Cosmic Microwave Background Polarization and Detection of Primordial Gravitational Waves

Wen Zhao @ Depart. of Astronomy, USTC 1 BACKGROUNDS

 Cosmic Microwave Background (CMB) radiation, including the temperature and polarization anisotropies, have proved to be a valuable tool to study the physics in the early Universe.

 The current observations of WMAP and Planck satellites have given fairly good results (especially the CMB TT and TE power spectra). These results have tightly constrained most of the cosmological parameters, and proved the existence of the inflationary stage in the very early Universe.

 Planck satellite, as well as various ground-based (BICEP, QUIET, CLOVER, POLARBEAR, QUIJOTE, ACTPOL, SPTPOL, QUBIC, ABS, CLASS, KECK, SPT, ACT … ), balloon-borne (EBEX, PIPER, Spider … ), and space-based experiments (CMBPOL, B-POL, LiteBird, COrE … ) have the much smaller noises than WMAP, and will give the much better results (especially the EE and BB polarization power spectra) in the near future, which will provide a great chance to study the physics of the very early Universe.

 CMB polarizations encode the cosmological information at recombination epoch and reionization epoch, which provides a great complementary channel for the CMB temperature anisotropy. In addition, B-mode polarization provides the unique way to detect the primordial gravitational waves . (tensor perturbations) 2 OUTLINE

 CMB field and Polarization

 Primordial gravitational waves & CMB

 Conclusion

3 Temperature and Polarization of the CMB

4 E and B types of polarization of the CMB field

5 Thomson Scatter and Generation of Polarization

Two factors: ** Temperature Anisotropies ** Free electrons

Two stages: @ recombination stage @ reionization stage

E-Polarization & B-Polarization

6 7 CMB power spectra

Density perturbation Gravitational waves

r = 1

8 9 Polarization observations (Before BICEP2)

(Chiang et al 2010)

10 Polarization observations (including BICEP2)

11 Primordial gravitational waves and CMB

12 Generation of RGWs

 In the curved space-time, the vacuum state at the stage 1 naturally corresponds to the multi-particle state at a different stage 2.

 If considering the Universe, we can assume the universe had a vacuum state of graviton at the initial stage. With the expansion of the Universe, but the radiation-dominant stage, the gravitons were naturally generated. (Grishchuk, 1974)

 If the Universe had an exact de-Sitter expansion in the inflationary stage, the generated primordial power spectrum of RGWs is scale- invariance. The amplitude of the spectrum directly depends on the Hubble parameter, which directly relates to the energy scale of the early Universe.

13  In the inflationary stage, the expansion of the scale factor is much faster than that of the horizon, so the gravitational waves in the small scale becomes the waves in the large scale, and the amplitudes were keeping constant (proportional to the Hubble parameter), when they crossed the horizon.

 After this stage, the expansion of the scale factor becomes slower than that of horizon, the waves reentered the horizon, (the smaller wavelength, the earlier entering). When they crossed the horizon, the amplitudes began to decrease with the expansion of the universe.

14 Why important?

 For Physics: * Gravitational wave detection * Gravity at high energy scale, Quantum Gravity

 For Cosmology: ------unique way for pre-recombination stage * Pre-inflationary stage, * Birth of Universe * Inflation physics (energy scale, evolution, et al.) * Phase transitions in early Universe (QCD, e+e-, supersymmetry, et al.) * Cosmic strings, cosmic walls, et al.

15 What? (properties)

 Stochastic signal – something random, Gaussian, noisy, unpredictable

 Background signal – something happening almost everywhere, in all directions, at all times

 All frequency – from 10^{-18}Hz to 10^{10}Hz

16 17 DETECTION OF RGWS

(Smith et al 2006 18 See also WZ & Zhang 2006b) Detecting in the CMB: WMAP  Planck  (BICEP) CMBPol

19 CMB power spectra

Density perturbations Relic gravitational waves

r=1

20 Detection in the CMB

 There are two sources to generate the CMB power spectra: density perturbations and relic gravitational waves

 Method a: Detecting the B-polarization, which is only generated by relic gravitational waves. But the signal of B-polarization is very small. When the noise is large, this channel is useless.

 Method b: Detecting in the TT, TE, EE data: Total TT = TT (dp,+) + TT (gw,+) Total TE = TE (dp,+) + TE (gw,-) Total EE = EE (dp,+) + EE (gw,+) This method is limited by cosmic variance of d.p. When r<0.05, these three channels will be useless.

