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Register a new userA perspective on the CMB acoustic peak
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T. A. Marriage
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CMB angular spectrum measurements suggest a flat universe. This paper clarifies the relation between geometry and the spherical harmonic index of the first acoustic peak ($ell_{peak}$). Numerical and analytic calculations show that $ell_{peak}$ is approximately a function of $Omega_K/Omega_M$ where $Omega_K$ and $Omega_M$ are the curvature ($Omega_K > 0$ implies an open geometry) and mass density today in units of critical density. Assuming $Omega_K/Omega_M ll 1$, one obtains a simple formula for $ell_{peak}$, the derivation of which gives another perspective on the widely-recognized $Omega_M$-$Omega_Lambda$ degeneracy in flat models. This formula for near-flat cosmogonies together with current angular spectrum data yields familiar parameter constraints.
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Cosmic microwave background (CMB) temperature anisotropies encode the history of the universe, which manifest itself in the angular power spectrum. We test the angular power spectra of small patches from the ESA $textit{Planck}$ data. Known variations in the power spectra from small patches reveal informative details such as the gravitational lensing and the Doppler boosting effect. We compute the relative shifts of power spectra via comparing patches selected randomly from the CMB. We visualize the relative shifts on a full-sky HEALPix grid (a feature map) and analyze the statistical properties on the full-sky map. We find the regions contain the Cold Spot and the Draco supervoid have large relative shifts to large scales. We also find a dipole on the generated feature map comparing with simulations. We discuss possible ways to resolve this dipole on the feature map, including foregrounds, solar dipole systematics, and the uncertainty in our method.
The correlation function of the distribution of matter in the universe shows, at large scales, baryon acoustic oscillations, which were imprinted prior to recombination. This feature was first detected in the correlation function of the luminous red galaxies (LRG) of the Sloan Digital Sky Survey (SDSS). The final release (DR7) of the SDSS has been recently made available, and the useful volume is about two times bigger than in the old sample. We present here, for the first time, the redshift space correlation function of this sample at large scales together with that for one shallower, but denser volume-limited subsample drawn from the 2dF redshift survey. We test the reliability of the detection of the acoustic peak at about 100 Mpc/h and the behaviour of the correlation function at larger scales by means of careful estimation of errors. We confirm the presence of the peak in the latest data although broader than in previous detections.
New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering data are compared to each other and with existing inelastic neutron scattering data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data show collective propagating excitations up to Q=3.5 nm^-1. The temperature behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak found in INS and RS. This supports the acoustic origin of the excess of vibrational states giving rise to the boson peak in this glass.
Experimental results on the density of states and on the acoustic modes of glasses in the THz region are compared to the predictions of two categories of models. A recent one, solely based on an elastic instability, does not account for most observations. Good agreement without adjustable parameters is obtained with models including the existence of non-acoustic vibrational modes at THz frequency, providing in many cases a comprehensive picture for a range of glass anomalies.
A theory for the vibrational dynamics in disordered solids [W. Schirmacher, Europhys. Lett. {bf 73}, 892 (2006)], based on the random spatial variation of the shear modulus, has been applied to determine the wavevector ($k$) dependence of the Brillouin peak position ($Omega_k)$ and width ($Gamma_k$), as well as the density of vibrational states ($g(omega)$), in disordered systems. As a result, we give a firm theoretical ground to the ubiquitous $k^2$ dependence of $Gamma_k$ observed in glasses. Moreover, we derive a quantitative relation between the excess of the density of states (the boson peak) and $Gamma_k$, two quantities that were not considered related before. The successful comparison of this relation with the outcome of experiments and numerical simulations gives further support to the theory.
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