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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 #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
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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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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.
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.