A theorist’s view of dark energy Andreas Albrecht (UC Davis) UCSC Colloquium Jan 19 2012

1. A theorist’s view of dark energy Andreas Albrecht (UC Davis) UCSC Colloquium Jan 19 2012

2. CONCLUSIONS • Cosmic acceleration has made life really exciting for the theorist • Hardly a closed case

3. CONCLUSIONS • Cosmic acceleration has made life really exciting for the theorist • Hardly a closed case

4. OUTLINE • The Basics: Data, Directions and Issues • Anthropics, Landscape & Critique • Alternative Viewpoints • Conclusions

5. OUTLINE • The Basics: Data, Directions and Issues • Anthropics, Landscape & Critique • Alternative Viewpoints • Conclusions 

6. Cosmic acceleration Accelerating matter is required to fit current data Preferred by data c. 2003  Amount of w=-1 matter (“Dark energy”) “Ordinary” non accelerating matter Supernova  Amount of “ordinary” gravitating matter (Includes Dark Matter)

7. Friedmann Eqn.

8. Friedmann Eqn. Scale factor Dark Energy Curvature Non-relativistic Matter Relativistic Matter

9. Friedmann Eqn. Scale factor Dark Energy Curvature Non-relativistic Matter Relativistic Matter A. Albrecht @ UCD 10/3/11

10. Friedmann Eqn.

11. Friedmann Eqn.

12. Cosmic acceleration Accelerating matter is required to fit current data Preferred by data c. 2003  Amount of w=-1 matter (“Dark energy”) “Ordinary” non accelerating matter Supernova  Amount of “ordinary” gravitating matter (Includes Dark Matter)

13. Cosmic acceleration Accelerating matter is required to fit current data Preferred by data c. 2003  Amount of w=-1 matter (“Dark energy”) “Ordinary” non accelerating matter Supernova  Amount of “ordinary” gravitating matter (Includes Dark Matter)

14. Cosmic acceleration Accelerating matter is required to fit current data Kowalski, et al., Ap.J.. (2008) Preferred by data c. 2008  Amount of w=-1 matter (“Dark energy”) “Ordinary” non accelerating matter BAO  Amount of “ordinary” gravitating matter (Includes Dark Matter)

15. Cosmic acceleration Accelerating matter is required to fit current data Suzuki, et al., Ap.J.. (2011) Preferred by data c. 2011  Amount of w=-1 matter (“Dark energy”) “Ordinary” non accelerating matter BAO  Amount of “ordinary” gravitating matter (Includes Dark Matter)

16. Positive acceleration requires • (unlike any known constituent of the Universe) or • a non-zero cosmological constant or • an alteration to General Relativity.

17. Positive acceleration requires • (unlike any known constituent of the Universe) or • a non-zero cosmological constant or • an alteration to General Relativity. 

18. Positive acceleration requires • (unlike any known constituent of the Universe) or • a non-zero cosmological constant or • an alteration to General Relativity. 

19. Positive acceleration requires • (unlike any known constituent of the Universe) or • a non-zero cosmological constant or • an alteration to General Relativity. 

20. Two “familiar” ways to achieve acceleration: 1) Einstein’s cosmological constant and relatives 2) Whatever drove inflation: Dynamical, Scalar field? • Positive acceleration requires • (unlike any known constituent of the Universe) or • a non-zero cosmological constant or • an alteration to General Relativity.

21. Two “familiar” ways to achieve acceleration: 1) Einstein’s cosmological constant and relatives 2) Whatever drove inflation: Dynamical, Scalar field? • Positive acceleration requires • (unlike any known constituent of the Universe) or • a non-zero cosmological constant or • an alteration to General Relativity.

22. Some general issues: Numbers: • Today, • Field models typically require a particle mass of from

23. Some general issues: Numbers: • Today, • Field models typically require a particle mass of from Where do these come from and how are they protected from quantum corrections?

24. Some general issues: Numbers: • Today, • Field models typically require a particle mass of from Where do these come from and how are they protected from quantum corrections?

25. Some general issues A cosmological constant • Nice “textbook” solutions BUT • Deep problems/impacts re fundamental physics • Vacuum energy problem  = 10120 Vacuum Fluctuations   0 ?

26. Some general issues A cosmological constant • Nice “textbook” solutions BUT • Deep problems/impacts re fundamental physics • Vacuum energy problem (not resolved by scalar field models)  = 10120 Vacuum Fluctuations   0 ?

