Nuclei & atomshow the cosmic microwave background began

The cosmic microwave background is the afterglow of the Big Bang. It formed about 380,000 years after the Big Bang, when electrons were captured by nuclei, the universe became transparent, and the first light began its journey to the future. That light can still be measured today. Before that, at 3 minutes, about 25% of the mass had turned into helium, and the recipe for the lightest elements was fixed by 20 minutes.

This era runs from 3 minutes to 150 million years and has three parts: nucleosynthesis, which makes the lightest elements; recombination and transparency, when light first travels freely; and finally the long dark ages, when the universe had no stars and only neutral hydrogen cooling.

Six events

• 3 min · Big Bang nucleosynthesis: About 25% of the mass becomes helium.
• 20 min · Lithium-7 formation: The recipe for the lightest elements is fixed. New elements will have to wait for stars.
• 47,000 years · Matter domination: Matter density exceeds radiation density, and matter outweighs light and radiation.
• 380,000 years · Recombination: Electrons are captured by nuclei.
• 380,000 years · Cosmic microwave background: The universe becomes transparent and the first light heads into the future, like a thick fog suddenly lifting.
• 380,000 years – 150 million years · Dark ages: No stars, only neutral hydrogen cooling.

The recipe for light elements

Ralph Alpher and George Gamow calculated how light elements formed in the Big Bang, and Hans Bethe's name was added to make the joke "αβγ". The paper is titled "The origin of chemical elements" (Alpher, Bethe and Gamow, Physical Review, 1948). Against the timeline above, helium was already abundant at 3 minutes and lithium-7 appeared by 20 minutes, which fixed the recipe for the lightest elements.

The afterglow, stumbled on by two people

Two Bell Labs researchers could not get rid of the noise in their antenna. They finally realized it was the afterglow of the Big Bang, and won the 1978 Nobel Prize. Their paper has an unassuming title, "A measurement of excess antenna temperature at 4080 Mc/s" (Penzias and Wilson, The Astrophysical Journal, 1965). By coincidence, a Princeton team was preparing to look for exactly this radiation and was beaten by their neighbors. They wrote up the theoretical explanation, titled "Cosmic black-body radiation" (Dicke, Peebles, Roll and Wilkinson, The Astrophysical Journal, 1965).

Measuring the afterglow more precisely

The spectrum measured by COBE matched the theoretical black-body curve almost perfectly, giving decisive support to Big Bang theory. The paper is "A preliminary measurement of the cosmic microwave background spectrum by the Cosmic Background Explorer (COBE) satellite" (Mather et al., The Astrophysical Journal Letters, 1990). Two years later, data from the same satellite showed temperature variations of one part in 100,000 in the background radiation. These tiny differences were the seeds of later galaxies. The paper is titled "Structure in the COBE Differential Microwave Radiometer first-year maps" (Smoot et al., The Astrophysical Journal Letters, 1992). Sound waves in the early universe also left a characteristic scale of about 500 million light-years in the distribution of galaxies, which became a ruler for measuring the universe. The paper is "Detection of the baryon acoustic peak in the large-scale correlation function of SDSS luminous red galaxies" (Eisenstein et al., The Astrophysical Journal, 2005). Finally, the Planck satellite measured the fine ripples of the afterglow: ordinary matter makes up about 5% of the universe's total energy, dark matter about 27%, and the rest is dark energy (Planck Collaboration, Astronomy & Astrophysics, 2020).

How to "see" the dark ages

With no stars there is no light to see by, so researchers rely on hydrogen. Humans first detected the 21-cm radio emission of neutral hydrogen, which made it possible to "see" gas in the Milky Way that is otherwise invisible. The paper's title is simply "Observation of a line in the galactic radio spectrum" (Ewen and Purcell, Nature, 1951). The EDGES experiment reported a 21-cm absorption signal left by the first generation of stars, in a paper titled "An absorption profile centred at 78 megahertz in the sky-averaged spectrum". The result is still disputed and awaits independent verification (Bowman et al., Nature, 2018).

What is inside an atom

Alpha particles are occasionally bounced back by gold foil, and from this Ernest Rutherford inferred that an atom's mass is concentrated in a tiny nucleus (Rutherford, Philosophical Magazine, 1911). The paper is titled "The scattering of α and β particles by matter and the structure of the atom".

How the game shows it

In the game, the time range of the Nuclei & atoms era is "3 min – 150 million years" and its resource is atoms. The generators are Proton capture, Deuteron fusion, Helium-4 synthesis and Electron capture, corresponding to the process from nucleosynthesis to the formation of atoms.