When we look at the distant Universe, we see galaxies everywhere - in all directions, millions and even billions of light years away. Since there are two trillion galaxies that we could observe, the sum of everything beyond them is bigger and cooler than our wildest imaginations. One of the most interesting facts is that all the galaxies we have ever observed follow (on average) the same rules: the farther they are from us, the faster they are moving away from us. This discovery, made by Edwin Hubble and his colleagues back in the 1920s, led us to the picture of an expanding universe. But what if it expands? Science knows, and now you will know too.
At first glance, this question may seem like a common sense question. Because anything that expands is usually made of matter and exists in the space and time of the Universe. But the Universe itself is space and time containing matter and energy within itself. When we say that “the Universe is expanding,” we mean the expansion of space itself, causing individual galaxies and clusters of galaxies to move away from each other. The easiest way would be to imagine a ball of dough with raisins inside, which is baked in an oven, says Ethan Siegel.
An expanding "bun" model of the Universe, in which relative distances increase as space expands
This dough is the fabric of space, and the raisins are connected structures (like galaxies or clusters of galaxies). From the point of view of any raisin, all other raisins will move away from it, and the further away they are, the faster. Only in the case of the Universe, the oven and air outside the dough do not exist, there is only dough (space) and raisins (matter).
It's not just receding galaxies that create redshift, but rather the space between us
How do we know that this space is expanding and not galaxies moving away?
If you see objects moving away from you in all directions, there is only one reason that can explain this: the space between you and these objects is expanding. You could also assume that you are near the center of the explosion, and many objects are simply further away and moving away faster because they received more energy from the explosion. If this were the case, we could prove it in two ways:
- At greater distances and high speeds there will be fewer galaxies because over time they would spread out greatly in space
- The relationship between redshift and distance will take on a specific shape at greater distances, which will be different from the shape if the fabric of space were expanding
When we look at great distances, we find that further out in the Universe the density of galaxies is higher than those closer to us. This is consistent with a picture in which space is expanding, because looking further is the same as looking into the past, where less expansion occurred. We also find that distant galaxies have a redshift-to-distance ratio consistent with the expansion of space, and not at all - if the galaxies were simply rapidly moving away from us. Science can answer this question in two different ways, and both answers support the expansion of the universe.
Has the Universe always expanded at the same rate?
We call it the Hubble constant, but it is constant only in space, not in time. The universe is currently expanding more slowly than in the past. When we talk about expansion speed, we are talking about speed per unit distance: about 70 km/s/Mpc today. (Mpc is a megaparsec, approximately 3,260,000 light years). But the rate of expansion depends on the densities of all the different things in the universe, including matter and radiation. As the Universe expands, the matter and radiation in it become less dense, and as the density drops, the expansion rate also decreases. The universe has expanded faster in the past and has been slowing down since the Big Bang. The Hubble constant is a misnomer; it should be called the Hubble parameter.
The distant fate of the universe offers different possibilities, but if dark energy is truly constant as the data suggests, we will follow the red curve
Will the Universe expand forever or will it ever stop?
Several generations of astrophysicists and cosmologists have puzzled over this question, and it can only be answered by determining the rate of expansion of the Universe and all the types (and amounts) of energy present in it. We have already successfully measured how much ordinary matter, radiation, neutrinos, dark matter and dark energy there is, as well as the rate of expansion of the Universe. Based on the laws of physics and what has happened in the past, it appears that the universe will expand forever. Although the probability of this is not 100%; if something like dark energy behaves differently in the future compared to the past and present, all our conclusions will have to be reconsidered.
Do galaxies move faster than the speed of light? Isn't this prohibited?
From our point of view, the space between us and the distant point is expanding. The further it is from us, the faster it seems to us that it is moving away. Even if the rate of expansion were tiny, a distant object would one day cross the threshold of any speed limit, because the rate of expansion (speed per unit distance) would multiply many times over with sufficient distance. OTO approves of this scenario. The law that nothing can travel faster than the speed of light applies only to the movement of an object through space, not to the expansion of space itself. In reality, the galaxies themselves move at speeds of only a few thousand kilometers per second, far below the 300,000 km/s limit set by the speed of light. It is the expansion of the Universe that causes the recession and redshift, not the true motion of the galaxy.
