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W. Zhao, D. Baskaran, L.P. Grishchuk School of Physics & Astronomy, Cardiff University

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12th Marcel Grossman Meeting,

Paris, 12-18 July 2009

Stable indications of relic gravitational waves in Wilkinson Microwave Anisotropy Probe data and forecasts for Planck mission

W. Zhao, D. Baskaran, L.P. Grishchuk

School of Physics & Astronomy, Cardiff University

Wales Institute of Mathematical & Computational Sciences

- Motivation
- Improved analysis of WMAP5 data
- Reasons why RGWs can be overlooked in data analysis
- Forecast for Planck
- Summary

The talk is based on:

W. Zhao, D. Baskaran and L. P. Grishchuk,

“Stable indications of relic gravitational waves in Wilkinson Microwave Anisotropy Probe data and forecasts for Planck mission”,

arXiv:0907.1169.

2) W. Zhao, D. Baskaran and L. P. Grishchuk,

“On the road to discovery of relic gravitational waves: the TE and BB correlations in the cosmic microwave background”,

Phys. Rev. D 79, 023002 (2009); arXiv:0810.0756.

Relic Gravitational Waves (RGWs) are necessarily generated by strong variable gravitational field of the very early Universe.

The generation mechanism for RGWs dependsonly on the validity of general relativity and quantum mechanics. (On the other hand, generation mechanisms for Density Perturbations (DPs) require additional assumptions.)

RGWs provide the cleanest probe of the very early Universe.

Theoretical analysis shows that the spectral amplitude of RGWs (at the very long wavelengths) is of the same order of magnitude as that of DPs. (In constrast to inflationary theory that typically predicts DP >> RGW.)

If the observed large-angular-scale anisotropies of CMB are caused by cosmological perturbations of quantum mechanical origin, then a significant fraction of the CMB anisotropies should be due to RGWs.

We revisit WMAP5 data in search of signature of RGWs, and find indications for the presence of RGWs at a level R=0.229. (We analyze the question of why our result is different from the analysis of the WMAP team.)

The recently launched Planck mission (and the ground based detectors) should see this signal at a sufficiently high SNR ( >3 ).

RGWs compete with DPs in generating CMB temperature and polarization anisotropies at relatively low multipoles .

We restrict our analysis to the WMAP5 TT and TE data at .

The RGW contribution to the CMB is quantified by the quadrupole ratio R:

The perturbations are characterized by the power spectra. In the simplest case - amplitude and spectral index : DPs ( ) and RGWs ( , ).

The free parameters in the data analysis are .

( and does not affect the results significantly.)

The parameters are estimated using the Maximum Likelihood method

WMAP5 TT & TE data at l<100.

Theoretical model ( )

WMAP noise estimate

The Maximum Likelihood parameters are

1d and 2d marginalized distributions

The peak value and the associated errors from the 1-dimensional posterior pdf is

Our results (from marginalized 1d pdf):

The stated results are markedly different from the results of the WMAP team

WMAP-team results:

Assumption about the constancy of the spectral index ns

WMAP-team data analysis assumes the constancy of spectral index over a large range of multipoles. The large multipoles predominantly constrain the spectral index (they cannot constrain RGWs since RGWs do not significantly contribute to CMB at these multipoles), and only indirectly constrain .

Exclusive focus on B-mode

RGWs contribute to all of the CMB power spectra : TT, TE, EE and BB. The detection of RGWs should not be equated with observing B-mode. The inclusion of all the information channels can make the difference between observing RGWs and not.

Viable RGW parameters:

Planck instrumental noise:

(significantly lower noise level than WMAP)

Astrophysical foregrounds:

(dust and synchrotron foregrounds)

Removal characterized by parameter

Other possible systematics:

(EB mixing, lensing, etc.)

pessimistic scenario

The detection ability is quantified by the signal-to-noise ratio

Multipole dependence of S/N for various channels

(optimistic noise levels)

(pessimistic noise levels)

The signature of RGWs is not restricted to B-mode alone.

The S/N of B-mode strongly depends on the ability to fight foregrounds and systematics.

S/N for B-mode is accumulated due to the reionization peak.

R=0.229 model will be seen by Planck at S/N:

S/N = 3.6 (pessimistic case)

S/N = 6.7 (optimistic case)

2-sigma threshold for Planck:

R>0.11 (pessimistic case)

R>0.04 (optimistic case)

The improved analysis of the WMAP5 TT and TE data lead to maximum likelihood value R=0.229. (This result is consistent with previous work which gave R=0.24)

We identified the reasons for possible overlooking RGWs in data analysis.

Assumption about constancy of spectral index.

Exclusive focus on B-mode for RGW signal.

Planck satellite mission will be able to see the R=0.229 model with S/N>3.6 (in the most pessimistic noise scenario)

The detection ability is quantified by the signal-to-noise ratio

For the default model R=0.229

In the most optimistic case:

S/N>9 (TT+TE+EE+BB)

S/N~6 (BB alone)

In the most pessimistic case:

S/N>6 (TT+TE+EE+BB)

S/N<2 (BB alone)

Minimally detectable R at 2-sigma

In the most optimistic case:

R>0.03 (TT+TE+EE+BB)

R>0.04 (BB alone)

In the most pessimistic case:

R>0.06 (TT+TE+EE+BB)

R>0.25 (BB alone)