How old is the universe?

People have been interested in the age of the universe since ancient times. And although she can not ask for a passport to see the date of birth, modern science was able to answer this question. However, only recently.

The sages of Babylon and Greece considered the universe eternal and unchanged, and the Hindu chroniclers in 150 BC. determined that he was exactly 1 972 949 091 (by the way, in order of magnitude they were not very mistaken!). In 1642, the English theologian John Lightfoot, through a meticulous analysis of the biblical texts, calculated that the creation of the world had taken place in 3929 BC; After a few years, the Irish bishop James Asher moved it to 4004. The founders of modern science, Johannes Kepler and Isaac Newton, also did not pass by this topic. Although they appealed not only to the Bible, but also to astronomy, their results turned out to be similar to the calculations of theologians – 3993 and 3988 years BC. In our enlightened time, the age of the universe is determined in other ways. To see them in a historical projection, first glance at your own planet and its cosmic environment.

Astronomers studied in detail the early biography of the universe. But as to her exact age, they had doubts, which could only be dispelled in the last couple of decades.

Divination by stones

Since the second half of the XVIII century, scientists began to evaluate the age of the Earth and the Sun on the basis of physical models. So, in 1787, the French naturalist Georges-Louis Leclerc concluded that if our planet at birth were a sphere of molten iron, it would need from 75 to 168 thousand years to cool to the current temperature. After 108 years, Irish mathematician and engineer John Perry re-calculated the thermal history of the Earth and determined its age in 2-3 billion years. At the very beginning of the 20th century, Lord Kelvin came to the conclusion that if the Sun gradually shrinks and shines solely due to the liberation of gravitational energy, then its age (and, consequently, the maximum age of the Earth and the other planets) may amount to several hundred million years. But at that time geologists could neither confirm nor disprove these estimates because of the lack of reliable methods of geochronology.

In the middle of the first decade of the twentieth century, Ernest Rutherford and the American chemist Bertram Boltwood developed the basics of radiometric dating of terrestrial rocks, which showed that Perry was much closer to the truth. In the 1920s, samples of minerals were found whose radiometric age was approaching 2 billion years. Later, geologists repeatedly increased this value, and by now it has more than doubled – up to 4.4 billion. Additional data are provided by the study of “celestial stones” – meteorites. Almost all radiometric estimates of their age fit within an interval of 4.4-4.6 billion years.

Modern helioseismology allows us to directly determine the age of the Sun, which, according to the latest data, is 4.56 – 4.58 billion years. Since the duration of gravitational condensation of the protosolar cloud was estimated to be only millions of years, we can confidently state that no more than 4.6 billion years have passed since the beginning of this process. At the same time, the solar matter contains many elements heavier than helium, which were formed in thermonuclear furnaces of massive stars of previous generations, burned out and exploded by supernovae. This means that the extent of the existence of the universe greatly exceeds the age of the solar system. To determine the extent of this excess, you must first go to our Galaxy, and then beyond.

Following the white dwarfs

The lifetime of our Galaxy can be determined in many ways, but we confine ourselves to the two most reliable. The first method is based on monitoring the glow of white dwarfs. These compact (roughly the size of the Earth) and initially very hot celestial bodies represent the final stage of life of almost all stars except for the most massive ones. To turn into a white dwarf star must completely burn all of its thermonuclear fuel and undergo several cataclysms – for example, for some time to become a red giant.

Natural watches

According to radiometric dating, the oldest rocks on Earth are now considered gray gneisses of the coast of the Great Slave Lake in northwest Canada – their age is estimated at 4.03 billion years. Even earlier (4.4 billion years ago), the smallest grains of zircon mineral, natural zirconium silicate, found in gneisses in the west of Australia crystallized. And once in those times the Earth’s crust already existed, our planet should be somewhat older.

