figure 11 J. Fluctuations in the microwave background from the BOOMERANG balloon experiment. The measurements map the variations in the microwave background The angular size of the fluetunions teHs the cosmic geometry, which agrees best with a fbt universe in which + S}„ = I Courtesy of the BOOMERANG collaboration (Also see color insert}

figure 11 J. Fluctuations in the microwave background from the BOOMERANG balloon experiment. The measurements map the variations in the microwave background The angular size of the fluetunions teHs the cosmic geometry, which agrees best with a fbt universe in which + S}„ = I Courtesy of the BOOMERANG collaboration (Also see color insert}

articles. If you give a lot of talks at conferences, you may end up using similar material more than once. But a shift in the level of conviction that the data generate can be detected by careful reading of the titles for these talks. In March 1998, Mike Turner wrote one called "Cosmology Solved? Maybe." In April, he recycled the text with the less reserved title "Cosmology Solved?" And in October 1998, Mike took the next step when he entitled his talk, "Cosmology Solved? Quite Possibly." I'm looking forward to his future work, "Cosmology Solved!" Taking the cosmological constant out of Einstein's wastebasket seems to be required by the supernova data. Combined with the CMB measurements, the measurements point to a universe that is approximately two-thirds dark energy and one-third dark matter

Contents of the Universe

Figure 11.4. The Universal Pie. Although we car be proud that we have filled up this diagram, die biggest slice of energy-density in the universe is dark energy, which we don't understand, and the next biggest is dark matter, which we don't understand. There is plenty of work to be done. Courtesy of Peter Garnavich, University of Notre Dame.

I guess we should be proud of the fact that we've been able to make any sense at all out of the universe, given our small brains, brief lives, and limited experiences, but there is something deeply unsettling about this picture. We may, quite possibly, have accounted for all of the matter and energy in the extravagant universe, but unfortunately, we don't know exactly what we are talking about. The dark energy could be the cosmoiogical constant, but it could be something else that has negative pressure. And if inflation is right, then this is the second time the universe has been dominated by dark energy—once at 1055 seconds, and now again at 10lfi seconds after the Big Bang. The dark matter includes neutrons and protons and neutrinos, hut most of it must be something else that is definitely not made of those familiar particles and is still unidentified. So, while we should take some pleasure in filling in the blanks, we've done it with things whose nature we only dimly grasp. This should be good news to somebody thinking of entering the field—the subject's not done, it is just starting.

Garnavich and Jha led the way for our team's modest foray into learning the nature of the dark energy. The cosmological constant will produce acceleration. We observed acceleration. This does not prove that the cosmological constant that Einstein imagined is responsible. What if there's something else that might cause acceleration? Could the dark energy be something else?

Looking at Einstein's equations, it is easy to see (remember I am not really a licensed theorist, but this is how they talk!) that you get acceleration from a component of the universe that has positive energy and negative pressure. While the cosmological constant does that, it has some other properties that are distasteful to theorists. First, the measured value is so small compared to theoretical estimates. They compute at least 10™. We measure QA - 0.7. They prefer very large or exactly zero. But we measure something that is not very large and is not zero.

Second, the number we measure for dark energy, = 0.7, is not so different from = 0.3, the value for the dark matter. But that wasn't true in the past and it won't be true in the future if the dark energy is the cosmological constant. In the past, was larger because the density was higher. We see evidence for that from the very high redshift supernovae, like SN 1997ff, where the data favor deceleration early in the history of the universe and acceleration only over the last 5 billion years. In the future, the energy density of dark matter will continue to fall, while a cosmological constant that stays constant would become a larger fraction of the total energy density in the universe. In other words, if there's a cosmological constant, it guarantees that will eventually dominate because the density of matter declines, while the energy density of the vacuum does not. Why do we live at the unique and cosmically brief moment when they are about the same?

