The Mystery of Dark Energy

1. Ogden at 5 The Mystery of Dark Energy Prof Shaun Cole ICC, Durham Photo: Malcolm Crowthers

2. Outline • What is the evidence for dark energy • Why is dark energy so mysterious • Prospects for learning more about dark energy

3. The Expanding Universe

5. Will the expansion continue forever? The mass density required to be on the borderline between perpetual expansion and collapse is called the critical density Measured density

6. Einstein’s Gravity Gravity is caused by space being curved Mass and Energy curve space

7. The Curvature of the Universe

8. If we can do a large scale surveying project we can measure the curvature of the Universe and hence deduce the average mass-energy density of the Universe

9. Last scattering surface

10. Photon-Baryon Plasma Oscillates

11. WMAP 5yr data (March 2008)

12. Angular size Open universe Flat universe with dark energy Fluctuation Strength 

13. Angular size 1 deg 0.1 deg Fluctuation Strength  180 deg

14. What is Dark Energy? • It isn’t any form of matter, dark or not. • It isn’t any visible form of energy. • It is spread uniformly through the Universe. • It is (dark) energy associated with empty space – the vacuum.

15. Properties of Dark Energy If the vacuum has energy it must also have pressure.

16. Properties of Dark Energy If the vacuum has energy it must also have pressure.

17. Negative Pressure Acceleratesthe expansion of the Universe • In general relativity, which is the theory of gravity, both mass-energy and pressure act as sources of gravity. • If the vacuum has negative pressure, it will produce negative gravity and hence push rather than pull.

19. Type Ia SN can outshine whole galaxies

20. Type 1a SN all have the same luminosity • At a given observed redshift distant SN are fainter than one would expect • Implies the expansion of the Universe is accelerating

22. A variety of measures consistently point at these parameter values • LCDM has become the paradigm

23. In Quantum Field theory the vacuum is dynamic and buzzing with virtual particle anti-particle pairs What is Dark Energy? • Energy of the vacuum! • If you think of the vacuum as being nothing then the only natural value for its energy is zero, but …

24. Mysterious Dark Energy • Natural (Planck) value: • Most inaccurate prediction in physics • Requires fine tuning! • Or new physics.

25. Mysterious Dark Energy The transition from mass to DE domination is short.

26. How can we learn more? • The Universe has a flat space-time geometry ( ). • Dark energy has a large negative pressure and is causing the expansion of the Universe to accelerate. • But is Dark Energy really vacuum energy or some other field? • How does if evolve with cosmic time • -what is its equation of state

27. Future Probes of Dark Energy • If we can probe the past rate of expansion of the Universe we can determine more about the DE. • This can be done mapping galaxies and detecting the lBAO at different redshifts/distances.

28. 221,000 redshifts 250 nights on 4m Anglo-Australian Telescope 1996-2002

29. Fluctuation amplitdue Galaxy separation

31. New Larger Surveys

32. Pan-STARRS Haleakala, Hawaii Dec 2008 500,000,000 galaxies Constrain time/redshift evolution of the dark energy

33. ESA Cosmic vision programme Launch 2015 SPectroscopic All-sky Cosmic Explorer

34. ESA Cosmic vision programme Launch 2015 DUNE SPectroscopic All-sky Cosmic Explorer EUCLID

35. Summary Measured curvature and acceleration implies Universe is dominated by dark energy. This is unexpected, but fascinating. New surveys can tell us more about the nature of dark energy.