The Structure of the Universe: Its Organization in Space and Time

The structure of the universe: its organization in space and time

Domingos Soares

Four hundred years ago, more precisely, in 1638, the Italian astronomer Galileo Galilei (1564-1642) put forward, for the ﬁrst time in the history of science, a method to measure the speed of light. The method is described in his book entitled “Discorsi e dimonstrazioni matematiche intorno a due nuove scienze, attinenti alla meccanica e i movimenti locali”, or, in short, “Discourse on the two new sciences, mechanics and kinematics”. An assistant is positioned at the peak of a mountain, some kilometers away, with a torchlight covered by a cloth. Galileo, also with a torchlight, in the same way, would, then, uncover his torchlight, and the assistant, as soon as he saw the light, would uncover his. Galileo would measure the time elapsed during the round trip of the light rays with a water clock. Actually, Galileo performed the experience and veriﬁed that the time was very small to be measured. His conclusion was simply that “if the light does not spread instantly, it is extraordinarily fast”. Today, we know that the light travels at the amazing speed of 300,000 km in each second! It is not surprising that Galileo could not measure the desired time interval! He should be able to measure some hundredths of thousandths of a second to reach his goal. Something that, even nowadays, is not an easy task. . . The distance traveled by light in one second is, therefore, very large. It corresponds almost to the distance from the Earth to the Moon. Light needs 500 seconds to come from the Sun to the Earth, and about 5 hours to travel from the Sun to the remote Pluto, which sits in the periphery of our planetary system. Light is, thus, quite useful for our purposes of investigating the structure of the universe. The goal is to know a little about the size of the universe, both in space and time. When we see something that is very far away, we

1 also see it as it was in the past. The Moon that we see now is the Moon of 1 second ago. The Sun that shines on the sky is the Sun 500 seconds ago, and so forth. The organization of the universe is revealed by the way galaxies are distributed in space. The Milky Way galaxy, also called the Galaxy, is our home. It is the site from where we observe the structure of the universe. The Galaxy is part of a small ensemble of galaxies, gravitationally bound to each other, which are distributed in a space of 3 million light-years! The universe is really big, we have not left home yet and have gone from the light- seconds to the millions of light-years. The light of the most remote galaxy in our immediate neighborhood departed towards us at least 2 million years ago! Galaxies, generally speaking, present themselves in pairs, in small and large groups with three to a dozen galaxies, and in enormous clusters formed by hundreds and even thousands of galaxies. The galaxies in a cluster are distributed in regions of 10 to 20 million light-years in diameter.

2 Pair of galaxies gravitationally bound. They are at 280 million light-years from us. The image was obtained in March 1991 at the Pico dos Dias Observatory (PDO), located in Brazpolis, Minas Gerais, Brazil. A 0.2 Megapixel CCD detector was used. The image has artiﬁcial colors. The catalog name of the larger galaxy is ESO-LV5100560. It is a spiral seen head-on, morphological type Sb. The other galaxy, ESO-LV5100550, is a barred spiral galaxy, type SBa, and is seen edge-on. There are many galaxy pairs like this one (Image: Domingos Soares and Paulo M. V. Veiga).

Groups of galaxies are constituted by galaxies that are bound to each other by their mutual gravitational attraction. In some groups, the galaxies are very close to each other, like in the Stephan’s Quintet, shown in the ﬁgure. These groups are called “compact groups”. In them, the separation between the galaxies is approximately equal to the average size of the galaxies that form them. Compact groups are important for the study of the evolution of galaxies because the group members tend to merge. Some of the galaxies in compact groups show the original characteristics of galaxies that have merged. This suggests that there is a probability that new mergers occur.

3 Some elliptical galaxies originated from the merging of two spiral galaxies. This mechanism for the formation of elliptical galaxies is not the only one, though. Astronomers observe that some ellipticals were already “born” with their typical features, not being in these cases formed by the merging of two — or more — galaxies. There are also groups in which the galaxies are further apart from each other, when compared to compact groups. In general, a catalog of galaxy groups has representatives of both types of groups.

Group of galaxies called “Stephan’s Quintet”, named after its discoverer, the French astronomer Edouard´ Stephan (1837-1923). The group is located in the constellation Pegasus (Winged Horse). There are controversies to this day whether or not the largest galaxy seen in the image belongs to the group. The sup- porters of the standard cosmological theory believe that it is a galaxy much closer to us, accidentally seen in the direction of the ﬁeld where the rest of the group is located, not being, for that reason, a member of the group (Image: N.A.Sharp/NOAO/AURA/NSF).

