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Why Are We Here?
Journey Through the Universe:
Cosmology
Gregory D. Wirth
W. M. Keck Observatory
May 22, 2017
What is Cosmology?
First, what is cosmology not? Cosmology is
not cosmetology. Note the distinction:
Cosmology is concerned with
the makeup of the Universe…
Cosmetology is concerned with
the universe of makeup.
- Rocky Kolb
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What is cosmology?
According to the dictionary:
cos·mol·o·gy (käz mäl’e jē)
the branch of philosophy and science that deals
with the study of the universe as a whole and of its
form, nature, etc. as a physical system
- Webster’s
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The Really
BIG Questions
Where are we?
 How did we get here?
 Where are we going?

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What is the Universe Like?
Pretend that you’re describing your Universe
to someone who has never seen it.
What words would you use to describe our
Universe?
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Big Questions about the
Universe
1.
2.
3.
4.
5.
6.
7.
8.
How big is the Universe?
How old is the Universe?
What is the Universe made of?
How did the Universe begin?
How did we get to where we are today?
What is the shape of the Universe?
What is the fate of the Universe?
How did mankind discover this?
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The Cosmic Zoo
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Stars
Planets
Gas Nebulae
Star Clusters
Galaxies
Clusters of Galaxies
Large-scale
structures
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Units of Distance in Astronomy

Astronomical Unit (AU)
Defined as distance from Earth to Sun
 Used within a solar system
 1 AU = 150,000,000 km

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Units of Distance in Astronomy
Astronomical Unit (AU)
 Light Year (l.y.)

Defined as distance light travels in 1 year at
300,000 km/sec
 Used within star systems
 1 l.y. = 9,500,000,000,000 km

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Units of Distance in Astronomy
Astronomical Unit (AU)
 Light Year (l.y.)
 Parsec (pc)
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1 pc = 3.258 light-years
 Used within star systems
 1 pc = 30,800,000,000,000 km

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Units of Distance in Astronomy
Astronomical Unit (AU)
 Light Year (l.y.)
 Parsec (pc)
 Kiloparsec (kpc)
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Used within galaxies
 1 kpc = 1,000 pc
 1 kpc = 30,800,000,000,000,000 km

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How far is the nearest star?
Consider a scale model with 1 AU = 1 meter.
In this model:
 Sun’s diameter: 10 mm (marble)
 Earth’s diameter: <0.1 mm (grain of sand)
 Distance to Pluto: 40 meters
 Distance to Oort cloud (comets): 50 km
 Distance to Proxima Centauri: 268 km
The Universe is a big place, and “space” is well-named!
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Activity: How old is the Universe?
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What if we compressed the entire history
of the Universe into a single year, so that:
The Big Bang occurred at midnight on Jan 1
 The present time is midnight on Dec 31
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At what dates on this calendar would you
place the key events in mankind’s history
of the Universe?
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Activity: How old is the Universe?
1.
2.
3.
4.
5.
6.
7.
8.
First stable atoms
First stars, quasars
Milky Way galaxy
Solar system
Life forms on Earth
Multi-cell life forms
Vertebrate life forms
Land plants
9.
10.
11.
12.
13.
14.
15.
First dinosaurs
Last dinosaurs
First hominids
First homo sapiens
Agriculture invented
Egyptian pyramids
America discovered
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Activity: How old is the Universe?
1.
2.
3.
4.
5.
6.
7.
8.
First stable atoms
First stars, quasars
Milky Way galaxy
Solar system
Life forms on Earth
Multi-cell life forms
Vertebrate life forms
Land plants
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Jan 1, 12:11am
Jan 19
March
August
September
November
December 17
December 18
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Activity: How old is the Universe?
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Dec 24
Dec 29
Dec 31, 1:33 pm
11:54 pm
11:59:20 pm
11:59:50 pm
11:59:59 pm
9.
10.
11.
12.
13.
14.
15.
First dinosaurs
Last dinosaurs
First hominids
First homo sapiens
Agriculture invented
Egyptian pyramids
America discovered
Our lives are the blink of an eye in the grand scheme of things…
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How is the Universe arranged?
We observe stars in all directions
 We are in the middle of a galaxy!
 Stars and nebulae lie within galaxies
 Star clusters orbit around galaxies
 Galaxies form groups, clusters, and
superclusters
 Galaxies lie in all directions, but are not
distributed uniformly

