These are the big questions of cosmology, directly related to the nature of space, the expansion of the universe, and the limits of human observational capabilities.
Does the universe have an end point?
This is one of the most perplexing questions in cosmology: does the universe have edges, and is the question itself even plausible?
Imagine humans possessing a spaceship capable of traveling faster than the speed of light. If they continued to fly through space, would the ship eventually reach a point where it could no longer go any further?
How vast is the universe really? If the universe has an end point, what lies beyond that boundary?
These are all very difficult problems to visualize. However, imagining an infinite universe is no easier.
People tend to think that everything must exist within a certain space.
Therefore, knowing that the universe is expanding, many people ask: if the universe is getting bigger, where is it expanding into?
To find the answer, we first need to distinguish between the observable part of the universe and the entire universe that actually exists.
The cosmic horizon: The boundary of what humans can see.
There is a concept that is relatively easier to visualize than the question of the edge of the universe, and that is the cosmological horizon – the limit of the universe that humans can observe.
Light from the very early moments after the Big Bang has traveled through space for approximately 13.8 billion years, which corresponds to the age of the universe.
This means we can only observe regions of space where light has had enough time to travel to Earth since the universe was formed.
However, as light moves, space itself is also constantly expanding.
Therefore, the current distance to the furthest regions we can observe is much greater than the distance light has traveled in 13.8 billion years.
This distance, typically defined in a uniformly moving frame of reference, is approximately 45 billion light-years in each direction.
Therefore, the diameter of the observable universe is estimated to be around 90 billion light-years.
Beyond this limit, light from distant regions of space hasn't had enough time to reach our telescopes.
Therefore, no matter how advanced observation technology becomes, humans will still not be able to see areas beyond the current cosmological horizon simply by building more powerful telescopes.
The important thing is that the cosmological horizon is not a real physical wall or boundary. It's more like the horizon that sailors see when standing in the middle of the ocean.
The horizon at sea makes the ocean seem to end at a certain distance. But in reality, the sea continues to stretch far beyond the horizon.
Similarly, space could also continue to extend beyond our observable universe.
There is no reason to assert that galaxies cannot exist at distances greater than 45 billion light-years. They may be present there, but their light has not yet reached us.
However, the fact that the universe continues to exist beyond the observable realm is not enough to determine whether it is finite or infinite in size.
The universe may be finite, but it has no boundaries.
According to Albert Einstein's theory of general relativity, the geometry of space can be affected by matter and energy.
In principle, three-dimensional space can curve in ways that make the universe finite in size but without end.
To illustrate this, imagine the surface of a beach ball. That surface has a finite area, but if an organism could only move along the surface of the ball, it would never find a boundary to reach the end.
The organism may continue moving in one direction and eventually return to its original position.
Of course, the surface of a ball only has two dimensions, while the space we live in has three dimensions. This comparison is simply meant to illustrate how a space can be finite without edges.
If the universe has positive curvature, similar to the geometry of a sphere's surface but extended to three spatial dimensions, its total volume could be finite without a physical boundary.
In a similar two-dimensional model, one wouldn't need an infinite amount of paint to cover the entire surface of the ball.
Conversely, a universe with negative curvature can be envisioned as a more complex geometric form, in which space curves in different directions.
Flat universes and negatively curvature universes, in the standard models considered, can extend infinitely.
Thus, the existence of a cosmic edge and whether the universe is finite are two completely different issues. A finite universe could still have no boundaries, while an infinite universe doesn't need any end point.
Measurements suggest the universe may be flat and infinite.
For decades, cosmologists have sought to measure the curvature of space on a large scale. Recent observational results, combined with theoretical arguments, generally suggest that the universe we live in is very close to a geometrically flat state.
A flat universe doesn't mean space is like a tabletop or a sheet of paper lying within a more multidimensional space.
This concept describes the geometry of space on a large scale, in which the usual rules of Euclidean geometry still apply approximately.
For example, in planar geometry, the sum of the three angles of a triangle is 180 degrees. If space has significant curvature, the sum of the angles of a triangle on a sufficiently large scale may differ from this value.
Determining the curvature of space is no simple task. Scientists must analyze the signatures left behind in the cosmic microwave background radiation, the distribution of galaxies, and various other astronomical observational data.
Current measurements show that the curvature of the universe is very small, consistent with the flat space model. However, it should be noted that a flat universe does not mean that scientists have proven the universe to be infinite.
Current observations can only examine a finite portion of space, so it's still not possible to completely rule out the possibility that the universe is finite in size but too large for us to perceive its curvature.
If the universe is truly flat and extends infinitely, the portion that humans can observe would only occupy an extremely small fraction of the entire universe.
At that time, what we saw through the telescope was only a limited part of a whole so vast that it was difficult to imagine.
(According to SciTechDaily, Earthsky, Skyatnightmagazine)