All parallel universes form the first level of the multiverse. --This is the least controversial level. Everyone accepts the fact that although we cannot see the other self at the moment, we can observe it later by moving to another place or simply waiting in place long enough. It's like looking at an approaching ship beyond the horizon of the sea - it's similar to looking at objects outside the horizon. As light travels, the radius of the observable universe expands by one light-year every year, so just sit there and wait and see. Of course, you probably won’t be able to wait until the day when the light from another you in another universe reaches here, but theoretically speaking, if the theory of universe expansion is tenable, your descendants may be able to see it with super telescopes they.

So, the concept of a Level 1 multiverse sounds mundane? Isn't space infinite? Who could imagine a sign somewhere saying "Space ends here, watch out for the ditch below"? If so, everyone will instinctively question: What is "outside" at the end? In fact, Einstein's gravity field theory turned our intuition into a problem. It is possible that space is not infinite, as long as it has some degree of curvature or is not topological (i.e., interconnected) as we intuitively think.

A spherical, donut-shaped or horn-shaped universe may have limited size but no boundaries. Observations of the cosmic microwave background radiation can be used to test these hypotheses. However, observations so far seem to contradict them. The model of the endless universe is consistent with the observational data, and there are strong restrictions.

Another possibility is that space itself is infinite, but all matter is limited to a limited area around us - the once popular "island universe" model. What's different about this model is that at large scales matter is distributed in fractal patterns and is constantly dissipated. In this case, almost every universe in the first level multiverse will eventually become empty and dead. However, recent observations on the three-dimensional galactic distribution and microwave background have pointed out that the organization of matter presents a vague uniformity on large scales, and no clear details can be observed on scales greater than 10^24 meters. Assuming this pattern continues, the space beyond our observable universe will also be filled with planets, stars, and galaxies.

There is data supporting the theory that space extends beyond the observable universe. The WMAP satellite recently measured fluctuations in the microwave background radiation. The strongest amplitude exceeds 0.5 kelvin, suggesting that space is very large, perhaps even infinite (middle image). In addition, WMAP and 2dF galaxy redshift detectors found that matter is evenly distributed in space at very large scales

Observers living in different parallel universes of the first level multiverse will perceive that they are different from ours Same physical laws, but different initial conditions. According to the current theory, matter was thrown out with a certain degree of randomness at a moment in the early days of the Big Bang. This process includes all possibilities for the distribution of matter, and each possibility is not 0. Cosmologists assume that our original universe with an approximately uniform distribution of matter and an initial wave state (one of 100,000 possibilities) is quite typical (at least among all parallel universes that have produced observers). typical) individual. Then the nearest person who is exactly like you will be 10^(10^28) meters away; and there will be an area with a radius of 100 light years only 10^(10^92) meters away, and everything in it It is exactly the same as the space we live in, which means that everything that happens in our world in the next 100 years will be completely reproduced in this area; and the area will only increase to 10^(10^118) meters away. Bo volume is so large, in other words, there will be a universe exactly like ours.

The above estimate is extremely conservative. It only enumerates all quantum states in a space with a temperature below 10^8 degrees and a size of one Hubble volume. One step in the calculation is this: How many protons can fit in a Hubble volume at that temperature? The answer is 10^118.

Each proton may exist or not exist, which is a total of ***2^(10^118) possible states. Now all it takes is a box that can hold 2^(10^118) Hubble spaces to exhaust all possibilities. If the box is larger—for example, a box with a side length of 10^(10^118) meters—the arrangement of the protons will inevitably repeat according to the drawer principle. Of course, the universe has more than just protons and more than two quantum states, but a similar method can be used to estimate the total amount of information the universe can hold.

The average distance of another universe that is exactly the same as ours. The "double" closest to you may not be as far away as theoretically calculated, but may be much closer. Because the way matter is organized is also subject to other physical laws. Given some laws such as planet formation processes and chemical equations, astronomers suspect that there are at least 10^20 planets inhabited by humans within our Hubble volume alone; some of them may be very similar to Earth.

The framework of the Level 1 multiverse is often used to evaluate modern cosmological theories, although the process is rarely articulated. Consider, for example, how our cosmologists use the microwave background to try to derive the geometry of the universe as "spherical space." With the difference in the radius of curvature of space, the size of those "hot areas" and "cold areas" on the cosmic microwave background map will show certain characteristics; and the observed areas indicate that the curvature is too small to form a spherical closed space. However, it is important to maintain statistical rigor. The average size of these regions in each Hubble space is completely random. So it's possible that the universe is fooling us - it's not that the curvature of space is insufficient to form a closed sphere making the observed area smaller, but rather that the average area of ??our universe is inherently smaller than others. So when cosmologists swear that their spherical space model has 99.9 confidence, what they really mean is that our universe is so unsociable that only one in 1,000 Hubble volumes would have something like that. .

The key point of this class is: even if we cannot observe other universes, the multiverse theory can still be verified in practice. The key is to predict the uniqueness of each parallel universe in the first-level multiverse and point out its probability distribution-what mathematicians call a "metric." Our universe should be one of those "most likely universes." Otherwise - and we unfortunately live in an unlikely universe - then the previously hypothesized theory would be in big trouble. As we will discuss next, how to solve this measurement problem will become quite challenging.