Infinite universe

　　When you stare at the night sky, what do you think of? When you see all these twinkling stars, you probably already know that many of them are stars and planets. But is this the entire universe? Do not! The universe is much larger.
　　Galileo Galilei discovered the “countless star clusters” hidden in the Milky Way in 1610. About three centuries later, astronomers used more advanced telescopes to discover that when the Milky Way was only one of “the many galaxies in the universe”, the boundaries of the universe “extended” again. Soon, researchers discovered that the universe was expanding and the distance between galaxies was getting farther and farther.
　　Now, larger and larger telescopes have told us that the observable universe has an unimaginable diameter of 92 billion light years, which may contain 2 trillion galaxies. However, astronomers still don’t know how many unknown universes there are beyond the known universe.
　　Virginia Trimble of the University of California, Irvine, said: “The universe is always bigger than what we see.”
　　”The telescope can only see the observable part. We can’t see the past before the universe was born.” Said John Mather, a Nobel Prize cosmologist, chief scientist of the James Webb Space Telescope, and NASA Goddard Space Flight Center.
　　”So, our sight is completely restricted. We have tried our best to see everything we can see.” At the edge of the universe, we saw the brilliance left by the Big Bang-the so-called cosmic microwave background radiation (CMB). But this is not the magical edge of the universe. Our universe is still advancing. We may never know where its edge is.
　　In recent decades, cosmologists have tried to solve this problem by first determining the shape of the universe, just as the ancient Greek mathematician Eratosz used simple trigonometric functions to calculate the size of the earth. Theoretically, our universe can have three shapes. Which shape depends on the curvature of the space itself: saddle-shaped (negative curvature), spherical (positive curvature), or flat (no curvature).
　　The saddle-shaped universe has fewer supporters, and the spherical universe seems to be easier for us on earth to understand. The earth is net-shaped, and the sun and other planets are also net-shaped. The spherical universe means that you can go in any direction in the universe and eventually return to the starting point, just like Ferdinand Magellan and his staff travel around the earth. Einstein called this model “a finite and unbounded universe.”
　　However, since the late 1980s, a series of orbital observatories built for the study of CMB have carried out more and more accurate measurements. The measurement data show that the universe has no curvature at all. At least, it is flat within the limited range that astronomers can measure-if it is a sphere, then it is a huge sphere, so that the entire universe we observe is not enough to show any curvature.
　　Mather said: “The universe is flat, like an endless piece of paper. According to this, you can go on indefinitely in any direction, and the universe will hardly change. You will never get here. At the edge of the flat universe, you will only find more and more galaxies.”
　　Most astronomers have no objection to this view. Therefore, flat cosmology is the core of modern cosmology.
　　But the problem is that, unlike a spherical universe, a flat universe can be infinite or finite. And there is no real way to tell the difference between the two.
　　Therefore, astronomers pin their hopes on theory, a model that can provide indirect proof. For example, the standard model of physics predicts the existence of many particles, such as the famous Higgs boson. Even if they have not been discovered, physicists still believe that these particles are real.
　　Trimble said: “If you have a good description of everything you have observed so far, and this description can also make some accurate predictions, then you will tend to believe that this is the case. This is what most Science in the eyes of scientists.”