In general, the planets we have found so far do not look like Earth. They tend to be very large (larger than Jupiter) and close to their star (closer than Mercury). They also seem to have highly elliptical orbits, rather than orbits that are nearly circular, as in our own system. Astronomers think that these so-called hot Jupiters formed far out in their solar systems and moved in to their present orbits.
GALAXIES
Stars are not scattered at random throughout the universe; all stars are gathered into clumps called galaxies. Our own sun, for example, is one of a group of about 100 billion stars called the Milky Way galaxy. About 120,000 light-years across, the Milky Way, like about three fourths of all galaxies, is a flattened rotating disk with bright spiral arms. On a good night, your unaided eye can see about 2,500 stars, all of them in the Milky Way. You can also see a few fuzzy patches of light called nebulae (clouds). Seemingly unimportant in the grand celestial display, they are actually other galaxies, with billions of stars, planets, and possibly life like our own.
The discovery that there were other “island universes” besides the Milky Way was made by the American astronomer Edwin Hubble in 1923, using the then new 100-inch telescope on Mount Wilson, near Los Angeles. Until this telescope was built, astronomers, like someone peering at fine print without glasses, had been trying to understand nebulae without much success. The advent of the Mount Wilson telescope changed all that. Hubble could pick out types of stars that astronomers used to establish distances within the Milky Way. Using measurements of these stars, Hubble showed that the Andromeda nebula is some 2 million light-years away—far outside the confines of the Milky Way. Because of his work, we now realize that our galaxy is only one among billions, each with billions of stars, in the universe.
The M 81 spiral galaxy, which contains billions of stars, is one of the closest galaxies to our own Milky Way. Viewed from 4 million light-years away, our home cluster of stars would look similar to this. PHOTO COURTESY OF THE CARNEGIE INSTITUTION OF WASHINGTON.
Most galaxies, like the Milky Way, are relatively sedate, homey places where the slow process of the stellar life cycle goes on quietly. A small number, however, seem to house a kind of violence unknown in our own peaceful neighborhood. Cataclysmic explosions rip through the cores of the galaxies, spewing huge jets of material hundreds of thousands of light-years into space. These active galaxies typically emit large amounts of energy as radio waves, and hence shine brightly in the radio sky.
The most interesting of the active galaxies are the quasars (the name comes from a contraction of “quasi-stellar radio source”). A quasar may easily emit more energy in a second than the sun has in its entire lifetime. The thousands of quasars known in the sky tend to lie at great distances from Earth—in fact, the most distant objects known are quasars. One current theory is that quasars are an early, violent stage in the evolution of galaxies. According to that theory, light from quasars has been traveling toward us for billions of years, in some cases from almost the very beginnings of the universe. The Milky Way may once have been a quasar and may appear to be one now to astronomers at the other end of the universe.
Telescopes
Hubble’s work illustrates an important point about astronomy. Our knowledge of the universe is intimately tied to our ability to build large (and expensive) telescopes to detect and record the radiation that comes to us from deep space. As a rule of thumb, a state-of-the-art ground-based telescope costs about as much as a major highway interchange. That was true when Mount Wilson was built in the early twentieth century, when its more illustrious 200-inch partner was built on Mount Palomar in the 1930s, and, given current rates of inflation, will be true when the next generation of telescopes is finished.
There are two major areas of telescope building going on today: ground-based and satellite. Palomar represents the pinnacle of building large “light buckets” from single blocks of glass. Today, telescopes use modern fast electronics to achieve the same result much more simply. The Keck telescope, built by the California Institute of Technology at Mauna Kea in Hawaii, typifies this design. It is made of many small mirrors—in fact, its working surface looks like a plateful of potato chips. Each of these small mirrors is individually controlled by a computer that constantly adjusts the total array to keep everything in focus. In this way, a series of small mirrors can take the place of a single large block of glass, producing a telescope that is, in terms of its light-gathering ability, more powerful than Palomar.
In the same way, radio telescopes no longer are being built as gigantic single dishes—overgrown versions of home TV antennae—but as series of individually controlled receivers whose positions are synchronized by a central computer. The largest such telescope is the VLA (Very Large Array) located in the desert near Socorro, New Mexico.
Only visible light and radio waves can penetrate through Earth’s atmosphere to ground-based telescopes. To monitor the other parts of the electromagnetic spectrum, we must lift receivers above the obscuring air. Instruments located in satellite observatories have greatly expanded our understanding of the universe. Over the past decades, astronomers using satellite instruments have been able to scan the heavens using the entire electromagnetic spectrum (see Chapter 3), a process that has added immeasurably to our understanding of the universe.
