In scientific research, every time a scientist discovers a new natural phenomenon or proposes a new scientific concept, he creates a proper noun to name it. It's named, it helps with understanding. Complex phenomena and difficult concepts will be remembered because of the intuitive and easy-to-understand name. Some scientific terms are fascinating and arouse curiosity and exploration. If the name is not good, it will cause misunderstanding or discouragement. In this article, we will examine some well-known physical terms in cosmology and astrophysics, examine their meanings, and explore the physical meanings behind them.
The Big Bang was not an explosion
Universe refers to the sum of space and time, which contains all matter and energy in space-time. The English word Universe has the meaning of "Unique", that is, there is only one universe, and the universe contains everything.
In physics, there is the theory of the Multiverse, which states that there are multiple parallel universes that are independent of each other. However, this is only a hypothesis based on mathematical logic and cannot be proven in the real world.
The universe we are in has its origins in The Big Bang, 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 piece of evidence for physicists to convince them of the Big Bang theory is the Cosmic Microwave Background. A large number of observations have confirmed that microwaves from all directions of the universe have a stable background temperature of 2.7 Kelvin. This background temperature was reserved for the Big Bang.
Hubble observations show that all celestial bodies in the universe are moving away from each other. An observer anywhere in the universe can conclude that other celestial bodies are far away from them. A plausible explanation for this fact is that the universe is expanding. As space gets bigger and bigger, the celestial bodies in space move away from each other. The expansion of three-dimensional space is difficult to imagine, so let's use two-dimensional space as an example. Mark any two dots on a balloon and inflate it. As the balloon expands, the surface area grows larger and the two marker points move farther and farther apart. The same is true for three-dimensional space.
Since the present universe is expanding, the earlier the universe, the smaller the space. It can be inferred from this that in the original universe, all matter and energy were gathered in a very small space, called the singularity. It was from this tiny singularity that the universe continued to expand, and it took about 13.78 billion years to form the universe we have today.
Figure 1. Schematic diagram of the expansion of the universe. Since the Big Bang, the universe has continued to expand over a period of about 13.78 billion years. | Source: Wiki
The Big Bang means that the universe starts from a very small space and expands extremely quickly in a very short period of time. This is not the same thing as the well-known bomb explosion. Whether it is the explosion of chemicals or the explosion of a nuclear bomb, it refers to the rapid diffusion of matter or energy in space, and 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, and it obviously fails to capture the essence. Either way, the English word The Big Bang and the Chinese term Big Bang are widely accepted and used.
There is no visible light during dark times
After the Big Bang began, space expanded rapidly and the temperature dropped rapidly. In the first 1 trillionth of a second, the four basic interactions are separated one after the other. Over the next 10 seconds, various elementary particles are formed. Some elementary particles transmit interactions, while others form the nucleus in about 17 minutes (1000 seconds).
Since most of the particles have been annihilated by the antiparticles, the energy of the universe will be mainly in the form of photons for the next 370,000 years or so. At this time, the universe contained a large mass of high-temperature, high-density plasma composed of atomic nuclei, electrons, and photons. Photons frequently collide with charged particles such as nuclei and electrons, so the average free path of photons is very short, causing the universe to be opaque. Although there are photons, 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 the nucleus to form atoms, a process called recombination. About 370,000 years later, the recombination process ended, and a large number of neutral atoms were formed in the universe. It is dominated by hydrogen atoms, but also has a small amount of helium atoms. The atoms produced by the recombination process are initially in an excited state and then quickly transition 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 of neutral atoms with photons is very small, and the density of the universe continues to decrease, so the average free path of photons becomes almost infinite. In other words, photons can pass through the universe unimpeded, and the universe becomes transparent.
The wavelength of the photons released from the deexcitation of hydrogen atoms is yellow-orange in the visible band. As the cosmic space expands, for observers on Earth, the light source moves in a direction away from itself. According to the Doppler Effect, when these photons travel to today's Earth, they produce a red shift, that is, the wavelength becomes larger, from visible light to microwaves, which is the cosmic microwave background radiation observed on Earth today. This is also the earliest cosmic event that humans can observe.
At this time, there was theoretically another mechanism to generate microwave radiation, namely the quantum transition between the two hyperfine structure levels of the ground state of the hydrogen atom, which released photons with a wavelength of 21 centimeters. Due to the large number of hydrogen atoms in the universe at this time, the 21 centimeter line should be observable. How to detect 21 cm spectral lines is currently a cutting-edge research area.
From 370,000 years after the Big Bang to hundreds of millions of years, for observers on Earth today, although microwave radiation can be observed, visible light cannot be observed, so this cosmic period is called "Dark Ages". It wasn't until hundreds of millions of years after the Big Bang that the first generation of stars was born and stars emitted visible light, and the universe began to have light and the dark ages ended.
Supernova explosions do not give birth to new stars
The first generation of stars has not been directly observed, and it is only theoretically speculated that they are non-metallic stars. This is because, the Big Bang process produced hydrogen and helium, but no metallic elements. Unlike metals, elements heavier than helium are collectively referred to as metallic elements in astrophysics.
A large amount of fusion occurs inside the star, and the fusion provides energy to resist gravitational collapse. Fusion constantly turns hydrogen into helium. For massive stars, when the hydrogen in their core is depleted, the hydrogen in the outer shell begins to fuse, which causes 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 collapse abruptly due to the inability of the electron degeneracy pressure to resist gravity. The process of collapse produces a huge explosion that throws most of the star's material outward at high speed, a process called a supernova. The supernova explosion produces enough energy to fuse some lighter non-metallic elements into heavier metallic elements. Projectiles containing metallic elements provide the raw materials for the formation of the next generation of stars.
