Redshift phenomenon

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red shift In physics and astronomy, of objects electromagnetic radiation For some reason, the frequency decreases visible light Spectral Spectral line Move a distance towards the red end, that is, the wavelength becomes longer and the frequency decreases. The phenomenon of redshift in 2021 is mainly used to predict the movement and laws of celestial bodies.

Chinese name: Redshift phenomenon

Foreign name: Redshift
Field: astronomy

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red shift （ Redshift ）

The redshift phenomenon is Doppler shift One of the blue shift Phenomenon. The change of light wave frequency makes people feel the change of color If the star moves away from us, the spectral line of light will move towards the red light, which is called red shift; If the star moves towards us, the spectral line of light will move in the direction of purple light, which is called blue shift.

The redshift phenomenon was originally aimed at Mechanical wave In other words, a relative Observer The farther away the moving object is, the more powerful the sound will be (longer wavelength). On the contrary, the closer the object is, the more desolate the sound will be (shorter wavelength).

Later, U.S.A The astronomer Hubble called the shift of a celestial body's spectrum to the low frequency (red) end Doppler Redshift. It is generally believed that it is Doppler effect As a result, that is, when a Wave source (Light wave or Radio wave ）The frequency change caused by rapid movement with an observer. American astronomer Hubble confirmed in 1929 that all distant galaxies are far away from the Galaxy And go, at the same time, their red shift They increase proportionally as their distance increases. This general rule is called Hubble's law , it becomes a galaxy Retrogression speed And its distance from the earth. That is to say, the greater the red shift shown by the light emitted by a celestial body, the Distance of celestial bodies The farther it goes, the greater its retrogression speed.

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The wavelength of radiation is Wave source And observer's relative motion And change. In front of the moving wave source, the wave is compressed, the wavelength becomes shorter, and the frequency becomes higher（ blue shift (blue shift)）。 The opposite effect occurs behind the moving wave source. The wavelength becomes longer and the frequency becomes lower (red shift). The higher the velocity of the wave source, the greater the effect. According to the degree of red/blue shift of light wave, the velocity of wave source moving along the observation direction can be calculated. fixed star Spectral line The displacement of 0 shows the speed of the star moving in the observation direction. Unless the speed of the wave source is very close to the speed of light Doppler shift Generally speaking, the degree is very small. All wave phenomena (including light waves) have Doppler effect.

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red shift There are 3 types: Doppler Redshift (due to radiation source The gravitational redshift (due to the photon escaping from gravitational field And cosmological redshift (due to Cosmic space Caused by its own expansion). For different research objects, different redshifts are involved. See the following table for details:

Redshift phenomenon
Celestial body type	Doppler redshift	Gravitational redshift	Cosmological redshift
planet	X	X	-
fixed star	X	-	-
Nebula	X	-	-
neutron star	X	X	-
White dwarf	X	X	-
Close galaxy	X	-	X
Distant galaxy	X	-	X
black hole	X	X	-

Generally, gravitational red shift is relatively small, only when neutron star Or the effect around the black hole will be relatively large. For distant galaxies, cosmological redshifts are easy to distinguish, but when galaxies are far away from us with space expansion, due to their own motion Cosmological redshift Will also be mixed into Doppler red shift 。

Generally speaking, in order to distinguish from other redshifts Gravitational redshift You can compare the size of this object with the size of the black hole with the same mass of this object. Celestial objects such as nebulae and galaxies have a radius of 100 billion times that of black holes of the same mass, so the magnitude of their redshift is about one hundred billion times of the stationary frequency. For ordinary stars, their radius is about 100000 times that of black holes with the same mass, which is close to the limit of spectral observation resolution. neutron star and White dwarf Its radius is about 10 and 3000 times that of a black hole with the same mass, and its gravitational redshift can reach the magnitude of 1/10 and 1/1000 of the stationary frequency.

Cosmology red shifts at 100 Millisecond gap The scale is very obvious. But for the nearer galaxies, because the galaxy itself Galaxy cluster Caused by the movement in Doppler red shift and Cosmological redshift You must carefully distinguish between the two. Generally, the velocity of the galaxy in the cluster is 3000 m/s, which is about the same as that of the galaxy at a distance of 50 million seconds Retrogression speed Equivalent.

Explanation of redshift phenomenon

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Celestial light or other electromagnetic radiation The wavelength may be longer and the frequency may be reduced due to the stretching of three effects. Because of the red light Frequency ratio Blue light is low, so this stretching affects the optical waveband Spectral characteristics The effect of is to move them to the red end of the spectrum, so all three processes are called‘ red shift ’。

The first type of red shift was changed from University of Prague Professor of Mathematics Christian Doppler It is explained that it is caused by movement. When an object, such as a star, is far away from Observer When moving, its spectrum will show relative to the static Stellar spectrum Because the moving star stretches the light emitted behind it. Similarly, the light of a star moving towards the observer will be compressed due to the motion of the star, which means that these light frequencies are high, so they are called blue shift Has. The stars are redshifted, indicating that the universe is expanding.