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Activity:
Journey Through the
Universe – The Movie!
What We Believe about the
Universe:
The universe is BIG
 The universe is OLD
 Matter is organized into structures of many
sizes (stars, star clusters, galaxies, etc.)
..but how do we know these things?
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Ideas About the Universe: Aristotle
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Aristotle (Greek philosopher, 340 B.C.)
determined that:
Earth was a sphere (by observing the shape
of Earth’s shadow during a lunar eclipse)
 The diameter of Earth was 4000 stadia (about
twice the actual size)
 Earth was the center of the Universe, with the
Sun, Moon, Planets, and stars moving around
it in circular orbits

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Ideas About the Universe: Ptolemy

Ptolemy (Greek
astronomer, 100 A.D.)
elaborated upon these
ideas to produce the
first Earth-centered
model of the Universe
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Ideas About the Universe:
Copernicus
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Copernicus (Polish priest, 1514) first
proposed that:
the Sun was stationary
 the planets orbit the Sun
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His idea was ignored for a century!
 Eventually it was supported by
astronomers Johannes Kepler (Germany)
and Galileo Galilei (Italy)
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Ideas About the Universe: Galileo
Galileo used a new tool to make a series
of key discoveries
 Most importantly, he discovered moons
orbiting Jupiter
 This was the first observation of something
which could not be orbiting the Earth
 He established that the Earth was not the
center around which all things traveled
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Ideas About the Universe: Newton
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Isaac Newton (English, 1687) first
described gravitation and
explained the motion of the
planets
The Sun was recognized as just
one of many stars in the Universe
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Ideas About the Universe:
The Picture 100 Years Ago
Around 1900, the
Universe was
believed to:
 Consist of one galaxy,
the Milky Way
 Be infinitely old
 Be static and
unchanging
But that would soon
change…
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Ideas About the Universe: Einstein

Einstein (Swiss physicist) made three
incredible breakthroughs in science:
Quantum theory
 Special relativity
 General relativity
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“As the creation of a single mind, it is
undoubtedly the highest intellectual
achievement of humanity.”
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Ideas About the Universe: Einstein
Einstein’s General Theory of Relativity:
 Describes time as part of space, yielding
the new concept of spacetime
 Explains gravity as the effect of curved
spacetime, caused by the presence of any
object
 Was experimentally verified in 1919 and
explained observations which were at
odds with Newton’s theories.
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Ideas About the Universe: Einstein
Another prediction of his theory: the entire
universe should either be expanding or
contracting
 Einstein added “anti-gravity” to his theory
so that it could explain a static universe
“The biggest blunder of my life.”

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Ideas About the Universe: Hubble
Edwin Hubble (American astronomer) made a
series of revolutionary discoveries:
1. There are other galaxies outside the Milky Way
2. Virtually all other galaxies are moving away
from us
3. Galaxies further away from us are receding
more quickly
The universe is expanding!
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Hubble’s Original Diagram
Fig. 1. Velocity-distance relation among extra-galactic nebulae
Velocity
The more distant the galaxy,
the faster it recedes from us!
Kirshner, Robert P. (2004) Proc. Natl. Acad. Sci. USA 101, 8-13
Distance
Journey Through the Universe -- Cosmology
Copyright ©2004 by the National Academy of Sciences
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Implications of Hubble’s Discovery
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If the Universe is now expanding, then…
At an earlier time, it must have been smaller and
denser…
The further back you go, the smaller and denser
the Universe must have been…
There must have been a beginning in which the
entire Universe was contained within an infinitely
small space:
The Big Bang!
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How To Test the Big Bang Theory?
To study the origins of the Universe, we
need information from earlier eras.
 The single most important piece of
information in astronomy is this:

The speed of light is finite.
As we look out into space,
we look back in time.
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Echoes of the Big Bang
The theory predicts that if we look back far
enough, we can observe radiation from the
Big Bang as microwaves
 In 1965, Penzias & Wilson accidentally
discovered this cosmic microwave
background radiation while testing a
microwave antenna in the U.S.
 For providing confirmation of the Big Bang
theory, the received the 1978 Nobel Prize

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Ideas About the Universe:
Summary
The Universe is expanding.
 The Universe is changing and evolving.
 The Universe had a beginning.
 Mankind’s discoveries about the Universe
are the result of centuries of observations
of the sky.
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The Big Bang in a Nutshell
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Prior to the Big Bang neither time nor space
existed.
The Big Bang occurred 13.7 billion years ago,
creating spacetime.
At the start, the Universe had zero size and was
infinitely hot. After 1 second, the temperature of
the Universe was 10 billion degrees
During the first three minutes, hydrogen (75%),
helium (25%), and trace amounts of other
elements (<1%) formed.
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The Big Bang in a Nutshell
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After 10 minutes, the temperature dropped to
one billion degrees.
The universe continued to expand and cool.
After a million years, the first stable atoms
formed.
Giant gas clouds collapsed to form galaxies.
Gas clouds within galaxies collapsed to form the
first stars.
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The Big Bang in a Nutshell
Some early stars destroyed themselves in
supernova explosions, forming all of the
elements found today on Earth and
dispersing them throughout the galaxies
 Subsequent generations of stars formed
from the remains of earlier stars.
 Eventually, stars, planets and life (at least
on Earth!) formed.

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What Does the Future Hold?
Several possibilities:
a) The Universe may eventually stop expanding
and recollapse in a “Gib Gnab”
b) The Universe may slow down but never quite
stop expanding
c) The Universe may continue to expand forever.
Recent evidence indicates (c).
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The Shape of the Universe
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We are not at the
center!
The raisin cake model
explains how all
galaxies can be
moving apart as the
Universe expands
The universe may be
finite or infinite; we
don’t yet know which
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Answers to the Big Questions
1.
2.
3.
4.
How big is the Universe?
Unimaginably big
How old is the Universe?
Unfathomably old
What is the Universe made of?
90% hydrogen, 10% helium
How did the Universe begin?
With a Big Bang
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Answers to the Big Questions
5.
6.
7.
8.
How did we get to where we are today?
Expansion and contraction
What is the shape of the Universe?
Good question…
What is the fate of the Universe?
Expanding forever
How do we know this?
Over 100 years of observations…and
counting!
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What We Still Don’t Know
What’s the unseen Dark Matter that
constitutes most of the mass in the
Universe?
 What’s the Dark Energy that propels the
Universe to expand?
 What caused the Big Bang?
 What happened to all of the anti-matter?
 Is there a Theory of Everything?
 In the Universe finite or infinite?
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Teaching Cosmology: Strategy
1.
2.
3.
Cosmology is fundamentally about
forming our "world view" .
Many of the main ideas in cosmology are
accessible even to very young children
and should be presented to them.
Cosmology can be challenging to teach -- provide connections to make it relevant
to the students.
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Teaching Cosmology: Content
1.
2.
Scientific methods (observations and
calculations) are used to study cosmology.
Our evolving ideas about the Universe
illustrate important problems in science:
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3.
Bad assumption: supreme importance of Man
Observational bias: blinded to evolution by our short
lives
Don’t be afraid to teach the Big Bang!
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Want More? Further Reading:
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Steven Hawking
The Illustrated Theory of Everything
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Bill Bryson,
A Short History of Nearly Everything
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Neutron Stars
If the star started out 6 to 30
times the mass of the sun,
the core of the exploding star
becomes a neutron star.
 As massive as the sun, but
only 16 km across.
 Neutron stars spin rapidly
and give off pulses of radio
waves
 If these radio waves come in
pulses it is called a pulsar
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Neutron Stars
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Black holes
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If the star was bigger than
30 times the mass of the
sun
The left over core becomes
so dense that light can’t
escape its gravity.
Becomes a black hole.
Grab any nearby matter
and get bigger
As matter falls in, it gives
off x-rays.
That’s how they find them
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The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc.
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M87: Bending Time and Space
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