The best-known orbiting observatory is the Hubble Space Telescope (HST), which was launched in April 1990. The Hubble detects visible and ultraviolet radiation and has given us data of unprecedented resolution, from pictures of the planets to photographs of distant galaxies. It does not see farther than ground-based telescopes, but because it is above the distorting atmosphere, it can often resolve finer details. As the Hubble nears the end of its useful life, astronomers are already drawing plans for its replacement instrument.
FRONTIERS
The Search for Earth-like Planets
As the roster of extrasolar planets grows, astronomers have not yet found planets that look like Earth. There are good reasons for this failure. If you detect the planet by the pull it exerts on its star, then small planets like Earth will be harder to see than large planets like Jupiter. Similarly, planets close to their star will exert a larger gravitational force and produce a larger and more easily measurable Doppler shift. It is not surprising, then, that the initial discoveries are of “hot Jupiters.” The question is whether, as our search techniques become more refined, we will find that for some as yet unexplained reason our own solar system, with Earth-sized planets in nearly circular orbits, is unique (or nearly unique) in the galaxy. Such an outcome would have profound consequences for our thinking about the prevalence of life and intelligence in the universe.
SETI
The search for extraterrestrial intelligence, or even extraterrestrial life, catches the imagination but does not have high priority among astronomers. We know from the space program that it is extremely unlikely that life will be found in our solar system. There are programs to monitor nearby stars (which may or may not have planets) for radio messages sent by extraterrestrials, but they tend to be rather small-scale operations. Some scientists argue that the human race is probably alone in the galaxy, because it is extremely unlikely that all the conditions necessary to produce intelligent life will be present nearby in the galaxy. For the record, however, we feel that these searches should be carried out. If we find someone out there, the implications will be staggering. If we don’t, the implications will be even more staggering.
CHAPTER ELEVEN
The Cosmos
WHERE DID THE UNIVERSE come from? Where is it going? How is it put together? How did it get to be the way it is?
These are Big Questions. Like others of their ilk, they are easy to ask and very hard to answer. Humans seek answers for deep philosophical reasons—reasons that have very little to do with the immediate applications of technology. No one is going to get rich from discovering the structure of the universe (unless, of course, she decides to write a book about it).
The branch of science devoted to these Big Questions is called cosmology. Modern thought in this area derives from the fact that:
The universe was born at a specific
time in the past, and it has been
expanding ever since.
THE EXPANSION OF THE UNIVERSE
Edwin Hubble established the existence of other galaxies, but this was not the most important result of his work. When he looked at the light coming from those galaxies, he found that it was shifted toward the red. That is, its wavelength was longer than that of the light emitted from corresponding atoms in laboratories. Furthermore, he found that the farther away the galaxy was, the greater was the shift in its light. Hubble attributed this redshift to the Doppler effect.
You experience the Doppler effect every time a speeding car passes you as you stand on the sidewalk. Sound waves have a regular pulse or frequency, and your ear interprets that frequency as a pitch. If a noisy object like a horn or a racing motor moves toward you, then you hear more pulses per second because the source moves a short distance toward you between sound-wave crests. Your ear hears a higher pitch. Once past, however, the receding vehicle imparts fewer pulses per second as the source moves away from you between crests. As a result, the pitch sounds lower. Listen for that characteristic change in pitch, from high to low, the next time a loud vehicle whizzes by.
Light can experience a Doppler shift just like sound. Light emitted by a star that is speeding away from us appears to be at a lower frequency (shifted toward the red end of the spectrum) than that emitted in a laboratory. Hubble saw this redshift and concluded that almost all galaxies are rushing away from us and the universe as a whole is expanding. Observations with modern instruments verify that this so-called Hubble expansion exists throughout the observable universe. This fact is central to our present ideas about the universe.
Imagine a piece of rising bread dough with raisins scattered throughout it. Each raisin represents a galaxy, the dough the space that separates galaxies. If you were standing on one raisin, you would see the neighboring raisin receding from you because the dough between you and it is expanding. A raisin twice as far away would be receding twice as fast because there is twice as much dough between you and it. The farther away the raisin, the faster it would be moving. This is exactly the sort of behavior that Hubble observed for galaxies.
The bread dough analogy illustrates several important points about the Hubble expansion. First, there is no significance to the fact that the Earth seems to be the center of the universal expansion. In the bread dough, you see the same thing: no matter which raisin you stand on, it appears that you are standing still and everyone is moving away from you. Thus, everyone sees himself as the center of the universal expansion, and the fact that we see everything moving away from the Earth in no way makes us special.
Second, the movement of the raisins is not like the explosion of an artillery shell. The raisins do not move through the dough, but are carried along by the general expansion. In the same way, the galaxies do not fly apart through space, but are carried along as space itself expands.
Third, the raisins do not themselves expand—only the space between them. In the same way, the solar system and the Milky Way galaxy are not expanding, even though distant galaxies are receding from us.
