Laniakia: Our exact address in the universe

It turns out that the Milky Way is part of a huge supercluster of galaxies that forms one of the largest structures known in the universe. This discovery is just the beginning of a new effort to map the cosmos

By Noam Y. Libeskind וRabbi Brent Talley, The article is published with the approval of Scientific American Israel and the Ort Israel network

• Similar to how stars clump together to form star clusters and galaxies, galaxies themselves gather into clusters, and galactic clusters group into superclusters.
• These galactic superclusters are the building blocks of massive filaments, sheets, and voids that make up the largest measurable structures in the universe.
• Recent studies of the motions of thousands of nearby galaxies have shown that the Milky Way's home supercluster is much larger than previously thought. Astronomers call this recently discovered giant supercluster "Laniakia".
• A more detailed mapping of Laniakia and its nearby superclusters could reveal new details about the formation of galaxies and help researchers solve the pair of cosmological mysteries, dark matter and dark energy.

Imagine visiting a galaxy far away and sending a postcard to your relatives back home. Recipient address Orion arm, a section of a spiral arm in the suburbs of the Milky Way, after which the location of the Milky Way will come inthe local group”, a collection of more than 50 nearby galaxies spanning about seven million light-years of space. The local group, in turn, is located on the outskirts the virgin cluster, a cluster whose center is 50 million light-years away from us and includes more than 1,000 galaxies. The Virgin cluster itself is a small part of the machine supercluster "The local supercluster", a collection of hundreds of galaxy clusters spread over more than 100 million light years. Such superclusters are thought to be the largest components of the large-scale structures of the universe, and form vast sheets and fibers (called galactic filaments) of galaxies, surrounding spaces where there are almost no galaxies at all.

Until recently, the local supercluster was the last line in your cosmic address. It was reasoned that beyond this scale, further guidance would be meaningless because the boundary between the voids and the sharp structure of the galactic sheets, decorated with supercluster lace, gives way to another region of the universe, a uniform realm devoid of larger distinct features. However, in 2014, one of us (Brent Talley) headed a team that discovered that we are part of such a large structure that he shattered that view to pieces. The local supercluster, it turns out, is but one lobe of a much larger supercluster, a collection of 100,000 large galaxies spanning 400 million light years. The team that discovered this monstrous supercluster called it "to Nyakea", a word in the Hawaiian language that means "unparalleled paradise", in honor of the ancient Polynesians who navigated the vast expanses of the Pacific Ocean with the help of the stars. The Milky Way is located far from the center of Laniakia, in its outermost book regions.

Lanyakya is more than another line in our cosmic inscription. If we study the architecture and dynamics of this vast structure, we can learn more about the universe's past and its future. Mapping the galaxies that make it up and how they behave could help us gain a better understanding of the way galaxies form and grow, and at the same time also tell us more about the nature of dark matter, the invisible substance that astronomers believe is responsible for about 80% of the universe's composition .

Nyakea may also be able to dispel the fog surrounding dark energy, a powerful force discovered only in 1998 that somehow drives the acceleration of the universe's expansion and thus shapes the ultimate fate of the cosmos. And it is also possible that the supercluster will not actually be the last line in our cosmic inscription, in fact, it may be part of an even larger structure that has yet to be discovered.

Exploring mysteries with the help of galactic currents

The team that discovered Laniakia didn't exactly go out looking for her. In fact, Nicea emerged from strenuous attempts to answer stubborn fundamental questions about the nature of the universe.

Scientists have known for almost a century that the cosmos is expanding and pushing galaxies apart like dots on the surface of an inflated balloon. But in recent decades it has become clear that most galaxies are not separating from each other as quickly as we would expect to see if expansion were the only force acting on them. Another, more localized force is also at work: gravitational pulls exerted by adjacent accumulations of matter on each other. These gravitational pulls may act against the flow of the galaxy as part of the expansion of the universe. The difference between the speed of movement of a certain galaxy caused by cosmic expansion and its movement as a result of gravity in its local environment is called "Unique speed".

If we take all the stars in all the galaxies we see and add to them the gas and the extra normal matter we know about, it still won't be enough to explain the gravitational origins of the observed singular velocities: the amount of matter will be an order of magnitude less than what is required. In our ignorance, we astronomers call what is missing "dark matter". We assume that this dark matter consists of particles whose interactions with the rest of the universe are carried out almost exclusively through gravity and not through other forces such as the electromagnetic force, and that it is the dark matter that activates the "missing" gravitational force required to explain the observed velocities. The scientists believe that the galaxies sit in deep "pools" of dark matter, meaning that the dark matter serves as invisible scaffolding around which galaxies form.

