The cosmic microwave background radiation (CMB) is one of the key pieces of evidence supporting the Big Bang theory. It is a faint glow of radiation that permeates the entire observable universe and is detected uniformly from every direction we observe. The existence of the CMB and its isotropy (the same in all directions) is indeed intriguing and requires an explanation within the framework of the Big Bang theory. Here's a brief overview of why we observe the CMB from all parts of the universe:
According to the Big Bang theory, the universe was in a hot and dense state in its early stages. As the universe expanded and cooled down, around 380,000 years after the Big Bang, it reached a critical temperature and density point known as recombination. At this moment, the primordial plasma consisting of protons and electrons combined to form neutral hydrogen atoms. This process made the universe transparent to light for the first time, allowing photons to travel freely without frequent scattering.
The cosmic microwave background radiation we observe today is the remnants of those primordial photons that were released at the time of recombination. However, since the universe has been expanding for billions of years since then, the once high-energy photons have significantly redshifted into the microwave region of the electromagnetic spectrum. This redshifted radiation is what we detect as the CMB.
Now, why is the CMB observed from every part of the universe? It is because the CMB radiation we observe today was emitted when the universe was in a highly uniform state. At the time of recombination, the distribution of matter and energy in the universe was remarkably homogeneous on large scales. As a result, the CMB photons were emitted uniformly from all directions, and as they traveled through space, they have remained largely unchanged, providing a snapshot of the early universe.
While the CMB is extremely isotropic on large scales, it does exhibit tiny temperature fluctuations on smaller scales. These fluctuations contain valuable information about the early universe's structure and have been extensively studied by cosmologists, leading to a wealth of insights into the formation of galaxies and large-scale structures.
In summary, the cosmic microwave background radiation is observed from all directions because it represents the uniform emission of photons from a highly homogeneous early universe at the time of recombination. The isotropy of the CMB provides strong evidence in support of the Big Bang theory and has allowed us to gain significant knowledge about the history and composition of our universe.