1. Cosmology with the extragalactic gamma-ray background Vasiliki Pavlidou University of Chicago

2. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 Outline oEGRB vs CMB: information content oCosmology with the EGRB: constraining the Cosmic Star Formation Rate oThe future

3. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 EGRB vs CMB CMBEGRB oOrigin: cosmologicalcosmological truly diffuseunresolved point sources oForegrounds:Milky WayMilky Way oEnergies: ~ 2.7 K ~ 10 9 eV (1GeV) oSpectrum:blackbodypower law

4. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 EGRB: how is it measured? what does it look like? oSpace-born gamma-ray telescopes: EGRET aboard CGRO (1990’s) LAT aboard GLAST (about to be launched) o Take all -sky map, subtract: Emission from the Milky Way Point sources Credit: Strong et al 2004

5. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 What makes up the EGRB? oGuaranteed contributions: established classes of gamma-ray emitters Normal galaxies Active galaxies Extragalactic unidentified sources oTruly diffuse emission? oExotic physics?

6. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 And now… cosmology

7. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 The Cosmic Star Formation Rate oThe Cosmic Star Formation Rate: how much gas mass is converted to stars per unit time per unit cosmic volume oAn essential measure of: baryonic energy production feedback processes in galaxy formation stellar contribution to reionization metal production oTraditional measures: SF makes stars - young stars emit in UV, IR compilation by Hopkins & Beacom 2006

8. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 The SF - gamma-ray connection oStar Formation -> Supernovae -> Cosmic ray acceleration - > interaction with ISM -> gamma rays Characteristic normal galaxy spectral feature imprinted on spectral shape of gamma-ray background oStar Formation -> Background starlight (EBL) -> interaction with gamma rays EBL imprinted on spectra of: individual  -ray sources,  -ray background Starforming galaxies (VP & Fields 2002) Unidentified sources (VP, Siegal-Gaskins, Fields, Olinto & Brown 2007) Blazars (VP & Venters 2007) EGRET gamma-ray background, conservative (Strong et al 2004) Maximal EGRET gamma-ray background (Sreekumar et al 1998) Stecker 1999

9. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 How do we utilize this connection? oUntil now: use knowledge of CSFR to predict signal/effects for gamma-ray telescopes oThe future: GLAST observations will allow inversion of the problem: use observations of gamma-ray signal/effects to constrain CSFR oUncertainties: significant, BUT largely uncorrelated with uncertainties of low-E methods Strigari, Beacom, Walker & Zhang 2005 Prodanovic, VP & Fields preliminary

10. Vasiliki Pavlidou Great Lakes Cosmology Workshop 8 Ohio State University, 01June 2007 Conclusions oCosmic star formation history imprinted on extragalactic gamma-ray background: Normal galaxy spectral feature @ ≈ 1GeV EBL absorption pileup/suppression @ ≥20GeV oGLAST will: resolve thousands of bright point sources (e.g. AGNs) but at most 3 normal galaxies -> normal galaxy feature expected to become visible Probe the >20GeV regime, map the shape of high-E absorption feature oA new era: observations of the EGRB can strongly constrain the cosmic history of star formation