The galaxies are flying apart. The speed of the sun and galaxy in the universe
At present, according to astronomical observations, it is established that The universe is homogeneous on a large scale, i.e. all its regions with a size of 300 million light years and more look the same. On a smaller scale, there are regions in the Universe where clusters of galaxies are found and, conversely, voids where there are few of them.
A galaxy is a system of stars that have a common origin and are connected by forces of attraction. The galaxy in which our Sun is located is the Milky Way.
Distances to celestial bodies in astronomy they are defined differently depending on whether these objects are close or far from our planet. In outer space, it is customary to use the following units for measuring distances:
1 AU( astronomical unit) = (149597870 2) km;
1 pc ( parsec) = 206265 a.u. = 3.086 10 m;
1 this year ( light year) \u003d 0.307 pc \u003d 9.5 10 m. A light year is the path that light travels in a year.
In this paper, we propose a method for determining the distances to distant galaxies using the "redshift", i.e. by increasing the wavelengths in the spectrum of the observed remote radiation source compared to the corresponding wavelengths of the lines in the reference spectra.
A light source is understood as radiation from distant galaxies (most bright stars or gas and dust nebulae in them). Under " redshift» - shift of spectral lines in spectra chemical elements, of which these objects are composed, to the long-wave (red) side, compared with the wavelengths in the spectra of reference elements on Earth. "Redshift" is due to the Doppler effect.
Doppler effect consists in the fact that the radiation sent by a source moving away from a stationary receiver will be accepted by it as longer wavelength than radiation from the same stationary source. If the source approaches the receiver, then the wavelength of the recorded signal, on the contrary, will decrease.
In 1924, Soviet physicist Alexander Fridman predicted that the universe was expanding. The data currently available show that the evolution of the universe began from the moment Big Bang. About 15 billion years ago, the Universe was a point (it is called point of singularity), to which, due to the strongest gravity in it, very high temperature and density, the known laws of physics do not apply. In accordance with the currently accepted model, the Universe began to inflate from the point of singularity with increasing acceleration.
In 1926, experimental evidence was obtained for the expansion of the universe. The American astronomer E. Hubble, while studying the spectra of distant galaxies with a telescope, discovered the redshift of spectral lines. This meant that the galaxies were moving away from each other, and at a rate that increased with distance. Hubble built a linear relationship between distance and speed associated with the Doppler effect ( Hubble law):
(1) , where
r– distance between galaxies;
v- galaxy removal rate;
H is the Hubble constant. Meaning H depends on the time elapsed from the beginning of the expansion of the Universe to the present moment, and varies in the range from 50 to 100 km/s Mpc. In astrophysics, as a rule, H = 75 km/s·Mpc is used. The accuracy of determining the Hubble constant is
0.5 km/s Mpc;
With is the speed of light in vacuum;
Z- redshift of the wavelength, the so-called. cosmological factor.
(2) , where
is the wavelength of radiation received by the receiver;
is the wavelength of the radiation emitted by the object.
Thus, by measuring the shift of lines, for example, of ionized hydrogen (H+) in the visible part of the spectrum, it is possible for a galaxy observed from the Earth to determine its redshift by formula (2) Z and, using the Hubble law (1), calculate the distance to it or the speed of its removal:
Work order
1. Call the program "Determination of distances to galaxies" on the computer desktop. A region of the Universe with nine different galaxies observed from the Earth's surface will appear on the monitor screen. The spectrum of visible light and the H+ ionized hydrogen wavelength marker appear at the top of the screen.
2. Place the cursor on the galaxy indicated by the teacher and click the key.
3. Record the wavelength and λ emitted by this galaxy as it recedes.
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This article discusses the speed of the Sun and the Galaxy relative to different frames of reference:
The speed of the Sun in the Galaxy relative to the nearest stars, visible stars and the center of the Milky Way;
Velocity of the Galaxy relative to the local group of galaxies, distant star clusters and cosmic background radiation.
Brief description of the Milky Way Galaxy.
Description of the Galaxy.
Before proceeding to the study of the speed of the Sun and the Galaxy in the Universe, let's get to know our Galaxy better.
We live, as it were, in a gigantic "star city". Or rather, our Sun “lives” in it. The population of this "city" is a variety of stars, and more than two hundred billion of them "live" in it. A myriad of suns are born in it, going through their youth, middle age and old age - they go through a long and difficult life path lasting billions of years.
The dimensions of this "star city" - the Galaxy are enormous. The distances between neighboring stars are, on average, thousands of billions of kilometers (6*1013 km). And there are more than 200 billion such neighbors.
If we raced from one end of the Galaxy to the other at the speed of light (300,000 km/sec), it would take about 100,000 years.
Our entire star system slowly rotates like a giant wheel made up of billions of suns.
Orbit of the Sun
At the center of the Galaxy, apparently, there is a supermassive black hole(Sagittarius A *) (about 4.3 million solar masses) around which, presumably, a black hole of average mass from 1,000 to 10,000 solar masses rotates with an orbital period of about 100 years and several thousand relatively small ones. Their combined gravitational action on neighboring stars causes the latter to move along unusual trajectories. There is an assumption that most galaxies have supermassive black holes in their core.
