Cosmic timeline 09

The photon epoch
The photon epoch is defined as “Between 10 seconds and 380,000 years after the Big Bang”. In physical cosmology, the photon epoch was the period in the evolution of the early universe in which photons dominated the energy of the universe. The photon epoch started after most leptons and anti-leptons were annihilated at the end of the lepton epoch, about 10 seconds after the Big Bang.

Atomic nuclei were created in the process of nucleosynthesis which occurred during the first few minutes of the photon epoch.

Big Bang nucleosynthesis begins about three minutes after the Big Bang, when the universe has cooled down sufficiently to form stable protons and neutrons, after baryogenesis which is supposed to have happened in the earlier inflationary epoch. However, nucleosynthesis only lasts for about seventeen minutes, after which time the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. At this time, there is about three times more hydrogen than helium and only trace quantities of other nuclei.

For the remainder of the photon epoch the universe contained a hot dense plasma of nuclei, electrons and photons. About 380,000 years after the Big Bang the temperature of the universe fell to the point where nuclei could combine with electrons to create neutral atoms. As a result, photons no longer interacted frequently with matter, the universe became transparent and the cosmic microwave background radiation was created.

The universe is dominated by photons. These photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 300,000 years.

What is a photon?
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic “unit” of light and all other forms of electromagnetic radiation. Like all elementary particles, photons are governed by quantum mechanics and will exhibit wave-particle duality, which means that they behave just like waves and at the same time behave like particles. For example, a single photon may be refracted by a lens, but also act as a particle giving a definite result when quantitative mass is measured.


Thomas Young‘s double-slit experiment in 1805 showed that light can act as a wave, helping to defeat early particle theories of light.

Theories of light
In most theories up to the eighteenth century, light was pictured as being made up of particles. One of the earliest particle theories was described in the Book of Optics (1021) by Alhazen, who held light rays to be streams of minute particles that “lack all sensible qualities except energy.” In classical antiquity, there were two major theories on vision. The first theory, the emission theory, was supported by such thinkers as Euclid and Ptolemy, who believed that sight worked by the eye emitting rays of light. The second theory, the intromission theory, supported by Aristotle and his followers, had physical forms entering the eye from an object. Alhacen argued on the basis of common observations (such as the eye being dazzled or even injured if we look at a very bright light) and logical arguments (such as how a ray could proceeding from the eyes reach the distant stars the instant after we open our eye) to maintain that we cannot see by rays being emitted from the eye nor through physical forms entering the eye. Alhacen instead developed a highly successful theory which explained the process of vision by rays of light proceeding to the eye from each point on an object, which he proved through the use of experimentation.

Alhacen (Ibn al-Haytham) proved that rays of light travel in straight lines, and carried out a number of experiments with lenses, mirrors, refraction, and reflection.

Since particle models cannot easily account for the refraction, diffraction and birefringence of light, wave theories of light were proposed by René Descartes (1637), Robert Hooke (1665), and Christian Huygens (1678); however, particle models remained dominant, chiefly due to the influence of Isaac Newton. In the early nineteenth century, Thomas Young and August Fresnel clearly demonstrated the interference and diffraction of light and by 1850 wave models were generally accepted. In 1865, James Clerk Maxwell‘s prediction that light was an electromagnetic wave—which was confirmed experimentally in 1888 by Heinrich Hertz‘s detection of radio waves—seemed to be the final blow to particle models of light.

However the modern concept of the photon was developed gradually by Albert Einstein to explain experimental observations that did not fit the classical wave model of light. In 1900, Max Planck was working on black-body radiation and suggested that the energy in electromagnetic waves could only be released in “packets” of energy; he called these quanta (singular quantum). Later, in 1905 Albert Einstein went further by suggesting that electromagnetic waves could only exist in these discrete wave-packets. The photon concept has led to momentous advances in experimental and theoretical physics.

Light does not travel in straight lines
In 1911 Einstein published a paper about the effects of gravity on light, about the way gravity bends or curves space and therefore light. So space is curved! He specifically looked at the gravitational redshift and the gravitational deflection of light. Redshift is looked at in our timeline about the discoveries by Hubble. Einstein’s paper challenged astronomers to detect the deflection during a solar eclipse.

In May 1919, a team led by the British astronomer Arthur Stanley Eddington claimed to have confirmed Einstein’s prediction of gravitational deflection of starlight by the Sun while photographing a solar eclipse with dual expeditions in Sobral, northern Brazil, and Príncipe, a west African island.

Eddington’s photograph of a solar eclipse, which confirmed Einstein’s theory that light “bends.” On 7th November 1919, the leading British newspaper The Times printed a banner headline that read: “Revolution in Science – New Theory of the Universe – Newtonian Ideas Overthrown.”

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