There are approximately 2 trillion galaxies within the observable universe (yellow circle). We will never be able to catch up with galaxies that are closer than a third of the way to this boundary due to the expansion of the Universe. Only 3% of the volume of the Universe is open to human exploration.
The expansion of the Universe is a necessary consequence of the fact that matter and energy fill space-time, which obeys the laws of general relativity. As long as there is matter, there is also gravitational attraction, so either gravity wins and everything contracts again, or gravity loses and expansion wins. There is no center of expansion and there is nothing outside the space that is expanding; it is the very fabric of the Universe that is expanding. What's most interesting is that even if we left Earth at the speed of light today, we would only be able to visit 3% of the galaxies in the observable Universe; 97% of them are already out of our reach. The universe is complex.
Just a hundred years ago, scientists discovered that our Universe is rapidly increasing in size.
Just a hundred years ago, ideas about the Universe were based on Newtonian mechanics and Euclidean geometry. Even a few scientists, such as Lobachevsky and Gauss, who accepted (only as a hypothesis!) the physical reality of non-Euclidean geometry, considered outer space eternal and unchanging
Alexey Levin
In 1870, the English mathematician William Clifford came to a very profound idea that space can be curved, and unequally at different points, and that over time its curvature can change. He even admitted that such changes were somehow related to the movement of matter. Both of these ideas, many years later, formed the basis of the general theory of relativity. Clifford himself did not live to see this - he died of tuberculosis at the age of 34, 11 days before Albert Einstein was born.
Redshift
The first information about the expansion of the Universe was provided by astrospectrography. In 1886, English astronomer William Huggins noticed that the wavelengths of starlight were slightly shifted compared to the terrestrial spectra of the same elements. Based on the formula for the optical version of the Doppler effect, derived in 1848 by the French physicist Armand Fizeau, the radial velocity of a star can be calculated. Such observations make it possible to track the movement of a space object.
Just a hundred years ago, ideas about the Universe were based on Newtonian mechanics and Euclidean geometry. Even a few scientists, such as Lobachevsky and Gauss, who assumed (only as a hypothesis!) the physical reality of non-Euclidean geometry, considered outer space eternal and unchanging. Due to the expansion of the Universe, it is not easy to judge the distance to distant galaxies. The light that arrived 13 billion years later from the galaxy A1689-zD1, 3.35 billion light years away (A), “reddens” and weakens as it travels through expanding space, and the galaxy itself moves away (B). It will carry information about the distance in redshift (13 billion light years), in angular size (3.5 billion light years), in intensity (263 billion light years), while the real distance is 30 billion light years. years.
A quarter of a century later, this opportunity was used in a new way by Vesto Slifer, an employee of the observatory in Flagstaff in Arizona, who, since 1912, had been studying the spectra of spiral nebulae with a 24-inch telescope with a good spectrograph. To obtain a high-quality image, the same photographic plate was exposed for several nights, so the project moved slowly. From September to December 1913, Slipher studied the Andromeda nebula and, using the Doppler-Fizeau formula, came to the conclusion that it was approaching the Earth by 300 km every second.
In 1917, he published data on the radial velocities of 25 nebulae, which showed significant asymmetries in their directions. Only four nebulae approached the Sun, the rest ran away (and some very quickly).
Slifer did not seek fame and did not promote his results. Therefore, they became known in astronomical circles only when the famous British astrophysicist Arthur Eddington drew attention to them.
In 1924, he published a monograph on the theory of relativity, which included a list of the radial velocities of 41 nebulae found by Slipher. The same four blue-shifted nebulae were present there, while the remaining 37 had spectral lines red-shifted. Their radial velocities varied between 150 and 1800 km/s and were on average 25 times higher than the known velocities of the Milky Way stars at that time. This suggested that nebulae participate in different movements than “classical” luminaries.
Space Islands
In the early 1920s, most astronomers believed that spiral nebulae were located on the periphery of the Milky Way, and beyond there was nothing but empty, dark space. True, back in the 18th century, some scientists saw giant star clusters in nebulae (Immanuel Kant called them island universes). However, this hypothesis was not popular, since it was impossible to reliably determine the distances to the nebulae.
