Recombination (and the briefest history of time)

The briefest history of time: illustrated edition

The Big Bang occurs; within 10^-37 seconds the universe undergoes inflation and expands exponentially as it becomes homogeneous; symmetry breaking follows and sets the laws of the universe in place as we now know them; by the time the universe is one-millionth of a second old it’s cooled enough for a (still very hot) plasma of protons, electrons etc. to form; the temperature plunges in the universe as it expands, when it reaches 5,000 degrees Fahrenheit it enters the Recombination Era and electrons and protons can finally stick together to make hydrogen atoms the epoch leaves behind the Cosmic Microwave Background-radiation (CMB) as evidence of its existence; eventually hydrogen leads to stars, planets and you and me… We all know the rest of the story.

(Stay tuned for my interpretative dance version next week)


The Era of Recombination part of this has been of great interest to physicists in recent years as it’s become pivotal to our understanding of how the universe formed. When Penzias and Wilson famously stumbled across the CMB in the 1960s, astronomers had already been expecting it for 2 decades. The CMB served as an early validation for the then controversial Big Bang theory and over the last decade, numerous NASA missions have successfully mapped its temperature and distribution in impressive detail. However, while the logic for why the CMB exists has been well established, the exact formation mechanism has been less well understood.

New research, published in the journal Physical Review A last month, should help to give a more accurate explanation of how that happened. The first simple atoms couldn’t form until 400,000 years after the Big Bang when the universe had finally cooled down to something like the temperature of the Sun’s surface. The first atoms would have had incredible difficulty forming.

As protons and electrons dropped energy states in the process of recombination, they would release a photon. That photon could easily excite a nearby atom and turn the whole process back to where it started, repeating the dance over and over as it coupled radiation with matter. As this paper details, radiation escaped coupling through a process called two-photon transitions. This is when two photons interact to excite a molecule from one state to a higher state. After this transition, recombination could occur, leading to what we now see as the Cosmic Microwave Background radiation.

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