Talley's group and others realized that if we mapped the galactic currents and singular velocities we could see the hidden cosmic distribution of dark matter, revealing the largest concentrations of this mysterious substance through their gravitational influence on the motions of galaxies. If, for example, it seems that all the streams of galaxies in a certain region flow towards a certain point, it can be assumed that the galaxies are drawn to that point by the force of gravity that activates a highly compressed region of matter.

They also realized that if we could accurately determine the density and distribution of all types of matter in the universe, this would help us solve another, deeper mystery: the fact that the cosmos is not only expanding, but is also doing so at an accelerated rate. This behavior is as contrary to common sense as a situation where a stone thrown upwards will begin to accelerate further towards the sky instead of falling back to the earth. The thing behind this strange phenomenon is called "dark energy" and it has fateful consequences for the future of the universe. The accelerated expansion raises the possibility that the end of the cosmos will be a frozen death, where most galaxies will race away from each other at ever-increasing speeds until a final darkness will descend on the universe, when every star in every galaxy will die and all matter will cool to absolute zero. But in order to know with certainty what the end of the universe will be, we must not only determine what dark energy actually is, but also how much matter there is in the universe: given a high enough matter density, our universe may in the distant future turn its expansion into a collapse in on itself as a result of the self-gravity of the total his mass. Or maybe it has a balanced critical density of matter that would lead to infinite expansion that slows down forever.

It was this mapping of the galactic currents, with the aim of mapping the cosmic density of normal matter and dark matter, that ultimately led to the removal of the lot above Laniakia.

Discovery of Nyakeya

Mapping galactic currents requires us to know two things: the speed of the galaxy's movement resulting from the cosmic expansion and the speed resulting from the gravitational force of nearby matter. In the first step, astronomers measure the The shift to red of the galaxy: the stretching of the light emitted by the galaxy as it recedes from us through the expanding universe. The sound of a whistle or the sound of a horn approaching us sounds louder than their sound moving away from us, because their sound waves are compressed into higher frequencies and shorter wavelengths. Similarly, light waves from a galaxy moving away from us undergo a shift to lower frequencies and longer, redder wavelengths. And the faster they recede, the more red their light shifts. Thus, the redshift of a galaxy's light gives astronomers a measure of its overall speed and a rough estimate of its distance from us.

Astronomers can infer what fraction of a galaxy's velocity is the result of local gravitational pull by measuring its distance from us using methods other than measuring redshift. For example, based on strict estimates of the expansion rate of the universe, the speed of a galaxy measured to be 3.25 million light years away from us should be about 70 kilometers per second. If instead the galaxy's redshift dictates a speed of 60 kilometers per second, astronomers can conclude that the concentrations of matter near the galaxy give it an additional speed of 10 kilometers per second. The methods used to provide a redshift-independent distance measurement rely primarily on the fact that light intensity decreases as the square of the distance from the light source. That is, if we see two lighthouses that are identical to each other, but the light of one seems four times weaker than the light of the other, we can know that the dimmer lighthouse is twice as far away. In astronomy, such identical lighthouses are called standard candles, astrophysical bodies that will always shine with the same brightness, no matter where they are in the universe. Examples of such bodies include certain types of exploding or pulsating stars, or even massive galaxies as first proposed by Talley and astronomer G. Richard Fisher in 1977. Tully-Fisher connection It relies on the fact that massive galaxies are both brighter and spin faster than small galaxies because they have more stars that they must spin faster to maintain stability in their stronger gravitational fields. If we measure the rotation rate of a galaxy, we can know the rate of light emission (theLuminosity") her self; We will compare this luminosity to the observed luminosity and we can conclude what its distance is.

Each distinct standard candle has its own range, in which it operates at its best. in the pulsating stars calledCepheid variables” can only be properly observed if the galaxies are close to the Milky Way, so they are not suitable for large-scale measurements. Many spiral galaxies can be used in the Tully-Fisher relation, but the distance estimate it yields has uncertainties of up to 20%. Exploding stars called Type Ia supernovae yield measurements with less than half the uncertainty and the glow they spread across great cosmic distances, but such supernovae are rare, occurring only once a century in a galaxy of significant size.

"Galaxies flow in currents, swirl in eddies and swim in pools and thus reveal the structure, dynamics, origins and future fates of the largest accumulations of matter in the universe."