The central regions of the Galaxy are characterized by a strong concentration of stars: each cubic parsec near the center contains many thousands of them. Distances between stars are tens and hundreds of times less than in the vicinity of the Sun.
The core of the Galaxy with great force attracts all other stars. But a huge number of stars are settled throughout the "star city". And they also attract each other in different directions, and this has a complex effect on the movement of each star. Therefore, the Sun and billions of other stars mostly move in circular paths or ellipses around the center of the Galaxy. But that's just "basically" - if we look closely, we'd see them moving in more complex curved, meandering paths among the surrounding stars.
Feature of the Milky Way Galaxy:
Location of the Sun in the Galaxy.
Where in the Galaxy is the Sun and does it move (and with it the Earth, and you and me)? Are we in the "city center" or at least somewhere close to it? Studies have shown that the sun and solar system located at a great distance from the center of the Galaxy, closer to the "urban outskirts" (26,000 ± 1,400 light years).
The Sun is located in the plane of our Galaxy and is removed from its center by 8 kpc and from the plane of the Galaxy by about 25 pc (1 pc (parsec) = 3.2616 light year). In the region of the Galaxy where the Sun is located, the stellar density is 0.12 stars per pc3.
model of our galaxy
The speed of the Sun in the Galaxy.
The speed of the Sun in the Galaxy is usually considered relative to different frames of reference:
relative to nearby stars.
Relative to all bright stars visible to the naked eye.
Regarding interstellar gas.
Relative to the center of the Galaxy.
1. The speed of the Sun in the Galaxy relative to the nearest stars.
Just as the speed of a flying aircraft is considered in relation to the Earth, not taking into account the flight of the Earth itself, so the speed of the Sun can be determined relative to the stars closest to it. Such as the stars of the Sirius system, Alpha Centauri, etc.
This velocity of the Sun in the Galaxy is relatively small: only 20 km/sec or 4 AU. (1 astronomical unit is equal to the average distance from the Earth to the Sun - 149.6 million km.)
The Sun, relative to the nearest stars, moves towards a point (apex) lying on the border of the constellations Hercules and Lyra, approximately at an angle of 25 ° to the plane of the Galaxy. Equatorial coordinates of the apex = 270°, = 30°.
2. The speed of the Sun in the Galaxy relative to the visible stars.
If we consider the movement of the Sun in the Galaxy Milky Way relative to all the stars visible without a telescope, then its speed is even less.
The speed of the Sun in the Galaxy relative to the visible stars is 15 km/sec or 3 AU.
The apex of the motion of the Sun in this case also lies in the constellation Hercules and has the following equatorial coordinates: = 265°, = 21°.
The speed of the Sun relative to nearby stars and interstellar gas
3. The speed of the Sun in the Galaxy relative to the interstellar gas.
The next object of the Galaxy, with respect to which we will consider the speed of the Sun, is interstellar gas.
The expanses of the universe are far from being as desolate as it was thought for a long time. Although in small quantities, interstellar gas is present everywhere, filling all corners of the universe. The interstellar gas, with the apparent emptiness of the unfilled space of the Universe, accounts for almost 99% of the total mass of all space objects. Dense and cold forms of interstellar gas containing hydrogen, helium and minimal amounts of heavy elements (iron, aluminum, nickel, titanium, calcium) are in a molecular state, combining into vast cloud fields. Usually, in the composition of the interstellar gas, the elements are distributed as follows: hydrogen - 89%, helium - 9%, carbon, oxygen, nitrogen - about 0.2-0.3%.
A tadpole-like cloud of interstellar gas and dust IRAS 20324+4057 that hides a growing star
Clouds of interstellar gas can not only rotate in an orderly manner around galactic centers, but also have unstable acceleration. Over the course of several tens of millions of years, they catch up with each other and collide, forming complexes of dust and gas.
In our Galaxy, the main volume of interstellar gas is concentrated in spiral arms, one of the corridors of which is located near the solar system.
The speed of the Sun in the Galaxy relative to the interstellar gas: 22-25 km/sec.
Interstellar gas in the immediate vicinity of the Sun has a significant intrinsic velocity (20-25 km/s) relative to the nearest stars. Under its influence, the apex of the Sun's motion shifts towards the constellation Ophiuchus (= 258°, = -17°). The difference in direction of movement is about 45°.
4. The speed of the Sun in the Galaxy relative to the center of the Galaxy.
In the three points above we are talking about the so-called peculiar, relative speed of the Sun. In other words, peculiar speed is the speed relative to the cosmic frame of reference.
But the Sun, the stars closest to it, and the local interstellar cloud are all involved in a larger movement - movement around the center of the Galaxy.
And here we are talking about completely different speeds.
The speed of the Sun around the center of the Galaxy is huge by earthly standards - 200-220 km / s (about 850,000 km / h) or more than 40 AU. / year.
It is impossible to determine the exact speed of the Sun around the center of the Galaxy, because the center of the Galaxy is hidden from us behind dense clouds of interstellar dust. However, more and more new discoveries in this area are decreasing the estimated speed of our sun. More recently, they talked about 230-240 km / s.
