Effect of galactic foregrounds on the recovery of the cosmic 21 cm signal. Galactic foregrounds due to synchrotron radiation is 10 percent polarized. This signal looks similar to the cosmological signal in frequency space and is much higher in magnitude. This makes recovery of the signal of interest very difficult.
See my in-prep manuscript with ETH group

Extraction of cosmological information in presence of leakage of polarized synchrotron signal