Since the speed of light travels at 3×10 8 m/s, information could only have travelled ~3×10 -37 metres during this time. The red line in the figure on the left shows that according to Big Bang theory, the Universe had a radius of more than 10 -10 metres at 10 -45 seconds after the Big Bang. A period of inflation is also necessary so that regions of the early Universe are close enough to thermally equalise. The Big Bang model alone cannot account for the uniform temperature of the CMB. However, this is not possible given standard Big Bang theory, the age of the Universe, and the finite speed of light. This suggests that regions of the Universe that are now widely separated, were once close enough to ‘communicate’ with each other in order to equalise their temperature. In particular, once we remove the dipole that arises due to our motion in the Universe, the CMB is incredibly uniform across the sky, varying by no more than one part in ten thousand. However, standard Big Bang theory does not account for all of the observed properties of the CMB. This agrees well with what Big Bang theory predicts. However, they have been cosmological redshifted to longer wavelengths during their ~13 billion year journey through the expanding Universe, and are now detected in the microwave region of the electromagnetic spectrum at an average temperature of 2.725 Kelvin. The photons of the CMB were emitted at the epoch of recombination when the Universe had a temperature of about 3,000 Kelvin.The agreement is so good that it is impossible to distinguish the data from the theoretical curve. The figure on the right plots a theoretical blackbody curve along with CMB data from the COsmic Background Explorer ( COBE) satellite. This is exactly what is observed for the CMB. The multiple scattering of photons by a hot plasma in the early Universe should result in a blackbody spectrum for the photons once they have escaped at the epoch of reionisation.In particular, Big Bang theory predicts certain characteristics for the radiation left over from the birth of the Universe, all of which are confirmed by the CMB: Credit: NASA/COBEįirst detected by Arno Penzias and Robert Wilson in 1965, the CMB is one of the most conclusive pieces of evidence in favour of the Big Bang. The data from COBE match the theoretical blackbody curve so exactly that it is impossible to distinguish the data from the curve. The ‘Cosmic Microwave Background radiation’ ( CMB) is the record of these photons at the moment of their escape. This is known as the epoch of recombination, and it is at this time that photons were finally able to escape the fog of the early Universe and travel freely. The result was that information ( photons) from the early Universe were effectively trapped in an inpenetrable ‘fog’ which, to this day, hides these early times from astronomers.Īs the Universe expanded, however, its temperature and density dropped to a point where the atomic nuclei and electrons were able to combine to form atoms. Matter was instead distributed as a highly ionised plasma which was very efficient at scattering radiation. Credit: NASA/COBEĪccording to Big Bang theory, temperatures and pressures for the first ~300,000 years of the Universe were such that atoms could not exist. The temperature differences (indicated by colour) are about one part in ten thousand of the average (2.725 Kelvin) temperature. An all-sky map of the cosmic microwave background, as detected by COBE.
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