As for meteorites, the most accurate information is given by the dating of calcium-aluminum impregnations in the substance of coal-bearing chondritic meteorites, which practically did not change after its formation from the gas-dust cloud surrounding the newborn Sun. The radiometric age of such structures in the meteorite Efremovka, found in 1962 in the Pavlodar region of Kazakhstan, is 4 billion 567 million years.

A typical white dwarf consists almost entirely of carbon and oxygen ions immersed in a degenerate electron gas, and has a subtle atmosphere, in which hydrogen or helium dominate. Its surface temperature ranges from 8,000 to 40,000 K, while the central zone is heated to millions and even tens of millions of degrees. According to theoretical models, dwarfs consisting mainly of oxygen, neon and magnesium (into which stars with a mass of 8 to 10.5 or even up to 12 solar masses are transformed under certain conditions) can also be born, but their existence has not yet been proved. The theory also asserts that stars, at least twice as massive as the Sun, end their lives in the form of helium white dwarfs. Such stars are very numerous, but they burn hydrogen very slowly and therefore live many tens and hundreds of millions of years. So far, they simply did not have enough time to exhaust hydrogen fuel (very few helium dwarfs, discovered so far, live in binary systems and originated in a completely different way).

Since a white dwarf can not support the reaction of thermonuclear fusion, it shines due to accumulated energy and therefore slowly cools. The rates of this cooling can be calculated and, on this basis, the time required to reduce the surface temperature from the original (for a typical dwarf it is approximately 150,000 K) to the observed one. Since we are interested in the age of the Galaxy, one should look for the longest-lived, and therefore the coldest, white dwarfs. Modern telescopes allow us to detect intragalactic dwarfs with a surface temperature of less than 4000 K, whose luminosity is 30,000 times inferior to the solar one. Until they are found – either they are not at all, or very few. It follows that our Galaxy can not be older than 15 billion years, otherwise they would be present in appreciable quantities.

For the dating of rocks, an analysis is made of the content of decay products of various radioactive isotopes in them. Depending on the type of rocks and dates of dating, different pairs of isotopes are used.

This is the upper limit of age. And what about the bottom? The coldest of the now-known white dwarfs were recorded by the Hubble Space Telescope in 2002 and 2007. Calculations showed that their age is 11.5 – 12 billion years. To this, we need to add the age of the predecessor stars (from half a billion to a billion years). It follows that the Milky Way is at least 13 billion years old. So the final estimate of his age, obtained on the basis of observations of white dwarfs, is about 13-15 billion years.

Ball certificates

The second method is based on the study of spherical star clusters located in the peripheral zone of the Milky Way and circling around its core. They contain from hundreds of thousands to more than a million stars connected by mutual attraction.

Globular clusters are found in virtually all large galaxies, and their number sometimes reaches many thousands. New stars there are almost never born, but the elderly luminaries are present in abundance. About 160 such globular clusters are registered in our Galaxy, and maybe two or three dozen more will be discovered. The mechanisms of their formation are not completely clear, however, most likely, many of them arose soon after the birth of the Galaxy itself. Therefore, the dating of the formation of the oldest globular clusters makes it possible to establish the lower boundary of the galactic age.

This dating is very difficult technically, but it is based on a very simple idea. All the stars of the cluster (from supermassive to the lightest) are formed from the same total gas cloud and are therefore born almost simultaneously. Over time, they burn out the main reserves of hydrogen – some earlier, others later. At this stage, the star leaves the main sequence and undergoes a series of transformations that result either in a complete gravitational collapse (followed by the formation of a neutron star or black hole) or the emergence of a white dwarf. Therefore, the study of the composition of the globular cluster allows us to determine its age sufficiently accurately. For reliable statistics, the number of clusters studied should be at least several tens.

Three years ago, a team of astronomers using the camera (Advanvced Camera for Survey) of the Hubble Space Telescope performed this work. Monitoring of 41 globular clusters of our Galaxy showed that their average age is 12.8 billion years. The record holders were NGC 6937 and NGC 6752, remote from the Sun at 7200 and 13 000 light years. They are almost certainly not younger than 13 billion years, and the most probable lifetime of the second cluster is 13.4 billion years (true, with an error of plus or minus a billion).

