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Science 3210 001 : Introduction to Astronomy
Lecture 13 : Cosmology, Interstellar Travel, and
the Search for Life in the Universe
Robert Fisher
Items
 Nathan Hearn guest lecture. Lunch at Frontera Fresco and the
Macy’s food court was amazing.
 Adler Planetarium field trip next week on May 4th. $16/person -to be collected today. Waiver forms to be signed today.
 Final projects due May 11th, along with a short (5-10 minute)
presentation that day. Important : If for whatever reason you are
unable to attend on May 11th, you MUST get your final project to
me before then.
 Return midterms and homeworks.
Final Project
 Your final project is to construct a creative interpretation a scientific
theme we encountered during the class. You will present your work in a
five minute presentation in front of the entire class on May 11.
 The project must have both a scientific component and a creative one.
 For instance, a Jackson Pollock-lookalike painting would fly, but ONLY if
you said that it was your interpretation of the big bang cosmological
model AND you could also demonstrate mastery of the basic
astrophysics of the big bang while presenting your work.
 Be prepared to be grilled!
 Ideas :
 Mount your camera on a tripod and shoot star trails.
 Create a “harmony of the worlds” soundtrack for the Upsilon Andromeda
system.
 Paint the night sky as viewed from an observer about to fall behind the
horizon of a black hole.
 Write a short science fiction story about the discovery of intelligent life in the
universe.
Major Astronomical News Item of the Week -Discovery of Gliese 581c
 Gliese 581c is the smallest extrasolar planet discovered to date, at about
5 times the mass of the Earth.
 It is believed based on models (not yet demonstrated by direct
observation) that this could very well be a massive Earthlike rocky
planet, unlike the gaseous giants discovered to date.
 What is more, even though the planet orbits Gliese 581 at a very close
distance (.07 AU), the star is a red dwarf of about a third the mass of our
sun, and so is far dimmer -- about 1/100 the luminosity.
 Because the intensity of light falls off with the square of the distance, it
turns out that the effective intensity at Gliese 581 is roughly the same as
that of the sun on the Earth -- it is possible that it Gliese 581c has a
surface temperature which can support liquid water.
Two Weeks Ago
 Black Holes, White Holes, Wormholes
 Galaxies
 Distances in the universe
 Types of galaxies
 Ellipticals
 Spirals
 Irregulars
Last Week
 Cosmology
 Why is the night sky dark?
 Newtonian Cosmology
 Einstein, Hubble and the Expansion of the Universe
Today
 Cosmology
 Cosmic Microwave Background
 Clusters of Galaxies, Superclusters
 Quasars
 Interstellar Travel
 The Search for Life in the Universe
Cosmic Microwave Background
 During the post-WWII era,
physics and astronomy grew
rapidly, fueled by discoveries
made during WWII.
 After taking their measured
signal and subtracting out all
known sources of noise (the sky
and the ground, the galaxy, and
the telescope itself), they were
still left with a background signal.
 In 1965, Robert Wilson and Arno
Penzias were conducting
observations of the Milky Way
galaxy using a radio telescope at  This background signal persisted
Bell Laboratories.
regardless of where they looked
on the sky.
 When conducting their
observations, they needed to
separate out the signal (the
microwave emission of the
galaxy) from the noise generated
by other sources.
Blackbody Radiation : A Review
 Each and every body in the universe radiates a characteristic
thermal spectrum which depends only upon its temperature.
 As a consequence, for instance, the interior of a kiln heated to a
white-hot uniform temperature, each surface glows with the same
characteristic spectrum, regardless of its composition and color in
reflected light. One sees only a uniform glow, and individual
objects are indistinguishable.
Cosmic Microwave Background
 Starting with a total signal measuring
several hundreds of degrees Kelvin
(degrees on the Centigrade scale
above absolute zero), and after
subtracting out all known signals,
Penzias and Wilson were left with a
residual background signal
corresponding to blackbody radiation
at the tiny temperature of 3 degrees
Kelvin.
 This signal persisted at the same
level regardless of where the
telescope were pointed on the sky.
 Many people would disregard the
tiny discrepancy as an unexplained
curiosity.
