Warped Passages - Lisa Randall [148]
Even in the absence of particles, the universe can carry energy known as vacuum energy. According to general relativity, this energy has a physical consequence: it stretches or shrinks space. Positive vacuum energy accelerates the expansion of the universe, while negative energy makes it collapse. Einstein first proposed such an energy in 1917 in order to find a static solution to his equations of general relativity in which the gravitational effect of the vacuum energy would cancel that of matter. Although he had to abandon this idea for many reasons, including Edwin Hubble’s observed expansion of the universe in 1929, there is no theoretical reason that such vacuum energy should not exist in our universe.
Indeed, astronomers have recently measured the vacuum energy in our cosmos (it is also known as dark energy or the cosmological constant) and found a small positive value. They have observed that distant supernovas are dimmer than you’d expect unless they were accelerating away. The supernova measurements and the detailed observations of relic photons created during the Big Bang tell us that the expansion of the universe is accelerating, which is evidence that the vacuum energy has a small positive value.
This measurement is very exciting. But it also introduces a significant puzzle. The acceleration is very slow, which tells us that the value of the vacuum energy, though nonzero, is extremely tiny. The theoretical problem with the observed vacuum energy is that it is far smaller than anyone would estimate. According to string theory estimates, the energy should be much bigger. But if it were, this energy wouldn’t just lead to the hard-to-measure supernova acceleration. If the vacuum energy were big, the universe would long ago have collapsed (if negative) or quickly expanded away to nothing (if positive).
String theory has yet to explain why the universe’s vacuum energy is as small as we know it must be. Particle physics has no answer to this problem either. However, unlike string theory, particle physics does not purport to be a theory of quantum gravity—it’s less ambitious. A particle physics model that cannot explain the energy is unsatisfying, but a string theory that gets the energy wrong is ruled out.
The question of why the energy density is so extraordinarily tiny is an entirely unsolved problem. Some physicists believe that there is no true explanation. Although string theory is a single theory with a single parameter—the tension of the plucked string—string theorists cannot use it to predict most features of the universe. Most physical theories cannot yet use it to predict most features of the universe. Most physical theories contain physical principles which allow you to decide which of the many possible physical configurations a theory would actually predict. For example, most systems will settle down into the configuration that has the lowest energy. But that criterion doesn’t seem to work for string theory, which looks as if it might give rise to an infinite number of different configurations that don’t have the same vacuum energy—and we don’t know which of them, if any, is preferred.
Some string theorists no longer try to find a unique theory. They look at the possible sizes and shapes of rolled-up dimensions and the different options for the energy a universe could contain, and conclude that string theory can only delineate a landscape that describes the huge number of possible universes in which we might live. These string theorists don’t think that string uniquely predicts the vacuum energy. They believe that the cosmos houses many different disconnected regions with different values of the vacuum energy, and we live in the portion of the cosmos that contains the right one. Of the many possible universes,