FEATURE ARTICLE
The Cosmic Web
Observations and simulations of the intergalactic medium reveal the largest structures in the universe
Robert Simcoe
Lumps and All
Technology has thus given us the tools to observe the remote corners of the intergalactic medium and to interpret these observations in the context of a cosmological model. Having described the methods, now let us step back to examine the model itself by offering a narrative that explains the formation of galaxies and intergalactic structure.
The story begins more than 13 billion years ago, roughly 380,000 years after the Big Bang, when the universe was very different from today. There were no stars, galaxies or webs yet, just a uniform soup of free-floating protons and electrons. In fact, the gas was so evenly distributed that its peak densities differed by only 1 part in 100,000 from the cosmic average. But sometime between then and now it evolved into a very lumpy place, where vast stretches of nearly empty space are interrupted by "dense" strands of galaxies and gas. Today, the range of densities is much greater: The difference between the atomic density of the Sun's interior and intergalactic space spans about 32 orders of magnitude!
Astronomers believe that this transition from smooth to lumpy was driven by gravity. Imagine a box containing a perfectly uniform distribution of matter, so that the density of the particles is constant. Suppose that at one location in the box the particles are somehow stirred, leading to a slight density enhancement at this particular spot. This tiny new concentration of mass will create a gravitational force, which tugs on the surrounding particles and causes them to fall inward. The infalling matter increases the clump's mass, which in turn increases its gravitational pull, allowing it to assemble even more material, and so on. Given enough time, this "gravitational runaway" transforms what was originally a tiny density enhancement into a dense clump, containing most of the mass that was distributed throughout the volume.
This simple phenomenon is the basis for theories of how the large-scale structure of the universe was formed. Yet in order for it to work, the universe must have been "imprinted" at some earlier time with a network of primordial density perturbations that would later collapse into the structures we see today. As it happens, the signature of these ripples has been observed—as tiny variations in the temperature distribution of microwave photons coming from different parts of the sky. Characterization of this microwave background is currently a major focus of astronomical research, as the ripples represent the ancient gravitational seeds of cosmic structure.
It would seem that we have all the elements needed to explain the origin of the cosmic web. We have observed density variations in the early universe, and we have a powerful model that explains how they could evolve into larger structures. However, there is one problem: The primordial variations were so small that 13.7 billion years is still not enough time to grow them into the assemblages we observe today! This puzzle received a great deal of attention during the 1970s, perhaps fueled by Cold War politics. Two competing theories of structure formation emerged, one devised by Yakov Zel'dovich at the School of Russian Astrophysics in Moscow, and the other by James Peebles and his collaborators at Princeton University. The ensuing debate exposed significant weaknesses in both theories. The solution required the introduction of an entirely new ingredient—ominously named dark matter—in the cosmological models. This proved to be one of the most important discoveries in modern cosmology.
This dark stuff is quite different from the ordinary matter that makes up stars, planets and people. Not only does dark matter not shine, it interacts with "our" kind of matter only through the force of gravity. It is largely believed to consist of exotic particles that have no other effects on ordinary atoms and molecules. Furthermore, dark matter appears to outweigh normal matter throughout the universe by a factor of four to one. This notion is indeed odd, and it has met with resistance since it was first suggested by the eccentric astronomer Fritz Zwicky in the 1930s. However, cosmologists have now grown to accept its existence as nearly certain in the face of overwhelming evidence from a variety of observations. Although we may not understand exactly what dark matter is, we do understand what it does—it holds galaxies together, bends light, slows down the universe's expansion and drives the formation of intergalactic structure.
To understand this last point, we need to return to the early history of the universe. During the first 380,000 years, the relic heat from the Big Bang kept the universe so hot (greater than 3,000 kelvins) that electrons and protons in the primordial soup could not combine to form neutral hydrogen atoms. Such ionized gas, in this case consisting of dissociated electrons and protons, is known as a plasma. When plasma particles are in their free-floating state, they can interact with light, exchanging energy and momentum. In the early universe, this scattering increased the gas pressure within the cosmic soup. So, when gravity tried to collapse the first density perturbations, the gas pressure pushed back—much as a balloon does when it is squeezed. As long as the electrons and protons were separated, the gas could not form larger structures. Instead, the potential structures churned and oscillated as the inward pull of gravity fought the outward push of gas pressure.
Then, when the universe was 380,000 years old, a major event took place. As the universe was expanding, it was also cooling, and at this point it became cold enough for electrons and protons to combine, forming hydrogen atoms. Suddenly, these new atoms became decoupled from the photons—they no longer interacted so strongly with light—which drastically reduced the pressure that had kept gravity at bay. With gravity free to work on all the newly formed hydrogen atoms, structures could form in earnest.
How did dark matter fit into the picture? While the protons, electrons and photons were oscillating under the competing influences of gravity and pressure, the dark matter followed a different storyline. Because dark matter interacts with normal matter only through gravity, the pressure that kept the normal gas from collapsing couldn't act on it. Particles of dark matter enjoyed an unimpeded assembly into large structures long before the normal gas could begin to get organized. By the time normal matter decoupled from the photons, the dark matter had already grown into a primitive web-like network. As soon as the normal matter lost its support from the photon pressure, the gravity from the pre-existing dark-matter structures quickly pulled normal gas into the web. In this way, normal matter was given a gravitational "head-start" by the dark matter.
Once this process was set in motion, the gravitational building blocks of the intergalactic medium were in place. Normal and dark matter continued to free-fall toward concentrations of mass until the rising gas pressure slowed the infall. The web-like lattice was taking shape, but stars had not yet begun to form and all of the gas in the universe was neutral. The universe had entered an age where matter drifted about in the darkness, quietly assembling under gravity's influence. So it continued until at some point—probably somewhere between 200 million years and one billion years after the Big Bang—a process began that would fundamentally alter the nature of the intergalactic medium and the universe as a whole: The first stars were born.
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