When we talk about observing light or radiation from the early universe, we are referring to the detection of ancient photons that were emitted during the early stages of the universe's existence. The key concept to understand here is that the expansion of the universe allows light to travel great distances over time.
In the early universe, the cosmos was hot and dense, filled with a plasma of particles. Photons, which are particles of light, were constantly interacting with this plasma through a process called scattering. As the universe expanded and cooled down over time, the plasma transitioned into a neutral gas, and the photons were able to travel more freely without being scattered.
Fast forward billions of years, and these ancient photons, which have been traveling through space since their initial emission, reach us and can be detected by telescopes and observatories. The expansion of the universe itself is what allows these photons to cover vast distances and reach us despite the fact that matter cannot travel faster than light within space.
It's important to note that the detection of light from the early universe is not a direct observation of the universe at that time. Instead, scientists study the cosmic microwave background radiation (CMB), which is the remnant glow from the hot, dense phase of the early universe. By analyzing the characteristics of the CMB, such as its temperature fluctuations, scientists gain valuable insights into the early universe's properties and its evolution over time.