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Adam G. Riess

Dark Energy and the History of Cosmic Expansion. Adam G. Riess. Space Telescope Science Institute, Johns Hopkins University For further reading: Scientific American Feb 2004, “ From Slowdown to Speedup ”, Riess and Turner The Runaway Universe by Donald Goldsmith

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Adam G. Riess

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  1. Dark Energy and the History of Cosmic Expansion AdamG. Riess Space Telescope Science Institute, Johns Hopkins University For further reading: Scientific American Feb 2004, “From Slowdown to Speedup”, Riess and Turner The Runaway Universe by Donald Goldsmith The Extravagant Universe by Robert Kirshner

  2. The Universe looks static… …but it’s not!

  3. Hubble’s Law: In an expanding Universe, velocity= x distance

  4. The Expanding Universe • Hubble (1929) • Velocities=Doppler shift=z • Distances: Brightest “stars” One of the original reasons for building HST was to measure sets the scale of the Universe and its age

  5. The Expansion Rate may be Slowing down or Speeding Up • (Dark) Matter provides attractive gravity which should • be decelerating Universe • But Einstein thought Universe static, GR allowed for • repulsive gravity, i.e., cosmological constant (Dark Energy), • that could repel gravity and keep Universe static

  6. “Standard” Candles How far away? We Need a Standard candle We can see Far away, Back in time Receding how fast? Bright=near dim=far Doppler Effect

  7. Measuring the Expansion History: Changes in scale/Changes in time

  8. At the End of its Life, a Star is Reborn Subrahmanyan Chandrasekhar 1910-1995 • as bright as 4 billion Suns • can be seen halfway across the Universe

  9. Bang! A star explodes… 1987: The galaxy next door

  10. Elements produced= Fe,Co,Ni,Si,Ca Four months in the life of a SN Ia On the Rise: Age: -6 days Maximum +26 days +47 days +102 days Powered by radioactivity Vejecta=10-20,000 km/s

  11. How do we Find Supernovae?

  12. Improving the Discovery Odds • 1 SN per galaxy per 100 years = • 1 SN per 100 galaxies per year = • 10 SN per 1000 galaxies per year, if monitor them frequently enough.

  13. Searching by Subtraction

  14. The Accelerating Universe By 1998 two teams measured ~100 SNe Ia Surprise! The Universe is accelerating, propelled by dark energy. High-z SCP

  15. Found: Ground, Followed: Hubble Dark Energy Dominated 1.0 Fainter Constant acceleration Constant acceleration High Redshift SN and Expansion History 0.5 Acceleration/ Deceleration 0.0 Freely expanding Freely expanding Relative Brightness (m) -0.5 Constant deceleration Constant deceleration Brighter -1.0 0.0 0.5 1.0 1.5 2.0 present past Redshift z

  16. Why is The Universe Accelerating? (Honestly, we have no idea…) • Dark Energy (e.g., “nothing weighs something”) • There are more spatial dimensions than we can see (string theory) • We don’t understand gravity (especially at long range) • Insert new ideas <here> • We need a sanity check: Was the Universe ever not accelerating? • (i.e., was it ever decelerating?)

  17. Before Acceleration there was Deceleration If the Universe was always accelerating apart, how did the galaxies, stars, and planets form? Hubble Space Telescope

  18. Searching for SNe Ia with ACS on HST ACS imaging, spaced by 45 days CDFS HDFN

  19. Hubble Spies an Ultra-distant SN Aphrodite (z=1.3) ACS F850lp ACS grism spectrum Highest z spectrum of a SN NICMOS F110W viz

  20. Color Images of SNe Ia

  21. Found: Hubble, Followed: Hubble Found: Ground, Followed: Hubble Dark Energy Dominated Dark Matter Dominated 1.0 Fainter Constant acceleration Constant acceleration High Redshift SN and Expansion History 0.5 Acceleration/ Deceleration 0.0 Freely expanding Freely expanding Relative Brightness (m) -0.5 Constant deceleration Constant deceleration Brighter -1.0 0.0 0.5 1.0 1.5 2.0 present past Redshift z

  22. A Primer on Dark Energy • It accelerates the expansion of the Universe • It makes up most (~2/3rds) of the Universe • It will determine the fate of the Universe • We do not understand what it is! • vacuum energy, mini-inflation, extra dimensions, super- horizon fluctuations, Chaplygin gas, new physics, ?? We need a way to measure the nature of Dark Energy

  23. Dark Energy is repulsive Gravity(how strong is it?) In GR, gravitational force Cosmological Constant (vacuum) (mini-inflation) +1/3 0 -1/3 <w<-1 -1 If w<-1/3 the Universe accelerates

  24. Determining the nature of Dark Energywill require many more Supernovae from HST and measuring very small (few %) differences in the expansion history. NASA Plans to service Hubble in 2007 and add a new camera.

  25. Can we keep Hubble alive? Need to send robots or astronauts in shuttle by 2008 or else…

  26. The physics of Dark Energy will determine the fate of the Universe.

  27. T-minus 10 minutes to Rip…

  28. The End Nearby SN Ia, SN 1994D, seen by HST

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