The solar system in the galaxy is moving towards the constellation Cygnus.
The motion of the Sun in the Galaxy occurs perpendicular to the direction to the center of the Galaxy. Hence the galactic coordinates of the apex: l = 90°, b = 0° or in more familiar equatorial coordinates - = 318°, = 48°. Since this is a reversal motion, the apex shifts and completes a full circle in a "galactic year", approximately 250 million years; its angular velocity is ~5" / 1000 years, i.e. the coordinates of the apex shift by one and a half degrees per million years.
Our Earth is about 30 such "galactic years" old.
The speed of the Sun in the Galaxy relative to the center of the Galaxy
By the way, an interesting fact about the speed of the Sun in the Galaxy:
The speed of rotation of the Sun around the center of the Galaxy almost coincides with the speed of the compression wave that forms the spiral arm. Such a situation is atypical for the Galaxy as a whole: the spiral arms rotate at a constant angular velocity, like spokes in wheels, and the movement of stars occurs with a different pattern, so almost the entire stellar population of the disk either gets inside the spiral arms or falls out of them. The only place where the speeds of stars and spiral arms coincide is the so-called corotation circle, and it is on it that the Sun is located.
For the Earth, this circumstance is extremely important, since violent processes occur in the spiral arms, which form powerful radiation that is destructive to all living things. And no atmosphere could protect him from it. But our planet exists in a relatively quiet place in the Galaxy and has not been affected by these cosmic cataclysms for hundreds of millions (or even billions) of years. Perhaps that is why life was able to originate and survive on Earth.
The speed of movement of the Galaxy in the Universe.
The speed of movement of the Galaxy in the Universe is usually considered relative to different frames of reference:
Relative to the Local Group of galaxies (speed of approach to the Andromeda galaxy).
Relative to distant galaxies and clusters of galaxies (the speed of movement of the Galaxy as part of the local group of galaxies to the constellation Virgo).
Regarding the relic radiation (the speed of movement of all galaxies in the part of the Universe closest to us to the Great Attractor - a cluster of huge supergalaxies).
Let's take a closer look at each of the points.
1. Velocity of movement of the Milky Way Galaxy towards Andromeda.
Our Milky Way Galaxy also does not stand still, but is gravitationally attracted and approaches the Andromeda galaxy at a speed of 100-150 km/s. The main component of the speed of approach of galaxies belongs to the Milky Way.
The lateral component of the motion is not precisely known, and it is premature to worry about a collision. An additional contribution to this motion is made by the massive galaxy M33, located approximately in the same direction as the Andromeda galaxy. In general, the speed of our Galaxy relative to the barycenter of the Local Group of galaxies is about 100 km / s approximately in the Andromeda/Lizard direction (l = 100, b = -4, = 333, = 52), however, these data are still very approximate. This is a very modest relative speed: the Galaxy is displaced by its own diameter in two or three hundred million years, or, very approximately, in a galactic year.
2. Velocity of movement of the Milky Way Galaxy towards the Virgo cluster.
In turn, the group of galaxies, which includes our Milky Way, as a whole, is moving towards the large cluster of Virgo at a speed of 400 km/s. This movement is also due to gravitational forces and is carried out relative to distant clusters of galaxies.
Velocity of the Milky Way Galaxy towards the Virgo Cluster
3. Speed of movement of the Galaxy in the Universe. To the Great Attractor!
Relic radiation.
According to the Big Bang theory, early universe was a hot plasma consisting of electrons, baryons and constantly emitted, absorbed and re-emitted photons.
As the universe expanded, the plasma cooled down certain stage slowed down electrons were able to combine with slowed down protons (hydrogen nuclei) and alpha particles (helium nuclei), forming atoms (this process is called recombination).
This happened at a plasma temperature of about 3,000 K and an approximate age of the universe of 400,000 years. There is more free space between particles, there are fewer charged particles, photons no longer scatter so often and can now move freely in space, practically without interacting with matter.
Those photons that were emitted at that time by the plasma towards the future location of the Earth still reach our planet through the space of the universe that continues to expand. These photons make up the relic radiation, which is thermal radiation that evenly fills the Universe.
The existence of relic radiation was predicted theoretically by G. Gamow within the framework of the theory big bang. Its existence was experimentally confirmed in 1965.
Velocity of movement of the Galaxy relative to the cosmic background radiation.
Later, the study of the speed of movement of galaxies relative to the cosmic background radiation began. This movement is determined by measuring the non-uniformity of the temperature of the relict radiation in different directions.
The radiation temperature has a maximum in the direction of motion and a minimum in the opposite direction. The degree of deviation of the temperature distribution from isotropic (2.7 K) depends on the magnitude of the velocity. It follows from the analysis of the observational data that the Sun moves relative to the cosmic microwave background at a speed of 400 km/s in the direction =11.6, =-12.
Such measurements also showed another important thing: all galaxies in the part of the Universe closest to us, including not only ours local group, but also the Virgo cluster and other clusters, move relative to the background cosmic microwave background at an unexpectedly high speed.
For the Local Group of galaxies, it is 600-650 km / s with an apex in the constellation Hydra (=166, =-27). It looks like that somewhere in the depths of the Universe there is a huge cluster of many superclusters that attract the matter of our part of the Universe. This cluster was named Great Attractor- from English word"attract" - to attract.