First-generation stars generally had a lifespan of millions of years, and when they died, small amounts of metal were produced by supernova explosions. These metallic elements, along with hydrogen and helium, form the elements that make up the next generation of stars, so the second generation of stars contains a small amount of metal. Second-generation stars have lifespans of hundreds of millions or billions of years, and some of them die with supernova explosions that produce more metallic elements. Third-generation stars utilize these metallic elements during their formation and are therefore rich in metals. The sun we know as it is the third generation of stars.
Supernova explosions produce extremely strong electromagnetic radiation that emits extremely bright visible light that can last for weeks, months, or even years. The English name for supernova is Supernova, where nova means "new" in Latin, that is, a new bright star has appeared. In fact, the star has been around for a long time, and it was only discovered by observers on Earth because of its sudden and dramatic increase in brightness, so it was mistaken for a nova.
A well-known example of a supernova explosion is SN 1054, the remnants of which form the Crab Nebula. In 1054 AD, astronomers from China, Arabia and Japan recorded the supernova explosion. "History of the Song Dynasty, Astronomical Chronicles-ninth" recorded: "In May of the first year of Zhihe, it was ugly, and it was a few inches southeast of Tianguan, and the years were not a little. It is recorded in "Song Hui Yao": "In March of the first year of Jiayou, Si Tian said: 'There is no guest star, and there is no sign of guest leaving. 'At the beginning, in the fifth month of the first year of He, the morning rises in the east, and the heavens are guarded, and the day is as white as it is too white, and the mangjiao is out in all directions, and the color is red and white, and it is seen on the twenty-third day. ”
Figure 2. Crab nebula photographed by the Hubble Telescope. The Crab Nebula is a remnant of a supernova explosion and is made up of expanding gas and dust that form a shell-like structure. | Source: Wiki
Are black holes holes?
The remaining stellar core of the supernova explosion continues to collapse, and the tremendous pressure causes protons to absorb electrons and convert them into electrically neutral neutrons. The core of a star eventually forms a compact object made of neutrons, called a neutron star. Typical neutron stars are only about 10 kilometers in diameter, which is smaller than a city. Neutron stars are like giant atomic nuclei, much denser than the atoms that are commonly found on Earth. The mass of a small cup of matter on a neutron star is greater than the mass of all humans on Earth combined.
If the mass of the star's core is large enough, it will continue to collapse to less 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 an area that cannot be escaped is called an Event Horizon.
In 1916, German physicist Carl Schwarzschild discovered a solution with the characteristics of a black hole when solving Einstein's equations of general relativity. In the early 20th century, physicists referred to black holes as Gravitationally Collapsed Objects. In the 1960s, American physicist Robert Dicke first used the term "black hole" to describe such objects. It was only later that the term black hole was widely adopted by the academic community due to the popularization of John Wheeler, the grandmaster of general relativity. (Editor's note: For the origin of the name of the black hole, readers can also refer to https://ar5iv.labs.arxiv.org/html/1811.06587)
Black holes cannot be directly observed by observers outside the event horizon. The existence of a black hole can be indirectly confirmed by observing the movement of celestial bodies near the black hole and inferring it according to gravity. When interstellar matter is absorbed by the black hole, it forms a high-speed rotating accretion disk that emits strong electromagnetic waves, so the accretion disk around the black hole can be observed. The event horizon at the center of the accretion disk is a non-luminous, spherical region that appears to be a hole from the outside.
At the center of the black hole is a singularity of infinite density. Inside the black hole, space-time is highly distorted, and all matter falls towards the singularity in the center. Some theories suggest that most of the space inside the horizon is empty. If this is the case, there is a "hole" in the black hole. Since the inside of a black hole cannot be observed from the outside, its true condition remains an unsolved mystery.
Figure 3. The first image of a black hole in history was released on April 10, 2019, and was observed through the Event Horizon Telescope. | Source: Wiki
Quasars are not the same as quasars
Finally, let's look at two commonly confused terms associated with black holes, quasar and quasi-star.
In the fifties and sixties of the 20th century, astrophysicists observed some radio waves from the distant universe, but the origin was puzzling, so they described them as quasi-stellar radio sources. The objects that emit these radio waves are called Quasi-stellar Objects, abbreviated as Quasar, which translates to quasar in Chinese. However, subsequent studies have shown that quasars are galaxies that are being absorbed by supermassive black holes, and are not actually similar to stars.
The peak of quasar activity was about 10 billion years ago, that is, quasars were more than 10 billion light-years away from Earth. At that time, some galaxies in the universe happened to be close to the black hole, and under the strong gravitational pull of the black hole, the matter in the galaxy 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). This process produces a much greater luminosity than that of any star, making the quasar one of the brightest objects in the universe, and even quasars in distant space can be observed by humans on Earth.
Figure 4: Hypothetical view of a quasar (not a telescope observation). | Source: webbtelescope.org
Unlike quasars, quasi-stars refer to a class of imaginary stars that formed in the early universe. Unlike modern stars, a star has a black hole at its center, hence its other name Black Hole Star. According to theoretical calculations, when a star collapses into a black hole, a star will be formed if the star's outer shell has enough mass to absorb the energy produced by the collapse without a supernova explosion. Stars of this mass could only exist in the early universe before hydrogen and helium fused into metallic elements, that is, in the first generation of stars.
Source | Back to basics