We do not know whether the universe will exist forever; we do not know whether the universe is limited or infinite in terms of scope; we only know that the physical size of the universe must be larger than the part we can observe; we do not know whether our universe contains all the things that exist, or is only one of the many universes that make up the multiverse. We also don’t know what happened at the earliest stage of the universe – the first moment of the big bang – and we didn’t find the evidence necessary to arrive at a reliable conclusion. < / P > < p > in the nearby universe, we see stars and galaxies that are very similar to the sun and the Milky way. But when we look further, we can see what the universe looked like in the distant past: less compact, hotter, younger, less evolved. In many ways, the history of the universe we can see is limited. < / P > < p > but one thing we can be sure of is that the universe has an edge: not in space, but in time. This is because the big bang happened in the past known limited time, that is, 13.8 billion years ago, and the uncertainty range is less than 1%. Even under the limit of the ultimate speed of light, the farthest distance we can see is also limited. There is a “edge” in the universe that we can understand its distance. Maybe we still need to sort out the time line, and the farther we look, the more we can see The earlier it is. < / P > < p > if you’re tired of the big bang theory and want to make your own interpretation of cosmology, that’s fine, but you have to figure out some problems, such as the expansion of the universe, and the spots in the early image of the universe. In other words, you have to find a better explanation than inflation. < / P > < p > this seems simple, but it’s not. Pressure, density and temperature differences in the early universe have plagued many other cosmic theories, including the most popular one, the ekpyrotic universe. It’s an ancient philosophical idea. The word “ekpyrotic” means “fire” in Greek. The Stoic school of ancient Greece believed that the universe was a fire, in the eternal cycle of birth, cooling and regeneration. < / P > < p > in the fire universe model, the universe is constantly circulating, and we are currently in the “explosion” phase, which will eventually (in some way) slow down, stop, reverse, and compress back to incredibly high temperatures and pressures. Then, the universe will bounce back (in some way) and rekindle in the new big bang phase. < / P > < p > the problem is that it is difficult to replicate the spots and patches in the early images of the universe in the fire universe. When we try to piece together some fuzzy physics ideas to explain the “squeeze rebound explosion” cycle (emphasis should be placed on “fuzziness”, because the known physics can not understand some of the energy and scale involved in the process), everything is too flat. There are no bumps, no oscillations, no spots, no differences in temperature, pressure and density. < / P > < p > this not only means that these theories are inconsistent with observations of the early universe, but also that they cannot explain a universe full of galaxies, stars and even humans. < / P > < p > artist’s logarithmic scale concept map of the observable universe. Galaxies give way to large-scale structures in the periphery, as well as the hot, dense plasma produced by the big bang. This “edge” is just the edge of time. < / P > < p > today, we see the universe as it looked 13.8 billion years after the big bang. Most of the galaxies we see come together to form clusters (such as the local cluster) and clusters (such as the Virgo Cluster), separated by so-called “holes.”. These clusters include both spiral and elliptical galaxies. Typical galaxies similar to the Milky way can form an average new star like the sun every year. < / P > < p > in addition, conventional matter in the universe is mainly composed of hydrogen and helium, but about 1% to 2% of the conventional matter is composed of heavy elements in the periodic table, which enables the formation of rocky planets such as the earth and produces complex, even organic chemical reactions. Although there are many differences between galaxies, for example, some galaxies are very active, and stars are constantly forming; some galaxies have active black holes, while others have not formed any new stars for billions of years. Generally speaking, the galaxies we see are very large, and they have accumulated after a long period of evolution. The evolution of the large-scale structure of the universe, from the early homogeneous state to the agglomerated universe as we know it today. If we change the matter in the universe today, the kind and amount of dark matter will make up a completely different universe. Note the fact that in all cases, small-scale structures appear in the early days, while large-scale structures do not appear until a long time later. < / P > < p > however, if we look beyond the universe, that is, looking back at the earlier universe, we can see how the universe has grown into what it is now. Further down in the universe, we’ll find clusters that are slightly smaller and more uniform, especially on larger scales. We’ll see galaxies with lower mass, fewer evolutionary features, more spiral galaxies, and fewer elliptical galaxies. On average, these galaxies have a larger proportion of blue stars and a higher rate of past star formation. Although the space between galaxies has become smaller, the overall mass of early clusters and clusters has also decreased. < p > < p > in this picture of the universe, today’s galaxies are formed by merging smaller and lower mass galaxies on the cosmic time scale, and gradually