This problem was solved by Edwin Hubble, working on the 100-inch reflecting telescope at California's Mount Wilson Observatory. In 1923-1924, he discovered that the Andromeda nebula consists of many luminous objects, including variable stars of the Cepheid family. It was already known then that the period of change in their apparent brightness is related to absolute luminosity, and therefore Cepheids are suitable for calibrating cosmic distances. With their help, Hubble estimated the distance to Andromeda at 285,000 parsecs (according to modern data, it is 800,000 parsecs). The diameter of the Milky Way was then believed to be approximately 100,000 parsecs (in reality it is three times less). It followed that Andromeda and the Milky Way must be considered independent star clusters. Hubble soon identified two more independent galaxies, which finally confirmed the “island universes” hypothesis.
To be fair, it is worth noting that two years before Hubble, the distance to Andromeda was calculated by the Estonian astronomer Ernst Opik, whose result - 450,000 parsecs - was closer to the correct one. However, he used a number of theoretical considerations that were not as convincing as Hubble's direct observations.
By 1926, Hubble had conducted a statistical analysis of observations of four hundred “extragalactic nebulae” (a term he used for a long time, avoiding calling them galaxies) and proposed a formula for relating the distance to a nebula to its apparent brightness. Despite the huge errors of this method, new data confirmed that nebulae are distributed more or less evenly in space and are located far beyond the boundaries of the Milky Way. Now there was no longer any doubt that space is not limited to our Galaxy and its closest neighbors.
Space fashion designers
Eddington became interested in Slipher's results even before the nature of spiral nebulae was finally clarified. By this time, a cosmological model already existed, which in a certain sense predicted the effect identified by Slipher. Eddington thought a lot about it and, naturally, did not miss the opportunity to give the observations of the Arizona astronomer a cosmological sound.
Modern theoretical cosmology began in 1917 with two revolutionary papers presenting models of the universe based on general relativity. One of them was written by Einstein himself, the other by the Dutch astronomer Willem de Sitter.
Hubble's laws
Edwin Hubble empirically discovered the approximate proportionality of redshifts and galactic distances, which he turned into a proportionality between velocities and distances using the Doppler-Fizeau formula. So we are dealing with two different patterns here.
Hubble didn't know how they were related to each other, but what does today's science say about it?
As Lemaître also showed, the linear correlation between cosmological (caused by the expansion of the Universe) redshifts and distances is by no means absolute. In practice, it is well observed only for displacements less than 0.1. So the empirical Hubble law is not exact, but approximate, and the Doppler-Fizeau formula is valid only for small shifts of the spectrum.
But the theoretical law connecting the radial speed of distant objects with the distance to them (with a proportionality coefficient in the form of the Hubble parameter V=Hd) is valid for any redshift. However, the speed V that appears in it is not at all the speed of physical signals or real bodies in physical space. This is the rate of increase in distances between galaxies and galaxy clusters, which is caused by the expansion of the Universe. We would be able to measure it only if we were able to stop the expansion of the Universe, instantly stretch measuring tapes between galaxies, read the distances between them and divide them into time intervals between measurements. Naturally, the laws of physics do not allow this. Therefore, cosmologists prefer to use the Hubble parameter H in another formula, which includes the scale factor of the Universe, which precisely describes the degree of its expansion in various cosmic epochs (since this parameter changes over time, its modern value is denoted by H0). The Universe is now expanding at an accelerating rate, so the value of the Hubble parameter is increasing.
By measuring cosmological redshifts, we obtain information about the extent of expansion of space. The light of the galaxy, which came to us with a cosmological redshift z, left it when all cosmological distances were 1+z times smaller than in our era. Additional information about this galaxy, such as its current distance or speed of removal from the Milky Way, can only be obtained using a specific cosmological model. For example, in the Einstein-de Sitter model, a galaxy with z = 5 is moving away from us at a speed equal to 1.1 s (the speed of light). But if you make a common mistake and simply equalize V/c and z, then this speed will turn out to be five times greater than light speed. The discrepancy, as we see, is serious.
Dependence of the speed of distant objects on redshift according to STR, GTR (depends on the model and time, the curve shows the present time and the current model). At small displacements the dependence is linear.
