Once and FutureRedshift Surveys

UK National Astronomy Meeting8 April 2005

Matthew CollessAnglo-Australian Observatory

Large-Scale Structure in Different Model Universes

• The large-scale structure of the galaxy distribution, on scales from millions to billions of light-years, depends on…– the amounts of the

various constituents of the universe (ordinary matter, dark matter, dark energy etc.)

– the recipe for how galaxies are formed (when, where, and with what bias relative to the dark matter)

100 million light-years

The universe in a computer

The million brightest galaxies on the sky

The Expansion of the Universe• In 1929, Edwin Hubble found that…

– all distant galaxies are moving away from our Milky Way galaxy;

– the further away they are, the faster they are receding from us.

• Hubble correctly interpreted this as the isotropic expansion of the universe.

E.H.

• Hubble’s Law: at low redshift, the recession velocity of a galaxy is proportional to its distance.

• Measuring redshifts (recession velocities) gives distances.

A redshift survey of a strip of sky is

a slice through the 3-D galaxy

distribution

You are here

Las Campanas Redshift Survey~25000 z’s

CfA Survey~15000

z’s

State of the Art in the mid-1990’s

Comparison of Redshift Surveys

2dFGRS

SDSS

The 2-degree Field spectrograph

AAO technology combining robotics and optical fibresObtains spectra for 400 galaxies at once2dF enabled a huge redshift survey

The 2dF Galaxy Redshift Survey

NGP strip

SGP strip

~2000 deg2

Random fields

~250,000 galaxies

NGP R.A. strip

SGP R.A. strip

z 0.3

You are here

The 2dF Galaxy Redshift Surveymap of 221,000

galaxies

Observed

CDMbias #1

SCDMbias #1

SCDMbias #2

CDMbias #2

Cosmology by eye!

Cole et al. 2005, astro-ph/0501174

The Galaxy Power

Spectrum• The final galaxy redshift-space power spectrum from full 2dFGRS

• The acoustic oscillations (“baryon wiggles”) are detected at the 4 significance level

Total matter, dark matter and baryon densities

Total matter and Hubble constant: mh = 0.168±0.016Baryonic matter fraction:b/m= 0.185±0.046

Direct from 2dFGRS alone2dFGRS and WMAP combined

Total matter density: m = 0.2310.021Cold dark matter density: CDM = 0.1890.020Baryonic matter density: b = 0.0420.002

Constraints on the neutrino mass

=0.05

=0.01=0

Elgaroy et al., 2002, PRL, 89, 061301

P(k) gives an upper limit on the total mass of all species

2dFGRS: /m < 0.13 m,tot < 1.8 eV (95% confidence)

Best previo

us bound

/m< 0.13

+WMAP: /m < 0.05 m,tot < 0.7 eV (95% confidence)

Cosmology and LSS results from the 2dFGRS

• The large-scale structure of the galaxy distribution is precisely determined on size scales from about 1 million light years to about 1 billion light-years

• The properties of the galaxy distribution confirm that the large-scale structure grows by gravitational instability… quantum(?) fluctuations emerging from the Big Bang are amplified by gravity to become galaxies, clusters and superclusters

• The total density of all matter in the universe is M = 0.230.02 there is only 23% of the matter needed to

make the universe flat• The total density in ordinary matter is B = 0.0420.002

baryons are 18% and CDM 82% of the matter in the universe

• Neutrinos make up less than 5% of all the matter in the universe the total mass of the 3 neutrino species is less than

0.7 eV

1919%%

7777%%

SNSNeeCMCMBB

SNe Hubble Relation

• Measure the brightnesses (distances) for many SNe at different redshifts to obtain the Hubble relation

• Look for deviations from a simple straight line out to nearly z~1 the expansion is seen to be accelerating Redshift

Rela

tive

Brig

htne

ss

Di

stan

ce

Hubble Relation for High-Redshift Supernovae

• However, using SNe at higher z, we see at first the expansion of the universe accelerating, then at larger distances the expansion decelerating

2003

WMAP

The CMB power spectrum from WMAP

Standard Model

Cosmic Microwave

Background +

Large-Scale Structure

+ Distant

Supernovae +

HST Key Project

the geometry of the universe is flat

there is less than the critical density of matter

the expansion of the universe is accelerating

the overall scale (size/age) of the universe

2dFGRS• The geometry of the universe is flat (CMB)

• Matter makes up 23% of the energy density in the universe (2dFGRS)

• The expansion of the universe is accelerating (SNe)

• Dark energy makes up 77% of the energy density in the universe (any two of the above)

The State of the Universe

??

??

??

