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1 Andreas Albrecht UC Davis APS Meeting Philadelphia, April 2003 Title Cosmic acceleration and fundamental physics

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2 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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3 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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4 As far as we can tell so far, the cosmic acceleration might be the result of I.1 The cosmological constant problem / fundamental gravity

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5 = 10 120 0 ? Vacuum Fluctuations Greatest unsolved problem in physics: Why is I.1 The cosmological constant problem / fundamental gravity

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6 Challenges: 0 dream may not work A more mature fundamental theory of gravity could overturn ideas about the origins of acceleration Huge potential for observations to impact fundamental theories I.1 The cosmological constant problem / fundamental gravity

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7 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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8 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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9 I.2 The “why this/why now” problems Why this? Where does this strange parameter come from? (not really a separate “why now problem”, with )

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10 Why now? If the source of acceleration is some dynamical matter (“quintessence”), then it has to acquire the value today (t=14.5 Gyr). (not some other time) Today Time I.2T he “why this/why now” problems

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11 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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12 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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13 I.3 Quantum corrections/long range forces If the acceleration is produce by a scalar field (quintessence): i) The quintessence field must have a mass How can this value stay safe from quantum corrections?

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14 If the acceleration is produce by a scalar field (quintessence): ii) If such a small mass is preserved, there is a new long range force mediated by the quintessence field. How can 5 th force bounds be evaded? (Carroll) I.3 Quantum corrections/long range forces

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15 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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16 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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17 ia) The entropy bound: If there really is a , the late-time asymptotic state of the universe will have a horizon with area: This implies a Hawking-Beckenstein type upper bound on the entropy: Are we then compelled to only consider only models of physics with a finite dimensional Hilbert space? (Excludes field theory, M-theory, SHO, etc.) T. Banks & W. Fischler II.1 The quantum physics of

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18 ib) Holography & the Causal Patch Two basic facts about de Sitter space: II.1 The quantum physics of Observer’s Worldline 1) Objects never appear to leave the horizon Const. t Const. r (Gibbons & Hawking) 2) A free field appears at Temp: [ Black Hole Analogy]

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19 ib) Holography & the Causal Patch Two basic facts about de Sitter space: II.1 The quantum physics of Observer’s Worldline 1) Objects never appear to leave the horizon Const. t Const. r See Kaloper (Davis Inflation Meeting ’03) (t’ Hooft: BH case) NB: Tollman taught us Increasing (divergent) T(r) 2) A free field appears at Temp:

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20 ib) Holography & the Causal Patch Causal Patch “unification” hypothesis: II.1 The quantum physics of 1) Objects never appear to leave the horizon 2) A free field appears at Temp. (divergent at horizon) Holographic (quantum gravity) “magic” gives Cutoff for “divergent” entropy at r H area entropy law “All Physics” inside causal patch*. Objects thermalize as they approach the horizon (no “outside” = no “inside” of Black Hole) Consistency of the above across different observers. *The end of inflation? (Dyson et al ’02) O This observer sees divergent entropy here, cut off by quantum gravity. Banks Bousso Fischler t’Hooft Susskind No physics out here

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21 ii) Other constraints on fundamental theories with : Do scattering states exist? (Banks, Fischler) Does string theory exist in de Sitter space? (see Kachru et al ’03) Does quantum de Sitter space exist? (Goheer et al ’03) II.1 The quantum physics of

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22 iii) Causal set theory of gravity: a prediction of causal set theory?! (Ahmed et al. Sep 2002) II.1 The quantum physics of

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23 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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24 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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25 Inflation has taught us how to accelerate the universe with a scalar field… why not try again? With f 10 18 GeV, M 10 -3 eV PNGB: Frieman, Hill, Stebbins, & Waga 1995 M=1 meV - 1GeV Zlatev, Wang & Steinhardt 1998 Scalar field domination/ acceleration II.2 Dark energy/quintessence, Stage I (adventures)

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26 Inflation has taught us how to accelerate the universe with a scalar field… why not try again? With f 10 18 GeV, M 10 -3 eV PNGB: Frieman, Hill, Stebbins, & Waga 1995 M=1 meV - 1GeV Zlatev, Wang & Steinhardt 1998 Scalar field domination/ acceleration N.B Resurrect the 0 dream II.2 Dark energy/quintessence, Stage I (adventures)

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27 Some more scalar field models of dark energy: Dodelson Kaplinghat & Stewart 2000 Time Albrecht & Skordis 1999 Oscillating Shadowing There are many other examples II.2 Dark energy/quintessence, Stage I (adventures)

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28 A note about the “why this/now” problem: Essentially all existing quintessence models “solve” it by relating to parameters in the quintessence equations Attractor behavior gets rid of initial conditions dependence How compelling this “solution” is depends on how convinced you are that nature has chosen those parameters. II.2 Dark energy/quintessence, Stage I (adventures)

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29 A note about Quantum corrections/long range forces: Almost all models of dynamical dark energy/quintessence completely ignore these issues. “adventures” II.2 Dark energy/quintessence, Stage I (adventures)

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30 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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31 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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32 II.3 Dark energy/quintessence, Stage II (getting serious) Stage II: Take quantum corrections/long range forces seriously *PNGB: Frieman, Hill, Stebbins, & Waga 1995 * Extra dimensions A.A, Burgess, Ravndal 2001 * Supergravity Kalosh et al. 2002 Challenging particle physics issues

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33 Stage II: Take quantum corrections/long range forces seriously *PNGB: Frieman, Hill, Stebbins, & Waga 1995 Challenging particle physics issues Also: Risky because more radical developments could invalidate these efforts II.3 Dark energy/quintessence, Stage II (getting serious) * Extra dimensions A.A, Burgess, Ravndal 2001 * Supergravity Kalosh et al. 2002

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34 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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35 I) Challenges for models of cosmic acceleration I.1 The cosmological constant problem / links to fundamental gravity. I.2The “why this/why now” problems I.3 Quantum corrections/long range forces III) Conclusions II) Exciting Directions II.1 The quantum physics of II.2 Dark energy/quintessence, Stage I (adventures) II.3 Dark energy/quintessence, Stage II (getting serious) Cosmic acceleration and fundamental physics, APS April 2003

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36 III) Conclusions Modeling cosmic acceleration leads to: Exciting challenges and new directions, many of which are connected with very fundamental questions. Real opportunity for dramatic observational advances Weller & AA

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37 z % deviation Parameters and Cosmic Confusion IV-37 NB: Don’t be afraid of degeneracies!! Data can still distinguish among many interesting models Maor et al.

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