Einstein, in the spirit of the times, believed that the Universe as a whole was static (he tried to make it also infinite in space, but could not find the correct boundary conditions for his equations). As a result, he built a model of a closed Universe, the space of which has a constant positive curvature (and therefore it has a constant finite radius). Time in this Universe, on the contrary, flows like Newton, in one direction and at the same speed. The space-time of this model is curved due to the spatial component, while the time component is not deformed in any way. The static nature of this world provides a special “insert” into the main equation, which prevents gravitational collapse and thereby acts as an omnipresent anti-gravity field. Its intensity is proportional to a special constant, which Einstein called universal (now called the cosmological constant).
Lemaître's cosmological model of the expansion of the Universe was far ahead of its time. Lemaître's universe begins with the Big Bang, after which the expansion first slows down and then begins to accelerate.
Einstein's model made it possible to calculate the size of the Universe, the total amount of matter, and even the value of the cosmological constant. To do this, we only need the average density of cosmic matter, which, in principle, can be determined from observations. It is no coincidence that Eddington admired this model and used it in practice by Hubble. However, it is destroyed by instability, which Einstein simply did not notice: at the slightest deviation of the radius from the equilibrium value, Einstein’s world either expands or undergoes gravitational collapse. Therefore, this model has no relation to the real Universe.
Empty world
De Sitter also built, as he himself believed, a static world of constant curvature, but not positive, but negative. It contains Einstein's cosmological constant, but completely lacks matter. When test particles of arbitrarily small mass are introduced, they scatter and go to infinity. In addition, time flows more slowly at the periphery of the de Sitter universe than at its center. Because of this, light waves from large distances arrive with a red shift, even if their source is stationary relative to the observer. So in the 1920s, Eddington and other astronomers wondered whether de Sitter's model had anything in common with the reality reflected in Slipher's observations.
These suspicions were confirmed, albeit in a different way. The static nature of the de Sitter universe turned out to be imaginary, since it was associated with an unsuccessful choice of the coordinate system. After correcting this error, de Sitter space turned out to be flat, Euclidean, but non-static. Thanks to the antigravitational cosmological constant, it expands while maintaining zero curvature. Because of this expansion, the wavelengths of photons increase, which entails the shift of spectral lines predicted by de Sitter. It is worth noting that this is how the cosmological redshift of distant galaxies is explained today.
From statistics to dynamics
The history of openly non-static cosmological theories begins with two papers by Soviet physicist Alexander Friedman, published in the German journal Zeitschrift fur Physik in 1922 and 1924. Friedman calculated models of universes with time-varying positive and negative curvature, which became the golden fund of theoretical cosmology. However, contemporaries hardly noticed these works (Einstein at first even considered Friedman’s first paper to be mathematically erroneous). Friedman himself believed that astronomy does not yet have an arsenal of observations that would allow one to decide which of the cosmological models is more consistent with reality, and therefore limited himself to pure mathematics. Perhaps he would have acted differently if he had read Slifer's results, but this did not happen.
The largest cosmologist of the first half of the 20th century, Georges Lemaitre, thought differently. At home, in Belgium, he defended his dissertation in mathematics, and then in the mid-1920s he studied astronomy - at Cambridge under the direction of Eddington and at the Harvard Observatory under Harlow Shapley (while in the USA, where he prepared a second dissertation at MIT, he met Slifer and Hubble). Back in 1925, Lemaître was the first to show that the static nature of de Sitter’s model was imaginary. Upon his return to his homeland as a professor at the University of Louvain, Lemaitre built the first model of an expanding universe with a clear astronomical basis. Without exaggeration, this work was a revolutionary breakthrough in space science.
Universal revolution
In his model, Lemaitre retained a cosmological constant with an Einsteinian numerical value. Therefore, his universe begins in a static state, but over time, due to fluctuations, it embarks on a path of constant expansion at an increasing rate. At this stage it maintains a positive curvature, which decreases as the radius increases. Lemaitre included in his universe not only matter, but also electromagnetic radiation. Neither Einstein nor de Sitter, whose work was known to Lemaitre, nor Friedman, about whom he knew anything at that time, did this.
