Real Physics Terms: From the Big Bang to Black Holes

In scientific research, every time scientists discover a new natural phenomenon or propose a new scientific concept, they will create a proper noun to name it. Good naming can help people understand. Complex phenomena and difficult concepts will be remembered because of intuitive and easy-to-understand names. Some scientific terms are fascinating and arouse people's curiosity and desire to explore. If the naming is not good, it will cause misunderstandings or discourage people. This article will "bite the words" of some well-known physical terms in cosmology and astrophysics, carefully study their meanings, and explore the physical meanings behind them.

Written by Chen Shaohao (Massachusetts Institute of Technology, USA)

The Big Bang was not an explosion

The universe refers to the sum of space and time, including all matter and energy in space and time. The word "universe" in English means "unique", which means there is only one universe and it contains everything.

In physics, there is a multiverse theory, which holds that there are multiple independent parallel universes. However, this is only a hypothesis based on mathematical logic and cannot be verified in the real world.

The universe we live in originated from a Big Bang event about 13.78 billion years ago. The Big Bang theory is currently recognized as the most reliable theory of the origin of the universe. The most important evidence that convinces physicists of the Big Bang theory is the Cosmic Microwave Background. A large number of observations have confirmed that microwaves from all directions in the universe have a stable background temperature of 2.7 Kelvin. This background temperature is reserved by the Big Bang.

Observations by the Hubble telescope show that all celestial bodies in the universe are moving away from each other. Any observer in the universe can conclude that other celestial bodies are moving away from them. A reasonable explanation for this fact is that the universe is expanding. As space becomes larger and larger, celestial bodies in space will move away from each other. The expansion of three-dimensional space is difficult to imagine, so let's take two-dimensional space as an example. Mark two points on a balloon at random, and then blow up the balloon. As the balloon expands, the surface area becomes larger and larger, and the distance between the two marked points becomes farther and farther. The same is true for three-dimensional space.

Since the universe is expanding today, then if time is reversed, the earlier the universe is, the smaller the space is. From this, we can infer that in the earliest universe, all matter and energy were gathered in a very small space, called a singularity. It was from this tiny singularity that the universe began to expand continuously, and after about 13.78 billion years, it formed our universe today.

Figure 1. Schematic diagram of the expansion of the universe. Since the Big Bang, the universe has continued to expand for about 13.78 billion years. ｜ Source: Wiki

The Big Bang means that the universe started from a very small space and expanded very quickly in a very short time. This is different from the bomb explosion that people are familiar with. Whether it is the explosion of chemical substances or nuclear bombs, it refers to the rapid diffusion of matter or energy in space, while the Big Bang refers to the rapid expansion of the entire universe itself. The word Bang in English refers to the sound of an explosion, which obviously does not capture the essence. In any case, the English word The Big Bang and the Chinese word Big Bang have been widely accepted and used by people.

There is no visible light during the dark ages

After the Big Bang, space expanded rapidly and the temperature dropped rapidly. In the first trillionth of a second, the four basic interactions separated one after another. In the following 10 seconds, various basic particles were formed one after another. Some basic particles transmitted interactions, while other basic particles formed atomic nuclei in about 17 minutes (1000 seconds).

Since most particles have annihilated with antiparticles, the energy of the universe will mainly exist in the form of photons for the next 370,000 years. At this time, the universe contains a large mass of high-temperature and high-density plasma composed of atomic nuclei, electrons and photons. Photons frequently collide with charged particles such as atomic nuclei and electrons, so the average free path of photons is very short, making the universe opaque. Although photons exist, they cannot be observed.

About 18,000 years after the Big Bang, the temperature in the universe began to drop to the point where electrons could be captured by atomic nuclei to form atoms. This process is called recombination. About 370,000 years later, the recombination process ended and a large number of neutral atoms were formed in the universe. They were mainly hydrogen atoms, with a small number of helium atoms. The atoms produced by the recombination process were initially in an excited state, and then quickly transitioned from the excited state to the ground state, releasing energy in the form of photons. This process of releasing photons is called photon decoupling. Unlike charged particles, the interaction between neutral atoms and photons is very small, and as the density of the universe continues to decrease, the mean free path of photons becomes almost infinite. In other words, photons at this time can travel unimpeded in the universe, and the universe becomes transparent.