21 Relic gravitational waves in WMAP data

The current WMAP observation gives a quite tight constraint on the relic gravitational waves, i.e. r<0.36 at 95% C.L. (Komatsu et al 2010)

 This constraint comes from the WMAP7 TT+TE observations.

 BB observation only gives r<2.1 at 95% C.L. (Komatsu et al 2010)

 (Planck)TT + (WMAP9)TE + highL even give the r<0.11 at 95% C.L. (Planck Collaboration, 2013)

This constraints are not stable, since they base on the strong assumption on the density perturbation, especially the spectral index ns, which can easily overlook the contribution of gravitational waves!

22 WMAP TE data

23 24 WMAP TT+TE data (l<=100)

 1. power-law forms for d.p. and g.w.

 2. d.p. with running

 5. piece-wise form for d.p. and g.w.

 3. power-law d.p. and g.w., but l<100 data (red curves)

 4. power-law d.p. and g.w., but l<100 data + SNIa + SDSS

25 Using Planck (TT) + WMAP (TE) data

26 Planck mission and RGWs

 Planck was launched in 14, May, 2009. It surveyed 28 months. The instrumental noise of Planck is about 2 order smaller than that of WMAP.

 The observations of TT, TE and EE power spectra by Planck satellite will be to multipole l~2000.

 Planck may detect the B-mode polarization if r>0.05.

27 Simulated Planck data (r=0.1)

28 Detecting relic gravitational waves by Planck satellite

 For Planck, TT+TE+EE information channel still plays crucial role for the detection of relic gravitational waves.

 For the optimistic case (optimistic foreground removal and excellent E-B mixture removal), Planck can B-polarization, only if r>0.05.

 If B-mode detectable, Planck can only detect the reionization peak at l<10. It may be the only chance to detect these largest scale B-mode before CMBPOL.

 Planck is not good at for the detection of gravitational waves (I will explain it later!).

29 Detecting relic gravitational waves by Planck satellite

30 Other Projects

S/N is determined by two factors: 1) sky survey area 2) noise level.

Planck: full sky but large noise Ground: lower noise but small partial survey CMBPOL or COrE: lower noise + full sky survey

31 Planck or Ground-based experiment?

32 Various ground-based experiments

 Ground-based experiments can well detect the CMB polarization by observing a small part of full sky for a long time.

 In the near future, there are several ground-based experiments will begin to works, including QUaD, BICEP, QUIET, POLARBEAR, ABS, CLASS, ClOVER, QUIJOTE, ACTPOL, SPTPOL, QUBIC, KECK and so on.

 The instrumental noises of the experiments in the near future are very close to the cosmic lensing limit.

 In addition to the space-based Planck satellites and the various ground-based experiments, some balloon-borne experiments, such as the EBEX, PIPER, Spider. They have the similar detection ability as the ground-based experiments.

33 Current Observations (Before BICEP2)

(Chiang et al 2010) 34 BICEP1 + QUaD data

35 BICEP1: Updated

(Barkast et al 2014) 36 BICEP2: Telescope in the mount

37 38 39 BICEP2: Abstract

40 41 BICEP2: Observed Q & U

42 BICEP2: Constructed E & B

43 44 BICEP2: BB Spectrum & RGWs

45 46 47 48 49 Noise levels

50 Detection limits

51 52 Fourth Generation: Planned CMBPOL experiment

(see CMBPol white book for details)

53 CMBPOL: Accurate measurements

54 Testing inflationary consistency relation

55 Ideal CMB experiment

 Cosmic lensing generates the E-B mixtures, and forms a nearly white B- mode spectrum.

 For the ideal experiment, where only the reduced cosmic lensing contamination is considered.

 Detection limit:

56 Conclusion

 CMB polarization was formed in the recombination stage (z~1100) and reionization stage (z~10). Now, with the observing of Planck mission, as well as other detectors, CMB polarization (including TE, EE and BB) becomes one of the key probes of the early Universe.

 Detection primordial gravitational waves, especially by the B-mode polarization, is one of the main tasks for the current and future CMB experiments, including Planck , ground-based experiments , CMBPol and ideal experiment .

 BICEP2 has detected the signal of gravitational waves with r=0.2, which corresponds the energy scale of inflation . In principle, it can be confirmed by the forthcoming Planck polarization data.

57 THANKS!

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