Associated coordinates
In cosmological calculations it is convenient to use accompanying coordinate systems, which expand in unison with the expansion of the Universe. In an idealized model, where galaxies and galaxy clusters do not participate in any proper motions, their accompanying coordinates do not change. But the distance between two objects at a given moment in time is equal to their constant distance in accompanying coordinates, multiplied by the value of the scale factor for this moment. This situation can be easily illustrated on an inflatable globe: the latitude and longitude of each point do not change, and the distance between any pair of points increases with increasing radius.
Using comoving coordinates helps us understand the profound differences between expanding universe cosmology, special relativity, and Newtonian physics. Thus, in Newtonian mechanics all movements are relative, and absolute immobility has no physical meaning. On the contrary, in cosmology, immobility in comoving coordinates is absolute and, in principle, can be confirmed by observations. The special theory of relativity describes processes in space-time, from which spatial and temporal components can be isolated in an infinite number of ways using Lorentz transformations. Cosmological space-time, on the contrary, naturally breaks down into a curved expanding space and a single cosmic time. In this case, the speed of retreat of distant galaxies can be many times higher than the speed of light.
Lemaitre, back in the USA, suggested that the redshifts of distant galaxies arise due to the expansion of space, which “stretches” light waves. Now he has proven it mathematically. He also demonstrated that small (much smaller units) redshifts are proportional to the distances to the light source, and the proportionality coefficient depends only on time and carries information about the current rate of expansion of the Universe. Since the Doppler-Fizeau formula implied that the radial speed of a galaxy is proportional to its redshift, Lemaître came to the conclusion that this speed is also proportional to its distance. After analyzing the speeds and distances of 42 galaxies from Hubble's list and taking into account the intragalactic speed of the Sun, he established the values of the proportionality coefficients.
Unsung work
Lemaitre published his work in 1927 in French in the little-read journal Annals of the Brussels Scientific Society. It is believed that this was the main reason why she initially went virtually unnoticed (even by his teacher Eddington). True, in the fall of the same year, Lemaitre was able to discuss his findings with Einstein and learned from him about Friedman’s results. The creator of General Relativity had no technical objections, but he resolutely did not believe in the physical reality of Lemetre’s model (just as he had previously not accepted Friedman’s conclusions).
Hubble graphs
Meanwhile, in the late 1920s, Hubble and Humason discovered a linear correlation between the distances of 24 galaxies and their radial velocities, calculated (mostly by Slipher) from redshifts. Hubble concluded from this that the radial speed of a galaxy is directly proportional to its distance. The coefficient of this proportionality is now denoted by H0 and is called the Hubble parameter (according to the latest data, it slightly exceeds 70 (km/s)/megaparsec).
Hubble's paper plotting the linear relationship between galactic speeds and distances was published in early 1929. A year earlier, the young American mathematician Howard Robertson, following Lemaitre, derived this dependence from the model of an expanding Universe, which Hubble may have known about. However, his famous article did not mention this model either directly or indirectly. Hubble later expressed doubts that the velocities appearing in his formula actually describe the movements of galaxies in outer space, but he always refrained from their specific interpretation. He saw the meaning of his discovery in demonstrating the proportionality of galactic distances and redshifts, leaving the rest to theorists. Therefore, with all due respect to Hubble, there is no reason to consider him the discoverer of the expansion of the Universe.
And yet it is expanding!
Nevertheless, Hubble paved the way for the recognition of the expansion of the Universe and Lemaître's model. Already in 1930, such masters of cosmology as Eddington and de Sitter paid tribute to her; A little later, scientists noticed and appreciated Friedman's work. In 1931, at the instigation of Eddington, Lemaitre translated his article into English (with small cuts) for the Monthly News of the Royal Astronomical Society. In the same year, Einstein agreed with Lemaître’s conclusions, and a year later, together with de Sitter, he built a model of an expanding Universe with flat space and curved time. This model, due to its simplicity, has been very popular among cosmologists for a long time.