Finally, if you ask where in the dough the expansion started, you can only answer “everywhere,” since a bit of dough at any point in the bread now was at the center when the expansion started. In the words of the fifteenth-century philosopher Nicholas of Cusa, “The universe has its center everywhere and its edge nowhere.”
THE BIG BANG
The Hubble expansion has one remarkable and inescapable consequence: it requires that the universe had a beginning. If you think of “running the film backward” on the current expansion, you find that a little over fourteen billion years ago the universe was a single geometrical point. The current expansion must have started at that time. The initial event, as well as the general model in which the universe started expanding from a highly condensed beginning, is called the big bang. It represents our best guess as to the origin and evolution of the universe.
Universal Freezings
When the universe was younger, it was denser—more compressed—than it is now. When matter and energy are concentrated in a small volume, temperature inevitably is higher. Consequently, when the universe was younger it was hotter. Tracing backward in time, we can recognize six crucial events—we like to call them freezings—where the fabric of the universe changed in a fundamental way, much as water changes when it freezes into ice. Understanding these freezings is the main task of modern cosmology.
The most recent freezing occurred when the universe was about 500,000 years old (i.e., about 14 billion years ago). After the first 500,000 years electrons and nuclei formed permanent attachments in the form of atoms, but prior to that time if an electron fell into orbit around an atom, it would be knocked off by a collision with another speeding particle. Before 500,000 years, matter existed as loose electrons and nuclei—the state of matter we have called plasma.
Moving backward in time, the next freezing occurred at about three minutes, when nuclei formed. Before this time, there were only elementary particles in the universe, and if a proton and neutron came together to form a nucleus, they would be torn apart by subsequent particle collisions. After three minutes nuclei could remain stable (although for reasons discussed below, only nuclei up to helium and lithium were formed in the big bang—everything else was made later in stars).
From three minutes back to about ten millionths of a second the universe was a seething mass of elementary particles—protons, neutrons, electrons, and all the rest of the particle zoo. At ten millionths of a second the universe had cooled off enough so that quarks could come together to form the elementary particles. Before this time there were only leptons and quarks, afterward there were leptons and the whole sea of elementary particles that live inside the nucleus.
The First Ten-Billionth of a Second
Ever since the universe was ten billionths of a second old, the great freezings involved changes in the fundamental state of matter. Before that time there were three more freezings, each involving forces rather than matter. When quarks “froze” to form the elementary particles, the forces acting in the universe were pretty much as we see them today. There were four distinct forces—the strong, electromagnetic, weak, and gravitational. But earlier in the history of the universe, when things were hotter, some or all of these forces must have been unified. One by one, they come together as we move backward in time until finally, at the very beginning, there is only a single, all-encompassing force.
The timetable for the unification of the forces as we theorize them today is as follows:
1/10,000,000,000 second: The weak and electromagnetic forces unify into one force, called electroweak. The temperatures of the universe at that time can be reproduced on Earth at accelerator laboratories. Thus, we can have some confidence in our understanding of the universe from the time it was a ten-billionth of a second old to the present, because we can test in the laboratory our theories of what happened then.
1/1,000,000,000,000,000,000
,000,000,000,000,000 (or 10-33) second: The strong force unifies with the electroweak, leaving only gravity as odd man out. During this freezing, two other important events occurred: the entire universe expanded rapidly from something smaller than an elementary particle to something the size of a grapefruit (a process known as inflation), and antimatter started to disappear, annihilating itself with matter to produce radiation. This unification is described by the standard model, which, as we discussed in Chapter 9, has been well verified in the laboratory (although not, of course, at the energies that prevailed in the universe at this time). Thus this freezing represents the frontier of our theoretical knowledge.
1/10,000,000,000,000,000,000,000
,000,000,000,000,000,000,000 (or 10-43) second: Known as the Planck
time (after one of the founders of quantum mechanics), this marks the ultimate unification. Until the universe was this old, all four forces were unified, and things were as beautiful and simple and elegant as they could be. Particles of matter in its most fundamental form interacted through the medium of a single unified force. It’s all been downhill since then.
EVIDENCE FOR THE BIG BANG
The Cosmic Microwave Background
No matter which direction you look from Earth, microwave radiation from deep space is raining down. This radiation, discovered in 1964, constitutes the first great verification of the big bang. The reason is this: every object gives off radiation, and the type it gives off depends on its temperature. Your body, for example, gives off infrared radiation because you are at a temperature of 98.6 degrees Fahrenheit. If the universe started from a hot beginning and has been expanding and cooling off ever since, it would now be at a temperature of about three degrees C above absolute zero. A body at this temperature emits microwaves, and it is this radiation that we detect on Earth.
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