Knocking on Heaven's Door (56 page)

The chief confirmation of inflation that WMAP gave us was the measurement of the universe’s extreme flatness. Einstein taught us that space can be curved. (See Figure 73 for examples of curved two-dimensional surfaces.) The curvature depends on the energy density of the universe. At the time when inflation was first proposed, it was known that the universe would suggest, but the measurements were far too imprecise to test the inflationary prediction that the universe would expand so much that any curvature would be stretched away. Microwave background measurements have now demonstrated that the universe is flat at the level of one percent, which would be extremely difficult to understand without some underlying physical explanation.

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FIGURE 73
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Zero, positive, and negative curvature on two-dimensional surfaces. The universe, too, can be curved, but in four-dimensional spacetime that is difficult to draw.

This flatness of the universe was a huge victory for inflationary cosmology. Had it not been true, inflation would have been ruled out. The WMAP measurements were also a victory for science. When theorists first proposed the detailed measurements of the microwave background that would eventually tell us about the geometry of the universe, everyone thought it interesting enough to throw out to the science community, but far too difficult technically to achieve any time soon. Within the decade, confounding all expectation, observational cosmologists made the necessary measurements and gave us amazing insights into how the universe has evolved. WMAP is still providing new results, performing detailed measurements of the variation in temperature across the sky. The Planck satellite in operation today is measuring these fluctuations more precisely still. The CMB measurements have proven to be a prime resource of insight into the early universe and will most likely continue to be so.

Recent detailed studies of the cosmic radiation left throughout the sky have led to other enormous leaps in our quantitative knowledge of the universe and its evolution. The details of the radiation have provided rich information about the matter and energy that surrounds us. In addition to telling us the conditions when the light first started heading toward us, the CMB tells us about the universe through which the light had to travel. If the universe had changed in the last 13.75 billion years, or if its energy were different than expected, relativity tells us that it would have affected the path that the light-ray took and consequently the measured properties of the radiation that was measured. Since it is such a sensitive probe of the energy content of the universe today, the microwave background gives information about what the universe contains. This includes the dark matter and dark energy we will now consider.

HEART OF DARKNESS

In addition to successfully confirming inflationary theory, CMB measurements presented a few major mysteries that cosmologists, astronomers, and particle physicists now want to address. Inflation tells us that the universe should be flat but it doesn’t tell us where the energy required to make it flat now resides. Nonetheless, based on Einstein’s equations of general relativity, we can calculate the energy needed for the universe to be flat today. It turns out that known visible matter alone provides a mere four percent of the energy required.

An additional puzzle that had already indicated the need for something new concerned the tininess of the fluctuations in temperature and density that COBE had measured. With only visible matter and such tiny perturbations, the universe wouldn’t have lasted long enough for the perturbations to have grown large enough for structure to have formed. The existence of galaxies and clusters of galaxies in conjunction with the tininess of the measured fluctuations pointed to the existence of matter that no one had yet directly seen.

In fact, scientists had already known that a new type of matter known as dark matter should exist well before COBE’s microwave radiation results. Other observations that we will get to soon that had already indicated additional unseen matter must exist. This mysterious stuff, which became known as dark matter, exerts gravitational forces, but it doesn’t interact with light. Because it neither emits nor absorbs light, it is invisible—not dark. Dark matter (we’ll keep using the term) has so far provided few tangible identifying features other than its gravitational influence and that it is so feebly interacting.

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FIGURE 74
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Pie chart illustrating the relative amounts of visible matter, dark matter, and dark energy of which the universe is composed.

Furthermore, gravitational influence and measurements indicate the presence of something even more mysterious than dark matter, known as dark energy. This is energy that permeates the universe, but doesn’t clump like ordinary matter or dilute as it expands. It is very much like the energy that precipitated inflation, but its density today is much smaller than it was back then.

Although we now live in a renaissance era of cosmology, in which theories and observations have advanced to the stage where ideas can be precisely tested, we also live in the dark ages. About 23 percent of the universe’s energy is carried by dark matter, and approximately another 73 percent is carried by the mysterious dark energy, as is illustrated in the pie chart. (See Figure 74.)

