The Big Bang theory explains the evolution of space-time, matter, and energy since the Planck era. It accounts for the hydrogen, most of the helium, and some of the lithium that exist today. But how do we explain the existence of superclusters of galaxies, clusters of galaxies, and individual galaxies?

Had the universe been perfectly uniform until the era of recombination (that is, everywhere perfectly homogeneous and isotropic), it would have been impossible for stars, galaxies, and larger groupings of matter to form, pulled together as they were by differences in mass density and, hence, gravitational attraction. Quantum theory is believed to have played a vital role during the inflationary epoch in creating the clumpiness that we observe today as superclusters of galaxies. In 1927, the British physicist Paul A. M. Dirac concluded that the laws of nature allow pairs of particles to spontaneously appear, provided that they are identical to each other except for having opposite electric charges, and that they annihilate each other and disappear within a very short period of time, typically 10^-21 seconds. As discussed in Section 12-18, these pairs are called virtual particles.

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Normally, quantum fluctuations (the spontaneous formation of pairs of virtual particles) occur on the size scales of pairs of particles. However, the quantum fluctuations on microscopic scales that were occurring during the inflationary period were stretched by the inflation until they were large enough to affect the spacetime around them. Those particles created tiny regions of slightly higher temperature, density, and pressure in the inflating universe, like little firecrackers going off all over the place. Also like firecrackers, those events sent out sound waves that traveled through the rapidly expanding universe. These sound waves are believed to have led to the formation of superclusters of galaxies. Here is how that may have taken place.

Whereas light waves, like ocean waves, change amplitude perpendicular to the direction they travel, sound waves change the density of the material through which they move in the direction of their motion. In other words, sound waves are waves that compress and rarefy (make thinner) the medium through which they travel. The rate at which these compressions and rarefactions in the air enter your ears each second, for example, determines the pitches of the sounds you hear. As quantum fluctuations stretched out in the early universe, they compressed the surrounding medium, sending out spherical sound waves, which traveled as long as the gas in the young universe could support them. The speed of these sound waves was impressive by everyday standards. Sound in our atmosphere travels at about 350 m/s (1100 ft/s). The sound waves generated in the inflating universe traveled at 1.75 × 10⁵ m/s (5.75 × 10⁵ ft/s).

The sound waves created at the end of inflation in the early universe traveled out through the plasma of the universe until decoupling, 380,000 years later. At that time, when photons no longer scattered quickly off the gas in the universe, the waves stopped moving.

Because those sound waves were compression and rarefactions of the gas in the young universe, when they stopped traveling, they established shells of higher-density gas (compressions) and interior regions of lower density (rarefactions). The slightly denser and slightly more rarefied regions of the universe that existed at decoupling were also slightly warmer and slightly cooler, respectively, than the average gas in the cosmos. Although those differences in temperature were incredibly small, they have been detected by careful analysis of the data from the COBE and WMAP satellites, which observed the cosmic microwave background temperature at the time of decoupling. Their data (Figure 14-15a) show that these variations in temperature were only about 1 part in 100,000, implying that the average density of matter in the early universe varied by a similar amount. Such tiny differences in density were nevertheless sufficient to start the process of supercluster, cluster, and galaxy formation.

Astrophysicists have plotted the angular sizes of the peaks (warm regions) created by the acoustic (sound) waves, as shown in Figure 14-15b. Their sizes are clustered around 1.2° in angle (Figure 14-15b). In other words, the bubbles of sound generated by the quantum fluctuations spread out to that angular size, as seen from Earth some 13.8 billion light-years away. Allowing for the expansion of the universe since decoupling, those bubbles are about 960 million light-years in diameter today. This size is consistent with predictions of their properties made by the Russian physicist Andrei Sakharov in 1965. Like the sound generated by a violin, there are overtones (higher pitches) in the peaks of the cosmic microwave background. These overtones have different amplitudes. Their relative amplitudes tell us the relative amounts of visible matter and dark matter in the universe, from which we conclude that there is about 5 times as much dark matter as there is visible matter.

