Fine. Thanks.Orodruin said:Gravitationally (or otherwise) bound systems do not expand.
Photon decoupling is the process in which photons, the particles of light, become free to travel through space without being constantly scattered by the hot plasma of the early universe. This occurred around 380,000 years after the Big Bang, when the universe had cooled enough for atoms to form and the plasma to become transparent.
Photon decoupling played a crucial role in the expansion of the universe. Before decoupling, the hot plasma of the early universe was opaque, making it difficult for light to travel freely. This slowed down the expansion of the universe. However, after decoupling, the universe became transparent, allowing light to travel freely and the expansion to accelerate.
One of the strongest pieces of evidence for photon decoupling is the cosmic microwave background (CMB) radiation. This is the leftover glow from the hot plasma of the early universe, which became visible after decoupling. The CMB radiation has been observed and studied extensively, providing valuable insights into the early universe and the process of photon decoupling.
Aside from affecting the expansion of the universe, photon decoupling also had other significant effects. It allowed for the formation of neutral atoms, which eventually led to the formation of stars and galaxies. It also created the large-scale structure of the universe, as the density fluctuations in the early universe were imprinted on the CMB radiation and later evolved into the structures we see today.
It is possible that photon decoupling could have occurred differently, leading to a different universe than the one we observe. However, the precise conditions and timing of photon decoupling were crucial in shaping the universe as we know it. Any significant changes to this process could have resulted in a vastly different universe, making it difficult to predict or imagine what it would look like.