Stable indications of relic gravitational waves in Wilkinson Microwave
  Anisotropy Probe data and forecasts for the Planck mission
  The relic gravitational waves (gw) are the cleanest probe of the violent
times in the very early history of the Universe. They are expected to leave
signatures in the observed cosmic microwave background anisotropies. We
significantly improved our previous analysis [1] of the 5-year WMAP $TT$ and
$TE$ data at lower multipoles $\ell$. This more general analysis returned
essentially the same maximum likelihood (ML) result (unfortunately, surrounded
by large remaining uncertainties): the relic gw are present and they are
responsible for approximately 20% of the temperature quadrupole. We identify
and discuss the reasons by which the contribution of gw can be overlooked in a
data analysis. One of the reasons is a misleading reliance on data from very
high multipoles $\ell$, another - a too narrow understanding of the problem as
the search for $B$-modes of polarization, rather than the detection of relic gw
with the help of all correlation functions. Our analysis of WMAP5 data has led
to the identification of a whole family of models characterized by relatively
high values of the likelihood function. Using the Fisher matrix formalism we
formulated forecasts for {\it Planck} mission in the context of this family of
models. We explore in details various `optimistic', `pessimistic' and `dream
case' scenarios. We show that in some circumstances the $B$-mode detection may
be very inconclusive, at the level of signal-to-noise ratio $S/N =1.75$,
whereas a smarter data analysis can reveal the same gw signal at $S/N= 6.48$.
The final result is encouraging. Even under unfavourable conditions in terms of
instrumental noises and foregrounds, the relic gw, if they are characterized by
the ML parameters that we found from WMAP5 data, will be detected by {\it
Planck} at the level $S/N = 3.65$.
