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

21 November 2005Bruce Bassett SA 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


SCALPEL AND SALT

’

SCALPEL AND SALT

BRUCE A. BASSETTICG/SAAO

Darragh O’Donoghue (SAAO)Ed Elson (SAAO/NASSP)

Kurt van der Heyden (SAAO)Ricky Olivier (SAAO)

Bob Nichol (ICG)Dan Carson (ICG)


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!


SCALPEL AND SALT

Correction for

Brightness-Decline

relation

reduces scatter

in nearby SN Ia

Hubble Diagram

Riess et al. 96

a(t)


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

0a


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


SCALPEL AND SALT

Cosmic chronometers

• Excellent constraints on DE come from measuring the Hubble expansion rate

where

Then…dt

dz

zzH

1

1)(

a

azH

)( )(/11 taz

redshift


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 liketo live here!


SCALPEL AND SALT

But why should this work?

Simulated spectrafor an LRG as a functionof age from 10 million years to 13 billion years(top to bottom)

Notice the steady Reddening with age… Z=0..2


SCALPEL AND SALT

The current state-of-the-art

CDM! 32 LRG spectra

Simon et al


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


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

%3100

%30~


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!


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!


SCALPEL AND SALT


SCALPEL AND SALT

Documents

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