Observing Cosmic Inflationwith

Precision MicrowaveBackground Polarimetry

H. Cynthia ChiangUniversity of KwaZulu-Natal

NITheP Seminar, WitsMay 20, 2014

Big Bangt = 0

End of inflationt = 1e-35 sec

EW symmetry breakingt = 1e-12 sec

Dark matter decouplingt = 1e-10 sec

Quark-hadron transitiont = 1e-5 sec

Neutrinodecoupling

t = 1 sec

Electron-positronannihilation

t = 5 secBBNt = 3 min

Matter-rad.equality

t = 56 kyr

Formation of CMBt = 400 kyr

Reionizationt = 0.2 gyr

Matter-lambdaequalityt = 9.5 gyr

You are heret = 13.7 gyr

Image: Planck

Gravitational waves

Image: Monty Python

History of the universe

Temperature fluctuations in the CMB

Image: COBE DMR

T = 2.728 K

T = 3.353 mK

T = 18 K

CMB: uniform “afterglow” of the Big Bang (2.73 K),snapshot of universe ~400 kyr after its birth

Temperature dipole: caused by Doppler shift from bulk motion of solar system (amplitude ~ 3.4 mK)

Tiny temperature fluctuations: first detected byCOBE DMR with an amplitude of ~18 K atlarge angular scales

Temperature hot and cold spots correspond to small density fluctuations in the early universe, the seeds for all structure visible today

A picture of the infant universe, courtesy of Planck

Quantifying temperature anisotropies

A power spectrum is a “blob sorter”: it describes how many spots of each size are present in the picture.

Small angular scaleLarge angular scale

Most of the CMBtemperature spots are ~1 degree wide(multipole ell ~ 200)

Number of spots at each size

What do the temperature fluctuations tell us?

We seem to understand the basic physics of the early universe...but it's a strange picture. The universe is mostly made of dark energy and dark matter, which we don't understand!

http://www.strudel.org.uk/planck

But there's more – the CMB is polarised! Does the polarisation give us the same six numbers, and what more can we learn?

Six numbers: the Lambda and Cold Dark Matter (LCDM) model

Dark matter density Baryon density Reionization optical depthSpectral index (tilt) Scalar amplitude Hubble constant

(LAMBDA)

Image: M. Hedman

Quadrupole moment inincident radiation field

Scattered radiationis linearly polarised

Cold spot

Hot spotElectron

Observer's lineof sight

Polarisation in the CMB

CMB is intrisically polarised because of temperature anisotropies

Mechanism: Thomson scattering within local quadrupole moments

Polarised signal is small: ~100x weaker than temperature anisotropies!

“E” or “gradient” mode polarisationhas no handedness

“B” or “curl” mode polarisation hashandedness, i.e. rotation direction

We can decompose a polarisation map...

Two flavors of polarisation

We expect them to be there because of scattering processes in the CMB Temperature anisotropies predict E-mode spectra with almost no extra information Not only that, but “standard” CMB scattering physics generates ONLY E modes.

E modes are the CMB's “intrinsic polarisation”

So then where do B modes come from?

Inflation: exponential expansion of universe (x 1025) at 10-35 sec after big bang. “Smoking gun” signature = gravitational wave background that leaves a B-mode imprint on CMB polarization!

Gravitational lensing by large scale structure converts some of the E-mode polarisation to B-mode. Use this to study structure formation, “weigh” neutrinos.

How can we tell the difference between the above two? Degree vs. arcminute angular scales.

The moral of the story: B modes tell us things about the universe that temperature and E modes can't.

The buzz about B modes

Gravitational waves of

Gravitational waves on (r = 1)

CMB polarisation power spectra

Arcminute-scale B-mode from weak gravitational lensing by large-scale structure, partial conversion of E-modes

Degree-scale B-mode from gravitational waves, amplitude described by the tensor-to-scalar ratio r.

Both flavors of B-mode polarisation are much fainter than E-mode, and they appear at distinct angular scales.

E-mode is mainly sourced by density fluctuations and is the intrinsic polarisation of the CMB

E-mode

B-mode

Current CMB polarisation measurements

E-mode polarisation measured with high precision: acoustic peaks have been detected and are consistent with LCDM

NEWS FLASH: the first detections of B-mode polarisation were reported just in the last year!

