Red shift means that the frequency observed at the observation point changes under the condition that the frequency of the star light source remains unchanged.It is also called Doppler effect.
Doppler effect
All matter in the universe is in motion.Far away galaxies are also moving away from us.For example,Virgo Galaxy ClusterIs leaving us at a speed of about 1210 kilometers per second,Backhair ClusterLeaving us at about 6700 kilometers per second,Hercules Cluster At a speed of about 10300 kilometers per second, the Northern Corona Galaxy Cluster is leaving us faster, about 21600 kilometers per second.Why did the galaxy leave us?How do we know they are moving?
This needs to be explained by the red shift of Doppler effect.Red shift refers to the change of the frequency observed at the observation point when the frequency of the star light source is constant, which is also called Doppler effect.The scientist Edwin Hubble used the Doppler effect to draw the conclusion that the universe is expanding.He found that the frequency of light emitted by objects far away from the Milky Way became lower, that is, they moved to the red end of the spectrum, which is called redshift. The faster the objects left the Milky Way, the greater the redshift, which indicates that these objects are far away from the Milky Way.On the contrary, if the celestial body is moving towards the Milky Way, the light will blue shift.
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Doppler effect applies not only to sound waves (mechanical waves), but also to light waves (electromagnetic waves).When the light from a fast-moving light source reaches our eyes, its wavelength and frequency also change, that is, its color will change.Although astronomers can use this principle to measure the movement of celestial bodies, in general, the movement speed of celestial bodies relative to observers is insignificant compared with the speed of light, so it is difficult to measure the change of light source color.
Galaxies are huge clusters of stars, but because they are very far away from us, each galaxy can only see a weak light spot on the negative film taken by a large telescope.The first astronomer to observe and measure the spectrum of galaxies was Oliver Sliver of the Lowell Observatory in the United States.From 1912 to 1925, he took spectral photos of 40 galaxies. Except for two galaxies, all of them showed a Doppler shift of reduced frequency, that is, a shift towards the red end of the spectrum. The measured departure speed was surprisingly high, up to 5700 km/s.
The research on the radial velocity of galaxies continues.Astronomers found that the spectral line displacement of galaxies is very different from that of stars.First of all, the spectral line shifts of stars have red shifts and purple shifts, which reflects that some stars are far away from us, and some are close to us, while the spectral line shifts of galaxies are mostly red shifts and very few purple shifts.Secondly, the spectral line displacement of stars, whether red or purple, is generally about tens of kilometers per second, and the maximum is no more than two or three hundred kilometers per second. However, only a few galaxies have spectral line redshifts below 1000 kilometers per second, most of which are 2000 to 3000 kilometers per second, and some even reach 10000 kilometers per second.
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In 1929, American astronomer Hubble found that not only almost all galaxies in the universe have the phenomenon of redshift of spectral lines, but also there is a relationship between the redshift of galaxies and the distance of the galaxy. That is, the farther the galaxy is, the faster it is flying away, which is called Hubble's Law.
With Hubble's Law, astronomers can calculate the apparent velocity of galaxies and their distance by observing the redshift of their spectral lines.For example, a galaxy moving away from us at a speed of 1700 km/s is about 100 million light years away;A galaxy moving away from us at the speed of 17000 km/s is about 1 billion light years away.The farthest galaxy observed so far is moving away from us at a speed similar to the speed of light, with a distance of more than 10 billion light years.Why are all galaxies leaving us?
What is the essence of redshift?Why does Hubble's Law exist?These problems have been debated for more than half a century, but they have not been satisfactorily explained, thus becoming a long-standing problem in astronomy.
Two years before the publication of Hubble's Law, Belgian astronomer Lehmet put forward the concept of expansion of the universe.In 1930, British astronomer Eddington linked Lehmet's model with Hubble's law and called the universe an expanding universe.In 1932, Lehmet further mentioned that the observed universe was a hugePrimitive fireballIn the late 1940s, after discovering that the huge energy of the sun came from thermonuclear reactions, American physicist Ga Yingfu linked the theory of cosmic expansion with the movement of elementary particles and proposed the cosmology of the thermal big bang.He believes that the universe originated from a big explosion of high-temperature, high-density, "primitive fireballs".In the period after the thermal big bang model was proposed, few people cared about it.It was not until 1965 that Penzias and Wilson of Bell Telephone Laboratory discovered the 3K microwave background radiation (also known as cosmic background radiation) that the Big Bang theory became the most influential theory. With the subsequent measurement of other researchers, the cosmic background radiation has becomeBig Bang ModelThe strong testimony of the effectiveness has become a useful "absolute framework" for considering the large scale flow in the universe, and because of its isotropy, it has become an important constraint for publishing the theory of galaxy formation.
Astronomers believe that the so-called Big Bang cannot be imagined as the high-density and high-energy cosmic matter rushing into the already empty space at a high speed after the explosion. If so, the original explosion center will leave a gradually increasing hole.At the same time, the radiation goes faster than the material during the explosion, so all radiation emitted during the explosion will be separated from the material.In fact, neither of these two phenomena exists, so the Big Bang must be imagined as the expansion of space itself at the speed of light from the Big Bang.
