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The Planck blackbody spectrum of the CMB, measured by the COBE satellite team. (NASA)

Blackbody radiation

In 1965 physicist Arno Allan Penzias and astronomer Robert Woodrow Wilson announced the discovery of the cosmic microwave background (CMB) radiation. This radiation is a relic of the “fireball” phase of the hot Big Bang universe, when the dominant form of energy was radiation. It has a perfect Planck blackbody spectrum, with a temperature of 2.7 Kelvin (2.7 K or –270.4 °C). The blackbody form of the spectrum tells us that at some time in the past the matter and radiation in the universe must have been locked together in thermal equilibrium.

If a piece of matter completely absorbs all the radiation falling upon it, or, conversely, behaves as a perfect radiator when heated, then the matter radiates as a black body, and the radiation has the characteristic blackbody spectrum, like the plot of intensity against wavelength in the figure. A practical example is the interior of an oven or furnace.

The spectrum peaks at a wavelength inversely proportional to the temperature of the black body. The Sun’s spectrum, approximately a black body of temperature 5800 K, peaks at a wavelength 0.6 μm, in yellow light. The Earth’s spectrum, at an average temperature of 288 K, peaks at 12 μm, at mid-infrared wavelengths. The cosmic microwave background peaks at 1 mm, corresponding to a temperature of 2.73 K.

Blackbody radiation is the signature of matter in thermal equilibrium with radiation, with the energy equally shared between the matter and the photons.


Particles and atoms

The universe appears to have formed in a single explosive event, the initial “singularity”, with essentially infinite density and temperature. As the universe expanded, the temperature and density dropped. Initially the universe was very simple, consisting of photons (particles of light), and particles of matter, divided into light particles (electrons and neutrinos) and quarks, which are the building blocks of heavier particles such as protons and neutrons. When the temperature dropped to 1012 K the quarks were confined to make protons and neutrons. When the temperature reached 1010 K, about 1 second after the Big Bang, nuclear reactions began and neutrons and protons fused together to make deuterium, helium and lithium. About 380,000 years after the Big Bang, when the temperature had dropped to 3000 K, electrons and protons combined together to make hydrogen atoms and the universe became transparent to radiation for the first time. This is the moment, called the epoch of recombination, we are looking at when we view the cosmic microwave background. Since this time the universe has expanded by a factor of about 1100 in size and this accounts for the drop in the observed temperature of the microwave background radiation from 3000 K to 2.73 K.


Formation of galaxies

During the fireball phase, the ordinary matter that we and the Earth are made of – protons, neutrons and electrons – was extremely smoothly distributed, to better than one part in 100,000. For there to be planets, stars and galaxies today there had to be some small fluctuations in density present and these are believed to have been randomly distributed through the universe. These fluctuations were eventually shaped by gravity, which tended to concentrate matter to ultimately become galaxies of stars, planets, gas and dust.

To make galaxies, much stronger fluctuations must have already developed in the dark matter, which ceased to be controlled by radiation at a much earlier epoch. After recombination the ordinary matter responded to the gravitational attraction of the dark matter lumps and fell towards them, making proto-galaxies consisting of ordinary matter concentrations, in which stars started to form, embedded in halos of dark matter.

These proto-galaxies then merged together to make the galaxies we see today, with these mergers being accompanied by the vigorous formation of new stars. The gravitational assembly process also eventually collected galaxies together into groups and clusters.


The nearby spiral galaxy NGC 3310 imaged by the Hubble Space Telescope. (NASA and the Hubble Heritage Team [STScI/AURA])


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