In the same 1931, Lemaitre published a brief (and without any mathematics) description of another model of the Universe, which combined cosmology and quantum mechanics. In this model, the initial moment is the explosion of the primary atom (Lemaitre also called it a quantum), which gave rise to both space and time. Since gravity slows down the expansion of the newborn Universe, its speed decreases - perhaps almost to zero. Lemaitre later introduced a cosmological constant into his model, which forced the Universe to eventually enter a stable regime of accelerating expansion. So he anticipated both the idea of the Big Bang and modern cosmological models that take into account the presence of dark energy. And in 1933, he identified the cosmological constant with the energy density of the vacuum, which no one had ever thought of before. It’s simply amazing how ahead of his time this scientist, certainly worthy of the title of discoverer of the expansion of the Universe, was!
MOSCOW, January 26 - RIA Novosti. An independent group of scientists has confirmed that the Universe is indeed expanding even faster now than calculations based on observations of the “echo” of the Big Bang showed, according to a series of five papers accepted for publication in the journal Monthly Notices of the Royal Astronomical Society.
“Discrepancies in the current rate of expansion of the Universe and what observations of the Big Bang show have not only been confirmed, but also strengthened by new data on how distant galaxies bend light. These discrepancies may be generated by “new physics” beyond the Standard Model of cosmology , in particular, some other form of dark energy,” said Frederic Coubrin from the École Polytechnique Federale in Lausanne (Switzerland).
Dark births of the Universe
Back in 1929, the famous astronomer Edwin Hubble proved that our Universe does not stand still, but is gradually expanding, observing the movement of galaxies far from us. At the end of the 20th century, astrophysicists discovered, by observing supernovae of the first type, that it is expanding not at a constant speed, but with acceleration. The reason for this, as scientists today believe, is dark energy - a mysterious substance that acts on matter as a kind of “antigravity”.
Last June, Nobel laureate Adam Reiss and his colleagues, who discovered the phenomenon, calculated the exact expansion rate of the Universe today using Cepheid variable stars in nearby galaxies, the distance to which can be calculated with ultra-high precision.
This clarification gave an extremely unexpected result - it turned out that two galaxies, separated by a distance of about 3 million light years, are flying away at a speed of about 73 kilometers per second. This figure is noticeably higher than data obtained using the WMAP and Planck orbital telescopes show - 69 kilometers per second, and it cannot be explained using our existing ideas about the nature of dark energy and the mechanism of the birth of the Universe.
Riess and his colleagues suggested that there was also a third "dark" substance - "dark radiation" (dark radiation), which caused it to accelerate faster than theoretical predictions in the early days of the Universe. Such a statement did not go unnoticed, and the H0LiCOW collaboration, which includes dozens of astronomers from all continents of the planet, began testing this hypothesis by observing quasars - the active nuclei of distant galaxies.
Game of cosmic candles and lenses
Quasars, thanks to the giant black hole at their center, bend the structure of space-time in a special way, amplifying the light passing through its surroundings, like a giant lens.
If two quasars are placed next to each other for observers on Earth, an interesting thing occurs - the light of the more distant quasar will be split up as it passes through the gravitational lens of the first galactic nucleus. Because of this, we will see not two, but five quasars, four of which will be light “copies” of a more distant object. Most importantly, each copy will represent a "photograph" of the quasar at different times in its life due to the different amounts of time it took for their light to escape the gravitational lens.
The duration of this time, as scientists explain, depends on the expansion rate of the Universe, which makes it possible to calculate it by observing a large number of distant quasars. This is what the H0LiCOW participants did, looking for similar “double” quasars and observing their “copies”.
In total, Kubrin and his colleagues found three such quasar “matryoshka dolls” and studied them in detail using the Hubble and Spitzer orbital telescopes and a number of ground-based telescopes in the Hawaiian Islands and Chile. These measurements, according to the researchers, allowed them to measure the Hubble constant at the “average” cosmological distance with an error level of 3.8%, which is several times better than previously obtained results.
These calculations showed that the Universe is expanding at a speed of about 71.9 kilometers per second, which generally corresponds to the result that Riess and his colleagues obtained at “close” cosmological distances, and speaks in favor of the existence of some third “dark” substance that accelerated The universe in her youth. Another option to explain the discrepancies with the data is that the Universe is not actually flat, but resembles a sphere or an “accordion.” It's also possible that the amount or properties of dark matter have changed over the past 13 billion years, causing the universe to grow faster.
