Space is like a sponge; long, shining filaments of thousands and millions of galaxies alternate with voids - black holes in which there are much fewer star clusters than the average. True, no one is allowed to see the Universe like this: no matter where the observer is located, the scattering of stars and galaxies will seem to be the inner surface of the sphere, in the center of which the beholder stands.
Astronomers in ancient times and up to the beginning of the 20th century seemed to have a flat sky: they knew how to determine the distance only to the closest astronomical objects - the Sun, Moon, planets of the Solar system and their large satellites; everything else was unattainable far away - so far away that there was no point in talking about what was closer and what was next. Only at the beginning of the 20th century, deep space began to acquire volume: new ways of measuring distances to distant stars appeared - and we learned that in addition to our galaxy, there are also countless star clusters. And by the end of the century, humanity discovered that its native galaxy is circling in one of the gaps between the filaments of the stellar "sponge" - in a place that is very empty even by cosmic standards.
From plane to volume
The human eye can distinguish a distant object from a near one only if these objects are not too far from the observer. A tree growing nearby and a mountain on the horizon; a person standing in line in front of the beholder - and a hundred people from him. Binocularity allows us to understand what is far and what is close (with one eye this can also be done, but with less accuracy) and the ability of the brain to evaluate parallax - the change in the apparent position of an object relative to a distant background.
When we look at the stars, all these tricks are useless. With a powerful telescope, you can estimate the distance to the stars closest to the Sun using parallax, but this is where our capabilities end. The maximum achievable with this method was achieved in 2007 by the Hipparcos satellite telescope, which measured the distance up to a million stars in the vicinity of the Sun. But if parallax is your only weapon, then anything beyond a few hundred thousand parsecs remains points on the inner surface of the sphere. Rather, it remained - until the twenties of the last century.
The Millenium simulation calculates 10 billion particles in a cube with an edge of about 2 billion light years. For its first launch in 2005, preliminary data from the WMAP mission, which studied the relic radiation of the Big Bang, were used. After 2009, when the Planck Space Observatory clarified the parameters of the CMB, the simulation was repeatedly restarted, each time it took a month for the Max Planck Society's supercomputer to run. The simulation showed the formation of galaxies and their distribution - the appearance of clusters of galaxies and voids between them.
Where in the space "sponge" is the Milky Way?
The Milky Way Galaxy is located 700 thousand parsecs from the nearest large galaxy - Andromeda - and together with the Triangulum galaxy and fifty dwarf satellite galaxies makes up the Local Group of Galaxies. The Local Group, along with a dozen other groups, is part of the Local Leaf - a galactic filament, part of the Local Supercluster of Galaxies (supercluster), otherwise known as the Virgo Supercluster; besides ours, there are about a thousand large galaxies in it. Virgo, in turn, is part of the Laniakei supercluster, which already contains about 100 thousand galaxies. Laniakea's closest neighbors are the Hair of Veronica supercluster, the Perseus-Pisces supercluster, the Hercules supercluster, the Leo cluster, and others. The closest piece of cosmic void to us, the Local Entrance, is on the other side of the Milky Way, which is not facing the Local Leaf. From the Sun to the center of the Local Void, it is about 23 Mpc, and its diameter is about 60 Mpc, or 195 million light years. And this is a drop in the ocean compared to the truly Great Void that possibly surrounds us.
In 2013, a group of astronomers came to the conclusion that the Milky Way, and with it the nearest galaxies - most of Laniakea - are located in the middle of a truly giant void about 1.5 billion light years long. Scientists have compared the amount of radiation reaching the Earth from nearby galaxies and from distant corners of the universe. The picture looked as if humanity lives on the far outskirts of a metropolis: the glow over a large city illuminates the night sky more than the light of windows in houses nearby. The giant area of relative emptiness was called the KVS void - after the first (Latin) letters of the names of the authors of the study, Ryan Keenan, Amy Barger and Lennox Cowie.
Void PIC is still the subject of debate in the astronomer community. Its existence would solve some fundamental problems. Recall that a void is not a void, but a region in which the density of galaxies is 15-50% lower than the average in the Universe. If the KBC void does exist, then this low density would explain the discrepancy between the values of the Hubble constant (characterizing the rate of expansion of the Universe) obtained with the help of Cepheids and through the cosmic microwave background radiation. This discrepancy is one of the most difficult problems of modern astrophysics, because in theory the Hubble constant, like any other constant, should not change depending on the method of measurement. If the Milky Way is in a giant void, then the relic radiation on the way to the Earth meets much less matter than the average in space; correcting for this,you can reconcile experimental data and accurately measure the expansion rate of the universe.
Theories of the origin of galactic superclusters and voids
Immediately after the discovery of superclusters of galaxies and voids, scientists wondered about their origin - and from the very beginning it became clear that one cannot do without the invisible mass of the Universe. A spongy structure cannot be a product of normal, baryonic matter, of which our familiar objects and ourselves are composed; according to all calculations, its movement could not lead to the macrostructure observed today during the time that has passed since the Big Bang. Galactic superclusters and voids could only be generated by the redistribution of dark matter, which began much earlier than the first galaxies formed.
However, when the first theory appeared to explain the existence of threads and voids, the Big Bang had not yet been discussed. The Soviet astrophysicist Yakov Zeldovich, who together with Jaan Einasto began studying the macrostructure, made his first calculations within the framework of the concept of dark matter as a neutrino, known as the theory of hot dark matter. Perturbations of dark matter that occurred in the early stages of the existence of the Universe, according to Zeldovich, caused the appearance of a cellular structure ("pancakes"), which later gravitationally attracted baryonic matter and, in a little over thirteen billion years, formed the observed structure of galactic superclusters, filaments and walls and voids between them.
By the mid-1980s, the theory of hot dark matter was abandoned in favor of the theory of cold dark matter. Among other things, it was distinguished from the neutrino theory by the scales at which the primary inhomogeneities arose - smaller and therefore, it would seem, do not explain the existence of the cosmic "sponge" with its elements hundreds of thousands of parsecs long. Over the next two decades, however, astrophysicists have succeeded in reconciling the "pancake" model with the mathematics behind "cold" dark matter.
Modern computer simulations show perfectly how fluctuations in the distribution of dark matter in the young universe gave rise to galactic filaments and voids. The most famous of these simulations, performed in the framework of The Millennium Simulation project in 2005 on a supercomputer at the Leibniz, shows the formation of structures comparable in size to the Laniakei supercluster - the one in which our galaxy rotates.
Anastasia Shartogasheva