 Because Penzias and Wilson took
extraordinary care in isolating the
residual background, they concluded
that they must be observing a real
feature of the universe -- the cosmic
microwave background.
Cosmological Redshift
 Wait… three degrees above absolute zero is an incredibly cold
temperature; why is it that we believe the universe began in a hot
big bang??
 The answer to this question has to do with the fact that in an
expanding universe, all distances are expanding in time.
 The hot plasma in the early universe emitted short-wavelength,
very hot photons.
 As the universe expanded, so too do all other distances within the
universe, including individual wavelengths of light.
Cosmological Redshift
 As the universe expanded, so too do all other distances within the
universe, including individual wavelengths of light.
Hot Big Bang Model of the Universe
 The detection of the cosmic microwave background by Penzias and
Wilson led to the development of the hot big bang model of the universe.
 In this model, the universe began from an incredibly dense hot plasma
state, and rapidly expanded.
 As the universe expanded, it cooled down. At the point that it had cooled
down sufficiently (to about 4000 K) to allow individual protons and
electrons to combine to form hydrogen atoms, it became transparent.
 The microwave background we see today are the same photons that left
the early universe at the point of recombination, and have been
streaming freely since.
 The cosmic microwave background demonstrates that at the point of
recombination, the universe was in an extremely homogeneous state.
Surface of Last Scattering
 When we see the microwave background, we are actually seeing
the surface of last scattering of the intense radiation field of the
early universe.
Cosmic Microwave Background Anisotropies
 The cosmic microwave background is incredibly uniform -- to
within one part in one hundred thousand. If the microwave
background were the blades of grass in a football field, then the
blades would all be identical to within one centimeter.
 The uniformity of the cosmic microwave background is due to the
fact that the early universe was itself nearly perfectly
homogeneous and smooth.
 Yet today, we have evidence of structure on all scales in the
cosmos -- from planets to stars to galaxies and even bigger.
 In the framework of the hot big bang model of the universe, these
structures must have originated from small fluctuations in the
early universe, and grown under the influence of gravity.
Detecting the Cosmic Microwave Background
Anisotropies -- The “Fingerprint of God”
 For nearly thirty years following Penzias and Wilson’s discovery,
scientists probed the cosmic microwave background with a series of
experiments of ever-increasing precision, in an effort to detect the
anisotropies in the cosmic microwave background.
 The issue came to a crux in the early 1990s, when scientists had not yet
detected any of the intrinsic anisotropies in the microwave background.
 Scientists studying galaxy formation knew that the anisotropies had to be
there, and be large enough that they could grow into galaxies in the
current age of the universe.
 However, if the level of anisotropy were very small, then no theory of
structure formation could have been able to account for the existence of
galaxies in the universe, and the hot big bang model itself would be in
serious jeopardy.
COBE And George Smoot
 In 1992, George Smoot led a team of
scientists to design and fly a
microwave detector onboard a
satellite which became known as the
Cosmic microwave Background
Explorer (COBE).
 Smoot and his colleagues gathered
their data and spent many months
carefully analyzing it.
 Finally, after a long wait, Smoot and
his colleagues announced the
discovery of the intrinsic anisotropies
in the cosmic microwave
background, from nearly 14 billion
years ago, and from which all
structures in the universe emerged.
 For this work, Smoot was awarded
the Nobel Prize in Physics in 2006.
George Smoot
COBE Results
 The COBE science team’s discovery of the anisotropy in the
microwave background revolutionized cosmology. For the first
time, scientists had a glimpse of structures of the early universe,
when it was less than 500,000 years old.
 To some, this breakthrough was just the tip of the iceberg. They
asked, where did these fluctuations come from in the first place?
COBE All-Sky Map of Temperature Anisotropies
Inflationary Cosmology
 The discovery of anisotropies in the microwave background came along
at a serendipitous time -- during the 1980s, theorists studying the very
early universe -- at times 10-32 s ! -- were making major progress.
 At these very early epochs, the universe was incredibly hot and dense,
and if one goes back far enough in time, the energies achieved exceed
those of any particle accelerator on the Earth.
 The consequences of the high energy state of the very early universe are
profound -- specifically, one cannot understand the largest scales in the
cosmos without an understanding of the smallest scales as well.

