The further we look, the closer we get to the time of the Big Bang. With the continuous improvement of astronomical observation methods, we may discover the earliest stars and galaxies.
The answer to any physical question must ultimately return to the universe itself. However, what if the answer no longer exists?
Of all the questions regarding the universe that man can think of, perhaps the grandest is: Where did the universe come from in the first place? This is not an easy question, because to understand where something came from, we first have to know exactly what it is. Likewise, we must fully understand the laws of physics in order to calculate the results of a physical system starting from a specific set of initial conditions. Only from these initial conditions can we identify possible pathways for things to evolve, understand exactly how they got to be what they are today, and find out which predictions match our universe.
The Milky Way over the hoodoo formation in North America. The starry sky has long been a source of human awe. In the depths of space, stars have their own planets, and they all obey the same laws of physics. Despite their differences in structure and composition, these stars are all very similar to the Sun.
However, what’s incredible regarding this way of thinking is that no matter if we ask this question at any time in the past or in the future, and solve it scientifically, we always end up with the same cosmic story. Today, humans have traced the origins of the universe to an incredible degree, and have even identified the origins of planets, stars, elements, atoms, and more. We’ve found a lot of evidence for the hot Big Bang in the universe, and even before the Big Bang. Despite these realizations, there are still many grand and unknown mysteries regarding the universe waiting to be solved. This is where we are today.
Today, when we look beyond Earth, a brilliant and fairly comprehensive picture emerges. The planet we inhabit, like every other planet in the universe, is made of atoms. A solid center of the densest and heaviest atoms is covered by a gaseous atmosphere. The lighter layer floats on top of the heavier layer, forming an onion-like structure, as is the case with every planet, dwarf planet, and moon that has been well-studied to date.
The cosmic web is the largest structure in the entire universe and is mostly made up of dark matter. On smaller scales, however, baryons can interact, as well as with photons, to form stellar structures, but at the same time release energy that can be absorbed by other objects. Neither dark matter nor dark energy can perform such a function.
Planets can either float freely in galaxies or orbit stars. The cores of stars are constantly undergoing nuclear fusion reactions, fusing lighter elements into heavier ones. When a star runs out of fuel, its core collapses and heats up. If the temperature is high enough and the density is high enough, the next group of elements in the reaction chain will continue to fuse; otherwise, the star will transform into a stellar remnant. In some cases, the remains are mild, but in others they react violently.
On larger scales, stars combine into larger collections, known as galaxies; galaxies aggregate into clusters, known as groups and clusters of galaxies, or even larger superclusters. Together, they form the so-called “cosmic web,” in which galaxies are arranged along large-scale fibrous structures that aggregate into superclusters at the connecting nodes of the fibers; at the same time, this structure is surrounded by a vast, empty cosmic space called “Empty” – separated.
The main source of the various elements found in the universe today. Among them, a small star is any star that is not massive enough to become a supergiant or supernova; many of the elements thought to be from a supernova are more likely created by neutron star mergers.
This is what the universe looks like today. If we want to know how the universe came to be this way, we have to apply the laws of physics to the universe and follow the laws we know regarding the evolution of physical systems. E.g:
(1) We know how gravitation works, and there is a general theory of relativity that governs gravitation, so as long as there is mass or energy, there is a phenomenon of gravitation;
(2) We know how the electromagnetic force works; when an object is charged, whether it is moving or stationary, or exists in the form of electromagnetic waves (such as photons), the electromagnetic force is involved.
(3) We know how the nuclear force works, including how quarks and gluons bind together to form protons and neutrons, how protons and neutrons bind together to form nuclei, and how unstable nuclei (including protons and neutrons) how other combinations of quarks and/or antiquarks other than quarks) decay radioactively;
(4) We know how to perform time-evolution operations on any physical system that we used in the beginning.
Here are 20 protoplanetary disks, all surrounding young stars, as measured by the High Angle Resolution Disk Structure Project (DSHARP). Observations such as these show that protoplanetary disks formed primarily in a single plane, consistent with theoretical predictions and the positions of planets in the solar system.
Simply put, if you give a physicist a set of initial conditions to describe your system, they can write the equations that govern the evolution of that system and tell you—up to the limit of uncertainty and indeterminacy inherent in nature— — What is the outcome (or set of probabilistic outcomes) of the system at any point in the future.
So where does all this come from?
