Shulou(Shulou.com)12/24 Report--

What is the universe? The famous statesman of the warring States period and the corpse of one of the hundred schools of the pre-Qin Dynasty put forward in his work Zi Zi, "the four directions are called Yu, from ancient times to the present." In today's words, "up and down" refers to space, and "from ancient times to the present" refers to time. The universe is time and space. From ancient times to the present, the vast universe has aroused people's strong curiosity. Time goes by and never returns. What happened in the world in the distant past? Where will everything go in the invisible future? Look up at the starry sky, the distant stars and the endless starry sky, people are in awe, resulting in infinite reverie.

With their rich imagination, the ancients weaved their own view of the universe through fairy tales in a very limited observation. In ancient China, there was a myth of "Pangu epoch-making earth", which believed that the universe was chaotic at first, and Pangu split the darkness with an axe to form heaven and earth; in western civilization, such as ancient Greek mythology, there was only Kaos (chaos) at first. Then Gaia (earth) was born, and the world began. In many civilizations, the sun, moon and stars are often associated with gods, showing people's respect for the universe and nature.

The ancients also had different opinions on the structure of the universe. In our country, there is the "theory of covering the sky" that "the sky is round"; there is the theory that heaven and earth are like eggs, that "heaven and earth are wrapped in earth, and that the shell is wrapped in yellow"; and that the universe is infinite and that "the stars of the sun and the moon" naturally float in the void.

Among them, Xuanye theory is strangely similar to some viewpoints of modern astronomy and cosmology, which makes people admire the ingenuity of the ancients. In the West, the famous "geocentric theory" holds that the universe has a spherical shell, the earth is located in the center of the universe, and the sun, moon and stars move around the earth in a circle.

However, after all, the ancients lacked practical means of observation, and the universe imagined by thinking inevitably deviated from the truth. In addition, the understanding of the universe is often influenced by theology. After Copernicus put forward heliocentric theory, and later Tycho, Kepler, Galileo and others made groundbreaking work, the theories on astronomy and the universe gradually began to get rid of the shackles of theology and use observation, reasoning and empirical methods to understand the world. Newton combined the great achievements of his predecessors and established classical mechanics based on a large number of observations and experimental data. Since then, the study of astronomy and the universe has entered the era of science.

From the end of the 18th century to the end of the 19th century, with the application of scientific methods, people's understanding of the universe has developed rapidly, and people have a certain understanding of stars, galaxies and so on. However, people's understanding of the universe as a whole is still very limited. Due to historical theology and other reasons, people believe that the universe is static and constant, so that in 1917, after Einstein used general relativity to calculate that the universe was not static, he added a cosmological constant to his equation to make the universe "static".

In 1929, Hubble found that the redshift of the spectra of extragalactic galaxies was roughly proportional to their distance from us. This means that these galaxies are moving away from us, and the farther away they are from us, the faster they move away from us. A good explanation is that the universe is expanding.

Imagine that the line in the picture is an elastic rope with several points marked on the rope; slowly lengthen the rope and the distance between the points becomes longer. Imagine that the rope is an one-dimensional universe with a kind of villain living on it; they will find that even if they stay in place quietly, the distance between villain 1 and villain 2 will slowly grow farther. And, because the rope stretches evenly around, the farther away the two villains are, the longer the length of the rope between them, the faster they move away from each other. This coincides with Hubble's observations. The observation of the expansion of the universe shows that the universe itself has been moving, not immutable, subverting people's belief in the static universe. This is considered to be the first great achievement in observational cosmology.

Since the universe is not static but expanding, what was the universe like in the past? In 1946, George Gamov (George Gamow) proposed that the universe was once an extremely hot and dense state, which became what we saw after a long period of expansion, cooling and evolution. At first, the theory was so unpopular that other theorists even jokingly called it the Big Bang. Yes, the name is actually ironic. Interestingly, when Gamov's theory became a widely accepted basic theory of cosmology, it was still called the "Big Bang cosmological model".

It was nearly 20 years after the Big Bang cosmology was proposed, and it was not until the cosmic microwave background radiation was observed that it began to be accepted. As early as 1948, Alf (A. Alpher) and Herman (R. Herman) analyzed according to Gamov's theory and proposed that there should be a kind of isotropic microwave radiation in the universe. In 1964, Bell Labs radio engineers Penzias (A. Penzias) and Wilson (R. Wilson) discovered a strange noise coming from the sky. They carefully eliminated the effects of atmospheric absorption and the antenna itself, and even cleaned up the bird's nest and dung inside the antenna, and the noise remained. And no matter which direction they point the antenna, the noise is the same. After a year of observation, they found that the noise corresponds to blackbody radiation of about 3.5k (Kelvin). This is the cosmic microwave background radiation predicted by the Big Bang cosmology! This is considered to be the second great achievement in observational cosmology.

At this point, people finally began to attach importance to Gamov's theory. Today's more accurate observations show that the cosmic microwave background radiation is about 2.725K, rounded to 3K, so it is also called 3K background radiation. In the famous science fiction novel "three-body", the "cosmic flicker" seen by Wang Miao is caused by the trimaran's influence on the earth's observation of background radiation. When you turn on the old TV, a small part of the "snowflake" noise is contributed by background radiation.

How does Big Bang cosmology explain the cosmic microwave background radiation? We know that charged particles interact with electromagnetic fields. When the early universe was extremely hot, it was filled with charged particles, which hindered the movement of photons (electromagnetic waves) and prevented them from being free. When the universe cools to a temperature of about 3000K, protons and electrons combine to form hydrogen atoms. At this time, most of the photons hardly interact with the electrically neutral hydrogen atom as a whole, so the photons are free. These photons are found all over the universe, and their frequencies are greatly redshifted due to the expansion of the universe, reducing them to the microwave band (the wavelength is on the order of millimeters).

The spectral redshift of galaxies and cosmic microwave background radiation have laid an observational foundation for the study of modern cosmology, so that human beings can finally establish a view of the universe in a scientific way. Nowadays, scientists' observation of the universe is not limited to electromagnetic waves, but also combined with cosmic rays, gravitational waves and other signals to obtain a more comprehensive and accurate understanding of the universe.

This article is from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), by Xylon

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