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Dark Energy and How to Find It:The SNAP Experiment

Dark Energy and How to Find It:The SNAP Experiment

•Stuart Mufson

IU Astronomy

June 2007

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Energy Content of the Universe Energy Content of the Universe

dark matter

dark energy

dark atomsvisible matterneutrinos

+ baryons + dark matter + dark energy = crit = 1 k = 0

• vis = 0.007

• baryons = 0.047

• dark matter = 0.24‡

• dark energy = 0.70‡

• = 0.006†

WMAP + SNIa + SDSS:

Astronomical Observations:

Super-K:

† no mass degeneracy‡ h = 0.70

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• Fundamental Principle of Cosmology:

The Copernican Hypothesis is valid

except for local irregularities, the Universe presents the same aspect from every place in it

no matter where you are in the Universe, IT LOOKS THE SAME! (on cosmological scales)

the laws of physics as seen on Earth apply throughout the Universe

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isotropichomogeneous

nonisotropichomogeneous

nonisotropicnonhomogeneous

Copernican Principle:Universe is homgeneous & isotropic

Copernican Principle:Universe is homgeneous & isotropic

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Universe uniform on a scale of approx. 1 billion light years

every dot represents a galaxy 1 billion light yearsEarth

Copernican Principle: looks okCopernican Principle: looks ok

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Rules of the Game:

• Universe must be described by 4-dimensional space-time

3 dimensions of space (length, width, height) 4th dimension is time

• Gravity is the force that governs the Universe on cosmological scales

Einstein’s Theory of Gravity (General Relativity) is the correct theory of gravity

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Shape #1: “spherical”, closed geometry

finite, unboundk = +1

l

General Relativity: the Universe can have only one of three possible “shapes”

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Shape #2: “saddle-shaped”, open geometry

infinite, unbound

k = -1

l

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Shape #3: “flat” or Euclidean geometry

infinite, unbound

(unbound = no edge)

k = 0

l

But the Universe has “shape” #3But the Universe has “shape” #3

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Worse! It has precisely the “right amount” of Dark Energy to make it happen!

Worse! It has precisely the “right amount” of Dark Energy to make it happen!

dark matter

dark energy

dark atomsvisible matterneutrinos

+ baryons + dark matter + dark energy = crit = 1 k = 0

• vis = 0.007

• baryons = 0.047

• dark matter = 0.24‡

• dark energy = 0.70‡

• = 0.006†

WMAP + SNIa + SDSS:

Astronomical Observations:

Super-K:

† no mass degeneracy‡ h = 0.70

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R(t2)

*

**

*

time = t1

time = t2

How the universe expands from time t1 to time t2 is described by the “expansion factor” R(t)

One more thing: the Universe is ExpandingOne more thing: the Universe is Expanding

R(t1)

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• distance formula + General Relativity

expansion/contraction/ of the Universe

strength of gravity in the Universe

22 28

3

R GR kc

t

π ρ⎛ ⎞ = −⎜ ⎟⎝ ⎠

geometry of the Universe

The Friedmann Equation:The Friedmann Equation:

Put it all together/mix well/add a bunch of astrophysicsts

plus

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Solutions without Dark EnergySolutions without Dark Energy

R(t)

t

scal

e fa

ctor

time

k= +1

k=0

k= -1

Big BangBig Bang Big CrunchBig Crunch

solutions to Friedmann’s equation

solutions to Friedmann’s equation

nownow

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When astronomers went looking, this is what they foundWhen astronomers went looking, this is what they found

R(t)

t

scal

e fa

ctor

time

Big BangBig Bang Big CrunchBig Crunchnownow

(1) the Universe first slows down (decelerates) because of gravity

(2) the Universe then speeds up forever (accelerates) because of Dark Energy

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Two “Outstanding” issues:

• What is the “stuff” that makes the Universe “flat”?

• What is the “stuff” that is causing the Universe to expand at an ever increasing rate?

A Possible answer: the “stuff” is Dark Energy

• What is Dark Energy? Good question

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Dark EnergyDark Energy

• FRW distance + General Relativity + Dark Energy

Modified Friedmann’s Equation:

( )2

2 28

3

R GR kc

t

πρ

Δ⎛ ⎞ = −⎜ ⎟Δ⎝ ⎠− Λ

expansion/contraction of the Universe

strength of gravity in the Universe modified by Dark Energy

geometry of the Universe

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( )2

2 28

3

R GR kc

t

πρ

Δ⎛ ⎞ = − Λ −⎜ ⎟Δ⎝ ⎠

matter density gets smaller (1/R3) as the Universe expands

Dark Energy:

acts like negative gravity but does not get weaker as the Universe expands!

Eventually Dark Energy

must rule!

Dark Energy Dark Energy

Dark EnergyDark Energy

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time

As the Universe expands, the galaxies do not grow in size but space gets filled up uniformly with more and more Dark Energy

It’s like Jello that just keeps filling up space as it expands

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R(t)

t

scal

e fa

ctor

time

Big BangBig Bang Big CrunchBig Crunchnownow

Dark Energy Causes the Universe to Accelerate Forever!Dark Energy Causes the Universe to Accelerate Forever!

(1) the Universe first slows down (decelerates) because of gravity

(2) the Universe then speeds up forever (accelerates) because of Dark Energy

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SuperNova Acceleration Probe

SNAP

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In the early 1990’s, a new type of standard candle was discovered – a kind of supernova technically called SNIa

(this kind of supernova had been discovered long ago – that it could be used as a standard candle was what was recognized in the early 1990’s)

super

These supernovae are immune from evolutionary effects because their brightness only depends on the unchanging parameters of nuclear physics

Standard CandlesStandard Candles

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Bright, SNe IaBright, SNe Ia

SNIa Milky Way-like galaxy

A supernova can outshine an entire galaxy! and so be seen from very far away

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SNe Ia are similar throughout the Universe SNe Ia are similar throughout the Universe

Nearby SNe Ia

Distant SNe Ia

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SNAP MissionSNAP Mission

Telescope: 1.8 meter aperture sensitive to light from distant SNe and galaxies.

Mission DesignMission DesignMission DesignMission Design

Mission DesignMission DesignMission DesignMission Design

Mission DesignMission DesignMission DesignMission Design

Mission DesignMission DesignMission DesignMission Design

Mission DesignMission DesignMission DesignMission Design

Mission DesignMission DesignMission DesignMission Design

Shutter

Particle/Thermal/

Light shield

CCDs/HgCdTe

Thermal links

Spectrograph

Cables/FE elec

Nearelectronics

Radiator

Guiders

Cold plate

Filters

Instrument DesignInstrument DesignInstrument DesignInstrument Design

Observing ConceptObserving ConceptObserving ConceptObserving Concept

• Step across survey field in a predetermined pattern.

• Fields revisited every four days.

• All objects see all 9 filters.

• Four exposures per position

—To implement dithering pattern This improves the spatial and photometric resolution

—To eliminate cosmic ray pollution. Defects in the image are eliminated by this procedure

• 300 sec fixed length exposures determined by a shutter.

• After the set of four exposures shift over by 0.05O – half the pitch of a detector.

• Follow-up spectroscopy on SNe at peak brightness.

6.5°

1.2°

*

Launch VehicleLaunch VehicleLaunch VehicleLaunch Vehicle

1600 kg satellite can be lifted by a Delta IV [recent first flight] to our orbit with margin. Can use equivalent Delta IV, Atlas, or Sea Launch.

L2 Lagrange point

~1,500,000 km

~374,000 km

~1,500,000 km

L2

Mission OrbitMission OrbitMission OrbitMission Orbit