Let’s start with the earth. Earth is full of complexity and diversity, even intelligent life, as well as an atmosphere and oceans, and layered internal structures such as crust, mantle, outer core, and inner core. In simple terms, the Earth is made of atoms, but at a more complex level, the Earth is made up of the entire set of atoms that make up the periodic table, mainly iron, oxygen, silicon, magnesium, sulfur, nickel, calcium, and aluminum.
Throughout the history of the universe, there have been numerous galaxies comparable to the Milky Way today, and they have continued to grow in mass and evolve in structure. Relative to today’s galaxies, young galaxies were smaller, bluer, more chaotic, richer in gas, and lower in density of heavy elements.
This is interesting because the vast majority of these elements are heavy elements, not the lightest hydrogen and helium. However, when we look at the universe, hydrogen and helium are found everywhere. In fact, these two elements are so abundant that they make up more than 99 percent of the atoms in the universe; if you count them by numbers, less than 1 percent of the atoms in the universe are elements heavier than hydrogen and helium.
So in order to make an Earth-like planet—made of rocks, metals, ices, and complex molecules—there needs to be some way to make these heavier elements, and then clump them together in sufficient numbers to to form planets. Fortunately, when we look into the universe, we can see the various processes necessary for this to happen.
Nuclear fusion occurs inside stars, forming heavier elements from lighter elements. At the end of a star’s life, their fate varies according to their mass:
(1) become red giants, with new nuclear reaction processes that do not occur for most of their lives;
(2) produce strong winds that blow away a large part of the star’s mass;
(3) Die in a planetary nebula, and the remnant core will shrink into a white dwarf;
(4) May die in the form of a core-collapsed supernova, and the remnants of the implosion either become neutron stars or black holes;
(5) These remnants, whether white dwarfs or neutron stars, then collide, triggering runaway fusion reactions that create more abundant heavy elements.
This explains why, in some stellar groups, we can only find very few previously formed stars – which is consistent with the observations. For example, stellar groups outside the Milky Way’s halo have relatively low abundances of heavy elements. Likewise, in some stellar groups, there are more generations of star formation, such as in the plane of the galaxy closer to the center of the galaxy, where the abundance of heavy elements is higher.
The lightest elements in the universe were created in the early days of the hot Big Bang, when primordial protons and neutrons fused together to form isotopes of hydrogen, helium, lithium and beryllium. Beryllium is unstable, and only the first three elements before star formation remain in the universe. By comparing baryon density and photon number density, the ratio of observable elements can be obtained, allowing us to quantify the extent of matter-antimatter asymmetry in the universe.
In addition, astronomers recently photographed directly the disks that form around new stars: protoplanetary disks. Inside the disk, they found voids and clumps large and small, as well as evidence of young and newborn planets. After generations of stars were born, existed, and died, a new generation of stars is enriched with material recycled from previously dead stars, from which planets have arisen, including rocky planets with ingredients for life.
In fact, when we look further back into the distant history of the universe, we will see that it is not only a large number of heavy elements that are evolving, but the galaxies themselves. In the nearby universe, we find huge spiral and elliptical galaxies, which are densely packed, have low star formation rates and large masses, and have relatively low gas content; in general, these galaxies have The proportion of red stars is greater than that of blue stars. However, as observations get farther and farther away, we notice two main differences between galaxies:
(1) The farther the galaxy, the lower the degree of evolution. They are less massive and less clustered, and star formation peaked around 11 billion years ago and has declined since; they are rich in gas, with lower abundances of heavy elements, and blue stars compared to today’s galaxies higher relative abundance than red stars;
(2) Furthermore, the further away a galaxy is, the more systematically its light will shift to longer wavelengths, a so-called “cosmological redshift.”
During inflation, the quantum-scale fluctuations of spacetime itself are stretched across the universe, leading to defects in density and gravitational waves. Although in many ways inflationary space can be called “nothing”, both in inflationary times and today, cosmic space has a positive, non-zero energy density.
The second point, within the framework of general relativity, would lead us to the conclusion that the universe is expanding. Inflation causes all light to exhibit a cosmic redshift as it travels through intergalactic space, so objects that are farther away have a greater redshift and appear to be moving away from us faster. And – and this is perhaps the most important point – we’ll see what they were like a longer time ago, since light can only travel at a finite speed. In special relativity, the speed of light is the upper limit of the speed at which all matter moves and information travels in the universe.
However, one clear fact is that galaxies grow and evolve over time. This gives us some profound implications: If we might look back far enough in the past, we might discover the “earliest” groups of stars and galaxies; before that node, no stars or galaxies existed in the universe. If the universe:
(1) has been expanding;
(2) Continuous cooling;
(3) Gravitational effects become “heavier” over time.
