Dark matter and Dark energy


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Shamit Kachru (Stanford Univ) at a LASER http://www.scaruffi.com/leonardo/aug2012.html

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Dark matter and Dark energy

  1. 1. Dark Energy & Dark Matter Shamit Kachru (Stanford and SLAC) LASER, Stanford, August 2012Tuesday, August 7, 2012
  2. 2. Introduction It was the burden of Newton to teach us that the same laws that we can infer from terrestrial experiments, also apply in the heavens:Tuesday, August 7, 2012
  3. 3. Fast foward 250+ years: We have learned a lot about the constitution and interactions of terrestrial matter!Tuesday, August 7, 2012
  4. 4. But, starting with Newton’s observation, we can infer that what we know about constitutes only a small fraction of what exists: In fact, Newtonian logic suffices to understand 1/4 of this. The other 3/4 requires Einstein.Tuesday, August 7, 2012
  5. 5. Dark Matter Galaxies are massive, gravitationally bound systems that consist of stars and stellar remnants, gas and dust, and, as it happens, a bit more.There are in excess of 170 billion galaxies in the observable Universe. Telescopes capable of imaging distant galaxies came online in the first quarter of the 20th century.Tuesday, August 7, 2012
  6. 6. Fritz Zwicky and Vera Rubin were the first two to argue that there is more to typical galaxies than meets the eye:Tuesday, August 7, 2012
  7. 7. The flatness of the curve indicates that there is more matter present outside the luminous core of the galaxy. Quantitative estimates, based by now on a bewildering variety of independent probes, indicate that there is five times more of this “dark matter” than there is stuff we know about! From indirect tests, as well as e.g. the Bullet Cluster event,Tuesday, August 7, 2012
  8. 8. we know the dark matter interacts with “our” stuff very weakly. (Its self-interactions are also bounded). Many candidates have been proposed for the composition of this stuff (and maybe more than contributes some Dark Matter Candidates - fraction): A Terribly Incomplete Survey Thermal Relics ! equilibrium This list is by no means at early times complete ! I will focus on SUSY ! neutralino several classes SUSY ! gravitino SUSY is perhaps the favored source of dark matter Neutrino candidates standard model + "sterile# Axions are the favored solution right-handed to the strong CP problem Non-thermal Relics ! Neutrinos are the least good everything else candidate, but are known to actually exist Axion Primordial black holes are a Primordial Black Holes possibility ! difficult to form Planck mass and larger There is a dirty little secret ! ??? if its relevant its more interesting than dark matterTuesday, August 7, 2012 Edward A. Baltz (KIPAC) XXXII SLAC Summer Institute ! Natures Greatest Puzzles 8/2/04
  9. 9. We aim to learn about it using either astrophysical “dark matter annihilation” events (e.g. in the center of the galaxy): or via production or direct detection here on Earth:Tuesday, August 7, 2012
  10. 10. Given that “our” 4% of the stuff gives rise to such diverse structures and consequences, it is perhaps reasonable to think that many things could be afoot in the dark sector. Dark Energy This still leaves us with a missing 75%. First of all, how do we know that? It is already perhaps surprising that we could infer the existence of the dark matter, which interacts so very weakly with us. The key to the discovery of dark energy lies back in the original observations of Einstein and Hubble.Tuesday, August 7, 2012
  11. 11. Einstein:Gravitation is geometry. The geometry of space is fixed by the matter within it. Hubble: Space-time at the largest scales illustrates this!Tuesday, August 7, 2012
  12. 12. Expanding Universe encoded in “redshifts”:Tuesday, August 7, 2012
  13. 13. In a simple equation, the spatial slices are expanding in time in a way governed by a(t), the “scale factor”: ds = −dt + a (t)(dx + dy + dz ) 2 2 2 2 2 2 The dynamics of the Universe at large scales is then determined by how the various matter sources “tell” a(t) to behave:Tuesday, August 7, 2012
  14. 14. * Very little mass leads to a Universe which expands forever, with the expansion slowed slightly by the gravitational pull of matter. * In contrast, above a critical amount, there is enough matter that its gravitational pull causes recollapse of the spatial slices -- a “big crunch”. What was completely unexpected in 1998, when the verdict came in, was the third possibility: a Universe that not only continues to expand, but whose expansion accelerates!Tuesday, August 7, 2012
  15. 15. But this is what was found by a detailed study of the properties of distant Type IA supernovae.Because of the way they arise, such supernovae are thought to be ~ “standard candles”.Tuesday, August 7, 2012
  16. 16. Old supernovae are brighter than they should be - because the expansion is accelerating!Tuesday, August 7, 2012
  17. 17. “Normal” matter -- stuff that behaves like dust particles -- and even photons, cannot make the Universe do this.In fact, as nearly as we can tell, the accelerated expansion is caused by a small and positive “cosmological constant” modifying Einstein’s theory -- an energy density of empty space.Tuesday, August 7, 2012
  18. 18. On the one hand, such a cosmological constant should not have been unexpected (though it was). “Virtual” quantum particles, if nothing else, would be expected to generate a cosmological term in our best theories. This would be on top of any “classical” vacuum energy present in our vacuum.Tuesday, August 7, 2012
  19. 19. The best estimates we get from such quantum loops would suggest a “natural” size for the vacuum energy roughly 1060 − 10120 times larger than what we see! How are we to understand this? We’re not sure yet. Amazingly, the most conservative answer today (still untested) relies on vacuum selection occurring in a complicated potential landscape:Tuesday, August 7, 2012
  20. 20. Such a potential landscape seems to arise naturally in string theory, one promising candidate for a theory unifying gravity with other interactions: But to do justice to this subject would require further discussion of eternal inflation, string theory, and even anthropic arguments; all would take us far afield, and we have no more time!Tuesday, August 7, 2012
  21. 21. Thanks for your attention! Shamit Kachru Department of Physics Stanford University and SLAC http://www.stanford.edu/dept/physics/people/faculty/ kachru_shamit.htmlTuesday, August 7, 2012