Defining the Observable Universe
The observable universe refers to the spherical region of the cosmos comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time. This is because light (or other signals) from these objects has had sufficient time to reach us since the Big Bang. It represents our cosmic horizon, not necessarily the entire universe.
Size and Expansion
The current estimated radius of the observable universe is approximately 46.5 billion light-years, making its total diameter around 93 billion light-years. This figure is significantly larger than the age of the universe (about 13.8 billion years) multiplied by the speed of light, due to the continuous expansion of space itself. This expansion has stretched the light waves and carried distant objects further away while their light was traveling towards us.
The Cosmic Horizon
The boundary of the observable universe is known as the cosmic light horizon. It defines the maximum distance from which light could have traveled to us given the finite age of the universe. Objects beyond this horizon exist, but their light has not yet had enough time to traverse the expanding space and reach Earth, rendering them currently unobservable to us.
Implications and Limitations
The concept of the observable universe highlights that our view of the cosmos is limited by two fundamental factors: the finite speed of light and the finite age of the universe. Every location in the universe has its own unique observable universe, which may or may not overlap with ours. As time progresses, light from increasingly distant objects will reach us, causing the observable universe to continuously expand.