The wavelength of the photons released from the deexcitation of hydrogen atoms is in the visible light band, which is yellow-orange. As the universe is expanding, for observers on Earth, the light source is moving away from them. According to the Doppler Effect, when these photons reach today's Earth, they are redshifted, that is, the wavelength becomes larger, from visible light to microwaves. This is the cosmic microwave background radiation observed on Earth today. This is also the earliest cosmic event that humans can observe.

During this period, theoretically, there is another mechanism that can produce microwave radiation, that is, the quantum transition between the two hyperfine structure energy levels of the ground state of hydrogen atoms, releasing photons with a wavelength of 21 cm. Since there are a large number of hydrogen atoms in the universe at this time, the 21 cm spectral line should be observable. How to detect the 21 cm spectral line is currently a cutting-edge research field.

From 370,000 years to hundreds of millions of years after the Big Bang, for observers on Earth today, although microwave radiation can be observed, visible light cannot be observed, so this period of the universe is called the "Dark Ages". It was not until hundreds of millions of years after the Big Bang that the first generation of stars were born and emitted visible light, and the universe began to have light, ending the Dark Ages.

Supernova explosions do not create new stars

The first generation of stars have never been directly observed, and are only theoretically speculated to be non-metallic stars. This is because the Big Bang produced hydrogen and helium, but no metal elements. Unlike the commonly known metals, in astrophysics, elements heavier than helium are collectively referred to as metal elements.

A large amount of hydrogen nuclear fusion occurs inside the star, and the fusion provides energy to resist gravitational collapse. Fusion continuously turns hydrogen into helium. For massive stars, when the hydrogen in the core is exhausted, the hydrogen in the outer shell begins to fuse, which will cause the star to gradually increase in size until it becomes a red super giant. If the mass of the star's core exceeds the Chandrasekhar limit, the core will suddenly collapse because the electron degeneracy pressure cannot resist gravity. A huge explosion occurs during the collapse, throwing most of the star's matter outward at high speed. This process is called a supernova. The energy generated by a supernova explosion is enough to fuse some lighter non-metallic elements into heavier metal elements. The ejecta containing metal elements provide raw materials for the formation of the next generation of stars.

The lifespan of the first generation of stars is generally less than a few million years, and when they die, they produce a small amount of metals through supernova explosions. These metals, together with hydrogen and helium, become the elements that make up the next generation of stars, so the second generation of stars contains a small amount of metals. The lifespan of the second generation of stars is hundreds of millions or billions of years, and some of these stars produce more metals through supernova explosions when they die. The third generation of stars used these metals during their formation process, so they are rich in metals. The sun we are familiar with is a third generation star.

Supernova explosions produce extremely strong electromagnetic radiation and extremely bright visible light, which may last for weeks, months, or even years. The English name of supernova is Supernova, where nova means "new" in Latin, that is, a new bright star appears. In fact, this star has existed for a long time, but it was discovered by observers on Earth because of its sudden and significant increase in brightness, so people once mistakenly thought it was a new star.

A famous example of a supernova explosion is SN 1054, whose remnant formed the Crab Nebula. In 1054 AD, Chinese, Arab, and Japanese astronomers all recorded this supernova explosion. The "History of the Song Dynasty·Astronomy Records-9" records: "In the first year of Zhihe, on May, Jichou, it appeared a few inches southeast of Tianguan, and disappeared a year later." The "Song Huiyao" records: "In March of the first year of Jiayou, the Sitianjian said: 'The guest star disappears, which is a sign of the guest's departure.' At first, in May of the first year of Zhihe, it appeared in the east in the morning, guarding Tianguan, and was seen as Taibai during the day, with four red and white horns, and was seen for 23 days."

Figure 2. The Crab Nebula photographed by the Hubble Telescope. The Crab Nebula is a remnant of a supernova explosion, with a shell-like structure formed by expanding gas and dust. ｜ Source: Wiki

Is a black hole a hole?