The Composition of the Universe

tot = 1.02 0.02 = 0.77 0.04DM = 0.19 0.02B = 0.040.002 < 0.01

The Cosmic Timeline

• Hubble constant: H0 = 71 4 km/s/Mpc (HST KP H0= 72 7 km/s/Mpc)

• CMB last scattering surface: tCMB = 379 8 kyr

• Epoch of Re-ionization: tEoR = 100 - 400 Myr

• Age of the universe today: t0 = 13.7 0.2 Gyr

Dark Matter and Dark Energy

• Although we now know the amounts of all the major constituents of the universe we still have two major gaps in our knowledge…

What is the dark matter?

What is the dark energy?

What more can z-surveys say about dark matter?

• Additional information comes from measuring the distances as well as the redshifts to obtain both the galaxy density and velocity distributions

• Since galaxy velocities are directly produced by gravity from the matter distribution, this jointly constrains both luminous and dark matter

The 6dF Galaxy Survey• The 6dFGS is

now mapping the mass & motions in the very local (z<0.1) universe

• Currently 2/3 of southern sky mapped (DR2 next week); complete mid-2005

• The survey aims to measure redshifts for a NIR-selected (2MASS) sample of 150,000 galaxies, plus velocities for 15,000 galaxies (10x previous surveys)

The AAO’s UK Schmidt Telescope

The 6dF fibre spectrograph

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The 6dFGS map of the local universe

Predicted 6dFGS galaxy power spectrum

Effective volume

shot noise/modePredict

ed errors

Non-linear regime

Predicted 6dFGS velocity power spectrum

Predicted errors

Larger errors reflect smaller size of survey and 1D peculiar velocities

• For cosmological models specified by:– the power spectrum amplitude

and shape (Ag, )– the redshift-space distortion ()– the galaxy-mass correlation (rg)

… the errors from the combined redshift and velocity surveys are 1-3% in all four parameters

… the velocity survey much improves the joint constraint on and rg, which are now only relatively weakly correlated

z-only z+v1 contours on pairs of parameters

Constraints from joint redshift-velocity survey

Burkey & Taylor (2004)

What can z-surveys say about dark energy?

• Is dark energy Einstein’s cosmological constant, or new physics?

• The nature of the dark energy affects the geometry of the universe, which can be measured by comparing the structure in the galaxy distribution (specifically, the apparent scale of the acoustic oscillations) at different times.

• The geometry of the universe is measured at early times by the CMB, and at late times by the 2dFGRS.

• Redshift surveys of ~106 galaxies at intermediate redshifts (say, z~1 and z~3) could map the geometry over the full span of cosmic time and see the transition from a dark matter dominated universe to a dark energy dominated universe.

The Dark Energy Equation of State

• The equation of state for dark energy is the ratio of pressure to density as a (potentially evolving) function of redshift, w(z)

• For cosmological constant , w-1 and does not change with z

• For new physics, this is not the case: w(z) = w0 + w1z +…

• Determining w(z), esp. if w0-1 and w10, would be a major step towards understanding the nature of the dark energy

• Measuring the geometry of the universe may therefore provide a window on new physics (quantum gravity? string theory?)

• w(z) requires high-precision versions of the classical cosmological tests, as likely effects on geometry are small

• Use the ‘standard rod’ provided by scale of acoustic oscillations in the galaxy power spectrum (‘Doppler peaks’ in CMB)

Equation of State from Acoustic

Oscillations

Current constraints on the EoS• Constraints from z-surveys, cluster

evolution, BBNS, SNe & CMB• The 95% confidence interval is

approximately -1.5<w<-0.8 (w=-1) is consistent with all data; Big

Rip (w<-1) is possible

ΩmΩm

2dFGRS2dFGRS

Allen et al., 2004, MNRAS, 353, 457

• Measure acoustic oscillations with redshift surveys of large-scale structure at high redshift:– z~1: 900,000 gals, 1000 deg2

– z~3: 600,000 gals, 150 deg2

• For interesting cases with w10, z-surveys give similar constraints to other methods (e.g. SNe)

• Combining methods (LSS, CMB, SNe) increases the precision in measuring both w0 and w1

• Redshift survey method has the advantage that it should be less subject to systematic errors

Redshift Surveys & Dark Energy

WFMOS on Gemini/Subaru

• WFMOS is ideal for massive surveys of high-redshift galaxies:– 8-metre telescope– 4500 optical fibres– 1.5 degree field of

view

Once and Future Redshift Surveys

• Wide-field spectroscopy has been, and will continue to be, a very powerful tool for studying large-scale structure and cosmology

• Combined with CMB, SNe and other observations, the 2dFGRS and SDSS provide a precise picture of the low-redshift universe

• The combined 6dF redshift-velocity survey will provide additional constraints on the relation between luminous and dark matter

• Future massive redshift surveys at high redshift with AAOmega, FMOS and WFMOS can trace the evolution of galaxies and large-scale structure, and reveal the nature of the dark energy