Question

What happened to all of the radiation that was produced during the first four hundred thousand years after the universe began?

Short answer

The radiation became the Cosmic Microwave Background (CMB).

Step-by-step solution

Understanding Initial Conditions

Initially, the universe was extremely hot and dense. During the first few minutes after the Big Bang, nucleosynthesis occurred, forming the lightest elements. The universe eventually cooled down enough for photons to decouple from matter.

Decoupling of Radiation and Matter

Approximately 400,000 years after the Big Bang, the universe cooled down sufficiently for electrons to combine with protons, forming neutral hydrogen atoms. This process is known as recombination. With electrons no longer free, photons could travel freely through space.

Formation of the Cosmic Microwave Background (CMB)

As photons decoupled from matter, they were able to travel through space without constant scattering. This release marked the creation of the Cosmic Microwave Background (CMB), a weak radiation that fills the universe and provides a snapshot of the universe at that time.

Observing the CMB Today

The radiation from the CMB has redshifted due to the expansion of the universe, changing its wavelength from visible to microwaves. This is why it is now detectable as microwave radiation, and it remains as evidence of the early state of the universe.

Key concepts

Big Bang nucleosynthesis

The birth of the universe began with the Big Bang, a colossal explosion that resulted in the formation of all matter we see today. In the first few minutes after this event, the universe was extremely hot and dense. This state was pivotal for nucleosynthesis, which is the process of forming new atomic nuclei.
During Big Bang nucleosynthesis, light elements were generated, such as hydrogen, helium, and trace amounts of lithium. These are called the primordial elements, and they make the universe's early chemical composition quite particular.

• Hydrogen and helium were the most abundant, making up nearly all of the universe's baryonic matter.
• The temperature and density during this time allowed these elements to form without any influence from stars, as they had not yet formed.

Big Bang nucleosynthesis set the stage for what would come next, as the universe started expanding and cooling over time.

Recombination

As the universe continued to expand, it cooled down substantially. This cooling process brought about a phase known as recombination, which happened approximately 400,000 years after the Big Bang.
During recombination, the universe had cooled to a point where electrons could combine with protons to form neutral hydrogen atoms. Before recombination, the universe was like a thick fog of charged particles where photons could not travel freely without scattering.

• Recombination drastically changed the universe's opacity, as free electrons were no longer prevalent.
• Photons, or light particles, were able to travel unimpeded for the first time.

This transformation was crucial because it marked the transition of the universe from an opaque state to a transparent one, setting up conditions for later cosmic developments.

Photon decoupling

Photon decoupling was a critical event that coincided with recombination. When electrons and protons joined to form neutral hydrogen atoms, photons were no longer obstructed.
This meant photons were free to travel across the universe without frequently bumping into particles.

• The moment photons decoupled from matter, they began their long journey across the cosmos.
• This transition led to the creation of the Cosmic Microwave Background (CMB), a blanket of radiation that provides a snapshot of the universe at that pivotal moment in time.

Photon decoupling is important because the resulting CMB allows astronomers to study and understand the state and composition of the early universe.

Universe expansion

From the moment the Big Bang occurred, the universe hasn't stopped expanding. This expansion has significant implications for various cosmic phenomena, including the redshift of the Cosmic Microwave Background (CMB).
The universe's expansion means that as the CMB radiation travels through space, its wavelength stretches. As such, the CMB is now mostly observed in the microwave part of the electromagnetic spectrum, as opposed to visible light.

• This redshift is a key piece of evidence supporting the theory of an expanding universe.
• As the universe grows, distances between galaxies increase, creating more space for cosmic evolution.

The expansion of the universe is a fundamental concept that continues to shape our understanding of the cosmos and guides observational astronomy today.