FSD: Frequency Space Differential measurement of CMB spectral
  distortions
  Although the Cosmic Microwave Background agrees with a perfect blackbody
spectrum within the current experimental limits, it is expected to exhibit
certain spectral distortions with known spectral properties. We propose a new
method, Frequency Space Differential (FSD) to measure the spectral distortions
in the CMB spectrum by using the inter-frequency differences of the brightness
temperature. The difference between the observed CMB temperature at different
frequencies must agree with the frequency derivative of the blackbody spectrum,
in the absence of any distortion. However, in the presence of spectral
distortions, the measured inter-frequency differences would also exhibit
deviations from blackbody which can be modeled for known sources of spectral
distortions like $y\,\&\,\mu$. Our technique uses FSD information for the CMB
blackbody, $y$, $\mu$ or any other sources of spectral distortions to model the
observed signal. Successful application of this method in future CMB missions
can provide an alternative method to extract spectral distortion signals and
can potentially make it feasible to measure spectral distortions without an
internal blackbody calibrator.
