The cosmic microwave background radiation (CMB) is the residual radiation from the early stages of the universe, specifically from about 380,000 years after the Big Bang. Here's why we can still detect it and why it appears to be uniform throughout the observable universe:
Expansion of the Universe: The universe has been expanding since the Big Bang. As the universe expands, the wavelengths of the radiation within it also stretch. The CMB, which was initially emitted as high-energy radiation, has been redshifted over time due to the expansion of space. As a result, it has shifted to lower-energy microwaves, making it detectable by microwave instruments.
Cosmic Transparency: About 380,000 years after the Big Bang, the universe cooled down enough for atoms to form. Prior to this period, the universe consisted of a hot, dense plasma that was opaque to electromagnetic radiation. However, once atoms formed, the universe became transparent to light, allowing the CMB to travel freely throughout space.
Observable Universe: The CMB radiation comes from all directions in the sky because it represents a snapshot of the entire universe at the time of its emission. Due to the expansion of the universe, the CMB photons have been traveling through space for approximately 13.8 billion years, reaching us from regions that were much closer at the time they were emitted.
Uniformity: The uniformity of the CMB across the observable universe is one of the significant observations that support the Big Bang theory. The high degree of isotropy (uniformity in all directions) indicates that the universe was once in a highly homogeneous state, where temperature and density were nearly the same in all regions. Small variations in the CMB temperature provide insights into the early universe's structure, which later led to the formation of galaxies and other cosmic structures.
In summary, the cosmic microwave background radiation is visible to us because it represents radiation emitted about 380,000 years after the Big Bang, when the universe became transparent. The uniformity of the CMB throughout the observable universe suggests that the early universe was highly homogeneous, and the expansion of the universe has allowed the CMB photons to reach us over billions of years.