The last time something was called “dark” in physics was in the mid-1800s, when Urbain Jean Joseph Le Verrier of France proposed an unseen dark planet, which he named Vulcan. Leverrier’s goal was to explain the peculiar trajectory of the planet Mercury. Le Verrier, along with John C. Adams of England, had previously deduced the existence of Neptune based on its effects on the planet Uranus. Yet he was wrong about Mercury. It turned out that the reason for Mercury’s strange orbit was much more dramatic than the existence of another planet. The explanation could be found only with Einstein’s theory of relativity. The first confirmation that his theory of general relativity was correct was that he could use it to accurately predict Mercury’s orbit.

It could turn out that dark matter and dark energy are a consequence of known theories. But it might also be that these missing elements of the universe presage a similar significant change of paradigm. Only time will tell which of these options will resolve the dark matter and dark energy problems.

Even so, I’d say that dark matter is very likely to have a more conventional explanation, consistent with the type of physical laws we now know. After all, even if novel matter acts in accordance with force laws similar to those we know, why should all matter behave exactly like familiar matter? To put it more succinctly, why should all matter interact with light? If the history of science has taught us anything, it should be the shortsightedness of believing that what we see is all there is.

Many people think differently. They find dark matter’s existence very mysterious and ask how it can possibly be that most matter—about six times the amount we see—is something we can’t detect with conventional telescopes. Some are even suspicious that dark matter is really some sort of mistake. Personally, I think quite the opposite (though admittedly not even all physicists see it this way). It would perhaps be even more mysterious if the matter we can see with our eyes is all the matter that exists. Why should we have perfect senses that can directly perceive everything? Again, the lesson of physics over the centuries is how much is hidden from view. From this perspective, it’s mysterious why the stuff we do know should constitute even as much as 1/6 of the energy of all matter, an apparent coincidence that my colleagues and I are currently trying to understand.

We know something with dark matter’s properties has to be there. Although we don’t exactly “see” it, we do detect dark matter’s gravitational influence. We know dark matter exists due to the extensive observational evidence of its gravitational effects in the cosmos. The first clue that it existed came from the speed with which stars rotated in galaxy clusters. In 1933, Fritz Zwicky observed that galaxies in galaxy clusters orbited faster than could be accounted for by the visible mass, and Jan Oort soon after observed a similar phenomenon in the Milky Way. Zwicky was convinced enough by his work to conjecture the existence of dark matter that no one could directly see. But neither of these observations was conclusive. A faulty measurement or some other galaxy dynamics seemed like a far more plausible explanation than some invisible substance invented solely to provide additional gravitational attraction.

At the time Zwicky made his measurements, he didn’t have the resolution to see individual stars. Much more solid evidence for dark matter came from Vera Rubin, an observational astronomer, who much later—in the late 1960s and early 1970s—made detailed quantitative measurements of stars rotating in galaxies. What first seemed to be a “boring” study of stars orbiting in a galaxy—a study Vera turned to since it provided less-well-trodden territory than other astronomical activities at the time—emerged as the first solid evidence of dark matter in the universe. Rubin’s observations with Kent Ford yielded incontrovertible evidence that Zwicky’s conclusion years earlier had been correct.

You might wonder how someone could look through a telescope and see something dark. The answer is that she could see its gravitational consequences. The properties of a galaxy, such as the rate at which its stars orbit around, are influenced by how much matter it contains. With only visible matter present, one would have expected those stars well beyond the galaxy to be rather insensitive to the galaxy’s gravitational influence. Yet stars ten times farther away than the luminous central matter rotated with the same velocity as stars closer to the galaxy’s center. This implied that the mass density did not fall off with distance, at least to distances as far from the galaxy’s center as ten times the distance of the luminous matter. Astronomers concluded that galaxies consisted primarily of unseen dark matter. The luminous matter we see is a significant fraction, but most of the galaxy is invisible, at least in the ordinary sense of the word.

We now have a good deal of other supplementary evidence for dark matter’s existence. Some of the most direct is from lensing, illustrated in Figure 75. Lensing is the phenomenon that occurs when light passes a massive object. Even if that object itself doesn’t emit light, it does exert a gravitational force. And that gravitational force can cause light emitted by a nondark object behind (as seen from our vantage point) to bend. Because the light bends in different directions according to the path it takes around the dark object and because we automatically project straight lines for light, this lensing can produce multiple images of the original bright object in the sky. These multiple images allow us to “see” the dark object—or at least infer its existence and properties by deducing the gravity needed to bend the observed light.

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FIGURE 75
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Light passing a massive object can bend, which from the perspective of the observer appears to create multiple images of the original object.