Inflationary Cosmology
 During the early 1980s, Alan Guth was attempting to understand several
surprising properties of the early universe -- including,
 Why is the microwave background so uniform?
 Why are strange particles predicted to exist by some cosmological theories absent or at
least very rare in nature?
 Why does the curvature of the universe appear to correspond to a “flat” cosmologiy?
 Guth’s proposal was radical. He suggested that instead of the expansion rate
expected from the regular Einstein equations, the universe expanded enormously
faster in the universe -- exponentially.

Inflationary Cosmology

Guth’s model was based on a heady mix
of general relativistic cosmology and
particle physics.

The mechanism producing the inflation
can best be understood by the analogy of
a supercooled container of water being
held beneath its freezing point.

Similarly, in an inflationary cosmology,
certain particles may find themselves in a
“false vacuum” as the universe expands
and cools.

During the period of time that the particles
remain in the false vacuum, the universe is
endowed with an enormous amount of
energy that causes the size of the
universe to expand exponentially with
time.

In this picture, the anisotropies which
develop are initially nothing more than
quantum fluctuations within a tiny region of
spacetime.
Wow… Space is so full of … Space
 Once it begins to sink in, one realizes the inflationary cosmology
picture is an amazingly startling one.
 The entire observable universe began with a tremendously small
region of space and then blew up by a huge amount in a very
short span of time.
 Every structure which we observe in the universe today…
superclusters and clusters of galaxies, stars, planets… originated
from quantum fluctuations in the very early universe.
Schematic Diagram of History of Universe
Inflationary Cosmology
 One of the most startling realizations of inflationary cosmology is
that the entire observable universe is only a tiny fraction of what
is really out there.
 As the universe expands, more and more of the rest of the
universe comes into our observable horizon. However,in this
picture, vast regions of space are entirely unknowable until very
distant points in the far future.
But Is Inflation Correct?
 The map of the anisotropies on the cosmic microwave
background can be used to constrain theories of the very early
universe, including inflation.
 The key idea here is that very distant regions on the microwave
background were not in causal contact at the surface of last
scattering. Hence, whatever anisotropies exist on these large
scales, they must be leftover from much earlier epochs still.
 It turns out that COBE-measured anisotropies on large scales are
consistent with those predicted by inflation.
 This provides a tantalizing hint that there may actually be
something to the inflationary cosmology and the radical ideas it
advances.
Large-Scale Structure of the Universe
Zel’dovich Pancakes and Filaments
 The structures seen in the largescale structure surveys such as
SDSS and in the cosmological
simulations were originally predicted
to exist by Yakov Zel’dovich (1914 1987).
 Zel’dovich was a monumental figure
in 20th century physics. He did much
of his early work on fundamental
contributions to chemical combustion
and detonation. During and after
WWII, he played a leading role in the
development of Soviet nuclear
weapons.
 In 1965, at the age of 49, he entered
the field of astrophysics, and
subsequently made so many
fundamental contributions that
Stephen Hawking once said,
“…before I met you here, I believed
you to be a collective author.”
Zel’dovich Pancakes and Filaments
 In the early 1970s, long before the advent of extensive largescale surveys or large computer simulations, Yakov Zel’dovich
and colleagues worked out much of the basic physics of structure
formation in the early universe using little more than pure thought.
 In order to understand the development of large-scale structure in
the early universe, they considered what would happen if one
perturbed an initially uniform density region.
Uniform Background
Overdense Region
Zel’dovich Pancakes and Filaments
 If the perturbed overdense region is nonspherical, it will collapse
the fastest along the shortest axis, which causes the region to
become even more distorted.
 From this line of thinking, it is clear the general outcome of a
gravitational collapse will be a pancake-like structure.
 Note that this mechanism does not rely upon rotation.
Overdense Region
Overdense Region
Zel’dovich Pancakes and Filaments
 Once a pancake is formed, a similar process begins to occur
within it.
 The thin pancake become unstable along its shortest dimension,
which results in the production of filamentary structures.
 The outcome of this process should be a network of highly
complex, interwoven filaments and pancakes containing most of
the mass in the universe, interspersed with enormous voids.
