Gravitational LensingThrough gravitational lensing, the large scale density fluctuations in the universe like galaxy clusters deflect the trajectory of CMB photons as they propagate from the surface of last scattering to us. Lensing of the CMB gives us a handle on the cosmic expansion and structure formation history. Since lensing turns E-mode to B-mode polarization, the polarization of the CMB is an excellent tracer of lensing. |
Primordial Gravitational WaveInflation generates tensor fluctuations in the gravitational metric, producing gravitational waves. The amplitude of the primordial gravitational waves is directly proportional to the square of the energy scale at inflation. B-mode polarizations in the CMB is a direct probe of the primordial gravitational wave. Thus detecting B-modes generated by GW is a 'smoking gun' evidence for inflation. |
Current upper limits on B-modes from CMB experiments |
The 1-σ error of Mν vs ωm from Fisher matrix forecasts of POLAR. The red ellipse denotes error constraints derived from the temperature and polarization power spectra while the blue ellipse has the lensing power spectrum added to the derivation.
Constraining Cosmological ParametersWith the B-polarization power spectrum, one can reconstruct the lensing potential power spectrum, hence infer the clustering of structures. With the information from the lensing power spectrum, several cosmological parameters are much better constrained. Total neutrino massNeutrinos contribute to the total dark matter content of the universe, however, unlike normal dark matter, on small scales neutrinos do not cluster due to their extremely high velocity (~150 (1eV/mν) km/s). This effect is called neutrino free-streaming. This directly alters the shape of the matter power spectrum. CurvatureA non-flat universe has important implications for inflation and the string landscape. With the primary CMB anisotropy alone, allowing Ωm + ΩΛ to deviate from 1 introduces parameter degeneracies between H0 and ΩΚ , as shown in the figure below. The lensing spectrum can break this degeneracy, because the amount of clustering varies for different ΩΚ values given H0, which leads to different lensing spectrum amplitudes. |
WMAP team: Solid line: Ωm + ΩΛ = 1; Dashed line: ΩK not equal 1.
Dynamical eark energyThe lensing potential spectrum provides significant constraining power, on top of the primary CMB anisotropies and polarization and external distance measurements (BAO, H0), to determine the character of dark energy, whether it is a cosmological constant or a field, and whether it has a constant or varying equation of state. Measuring r: the tensor to scalar ratioWith a well-measured B-polarization spectrum, one can separate the lensing B-modes and the primordial B-modes and infer the energy scale at which inflation happened. |
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