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waynehu

Professor, Department of Astronomy and Astrophysics
University of Chicago

Group Contact CV SnapShots
CMB Introduction '96   Intermediate '01   Polarization Intro '01   Cosmic Symphony '04   Polarization Primer '97   Review '02   Power Animations   Lensing   Power Prehistory   Legacy Material '96   PhD Thesis '95 Baryon Acoustic Oscillations Cosmic Shear Clusters
Transfer Function WMAP Likelihood Reionization PPF for CAMB Halo Mass Conversion Cluster Abundance
Cosmology I [legacy 321] Cosmology II [321] Current Topics [282] Galaxies and Universe [242] Radiative Processes [305] Research Preparation [307] GR Perturbation Theory [408] CMB [448] Cosmic Acceleration [449]

Soundscape

Key Concepts

Okay, lets put this all together.

Modes caught at extrema of their oscillations become the peaks in the CMB power spectrum.  They form a harmonic series based on the distance sound can travel by recombination, called the sound horizon.  The first peak represents the mode that compressed once inside potential wells before recombination, the second the mode that compressed and then rarefied, the third the mode that compressed then rarefied then compressed, etc:

We see these spatial variations in the temperature as angular fluctuations on the sky since the CMB photons stream unimpeded to us from the time of recombination.  The fundamental physical scale is then converted to a fundamental angle that is about a degree on the sky today.  By analogy to spatial wavenumber k, we define an angular wavenumber, or multipole l as roughly the inverse of this angular scale (in radians).  The fundamental angular wavenumber or multipole is l~200.