If inflation was preceded by a radiation era then at the time of inflation there will exist a decoupled thermal distribution of gravitons. Gravitational waves generated during inflation will be amplified by the process of stimulated emission into the existing thermal distribution of gravitons. Consequently the usual zero temperature scale invariant tensor spectrum is modified by a temperature dependent factor. This thermal correction factor amplify the B-mode polarization of the CMB by an order of magnitude at large angles, which may now be in the range of observability of WMAP.PACS numbers: PACS No.: 04.30. Db, 04.62.+v, 98.80.Cq Inflation [1], in addition to solving the horizon and flatness problems of the standard hot big-bang model, generates a nearly scale invariant density perturbations, which has been tested in the observations of the CMBR angular spectrum. One prediction of inflationary models which has not yet been tested is the existence of a nearly scale invariant spectrum of gravitational waves [2,3]. The definitive test of the existence of these cosmological gravitational waves would be the observation of B mode polarization in the CMB. The recent WMAP three year results [4] give only an upper bound on the B mode polarization, (l+1)l 2π C BB l=(2−6) < 0.05(µK) 2 .In this paper we show that if inflation was preceded by a radiation era then there would be a thermal background of gravitons at the time of inflation. This thermal distribution of gravitons would have decoupled close to Plank era. The generation of tensor perturbation during inflation would be by stimulated emission into this existing thermal background of gravitational waves. This process changes the power spectrum of tensor modes by an extra temperature dependent factor coth(k/2T ). At large angular scales (l ≤ 30) the power spectrum P T = A T k nT of gravitational waves generated during inflation would have a spectral index n T = −1 − 2ǫ, instead of the standard slow roll inflation prediction n T = −2ǫ which implies that C BB l=3 ≃ 10 × C BB l=30 . If a thermal enhancement of low l BB modes exists then it will be observable with WMAP or in the upcoming Planck [5] experiment. In the conclusion, we discuss the implications of inflationary models from the observations or non-observation of this low l thermal enhancement.

In warm inflation models there is the requirement of generating large dissipative couplings of the inflaton with radiation, while at the same time, not de-stabilising the flatness of the inflaton potential due to radiative corrections. One way to achieve this without fine tuning unrelated couplings is by supersymmetry. In this paper we show that if the inflaton and other light fields are Pseudo-Nambu-Goldstone Bosons then the radiative corrections to the potential are suppressed and the thermal corrections are small as long as the temperature is below the symmetry breaking scale. In such models it is possible to fulfill the contrary requirements of an inflaton potential which is stable under radiative corrections and the generation of a large dissipative coupling of the inflaton field with other light fields. We construct a warm inflation model which gives the observed CMB-anisotropy amplitude and spectral index where the symmetry breaking is at the GUT scale.Comment: Some typos and numerical errors corrected. Version to be published in Physics Letters

Natural inflation driven by pseudo-Nambu-Goldstone bosons have a problem that the nearly scale invariant spectrum of density perturbations is attained only when the symmetry breaking scale is of the order of Planck scale. We show here that if one couples the PNGB to a thermal bath as in warm inflation models, the amplitude and spectral index which agrees with the Wilkinson Microwave Anisotropy Probe (WMAP) data is obtained with the symmetry breaking in the GUT scale. We give a GUT model of PNGB arising out of spontaneously broken lepton number at the GUT scale which gives rise to heavy Majorana masses for the right handed neutrinos which is needed in seesaw models. This model also generates a lepton asymmetry because of the derivative coupling of the PNGB to the lepton current. A characteristic feature of this model is the prediction of large non-gaussianity which may be observed in the forthcoming PLANCK experiment.

In this work, we consider a network of cosmic strings to explain possible deviation from ΛCDM behaviour. We use different observational data to constrain the model and show that a small but non zero contribution from the string network is allowed by the observational data which can result in a reasonable departure from ΛCDM evolution. But by calculating the Bayesian Evidence, we show that the present data still strongly favour the concordance ΛCDM model irrespective of the choice of the prior.

If the universe had a large curvature before inflation there is a deviation from the scale invariant perturbations of the inflaton at the beginning of inflation. This may have some effect on the CMB anisotropy at large angular scales. We calculate the density perturbations for both open and closed universe cases using the Bunch-Davies vacuum condition on the initial state. We use our power spectrum to calculate the temperature anisotropy spectrum and compare the results with the WMAP three year data. We find that our power spectrum gives a lower quadrupole anisotropy when $\Omega-1 >0$, but matches the temperature anisotropy calculated from the standard Ratra-Peebles power spectrum at large $l$. The determination of spatial curvature from temperature anisotropy data is not much affected by the different power spectra which arise from the choice of different boundary conditions for the inflaton perturbation.Comment: 17 pages, 4 figures, revtex4; section on comparison with WMAP3 data adde