In the depths of the cosmos, at a time when the Universe was in a state of transformation, a crucial phenomenon changed the very shape of everything that would become our cosmic environment. This phase, known as the cosmological recombination epoch, marks a decisive turning point in the history of the primordial Universe. Several hundred thousand years after the Big Bang, elementary particles, previously free and in constant interaction with the ubiquitous radiation, combined to form the first neutral atoms. This transformation paved the way for the formation of galaxies, stars, and ultimately life as we know it.
Thanks to recombination, the Universe transitioned from an opaque to a semi-transparent state to a cosmos where light could freely circulate, giving birth to the cosmic microwave background, a historical luminous witness and the main object of study in cosmology. Through this process, photon-matter decoupling emerged as an essential phenomenon, revealing the first images of the cosmic architecture. It was also at this moment that the complex entanglement between atom formation and the evolution of the universe began, marking a transition between the ionized era and the establishment of predominantly neutral matter. Our understanding of the underlying mechanisms of this epoch remains at the core of the most advanced research, illustrating how recombination constitutes a key bridge between the Big Bang and the current structure of the cosmos.
In short :
- The cosmological recombination occurs about 380,000 years after the Big Bang and corresponds to the formation of the first neutral atoms.
- It causes photon-matter decoupling, making the Universe transparent and emitting the cosmic microwave background radiation.
- The ionized era, characterized by free electrons scattering light, gives way to a predominantly neutral matter.
- The recombination directly impacts hydrogen formation, the dominant element that structures matter.
- It constitutes a central stage in the evolution of the universe, laying the groundwork for the cosmic structures observable today.
The cosmological recombination: foundation of the primordial Universe and emission of the cosmic microwave background
The so-called epoch of cosmological recombination is a key episode where the matter of the Universe underwent profound transformation, enabling the transition from an ionized era to a state dominated by neutral atoms. According to the standard models of cosmology based on the Big Bang, in the early moments, the Universe was a dense and hot soup of protons, neutrons, and free electrons, bathing in an uninterrupted sea of high-energy photons.
During this era, electrons could not assemble with atomic nuclei without being immediately ionized by energetic photons. The Universe behaved like an opaque fog, analogous to an “electron mist,” causing intense and repeated scattering of photons according to the so-called Thomson scattering phenomenon. This is why light was then unable to travel great distances – it was constantly scattered, preventing the formation of clear luminous images.
Recombination occurs when the overall temperature falls sufficiently for the average energy of photons to be insufficient to ionize the newly formed atoms. This cosmic crystallization gives rise to the first hydrogen and helium atoms, and in smaller quantities, lithium — the light elements resulting from primordial nucleosynthesis. Free protons and electrons were thus able to combine to create electrically neutral matter.
This event is commonly placed at about 380,000 years after the Big Bang, with an average temperature around 3,000 Kelvin. The major outcome is the appearance of the cosmic microwave background (CMB), a true photograph of the young Universe at this stage. This fossil radiation, detected via its microwaves, today constitutes a fundamental source of information about the composition, geometry, and early dynamics of the cosmos.
Physical mechanisms of recombination: atom formation and photon-matter decoupling
The cosmological recombination is based on extremely precise atomic processes governed by quantum physics and thermodynamics. The main issue lies with photon-matter decoupling, the moment when photons cease to be continuously scattered by free electrons and can thus travel freely through space.
Before recombination, the temperature of the Universe is so high that photons carry enough energy to systematically ionize any atom they encounter. Thus, these remain dissociated into protons and electrons. As cosmic cooling progresses, the energy of photons decreases, and around 380,000 years, hydrogen recombination begins:
- Free electrons bind to protons to form neutral hydrogen atoms.
- This drastically reduces the density of free electrons, minimizing photon scattering.
- A surface thus emerges from which the CMB radiation is emitted, called the last scattering surface.