about Mechanical wave For example, the wavelength (tone) of the sound wave emitted by a moving object has a completely similar change. The sound wave emitted by the object moving towards you is compressed, so the tone is bleak (shorter wavelength); The sound wave of the object away from you is stretched, so the tone is stronger (longer wavelength). Anyone who has met a car passing by and has a strong sense of music will be familiar with the above two situations. The above phenomena of sound wave and electromagnetic radiation are called Doppler effect 。

The measurement of red shift and blue shift caused by Doppler effect enables astronomers to calculate Space motion of stars How fast, and can also determine, for example, the way galaxies rotate. The measurement of celestial body redshift is expressed by the relative change caused by redshift, which is called z. If z=0.1, the frequency is reduced by 10%, etc. As long as the speed involved is much lower than the speed of light, z will also be equal to the speed of moving objects divided by the speed of light. Therefore, a red shift of 0.1 means that the star moves away from us at 1/10 of the speed of light.

In 1914, he worked in Lowell observatory Of Vesto Slipher It was found that 15 Spiral nebula Eleven of the objects (called galaxies in 2021) show redshifts in light. In 1922, Mount Wilson Observatory Of Edwin Hubble More similar observations were made with Milton Hammarson. Hubble first identified the nebula as Galaxy The same other galaxy. And then they found that a lot of galaxies have light red shift 。 By 1929, Hubble was mainly responsible for Apparent brightness By comparison, it is established that the redshift of galaxies is proportional to their distance from us (called Hubble's law )。 This law is not true only for a few galaxies that are spatially closest to the Milky Way, such as Andromeda galaxy The spectrum of blue shift 。

At first, the redshift of distant galaxies was interpreted as the motion of galaxies in space Doppler effect , it seems that they are all due to Galaxy They flew away for an explosion in the center. However, he soon realized that this expansion was already implicit in the general relativity equation published more than ten years before the discovery of Hubble's law. When Albert Einstein I first used those equations to derive a description of the universe in 1917（ Cosmic model ）It found that the equation requires that the universe must be in the motion state ——Either expand or contract. The equation excludes the possibility of a stable model. Since no one knew that the universe was expanding at that time, Einstein introduced a false factor in the equation to keep the model static; He later said that this was the 'biggest mistake' of his life.

After removing the false factor, Einstein equation It can accurately describe the phenomena observed by Hubble. The equation shows that the universe should expand, not because galaxies are moving in space, but because the empty space between galaxies (strictly speaking, space-time) is expanding. This cosmology red shift The reason is that the light of distant galaxies is pulled apart by the expanding space during its propagation, and the degree of pulling apart is the same as that of space expansion.

Due to redshift Proportional At the distance, this gives cosmology Home provides a measure of the universe. The measuring rod must be calibrated by measuring the nearest galaxy, although this calibration has some uncertainties (see the universe Distance scale ）But it is still an important discovery of cosmology. No, Measuring distance Method, Cosmologist You can't really start Understand the essence of the universe , and Hubble's law Of accuracy It shows that general relativity is an excellent description of how the universe works.

For historical reasons, the redshift of galaxies is still expressed in terms of speed, although astronomers know that redshift is not caused by motion through space. The distance of a galaxy is equal to its redshift 'velocity' divided by a constant, which is called Hubble constant , its value is about 60 kilometers per second Millisecond gap This means that every millisecond difference between the galaxy and us will cause red shift Speed. For our nearest neighbor, the cosmological red shift is very small Andromeda galaxy Galaxies like that show blue shift It's really caused by their space movement Doppler effect Blue shift. Galaxies in a distant galaxy cluster (like a swarm of bees) appear to surround a Intermediate value Redshift diffusivity of; This intermediate value is the Galaxy cluster The deviation of the intermediate value is the Doppler effect caused by the motion of the galaxy inside the cluster.

Hubble's law It is the law of redshift/distance (except for the stable universe). No matter which galaxy in the universe is observed, this law 'looks the same'. The motion of each galaxy (except for its very close neighbor) retreating from another galaxy follows this law, and there is no 'center' for expansion. This is usually more than the spots painted on the surface of the balloon. When the balloon inflates, the spots are farther apart from each other, because the balloon wall expands, not because the spots move on the surface of the balloon. The measurement from any spot will prove that the regression of all other spots is uniform and fully complies with Hubble's law.

When red shift When it is larger than about 1/3 of the light speed, or when the redshift z is greater than 0.1, the redshift calculation must consider Special relativity Requirements. So the redshift equal to 2 does not mean that the cosmology 'Speed' is twice the speed of light. In fact, the cosmological speed corresponding to z=2 is equal to 80% of the speed of light. The farthest known a quasar The red shift of is slightly greater than 4, and the corresponding 'speed' just exceeds 90% of the speed of light; Galaxy redshift The highest record of 8% belongs to an object called 8C1435+63, whose redshift is equal to 4.25. Cosmic microwave Background radiation Of red shift It's 1000.

