1 Dark Energy and How to Find It: The SNAP Experiment Stuart Mufson IU Astronomy June 2007

2 Energy Content of the Universe  +  baryons +  dark matter +  dark energy =  crit = 1  k = 0  vis =  baryons =  dark matter = 0.24 ‡  dark energy = 0.70 ‡  = † WMAP + SNIa + SDSS: Astronomical Observations: Super-K: † no mass degeneracy ‡ h = 0.70

3 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

4 isotropic homogeneous nonisotropic homogeneous nonisotropic nonhomogeneous Copernican Principle: Universe is homgeneous & isotropic Copernican Principle: Universe is homgeneous & isotropic

5 Universe uniform on a scale of approx. 1 billion light years every dot represents a galaxy 1 billion light years Earth Copernican Principle: looks ok

6 Rules of the Game: Universe must be described by 4-dimensional space-time  3 dimensions of space (length, width, height)  4 th 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

7 Shape #1: “spherical”, closed geometry finite, unbound k = +1  l General Relativity: the Universe can have only one of three possible “shapes”

8 Shape #2: “saddle-shaped”, open geometry infinite, unbound k = -1  l

9 Shape #3: “flat” or Euclidean geometry infinite, unbound (unbound = no edge) k = 0  l But the Universe has “shape” #3

10 Worse! It has precisely the “right amount” of Dark Energy to make it happen!  +  baryons +  dark matter +  dark energy =  crit = 1  k = 0  vis =  baryons =  dark matter = 0.24 ‡  dark energy = 0.70 ‡  = † WMAP + SNIa + SDSS: Astronomical Observations: Super-K: † no mass degeneracy ‡ h = 0.70

11 R(t 2 ) * * * * time = t 1 time = t 2 How the universe expands from time t 1 to time t 2 is described by the “expansion factor” R(t) One more thing: the Universe is Expanding R(t 1 )

12 distance formula + General Relativity expansion/contraction/ of the Universe strength of gravity in the Universe geometry of the Universe The Friedmann Equation: Put it all together/mix well/add a bunch of astrophysicsts  plus

13 Solutions without Dark Energy R(t) t scale factor time k= +1 k=0 k= -1 Big BangBig Crunch solutions to Friedmann’s equation now

14 When astronomers went looking, this is what they found R(t) t scale factor time Big BangBig Crunchnow (1) the Universe first slows down (decelerates) because of gravity (2) the Universe then speeds up forever (accelerates) because of Dark Energy

15 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

16 Dark Energy FRW distance + General Relativity + Dark Energy  Modified Friedmann’s Equation: expansion/contraction of the Universe strength of gravity in the Universe modified by Dark Energy geometry of the Universe

17 matter density gets smaller (1/R 3 ) 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

18                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

19 R(t) t scale factor time Big BangBig Crunchnow 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

20 SuperNova Acceleration Probe SNAP

21 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 Candles

22 Bright, SNe Ia SNIa Milky Way-like galaxy A supernova can outshine an entire galaxy! and so be seen from very far away

23 SNe Ia are similar throughout the Universe Nearby SNe Ia Distant SNe Ia

24 SNAP Mission  Telescope: 1.8 meter aperture sensitive to light from distant SNe and galaxies.

Mission Design

Shutter Particle/ Thermal/ Light shield CCDs/ HgCdTe Thermal links Spectrograph Cables/ FE elec Near electronics Radiator Guiders Cold plate Filters Instrument Design

Observing 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 0.05 O – half the pitch of a detector. Follow-up spectroscopy on SNe at peak brightness. 6.5° 1.2° *

Launch 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 Orbit