The first light in the universe

The first light in the universe Aniello Mennella Università degli Studi di Milano Dipartimento di Fisica

Photons in the early universe Early universe is a hot and dense expanding plasma 14 May 1964, 11:15 am Photons are coupled to matter via Compton scattering At ~300000 yrs T falls below 3000 K and matter becomes neutral At that time photons freely travel carrying the imprinting of the baby universe We detect it today as a microwave signal, red shifted by a factor ~1000

Perfect black body Radiation and matter in the primordial plasma are in thermal equilibrium Expansion provides a redshift in frequency but maintains the shape unaltered This implies a black body spectrum of the CMB today at a temperature 1000 times smaller

Seeds in the universe CMB angular distribution closely tracks matter density distribution at the last scattering surface COBE T» 10 5 T DMR

CMB polarisation d¾t 0 2 / j^ ² ²^ j d Thompson scattered light is polarised. Only polarisation perpendicular to propagation direction is maintained To produce average polarisation in the CMB photon (and therefore matter) distribution must have a non zero quadrupole.

CMB polarisation Three mechanisms can produce quadrupolar anisotropy Scalar perturbations (perturbations of the energy density at last scattering) Vector perturbations (matter vortical motions) Tensor perturbation (metric perturbations, i.e. gravitational waves) Stokes Q; U! E; B modes

Spherical harmonics T (µ; Á) = 1 X X a`;m Y`;m (µ; Á) `=1 m= ` COBE T» 10 5 T DMR

Power spectrum 180±» 20±» 2±» 100» 10 4 The power spectrum provides a statistical description of `(` + 1)C` (¹K2 ) 104 the angular distribution of anisotropies in the sky 3 2 C` = hja`;m j i 2 1 0 1 10 100 ` 1000 10000

Power spectrum 180±» 20±» 2± 4 Large angular scales (> 1 ): CMB» 100» 10 1000 10000 `(` + 1)C` (¹K2 ) 104 anisotropies trace scalar perturbations in gravitational potential before inflation 3 T 1 ±Á = T 3 Á 2 1 0 1 10 100 `

Power spectrum ± ± ± 180» 20» 2 Medium to small angular scales 4» 10 1000 10000 `(` + 1)C` (¹K2 ) 104 (1 < δθ < 5') trace perturbations driven by acoustic oscillations in the expanding plasma before 3 decoupling» 100 2 2 k c Ä 2 + '0 3 ±T 1 ±Á 1 = T c2 Á 0 1 10 100 `

Power spectrum 180± 4» 20±» 2±» 100» 10 1000 10000 `(` + 1)C` (¹K2 ) 104 Below 5' anisotropies are washed out by photon diffusion 3 happening during recombination. 2 1 0 1 10 100 `

Experimental requirements for high precision anisotropy experiments ±C` = C` s " 2 µpix ¾pix 2 1+ fsky (2` + 1) C` W` # As a first approximation power spectrum uncertainty depends on surveyed area, optical and noise performances

Low multipoles ±C` = C` s " 2 µpix ¾pix 2 1+ fsky (2` + 1) C` W` # Large sky surveys required for accuracy at low multipoles

High multipoles Low noise detectors ±C` = C` s " 2 µpix ¾pix 2 1+ fsky (2` + 1) C` W` #

High multipoles Sub degree angular resolution ±C` = C` s " 2 µpix ¾pix 2 1+ fsky (2` + 1) C` W` #

High multipoles ±C` = C` s " 2 µpix ¾pix 2 1+ fsky (2` + 1) C` W` # Optimal optical response reconstruction

An example from Planck ±C1500 =C1500 Expected Planck performances

Requirements for polarisation measurements `(` + 1) C` (¹K2 ) 106 C`TT 104 102 C`EE 10 C`BB 10 2 10 4 r = 0:1 10 6 1 10 100 1000 ` 10000

Requirements for polarisation measurements `(` + 1) C` (¹K2 ) 106 C`TT 104 102 Sensitive to reionisation C EE ` 10 C`BB 10 2 10 4 r = 0:1 10 6 1 10 100 1000 ` 10000

Requirements for polarisation measurements `(` + 1) C` (¹K2 ) 106 104 102 10 10 2 E mode polarisation between TT 1 2 order of C magnitudes smaller than` T anisotropies B mode even smaller, depending on tensor scalar ratio EE C` Requires new generation of instruments and experiments C`BB Key issues: Low noise detectors r= 0:1 10 4 10 6 Clean optics Modular, scalable instrument architecture 1 10 100 1000 ` 10000

15 years testing the universe 1992 COBE DMR First evidence of temperature anisotropies First points on the power spectrum C2 is somewhat surprisingly low

15 years testing the universe 2000 waiting for WMAP First evidence of temperature anisotropies First points on the power spectrum C2 is somewhat surprisingly low

