Cosmologists are able to differentiate between microwave radiation from the Sun and the cosmic microwave background radiation (CMB), which is believed to be the remnants of the Big Bang, using several key characteristics:
Spectrum: The spectrum of the two types of radiation is different. The Sun emits a thermal spectrum with a peak intensity in the visible light range. On the other hand, the cosmic microwave background radiation has a distinct blackbody spectrum with a peak intensity in the microwave range. The precise measurement of the spectrum of the CMB allows cosmologists to distinguish it from other sources of radiation.
Uniformity: The CMB is highly uniform and isotropic, meaning it appears nearly the same in all directions. This uniformity is a key prediction of the Big Bang theory. In contrast, the microwave radiation from the Sun exhibits variations due to its localized source and the presence of features such as sunspots and solar flares.
Anisotropies: While the CMB is largely uniform, it does contain subtle variations known as anisotropies. These anisotropies represent small temperature fluctuations across the sky, which provide valuable information about the early universe. These fluctuations have been precisely measured by instruments like the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite.
Redshift: The CMB has been redshifted over billions of years of cosmic expansion. This redshift is consistent with the predictions of the Big Bang theory and provides evidence for the radiation's origin from an early hot and dense phase of the universe. The microwave radiation from the Sun, on the other hand, does not exhibit the same redshift characteristics.
By carefully studying the spectrum, uniformity, anisotropies, and redshift of the radiation, cosmologists can distinguish the cosmic microwave background radiation from other sources of microwave radiation, such as the Sun. These characteristics provide strong evidence for the Big Bang theory and our understanding of the early universe.