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Dark Energy & Dark Matter




                             Shamit Kachru (Stanford and SLAC)

                                LASER, Stanford, August 2012




Tuesday, August 7, 2012
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
Fast foward 250+ years:




              We have learned a lot about the constitution and
                    interactions of terrestrial matter!




Tuesday, August 7, 2012
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
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
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
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
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 it's relevant it's more
                                                                           interesting than dark matter
Tuesday, August 7, 2012   Edward A. Baltz (KIPAC)     XXXII SLAC Summer Institute ! Nature's Greatest Puzzles   8/2/04
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
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
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
Expanding Universe encoded in “redshifts”:




Tuesday, August 7, 2012
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
* 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
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
Old supernovae are brighter than they should be -
                because the expansion is accelerating!




Tuesday, August 7, 2012
“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
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
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
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
Thanks for your attention!

                                 Shamit Kachru
                             Department of Physics
                          Stanford University and SLAC

          http://www.stanford.edu/dept/physics/people/faculty/
                          kachru_shamit.html




Tuesday, August 7, 2012

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Dark matter and Dark energy

  • 1. Dark Energy & Dark Matter Shamit Kachru (Stanford and SLAC) LASER, Stanford, August 2012 Tuesday, August 7, 2012
  • 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. Fast foward 250+ years: We have learned a lot about the constitution and interactions of terrestrial matter! Tuesday, August 7, 2012
  • 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. 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. 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. 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. 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 it's relevant it's more interesting than dark matter Tuesday, August 7, 2012 Edward A. Baltz (KIPAC) XXXII SLAC Summer Institute ! Nature's Greatest Puzzles 8/2/04
  • 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. 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. 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. Expanding Universe encoded in “redshifts”: Tuesday, August 7, 2012
  • 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. * 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. 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. Old supernovae are brighter than they should be - because the expansion is accelerating! Tuesday, August 7, 2012
  • 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. 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. 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. 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. Thanks for your attention! Shamit Kachru Department of Physics Stanford University and SLAC http://www.stanford.edu/dept/physics/people/faculty/ kachru_shamit.html Tuesday, August 7, 2012