You could say, "Well, that's just the way it is." But most theorists are, quite rightly, suspicious of coincidences. They don't like the smell of an idea that places us at a special time in the history of the universe. They would be happier if the dark energy were somehow related to the dark matter, so there would be a reason why they were so nearly the same. Paul Steinhardt, who pointed out that we might need A while the early LBL supernova results pointed the other way, has gone on to sketch a replacement for A he calls "quintessence." Quintessence is a form of vacuum energy that evolves with time, so the near agreement of the energy density in matter and energy is not a coincidence—it is just what you should expect. Paul has also gone on to challenge our imaginations and spelling ability with what he calls the Ekpyrotic universe, which replaces inflation with colliding space-time membranes at the beginning of time. The cosmological constant was invented by Einstein and it is completely consistent with general relativity as he formulated it. Quintessence and other forms of dark energy move beyond general relativity into new realms of physics. It is exciting to think that the ultimate origin of this effect, which is detected only by astronomical measurement over billions of light-years, is connected to the pursuit of understanding the universe on the smallest imaginable scales.

Is there a way to move the discussion of dark energy away from esthetics and rhetoric to measurement? Yes. We can distinguish some of the possibilities by measurement. The key ingredient is the way the pressure is related to the density. We call that the "equation of state." For ordinary gases like the carbon dioxide cartridge for a paint ball gun, as you increase the density, by stuffing more C02 molecules into the same-sized cylinder, the pressure goes up. For the cosmological constant, as the universe expands, the pressure (which is negative) and the energy density do not change. For other sources of acceleration, for example, quintessence, the pressure may change as the universe expands. This will leave a signature in the Hubble diagram if it makes cosmic acceleration occur at a different redshift. Garnavich showed from our early data that some forms of dark energy were ruled out, while the cosmological constant was completely consistent with the observations. Hut the present data are very sketchy. To do a good job on the cosmic equation of state, to find out what the dark energy really is, will require more numerous and more precise measurements of supernovae over a range of redshifts.

The Lawrence Berkeley Lab group has embarked on an ambitious plan to build a specialized satellite, deftly named SNAP (Su-perNova Acceleration Probe) to discover and measure thousands of supernovae. This satellite would have a telescope with a wide field of view focused on an immense CCD array, far larger than anything ever sent into space on a civilian satellite. By concentrating on finding and measuring supernovae, SNAP advocates say they can pin down the nature of the dark energy. This is certainly worth doing, and I wish them well. One thing is for sure, they are going to go to a lot of boring meetings over the next ten years. I hope they know somebody down the hall who is working on projects with a shorter horizon.

But I suspect there will also be a role for less comprehensive, less expensive, and more rapid projects. You wouldn't want a quest for doing the perfect measurement to interfere with doing something that was pretty good. Perhaps by finding and measuring a few hundred supernovae from the ground we could learn the details of cosmic acceleration from redshift 0.5 up to redshift 1. This is the region where the effects of acceleration appear to be the largest, and we already know we can make the measurements from the ground, so detailed observations might pin down the equation of state for dark energy. Like the balloon-based experimenters measuring the CMB, we might find it possible to anticipate the main results of the SNAP satellite by a more modest route. That's part of our plan for the next few years. Maybe later we can check our results against the findings from SNAP.

While I've been sitting at my desk finishing this manuscript, a crew of astronauts has taken a bone-shaking ride on the Shuttle up to the I lubble Space Telescope. They skillfully installed a new camera with a larger field of view that takes sharper images. The new camera makes a practical proposition out of searching for distant supernovae with HST itself, as Stirling Colgate and Gustav Tam-mann prophesied 23 years ago. With this camera, a team led by Adam Riess hopes to find several objects beyond redshift 1, like SN 1997ff, to see if the universe really does have the stop-and-go property that signals the effect of dark energy at work. Even better, those deft astronauts plumbed in a new refrigerator for NICMOS, so, if all goes well, we will measure the light curves for these very distant supernovae in the infrared where cosmic expansion shifts their light. The next few years should be very exciting.

To see SN la even deeper into the cosmic past than we can observe with IiST will require a telescope that has the power of the Space Telescope, but that is designed to work in the infrared. This telescope is already in development. The next generation space telescope will be a large, cold, space-based telescope that can see SN la (if there are any) back to redshift 5! Then we will certainly see how the ages of the stars in a galaxy affect the properties of supernovae, and we will be able to use supernovae to trace cosmic-history right back to the very first stars That's a telescope I've been willing to sit in meetings to help build.

What are the implications if this story is correct? If there is a cosmological constant causing acceleration over the last 5 billion years, then the universe will continue to accelerate indefinitely into the future. The expansion will literally be exponential: the bigger it gets, the more it speeds up. The universe will run away in headlong expansion. One curious effect is that galaxies we can observe today will get redshifted beyond our detection in the future. Instead of seeing more of the contents of the universe as time passes, we will see less and less. The universe could become a lonely, dull, cold, dark place. This is a good reason to do this work now. In a few-hundred billion years, perhaps we won't be able to.