The third kind of redshift is caused by gravity and is also explained by Einstein's general theory of relativity. The light moving outward from a star is gravitational field Do 'mountaineering' sports, so it will lose energy. When an object, such as a rocket, moves upward in the gravitational field, it loses energy and decelerates (which is why rocket engine The reason why it must be ignited to push it onto the track). But light cannot slow down; Light always travels at the same rate c, a little less than 300000 kilometers per second. Since light does not slow down when it loses energy, it has to reduce the frequency, that is, redshift.

In principle, the light escaping from the sun, even the light emitted by the torches on the earth, has this gravitational red shift. However, only when White dwarf The surface is so strong gravitational field The gravitational redshift is large enough to be measured. A black hole can be seen as a gravitational field so strong that the light trying to escape it will produce infinity red shift Object.

All three types of redshifts may work at the same time. If our telescope is very sensitive and can see white dwarfs in distant galaxies, then the red shift of the light of white dwarfs will be Doppler The combined effect of redshift, cosmological redshift and gravitational redshift.

majority a quasar The redshift of is greater than 1. If you put Quasar redshift Z is interpreted as Doppler redshift, then Retrogression speed V can be calculated as follows:

Quasar redshift z, Doppler redshift

Where c is the speed of light, when z=3.5, v can reach 0.9c. (Is there a problem here? How much z must be Superluminal speed ？)

red shift It is a common feature of extragalactic objects. Therefore, most astronomers believe that, a quasar It's an extragalactic object. The statistical results of redshift apparent magnitude relationship show that: Hubble's law about Extragalactic galaxy Yes. That is to say, their redshifts are cosmological redshifts, and their distances are Cosmological distance , their redshifts and Apparent magnitude It is statistically related. However, for quasars, the redshift and apparent magnitude Statistical correlation Very bad, which leads to two interrelated questions: whether the redshift of quasars is the cosmological redshift, and whether the distance of quasars is the cosmological distance. Most astronomers believe that the red shift of quasars is cosmological red shift. Therefore, the redshift reflects the retrogression of quasars and conforms to Hubble's law. According to this view, as a type of celestial body, a quasar It is the most distant celestial body observed by human beings so far. Those who hold this view believe that quasars red shift -Statistics of apparent magnitude relevance The reason is that the quasar Absolute magnitude The dispersion is too large. If quasars are classified according to certain standards, and red shift apparent magnitude statistics are carried out for certain types of quasars, the correlation will be significantly improved. The observational facts supporting the cosmological redshift are as follows: three quasars have been found in three galaxy clusters, and the redshift and Galaxy cluster The red shift of is about the same; Quasars and some Perturbed galaxy (e.g Seyfert galaxies ）Very similar; BL Lacertae objects It is a kind of celestial body similar to stars in shape. It was previously thought that they were Galaxy Internal Variable star It has been confirmed that they are distant extragalactic objects.

A few astronomers believe that the redshift of quasars is not cosmological redshift. The observational facts on which this view is based are: a quasar and Bright galaxy (their red shift The sampling statistical results show that there are some Statistical correlation ； Some quasars (such as Makaryan galaxy 205) seems to have a matter bridge connection with bright galaxies, and the redshifts of the two differ greatly. Those who hold this view have put forward some explanations for the redshift. For example, it is believed that quasars are ejected from the Milky Way or nearby galaxies, so it is believed that Quasar redshift yes Doppler Redshift, not cosmological redshift. Others believe that the red shift of quasars is Massive celestial body The gravitational redshift of. Other theories believe that the redshift of quasars may be caused by some unknown physical laws, which poses a so-called redshift challenge to modern physics.

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Redshift law It has been confirmed by later research and is believed that Cosmic expansion Modern Relativistic cosmology Theory provides the cornerstone. Since the early 1960s, astronomers have discovered that a quasar , their red shift Farthest than previously observed Galaxy The redshift of is greater. The great redshift of various quasars makes us think that they are all away from the Earth at a great speed (that is, close to 90% of the speed of light); It also makes us imagine that they are the most distant celestial bodies in the universe.

In other words, because Doppler red shift In this sense, the universe is not infinite, but bounded, that is, the area where the redshift speed of the celestial body is equal to the speed of light is the edge and boundary of the universe. Beyond this limit, the speed of light will be exceeded, and light will never reach our horizon. That is not our world. Only God knows how it is.

In 2021, according to scientific measurement, the age of the universe is about 13.8 billion years^{[1 ]} This is not only its age (time), but also its space length, that is, 15 billion light years is the longest distance we can theoretically reach when observing space. The place we see 15 billion light years away from the Earth in 2021 is exactly the mirror image of the birth of the universe. Fifteen billion years ago, at the beginning of the Big Bang, the unity of time and space was the beginning of our entire universe.

Light is composed of different frequencies electromagnetic wave Composed of spectral analysis Medium, Spectrogram The light emitted by a star is divided into light of different frequencies to form a color band, which is called a spectrogram. The gas in a star will absorb light of certain frequencies, so it will form dark in the spectrum Absorption line 。 Each element will produce a specific absorption line. Astronomers can know which kind of star is formed by studying the absorption line in the spectral map Element composition Of. Comparing the position of the absorption line in the stellar spectrogram with the position of the same absorption line under the laboratory light source, we can know that the star is relatively Earth movement The situation.

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