We can now outline the scenario for how matter clumped in the early universe. When the universe was radiation-dominated, the particles in it were moving too fast to be pulled together and form structures, like stars, galaxies, clusters, or superclusters. During that time, sound waves from quantum fluctuations created growing shells of higher-density gases, surrounding regions of lower-density gases. Following decoupling, those shells stopped growing. The chemical composition of the early universe was 76% hydrogen and 24% helium, with trace amounts of lithium gas. It took time for the gas to cool sufficiently to start forming stars, a period called the dark ages. (Despite the name of that era, the hydrogen in the dark ages was emitting 21-cm photons, as discussed in Section 13-3. These photons have now been redshifted by the Hubble flow to tens of meters (or yards) in length, which is where astronomers are looking for them.) Gravitational attraction in the gases on the shells ultimately led to the formation of superclusters, clusters of galaxies, and galaxies.

The voids seen throughout the cosmos (see Section 13-14) are the relatively empty spaces surrounded by shells of superclusters. The diameters of some of the voids have been measured. They are all 1 billion light-years or smaller, consistent with the 960 million light-year diameters of the shells on which superclusters formed. That not all voids are the same size comes from the fact that many shells of sound crossed each other before decoupling.

The first generation of stars is called Population III stars. Except for a trace of lithium, they were completely devoid of metals. Computer simulations indicate that many of these stars had masses in the range 100–500 M_⊙. These behemoths were a million times or more as bright as the Sun, and they lived for no more than a few million years. Stars with between 100 and 250 M_⋯ then exploded, while more massive stars collapsed and formed black holes without exploding. The stars in this early cohort that did explode provided the first metals in the universe (other than the primordial lithium). During the first billion years, it appears that clumps of gas that contained millions or billions of solar masses also collapsed to create supermassive black holes. These black holes would have attracted other matter into orbit around them and thereby served as the seeds for the formation and growth of some galaxies. Recent observations reveal that massive and supermassive black holes typically grow until they have about 0.2% of the mass of a disk galaxy’s central bulge.

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If, as is now believed, some of the earliest stars had masses in the range of stars found in our Galaxy, it is possible that the low-mass (red dwarf) ones are still on the main sequence. In that case, some of these stars are still orbiting in galaxies like the Milky Way. Because they are Population III stars, astronomers are looking for them through their spectroscopic signature of having virtually no metal. At least one such star has been located. Analogously, astronomers have discovered two intergalactic clouds of gas that, to the precision of the observations, are composed just of hydrogen, helium, and a trace of lithium. They are remnants of the Big Bang that never became parts of galaxies, nor did they ever undergo star formation, which would have added metals to them.

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By observing remote galaxies, astronomers have discovered that most galaxies initially emitted energy we associate with quasars and other active galaxies. This implies that most galaxies have supermassive black holes at their centers. The earliest quasars dating back to when the universe was only 900 million years old provide information about when the first stars formed. Those quasars have spectral lines created by iron and magnesium. In order for those elements to be in those quasars, the iron and magnesium had to be created in stars that formed 700 million years earlier (then exploded and put the metals into the interstellar medium). These metals were then attracted by the black hole. This scenario means that the first stars existed within 200 million years of the Big Bang (Figure 14-16a), thus defining the end of the dark ages.

By 500 million years after the Big Bang, galaxies were beginning to coalesce (Figure 14-16b). We also have observational evidence that galaxies were in clusters within 2 billion years of the Big Bang (Figure 14-16c).

From about 600 million years to about 2.6 billion years after it formed, the universe underwent heavy star formation. Galaxies grew in size during this period, some continuously, others suddenly and quickly. Star formation decreased as the amount of interstellar hydrogen diminished. One observed galaxy, with about 8 times the mass in stars as the Milky Way has, reached its full star-forming potential when the universe was only about 800 million years old. About 2.6 billion years after the Big Bang there were so many quasars emitting so much energy that much of the gas in the universe became too hot to form new stars, as discussed in Section 13-22. For 500 million years, the star formation rate in galaxies back then decreased. As many quasars became quiescent, the star formation reignited and continued at very high levels until about 6 billion years after the Big Bang.