To model this recombination, physicists initially applied Saha’s equation, which summarizes the balance between ions, free electrons, and neutral atoms in thermal equilibrium. However, the equilibrium approximation used does not fully reflect reality as the medium is dynamic and recombination is a gradual, not instantaneous, process.
Major advancements were made by the Peebles model established in 1968. This model takes into account the excited states of atoms and the photons emitted during transitions. For example:
- Electrons preferentially recombine at excited states, particularly at the level n=2.
- From this state, the transition to the ground state can occur via two mechanisms: emission of a Lyman-α photon or emission of two photons.
- These photons can reionize other atoms, slowing down recombination.
This model leads to a recombination that is slower and more gradual than initially expected. It is central to modern analyses of data from observations of the cosmic microwave background.
Intermediate states: recombination of lithium and helium in the primordial Universe
Before hydrogen recombination takes over, lithium and helium already begin their recombination during cosmic cooling. These elements, while less abundant, play a specific role in the chemical evolution of the cosmos.
The recombination occurs in a descending order of ionization energy:
| Ion | Ionization Energy (eV) |
|---|---|
| Li3+ → Li2+ | 122.4 |
| Li2+ → Li+ | 75.6 |
| He2+ → He+ | 54.4 |
| He+ → He0 | 24.6 |
| H+ → H0 | 13.6 |
| Li+ → Li0 | 5.4 |
The first atoms to recombine are triply ionized lithium and then doubly ionized, at very high redshifts (z ~ 14,000 and 8,600 respectively). Then comes the recombination of helium, first from doubly ionized to singly ionized (z ~ 6,000), then from ionized to neutral (z ~ 2,500).
On the other hand, lithium fails to fully reach its neutral state due to Lyman-α photons produced during hydrogen recombination that reionize neutral lithium atoms. Eventually, hydrogen, recombining at z ~ 1,300, becomes the primary neutral and dominant component of cosmic matter.
This sequence of recombinations shows that the formation of the first atoms is not a simple one-time event but a complex succession governed by the physical properties of the elements themselves and the thermal evolution of the Universe.
First chemical reactions after recombination and implications for cosmic structure formation
Recombination also constitutes the preamble to what is known as primordial chemistry. After the Universe became transparent thanks to recombination, more complex atomic interactions could take place, paving the way for the birth of simple molecules that would be precursors to future galactic and stellar structures.
The phases of primitive chemistry extend to redshifts ranging from about 2,000 to 800, a period during which neutral helium and hydrogen atoms interact with rare residual protons and ions to form simple compounds:
- Formation of the ion HeH+ through reaction between neutral helium and a proton.
- Formation of ionized molecular hydrogen H₂⁺ and neutral H₂, particularly from the combination of neutral atomic hydrogen.
- Formation of isotopic species such as HD.
These molecules, in modest yet crucial quantities, have sensitized matter to new radiative processes and allowed the onset of star formation in the first galactic clusters.
Primordial chemistry is very rich despite the apparent simplicity of the elements initially involved (hydrogen, helium, and lithium). In reality, no fewer than 250 chemical reactions are considered in current models to understand this initial chemical complexity. This illustrates the growing sophistication of our understanding of the beginnings of structured matter in the evolution of the universe.
Timeline of cosmological recombination
What is cosmological recombination?
Cosmological recombination is the period when free electrons combined with nuclei to form the first neutral atoms, making the Universe transparent to light.
Why is recombination essential for the universe?
It allows for photon-matter decoupling, which led to the formation of the cosmic microwave background, a key image of the primordial Universe.
What role do lithium and helium play in recombination?
They recombine before hydrogen according to decreasing ionization energies, thus influencing the course of the first chemical reactions in the universe.
What is the last scattering surface?
It is the spatial instant when photons were able to travel freely after recombination, forming the direct source of the observable cosmic microwave background today.
How does recombination affect cosmic structure formation?
It initiates primordial chemistry, which is critical for forming the first molecules, and then structures such as stars and galaxies.