If it is possible to determine the unique velocities of a large sample of galaxies in the universe, astronomers will be able to map the galactic streams on the largest scales. On these vast scales, the flow of galaxies can be likened to rivers meandering through what we call cosmic drainage basins, their movements determined by gravitational pulls from nearby structures, rather than topography. In these "cosmographic" maps, the galaxies flow in currents, swirl in eddies and clear in pools and thus indirectly reveal the structure, dynamics, origins and future fates of the largest accumulations of matter in the universe [see box].

In order for us to create a map at the scale necessary to answer our questions about dark matter and dark energy, we must catalog all the best available data from many observational projects. In 2008, Talley published, Alain M. Courtois who currently works at the Institute of Nuclear Physics in the city of Lyon in France, and their colleagues the The catalog of cosmic currents, where several datasets were collected and compared to detail the dynamics of 1,800 galaxies within 130 million light years of the Milky Way. The team expanded its efforts in 2013 and released the Cosmic Stream Catalog 2, which maps the motions of about 8,000 galaxies in a volume of about 650 million light years. one of the group members Yehuda Hoffman from the Hebrew University in Jerusalem, developed methods for the precise derivation of the distribution of dark matter from the unique velocities of cosmic stream data.

As the catalog evolved, we were amazed to discover an unexpected pattern hidden in the mountains of data: outlines of a new cosmic structure unseen before. Clusters of galaxies spanning more than 400 million light-years, all moved together within a local "basin of attraction" corresponding to a watershed at the lowest point of a landscape's topography. Were it not for the constant expansion of the universe, these galaxies would eventually collapse and become a tight structure held together by gravity. The vast swarm of these galaxies, all together, makes up the Laniakia supercluster.

So far, the studies of the motions of the Laniakia galaxies show that they behave exactly as we would expect from the leading models of the cosmic distribution of dark matter. Although we cannot see it, we can predict with reasonable accuracy where this invisible matter of the universe is accumulating. Moreover, the total density of visible and dark matter in Laniakia shows that, just as dark energy theorists believe, the fate of the universe, for better or worse, is indeed a frozen death due to an eternally accelerating expansion.

These conclusions are not yet conclusive. The daunting task of mapping galactic currents is still in its infancy. Currently, only for 20% of the galaxies within 400 million light years has the unique velocity also been calculated, and many standard candles for measuring distance still have large uncertainties. And yet, the emerging map of our galactic neighborhood gives us a new appreciation of our place in the cosmographic basins and the expanses of the universe.

Our cosmographic context

Let's take a tour of the flowing and flowing components of Laniakia, our recently discovered home. Let's start with the most familiar part: you. No matter how fast or slow you are moving on Earth as you read this article, you are orbiting the Sun along with the rest of our planet at about 30 kilometers per second. The Sun, in turn, orbits the galactic center at a speed of about 200 kilometers per second, and the entire local group, including the Milky Way, rushes towards a mysterious concentration of mass in the direction Centaurus At a speed greater than 600 kilometers per second (more on that later.) You probably never imagined that you could move so fast while reading an article, or even without doing anything at all.

Now we will look away from the Milky Way, and begin our journey around Laniakia with two dwarf galaxies, two Magellanic clouds, which are "only" at a distance of 180,000 to 200,000 light years from us. You can catch a glimpse of the Magellanic Clouds from the Southern Hemisphere, but to get the best vantage point you have to travel all the way to Antarctica, during the winter. The only other galaxy that can be seen with the naked eye is the giant spiral galaxy Andromeda, even though it looks like nothing more than a blurry speck in a very dark sky.

Andromeda is located at a distance of two and a half million light years from us, and it is rushing towards us at a unique speed of about 110 kilometers per second. In about four billion years it will collide in a head-on collision with the Milky Way, turning the two galaxies into a single, featureless ellipsoid composed of red, old stars. Our solar system will probably not be affected by this cosmic head-on accident because the distance between the stars is too great and therefore, it is unlikely that two stars will be close enough to each other to collide. The Milky Way, Andromeda and about 50 other galaxies belong to the local group, a region where gravity has won the fight against the cosmic expansion and therefore a collapse occurs there. Like the Milky Way itself, with its Magellanic Clouds, all these large galaxies have their own retinue of dwarf galaxies.