27. OUTLINE • The Basics: Data, Directions and Issues • Anthropics, Landscape & Critique • Alternative Viewpoints • Conclusions 

28. OUTLINE • The Basics: Data, Directions and Issues • Anthropics, Landscape & Critique • Alternative Viewpoints • Conclusions 

29. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form

30. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form Density Structure forming zone Time 

31. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form Density Structure forming zone Time 

32. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form Density Structure forming zone Time 

33. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form • Can we input that data that we have cosmic structure and predict the (very small) value of Λ? (Life?!) • To do this one requires: • A theory with an ensemble of values of Λ • A way to quantify “having structure” sufficiently

34. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form • Can we input that data that we have cosmic structure and predict the (very small) value of Λ? (Life?!) • To do this one requires: • A theory with an ensemble of values of Λ • A way to quantify “having structure” sufficiently • Weinberg used some simple choices for 1) and 2) and “predicted” a value of Λ in 1987 similar to the value discovered ~10 years later. • Since then string theorists have argued that the string theory landscape delivers a suitable ensemble of Λ’s (Bousso & Polchinski)

35. Anthropics and the value of Λ • Basic idea: • When Λ or radiation dominates the universe structure (i.e. galaxies) cannot form • Can we input that data that we have cosmic structure and predict the (very small) value of Λ? (Life?!) • To do this one requires: • A theory with an ensemble of values of Λ • A way to quantify “having structure” sufficiently • Weinberg used some simple choices for 1) and 2) and “predicted” a value of Λ in 1987 similar to the value discovered ~10 years later. • Since then string theorists have argued that the string theory landscape delivers a suitable ensemble of Λ’s (Bousso & Polchinski)

36. LAB LAB LAB LAB LAB LAB Comment on how we use knowledge (“A” word!) Total knowledge about the universe Input Theory Output

37. LAB LAB LAB LAB LAB LAB Comment on the “A” word: Total knowledge about the universe Input Theory Output

38. LAB LAB LAB LAB LAB LAB Comment on the “A” word: Total knowledge about the universe Input Theory Output

39. LAB LAB LAB LAB LAB LAB Comment on the “A” word: Total knowledge about the universe Input Theory Output

40. LAB LAB LAB LAB LAB LAB LAB PREDICTIONS Comment on the “A” word: Total knowledge about the universe Input Theory Output

41. PRED LAB LAB LAB LAB LAB LAB LAB The best science will use up less here and produce more here Input Theory Output

42. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial)

43. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis

44. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis • These ingredients still not well developed in case of Λ anthropics: • A theory with an ensemble of values of Λ • A way to quantify “having structure” (or alternative condition) sufficiently

45. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis • These ingredients still not well developed in case of Λ anthropics: • A theory with an ensemble of values of Λ • A way to quantify “having structure” (or alternative condition) sufficiently Can get very different answers depending on how these ingredients are realized Banks, Dine & Motl

46. Can get very different answers depending on how these ingredients are realized • Use "entropy production weighting” (Causal Entropic Principle, Bousso et al) • Include variability of world lines due to cosmic structure • Two different behaviors for late time entropy producing in halos Un-normalized probability density Phillips & Albrecht 2011

47. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis • These ingredients still not well developed in case of Λ anthropics: • A theory with an ensemble of values of Λ • A way to quantify “having structure” (or alternative condition) sufficiently Can get very different answers depending on how these ingredients are realized Banks, Dine & Motl

48. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis • These ingredients still not well developed in case of Λ anthropics: • A theory with an ensemble of values of Λ • A way to quantify “having structure” (or alternative condition) sufficiently Can get very different answers depending on how these ingredients are realized Banks, Dine & Motl

49. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis • These ingredients still not well developed in case of Λ anthropics: • A theory with an ensemble of values of Λ • A way to quantify “having structure” (or alternative condition) sufficiently

50. Further comments on anthropics: • Replace “life” with more humble “correlations” and one has a commonplace part of physics (non-controversial) • In my view 2nd law is most robust candidate for anthropic analysis • These ingredients still not well developed in case of Λ anthropics: • A theory with an ensemble of values of Λ • A way to quantify “having structure” (or alternative condition) sufficiently • In my view the string theory landscape is unlikely to survive as a compelling example of 1)