Observations also reveal that galaxies were bluer and brighter in the past than they are today. These changes in color and brightness suggest a high abundance of young, bright, hot, massive stars in newly formed galaxies (Figure 14-17a). As galaxies age, these blue O and B stars become supergiants and eventually die. Therefore, galaxies grow somewhat redder and dimmer. This is especially true of those elliptical galaxies that formed early on. They appear to form nearly all of their stars in one vigorous burst of activity that lasts for about a billion years (Figure 14-17b), after which star formation diminishes drastically and their massive stars evolve and explode so that today we see in them primarily red, intermediate-mass stars. Astronomers say that such elliptical galaxies are “red and dead.”

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In contrast, spiral galaxies have been forming stars for at least the past 10 billion years, although at a gradually decreasing rate. There is still plenty of interstellar hydrogen in the disks of spiral galaxies, like our Milky Way, to fuel star formation today. That is why O and B stars still highlight their spiral arms. Figure 14-17b compares the rates at which spiral and elliptical galaxies form stars.

Imagine a developing galaxy, called a protogalaxy, forming from a cloud of gas. Theory proposes that the rate of star formation determines whether this protogalaxy becomes a spiral or an elliptical galaxy. If stars form slowly enough, then the gas surrounding them has plenty of time to settle by collision with other infalling gas into a flattened disk, just like the early solar system. Star formation continues because the protogalactic disk contains an ample supply of hydrogen, and a spiral or lenticular (disk-shaped but without spiral arms) galaxy is created. If, however, the initial stellar birthrate is high in the protogalaxy, the theory predicts that virtually all pregalactic gas is used up in the creation of stars before a disk can form. In this case, an elliptical galaxy is created. Figure 14-18 depicts these contrasting sequences of events.

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One intriguing nearby galaxy, NGC 1277, apparently didn’t play by the normal rules of galactic evolution. It is a lenticular galaxy: It has a disk, but no spiral arms, with a 17 billion M_⊙ supermassive black hole in its nucleus. Observations suggest that it formed very rapidly (over a 100 million year period) early in the life of the universe and has not changed since then. Normally, such massive black holes take much longer to form. Other lenticular galaxies evolve due to gravitational interactions with neighboring galaxies, but not NGC 1277. Understanding both the stable lenticular structure and the rapidly formed, extremely massive central black hole are works in progress.

Not all galaxies maintain their initial structure over the evolution of the universe. As we discussed in Section 13-14, some galaxies collide, and these collisions can change a galaxy’s structure from spiral, for example, to elliptical. Indeed, astronomers who use extremely long exposures, called Hubble Deep Field images (for example, see the inset on Figure 14-11), have observed elliptical galaxies during the first few billion years of the universe’s existence that are far less uniform in color than are closer ellipticals. They observed blue stars in the young ellipticals consistent with the merger of spirals and a resulting burst of star formation, as well as lots of lower-mass (yellow and red) stars. Our understanding of galactic formation and evolution is far from complete.

Determining the location and nature of the dark matter in the universe (see Chapter 13) is still of paramount importance in understanding the cosmos. The observable stars, gas, and dust in a galaxy or cluster of galaxies account for only about 10% to 20% of each object’s mass. (Recall that this observable matter does not have enough mass to hold galaxies or clusters of galaxies together.) We have very little idea of what the composition of the remaining 80% to 90% of each galaxy or cluster of galaxies is. However, some progress in understanding the distribution of this dark matter is being made. In 2002, astronomers who map large numbers of galaxies and use computer simulations of the effects of dark matter determined that on the scales of clusters of galaxies, the locations of the galaxies often coincide with the concentrations of dark matter, while voids between clusters coincide with voids of dark matter. It is likely that the gravitational force of the dark matter caused gas to concentrate in the same regions, thereby stimulating formation of supermassive black holes and galaxies. In 2007, using images of galaxies that were focused by a gravitational lens of dark matter, astronomers were able to plot the locations of this dark matter (Figure 14-19).