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

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

The Expanding Universe

The Expanding Universe

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

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

The Curvature of the Universe

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

Last scattering surface

Photon-Baryon Plasma Oscillates

WMAP 5yr data (March 2008)

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

 Angular size 1 deg 180 deg 0.1 deg

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

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

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

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

Type Ia SN can outshine whole galaxies

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

A variety of measures consistently point at these parameter values A variety of measures consistently point at these parameter values  CDM has become the paradigm  CDM has become the paradigm

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

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

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

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 How can we learn more?

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

250 nights on 4m Anglo-Australian Telescope ,000 redshifts

Galaxy separation Fluctuation amplitdue

New Larger Surveys

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

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

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

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