By engaging with all the videos within this series, you will effectively complete a full undergraduate course in astronomy, equipping yourself with the knowledge and skills necessary to navigate the night sky with confidence, learning all the basics and many advanced topics! I explore the Cosmic Microwave Background (CMB), a crucial observational evidence confirming the Big Bang theory. It’s one of its three main pillars. The CMB is relic radiation from the early hot and dense phases of the Big Bang, providing a glimpse into the universe’s condition 360,000 years after the initial explosion. Five key observations underscore its importance: 1. The CMB spectrum closely resembles a perfect black body at 2.725 ± 0.0101 Kelvin. 2. The temperature is remarkably isotropic, exhibiting only minor anisotropies. 3. A significant dipole anisotropy is noted due to our motion through space. 4. Minute temperature differences exist between any two points in the sky, measured at about one part in 100,000. 5. These observations align with the first law of thermodynamics, reinforcing the CMB’s validity. To explain the CMB, we use the principles of thermodynamics, assuming homogeneity and isotropy. The heat content of the universe remains constant, leading to the conclusion that the temperature scales inversely with the scale factor. This framework explains why the universe was hotter in its infancy. The CMB is black body radiation, characterized by temperature-dependent spectra. Stars emit such radiation due to their hot, dense, and opaque nature. The CMB’s black body characteristics indicate a uniformly hot early universe. As the universe expanded, the wavelengths of this radiation stretched into the microwave region we now observe. In 1965, Arno Penzias and Robert Wilson discovered the CMB while mapping the sky at microwave wavelengths. Suspecting equipment malfunction, they confirmed the persistent radiation. Concurrently, researchers at Princeton developed instruments to detect the CMB. Over time, the CMB’s temperature cooled from its original hot state, as predicted. NASA’s Cosmic Background Explorer (COBE) satellite confirmed the CMB’s characteristics as a perfect black body at 2.725 Kelvin. The Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite provided enhanced resolution and detailed mappings of the anisotropies, revealing insights into the early universe’s structure. Understanding the anisotropies is crucial for comprehending the universe’s architecture at the time of last scattering, when atoms formed and photons could travel freely. The CMB offers a snapshot of the universe at 360,000 years old, providing valuable information about its development. The power spectrum of the CMB illustrates temperature fluctuations at various angular scales. The largest peak, corresponding to approximately one degree, reveals the scale of the universe at last scattering. Additional peaks yield insights into the proportions of normal matter, dark matter, and dark energy. The detailed examination of CMB anisotropies determines the universe’s age and composition, estimated at approximately 13.799 billion years. Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.