The Fabric of the Cosmos: Space, Time, and the Texture of Reality (54 page)

Before 1996, most theoretical models that incorporated extra dimensions imagined that their spatial extent was roughly Planckian (10
^-33
centimeters). As this is seventeen orders of magnitude smaller than anything resolvable using currently available equipment, without the discovery of miraculous new technology Planckian physics will remain out of reach. But if the extra dimensions are "large," meaning larger than a hundredth of a billionth of a billionth (10
^-20
) of a meter, about a millionth the size of an atomic nucleus, there is hope.

As we discussed in Chapter 13, if any of the extra dimensions are "very large"—within a few orders of magnitude of a millimeter—precision measurements of gravity's strength should reveal their existence. Such experiments have been under way for a few years and the techniques are being rapidly refined. So far, no deviations from the inverse square law characteristic of three space dimensions have been found, so researchers are pressing on to smaller distances. A positive signal would, to say the least, rock the foundations of physics. It would provide compelling evidence of extra dimensions accessible only to gravity, and that would give strong circumstantial support for the braneworld scenario of string/M-THEORY.

If the extra dimensions are large but not very large, precision gravity experiments will be unlikely to detect them, but other indirect approaches remain available. For example, we mentioned earlier that large extra dimensions would imply that gravity's intrinsic strength is greater than previously thought. The observed weakness of gravity would be attributed to its leaking out into the extra dimensions, not to its being fundamentally feeble; on short distance scales, before such leakage occurs, gravity would be strong. Among other implications, this means that the creation of tiny black holes would require far less mass and energy than it would in a universe in which gravity is intrinsically far weaker. In Chapter 13, we discussed the possibility that such microscopic black holes might be produced by high-energy proton-proton collisions at the Large Hadron Collider, the particle accelerator now under construction in Geneva, Switzerland, and slated for completion by 2007. That is an exciting prospect. But there is another tantalizing possibility that was raised by Alfred Shapere, of the University of Kentucky, and Jonathan Feng, of the University of California at Irvine. These researchers noted that cosmic rays—elementary particles that stream through space and continually bombard our atmosphere—might also initiate production of microscopic black holes.

Cosmic ray particles were discovered in 1912 by the Austrian scientist Victor Hess; more than nine decades later, they still present many mysteries. Every second, cosmic rays slam into the atmosphere and initiate a cascade of billions of downward-raining particles that pass through your body and mine; some of them are detected by a variety of dedicated instruments worldwide. But no one is completely sure what kinds of particles constitute the impinging cosmic rays (although there is a growing consensus that they are protons), and despite the fact that some of these high-energy particles are believed to come from supernova explosions, no one has any idea of where the highest-energy cosmic ray particles originate. For example, on October 15, 1991, the Fly's Eye cosmic ray detector, in the Utah desert, measured a particle streaking across the sky with an energy equivalent to 30 billion proton masses. That's almost as much energy in a single subatomic particle as in a Mariano Rivera fastball, and is about 100 million times the size of the particle energies that will be produced by the Large Hadron Collider.
⁶
The puzzling thing is that no known astrophysical process could produce particles with such high energy; experimenters are gathering more data with more sensitive detectors in hopes of solving the mystery.

For Shapere and Feng, the origin of super-energetic cosmic ray particles was of secondary concern. They realized that regardless of where such particles come from, if gravity on microscopic scales is far stronger than formerly thought, the highest-energy cosmic ray particles might have just enough oomph to create tiny black holes when they violently slam into the upper atmosphere.

As with their production in atom smashers, such tiny black holes would pose absolutely no danger to the experimenters or the world at large. After their creation, they would quickly disintegrate, sending off a characteristic cascade of other, more ordinary particles. In fact, the microscopic black holes would be so short-lived that experimenters would not search for them directly; instead, they would look for evidence of black holes through detailed studies of the resulting particle showers raining down on their detectors. The most sensitive of the world's cosmic ray detectors, the Pierre Auger Observatory—with an observing area the size of Rhode Island—is now being built on a vast stretch of land in western Argentina. Shapere and Feng estimate that if all of the extra dimensions are as large as 10
^-14
meters, then after a year's worth of data collection, the Auger detector will see the characteristic particle debris from about a dozen tiny black holes produced in the upper atmosphere. If such black hole signatures are not found, the experiment will conclude that extra dimensions are smaller. Finding the remains of black holes produced in cosmic ray collisions is certainly a long shot, but success would open the first experimental window on extra dimensions, black holes, string theory, and quantum gravity.

