Inquiring into Dark Energy
Dr. James Lochner
USRA & NASA/GSFC
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Science is about Questions and Tools
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What are some questions we might ask?

And what are the tools we might use?
Inquiring into Dark Energy
NSTA - Baltimore 2006
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What is Gravity?
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
Einstein asked about the nature of
gravity.
He discovered that gravity is curved
space-time.
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His theory predicted that light would bend
when passing near a massive object.
Tool Used: 1919 Solar Eclipse verified
Einstein’s prediction.

Inquiring into Dark Energy
NSTA - Baltimore 2006
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How Far Away are “Spiral Nebulae”?
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In 1920, astronomers pondered the
distance to the “spiral nebulae.”
Harlow Shapley and Heber Curtis debated whether
they were within our own Galaxy or outside our
Galaxy.
 The question was settled when Edwin Hubble
determined the distance to Andromeda Galaxy.

Inquiring into Dark Energy
NSTA - Baltimore 2006
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Tools for answering
“How Far Away are Spiral Nebulae?”
Cepheid Variables
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These stars vary in brightness
due to pulsations.
The period of brightness
variation is related to star’s
intrinsic luminosity.
By measuring the observed
luminosity, and knowing
intrinsic luminosity we can
determine distance
Lo  Li / r2
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Tools for answering
“How Far Away are Spiral Nebulae?”

100” Telescope at Mt Wilson, CA (commissioned 1917)

provided the added aperture and resolution to resolve the stars.
Hubble determined distance to Andromeda to be 800,000 LY
(actual distance is 2.8 million LY)
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Consequence of asking
“How Far Away are Spiral Nebulae?”
Early observations showed the “nebulae” were red-shifted.
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I.e. moving very fast away from us.
Hubble put together the redshifts with their distances.
Universe is expanding !
Velocity

Distance
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Is Universe a “Steady State” or
Did it originate from a “Big Bang?”

Steady State Theory: As universe expands, matter is
created.

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Creation rate - a few hundred atoms per year per galaxy
Big Bang: running expansion backwards leads us to a point
of high density and high temperature from which universe
originated. (Create everything all at once)
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Tool for Determining
“Steady State” vs. “Big Bang”
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Penzias and Wilson were using a
20-foot horn detector to make
radio observations of the Milky
Way.
Effort to reduce noise in the
detector left them with a 3 K
residual. But they didn’t know
its origin.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Tool for Determining
“Steady State” vs. “Big Bang”
Peebles and Dicke (Princeton) had just calculated an estimate
for the temperature of the residual background temperature, and
found it was detectable in the microwave region.
 Peebles and Dicke were convinced that Penzias and Wilson
had found it.
This solved the Steady State vs Big Bang question.

Inquiring into Dark Energy
NSTA - Baltimore 2006
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How Fast is the Expansion Slowing Down?
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Saul Perlmutter (UC Berkeley) wanted to determine the
deceleration rate of the expansion.
Amount of deceleration depends on average mass density.
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So we’d be “weighing the universe”
This would lead to determining the curvature of the
universe and whether the universe is infinite or not.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Tools for Determining
“How Fast is the Expansion Slowing Down?”
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Compare a galaxy’s measured distance with its redshift.
Get distance by comparing observed and intrinsic
luminosity of an object in the galaxy.
Enter - Supernovae!
(But we need a special kind of supernova)
Inquiring into Dark Energy
NSTA - Baltimore 2006
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QuickTime™ and a
H.264 decompressor
are needed to see this picture.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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1. Create a White Dwarf
A dying star becomes a white dwarf.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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2. Dump more mass onto it
The white dwarf strips gas from its stellar companion….
Inquiring into Dark Energy
NSTA - Baltimore 2006
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3. Until it explodes
….and uses it to become a hydrogen bomb. Bang!
Inquiring into Dark Energy
NSTA - Baltimore 2006
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4. Observe it in a distant galaxy
The explosion is as bright as an entire galaxy of stars….
…..and can be seen in galaxies across the universe.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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5. Compare its distance to its velocity
Velocity
More distant galaxies recede from us
more rapidly.
These supernovae are more distant
than expected.
Space-time has expanded more than
expected.
Distance (via SN Ia)
Inquiring into Dark Energy
NSTA - Baltimore 2006
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History of the Universe’s Expansion
QuickTime™ and a
Sorenson Video 3 decompressor
are needed to see this picture.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Dark Energy Comprises 73% of Universe
Dark Energy
73%
“Normal Matter”
4%
Dark Matter
23%
Inquiring into Dark Energy
NSTA - Baltimore 2006
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QuickTime™ and a
H.264 decompressor
are needed to see this picture.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Common Thread: Standard Candle!

A Standard Candle is an object whose intrinsic brightness
does not vary.
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Measuring its observed brightness gives us its distance.
A Standard Candle utilizes 1/r2 property of light.
Lo Li / r2
Inquiring into Dark Energy
NSTA - Baltimore 2006
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A 1/r2 Classroom Activity

Measure area of an illuminated circle as distance increases.
distance
A
r
e
a
Inquiring into Dark Energy
NSTA - Baltimore 2006
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A 1/r2 Classroom Activity
Distance
(cm)
Measured Circle Avg Diameter Avg Radius
Diameters (cm)
(cm)
(cm)
Circle Area
(cm2)
10.0
6.4, 6.2, 6.1
6.23
3.12
30.5
20.0
11.8, 12.0, 12.1
11.97
5.98
112.5
30.0
17.5, 17.3, 17.5
17.43
8.72
238.7
40.0
22.4, 23.0, 22.8
22.73
11.37
405.9
Inquiring into Dark Energy
NSTA - Baltimore 2006
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A 1/r2 Classroom Activity
450
400
350
Area
300
250
200
150
100
50
0
0
10
20
30
40
50
Distance
Inquiring into Dark Energy
NSTA - Baltimore 2006
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What is the Dark Energy?
Einstein introduced the Cosmological Constant
to explain what was then thought to be a static
Universe, “my biggest mistake . . .”
Empty space has energy. Its
gravitational effect pushes the universe
apart.
Need a form of energy that is elastic:
• Vacuum energy (= Cosmol. Const.)
• But it’s effect may be too large
• Quintessence (particle field)
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Dark Energy is an Unfinished Story
WE DON’T KNOW WHAT IT IS!
But it traces the story of our understanding
of the nature of the universe.

An ideal setting for illustrating the process of science:
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Science is alive and on-going.
Our ideas change as the data changes.
Scientific debate differs from social/political debate.
Progress in science results from both individual and group efforts.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Preview: Cosmic Times
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Our “Cosmic Times” project will trace the understanding
of the nature of the universe through the 20th century.
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Start in 1919 with confirmation of Einstein’s theory of gravity (and
its implications for the nature of the universe).
Continue through to discovery of Dark Energy, our current state of
knowledge, and stepping stones to future.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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Scientific Themes
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Our understanding of the Expansion of the Universe
Nature of Supernovae
The size and scale of the Universe
A number of other themes will also appear.
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Role of Women in early astronomy.
Impact of improved technology.
Inquiring into Dark Energy
NSTA - Baltimore 2006
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“Newsletter
version” for individual studentInquiring
use into Dark Energy
NSTA - Baltimore 2006
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