Since the galaxies that make up the Great Attractor are hidden by interstellar dust that is part of the Milky Way, mapping of the Attractor was only possible in last years using radio telescopes.
The Great Attractor is located at the intersection of several superclusters of galaxies. The average density of matter in this region is not much greater than the average density of the Universe. But due to its gigantic size, its mass turns out to be so great and the force of attraction is so huge that not only our star system, but also other galaxies and their clusters nearby move in the direction of the Great Attractor, forming a huge stream of galaxies.
The speed of movement of the Galaxy in the Universe. To the Great Attractor!
So, let's sum up.
The speed of the Sun in the Galaxy and the Galaxy in the Universe. Pivot table.
Hierarchy of movements in which our planet takes part:
The rotation of the Earth around the Sun;
Rotation together with the Sun around the center of our Galaxy;
Movement relative to the center of the Local Group of galaxies together with the entire Galaxy under the influence of the gravitational attraction of the constellation Andromeda (galaxy M31);
Movement towards a cluster of galaxies in the constellation Virgo;
Movement to the Great Attractor.
The speed of the Sun in the Galaxy and the speed of the Milky Way Galaxy in the Universe. Pivot table.
It is difficult to imagine, and even more difficult to calculate, how far we move every second. These distances are huge, and the errors in such calculations are still quite large. Here is what science has to date.
Even astronomers don't always get the expansion of the universe right. An inflating balloon is an old but good analogy for the expansion of the universe. The galaxies located on the surface of the ball are motionless, but as the Universe expands, the distance between them increases, and the sizes of the galaxies themselves do not increase.
In July 1965, scientists announced the discovery obvious signs expansion of the universe from a hotter, denser initial state. They found the cooling afterglow of the Big Bang - the CMB. From that moment on, the expansion and cooling of the Universe formed the basis of cosmology. Cosmological expansion allows us to understand how simple structures were formed and how they gradually developed into complex ones. 75 years after the discovery of the expansion of the universe, many scientists cannot penetrate its true meaning. James Peebles, a cosmologist at Princeton University who studies the CMB, wrote in 1993: "It seems to me that even experts do not know what the significance and possibilities of the hot Big Bang model are."
Famous physicists, authors of textbooks on astronomy and popularizers of science sometimes give an incorrect or distorted interpretation of the expansion of the Universe, which formed the basis of the Big Bang model. What do we mean when we say that the universe is expanding? Undoubtedly, the circumstance that they are now talking about the acceleration of expansion is confusing, and this puzzles us.
OVERVIEW: A COSMIC MISTAKE
* The expansion of the universe is one of the fundamental concepts modern science- still receives various interpretations.
* The term "Big Bang" should not be taken literally. He was not a bomb that exploded at the center of the universe. It was an explosion of space itself, which took place everywhere, just as the surface of an inflated balloon expands.
* Understanding the difference between space expansion and space expansion is critical to understanding the size of the universe, the speed at which galaxies are receding, as well as the possibilities of astronomical observations, and the nature of the expansion acceleration that the universe is likely to experience.
* The Big Bang model only describes what happened after it.
What is an extension?
When something familiar expands, such as a wet spot or the Roman Empire, they become larger, their boundaries move apart, and they begin to occupy a larger volume in space. But the universe seems to have no physical limits, and it has nowhere to move. The expansion of our universe is very much like inflating a balloon. Distances to distant galaxies are increasing. Astronomers usually say that galaxies are receding or running away from us, but they do not move through space like fragments of a "Big Bang bomb". In reality, the space between us and the galaxies is expanding, moving chaotically inside practically immobile clusters. CMB fills the universe and serves as a reference frame, like the rubber surface of a balloon, against which motion can be measured.
Being outside the ball, we see that the expansion of its curved two-dimensional surface is possible only because it is in three-dimensional space. In the third dimension, the center of the ball is located, and its surface expands into the volume surrounding it. Based on this, one could conclude that the expansion of our three-dimensional world requires the presence of a fourth dimension in space. But according to general theory Einstein's relativity, space is dynamic: it can expand, contract and bend.
Traffic jam
The universe is self-sufficient. It does not require a center to expand from it, nor free space on the outside (wherever it is) to expand there. True, some latest theories, such as string theory, postulate the presence of extra dimensions, but they are not required when our three-dimensional universe expands.
In our universe, as on the surface of a balloon, every object moves away from all the others. Thus, the Big Bang was not an explosion in space, but rather an explosion of space itself that did not occur at a specific location and then expand into the surrounding void. It happened everywhere at the same time.
WHAT WAS THE BIG BANG LIKE?
WRONG: The universe was born when matter, like a bomb, exploded in a certain place. The pressure was high in the center and low in the surrounding void, which caused the matter to expand.
RIGHT: It was an explosion of space itself that set matter in motion. Our space and time originated in the Big Bang and began to expand. There was no center anywhere, because conditions were the same everywhere, there was no pressure drop characteristic of an ordinary explosion.