Beyond black hole production, there is another, accelerator-based way that researchers will be looking for extra dimensions during the next decade. The idea is a sophisticated variant on the "space-between-the-cushions" explanation for the loose coins missing from your pocket.

A central principle of physics is conservation of energy. Energy can manifest itself in many forms—the kinetic energy of a ball's motion as it flies off a baseball bat, gravitational potential energy as the ball flies upward, sound and heat energy when the ball hits the ground and excites all sorts of vibrational motion, the mass energy that's locked inside the ball itself, and so on—but when all carriers of energy have been accounted for, the amount with which you end always equals the amount with which you began.
⁷
To date, no experiment contradicts this law of perfect energy balance.

But depending on the precise size of the hypothesized extra dimensions, high-energy experiments to be carried out at the newly upgraded facility at Fermilab and at the Large Hadron Collider may reveal processes that appear to violate energy conservation: the energy at the end of a collision may be less than the energy at the beginning. The reason is that, much like your missing coins, energy (carried by gravitons) can seep into the cracks—the tiny additional space—provided by the extra dimensions and hence be inadvertently overlooked in the energy accounting calculation. The possibility of such a "missing energy signal" provides yet another means for establishing that the fabric of the cosmos has complexity well beyond what we can see directly.

No doubt, when it comes to extra dimensions, I'm biased. I've worked on aspects of extra dimensions for more than fifteen years, so they hold a special place in my heart. But, with that confession as a qualifier, it's hard for me to imagine a discovery that would be more exciting than finding evidence for dimensions beyond the three with which we're all familiar. To my mind, there is currently no other serious proposal whose confirmation would so thoroughly shake the foundation of physics and so thoroughly establish that we must be willing to question basic, seemingly self-evident, elements of reality.

Beyond the scientific challenges of searching into the unknown, and the chance of finding evidence of extra dimensions, there are a couple of specific motivations for recent upgrades on the accelerator at Fermilab and for building the mammoth Large Hadron Collider. One is to find Higgs particles. As we discussed in Chapter 9, the elusive Higgs particles would be the smallest constituents of a Higgs field—a field, physicists hypothesize, that forms the Higgs ocean and thereby gives mass to the other fundamental particle species. Current theoretical and experimental studies suggest that the Higgs should have a mass in the range of a hundred to a thousand times the mass of the proton. If the lower end of this range turns out to be right, Fermilab stands a reasonably good chance of discovering a Higgs particle in the near future. And certainly, if Fermilab fails and if the estimated mass range is nonetheless correct, the Large Hadron Collider should produce Higgs particles galore by the end of the decade. The detection of Higgs particles would be a major milestone, as it would confirm the existence of a species of field that theoretical particle physicists and cosmologists have invoked for decades, without any supporting experimental evidence.

Another major goal of both Fermilab and the Large Hadron Collider is to detect evidence of supersymmetry. Recall from Chapter 12 that supersymmetry pairs particles whose spins differ by half a unit and is an idea that originally arose from studies of string theory in the early 1970s. If supersymmetry is relevant to the real world, then for every known particle species with spin-
¹
/2 there should be a partner species with spin-0; for every known particle species of spin-1, there should be a partner species with spin-
¹
/2. For example, for the spin-
¹
/2 electron there should be a spin-0 species called the
supersymmetric electron,
or
selectron
for short; for the spin-
¹
/2 quarks there should be supersymmetric quarks, or squarks; for spin
¹
/2 neutrinos there should be spin-0 sneutrinos; for spin-1 gluons, photons, and W and Z particles there should be spin-
¹
/2 gluinos, photinos, and winos and
zinos.
(Yes, physicists get carried away.)

