is 13.7 billion years to
within 1%.
– That the Universe is flat and that the visible
universe makes up only 4% of the Universe.
WMAP Composition of the
Universe
Theoretical Models for the
Expansion of the Universe
 Until
recently, precise, unambiguous,
experimental measurements of H0 with
distance did not exist.
 Theory indicates that there are three basic
models for our Universe:
– Closed Universe (WM > 1)
– Flat Universe (WM = 1)
– Open Universe (WM < 1)
Diagram of Theoretical Models
for the Expansion of the Universe
The Future in Different Models
In a “closed” universe, the recessional speed of
galaxies would decrease to zero and that universe
would begin to collapse as gravity overcame the
initial expansion from the Big Bang. This
universe would eventually collapse completely –
or perhaps oscillate.
 In a “flat” universe, the recessional speed
continues to decrease and approach zero only as
time approaches infinity.
 In an “open” universe, the recessional speed
decreases, but never reaches zero.

A Surprise

Recent measurements of large-z Type Ia
Supernovae indicate that the recession rate follows
none of the previous patterns, but actually are
consistent with an “accelerating” universe which
would require some sort of “anti-gravity”
repulsive force which is now associated with
Einstein’s cosmological constant L.
 Many suggest that the source of this “anti-gravity”
is a “vacuum energy” or “dark energy” due to
quantum fluctuation.
End of Part XIV