Cosmic microwave background

In physical cosmology, the cosmic microwave background (CMB, CMBR) is electromagnetic radiation as a faint, very cold relic from the very early Big Bang. The CMB is also known as the cosmic microwave background radiation (CMBR) or "relic radiation". The radiation of the CMB is everywhere in space and is most detectable in the microwave spectrum.

Full-sky image derivedfrom nine years of WMAP data

Quotes

• It is now clear that astronomers came across indirect manifestations of the CMB long before the 1960s. In 1941, Andrew McKellar discovered cyanide molecules (HCN) in interstellar space. ... As the object of research, McKellar chose absorption lines caused by cyanide molecules in the star 'ε' of Ophiuchis. He concluded that these lines could only be caused by absorption of light by rotating molecules. Relatively simple calculations allowed McKellar to conclude that the excitation of rotational degrees of freedom of cyanide molecules required the presence of external radiation with an effective temperature of 2.3 K. Neither McKellar himself, nor anyone else, suspected that he had stumbled on a manifestation of the cosmic microwave background.
  • Pavel D. Naselsky, Pavel D., Dmitry I. Novikov, and Igor D. Novikov, The Physics of the Cosmic Microwave Background. Cambridge Astrophysics Series. 41. Cambridge University Press. 2006. 

• The standard Friedmann-Lemaître-Robertson-Walker (FLRW) cosmology is based on the cosmological principle, which posits that the universe is homogeneous and isotropic on large scales. This assumption is supported by the smoothness of the CMB, which has temperature fluctuations of only ∼ 1 part in 100,000 on small angular scales. These higher multipoles of the CMB angular power spectrum are attributed to Gaussian density fluctuations created in the early universe with a nearly scale-invariant spectrum, which have grown through gravitational instability to create the large-scale structure in the present universe. The dipole anisotropy of the CMB is however much larger, being ∼ 1 part in 1000 as observed in the heliocentric rest frame. This is interpreted as due to our motion with respect to the rest frame in which the CMB is isotropic, and is thus called the kinematic dipole.
  • Nathan Secrest, Sebastian von Hausegger, Mohamed Rameez, Roya Mohayaee, Subir Sarkar, and Jacques Colin, (2020). "A Test of the Cosmological Principle with Quasars". arXiv preprint arXiv:2009.14826.

• ... we know in particular that for the first 380,000 years of the universe it was filled with a fireball. And we know this for sure because we've seen the fireball. In fact we've seen it , and we've take a photograph of it. This is called the cosmic microwave background radiation, but a much better name for it is the Fireball That Filled the Universe When It Was Much Younger.
  • David Tong, Quantum Fields: The Real Building Blocks of the Universe - with David Tong YouTube, 15 February 2017, (quote from 45:33 of 1:00:17).