The Big Bang theory naturally explains many observed phenomena, and the theory is in good agreement with the observed results.But there are also some problems, the most prominent of which is“Primitive fireball”Where did you come from?Some astronomers believe that at first the universe was an extremely thin gas, which gradually contracted into a super dense mass due to the action of gravity.Then it explodes again, passes through the expansion stage, and returns to thin, which is almost as thin as absolute vacuum.We happen to live in a very short period when the universe is relatively full.Naturally, it can shrink, explode and expand again in the future.In 1965, American astronomer Sandwich even estimated that every oscillation of this "pulsating universe" would take more than 80 billion years.Whether this theory is a reality or a "myth" cannot be easily concluded at present.
Other astronomers do not think that the redshift of galaxy spectral lines is caused by their retrogression speed, so there is no problem of cosmic expansion.However, it is too difficult to find another explanation for redshift beyond the Doppler effect, at least for now.
One explanation is that the objects emitting spectra have red shifts due to their different physical states.For example, because the gravity of the galaxy is particularly strong, the red shift in the emitted spectrum is particularly large, which is called gravitational red shift.Gravitational redshift is one of the predictions of general relativity.According to general relativity, when an observer observes the light emitted by a radiation source in the gravitational field from a place far away from the gravitational field, the spectral line will move to the low-frequency direction, and the amount of movement is proportional to the size of the gravitational potential difference between the radiation source and the observer.This effect was initially confirmed in white dwarfs.However, according to the calculation results of gravitational theory, the influence of gravity on the redshift is very small, which is not enough to explain the observed redshift phenomenon of galaxies.
The other explanation is that the interaction between light and matter in the process of propagation produces redshift.After the light is emitted from the galaxy, it takes tens of thousands of light years to reach the earth, and the light has to penetrate manyIntergalactic mediumThere is some interaction between the light and the medium, which makes the spectrum redshift.The farther away a galaxy is, the more media it encounters on its way, and thus the greater the redshift. However, there is no satisfactory explanation for how light interacts with media to produce redshift.
Another explanation is that the light itself changes and causes redshift.During the propagation of light for tens of millions of years, photons have aged and their frequency has decreased, resulting in redshift. It can be inferred that the farther the galaxy light goes, the longer the redshift is seen.This hypothesis has not been experimentally confirmed.
At a time when the problem of galaxy redshift was so serious, the 1960s camea quasarThe redshift phenomenon of makes the problem more complicated.According to the research on the physical properties of quasars.It is certain that quasars are extragalactic objects.It belongs to the level of galaxies.In that case, can their redshifts also be interpreted as retrogression and meet Hubble's law, as normal galaxies do?It is difficult to verify this directly, because it is still impossible to calculate the distance of quasars.The statistics of quasars shows that their distribution is irregular on the redshift apparent magnitude diagram. What is the reason?
Most astronomers insist that the redshift of quasars iscosmological redshift That is, redshift reflects retrogression, and there is a Hubble relationship between redshift and distance.The evidence is that the physical properties of quasars and someActive galaxySimilarly, active galaxies have been shown to satisfy Hubble's law.In addition, it has been found that several quasars are close to a base cluster or within a galaxy cluster, and the redshifts of quasars and galaxy clusters are approximately equal.It is also found that some quasars are very close to some galaxies, and the redshifts of quasars and galaxies are roughly the same.They believe that the reason why quasars disperse on the redshift apparent magnitude diagram is that the absolute magnitude dispersion of quasars is too large, rather than that Hubble's law is not tenable.
A few astronomers believe thatQuasar redshiftNot cosmological.The sampling statistical study of some quasars and bright galaxies shows that some galaxies that are associated with each other (that is, linked together) or the redshifts of associated galaxies and quasars are completely different or very different from each other.In addition, it is found that in the spectra of some quasars, the redshifts of their absorption lines and emission lines are different from each other, and different absorption lines also have different redshifts, that is, multiple redshifts.However, the different redshifts of the associated objects and the multiple redshifts of the same object cannot be explained by the Doppler effect, so it is necessary to find a new redshift mechanism.In addition to the proposed red shift mechanisms such as gravitational red shift, photon aging, and physical constant change, there is also a so-called "transverse Doppler effect".The huge redshift of a quasar may indicate its great lateral velocity.
Some of these views are just hypotheses, and some have theoretical basis, but they cannot explain the redshift of quasars well. Those who hold the view of non cosmological redshift believe that the redshift of quasars is a challenge to modern physics.
The observation and study of the redshift of spectral lines, which is common in galaxies, has strongly promoted the rapid development of modern cosmology, which focuses on the structure, origin and evolution of the entire observable universe.Once the truth of galaxy redshift is revealed, human understanding of the universe will have a greater leap.
