# Presentation: 482675

## Inquiring into Dark Energy

- Dr. James Lochner
*USRA & NASA/GSFC*

## Science is about Questions and Tools

- What are some questions we might ask?
- And what are the tools we might use?

**Notes:**
This is one way to view Science as Inquiry: Asking questions and designing experiments to answer those questions.

## 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.

## 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.

## 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
*L**o** **** L**i** / r**2*

**Notes:**
The cepheids could not be resolved using the 60” at Mt. Wilson.  Even with the 100”, it took long exposures

Cepheids - Periods are in the 1-100 day range.  The variability is due to stellar pulsations.

## 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)

**Notes:**
The cepheids could not be resolved using the 60” at Mt. Wilson.  Even with the 100”, it took long exposures

Cepheids - Periods are in the 1-100 day range.  

## 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.

- Universe is expanding !

## 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)

**Notes:**
This debate was particularly relevant in the 1950s

Re: Hoyle coined the phrase “Big Bang” as a derogatory term 

## 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.

**Notes:**
Wilson was particularly interested in radio emission from the halo of the galaxy.  Previous observations had hinted at its existence, but the 20-ft horn was ideal for studying it.

## 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.

**Notes:**

Accordng to Wilson’s Nobel Prize Lecture:
Penzias talked with Burke at MIT.  Burke mentioned that Peebles had worked on theoretical work on radiation in the universe.  Penzias called Dicke, who sent a copy of Peeble’s preprint.  Shortly after, Dicke et al visited Penzias and Wilson, discussed their measurements and were convinced of its accuracy.  They agreed to side-by-side publication of two letters in APJ. 

Penzias Nobel Prize Lecture:
Of course, Alpher and Herman had much earlier (1949) had traced the temperature of the relic radiation to be a few degrees K, but they thought its detection would be difficult because it would be masked by cosmic rays and integrated starlight 

## 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.

## 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)

## 1. Create a White Dwarf

*1. Create a White Dwarf*

**Notes:**
Slide 1 of 5 about Type 1a supenovae.  Big Idea: setting the stage for the supernova - creating a white dwarf.

There are different types of supernova, but the type we are interested in comes from a star like our Sun.  When the Sun runs out of its nuclear fuel in about five billion years time, it will go through a beautiful death ritual, shedding its outer layers in a blaze of color while its inside squeezes down into a dense white hot ball about the size of the Earth. This ball, aptly named a white dwarf, is where the story ends for the Sun, but not so other white dwarfs.  About 2/3 of all stars in a galaxy live in pairs (called binaries), a union that becomes dangerous when one of the pair turns into a white dwarf.

This image shows the Ring Nebula, Messier 57, with the white dwarf remnant at its core.

## 2. Dump more mass onto it

*2. Dump more mass onto it*

**Notes:**
Slide 2 of 5.  Big Idea: setting the stage for the supernova - white dwarf binary

About 2/3 of all stars in a galaxy are binaries. One star may run through its life cycle faster, becoming a white dwarf while the other star continues to shine normally.  This is true of the brightest star in the sky, Sirius, which has a faint white dwarf companion.   The white dwarf is greedy, and if the orbit of the white dwarf and its companion is close,  the white dwarf’s strong gravity begins to tug the outer layers of hydrogen gas from the companion and wrap the gas around itself, in the process creating a ticking time bomb. 

This artists’ impression shows a star-white dwarf binary system.  The two objects are close enough that the gravity of the white dwarf strips the outer hydrogen envelope off of its companion, spinning it into a flat disc because of white dwarf’s  rapid rotation.  The gas in the disc will eventually accumulate on the surface of the white dwarf creating a dense hot hydrogen atmosphere.

## 3. Until it explodes

*3. Until it explodes*

**Notes:**
Slide 3 of 5.  Big Idea: Supernova

The hydrogen layer grows, getting hotter and hotter, until at a critical temperature the bomb goes off – a thermonuclear explosion as bright as a billion stars, and a flash that can be seen across the observable universe.  Because the process is identical for each white dwarf supernova, the event has a predictable, and reproducible, brightness.  

A type Ia supernova  was seen in our own galaxy by Johannes Kepler in 1604, five years before the invention of the telescope.  This image is a composite of observations from three space telescopes, Hubble (yellow), Chandra (blue and green) and Spitzer (red) of the remnants of the star Kepler saw in the sky four centuries ago. The ball of debris is now about 8 light years across and still expanding.

## 4. Observe it in a distant galaxy 

- .....and can be seen in galaxies across the universe.

*4. Observe it in a distant galaxy**** ***

**Notes:**
Slide 4 of 5.  Big Idea: Supernova

A supernova can shine as bright as a billion stars for several days, rivaling an entire galaxy of stars.  This image shows a type Ia supernova  explosion in the galaxy NGC 4526. Because the explosion is just a point of light in the sky looks like a star. The processes and conditions that create this type of explosion are effectively identical for each white dwarf supernova, so the event has a predictable, and reproducible, brightness.  This makes them ideal distance indicators - the fainter they are the further they are.

By measuring the brightness of the supernova flash in NGC 4526, the distance to the host galaxy can be calculated.  Note to the presenter: a discussion of how this distance is determined (I.e. how we know it is “further away than it should be”) is not required, but if asked, you can use the comparison of a flashlight. This type of supernova has a known brightness (like a flashlight with a certain watt bulb) and can only be that bright. Just as a flashlight shining from further away appears dimmer than one nearby, measuring the light from this type of supernova tells scientists how far away it is.

## 5. Compare its distance to its velocity

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

**Velocity**

**Distance (via SN Ia)**

- More distant galaxies recede from us more rapidly.

**Notes:**
Discovery in 1998
Discovery via Type Ia supernovae (white dwarfs blowing up)
  - used to study the expansion rate of the universe at earlier times.
In decelerating universe, supernovae would be brighter (i.e closer) than their redshifts would indicate.
These SN appear more distant than expected.  I.e. they are fainter (I.e. farther) than their redshifts would predict.  [Perlmuter - you have to look back further in time (ie greater distance, fainter SNIa) to find a “given integrated expansion (redshift)”
  - expansion of universe is speeding up, not slowing down!
Flatness of universe and its known makeup leave 65% as “unknown” - Dark Energy fits into this slot.

## History of the Universe’s Expansion

**Notes:**
Insert “expanding_univ.mov”

## Dark Energy Comprises 73% of Universe 

- Dark Energy
- 73%

- Dark Matter
- 23%

- “Normal Matter”
- 4%

## 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.
*L**o**** L**i** / r**2*

## A 1/r2 Classroom Activity

- Measure area of an illuminated circle as distance increases.

- distance

- Area

## 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 |

## A 1/r2 Classroom Activity

## 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)

**Notes:**
 Need a form of energy that is elastic (neg. pressure)
  - Maybe the quantum vacuum (but it currently is way too big).  Einstein’s cosmological constant was identified w/ the zero-pt vacuum energy of quantum fields in later years after he rejected it)
  - Quintessence (a particle field) - topological defects from phase transitions could get tangled and exert a neg. pressure
Relation to Cosmological constant: Einstein’s _x0008_Lamda - empty space has energy and couples to gravity.

## 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.

## 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.

## 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.

- “Newsletter version” for individual student use