The Cosmic Microwave Background (CMB) is a form of electromagnetic radiation dating from an early stage of the universe. is a faint afterglow of the Big Bang, a relic from the very early universe.
- The CMB is the oldest known form of radiation and is considered to be evidence for the Big Bang theory of the formation of the Universe.
- With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope can detect the CMB as a faint glow that is not associated with any star, galaxy, or other object.
- The CMB was initially composed of extremely high-energy gamma rays. However, as the universe expanded and cooled, these gamma rays have been redshifted, meaning that their wavelengths have been stretched. Today, the CMB appears as microwave radiation.
- The CMB is detected as a faint glow of uniform thermal energy coming from all parts of the sky.
- The CMB is a relic of the Big Bang, dating back to about 13.8 billion years ago in look-back time.
- The phrase look-back time refers to the time it takes for light to travel from its point of origin to our here-and-now.
- This means that the radiation that we see today from the CMB was emitted from a point in time that is 13.8 billion light-years away.
- However, the light from the CMB may have taken far longer than 13.8 billion years to reach us, because we believe that the universe is expanding.
- However, we do not know the exact rate of expansion of the universe during the time since the CMB was emitted.
- It is therefore not possible to know for sure how long the light has taken to reach us or for that matter, the distance it has travelled.
A timeline of the Universe. The far-left shows the Big Bang followed by a period of “inflation” that caused exponential growth in the scale of the Universe. The afterglow of the Big Bang, the Cosmic Microwave Background, began to be emitted about 400,000 years later.
Attribution: Adapted from NASA / WMAP Science Team. https://www.nasa.gov/sites/default/files/thumbnails/image/wmap_0.jpg