Astronomers used to believe that the universe was infinite, but Einstein’s theory of relativity suggests that our universe actually has a boundary, but this boundary exists in a higher-dimensional space.
Before discussing the universe’s boundary, we must first understand boundaries in the universe. From the Sun to the Milky Way, and then to the nearest Large and Small Magellanic Clouds in the Andromeda galaxy, which is 2.54 million light-years away, all these independent and enclosed celestial systems have clear boundaries. There are light-years of empty space between stars and millions of light-years of voids between galaxies that separate them distinctly.
However, according to Einstein’s perspective, as the universe’s spacetime itself belongs to three-dimensional space plus time, it all falls into a macroscopic four-dimensional closed spherical structure. This means that if someone pilots a spacecraft at the speed of light in any direction, they will eventually return to their starting point. The spacecraft controller might think they are moving straight up, but in a closed universe, the geodesic path is a closed curve, and they will return to the original point before they even realize it.
In traditional geometry, we often think of space as flat, but in reality, according to the theory of general relativity, space can actually be curved. This is because mass and energy change the space around them, causing it to curve. We often use Einstein’s famous equation E=mc² to describe this phenomenon, meaning that the presence of mass and energy affects the properties of spacetime. The curvature of space is considered the foundation for the wonders of the universe.
This finite universe model is similar to a human journey on Earth. Both move forward straight but eventually return to the starting point. Similarly, to get a full view of Earth, you must go straight up into space; only then can you see the entire Earth and evaluate its limitations.
So, if humans want to see the universe’s boundary in the future, they must first open a new direction or dimension in three-dimensional space, then fly at the speed of light or faster in that direction to escape our current world.
In addition to its colossal scale, the mass in the universe is unimaginable. Stars are the most basic building blocks of the universe, and they combine to form star clusters, nebulae, and galaxies. However, the mass of individual stars is relatively insignificant. Scientists have discovered colossal supernova explosions, the gigantic flares of these explosions releasing more energy than all the stars in the entire galaxy combined.
According to the theory of parallel universes, our current universe is just one of many parallel universes. There are countless universes in the vast multiverse. Each universe is the product of a big bang. Different universe bubbles have different laws of the universe, such as our universe, where the speed of light is 300,000 km/s, but in other universes, the speed of light could be 30,000 or 300 km/s.
However, with current human technology, it is still too far-fetched to talk about the universe’s boundary because even the James Webb Space Telescope, the most powerful human telescope, cannot see the universe’s boundary. It can only roughly estimate its existence. The universe has a diameter of 93 billion light-years, and at the current moment, humans can only see 2 trillion galaxies within it, and these cosmic structures are still quite vague at the macroscopic level.
The universe is a vast and mysterious existence, filled with countless wonders and puzzles that continue to perplex humans in their exploration. With advances in technology and the development of scientific theories, we are gradually realizing that the universe may not only exist in the familiar three-dimensional space but could also exist in curved spacetime and parallel universes.
In some science fiction films, the Milky Way, with a diameter of 100,000 light-years, is just a string of beads worn around the neck of a kitten, and these beads are just marbles that advanced extraterrestrial civilizations use to play games. Perhaps in the future, we will discover that the entire universe is just a microscopic part of a higher-level macroscopic object.
In this case, our universe could be just a small part of a larger universe, like Russian Matryoshka dolls, or the universe could be contained within a grain of sand, located on the beach of an unimaginably vast world. Perhaps intelligent civilizations from other dimensions are currently observing the Sun and Earth, just as we observe electrons and protons.
The universe remains the greatest mystery for human civilization, and questions about its boundaries will undoubtedly be discussed many times in the future. However, regardless of where the universe’s boundary lies or what it may conceal, human civilization has already emerged on Earth.