Overdense Region
Overdense Region
Clusters of Galaxies
 Zel’dovich realized that when one combined numerous
perturbations on the background, the resulting pattern would
resemble a complex spiderwork of filaments.
 In this picture, the intersection of filaments provide seeds for the
growth of clusters of galaxies.
 At these intersections, the density is significantly higher than the
background density or even the density in individual filaments.
 As a consequence, the regions of intersection collapse the
fastest, and produce some of the first galaxies in the universe.
Superclusters of Galaxies
 In addition to individual clusters, clusters of galaxies themselves
aggregate into even larger-scale bound structures known as
superclusters.
 The superclusters are bound together by the complex network of
filaments in large-scale structure.
 Over time, these superclusters will tend to swallow one another
up, along with mass contained in the filaments.
 The resulting picture of structure formation is fasctinating, and
suggests that we are looking at the universe at unique epoch, just
as structure formation has really begun in earnest, but before it
has fully quenched itself out.
Large Scale Structure
 As large-scale structure surveys
of the nearby universe were
completed in the 1980s,
structures similar to those
predicted by Zel’dovich and
others began to emerge :
 Clusters of Galaxies
 Superclusters of galaxies
 Walls, Sheets, and Filaments
 Voids
Cfa Survey of Galaxies
LCDM Animation
Zoom-in of Millenium Simulation
Flythrough of Large-Scale Structure of the
Millenium Simulation
Interstellar Space Travel
Interstellar Space Travel
 Travel through interstellar space has become a staple in science
fiction -- often by fictional devices with little or no basis in real
science.
 Is interstellar travel scientifically possible?
The Slow Boat to… Aldebaran
 Even with today’s technology, it is possible to travel to the nearest
stars.
 Pioneer 10 was the first device created by humanity to leave the
system, after its encounter with Jupiter in 1973.
 It is so distant, and its radioactive power sources have become so
weak, that it has not been heard from since 2002 when it was 80
AU away from the sun.
 It is nearly 90 AU away today.
Pioneer 10 Record Plaque Cover
 As humanity’s first envoy outside of the solar system, Pioneer 10
carried with it a record with sounds of Earth, along with a cover
etched with a kind of scientific Rosetta stone explaining who we
are, and when and where the spacecraft was launched.
The Slow Boat to… Aldebaran
 Pioneer 10 is headed towards the red giant star Aldebaran in the
constellation of Taurus.
 At its current speed, it will take nearly 2 million years to reach
Aldebaran.
 By aiming towards a nearer system (for instance, Alpha Centauri,
our nearest neighbor at about 4 light years distance), it would be
possible to cut the travel time to about 100,000 years.
 An interstellar voyage lasting such a long time would require a
enormous spacecraft ark capable of sustaining itself over many
generations.
Interstellar Arks
 An interstellar ark would be an
enormous spaceship (possibly
hollowed out of a smaller asteroid)
with everything needed to sustain a
society for many generations on its
voyage.
 While technically feasible even today,
it seems difficult to imagine a small
society could be sociologically stable
for such a long time when few
societies on Earth have been stable
for hundreds of years.
 Long a staple of science fiction,
many authors describe inhabitants in
“suspended animation” and a
spaceship run by robots.
 What is more probelmatic is the
likelihood that if humanity survives,
the ark will likely be long superceded
by the time of its arrival.
Can’t Wait for Another Lifetime? We Have Faster
Rocket Technologies Just for You
 Many other concepts for faster rocket technologies exist, using
varying degrees of existing and far-fetched science.
 Perhaps the most unusual example using an existing technology
is the Orion spacecraft concept, designed by General Atomics in
the late 1950s, and led by the physicist Freeman Dyson.
 By detonating a succession of hydrogen bombs and absorbing
part of the blastwave energy through a pusher plate, in principle it
can achieve about 10 percent of the speed of light.
Greener Energy to the Stars
 Another spaceship concept design
using existing technology is the light
sail.
 The sail reflects light over a wide
area (many square kilometers) using
a lightweight material.
 The reflection of individual photons
produces a thrust, by Newton’s third
law.
 The simplest source of light is the
sun itself, though this can only be
used in the inner solar system.