Inflationary: BICEP2 detected r = 0.2

Lensing:Detections by SPT and Polarbear, consistent with theoretical expectations

BICEP2

South PoleTelescope

March 17, 2014

October 4, 2013

CMB experiments at the South Pole

BICEP: 2005 – 2008BICEP2: 2010 – 2012

SPT: 2006 – 2011SPTpol: 2012 –

ACBAR:1998 – 2005

DASI: 1999 – 2003QuaD: 2004 – 2007KECK: 2011 –

Photo: Stefen Richter

South Pole is high and dry: atmospheric water vapor is a source of noise

Long winter night means stable atmospheric conditions

Sky never sets – can observe the same field 24 hours a day

Excellent infrastructure and support staff

The BICEP2 experiment

Minimize polarization systematicsBoresight rotation

Simple refractor, no mirrors

Optimize to 30 < < 300Beam size ~ 0.5 deg FWHM

Focal planeFrequency: 150 GHz

Field of view ~ 17 deg

Observed sky fraction ~ 2%

Small aperture (26 cm)

Entire telescope is cooled

256 orthogonal TES pairs(512 total detectors)

E- and B-mode maps from BICEP2

BICEP2 data Simulations with r = 0

E-m

od

eB

-mo

de

Note factor of ~6 difference in color scaleB-mode data show excess structure

compared to r=0 simulations

B-mode power spectrum and implications

B-mode power spectrumtemporal split jackknifelensed-ΛCDM r=0.2

5.3 sigma significance in excess B-mode power

Measured r is directly related to potential energy of field driving Inflation:r = 0.2 implies 2 x 1016 GeV

Field driving Inflation is moved by ~5x Planck mass, which is a challenge for model building

Would you bet your dog, house, or life...?

BICEP2 measurement is r = 0.2. Previous (temperature) data from Planck suggests that r < 0.1 at 95% conf. Tension is highly significant; ~0.1% unlikely (Smith et al., arXiv:1404.0373)

A few miscellaneous oddities in the power spectra, e.g. high points in E- and B-mode, nonzero points in EB cross-spectrum...

What about Galactic foregrounds?

Are we seeing systematic effects from the instrument or from the data processing? Doesn't take much leakage to cause a lot of trouble...

Reasons to pause and scratch your head:

The biggest fears of CMB experimentalists:

The trouble with foregrounds

30 GHz 44 GHz 70 GHz

100 GHz 143 GHz 217 GHz

343 GHz 545 GHz 857 GHz

“It's like more than just bugs on a windshield that we want to remove to see the light, but a storm of bugs all around us in every direction.” – Charles Lawrence re: foreground removal

Main contaminants: Galactic dust and synchrotron

Thermal dust emission: dust grains are nonspherical, emit along their longest axis, and align perpendicular to Galactic magnetic field. Emission increases with frequency.

Synchrotron emission: electrons spiral around Galactic magnetic field lines and radiate. Emission decreases with frequency.

Others: free-free, anomalous “spinning dust,” point sources...all expected to have low polarization

Foreground tests from BICEP2

BICEP2 cross-correlated maps with several foreground models. Resulting amplitude is small, and subtracting the most conservative model reduces r down to ~0.1.

Cross-correlated BICEP2 with BICEP1: different instrument operating at 100 GHz. B-mode signal persists at 3 sigma. (But cross-correlation with BICEP1 150 GHz is not inconsistent with r = 0...)

The bottom line: constraint on spectral index from BICEP2 x BICEP1 disfavours pure dust or synchrotron at ~2 sigma level, i.e. only 2-sigma confidence that the origin of the signal is cosmological.

(Also, spectral index constraint doesn't consider dust PLUS synchrotron, which could potentially mimic a flatter, CMB-like spectrum at 150 GHz.)

We've seen B modes. Time to kick back and relax?

BICEP1 winterover at the South Pole

– 60°C

Inflationary B modes: “There is no strong evidence that the detected B modes are not cosmological. However, there is no strong evidence that the detected B modes are cosmological, either.” – Eiichiro Komatsu. We need independent confirmation.

Lensing B modes: SPT and Polarbear have first detections = proof of principle.

We need more S/N to use this as a tool for constraining neutrino mass.

Nope. The party's just starting...