If we imagine that we are scrolling through a film in reverse order, we will see how all regions of the Universe are compressed, and galaxies approach each other until they all collide together in the Big Bang, like cars in traffic jam. But the comparison is not complete. If it was an accident, then you could avoid the traffic jam by hearing reports about it on the radio. But the Big Bang was a catastrophe that could not be avoided. It is as if the surface of the Earth and all the roads on it were crumpled, but the cars remained the same size. Eventually the cars would collide, and no amount of radio communication could have prevented it. So is the Big Bang: it happened everywhere, unlike a bomb explosion, which occurs at a certain point, and the fragments scatter in all directions.
The Big Bang theory does not give us information about the size of the universe, or even whether it is finite or infinite. The theory of relativity describes how each region of space expands, but says nothing about size or shape. Cosmologists sometimes claim that the universe was once no bigger than a grapefruit, but they only mean the part of it that we can now observe.
The inhabitants of the Andromeda Nebula or other galaxies have their own observable universes. Observers in Andromeda can see galaxies that are inaccessible to us, simply because they are a little closer to them; but they cannot contemplate those which we consider. Their observable universe was also the size of a grapefruit. One can imagine that the early universe was like a bunch of these fruits, stretching out indefinitely in all directions. So the notion that the Big Bang was "small" is wrong. The space of the universe is limitless. And no matter how you compress it, it will remain so.
faster than light
Misconceptions are also associated with a quantitative description of the extension. The rate at which the distances between galaxies are increasing follows a simple pattern identified by the American astronomer Edwin Hubble in 1929: the receding velocity of a galaxy v is directly proportional to its distance from us d, or v = Hd. The coefficient of proportionality H is called the Hubble constant and determines the rate of expansion of space both around us and around any observer in the Universe.
Some are confused by the fact that not all galaxies obey Hubble's law. The nearest large galaxy to us (Andromeda) generally moves towards us, and not away from us. There are such exceptions, since Hubble's law describes only the average behavior of galaxies. But each of them can also have a small motion of its own, since the gravitational influence of the galaxies on each other, like our Galaxy and Andromeda, for example. Distant galaxies also have small chaotic velocities, but at a large distance from us (at great importance d) these random velocities are negligible against the background of large removal velocities (v). Therefore, for distant galaxies, Hubble's law is fulfilled with high accuracy.
According to Hubble's law, the universe is not expanding at a constant rate. Some galaxies are moving away from us at a speed of 1 thousand km / s, others that are twice as far away at a speed of 2 thousand km / s, etc. Thus, Hubble's law indicates that, starting from a certain distance, called the Hubble distance, galaxies move away at a superluminal speed. For the measured value of the Hubble constant, this distance is about 14 billion light years.
But doesn't Einstein's theory of special relativity say that no object can travel faster than the speed of light? This question has baffled many generations of students. And the answer is that the special theory of relativity is applicable only to "normal" velocities - to motion in space. Hubble's law is about the rate of removal caused by the expansion of space itself, not motion through space. This effect of the general theory of relativity is not subject to the special theory of relativity. The presence of a removal velocity above the speed of light does not in any way violate the private theory of relativity. It is still true that no one can catch up with a beam of light.
CAN GALAXIES RETIRE AT A SPEED HIGHER THAN THE SPEED OF LIGHT?
WRONG: Einstein's special theory of relativity forbids this. Consider a region of space containing several galaxies. Due to its expansion, galaxies are moving away from us. The farther away the galaxy, the greater its speed (red arrows). If the speed of light is the limit, then the speed of removal should eventually become constant.
RIGHT: Of course they can. The private theory of relativity does not consider the speed of removal. The speed of removal increases infinitely with distance. Beyond a certain distance, called the Hubble distance, it exceeds the speed of light. This is not a violation of the theory of relativity, since the removal is caused not by movement in space, but by the expansion of space itself.
IS IT POSSIBLE TO SEE GALAXIES RETRAVING FASTER THAN LIGHT?
WRONG: Of course not. Light from such galaxies travels with them. Let the galaxy be outside the Hubble distance (sphere), i.e. moving away from us faster speed Sveta. It emits a photon (marked in yellow). As the photon flies through space, space itself expands. The distance to the Earth increases faster than the photon travels. He will never reach us.
RIGHT: Of course you can, because the rate of expansion changes with time. At first, the photon is actually blown away by the expansion. However, the Hubble distance is not constant: it increases, and eventually the photon can fall into the Hubble sphere. Once this happens, the photon will travel faster than the Earth is moving away, and it will be able to reach us.
Photon stretching
The first observations showing that the universe is expanding were made between 1910 and 1930. In the laboratory, atoms emit and absorb light always at certain wavelengths. The same is observed in the spectra of distant galaxies, but with a shift to the long wavelength region. Astronomers say that the galaxy's radiation is redshifted. The explanation is simple: as space expands, the light wave stretches and therefore weakens. If during the time that the light wave reached us, the Universe doubled, then the wavelength doubled, and its energy weakened by half.
FATIGUE HYPOTHESIS
Every time Scientific American publishes an article on cosmology, many readers write to us that they think galaxies are not really moving away from us and that the expansion of space is an illusion. They believe that the redshift in the spectra of galaxies is caused by something like "fatigue" from a long trip. Some unknown process causes the light, propagating through space, to lose energy and therefore turn red.
