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

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

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?

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.

16+34 SN data High-Z Supernova Search Team (HZSST) Riess et al. Astron.J. 116 (1998) 1009 [arXiv:astro-ph/9805201] Citations:3104

Supernova Cosmology Project (SCP) 42 SN data Perlmutter et al. Astrophys.J. 517 (1999) 565 [arXiv:astro-ph/9812133] Citations:3220

The Gold Sample of 157 SNe Ia Riess et al. Astrophys.J. 607 (2004) 665 [arXiv:astro-ph/0402512] Citations:1048 distance modulus Hubble diagram

[arXiv:astro-ph/0402512]

The Supernova Legacy Survey (SNLS) [arXiv:astro-ph/0510447]

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

Three Components

Evolutions of Components

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

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

[arXiv:astro-ph/0005111]

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

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

Observational Data 71 SN Ia from the Supernova Legacy Survey (SNLS) [arXiv:astro-ph/0510447] the baryon acoustic oscillation (BAO) peak found in the Sloan Digital Sky Survey (SDSS) [arXiv:astro-ph/0501171] The cosmic microwave background (CMB) shift parameter from the 3-year Wilkinson Microwave Anisotropy Probe (WMAP) [arXiv:astro-ph/0604051]

Theχ2 Statistic Method distance modulus luminosity distance

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

[arXiv:astro-ph/0702015]

[arXiv:astro-ph/0702015]

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

[arXiv:astro-ph/0702015]

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).

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.

Thank You!