Knocking on Heaven's Door - Lisa Randall [155]
Of course, missing energy is also characteristic of supersymmetric models. The signals could even appear so similar that even if a discovery is made, people from both extra-dimensional and supersymmetry camps are likely to interpret the data as supporting their expectations—at least initially. But with detailed understanding of the consequences and predictions of both types of models, we will be able to determine which idea—if either—is correct. One of our goals in building models is to match experimental signatures and details to their true implications. Once we have characterized different possibilities, we know the rate and features of the signatures that follow, and we can use subtle features to distinguish among them.
In any case, at this point, along with most of my colleagues, I doubt that the large-extra-dimensional scenario is truly the solution to the hierarchy problem, though we will soon see a very different extra-dimensional example that seems much more promising. For one thing, we don’t expect extra dimensions to be so large. It turns out that the extra dimensions would have to be enormous relative to the other scales posed in the problem. Even though the hierarchy between the weak scale and the gravity scale is in principle eliminated, a new hierarchy involving the new dimensions’ size gets introduced in this scenario.
Even more worrisome is that in this scenario, we would expect the evolution of the universe to be very different from what has been observed. The problem is that these very large dimensions would expand along with the rest of the universe until the temperatures are very low. For a model to be a potential candidate for reality, the evolution of the universe it predicts would have to mimic that which has been observed that is consistent with only three dimensions of space. That poses a difficult challenge for scenarios with such large additional dimensions.
These challenges are not enough to definitively rule out the idea. Clever enough model builders can find solutions to most problems. But the models tend to become overly complicated and convoluted in order to agree with all observations. Most physicists are skeptical about such ideas on aesthetic grounds. Many have therefore turned to more promising extra-dimensional ideas such as the ones described in the following section. Even so, only experiments will tell us for certain whether models with large extra dimensions apply to the real world or not.
A WARPED EXTRA DIMENSION
Large extra dimensions are not the only potential solution to the hierarchy problem, even in the context of an extra-dimensional universe. Once the door was opened to extra-dimensional ideas, Raman Sundrum and I identified what seems to be a better solution66—one that most physicists would agree is much more likely to exist in nature. Mind you, that doesn’t mean that most physicists think it is likely to be true. Many suspect that anyone would be lucky to correctly predict what the LHC will reveal or to get a model completely correct without further experimental clues. But it’s an idea that probably stands as good a chance as any of being right, and—like most good models—presents clear search strategies so that theorists and experimenters can more fully exploit all the LHC’s capabilities—and maybe even discover evidence that the proposal is true.
The solution that Raman and I proposed involves only a single extra dimension, and that dimension need not be large. No new hierarchy involving the dimension’s size is necessary. And—as opposed to large-extra-dimensional scenarios—the universe’s evolution automatically agrees with late time cosmological observations.
Although our focus is the single new dimension, additional dimensions of space might exist as well—but in this scenario they won’t play any discernible role in explaining particle properties. Therefore, we can justifiably ignore them when investigating the hierarchy solution—in accordance with the effective