Determination of tensor spectral index in the CMB

Wen Zhao @ IHEP . 2012

Introduction• The models of inflation predict the primordial power spect

rum of the primordial (relic) gravitational waves.

• Detection of RGWs provides a chance to study how the Universe was born.

• The only way to detect very low frequency RGWs is by studying the Cosmic Microwave Background Radiation (CMB).

• This talk will discuss how well we can determine the RGWs by the CMB observations.

The choice of pivot wavenumber

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, POLARBERA, QUIET, ClOVER, QUIJOTE, ACTPOL, SPTPOL, QUBIC 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 (EBEX, PIPER, Spider). They have the similar detection ability as the ground-based experiments.

Planck or ground-based experiment?

Difference of Planck and Ground-based Experiments

• If detectable, Planck can only detect the reionization peak at l~6.

• Ground-based (or balloon-borne) experiments can only detect the recombination peak at l~100.

• CMBPol will detect both peaks!

Planck + PolarBear

(WZ & Zhang 2009)

Planned CMBPol (COrE, LiteBird) experiment

(see CMBPol white book for details)

(WZ, 2011)

CMBPol------ instrumental noises + lensing

CMBPol------ foregrounds

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:

(WZ & Baskaran 2009)

Testing inflationary consistency relations(WZ & Huang, 2011)

Single-field slow-roll inflations• consistency relation

• Results: r > 0.14 for EPIC-2m r > 0.06 for Ideal

Potential-driven G-inflations

Conclusions• The CMB observations provide an excellent opportunity to determine

the RGWs.

• We have derived the analytic formulae for: best-pivot wavenumber; signal-to-noise ratio and uncertainty of nt.

• The uncertainties of r and nt strongly depends on the cosmic reionization, especially for nt.

• Ignoring the foreground contaminations, Planck: nt~0.25 for r=0.1.△ Ideal: nt~0.007 for r=0.1△• The inflationary consistency relations are quite difficult to be tested.

Thank you!