develop into one of the largest structures in the universe. Compared with today’s galaxies, the characteristics of galaxies in the early universe include the following aspects: < / P > < p > there are many galaxies comparable to the present Milky way, but compared with the galaxies seen today, the younger galaxies similar to the Milky way are smaller, bluer, more chaotic and richer in gas. For the first galaxies, the effect was extreme. In the universe we can observe, galaxies follow these rules. < / P > < p > but as we look farther and farther into the universe, and towards earlier and earlier times, this gradually changing picture suddenly begins to change rapidly. When we look at a place about 19 billion light-years away from us, it corresponds to the universe only 3 billion years after the big bang; we can see that the rate of star formation in the universe has reached its maximum, which is about 20 to 30 times the rate of new star formation today. A large number of supermassive black holes are very active at this time. Due to the consumption of surrounding materials, they release a lot of particles and radiation. < / P > < p > in the past 11 billion years or so, the evolution of the universe has been slowing down. Of course, gravity continues to cause the structure of the universe to collapse, but dark energy begins to fight against it and dominated the expansion of the universe six billion years ago. New stars continue to form, but the peak of star formation is in the distant past. Supermassive black holes continue to grow, but their brightest days are long past. More black holes are now dimmed and inactive than in the early days. < / P > < p > the Fermi Space Telescope team reconstructed the history of star formation in the universe and compared it with data points from other alternative methods in the literature. Astronomers have achieved consistent results through many different measurements, and Fermi’s data represent the most accurate and comprehensive part of this history to date. < / P > < p > as we move farther and farther away from the earth and closer to the “edge” defined by the beginning of the big bang, we start to see more significant changes. A distance of 19 billion light-years corresponds to a time when the universe is only 3 billion years old, which is the peak of star formation. At this time, there may be 0.3% to 0.5% heavy elements in the universe. < / P > < p > when we are close to 27 billion light-years away, the corresponding cosmic age is only 1 billion years. The stellar structure is much smaller at this time, because the rate of new star formation is about a quarter of that at the later peak. The proportion of conventional substances composed of heavy elements dropped sharply: 0.1% at 1 billion years old and 0.01% at 500 million years old. In such an early environment, rocky planets may not have formed. < / P > < p > at this distance, not only will the cosmic microwave background radiation be stronger – it should be infrared rather than microwave – but every galaxy in the universe should be young and full of young stars; elliptical galaxies may not have appeared in such an early universe. < / P > < p > a sketch of the history of the universe, emphasizing the period of reinonization. Before the formation of a star or galaxy, the universe was filled with light shielding neutral atoms. Although most of the universe did not re ionize until 550 million years later, a few regions had been largely re ionized at an earlier time. < / P > < p > to see the universe earlier than this is indeed beyond the limits of existing instruments, but the Keck, Spitzer and Hubble space telescopes have begun to bring us there. Once we reach a distance of about 29 billion light-years or more – corresponding to the age of the universe of 700 to 800 million years – we begin to enter the first “edge” of the universe: the transparent edge. < / P > < p > today we take for granted that this space is transparent to visible light, but only because it is not filled with materials that block light, such as dust or neutral gas. But in the early days, before enough stars formed, the universe was filled with neutral gases that were not completely ionized by the ultraviolet radiation of the stars. As a result, a lot of the light we see is obscured by these neutral atoms, and only when enough stars are formed will the universe be fully ionized. < / P > < p > that’s part of why infrared telescopes, such as NASA’s James Webb Space Telescope, were crucial to the study of the early universe. With this kind of telescope, we can see the “edge” at the familiar wavelength. < / P > < p > as we explore the universe more and more deeply, we see more and more distant in space, and the time we can trace is getting earlier and earlier. The James Webb Space Telescope will take us directly to the depths of the universe that cannot be compared with the current observation equipment, and reveal the super distant starlight that the Hubble Space Telescope cannot see. < / P > < p > at a distance of 31 billion light-years, equivalent to 550 million years after the big bang, we have reached the so-called reinonization edge: here, most of the universe is almost transparent to visible light. Reinonization is a gradual and uneven process; in many ways, it’s like an uneven wall filled with holes. Re ionization occurred earlier in some places, which led to the discovery of the most distant galaxy so far – 32 billion light-years away, only 407 million years after the big bang. But other regions remained partially neutral until nearly a billion years later. < / P > < p > if the answer is yes, the universe may have been in an endless great rebound cycle; in this cycle, all