1. 1 On the road to discovery of relic gravitational waves: From cosmic microwave background radiation Wen Zhao Department of Astronomy University of Science and Technology of China The 6th KIAS Workshop on Cosmology and Structure Formation

2. 2 Question: Why do RGWs Exist?  In the curved space-time, the vacuum state at “time 1” naturally corresponds to the multi-particle state at “time 2”.  Assuming 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- invariant. The amplitude of the spectrum directly depends on the energy scale of inflation:. Chaotic inflations predict r~O(0.1), (Linde, 1986)

3. 3

4. 4 CMB power spectra Density perturbationsRelic gravitational waves r=1

5. 5 Detection in the CMB  Method a: BB channel But B-polarization is very small. When the noise is large, this channel is useless.  Method b: TT+EE+TE channels Total TT = TT (dp,+) + TT (gw,+) Total EE = EE (dp,+) + EE (gw,+) Total TE = TE (dp,+) + TE (gw,-) This method is limited by cosmic variance. When r<0.05, these three channels will be useless.

6. 6 Detection in the CMB 1. BICEP2 2. WMAP & Planck 3. Forecasts 3.1 Planck B-mode polarization 3.2 Ground-based experiments 3.3 Next-generation satellites

7. 7 BICEP2: Constructed E & B

8. 8 BICEP2: BB Spectrum & RGWs

9. Alternative Explanations

11. Recently, Colley & Gott (arXiv:1409.4491) cross-correlated genus statistic in BICEP2 and Planck 353GHz data, and got:

12. 12 Relic gravitational waves in WMAP (Planck) data WMAP9 gives: r<0.13 at 95% C.L. (Komatsu et al 2012)  This constraint comes from TT+TE observations.  (Planck)TT + (WMAP9)TE + highL even give: r<0.11 at 95% C.L. (Planck Collaboration, 2013)  BB only gives r<2.1 at 95% C.L. (WMAP Collaboration, 2010)

13. Why?  1. All TT+TE data, the multipoles till to around 10,000  2. Power-law form for density perturbations is correct at least for four orders in scale

15. 15 WMAP low-multipole TE data

16. 16 Low-multipole TT+TE data (l<100) 1. power-law forms for d.p. and g.w. : (WMAP/Planck case) 2. power-law d.p. and g.w., but l<100 data (Our case)

17. 17 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  4. power-law d.p. and g.w., but l<100 data + SNIa + SDSS

18. FORECAST: Planck B-mode Polarization Ground-based and balloon-borne experiments Next generations (CMBPOL, COrE, LiteBird et al.)

19. 19 Detecting “B-mode” by “Planck” 1. Planck can detect B-mode, if r>0.05. 2. Planck can only detect the reionization peak at l<10. However, 1) “cosmic variance” is too large: 2) low-multipole TT data 3) depends on the nearly "full-sky" show a lot of “anomalies”; map (but most regions are dirty) 3. It seems that Planck CANNOT directly confirm or exclude the BICEP2 results at recombination peak (multipole l~80), because the instrumental noises in this region are too large! 4. Indirect comparison may be possible. For example, Planck HF data  dust template  dust map at 150GHz (BICEP2 region)  comparing with BICEP2 map

20. 20

21. 21 Other CMB experiments Ground-based experiments: KECK, BICEP3, QUIET, POLARBEAR, SPTPOL, ABS, QUIJOTE, ACTPOL, QUBIC, CLASS, et al. Balloon-borne experiments: EBEX, PIPER, Spider, et al.  Survey plan: a small part of full sky for a long time.  Noise level: cosmic lensing limit.  Frequency channels: multi-frequency channels. http://en.wikipedia.org/wiki/List_of_cosmic_microwave_background_experiments

22. 22 Noise levels

23. 23 Detection limits

24. 24 Fourth Generation: Planned CMBPOL (COrE, LiteBird, PRISM, PIXIE) experiments

25. 25 Testing inflationary consistency relation: nt=-r/8 ------ key issue for inflation

26. 26 Summary, but not conclusion!  Detections of CMB B-mode polarization are mainly at two peaks: 1. recombination peak at l~80; 2. reionizatin peak at l<10. It is the unique window for the low-frequency GW detection.  BICEP2 claimed r=0.2, but other explanation is also possible; WMAP (Planck) low-multipole data favor r~O(0.1) (but large uncertainty).  Forthcoming observations: 1. Planck (alone): recombination peak seems difficult to be detected (limited by instrumental noises); reionization peak is possible (limited by CV, contamination in Galactic plane); cross-correlation with BICEP2 is helpful. But if it can say "YES" or "NO" for BICEP2? 2. Ground-based experiments (KECK, BICEP3, SPTPOL, POLARBEAR, et al.): recombination peak is possible, so long as r>0.01. 3. Next generations (CMBPOL, COrE, LiteBird et al.): detection is possible for r>0.001; inflationary consistency relation is possible to be tested, if r>0.1.

27. 27 THANKS!