The observable universe refers to the portion of the universe that we can potentially observe from our vantage point on Earth or any other location within the universe. It is the region from which light and other forms of radiation have had sufficient time to reach us since the beginning of the universe.
Given that light has a finite speed, it takes time for light to travel from distant objects to our telescopes or detectors. Therefore, the observable universe is limited by the distance that light has had the opportunity to travel since the Big Bang, which is estimated to have occurred approximately 13.8 billion years ago. This distance is often referred to as the cosmic light horizon or simply the observable horizon.
Currently, the most distant objects we can observe, such as galaxies and cosmic microwave background radiation, are seen as they were about 13.8 billion years ago, representing the age of the universe at the time the light was emitted. As time goes on, we are able to observe more distant objects because the light they emit continues to reach us. However, there is a limit to how far we can observe, as there are regions beyond the observable universe from which light has not yet had enough time to reach us.
The particle horizon, on the other hand, is a related concept that refers to the maximum distance from which particles or signals could have reached us since the beginning of the universe. It represents the boundary beyond which we have no observational access due to the finite speed of light. The particle horizon is larger than the observable universe because it takes into account the distances that particles or signals could have traveled, not just the light.
Both the observable universe and the particle horizon are important concepts in cosmology and help us understand the limits of our observations and the size and age of the universe as we currently know it.