Inquiring into Dark Energy

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NSTA - Baltimore 2006

Inquiring into Dark Energy

Dr. James Lochner

USRA & NASA/GSFC

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NSTA - Baltimore 2006

Science is about Questions and Tools

What are some questions we might ask?

And what are the tools we might use?

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What is Gravity?

Einstein asked about the nature of gravity.

He discovered that gravity is curved space-time. His theory predicted that light would bend

when passing near a massive object.

Tool Used: 1919 Solar Eclipse verified Einstein’s prediction.

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How Far Away are “Spiral Nebulae”?

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.

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Tools for answering “How Far Away are Spiral Nebulae?”

Cepheid Variables

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

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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)

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Consequence of asking “How Far Away are Spiral Nebulae?”

Early observations showed the “nebulae” were red-shifted. I.e. moving very fast away from us.

Hubble put together the redshifts with their distances.

Ve

loc

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Distance

Universe is expanding !

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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. 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)

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Tool for Determining “Steady State” vs. “Big Bang”

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.

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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.

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How Fast is the Expansion Slowing Down?

Saul Perlmutter (UC Berkeley) wanted to determine the deceleration rate of the expansion.

Amount of deceleration depends on average mass density. 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.

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Tools for Determining “How Fast is the Expansion Slowing Down?”

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)

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A dying star becomes a white dwarf.

1. Create a White Dwarf

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The white dwarf strips gas from its stellar companion….

2. Dump more mass onto it

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….and uses it to become a hydrogen bomb. Bang!

3. Until it explodes

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The explosion is as bright as an entire galaxy of stars….…..and can be seen in galaxies across the universe.

4. Observe it in a distant galaxy

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5. Compare its distance to its velocity

These supernovae are more distant than expected.Space-time has expanded more than expected.

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Distance (via SN Ia)

More distant galaxies recede from us more rapidly.

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History of the Universe’s Expansion

QuickTime™ and aSorenson Video 3 decompressorare needed to see this picture.

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Dark Energy Comprises 73% of Universe

Dark Energy73%

Dark Matter23%

“Normal Matter”4%

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Common Thread: Standard Candle!

A Standard Candle is an object whose intrinsic brightness does not vary. Measuring its observed brightness gives us its distance.

A Standard Candle utilizes 1/r2 property of light.

Lo Li / r2

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A 1/r2 Classroom Activity

Measure area of an illuminated circle as distance increases.

distance

Area

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A 1/r2 Classroom Activity

Distance (cm)

Measured Circle Diameters (cm)

Avg Diameter (cm)

Avg Radius (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

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A 1/r2 Classroom Activity

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Distance

Area

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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)

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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: 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.

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Preview: Cosmic Times

Our “Cosmic Times” project will trace the understanding of the nature of the universe through the 20th century. 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.

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Scientific Themes

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. Role of Women in early astronomy. Impact of improved technology.

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NSTA - Baltimore 2006“Newsletter version” for individual student use