nithep wits node seminar: dr. h. cynthia chiang (university of kwa-zulu natal)
DESCRIPTION
TITLE: "Observing Cosmic Inflation with Precision Microwave Background Polarimetry" http://www.nithep.ac.za/4i2.htmTRANSCRIPT
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