In any case, scientists plan to study about a hundred more similar quasars in order to verify the reliability of the data they obtained, and to understand how such unusual behavior of the Universe can be explained, which does not fit into standard cosmological theories.
Analyzing the results of observations of galaxies and cosmic microwave background radiation, astronomers came to the conclusion that the distribution of matter in the Universe (the region of the studied space exceeded 100 Mpc in diameter) is homogeneous and isotropic, i.e. does not depend on the position and direction in space (see Cosmology) . And such properties of space, according to the theory of relativity, inevitably entail a change over time in the distances between the bodies filling the Universe, i.e. the Universe must expand or contract, and observations indicate expansion.
The expansion of the Universe differs significantly from the ordinary expansion of matter, for example, from the expansion of a gas in a cylinder. The gas, expanding, changes the position of the piston in the cylinder, but the cylinder remains unchanged. In the Universe, there is an expansion of all space as a whole. Therefore, the question in which direction the expansion occurs becomes meaningless in the Universe. This expansion takes place on a very large scale. Within stellar systems, galaxies, clusters and superclusters of galaxies, expansion does not occur. Such gravitationally bound systems are isolated from the general expansion of the Universe.
The conclusion that the Universe is expanding is confirmed by observations of the red shift in the spectra of galaxies.
Let light signals be sent from a certain point in space at two moments and observed at another point in space.
Due to a change in the scale of the Universe, i.e., an increase in the distance between the points of emission and observation of light, the second signal must travel a greater distance than the first. And since the speed of light is constant, the second signal is delayed; the interval between signals at the observation point will be greater than at the point of their departure. The greater the distance between the source and the observer, the greater the delay. A natural standard of frequency is the frequency of radiation during electromagnetic transitions in atoms. Due to the described effect of the expansion of the Universe, this frequency decreases. Thus, when observing the emission spectrum of some distant galaxy, all its lines should be red-shifted compared to laboratory spectra. This phenomenon of redshift is the Doppler effect (see Radial velocity) from the mutual “scattering” of galaxies and is observed in reality.
The magnitude of the red shift is measured by the ratio of the changed frequency of radiation to the original one. The greater the distance to the observed galaxy, the greater the frequency change.
Thus, by measuring the red shift from the spectra, it turns out to be possible to determine the speed v of galaxies with which they move away from the observer. These speeds are related to distances called the Hubble constant.
Accurate determination of the value is fraught with great difficulties. Based on long-term observations, the currently accepted value is .
This value corresponds to an increase in the speed of galaxy recession equal to approximately 50-100 km/s for each megaparsec of distance.
Hubble's law makes it possible to estimate the distances to galaxies located at enormous distances based on the redshift of lines measured in their spectra.
The law of galaxy recession is derived from observations from the Earth (or, one might say, from our Galaxy), and thus it describes the distance of galaxies from the Earth (our Galaxy). However, one cannot conclude from this that it is the Earth (our Galaxy) that is at the center of the expansion of the Universe. Simple geometric constructions convince us that Hubble's law is valid for an observer located in any of the galaxies participating in the recession.
Hubble's expansion law indicates that matter in the Universe was once at very high densities. The time separating us from this state can be conventionally called the age of the Universe. It is determined by the value
Since the speed of light is finite, the finite age of the Universe corresponds to the finite region of the Universe that we can currently observe. Moreover, the most distant observable parts of the Universe correspond to the earliest moments of its evolution. At these moments, a variety of elementary particles could be born and interact in the Universe. By analyzing the processes that occurred with the participation of such particles in the first second of the expansion of the Universe, theoretical cosmology finds, based on the theory of elementary particles, answers to the questions of why there is no antimatter in the Universe and even why the Universe is expanding.
Many of the theory's predictions about the physical processes of elementary particles relate to energy regions that are unattainable in modern terrestrial laboratory conditions, for example, in accelerators.
However, in the period before the first second of the expansion of the Universe, particles with such energy should have existed. Therefore, physicists view the expanding Universe as a natural laboratory of elementary particles.
In this laboratory, you can carry out “thought experiments”, analyze how the existence of a particular particle would affect physical processes in the Universe, how this or that prediction of the theory would manifest itself in astronomical observations.
