1. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT SA COSMO Main aim is to provide a place where students and post-docs can meet and learn from each other, give talks, and start collaborations Usually about 50-70% of talks given by students/post-docs Meeting is very informal, so please ask questions Speakers – remember your audience is very wide! Should happen every ~4 months

2. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT     ’      SCALPEL AND SALT BRUCE A. BASSETT ICG/SAAO Darragh O’Donoghue (SAAO) Ed Elson (SAAO/NASSP) Kurt van der Heyden (SAAO) Ricky Olivier (SAAO) Bob Nichol (ICG) Dan Carson (ICG)

3. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT The problem with modern cosmology It doesn’t make sense – cosmos is accelerating! The Universe on very large scales is dominated by an anti-gravity we call “dark energy” What is this dark energy? It requires negative pressure…a new form of energy!

4. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT Correction for Brightness-Decline relation reduces scatter in nearby SN Ia Hubble Diagram Riess et al. 96 a(t)

5. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT How can we hunt dark energy? If we can measure a(t), the “size” of the universe as a function of time, we can learn about DE Acceleration implies that currently

6. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT Probes of a(t) Type Ia supernovae – good but not well- understood CMB – good, but not really sensitive to DE Clusters – nice, but exponentially sensitive to systematic errors Weak lensing – nice, but requires good seeing

7. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT Cosmic chronometers Excellent constraints on DE come from measuring the Hubble expansion rate where Then… redshift

8. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT But how do we measure dt? Choose galaxy pairs at nearby redshifts (dz) Estimate the difference in their ages, dt, from the difference of their spectra But, can’t use any old galaxies! Have to select very simple ones – Luminous Red Galaxies are the best…``red and dead” LRGs like to live here!

9. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT But why should this work? Simulated spectra for an LRG as a function of age from 10 million years to 13 billion years (top to bottom) Notice the steady Reddening with age… Z=0..2

10. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT The current state-of-the-art  CDM! 32 LRG spectra Simon et al

11. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT SALT Cosmic Ages of Luminous Passive ELipticals survey Use the fact that we can take many (~5) spectra simultaneously with RSS on SALT Primary target: LRG’s at z=0.5 (optimal redshift window) We will get 20-40 LRG spectra per night on SALT Hence in 20 nights we should get 400-800 high-S/N LRG spectra LRG Target Catalog

12. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT So what? We want to dissect the Universe at z=0.5, hence the name – SCALPEL With 300 pairs of LRG spectra we get ~300 estimates of H(z) If each estimate of H(z) is independent and accurate to ~30% we get an estimate of H(z) at z=0.5 accurate to

13. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT Conclusions Aim: to achieve an estimate of H(z) at z=0.5 that is as accurate as we have at z=0 Since LRG’s are primarily found at the center of clusters we can estimate the cluster masses at the same time. We will learn a lot about LRG’s May make a key breakthrough regarding dark energy!

14. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT Thanks to… Joao, Robert and the CTP for hosting this event which we hope will be regular The local organising committee for doing all the hard work: Martin Cook Norman Ives and the rest of the LOC… All the speakers and participants… See you at the next meeting!

15. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT

16. 21 November 2005Bruce BassettSA COSMO SCALPEL AND SALT