Stars of mass of the order of solar as the hydrogen reserves are exhausted swell and go over to the category of red dwarfs, after which their helium nucleus is heated up during compression and the burning of helium begins. After a while, the star dumps the shell, forming a planetary nebula, and then turns into the category of white dwarfs and then cools down.

However, our Galaxy should be older than its clusters. Its first supermassive stars exploded with supernovae and emitted into the cosmos the nuclei of many elements, in particular, the nucleus of the stable isotope beryllium-beryllium-9. When the globular clusters began to form, their newborn stars already contained beryllium, and the more, the later they arose. By the content of beryllium in their atmospheres, it is possible to determine how much clusters are younger than the Galaxy. As evidenced by data on the accumulation of NGC 6937, this difference is 200 – 300 million years. So, without much exaggeration, we can say that the age of the Milky Way exceeds 13 billion years, and possibly reaches 13.3 – 13.4 billion. This is almost the same estimate as was made on the basis of observations of white dwarfs, but it was obtained quite differently way.

Hubble’s Law

The scientific formulation of the question of the age of the universe became possible only at the beginning of the second quarter of the last century. In the late 1920s, Edwin Hubble and his assistant, Milton Humason, began to refine the distances to dozens of nebulae beyond the Milky Way, which only a few years earlier had been considered independent galaxies.

These galaxies are removed from the Sun with radial velocities, which were measured from the magnitude of the red shift of their spectra. Although it was possible to determine the distances to most of these galaxies with a large error, Hubble did find out that they were approximately proportional to the radial velocities, as he wrote in an article published in early 1929. Two years later, Hubble and Humason confirmed this conclusion on the basis of observations from other galaxies-some of them more than 100 million light-years away.

These data formed the basis of the famous formula v = H0d, known as the Hubble law. Here v is the radial velocity of the galaxy with respect to the Earth, d is the distance, H0 is the proportionality coefficient, whose dimension is easily seen to be inverse to the dimensionality of time (before it was called the Hubble constant, which is incorrect, because in previous epochs the value of H0 was different than in our time). Hubble himself and many other astronomers for a long time refused to make assumptions about the physical meaning of this parameter. However, in 1927, Georges Lemaitre showed that the general theory of relativity makes it possible to interpret the expansion of galaxies as evidence of the expansion of the universe. Four years later, he had the courage to bring this conclusion to its logical conclusion, putting forward the hypothesis that the universe originated from an almost point-like germ, which he, for want of a better term, called an atom. This original atom could stay in a static state any time to infinity, but its “explosion” gave birth to an expanding space filled with matter and radiation, which in a finite time gave rise to the present universe. Already in his first article, Lemaître derived a complete analog of the Hubble formula and, having known at that time data on the velocities and distances of a number of galaxies, obtained approximately the same value of the proportionality coefficient between distances and velocities as Hubble. However, his article was printed in French in an obscure Belgian magazine and at first remained unnoticed. Most astronomers, she became known only in 1931 after the publication of her English translation.

The evolution of the universe is determined by the initial speed of its expansion, as well as the impact of gravity (including dark matter) and antigravitation (dark energy). Depending on the relationship between these factors, the size of the universe has a different shape in the future, and in the past, which affects the evaluation of its age. Current observations show that the universe expands exponentially (red graph).

Hubble time

From this work of Lemaitre and later works of both Hubble himself and other cosmologists, it follows directly that the age of the universe (naturally counted from the initial moment of its expansion) depends on the quantity 1 / H0, which is now called the Hubble time. The nature of this dependence is determined by the specific model of the universe. If we assume that we live in a flat universe filled with a gravitating substance and radiation, then to calculate its age, 1 / H0 must be multiplied by 2/3.