The theory of elementary particles is invoked to explain the “hidden mass” of the Universe. To explain how galaxies formed, how they move in galaxy clusters, and many other features of the distribution of visible matter, it turns out to be necessary to assume that more than 80% of the mass of the Universe is hidden in the form of invisible weakly interacting particles. In this regard, neutrinos with non-zero rest mass, as well as new hypothetical particles, are widely discussed in cosmology.
When astrophysicist Edwin Hubble determined almost a century ago that the Universe was expanding uniformly in all directions, the discovery came as a real surprise. Then, in the mid-1990s, another unexpected thing became clear: it turns out that the Universe is expanding faster and faster, that is, with acceleration. The reason for this was considered to be the repulsive properties of a substance called “dark energy”.
Now, using the Hubble Space Telescope, NASA astrophysicists have determined that the Universe is expanding faster than expected. It is still unclear how to interpret this discovery, but the Hubble constant will have to be reconsidered.
“This unexpected discovery could be an important clue to understanding what the 95% of the universe's mass is that doesn't emit light, including dark energy, dark matter and dark radiation,” explained the study's lead author and Nobel laureate. Laureate Adam Riess of the Space Telescope Institute and Johns Hopkins University.
The so-called “dark radiation” that the Nobel laureate talks about is probably one of the hypothetical forms of dark energy.
Scientists offer several explanations for what is happening. Perhaps dark energy is pushing galaxies apart more than expected. Or early space may contain a new type of elementary particle called “dark radiation,” meaning that more energy from dark radiation should be added to the formula for the expansion of the Universe after the Big Bang.
A third possibility is that dark matter, the invisible form of matter that makes up most of the mass of our Universe, has some strange, unexpected characteristics. After all, Einstein's theory of gravity may be incomplete.
Adam Riess and colleagues developed a new technique for estimating the expansion rate of the Universe in 2005. Innovative technology allows us to better determine the distance to distant galaxies.
The method consists of three steps, which are shown in the diagram. It involves searching for galaxies with type Ia supernovae and Cepheid stars. Cypheids pulsate in precise proportions to their intrinsic luminosity, which can be compared to their apparent luminosity to accurately estimate distance. Type Ia supernovae, in turn, are formed by the explosions of white dwarfs and are bright enough to be observed from a relatively large distance.
Over the course of ten years, scientists measured about 2,400 Cepheids in 19 galaxies, estimated their apparent brightness, accurately measured their true brightness, and calculated the distance to about 300 Type Ia supernovae in distant galaxies.
Until now, the most reliable estimate of the Hubble constant was 67.80 ± 0.77 (km/s)/Mpc, that is, in the modern era, two galaxies separated by a distance of 1 megaparsec are on average moving away at a speed of ~68 km/s.
According to new measurements, the Hubble constant is 73.2 (km/s)/Mpc, that is, two galaxies separated by a distance of 1 megaparsec are on average moving away at a speed of ~73 km/s.
The proposed method is more accurate than previous methods: the expansion rate is determined with an error of 2.4%. But even taking this error into account, the new Hubble constant is significantly larger than the old one.
The results of the ten-year study will be published in an upcoming issue The Astrophysical Journal.
Calculating the true value of the Hubble constant is not an easy task. For example, analysis of the afterglow from the Big Bang carried out by the Wilkinson Microwave Anisotropy Probe (WMAP) and the results of observations by the European Space Agency's Planck satellite mission gave opposite results: according to the predicted trajectory, the expansion rate of the Universe should now be 5% and 9% less than that obtained value of the Hubble constant.
Further research will help clarify and measure the rate at which galaxies are moving away more accurately over different time periods.
“We know so little about the dark parts of the Universe that it is very important to measure the force with which they have been attracted and repelled throughout cosmic history,” said Lucas Macri, one of the authors of the scientific paper.
Before the launch of the Hubble telescope, estimates of the expansion rate of the Universe differed by two orders of magnitude. Measurements in the late 1990s helped reduce the error to 10%. Now scientists from the Supernova H0 for the Equation of State (SH0ES) group are working on new calculation methods that will reduce the error to 1%.
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