How big is the universe? How long has the universe existed? What is the end of the universe? These major problems have always plagued humanity. But with the continuous development of modern astronomy, there are already preliminary answers to these questions.
According to the Standard Model of cosmology, the universe has existed not indefinitely, but about 13.8 billion years. The observable universe, on the other hand, is about 93 billion light-years in diameter. So does this mean that 46.5 billion light-years away is the end of the universe?
The answer is no. We observe in any direction on Earth, and at most we can only see objects 46.5 billion light-years away. But the edge of the observable universe is only the limit of what we can observe at the moment, not the edge of the universe. Since the universe has existed for a finite time, and the speed of light is not infinitely fast, there is still a part of the universe that we cannot observe now, and the light there does not have enough time to complete the journey to Earth.
The end of the observable universe is the unobservable universe, so what is the end of the unobservable universe?
At present, human beings cannot know the exact size of the entire universe, and it is impossible to observe whether the universe is limited or not. If the universe were infinitely large, there would be no concept of an end to the universe. If the universe is finite, there won't necessarily be an end to the universe. As for whether there is an end to the universe and what is beyond the end of the universe, it is necessary to figure out what the geometry of the universe is.
The geometry of the universe is determined by matter (ordinary matter and dark matter) and energy (photons, neutrino radiation, and dark energy) throughout the universe, and the magnitude of the mass-energy density affects the curvature of space-time. According to Friedman's equations, a critical density can be calculated. By comparing mass-energy densities with critical densities, the shape of the universe can be inferred, with a total of three possibilities:
(1) If the mass density of the universe is less than the critical density, and the space-time curvature of the universe is less than 0, the shape of the entire universe is an open hyperbolic shape, and the universe will expand indefinitely in the future.
(2) If the mass density of the universe is equal to the critical density, and the space-time curvature of the universe is equal to 0, the entire universe is flat, space extends infinitely, and the universe will gradually stop expanding in the future.
(3) If the mass density of the universe is greater than the critical density, and the space-time curvature of the universe is greater than 0, the entire cosmic space will be compressed into a closed state, and the universe will change from expansion to collapse in the future.
It follows that no matter what the geometry of the universe is, there is no concept of the universe ending. In an open universe, the universe has no end and is also infinite. In a closed universe, there is no end to the universe, but it is finite.
The above is based on the situation of four-dimensional space-time (three-dimensional space + one-dimensional time), which is what we can understand and recognize at present. Perhaps only by entering the higher dimensions of space-time can we see the whole picture of the universe and determine what the end of the universe is.