Then there was a snag. From the measurements of Hubble and Humanson it follows that the numerical value of 1 / H0 is approximately equal to 1.8 billion years. It followed that the universe was born 1.2 billion years ago, which clearly contradicted even greatly underestimated at that time estimates of the age of the Earth. It was possible to get out of this difficulty, assuming that the galaxies fly off more slowly than Hubble thought. Over time, this assumption was confirmed, but the problem did not resolve. According to data obtained by the end of the last century with the help of optical astronomy, 1 / H0 is from 13 to 15 billion years. So the discrepancy still remained, since the space of the Universe was considered to be flat, and two-thirds of the Hubble time is much less than even the most modest estimates of the age of the Galaxy.

Empty world

According to the latest measurements of the Hubble parameter, the lower boundary of the Hubble time is 13.5 billion years, and the upper limit is 14 billion. It turns out that the current age of the universe is roughly equal to the current Hubble time. Such equality must be strictly and invariably observed for an absolutely empty universe, where there is neither gravitating matter nor anti-gravity fields. But in fact in our world there is enough of both. The fact is that the space initially expanded with deceleration, then the rate of its expansion began to grow, and in the present era, these opposite tendencies almost compensated each other.

In general terms, this contradiction was eliminated in 1998 – 1999, when two teams of astronomers proved that the last 5-6 billion years of space expands not with a falling but increasing speed. This acceleration is usually explained by the fact that in our universe the influence of the anti-gravity factor, the so-called dark energy, whose density does not change with time, grows. Since the density of gravitating matter falls with the expansion of the Cosmos, dark energy is increasingly successful in competing with gravity. The lifetime of the universe with an anti-gravity component does not have to be equal to two thirds of the Hubble time. Therefore, the discovery of the accelerating expansion of the universe (noted in 2011 by the Nobel Prize) allowed to eliminate the separation between cosmological and astronomical estimates of the time of its life. It also became a prelude to developing a new method for dating her birth.

Cosmic rhythms

On June 30, 2001, NASA sent Explorer 80 into space, two years later renamed WMAP, Wilkinson Microwave Anisotropy Probe. Its equipment allowed recording temperature fluctuations of microwave background radiation with an angular resolution of less than three tenths of a degree. Then it was already known that the spectrum of this radiation almost completely coincides with the spectrum of an ideal black body heated to 2.725 K, and its temperature fluctuations for “coarse” measurements with an angular resolution of 10 degrees do not exceed 0.000036 K. However, on the “fine-grained” scale of the WMAP probe, the amplitudes of such fluctuations were six times larger (about 0.0002 K). The relic radiation turned out to be spotted, closely spaced with slightly more and slightly less heated sections.

Fluctuations of the relic radiation are generated by fluctuations in the density of the electron-photon gas, which once filled outer space. It fell almost to zero approximately 380,000 years after the Big Bang, when virtually all free electrons connected to the nuclei of hydrogen, helium and lithium, and thereby laid the foundation for neutral atoms. Until this happened, acoustic waves propagated in the electron-photon gas, which were influenced by the gravitational fields of particles of dark matter. These waves, or, as astrophysicists say, acoustic oscillations, have left an imprint on the relict radiation spectrum. This spectrum can be deciphered using the theoretical apparatus of cosmology and magnetic hydrodynamics, which makes it possible to re-evaluate the age of the universe in a new way. As the latest calculations show, its most probable length is 13.72 billion years. It is now considered the standard estimate of the lifetime of the universe. If we take into account all possible inaccuracies, tolerances and approximations, we can conclude that, according to the results of the WMAP probe, the universe exists from 13.5 to 14 billion years.

Thus, astronomers, estimating the age of the universe in three different ways, obtained quite compatible results. Therefore, now we know (or, to put it more cautiously, we think we know) when our universe was created – at least, to within a few hundred million years. Probably, the descendants will make the decision of this age-old puzzle in the list of the most remarkable achievements of astronomy and astrophysics.

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