This hypothesis is more than half a century old, and at first glance it looks reasonable. But it is completely inconsistent with observations. For example, when a star explodes as a supernova, it flares up and then dims. The whole process takes about two weeks for a supernova of the type that astronomers use to determine distances to galaxies. During this period of time, the supernova emits a stream of photons. The light fatigue hypothesis says that photons will lose energy during the journey, but the observer will still receive a stream of photons lasting two weeks.
However, in expanding space, not only are the photons themselves stretched (and therefore lose energy), but their stream is also stretched. Therefore, it takes more than two weeks for all the photons to reach the Earth. Observations confirm this effect. A supernova explosion in a galaxy with a redshift of 0.5 is observed for three weeks, and in a galaxy with a redshift of 1 - a month.
The hypothesis of light fatigue also contradicts observations of the CMB spectrum and measurements of the surface brightness of distant galaxies. It's time to put the "weary light" (Charles Lineweaver and Tamara Davis) to rest.
Supernovae, like this one in the Virgo cluster of galaxies, help measure cosmic expansion. Their observable properties rule out alternative cosmological theories in which space does not expand.
The process can be described in terms of temperature. The photons emitted by a body have an energy distribution that is generally characterized by a temperature indicating how hot the body is. As photons move through expanding space, they lose energy and their temperature decreases. Thus, the universe cools as it expands, like compressed air escaping from a scuba diver's balloon. For example, the CMB now has a temperature of about 3 K, while it was born at a temperature of about 3000 K. But since that time, the Universe has increased in size by a factor of 1000, and the temperature of photons has decreased by the same factor. By observing gas in distant galaxies, astronomers directly measure the temperature of this radiation in the distant past. Measurements confirm that the universe is cooling over time.
There are also some controversies in the relationship between redshift and speed. Redshift caused by expansion is often confused with the more familiar redshift caused by the Doppler effect, which generally makes sound waves longer if the sound source is removed. The same is true for light waves, which become longer as the light source moves away in space.
Doppler redshift and cosmological redshift are completely different things and are described by different formulas. The first follows from the special theory of relativity, which does not take into account the expansion of space, and the second follows from the general theory of relativity. These two formulas are almost the same for nearby galaxies, but differ for distant ones.
According to the Doppler formula, if the speed of an object in space approaches the speed of light, then its redshift tends to infinity, and the wavelength becomes too large and therefore unobservable. If this were true for galaxies, then the most distant visible objects in the sky would be receding at a speed noticeably less than the speed of light. But the cosmological formula for redshift leads to a different conclusion. As part of the standard cosmological model galaxies with a redshift of about 1.5 (i.e., the received wavelength of their radiation is 50% greater than the laboratory value) are removed at the speed of light. Astronomers have already discovered about 1000 galaxies with a redshift greater than 1.5. So, we know about 1000 objects moving away faster than the speed of light. The CMB comes from an even greater distance and has a redshift of about 1000. When the hot plasma of the young Universe emitted the radiation we receive today, it was moving away from us at almost 50 times the speed of light.
Running in place
It is hard to believe that we can see galaxies moving faster than the speed of light, but this is possible due to a change in the expansion rate. Imagine a beam of light coming towards us from a distance greater than Hubble's distance (14 billion light years). It is moving towards us at the speed of light relative to its location, but it is moving away from us faster than the speed of light. Although light rushes towards us at the highest possible speed, it cannot keep up with the expansion of space. It's like a child trying to run into reverse side along the escalator. Photons at the Hubble distance move at their maximum speed to stay in the same place.
One might think that light from regions farther than the Hubble distance could never reach us and we would never see it. But the Hubble distance does not stay the same, because the Hubble constant, on which it depends, changes over time. This value is proportional to the recession speed of two galaxies divided by the distance between them. (Any two galaxies can be used for the calculation.) In models of the universe consistent with astronomical observations, the denominator increases faster than the numerator, so the Hubble constant decreases. Therefore, the Hubble distance is increasing. And if so, the light that did not initially reach us may eventually be within the Hubble distance. Then the photons will find themselves in a region that is moving away more slowly than the speed of light, after which they will be able to get to us.
IS COSMIC REDSHIFT REALLY DOPPLER SHIFT?
WRONG: Yes, because receding galaxies are moving through space. In the Doppler effect, light waves stretch (become redder) as their source moves away from the observer. The wavelength of light does not change as it travels through space. The observer receives the light, measures its redshift, and calculates the speed of the galaxy.
RIGHT A: No, redshift has nothing to do with the Doppler effect. The galaxy is almost stationary in space, so it emits light of the same wavelength in all directions. Over the course of the journey, the wavelength gets longer as space expands. Therefore, the light gradually turns red. The observer receives the light, measures its redshift, and calculates the speed of the galaxy. The cosmic redshift differs from the Doppler shift, which is confirmed by observations.
However, the galaxy that sent out the light can continue to move away at superluminal speeds. Thus, we can observe light from galaxies, which, as before, will always move away faster than the speed of light. In a word, the Hubble distance is not fixed and does not indicate to us the boundaries of the observable universe.
