1. Swiss-Chees Cosmology & the Dark Energy Problem Tirthabir Biswas Penn-State University, University Park With Alessio Notari and Reza Mansouri, astro-ph/0606703 With Alessio Notari, astro-ph/0702555 More to come

2. The Dark Energy Problem The Dark Energy Problem The Universe seem to be accelerating today Why is this such a big puzzle? Smallness Problem Smallness Problem Coincidence or Why now Problem Coincidence or Why now Problem

3. The Inhomogeneous Universe Is homogeneity a good assumption? What do we really measure? Redshift Redshift Luminocity Distance Luminocity Distance Equivalently, Equivalently, Evolution history of universe effects D L (z), but Evolution history of universe effects D L (z), but Inhomogeneities also effect light paths, or D L (z) Inhomogeneities also effect light paths, or D L (z)

4. Why and why not ? First Order perturbation theory yields No “average” corrections: Local corrections, Non-linear effects First Order perturbation theory yields No “average” corrections: Local corrections, Non-linear effects Φ is small: Φ is small: At At How are corrections related to Φ ? How are corrections related to Φ ? Cancellation between voids and structures: More of voids Cancellation between voids and structures: More of voids

5. Swiss-Cheese toy Model Spherically symmetric LTB metric Characterized by a single `curvature function’ k(r)

6. How can it work? Scenario I, Average or integrated effect: Scenario I, Average or integrated effect: Light passes through hundreds of patches ~ 30 Mpc and pick up net correction Light passes through hundreds of patches ~ 30 Mpc and pick up net correction Doesn’t work Doesn’t work Scenario II, Local Effect: Scenario II, Local Effect: We are sitting inside a large ~ 300 Mpc void! We are sitting inside a large ~ 300 Mpc void! z<0.1, nearby supernova’s inside the patch, experience a faster local Hubble expansion z<0.1, nearby supernova’s inside the patch, experience a faster local Hubble expansion z>0.1 outside the patch, see average expansion z>0.1 outside the patch, see average expansion

9. Assessment Assessment Supernova works Supernova works Host of other measurements: Host of other measurements: CMB (fits 1 st peak), baryon oscillation (fits the data), local density measurements (preliminary check) CMB (fits 1 st peak), baryon oscillation (fits the data), local density measurements (preliminary check) Needs more comprehensive analysis. Needs more comprehensive analysis. How likely is our void and our central position? How likely is our void and our central position? Large Angle anomalies in CMB can be explained! [Silk, Inoue] Large Angle anomalies in CMB can be explained! [Silk, Inoue] Reports 25% less galaxy around a 300 Mpc region Reports 25% less galaxy around a 300 Mpc region [Frith et al] [Frith et al]