Just beyond the local group, in areas of volume of about 25 million light years, three distinct features appear on our maps. Most of the galaxies here, including our own, reside in a place with an uninspiring name."The local sheet". From the word "sheet" we can learn that it is very thin: most of its galaxies are within a range of three million light years from this structure, which serves as the equatorial plane of what is known as the supergalactic coordinate system. Below this plane, after a gap, there is a galactic filament, called Arm to the left, and also galaxies in the so-called locations Antalya cloud and cloud dorados. Above the plain, next to it, mostly there is nothing. This emptiness is the realm of the "local space".

If only the galaxies in the local sheet are taken into account, the situation looks very peaceful. These galaxies are moving away from each other at the rate of cosmic expansion, with only small unique velocities caused by local interactions. Beneath the local sheet, the Antalya and Dorados Cloud and Leo Arm galaxies also have small peculiar velocities. However, they approach the local sheet at high speed. The culprit of this behavior is apparently the local space. Spaces expand like inflating balloons, and matter moves from areas of low density to areas of high density and accumulates at their boundaries. According to our assessment today, the local sheet is a wall of the local space and this space is spreading and pushing us downwards, towards Antalya, Dorados and Lao.

If we continue to zoom out, we will come across the Virgo cluster, which has as many galaxies as 300 local groups crammed into a volume 13 million light-years in diameter. These galaxies fly back and forth at typical speeds of 700 kilometers per second, and every galaxy within 25 million light years outside the cluster falls in and will become part of it within 10 billion years. The full extent of the Virgin Kingdom, the territory it will eventually conquer, currently spans a radius of 35 million light years. It is interesting to note that our Milky Way, 50 million light years away, lies just outside the occupied zone.

The Great Galactic Stream

The larger area around the Virgo Cluster, which reaches all the way to our place, is called the Local Supercluster. Almost 30 years ago, a group of astronomers who were amusingly nicknamed the "Seven Samurai" discovered that not only the Milky Way was moving at hundreds of kilometers per second towards Centaurus, but the entire supercluster. They named the mysterious mass that pulls all these galaxies together as “The big puller". In many ways, the Great Attractor is not so mysterious: it is clear that the density of matter in this direction of the universe is high because it contains seven clusters the size of the Virgo cluster, and which lie within a sphere 100 million light-years in diameter. Three of the largest clusters are called Norma, Centaurus and Hydra.

In keeping with the way we think of superclusters as cosmic drainage basins, a view that delineates their boundaries based on the drifting motions of galaxies, the name "local supercluster" is inappropriate. It is only a part of something bigger: Laniakia, which folds into it other large structures such as for example the Pabo-Indus gyrus and the Ophiuchus cluster. If we imagine Laniakia as a city, then the congested city center would be the area of ​​great attraction. As with many urban centers, it is difficult to diagnose an exact center, but we can roughly say that it is located somewhere between the Norma and Centaurus clusters. This location means that our Milky Way is far out in the suburbs, near the borders of the nearby supercluster, called Perseus-Pisces. This boundary is so relatively close in cosmic terms that we can study it in detail to define the fuzzy, circle-like boundary of Laniakia, which is half a billion light-years in diameter. In total, the boundaries of Laniakia embrace a total mass of normal matter and dark matter weighing about one hundred million billion suns.

Astronomers have been getting glimpses of the outlines of what might reside beyond Laniakea for decades. Shortly after the discovery of the Great Attractor by the Seven Samurai, something greater emerged from the intergalactic nebulae. Just behind the large attractor region, but three times as far away, there is a monstrous accumulation of clusters, the densest we know of in the local universe. Because the astronomer Harlow Shaifley He was the first to identify evidence for the existence of this huge and distant structure in the 30s, he called it the Scheifelli Supercluster. (Incidentally, just like the local sheet, the Virgo cluster, and the main bar of the local supercluster, all lie on the supergalactic equator. Imagine a huge blob of galactic superclusters, and you'll get a good picture of our local environment on a large scale.)

So what makes our local supercluster move at a unique speed of 600 kilometers per second? To some extent, the culprit is the large attractor complex. But we must also take into account the gravitational pull of the Scheifelli supercluster, which is three times more distant but has four times more rich clusters. However, according to the Cosmic Currents Guide 2, the catalog that revealed Laniakia, the last word has not yet been said. The unique velocities of the 8,000 galaxies in this catalog dictate a uniform flow toward the Scheifelli supercluster. This flow embraces the entire volume surrounded by the catalog of cosmic currents 2, 1.4 billion light years from end to end. Only larger surveys, sampling even larger chunks of the universe, will be able to reveal the ultimate source and structure behind the massive flow of galaxies in our local universe.