And what actually marks the boundary of the observable space? Here, too, there is some confusion. If space did not expand, then we could observe the most distant object now at a distance of about 14 billion light years from us, i.e. the distance light has traveled in the 14 billion years since the Big Bang. But as the universe expands, the space traversed by the photon expanded during its journey. Therefore, the current distance to the most distant of the observed objects is approximately three times greater - about 46 billion light years.
Cosmologists used to think that we live in a slowing down universe and therefore we can observe more and more galaxies. However, in the accelerating Universe, we are fenced off by a boundary beyond which we will never see the events taking place - this is the cosmic event horizon. If light from galaxies receding faster than the speed of light reaches us, then the Hubble distance will increase. But in an accelerating universe, its increase is prohibited. A distant event may send a beam of light in our direction, but this light will forever remain outside the Hubble distance due to the acceleration of the expansion.
As you can see, the accelerating Universe resembles a black hole, which also has an event horizon, from outside of which we do not receive signals. The current distance to our cosmic event horizon (16 billion light years) lies entirely within our observable region. The light emitted by galaxies that are now beyond the cosmic event horizon will never be able to reach us, because. the distance, which now corresponds to 16 billion light years, will expand too quickly. We will be able to see the events that took place in the galaxies before they crossed the horizon, but we will never know about subsequent events.
Is everything in the universe expanding?
People often think that if space expands, then everything in it expands too. But this is not true. Expansion as such (i.e. by inertia, without acceleration or deceleration) does not produce any force. The wavelength of a photon increases along with the growth of the Universe, since, unlike atoms and planets, photons are not connected objects, the dimensions of which are determined by the balance of forces. The changing rate of expansion does introduce a new force into the equilibrium, but it cannot cause objects to expand or contract.
For example, if gravity were to get stronger, your spinal cord would shrink until the electrons in the spine reached a new equilibrium position, a little closer together. Your height would decrease a little, but the contraction would stop there. Similarly, if we lived in a gravitational-dominated universe, as most cosmologists believed a few years ago, then the expansion would slow down, and all bodies would be subjected to a slight contraction, forcing them to reach a smaller equilibrium size. But, having reached it, they would no longer shrink.
HOW BIG IS THE OBSERVABLE UNIVERSE?
WRONG: The Universe is 14 billion years old, so the observable part of it should have a radius of 14 billion light years. Consider the most distant of the observed galaxies - the one whose photons emitted immediately after the Big Bang have only now reached us. A light year is the distance traveled by a photon in a year. This means that the photon has overcome 14 billion light years
RIGHT: As space expands, the observable region has a radius greater than 14 billion light years. As the photon travels, the space it traverses expands. By the time it reaches us, the distance to the galaxy that emitted it becomes more than just calculated from the flight time - approximately three times more
In fact, the expansion is accelerating, which is caused by a weak force that “inflates” all bodies. Therefore, bound objects are slightly larger than they would be in a non-accelerating universe, since the balance of forces is achieved with them at a slightly larger size. On the Earth's surface, the outward acceleration from the center of the planet is a tiny fraction ($10^(–30)$) of the normal gravitational acceleration toward the center. If this acceleration is constant, then it will not cause the Earth to expand. It's just that the planet takes on a slightly larger size than it would without the repulsive force.
But things will change if the acceleration is not constant, as some cosmologists believe. If the repulsion increases, then this may eventually cause the destruction of all structures and lead to a "Big Rip", which would not be due to expansion or acceleration per se, but because the acceleration would be accelerating.
DO OBJECTS IN THE UNIVERSE ALSO EXPAND?
WRONG: Yes. Expansion causes the universe and everything in it to expand. Consider a cluster of galaxies as an object. As the universe gets bigger, so does the cluster. The cluster boundary (yellow line) is expanding.
RIGHT: Not. The universe is expanding, but the related objects in it don't. Neighboring galaxies first move away, but eventually their mutual attraction overpowers the expansion. A cluster is formed of such a size that corresponds to its equilibrium state.
As new precise measurements help cosmologists better understand the expansion and acceleration, they may be asking even more fundamental questions about the earliest moments and largest scales of the universe. What caused the expansion? Many cosmologists believe that a process called "inflation" (bloat), a special type of accelerating expansion, is to blame. But perhaps this is only a partial answer: in order for it to begin, it seems that the Universe must already have been expanding. And what about the largest scales beyond our observations? Do different parts of the universe expand differently, such that our universe is just a modest inflationary bubble in a giant superuniverse? No one knows. But we hope that over time we will be able to come to an understanding of the process of expansion of the Universe.
ABOUT THE AUTHORS:
Charles H. Lineweaver and Tamara M. Davis are astronomers at Australia's Mount Stromlo Observatory. In the early 1990s At the University of California at Berkeley, Lineweaver was part of a group of scientists who discovered fluctuations in the CMB using the COBE satellite. He defended his dissertation not only in astrophysics, but also in history and English literature. Davis is working on building the Supernova/Acceleration Probe space observatory.