 More advanced designs rely upon
banks of lasers to provide the
impulse -- up to 20 percent the
speed of light.
Cosmos 1 Sail
Relativistic Spaceflight
 The ultimate limit for any spacecraft is the speed of light itself.
 Approaching this limit requires vast amounts of energy, beyond any
known technology -- even matter/antimatter engines.
 If it is indeed possible to achieve relativistic spaceflight, the possibilities
are amazing.
 As we learned, according to Einstein’s theory of relativity, the time of a
moving observer is slowed down relative to a stationary observer.
 Interstellar rocket travel is an example of a famous paradox in relativity
theory.
Twin Paradox
 One of the famous apparent paradoxes of special relativity theory is the
twin paradox.
 Imagine we have two twins, Albert and Heindrik. Heindrik remains on
Earth, while Albert sets off on a relativistic spaceflight to Alpha Centauri
at a speed close to that of light, and returns.
 In the middle of his flight, according to Albert, Heindrik’s clock is running
slowly relative to his own.
 According to Heindrik, Albert’s clock is running slowly relative to his own.
 Yet when Albert returns to Earth, surely he is either older or younger than
Heindrik.
Twin Paradox
 Which twin is older on return to Earth? Albert or Heindrik? Why?
Relativistic Spaceflight
 If relativistic spaceflight is indeed possible, then it is possible to
travel to very distant parts of the galaxy with a single lifetime.
 Relativistic spaceflight results in a strange kind of time travel into
the future.
 For instance, special relativity predicts that if one can move at
99.9999% the speed of light, then (by one’s own clock) one will
be able to traverse the 50,000 LY radius of the Milky Way in just
70 years.
 However, the clocks on Earth back home will advance 50,000
years in the same time -- highly relativistic spaceflight is in a
sense an enormous leap into the future.
Life in the Cosmos
 It has taken about 14 billion years in the time since the Big Bang
for the universe to produce conditions suitable for life and
intelligence on Earth.
 When viewed on a broad scope, it appears the rate of “progress”
of the universe is accelerating rapidly.
Question : What is Life?
Definition of Life
 It makes sense to first address the question of what life is,
specifically.
 A reasonable (though not necessarily exhaustive) definition of life
is that
 A) A lifeform can react to its environment.
 B) It can grow by taking in nourishment from its surroundings.
 C) It can reproduce.
 D) It can pass on its genetic structure to its heirs.

Urey-Miller Experiment
 In a classic experiment
conducted at the University of
Chicago in the 1940s. Urey and
Miller demonstrated that
beginning with very simple
chemicals (which were chosen to
approximate the unevolved state
of the early Earth atmosphere),
and adding some energy in the
form of a spark, highly complex
organic chemicals fundamental
to life are rapidly produced.
Urey-Miller Experiment
 Analysis of the chemical
products revealed the presence
of amino acids, which are the
building blocks of DNA.
 However, not the original
experiment, and not any
repetitions of the experiment
have ever been able to
demonstrate the manufacture of
DNA in this process.
 It remains a major scientific
mystery how DNA was itself
produced, and how single-celled
beings came to exist.
Planetary Habitable Zones
 The range of distances from a central star over which a planet
can support liquid water is known as the stellar habitable zone.
 The more massive the central star, the wider the habitable zone,
and the further out it becomes.
 Gliese 581c is in the habitable zone of its parent star.
The Fermi Paradox -- Where are They??
 If planets are common, it seems quite plausible that life should
arise on many worlds.
 This poses an immediate paradox -- if life (and quite possibly
intelligent life) is common throughout the universe, why is it that
we have no evidence for extraterrestrial civilizations?
 There are many clever proposed solutions to this paradox,
including
 We are too meager for aliens to contact.
 The aliens aren’t talkative.
 They are far from us, and haven’t yet received our radio signals.
 They are here… among us…
Gathering at the ol’ water hole
 The most natural way to communicate in interstellar space that
we currently know of is through radio wave transmissions.
 There is a preferred range of frequencies in which the
background from astrophysical sources and atmospheric
absorption is minimal, and which some astronomers believe may
be a natural place for interstellar civilizations to communicate.
Who Speaks for Earth?
Next Week : Field Trip!