Introduction to The Formation of Galaxies and Other Structures
Wherever in the sky astronomers point radio telescopes, they pick up a hiss of microwaves. What they are detecting is the cosmic background radiation, low-energy electromagnetic radiation that is the “fossil record” of the infant universe. By studying both that radiation record and the present universe, cosmologists have developed an explanation for how the universe's galaxies and other large structures must have formed.
The universe began as a hot, dense “soup” of subatomic particles. As the universe expanded and cooled, it evolved more and more structure, from quarks and electrons to atomic nuclei to atoms, and finally to macrostructure—stars, galaxies, clusters of galaxies, and superclusters. The early stages in the development of structure were driven by the strong and electroweak forces, which operate at subatomic levels. Later, gravity played the dominant role.
Understanding the Evolution of Matter
Researchers have been trying for nearly 30 years to understand how all this happened. In the 1960's, physicists P. J. E. Peebles of Princeton University in New Jersey and the late Yakov B. Zel'dovich of the Soviet Union theorized that the only thing required to get the ball rolling would have been a very slight amount of “lumpiness” in the early universe. By that, they meant small variations in the universe's density—some regions where there was slightly more matter than average and others where there was slightly less.
The regions of higher density would have had more gravitational pull than the regions of lower density and thus would have expanded less rapidly. The matter density in these regions would then have grown, relative to the rest of the universe, further increasing the regions' gravitational pull, slowing their expansion even more and increasing their density even further. In this way, gravity made the universe lumpier and lumpier, doubling the lumpiness every time the universe doubled in size. Ultimately, regions of higher density stopped expanding and formed bound clumps of matter that evolved into stars and other large structures.
The Nature of Dark Matter
Until the 1980's, physicists thought the density variations in the early universe would have had to be at least 0.1 percent for this series of events to get started. That is, if a given volume of space contained an average of 1,000 particles of matter, some regions would contain 1,001 particles and others only 999.
But that estimate was made before researchers became convinced that the great bulk of matter in the universe is unseen dark matter. Many cosmologists now suspect that dark matter is in the form of subatomic particles created in the big bang. If that is the case, then only a 0.001 percent variation in density in the infant universe—100,001 particles in some areas and 99,999 in others—would have been required for gravity to do its work. A smaller variation would be required because in a universe containing dark matter, the process of clumping began sooner.
If the universe contained just ordinary matter, gravity could not have begun amplifying the initial lumpiness until about 300,000 years after the big bang. Before then, electrons and nuclei were constantly colliding with photons of electromagnetic radiation. But atoms interact much less often with photons than electrons and nuclei do. So when atoms formed at 300,000 years after the big bang, matter and radiation “decoupled,” freeing the matter so that gravity could begin its work.
But dark matter particles—if indeed dark matter is in the form of subatomic particles—would not be affected by electromagnetic radiation. Thus, dark matter would have begun responding to gravity much earlier as soon as about 1,000 years after the big bang. With a longer time for structure to grow, a smaller degree of lumpiness would be required to begin with.
A Snapshot of the Early Universe
Until recently, cosmologists had little evidence to support their ideas about the development of structure in the universe other than the existence of the structure itself. That situation changed in April 1992 with the announcement that the Cosmic Background Explorer satellite (COBE)—looking far across the universe and far back in time—had detected the tiny density variations as differences in the intensity of the cosmic background radiation. This finding provided direct evidence for the lumpiness of the infant universe.
The COBE satellite, launched by NASA in November 1989, was designed specifically to study the cosmic background radiation, which gives us a “snapshot” of the universe at an age of 300,000 years. The COBE project is headed by John Mather of NASA's Goddard Space Flight Center in Greenbelt, Md.
COBE data revealed energy variations of 0.001 percent in the microwave radiation, corresponding to regions of lesser or higher density in the early universe. Gravity caused the energy variations. In regions of higher density in the early universe—areas where there was slightly more matter—gravity was a bit stronger, and the radiation from those areas lost more energy.
Cosmologists were ecstatic when the COBE findings were reported. Finally scientists had convincing evidence that there had indeed been the lumpiness in the early universe required to account for the development of all the structure we now see. And the amount of lumpiness was the amount predicted by the dark matter theories.
The next question to be answered was: Where did that lumpiness come from? Theorists have suggested a number of ideas to answer that question, almost all of which involve events that took place in the first moments of creation. The most promising of these ideas is the theory of inflation, which holds that the universe went through a brief period of very rapid expansion at about 10-34 second after the big bang. In 1980, physicist Alan Guth of the Massachusetts Institute of Technology proposed this theory, which was later modified by other physicists, including Andrei Linde of the former Soviet Union and Paul Steinhardt of the University of Pennsylvania. According to the theory, during inflation, slight variations in the universe's density developed. The theory also predicts that the universe's dark matter consists of subatomic particles left over from the big bang.
Hot or Cold Dark Matter?
If these particles are truly the most abundant form of matter in the universe, how might they have guided the development of large scale structure? Cosmologists have proposed two theories. One, known as the hot dark matter theory, says that dark matter particles are fast moving neutrinos with a tiny amount of mass. The competing cold dark matter theory is based on hypothetical particles called axions or photinos. These particles are referred to as cold dark matter because they move very slowly.
Computer simulations have all but eliminated the hot dark matter theory. With hot dark matter, the simulations showed, superclusters of gas and neutrinos would have formed first and then fragmented to form galaxies and stars. The problem with this theory is that the oldest galaxies we can see today would not have had time to develop.
In simulations involving cold dark matter, structure forms from the bottom up–first galaxies, then galaxy clusters, and so on. In this view, gravity begins to amplify the tiny amount of lumpiness in the cold dark matter at about 1,000 years after the big bang. About 300,000 years later, ordinary matter is freed from the effects of radiation and is drawn by gravity to the lumps of dark matter. After about 2 billion years, the first protogalaxies (galaxies in formation) take shape. The protogalaxies contain many newly formed stars, though the details of how the stars would have formed are not well understood. Later, larger and larger structures form—fully developed galaxies, clusters of galaxies, superclusters, and even bigger structures in the future.
In these computer studies, cold dark matter seems to do much better than hot dark matter at reproducing the universe we see today. But simulations with cold dark matter have not worked perfectly. Consequently, some cosmologists have come to believe that the cold dark matter theory will have to be altered or even abandoned.
Some theorists have proposed, for example, that the early universe contained a mixture of both cold and hot dark matter. Others have suggested that the initial lumpiness was due not to inflation but rather to lumps or long strands of concentrated energy that were produced in the first moments of the universe and that served as gravitational “seeds” for the development of structure.
The debate shows that although our quest to understand the development of large scale structure in the universe has come a long way, it is far from over. Cosmologists have a general understanding of how gravity shaped the universe but they are still uncertain about where the initial lumpiness came from and the details of how the universe's structure developed.