The American astronomer George Abell (1927-1983) was the ﬁrst to determine a catalog of galaxy clusters. The catalog was originally published

4 in 1958 as part of his doctoral thesis in astronomy. The Abell catalog has 2,712 clusters, observed on the northern hemisphere sky. Later, in 1989, it was supplemented, using the same criteria for the definition of a cluster, with further 1,361 clusters, observed on the southern hemisphere sky. The whole Abell catalog has, therefore, 4,073 clusters of galaxies. One of Abell’s criteria was that a cluster should have at least 50 galaxies. Another crite- rion was that these galaxies must be distributed in a predefined interval of apparent brightness: the third brightest galaxy should be, at most, about 10 times as bright as the fainter galaxy. Furthermore, the galaxies must be between 300 million and 3 billion light-years away. These distances imply on a maximum and minimum apparent brightnesses of the galaxies listed in the catalog. Abell discovered the clusters by examining sky photographs obtained at the Mount Palomar Observatory, located in the state of Cali- fornia, United States. His work was extremely meticulous and he examined the photographs over and over, with the aid of a 3.5 times magnifying glass. As we can see, the scientific work may become, in many cases, strenuous. In George Abell’s case it was worth the work, because his catalog is still of great use today in the astronomical research. The cluster shown in the figure is Abell 2151, i.e., the cluster of number 2151 in the complete Abell list. The Abell catalog, described above, was the forerunner of countless other later catalogs of clusters. Nowadays, astronomers have many catalogs of clusters at his disposal. The Local Group of galaxies, to which the Galaxy belongs, is part of a cluster of galaxies, the so-called “Virgo cluster”. Its name comes from the name of the Virgo constellation, the region of the sky where it is located. It is always instructive to remember that a constellation is an arbitrary arrange- ment of stars of our own galaxy, that serves to delimit an area of the sky. The Virgo cluster is located on this region, but the majority of its galaxies are at millions of light-years from us, and are seen in projection over the Virgo constellation. The Galaxy sits in the periphery of the cluster, whose central region can be perfectly observed with low magnification telescopes. Most of the Virgo cluster galaxies, those that are at the central regions, are giant elliptical galaxies. This cluster is “older” than the Hercules cluster, which is illustrated here. In general, the clusters that are rich in spiral galaxies are younger than those rich in elliptical galaxies. Several giant ellipticals of the Virgo cluster were cataloged by the famous “comet hunter”, the French astronomer Charles Messier (1730-1817), and are designated by the letter

5 “M” followed by an order number. They were included in the catalog with the goal of not being mistaken with comets. The most famous giant elliptical galaxy of the Virgo cluster is M87. It is truly a cosmic “monster”! It is a very strong source of radio waves, X-rays, and has a jet of relativistic particles — that is, which move with velocities close to the speed of light —, that emerges from its center at opposite directions. The clusters of galaxies, by their turn, are gravitationally bound in larger groups called “superclusters”. There exists some catalogs of superclusters, each one with dozens of galaxy clusters. The superclusters extend over vast- nesses of 50 to 300 million light-years. The pioneering work in the discovery of the superclusters was due to a French astronomer, born in Paris. In 1953, even before the first catalog of galaxies, Gérardde Vaucouleurs (1918-1995) discovered that the Virgo cluster is approximately located at the center of a great group of galaxies — a supercluster — about 50 million light-years wide. He named it “Local Supercluster”. It is a great flattened structure, with the approximate shape of a “fat” pancake. The Local Group — and the Galaxy — sit at the periphery of the Virgo cluster, and also at the periphery of the Local Supercluster structure.

6 Galaxy cluster located in the Hercules constellation. It was cataloged by the American astronomer George Abell (1927-1983), in 1958, with the number 2151, hence its name Abell 2151. Abell’s catalog has 4,073 clusters. The total area of the image is about half of the full Moon area on the sky. The cluster has tens of galaxies, the majority of them of the spiral type. A little above the center of the image, one may notice a pair of spiral galaxies in gravitational interaction. This cluster is located at hundreds of millions of light-years from us, being relatively close (Image: Victor Andersen/Kitt Peak National Observatory, United States).

7 Abell himself used his 1958 galaxy cluster catalog to determine a catalog of superclusters, which was published in 1961. The hierarchy of the distribution of galaxies seems to end in the dimen- sions of superclusters. But it can also be just a reﬂection of our inability of observing further away, in space and time. Or, alternatively, if the standard model of cosmology — the science of the universe — is correct, when we see very far, we are going to approach a cosmic epoch — the time — when the galaxies themselves did not exist yet!