The term "cosmic microwave background" refers to a faint radiation left over from the Big Bang, which permeates the entire universe. In IPA phonetic transcription, this term is spelled /ˈkɑz.mɪk ˈmaɪ.krɵ.weɪv ˈbækˌɡraʊnd/. The first syllable, "cos," is pronounced like "cause." The "microwave" portion is pronounced with an emphasis on the "mi" syllable, which is pronounced like "my." The final syllables, "back" and "ground," are pronounced as they appear, with the emphasis on the "ground" syllable.
The cosmic microwave background (CMB) refers to a faint, uniform, and pervasive radiation that permeates all parts of the observable universe. It is often regarded as the remnant energy from the Big Bang, the event that marked the beginning of our universe approximately 13.8 billion years ago. The CMB consists of electromagnetic radiation, specifically microwaves, and derives its name from being spread across the entire cosmos.
The origins of the CMB can be traced back to a critical moment in the early universe called recombination, which occurred roughly 380,000 years after the Big Bang. Prior to recombination, the universe was a seething plasma of particles and radiation that were too energetic to combine and form atoms. However, as the universe expanded and cooled down, photons and electrons had the opportunity to combine to form stable hydrogen atoms. This process radically altered the behavior of the radiation, allowing it to travel freely across the cosmos.
Today, the CMB constitutes the most ancient light that can be detected by scientific instruments. Its presence provides valuable insight into the early universe, aiding astronomers in understanding the formation and evolution of cosmic structures. The CMB radiation is highly uniform, but tiny temperature fluctuations exist across the sky, representing slight density variations. Modern experiments, such as those conducted by satellite missions like the Planck spacecraft, have enabled precise measurements of these fluctuations, supporting various cosmological models and theories.