1. Probing the Coupling between Dark Components of the Universe
Zong-Kuan Guo
Department of Physics,
Kinki University
13 April 2007

2. Outline
Introduction
Coupled Dark Energy with Dark Matter (physical motivations)
Observational Constraints
Summary

3. Abstract
Recent observations of SN Ia suggest that the universe has entered a stage of an accelerated expansion.
Given this, a dark energy component with negative pressure was suggested to account for the invisible fuel that drives the current accelerated expansion.
However, in the LCDM model there exists a theoretical problem of cosmic coincidence: why is the vacuum density comparable with the critical density at the present epoch in the long history of the universe?

4. Abstract
Cosmological models with a coupling between dark energy and dark matter could provide a phenomenological explanation to the cosmic coincidence problem.
We place observational constraints on a coupling between dark energy and dark matter by using SNLS data, CMB shift parameter and BAO.
Our results indicate that the uncoupled LCDM model still remains a good fit to the data, but the negative coupling with a phantom equation of state of dark energy is slightly favored over the LCDM model.

5. 16+34 SN data
High-Z Supernova
Search Team (HZSST)
Riess et al.
Astron.J. 116 (1998) 1009
[arXiv:astro-ph/ ]
Citations:3104

6. Supernova Cosmology Project (SCP)
42 SN data
Perlmutter et al.
Astrophys.J. 517 (1999) 565
[arXiv:astro-ph/ ]
Citations:3220

7. The Gold Sample of 157 SNe Ia
Riess et al. Astrophys.J. 607 (2004) 665
[arXiv:astro-ph/ ]
Citations:1048
distance modulus
Hubble diagram

8. [arXiv:astro-ph/ ]

9. The Supernova Legacy Survey (SNLS)
[arXiv:astro-ph/ ]

10. Components of the Universe
Dark energy ( ~72% )
Dark matter ( ~23.8% )
Baryons, photons and others( ~4.2% )

11. Three Components

12. Evolutions of Components

13. Dark Energy Models Cosmological Constant
Dynamical Dark Energy, Quintessence
Many other ideas……

14. Problems of Dark Energy
The Fine-tuning Problem
The Cosmic Coincidence Problem

15. [arXiv:astro-ph/ ]

16. The Cosmic Coincidence Problem
The equation of state,
is a constant.
For dark energy
For dark matter
Thus, the ratio
where

17. Coupled Dark Energy with Dark Matter
The flat FRW metric:
The Friedmann equation:
The conservation equations:

18. Observational Data
71 SN Ia from the Supernova Legacy Survey (SNLS) [arXiv:astro-ph/ ]
the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS)
[arXiv:astro-ph/ ]
The cosmic microwave background (CMB) shift parameter from the 3-year Wilkinson Microwave Anisotropy Probe (WMAP)
[arXiv:astro-ph/ ]

19. Theχ2 Statistic Method
distance modulus
luminosity distance

20. We have three parameters
Model I
Assuming
where ξis a constant,
where ,
We have three parameters or

21. [arXiv:astro-ph/ ]

22. [arXiv:astro-ph/ ]

23. Model II Assumingδis a constant,
In this model we have three parameters

24. [arXiv:astro-ph/ ]

25. Summary
The CMB shift parameter provides a stringent constraint on the coupling.
Our results indicate that the uncoupled LCDM model is still remains a good fit to the data.
Perturbation of the coupled dark energy (under consideration).
Construction of the coupling between dark components (under consideration).

26. Future Directions for Cosmology
To release more and more precise observational data and to probe new observable effects.
Based on fundamental physics to propose new models or to find new physics.
To differentiate some models, to exclude some by using current data, and to find new observable effects that can be tested at least by the future experiments.

27. Thank You!