15 years testing the universe First sub degree CMB measurements 2000 waiting for WMAP Boomerang and MAXIMA measure first peak the universe is FLAT! Several measurements all over the world all speak the same language. The power spectrum actually looks like what it should

15 years testing the universe 2002 DASI detects polarization First faint detection of E mode polarisation B modes still out of reach Ground measurements show their potential for the forthcoming polarisation challenges

2001 WMAP: the baby universe smiles again Instrument Front-end Assembly WMAP W-band feed horn

Systematic error control: the key factor 1. Window function determination WMAP 3yrs: six Jupiter-crossing data sets Beam solid angles found 1% larger than assumed in 1st year (1.5% effect on power spectrum). 2. Radiometer gain model If not corrected for, it would lead to mk effect on maps 1st yr model Gain data New model From WMAP 3 years data (Jarosik et al 2006)

Foregrounds: a complex issue Foreground minumum contamination at ~70 GHz; Polarisation: even at optimal frequencies, signal (on average) is dominated by polarized galactic signal

2003-2008: 5 years of WMAP

2003-2008: 5 years of WMAP C2 is always low...

2003-2008: 5 years of WMAP T E correlation clearly shows a signal

2003-2008: 5 years of WMAP E modes start outlining

2003-2008: 5 years of WMAP B modes still out of reach...

Planck, the next big step ESA space mission, unprecedented combination of sensitivity, angular resolution, sky coverage and systematic error control Successful cold vacuum testing completed this summer Launch with Herschel foreseen first half 2009 14 month continuous observations from L2

Low Frequency Instrument 20 K radiometric array 30 44 70 GHz All polarisation sensitive Single receiver

Spider web PSB High Frequency Instrument 0.1 K bolometric array 100 857 GHz Polarisation sensitive between spaziale I 100 and 353

WMAP Angular resolution Average T/T per pixel/yr Average P/P per pixel/yr Mission lifetime Spectral coverage Detector technology Detector temperature Cooling 14'-56' 40 10-6 56 10-6 4+ yr 23-95 GHz HEMT 90 K Passive PLANCK 5'-33' 2 10-6 4 10-6 >14 months 27-900 GHz HEMT+BOL 20K/4K/0.1K Active 27/10/2008

Imaging the first light ESA-SCI(2005)1 ( Blue book ) COBE resolution

Imaging the first light ESA-SCI(2005)1 ( Blue book ) Planck resolution

Imaging the first light ESA-SCI(2005)1 ( Blue book ) WMAP 2 yrs

Imaging the first light ESA-SCI(2005)1 ( Blue book ) WMAP 8 yrs

Imaging the first light ESA-SCI(2005)1 ( Blue book ) Planck 1 yr

T power spectrum ESA-SCI(2005)1 ( Blue book )

E-mode polarisation ESA-SCI(2005)1 ( Blue book )

B-mode polarisation ESA-SCI(2005)1 ( Blue book ) r = 0:1

Cosmological parameters ESA-SCI(2005)1 ( Blue book ) WMAP, 4yr observation PLANCK, 1yr observation Marginalized posterior distributions for each parameter

Foregrounds WMAP Planck

What after Planck? Many of the outstanding questions standing out from current theories will require new measurements What is dark matter? Does dark energy exist and what is its nature? Did inflation actually happened and what were the conditions? Did the primordial gravitational potential present tensor fluctuations (gravitational waves)? How did structures start forming? When and how the universe reionised on large scales? Many of these issues can be addressed by precision CMB polarisation measurements

The quest for polarisation: experimental challenges Sub microk sensitivity (need for thousand sized arrays of very low noise detectors) Micro packaged coherent receivers (QUIET experiment)

The quest for polarisation: experimental challenges Sub microk sensitivity (need for thousand sized arrays of very low noise detectors) Resistor Antenna Antenna coupled bolometers (Boch, 2002, POLARBEAR exp.) ~ 1 mm Superconducting Nb microstrip

The quest for polarisation: experimental challenges Clean, modular, scalable optical front ends W band 2x2 feed array developed from machined platelets (UniMi, UniMib, 2007)

The quest for polarisation: experimental challenges Polarised foregrounds, systematic error control, calibration... Strong galactic contamination at all latitudes. Need precise measurements of galactic polarisation Hinshaw et al, 2008

Conclusions The CMB is like the snapshot of a newborn baby from which we want to know about his past and infer his future Clearly we want the picture to be a GOOD one! In 50 years the CMB has proved a powerful means to understand the universe. Measurements and theories often wonderfully match But nature never gets short of surprises. And the future is rich with profound and interesting questions. The game is not a quest for the ultimate answer, but to establish a dialogue with the surrounding world which is, ultimately, a part of ourselves