However, it is always unwise to think that today's best approximation to understanding the universe is really the whole story. The prevailing wisdom is always spoken in the same authoritative tone of voice, with the same degree of confidcnce. It's the content that changes. Ten years ago, a universe dominated by cold dark matter was very strongly advocated by many astrophysicists. This implied a "just right" universe with Cl^ equal to one that would expand and decelerate forever. Today, we say (in the same godlike tone of voice used to narrate planetarium shows and documentary films) that matter is only a fraction of the total energy density of the universe and that dark energy determines the future of cosmic expansion, which will accelerate forever. Since new evidence can change our best understanding on a timescale often years, we probably should be cautious in predicting what will happen in the next 100 billion years. If the present acceleration is caused by a variable sort of dark energy, it might go away at some distant time, ending the era of acceleration. And we shouldn't be too confident that there were not earlier episodes of acceleration that don't show up in the present data. The universe is wilder than we ordinarily dare to imagine. Although there is a sense today that the tumblers are all clicking into place as we unlock the secrets of the universe, this is not the end of the investigation, just the beginning.

It's a strange picture we have painted. The universe has dark energy and dark matter, neither of which is familiar to us from our everyday experience, or detected from any experiment on Earth. The visible part of the universe and the beautifully elaborate atoms that make up our bodies and our world are not the main material constituents of the cosmos. We have gone from thinking of ourselves as the centerpiece of creation though a series of cosmic leaps of understanding to seeing ourselves as observers and beneficiaries of a great pageant in space and time that we don't affect, but that has affected us greatly. We are not made of the type of particles that make up most of the matter in the universe, and we have no idea yet how to sense directly the dark energy that determines the fate of the universe. If Copernicus taught us the lesson that we are not at the center of things, our present picture of the universe rubs it in.

On the other hand, maybe the fact that we are not made of the stuff that forms most of the universe should make us feel special. Our origin is in the universe, with the atoms we're made of an unusual form of matter, baryons processed through stars. We're not the same as the dark matter or the dark energy, we're made of more versatile stuff that has more potential for complex, unpredictable outcomes like a human life

A year ago, I was at a meeting of the American Physical Society, the biggest physics association in the United States. Up on the podium were a collection of presidential science advisers from the past, going all the way back to the Truman administration. Listening to them talk about the role of science in the United States, I grew irritated, impatient, and cross. They were talking about the value of science for economic growth through technical innovation. Science as the golden goose. They were talking about the value of science for national defense. Science replenishing secrets faster than they leak out. They were talking about the value of science to cure diseases and increase the span of human life. Science as the fountain of youth.

Now I suppose everybody wants to be rich, safe, and immortal. Or at least every Congressman. So I guess this is a reasonable set of goals for a president's science adviser to advocate. But the science of the universe is not aimed at creating wealth, improving defenses, or curing disease. It is aimed at increasing our understanding, and nobody on the platform was talking about that.

We have little brains and brief lives, but as I have tried to show in this book, we are beginning to build a rational picture of the universe in which we find ourselves. By combining the clues from ancient light and a hard-won understanding of how the world works, we are beginning to see the big picture for the history of the universe.

Is this important to people? Of course it is important to those of us who have the joy of working together to find things out. For us, it's an adventure. But is it important to others? I think so. People are curious and people have imagination. People want to know. Where did we come from? Where are we going? And when do we get there? Cosmology tries to answer those questions about the physical world using the best tools of modem technology and the best ideas that have been sifted and tested in the laboratory.

Part of the fun of cosmology is that it takes us into realms where laboratory physics doesn't yet reach. The accelerating universe is a new phenomenon, not discovered in a laboratory, which may open up a new area of physical understanding. It might lead to a new understanding of what the vacuum is, how gravity is related to the other forces, and how the contents of the universe shape its destiny, The elements of adventure, exploration, and the discovery of real things that are stranger than we dared imagine makes astronomy a well of inspiration, and not just for the experts. Our highest aspiration is not perfect comfort. If we were rich, safe, immortal, and bored, this would not be a vision of paradise. Cosmic discovery nourishes our deep desire to learn what the world is and how it works.

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