We can then conclude that the early universe was smaller, denser, warmer, and more homogeneous than it is today. Using this logic, we can infer the original conditions of the universe using the appropriate principles of physics.
Astronomers did just that, and came up with a series of unusual predictions:
(1) According to the law of the growth of gravitational effects in the expanding universe, the universe will only develop structures such as galaxies, galaxy clusters, and cosmic webs;
(2) There was a period in which stars and galaxies first formed, and before that, there was only primordial gas in the universe;
(3) Before this period, the radiation of the universe would have been so intense and so hot that it would be impossible to form neutral atoms, so there should be some kind of sign when stable neutral atoms first formed;
(4) In earlier times, the universe would have been too hot to form stable nuclei, so when the universe cooled below this threshold, there should have been a set of elements in specific abundances that were produced by the early formed by the fusion reaction of the universe.
All of these predictions were confirmed by observations, but other findings were more impressive. For example, the cosmic microwave background radiation, which is only 2.725K above absolute zero, is consistent with the followingglow of the Big Bang that scientists expect. Astronomers also detected the first evidence of primordial gas clouds and found that they were composed entirely of hydrogen, helium and a small amount of lithium. We even indirectly detect the expected neutrino and antineutrino background remnants in the large-scale structure of the universe and the imprint of temperature defects in the cosmic microwave background.
According to the cosmic facts observed so far, the universe must have been born on the basis of the “seeds” of its large-scale structure, and these “seeds” were originally composed of a series of high-density regions and low-density regions.
So, what caused the initial high-density and low-density regions? This is where the brilliance of the inflationary theory of the universe comes into play. The theory not only provides a quantum fluctuation mechanism that produces these “seeds” of cosmic structure, but also explains the cosmic features that have been observed so far (same temperature everywhere, spatial flatness, large-scale uniformity, etc.), and can Make new predictions regarding what these quantum fluctuations should look like.
The inflationary theory of the universe states that at the time of the hot Big Bang, the hot, dense, largely uniform and rapidly expanding universe was filled with matter and radiation, whereas before that, the universe was completely empty. However, the universe at this time is not without energy (or very little energy, like today’s dark energy), but contains huge energy in the structure of space. As the universe expands, more space is created so that the energy density remains the same. As a result, the universe is endowed with the same properties everywhere, and it is stretched to the point where the curvature is extremely flat—the density of matter in the universe is very close to the critical density required for a flat universe. On the other hand, quantum fluctuations, which normally spread across all space on tiny scales, are stretched by inflation to enormous cosmic scales.
Starting from a state that already exists, inflation theory predicts that as inflation continues, a series of universes will emerge, each completely separate from the others, separated by more expanding space.
According to the predictions of inflation theory, these quantum fluctuations created the seeds for the large-scale structure in the universe today, which should have the following characteristics:
(1) have nearly the same magnitude at all scales;
(2) produced on a scale larger than the cosmic event horizon (that is, larger than the range over which light might have traveled since the beginning of the hot Big Bang);
(3) 100% adiabatic (constant entropy), equal curvature 0 (constant spatial curvature).
Inflation theory also predicts that the properties of the Big Bang’s residual glow might indicate the hot Big Bang’s maximum temperature, which is well below the highest possible temperature, the Planck temperature. According to the standard cosmological model, the Planck temperature is the fundamental upper limit of temperature at which modern physical theories fail, and there is currently no widely accepted theory of quantum gravity to explain it. In other words, this is a fundamental limit on the combination of quantum theory and gravity, where quantum gravitational effects kick in when the temperature reaches the Planck temperature.
Sadly, this is the furthest time we can trace our understanding of the universe today. Due to the very nature of inflation, it must erase any information in the universe that existed before it happened. In fact, we can only hope to see what happened in the final phase of the inflationary period — regarding 10^-32 seconds following the Big Bang —; anything that happened before is undetectable in today’s universe. While we can speculate with high confidence where the observable universe came from and explain the origins of many phenomena in the universe, similar questions, including the origins of space, time, energy, or the laws of physics, remain unanswered.
To be sure, everything we know right now is limited. A finite number of particles, encoding a finite amount of information, exist in the visible universe for a finite amount of time. But why is the universe full of matter and antimatter? Why is there dark matter and dark energy? And why are there physical constants with fixed values? We cannot guarantee that the universe today will give us enough information to find answers to all these questions. Of course, there is a long way to go. As long as we don’t give up searching, human beings will slowly approach the truth of the universe. (Nintendo)