No one has ever detected any of these purported doppelgängers, and the explanation, physicists hope with fingers crossed, is that the supersymmetric particles are substantially heavier than their known counterparts. Theoretical considerations suggest that the supersymmetric particles could be a thousand times as massive as a proton, and in that case their failure to appear in experimental data wouldn't be mysterious: existing atom smashers don't have adequate power to produce them. In the coming decade this will change. Already, the newly upgraded accelerator at Fermilab has a shot at discovering some supersymmetric particles. And, as with the Higgs, should Fermilab fail to find evidence of supersymmetry and if the expected mass range of the supersymmetric particles is fairly accurate, the Large Hadron Collider should produce them with ease.

The confirmation of supersymmetry would be the most important development in elementary particle physics in more than two decades. It would establish the next step in our understanding beyond the successful standard model of particle physics and would provide circumstantial evidence that string theory is on the right track. But note that it wouldn't prove string theory itself. Even though supersymmetry was discovered in the course of developing string theory, physicists have long since realized that supersymmetry is a more general principle that can easily be incorporated in traditional point-particle approaches. Confirmation of supersymmetry would establish a vital element of the string framework and would guide much subsequent research, but it wouldn't be string theory's smoking gun.

On the other hand, if the braneworld scenario is correct, upcoming accelerator experiments
do
have the potential of confirming string theory. As mentioned briefly in Chapter 13, should the extra dimensions in the braneworld scenario be as large as 10
^-16
centimeters, not only would gravity be intrinsically stronger than previously thought, but strings would be significantly longer as well. Since longer strings are less stiff, they require less energy to vibrate. Whereas in the conventional string framework, string vibrational patterns would have energies that are more than a million billion times beyond our experimental reach, in the braneworld scenario the energies of string vibrational patterns could be as low as a
thousand
times the proton's mass. Should this be the case, high-energy collisions at the Large Hadron Collider will be akin to a well-hit golf ball ricocheting around the inside of a piano; the collisions will have enough energy to excite many "octaves" of string vibrational patterns. Experimenters would detect a panoply of new, never before seen particles— new, never before seen string vibrational patterns, that is—whose energies would correspond to the harmonic resonances of string theory.

The properties of these particles and the relationships between them would show unmistakably that they're all part of the same cosmic score, that they're all different but related notes, that they're all distinct vibrational patterns of a single kind of object—a string. For the foreseeable future, this is the most likely scenario for a direct confirmation of string theory.

As we saw in earlier chapters, the cosmic microwave background radiation has played a dominant role in cosmological research since its discovery in the mid-1960s. The reason is clear: in the early stages of the universe, space was filled with a bath of electrically charged particles—electrons and protons—which, through the electromagnetic force, incessantly buffeted photons this way and that. But by a mere 300,000 years after the bang (ATB), the universe cooled off just enough for electrons and protons to combine into electrically neutral atoms—and from this moment onward, the radiation has traveled throughout space, mostly undisturbed, providing a sharp snapshot of the early universe. There are roughly 400 million of these primordial cosmic microwave photons streaming through every cubic meter of space, pristine relics of the early universe.

Initial measurements of the microwave background radiation revealed its temperature to be remarkably uniform, but as we discussed in Chapter 11, closer inspection, first achieved in 1992 by the Cosmic Background Explorer (COBE) and since improved by a number of observational undertakings, found evidence of small temperature variations, as illustrated in Figure 14.4a. The data are gray-scale coded, with light and dark patches indicating temperature variations of about a few ten-thousandths of a degree. The figure's splotchiness shows the minute but undeniably real unevenness of the radiation's temperature across the sky.

While an impressive discovery in its own right, the COBE experiment also marked a fundamental change in the character of cosmological research. Before COBE, cosmological data were coarse. In turn, a cosmological theory was deemed viable if it could match the broad-brush features of astronomical observations. Theorists could propose scheme after scheme with only minimal consideration for satisfying observational constraints. There simply weren't many observational constraints, and the ones that existed weren't particularly precise. But COBE initiated a new era in which the standards have tightened considerably. There is now a growing body of precision data with which any theory must reckon successfully even to be considered. In 2001, the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, a joint venture of NASA and Princeton University, was launched to measure the microwave background radiation with about forty times COBE's resolution and sensitivity. By comparing WMAP's initial results, Figure 14.4b, with COBE's, Figure 14.4a, you can immediately see how much finer and more detailed a picture WMAP is able to provide. Another satellite,
Planck,
which is being developed by the European Space Agency, is scheduled for launch in 2007, and if all goes according to plan, will better WMAP's resolution by a factor of ten.

