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Register a new userLimits on the detectability of the CMB B-mode polarization imposed by foregrounds
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We investigate which practical constraints are imposed by foregrounds to the detection of the B-mode polarization generated by gravitational waves in the case of experiments of the type currently being planned. Because the B-mode signal is probably dominated by foregrounds at all frequencies, the detection of the cosmological component depends drastically on our ability for removing foregrounds. We provide an analytical expression to estimate the level of the residual polarization for Galactic foregrounds, according to the method employed for their subtraction. We interpret this result in terms of the lower limit of the tensor-to-scalar ratio r that allows to disentangle the cosmological B-mode polarization from the foregrounds contribution. Polarized emission from extragalactic radio sources and gravitational lensing is also taken into account. As a first approach, we consider the ideal limit of an instrumental noise--free experiment: for a full--sky coverage and a degree resolution, we obtain a limit of r~10^(-4). This value can be improved by high--resolution experiments and, in principle, no clear fundamental limit on the detectability of gravitational waves polarization is found. Our analysis is also applied to planned or hypothetical future polarization experiments, taking into account expected noise levels.
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QUIJOTE (Q-U-I JOint TEnerife) is an experiment designed to achieve CMB B-mode polarization detection and sensitive enough to detect a primordial gravitational-wave component if the B-mode amplitude is larger than r = 0.05. It consists in two telescopes and three instruments observing in the frequency range 10-42 GHz installed at the Teide Observatory in the Canary Islands, Spain. The observing strategy includes three raster scan deep integration fields for cosmology, a nominal wide survey covering the Northen Sky and specific raster scan deep integration observations in regions of specific interest. The main goals of the project are presented and the first scientific results obtained with the first instrument are reviewed.
We consider the effectiveness of foreground cleaning in the recovery of Cosmic Microwave Background (CMB) polarization sourced by gravitational waves for tensor-to-scalar ratios in the range $0<r<0.1$. Using the planned survey area, frequency bands, and sensitivity of the Cosmology Large Angular Scale Surveyor (CLASS), we simulate maps of Stokes $Q$ and $U$ parameters at 40, 90, 150, and 220 GHz, including realistic models of the CMB, diffuse Galactic thermal dust and synchrotron foregrounds, and Gaussian white noise. We use linear combinations (LCs) of the simulated multifrequency data to obtain maximum likelihood estimates of $r$, the relative scalar amplitude $s$, and LC coefficients. We find that for 10,000 simulations of a CLASS-like experiment using only measurements of the reionization peak ($ellleq23$), there is a 95% C.L. upper limit of $r<0.017$ in the case of no primordial gravitational waves. For simulations with $r=0.01$, we recover at 68% C.L. $r=0.012^{+0.011}_{-0.006}$. The reionization peak corresponds to a fraction of the multipole moments probed by CLASS, and simulations including $30leqellleq100$ further improve our upper limits to $r<0.008$ at 95% C.L. ($r=0.01^{+0.004}_{-0.004}$ for primordial gravitational waves with $r=0.01$). In addition to decreasing the current upper bound on $r$ by an order of magnitude, these foreground-cleaned low multipole data will achieve a cosmic variance limited measurement of the E-mode polarizations reionization peak.
A linear polarization field on a surface is expressed in terms of scalar functions, providing an invariant separation into two components; one of these is the B mode, important as a signature of primordial gravitational waves, which would lend support to the inflation hypothesis. The case of a plane already exhibits the key ideas, including the formal analogy with a vector field decomposed into gradient plus curl, with the B mode like the latter. The formalism is generalized to a spherical surface using cartesian coordinates. Analysis of global data provides a path to vector and tensor spherical harmonics.
We develop a systematic and unified approach to estimate all possible secondary (i.e. non-primordial) nonlinear effects to the cosmic microwave background (CMB) polarization, named curve-of-sight integration approach. In this approach, the Boltzmann equation for polarized photons is rewritten in a line-of-sight integral along an exact geodesic in the perturbed universe, rather than a geodesic in the background universe used in the linear-order CMB calculation. This approach resolves the difficulty to solve the Boltzmann hierarchy with the nonlinear gravitational effects in the photon free-streaming regime and thus unifies the standard remapping approach for CMB lensing into the direct approach solving the Boltzmann equation for the nonlinear collisional effects. In this paper, we derive formulae that: (i) include all the nonlinear effects; (ii) can treat extended sources such as the contributions after the reionization. It offers a solid framework to discuss possible systematics in the standard estimation of CMB lensing by the remapping approach. As an explicit demonstration, we estimate the secondary B-mode power spectrum induced by all foreground gravitational effects: lensing, redshift, time-delay, emission-angle, and polarization-rotation effects. We define these effects properly so that they do not have any overlap, also without overlooking any effect. Then, we show that these effects only give corrections of the order of 0.001-0.01% to the standard lensing-induced B-mode power spectrum in the concordance $Lambda$ cold dark matter model. Our result confirms the reliability of using the remapping approach in upcoming CMB experiments aiming to detect the primordial gravitational waves with the tensor-to-scalar ratio of $r sim 10^{-3}$.
The CMB polarization promises to unveil the dawn of time measuring the gravitational wave background emitted by the Inflation. The CMB signal is faint, however, and easily contaminated by the Galactic foreground emission, accurate measurements of which are thus crucial to make CMB observations successful. We review the CMB polarization properties and the current knowledge on the Galactic synchrotron emission, which dominates the foregrounds budget at low frequency. We then focus on the S-Band Polarization All Sky Survey (S-PASS), a recently completed survey of the entire southern sky designed to investigate the Galactic CMB foreground.
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