REMARKS TO THE ARTICLE "PARADOXES OF THE BIG BANG"
Professor Zasov Anatoly Vladimirovich, phys. Faculty of Moscow State University: All the misunderstandings with which the authors of the article argue are related to the fact that, for clarity, they most often consider the expansion of a limited volume of the Universe in a rigid frame of reference (moreover, the expansion of a small enough area not to take into account the difference in the course of time on Earth and on distant galaxies in the Earth's frame of reference). Hence the idea of both an explosion and a Doppler shift, and a widespread confusion with the speeds of movement. The authors, on the other hand, write, and write correctly, how everything looks in a non-inertial (comoving) coordinate system in which cosmologists usually work, although the article does not directly say this (in principle, all distances and velocities depend on the choice of the reference frame, and here always there is some arbitrariness). The only thing that is not clearly written is that it is not defined what is meant by distance in the expanding Universe. First, the authors say that this is the speed of light multiplied by the propagation time, and then it is said that it is also necessary to take into account the expansion, which removed the galaxy even more while the light was on the way. Thus distance is already understood as the speed of light multiplied by the propagation time it would take if the galaxy stopped receding and emitted light now. In reality, everything is more complicated. Distance is a model-dependent quantity and cannot be obtained directly from observations, so cosmologists do fine without it, replacing it with redshift. But perhaps a more rigorous approach is inappropriate here.
The great physicists of the past I. Newton and A. Einstein saw the Universe as static. The Soviet physicist A. Fridman in 1924 came up with the theory of "receding" galaxies. Friedman predicted the expansion of the universe. This was a revolutionary upheaval in the physical representation of our world.
American astronomer Edwin Hubble explored the Andromeda nebula. By 1923, he was able to consider that its outskirts are clusters of individual stars. Hubble calculated the distance to the nebula. It turned out to be 900,000 light years (a more accurately calculated distance today is 2.3 million light years). That is, the nebula is located far beyond the Milky Way - Our Galaxy. After observing this and other nebulae, Hubble came to a conclusion about the structure of the Universe.
The universe is made up of a collection of huge star clusters - galaxies.
It is they who appear to us in the sky as distant foggy "clouds", since we simply cannot consider individual stars at such a great distance.
E. Hubble noticed an important aspect in the data obtained, which astronomers had observed before, but found it difficult to interpret. Namely: the observed length of the spectral light waves emitted by the atoms of distant galaxies is somewhat longer than the length of the spectral waves emitted by the same atoms under the conditions of terrestrial laboratories. That is, in the emission spectrum of neighboring galaxies, the light quantum emitted by an atom during an electron jump from orbit to orbit is shifted in frequency in the direction of the red part of the spectrum compared to a similar quantum emitted by the same atom on Earth. Hubble took it upon himself to interpret this observation as a manifestation of the Doppler effect.
All observed neighboring galaxies are moving away from the Earth, since almost all galactic objects outside the Milky Way have a red spectral shift proportional to the speed of their removal.
Most importantly, Hubble was able to compare the results of his measurements of the distances to neighboring galaxies with the measurements of their removal rates (by redshift).
Mathematically, the law is formulated very simply:
where v is the speed of the galaxy moving away from us,
r is the distance to it,
H is the Hubble constant.
And, although initially Hubble came to this law as a result of observing only a few galaxies closest to us, not one of the many new galaxies of the visible Universe discovered since then, more and more distant from the Milky Way, does not fall out of this law.
So, the main consequence of Hubble's law:
The universe is expanding.
The very fabric of world space is expanding. All observers (and we are no exception) consider themselves to be at the center of the universe.
4. The Big Bang Theory
From the experimental fact of the recession of galaxies, the age of the Universe was estimated. It turned out to be equal - about 15 billion years! Thus began the era of modern cosmology.
Naturally, the question arises: what happened in the beginning? In total, it took scientists about 20 years to completely turn over the ideas about the Universe again.
The answer was proposed by the outstanding physicist G. Gamow (1904 - 1968) in the 40s. The history of our world began with the Big Bang. This is exactly what most astrophysicists think today.
The Big Bang is a rapid drop in the initially huge density, temperature and pressure of matter concentrated in a very small volume of the Universe. All the matter of the universe was compressed into a dense lump of protomatter, enclosed in a very small volume compared to the current scale of the Universe.
The idea of the Universe, which was born from a superdense clot of superhot matter and has been expanding and cooling since then, is called the Big Bang theory.
There is no more successful cosmological model of the origin and evolution of the Universe today.
According to the Big Bang theory, the early universe consisted of photons, electrons, and other particles. Photons constantly interacted with other particles. As the universe expanded, it cooled, and at a certain stage, electrons began to combine with the nuclei of hydrogen and helium and form atoms. This happened at a temperature of about 3000 K and the approximate age of the universe is 400,000 years. From that moment on, photons were able to move freely in space, practically without interacting with matter. But we are left with "witnesses" of that era - these are relic photons. It is believed that the relic radiation has been preserved from the initial stages of the existence of the Universe and evenly fills it. As a result of further cooling of the radiation, its temperature decreased and now is about 3 K.
The existence of the CMB was predicted theoretically within the framework of the Big Bang theory. It is regarded as one of the main confirmations of the Big Bang theory.
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