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Register a new userImplications of the B-mode Polarization Measurement for Direct Detection of Inflationary Gravitational Waves
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The prospects for direct measurements of inflationary gravitational waves by next generation interferometric detectors inferred from the possible detection of B-mode polarization of the cosmic microwave background are studied. We compute the spectra of the gravitational wave background and the signal-to-noise ratios by two interferometric detectors (DECIGO and BBO) for large-field inflationary models in which the tensor-to-scalar ratio is greater than the order of 0.01. If the reheating temperature $T_{rm RH}$ of chaotic inflation with the quadratic potential is high ($T_{rm RH}>7.9times10^6$ GeV for upgraded DECIGO and $T_{rm RH}> 1.8times 10^{6}$ GeV for BBO), it will be possible to reach the sensitivity of the gravitational background in future experiments at $3sigma$ confidence level. The direct detection is also possible for natural inflation with the potential $V(phi)=Lambda^4 [1-cos(phi/f)]$, provided that $f>4.2 M_{rm pl}$ (upgraded DECIGO) and $f>3.6 M_{rm pl}$ (BBO) with $T_{rm RH}$ higher than $10^8$ GeV. The quartic potential $V(phi)=lambda phi^4/4$ with a non-minimal coupling $xi$ between the inflaton field $phi$ and the Ricci scalar $R$ gives rise to a detectable level of gravitational waves for $|xi|$ smaller than the order of 0.01, irrespective of the reheating temperature.
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We investigate the effect of the stochastic gravitational wave (GW) background produced by kinks on infinite cosmic strings, whose spectrum was derived in our previous work, on the B-mode power spectrum of the cosmic microwave background (CMB) anisotropy. We find that the B-mode polarization due to kinks is comparable to that induced by the motion of the string network and hence the contribution of GWs from kinks is important for estimating the B-mode power spectrum originating from cosmic strings. If the tension of cosmic strings mu is large enough i.e., Gmu >~ 10^{-8}, B-mode polarization induced by cosmic strings can be detected by future CMB experiments.
(abridged for arXiv) We report results from the BICEP2 experiment, a cosmic microwave background (CMB) polarimeter specifically designed to search for the signal of inflationary gravitational waves in the B-mode power spectrum around $ellsim80$. The telescope comprised a 26 cm aperture all-cold refracting optical system equipped with a focal plane of 512 antenna coupled transition edge sensor 150 GHz bolometers each with temperature sensitivity of $approx300mumathrm{K}_mathrm{CMB}sqrt{s}$. BICEP2 observed from the South Pole for three seasons from 2010 to 2012. A low-foreground region of sky with an effective area of 380 square deg was observed to a depth of 87 nK deg in Stokes $Q$ and $U$. We find an excess of $B$-mode power over the base lensed-LCDM expectation in the range $30< ell< 150$, inconsistent with the null hypothesis at a significance of $> 5sigma$. Through jackknife tests and simulations we show that systematic contamination is much smaller than the observed excess. We also examine a number of available models of polarized dust emission and find that at their default parameter values they predict power $sim(5-10)times$ smaller than the observed excess signal. However, these models are not sufficiently constrained to exclude the possibility of dust emission bright enough to explain the entire excess signal. Cross correlating BICEP2 against 100 GHz maps from the BICEP1 experiment, the excess signal is confirmed and its spectral index is found to be consistent with that of the CMB, disfavoring dust at $1.7sigma$. The observed $B$-mode power spectrum is well fit by a lensed-LCDM + tensor theoretical model with tensor-to-scalar ratio $r=0.20^{+0.07}_{-0.05}$, with $r=0$ disfavored at $7.0sigma$. Accounting for the contribution of foreground dust will shift this value downward by an amount which will be better constrained with upcoming data sets.
Stochastic gravitational wave backgrounds (SGWBs) receive increasing attention and provide a new possibility to directly probe the early Universe. In the preheating process at the end of inflation, parametric resonance can generate large energy density perturbations and efficiently produce gravitational waves (GWs) which carry unique information about inflation. Since the peak frequency of such GWs is approximately proportional to the inflationary energy scale, $Lambda_{mathrm{inf}}$, GWs from preheating are expected to be observed by interferometer GW detectors in low-scale inflationary models. We investigate the dependence of the amplitude of such GWs on $Lambda_{mathrm{inf}}$, and find that the present energy spectrum of these GWs does not depend on $Lambda_{mathrm{inf}}$ only in the case of $Lambda_{mathrm{inf}}$ is above a critical value $Lambda_{c}$, a parameter depending on the resonance strength. We numerically obtain $Lambda_{c}$ in terms of the model parameters in linear approximation and then conduct lattice simulations to verify this result. For $Lambda_{mathrm{inf}}lesssimLambda_{c}$, the amplitude of GWs quickly decreases with $Lambda_{mathrm{inf}}$ and becomes challenging to observe. In turn, observing such GWs in interferometer detectors also helps to determine $Lambda_{mathrm{inf}}$ and the resonance strength during the preheating.
We explore the impact of modified gravity on B-modes, identifying two main separate effects: lensing and propagation of tensor modes. The location of the inflationary peak of the BB spectrum depends on the speed of gravitational waves; the amplitude of the lensing contribution depends on the anisotropic stress. We single out these effects using the quasi-static regime and considering models for which the background and the growth of matter perturbations are standard. Using available data we obtain that the gravitational wave speed is compatible with the speed of light and constrained to within about 10%.
A second generation of gravitational wave detectors will soon come online with the objective of measuring for the first time the tiny gravitational signal from the coalescence of black hole and/or neutron star binaries. In this communication, we propose a new time-frequency search method alternative to matched filtering techniques that are usually employed to detect this signal. This method relies on a graph that encodes the time evolution of the signal and its variability by establishing links between coefficients in the multi-scale time-frequency decomposition of the data. We provide a proof of concept for this approach.
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