SPIDER: a new instrument for CMB polarimetry

SPIDER science goals

Measure inflationary B modes with sensitivity of r < 0.03 at 3

Characterize polarized foregrounds

Instrumental approach

Need high sensitivity, fidelity

Long duration balloon platform (2 flights, 20+ days each)

0.5 deg resolution over 8% of the sky, target 10 < ell < 300

6 compact, monochromatic refractors in LHe cryostat

2600 detectors split between 90,150, 280 GHz

Polarization modulation: HWPs

Balloon launch pad, McMurdo station, Antarctica

SPIDER test integration in Texas, USA

Flight track Launch from McMurdo station, circumnavigate continent in ~2 weeks

Float altitude: 40 kmVolume: 1 million m3

Max payload weight: 3600 kg More info: BLAST the movie,

EBEX launch on youtube

Antarctic long-duration ballooning

Flight cryostat2.

4 m

eter

s

Dry weight: 850 kg

Hold time: 20+ days

Main tank: 1200 liters LHe, 4K

Capillary-fed superfluid tank: 16 liters LHe, 1.4K

Two vapor cooled shields, 30K and 150K

Waveplates

Sapphire HWP, AR-coated with Cirlex on both sides Invar mounting ring

Cold encoders, +/- 0.1 deg absolute accuracy

Worm gear drive,+/- 0.05 deg backlash

Cold stepper motor

Five waveplates installed during July 2013 commissioning

SPIDER's six telescopes

Superfluid tankMain tankVapor cooled shieldsThermal contact pads

Capillary system

Six independent, monochromatic telescopes: 3 each at 90 and 150 GHz

Telescope insert

Each insert tuned for a single frequency band

90 lbs each: lightweighting + stiff carbon fiber truss

Two-lens optical design (based on BICEP)

Extensive efforts to optimize magnetic shielding

Focal plane: antenna-coupled TES bolometers

8mm

Each spatial pixel:Two orthogonal antenna arrays16 x 16 dipole slot antennas

Detectors: Al / Ti TES bolometers

Each focal plane: 4 tiles x 64 pixels x 2 polarizations = 512 detectors

SPIDER flight plan

Flight dateFocal plane and detector distribution Cumulative noise, K/deg2

90 GHz 150 GHz 280 GHz 90 GHz 150 GHz 280 GHz

Dec 20143 x FPs =

8643 x FPs =

1536– 0.27 0.20 –

Dec 2015?2 x FPs =

5762 x FPs =

10242 x FPs =

10240.21 0.16 0.62

SPIDER will map 8% of the sky in an exceptionally clean region (encompasses the “southern hole”)

First flight: 90 GHz and 150 GHz to maximize sensitivity for a B-mode detection

Second flight: expand frequency coverage to further characterize the signal

First flight: December 2014!

Potential instrument systematics

Relative gain uncertaintyDifferential pointingDifferential beam sizeDifferential ellipticityAbsolute polarization angleRelative polarization angleTelescope pointing uncertaintyBeam centroid uncertaintyPolarized sidelobes (150 GHz)Optical ghostingHWP differential transmissionMagnetic shielding at focal plane

0.5%5%

0.5%0.6%

1°1°

10 arcmin1.2 arcmin

-17 dBi-17 dB

0.7%10 K/B

e

Instrument property Benchmark (r = 0.03) Status

0.1% in Boomerang1% in SPIDER

0.3% in SPIDER0.15% in BICEP2

0.7° in BICEP0.1° in BICEP

2.4 arcmin in BoomerangAchieved by BICEPAchieved by BICEP

Achieved by BICEP2Achieved by SPIDERAchieved by SPIDER

Uncertainties in calibration quantities can leak T, E into B

Define r = 0.03 benchmark for systematics: false BB < 0.002 K2 at ell ~ 100

Use signal simulations to calculate false BB from systematic errors

Instrument characterization is still work in progress, but we are cautiously optimistic based on experience with other similar experiments

What will Spider do for you?

SPIDER has enough sensitivity to constrain r < 0.03 at 3 (even with foregrounds).

With high sensitivity, multiple frequencies, and extended sky/ell coverage, SPIDER will clearly distinguish primordial B modes and Galactic foregrounds.

If r = 0.2, we still have sensitivity to spare to restrict our analysis to a clean patch of sky.

SPIDER status: counting down to a December flight!

Preparing for cooldown

Team SPIDER owns the machine shop!

Insert assembly LDB cryostat on the gondola

The extended CMB polarimetry family

EBEX

PIPER

QUBIC

QUIJOTE

Planck

ACTPol SPTpol

ABS BICEP2/Keck

GroundBIRD

Polarbear

SPIDER

CLASS

POLAR-1

Large angular scales Medium angular scales Small angular scales