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Dark Energy

Dark Energy. Professor Bob Nichol (ICG, Portsmouth). Overview. General Relativity and  Hubble Diagram Sloan Digital Sky Survey (SDSS) Supernovae Dark Energy. Space and Time. Every event in the Universe has a coordinate (x,y,z,t) e.g. May 18th 2007 @ 2pm in Reading.

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Dark Energy

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  1. Dark Energy Professor Bob Nichol (ICG, Portsmouth) RAL Seminar

  2. Overview • General Relativity and  • Hubble Diagram • Sloan Digital Sky Survey (SDSS) • Supernovae • Dark Energy RAL Seminar

  3. Space and Time • Every event in the Universe has a coordinate (x,y,z,t) e.g. May 18th 2007 @ 2pm in Reading. • Newton believed space and time was absolute, a big clock and ruler in the sky • Einstein’s genius was to say the “speed of light” was absolute, not space-time. This opened the door to dynamic space-time! RAL Seminar

  4. M r m F = GMm/r2 Newtonian Gravity • Force between two masses (M, m) • Works well on scale of our planet • But action at a distance? RAL Seminar

  5. General Relativity I In GR, a mass “warps” space-time and then other masses travel the shortest distance in this “curved” space-time: Looks like gravity! RAL Seminar

  6. General Relativity II • Einstein’s genius was to describe gravity as a geometrical effect of mass • No problem with action at a distance • We are presently sitting in the “warped” space-time of the Earth. If the floor and crust wasn’t there you would fall towards the core of Earth in this curved space-time! • Not just mass, energy can warp space-time (E=mc2) RAL Seminar

  7. Cassini: 1 / 40000 General Relativity III GPS system would loose 10km a day! RAL Seminar

  8. Friedman’s Equation • Using GR, we can write a simple equation for the dynamics of the Universe R is the scale factor (“radius”) of the Universe k is the curvature of space-time of the Universe (a constant) Hubble Parameter (see later) Average density of matter RAL Seminar

  9. 3 Solutions to Friedman Never stop! Stop at infinity R Larger universe Big crunch! Bang time Later in Universe Value of  decides the fate of Universe! Like throwing a stone into space RAL Seminar

  10. Einstein didn’t like it • Remember in ~1920’s the Universe was thought to be static and infinite! Therefore, he introduced the cosmological constant  which acts as a negative pressure counteracting  RAL Seminar

  11. 1929 - Enter Hubble Less than 80 years ago! Recession Velocity due to doppler shift of light velocity = Ho x distance Ho = 500 km/s/Mpc Distance from Cepheid variable stars RAL Seminar

  12. Biggest Blunder! • Hubble’s Law is a natural consequence of Friedman equations • When Einstein heard of Hubbles discovery he removed calling it “his biggest blunder” RAL Seminar

  13. Hubble Flow RAL Seminar

  14. Cosmology 1930-1990’s • Since 1930’s, cosmologists have used Hubbles Law to map more and more galaxies, driven by technology • Today, SDSS gives us a million galaxies! 100x more than pre-1990’s • Digital “data flood” RAL Seminar

  15. SDSS www.sdss.org.uk/dr5

  16. SDSS

  17. 1998 - Enter Supernovae • Supernovae are “standard candles” - like having a 100W light bulb in the Universe! • As you know they true brightness, you can estimate their distance from us • Two projects used tens of supernovae to remake Hubbles Diagram but over much larger distances in the Universe; critical breakthrough! RAL Seminar

  18. Supernovae II RAL Seminar

  19. Supernovae III R Larger universe Today Bang time Later in Universe What is accelerating the Universe? “Dark Energy!” RAL Seminar

  20. Dark Energy • Repulsive force - “anti-gravity” accelerating the late Universe • Could be a constant - maybe not! • Tiny signal so needs lots of data and clear understanding of errors • Like particle physics! RAL Seminar

  21. Dark Energy II • Energy associated with empty space-time (“vacuum”), but value 10120 too small • GR could be wrong on the scale of the Observable Universe e.g. extra dimensions No compelling theory - therefore we must make progress observational RAL Seminar

  22. What next? • Measure density of dark energy (and dark matter) to higher precision. • Also, does it vary with space and time? • This demands massive new surveys of the sky RAL Seminar

  23. Dark Energy Survey (DES) Gigapixel camera RAL Seminar

  24. 5000 sq degrees in SGP Overlap with VISTA A billion galaxies RAL Seminar

  25. Next generation of radio telescope using an array of phased omni-directional antennas • Nearly 100 stations over europe • Fore-runner to SKA RAL Seminar

  26. Aliens Lofar is 10-90MHz & 110-220MHz (4MHz) Earth-bound signals are a problem for next generation of radio arrays • Motion of host planet (spin & rotation) • d-6 to 10-4 • Modulation of strength? • All this in petabytes of data RAL Seminar

  27. military radar emission (isotropic) SKA could see 108! 50kpc ~10000 stars Sensitivity for 1 month of LOFAR assuming 8kHz res RAL Seminar

  28. Summary • Cosmology is a young subject and continues to grow; by 2010 we will be studying billions not millions • Universe is full of a mysterious “dark energy” pushing the acceleration of the expansion of the Universe. Ironically, it smells a lot like “Einstein's Biggest Blunder” his greatest legacy? • Only gets better - petabytes of data soon in the hunt for ET RAL Seminar

  29. Traditional to think speed is variable but space-time is fixed. But what if the speed of light was fixed? Opens up a new way of thinking of space-time, as a “dynamical” quantity Time to play! Speed = distance/time TIME DISTANCE (SPACE) RAL Seminar

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