Figure 14.4 (a) Cosmic microwave background radiation data gathered by the COBE satellite. The radiation has been traveling through space unimpeded since about 300,000 years after the big bang, so this picture renders the tiny temperature variations present in the universe nearly 14 billion years ago. (b) Improved data collected by the WMAP satellite.

The influx of precision data has winnowed the field of cosmological proposals, with the inflationary model being, far and away, the leading contender. But as we mentioned in Chapter 10, inflationary cosmology is not a unique theory. Theorists have proposed
many
different versions (old inflation, new inflation, warm inflation, hybrid inflation, hyperinflation, assisted inflation, eternal inflation, extended inflation, chaotic inflation, double inflation, weak-scale inflation, hypernatural inflation, to name just a few), each involving the hallmark brief burst of rapid expansion, but all differing in detail (in the number of fields and their potential energy shapes, in which fields get perched on plateaus, and so on). These differences give rise to slightly different predictions for the properties of the microwave background radiation (different fields with different energies have slightly different quantum fluctuations). Comparison with the WMAP and Planck data should be able to rule out many proposals, substantially refining our understanding.

In fact, the data may be able to thin the field even further. Although quantum fluctuations stretched by inflationary expansion provide a compelling explanation for the observed temperature variations, this model has a competitor. The cyclic cosmological model of Steinhardt and Turok, described in Chapter 13, offers an alternative proposal. As the two three-branes of the cyclic model slowly head toward each other, quantum fluctuations cause different parts to approach at slightly different rates. When they finally slam together roughly a trillion years later, different locations on the branes will make contact at slightly different moments, rather as if two pieces of coarse sandpaper were being slapped together. The tiny deviations from a perfectly uniform impact yield tiny deviations from a perfectly uniform evolution across each brane. Since one of these branes is supposed to be our three-dimensional space, the deviations from uniformity are deviations we should be able to detect. Steinhardt, Turok, and their collaborators have argued that the inhomogeneities generate temperature deviations of the same form as those emerging from the inflationary framework, and hence, with today's data, the cyclic model offers an equally viable explanation of the observations.

However, the more refined data being gathered over the next decade may be able to distinguish between the two approaches. In the inflationary framework, not only are quantum fluctuations of the inflaton field stretched by the burst of exponential expansion, but tiny quantum ripples in the spatial fabric are also generated by the intense outward stretching. Since ripples in space are nothing but gravitational waves (as in our earlier discussion of LIGO), the inflationary framework predicts that gravitational waves were produced in the earliest moments of the universe.
⁸
These are often called
primordial gravitational waves,
to distinguish them from those generated more recently by violent astrophysical events. In the cyclic model, by contrast, the deviation from perfect uniformity is built up gently, over the course of an almost unfathomable length of time, as the branes spend a trillion years slowly heading toward their next splat. The absence of a brisk and vigorous change in the geometry of the branes, and in the geometry of space, means that spatial ripples are
not
generated, so the cyclic model predicts an absence of primordial gravitational waves. Thus, if primordial cosmological gravitational waves should be detected, it will be yet another triumph for the inflationary framework and will definitively rule out the cyclic approach.

It is unlikely that LIGO will be sensitive enough to detect inflation's predicted gravitational waves, but it is possible that they will be observed indirectly either by Planck or by another satellite experiment called the Cosmic Microwave Background Polarization experiment (CMBPol) that is now being planned. Planck, and CMBPol in particular, will not focus solely on temperature variations of the microwave background radiation, but will also measure
polarization,
the average spin directions of the microwave photons detected. Through a chain of reasoning too involved to cover here, it turns out that gravitational waves from the bang would leave a distinct imprint on the polarization of the microwave background radiation, perhaps an imprint large enough to be measured.

So, within a decade, we may get sharp insight into whether the bang was really a splat, and whether the universe we're aware of is really a three-brane. In the golden age of cosmology, some of the wildest ideas may actually be testable.