The remaining stellar core of a supernova explosion continues to collapse, and the huge pressure causes protons to absorb electrons and turn into electrically neutral neutrons. The core of the star eventually forms a dense celestial body composed of neutrons, called a neutron star. A neutron star is generally only about 10 kilometers in diameter, smaller than a city. A neutron star is like a huge atomic nucleus, and its density is much greater than the common atomic matter on Earth. The mass of a small cup of matter on a neutron star exceeds the total mass of all humans on Earth.

If the mass of the star's core is large enough, it will continue to collapse to a size smaller than the Schwarzschild radius, eventually forming a black hole. The gravitational field of a black hole is so strong that neither matter nor electromagnetic waves (including visible light) can escape. The boundary of the area where no escape can occur is called the event horizon.

In 1916, German physicist Karl Schwarzschild discovered a solution with black hole characteristics when solving Einstein's general relativity equations. In the early 20th century, physicists called black holes gravitationally collapsed objects. In the 1960s, American physicist Robert Dicke first used the term "black hole" to describe this type of object. Later, due to the promotion of John Wheeler, the master of general relativity, the term black hole was widely used in academia. (Editor's note: For the origin of the name black hole, readers can also refer to
https://ar5iv.labs.arxiv.org/html/1811.06587)

Observers outside the event horizon cannot directly observe black holes. By observing the movement of celestial bodies near black holes and inferring the gravitational effect, the existence of black holes can be indirectly confirmed. When interstellar matter is absorbed by a black hole, a high-speed rotating accretion disk is formed, which emits strong electromagnetic waves. Therefore, the accretion disk surrounding the black hole can be observed. The event horizon at the center of the accretion disk is a non-luminous spherical area that looks like a hole from the outside.

The center of a black hole is a singularity with infinite density. The space-time inside the black hole is highly distorted, and all matter falls toward the singularity at the center. Some theories believe that most of the space inside the event horizon is empty. If this is true, there is really a "hole" in the black hole. Since the interior of a black hole cannot be observed from the outside, its true situation remains an unsolved mystery.

Figure 3. The first black hole photo in history, released on April 10, 2019, was observed by the Event Horizon Telescope. ｜ Source: Wiki

Quasars are different from quasars

Finally, let’s look at two easily confused terms related to black holes: quasar and quasi-star.

In the 1950s and 1960s, astrophysicists observed some radio waves coming from distant space, but the origin was puzzling, so they described them as quasi-stellar radio sources. The celestial bodies that emit these radio waves are called quasi-stellar objects, abbreviated as Quasar, which is translated into quasar in Chinese. However, subsequent studies have shown that quasars are galaxies being absorbed by supermassive black holes, and are actually not similar to stars.

The peak period of quasar activity was about 10 billion years ago, which means that quasars were more than 10 billion light years away from the Earth. At that time, some galaxies in the universe happened to be close to black holes. Under the strong gravitational force of the black holes, the matter in the galaxies rotated and fell at high speed, forming an accretion disk, and huge amounts of energy were released in the form of electromagnetic radiation (see Figure 4). The luminosity generated by this process is far greater than that of any star, which makes quasars one of the brightest celestial bodies in the universe. Even quasars in distant space can be observed by humans on Earth.

Figure 4: Imaginary picture of a quasar (not a telescope observation photo). | Source: webbtelescope.org

Unlike quasars, quasi-stars refer to a type of hypothetical star formed in the early universe. Unlike modern stars, quasi-stars have a black hole at their center, so another name for them is black hole stars. According to theoretical calculations, when a star collapses into a black hole, if the outer shell of the star has a large enough mass to absorb the energy generated by the collapse without a supernova explosion, a quasi-star will be formed. Stars with such a large mass can only exist in the early universe before hydrogen and helium have fused into metal elements, that is, they may exist in the first generation of stars.

About the Author

Shaohao Chen holds a bachelor's degree in physics and a doctorate in atomic and molecular physics from Tsinghua University. He was a postdoctoral researcher at the University of Colorado Boulder, and has worked at Louisiana State University and Boston University. He is currently working at the Massachusetts Institute of Technology, engaged in high-performance computing.

This article is supported by the Science Popularization China Starry Sky Project

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.

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