Exploring the Boundaries of the Visible Universe
The question of what lies beyond the boundary of the visible universe is a fundamental one, inviting us to probe the limits of our understanding of the cosmos. To begin, we must first define what we mean by the observable universe, a concept defined by the reach of light that has traveled since the Big Bang.
Defining the Observable Universe
The diagram provided offers a visual representation of the concept. The visible light cone is plotted relative to the time of light emission on the vertical axis and the current position of the object on the horizontal axis. For instance, an object that emitted its light 2.5 billion years after the Big Bang (BG) is currently about 18 billion light years (Blyr) away. This distance is relative to our current position in the universe.
The Hubble Sphere
The Hubble sphere, a term derived from the work of astronomer Edwin Hubble, defines an object that would be moving at the speed of light. Therefore, an object 18 Blyr away is moving towards us at the speed of light, meaning that its light will never reach us. However, it was not at this speed when it emitted its light, making it visible to us now.
The Event Horizon and the Observable Universe
The event horizon is the distance to an object that emitted its light at a specific time that we would be able to observe in the infinite future. For instance, taking the time t0, the event horizon would place the observable universe 65 Blyr in the infinite future. This concept is critical in understanding the boundaries beyond which light will never reach us.
Understanding the Expansion of the Universe
Some objects at the edge of the observable universe are moving away from us at speeds greater than the speed of light due to the accelerated expansion of the universe. This expansion means that the light emitted from these objects will never reach us, as the distance between us and these objects is increasing faster than the speed of light. This phenomenon is often referred to as 'cosmic inflation.'
The theory of cosmic inflation, which occurred approximately 10^-32 seconds after the Big Bang, suggests an exponential expansion of the universe by a factor of 10^75. This means that the portion of the universe we can observe today is only a small fraction of the universe as a whole. However, due to the fact that the universe started from a single point and expanded uniformly, we can assume that the properties of the universe beyond our observable region are very similar to what we observe within it.
The Infinite Universe and Multiverses
It is important to note that, as far as our current understanding goes, the universe does not have a boundary. The concept of a center of the observable universe is also misleading, as the observable universe is not a fixed entity but is relative to the observer. Every point in the universe can be considered a center of its own observable universe.
Finally, the idea of the universe being infinite in size adds an interesting layer to this discussion. Given the vastness and uniformity of the early universe, it is reasonable to assume that the universe beyond our observable region is similar to what we observe within it. Additionally, the concept of a multiverse, where multiple universes exist beyond our observable universe, remains a topic of speculation and research.
Further Reading
For those interested in a more in-depth exploration of these topics, I have written several other Quora answers on this subject. One of these can be accessed here.