Once Before Time - Martin Bojowald [83]
OBSERVING QUANTUM GRAVITY? DESPERATE FOR CLUES
In the universe one directly observes or infers not negative pressure, but accelerated expansion. Negative pressure is the only classical way by which general relativity can sometimes, in special situations, generate repulsive forces despite its normally attractive character. Primarily, however, we are interested in an explanation of the acceleration and of the repulsive force itself, and here, as in the case of the singularity problem, quantum gravity can come to our aid. If this works, intricacies encountered in attempts to justify negative pressure can possibly be avoided.
In contrast to dark energy, which must be explained for a big universe in which no significant role is expected for quantum gravity effects, quantum aspects of space-time may well have been important in the early and hot universe. In this phase, after all, a repulsive quantum gravity force, at least according to loop quantum cosmology, acts to stop the collapse of the preceding universe at the big bang and turn it into the expansion visible now. But a force that stops the collapse can also drive an already expanding universe to accelerated growth, just as a rock thrown up in the air first slows down and stops for an instant, then falls down back toward the ground with increasing speed. Effective forces of loop quantum cosmology, active when its discrete space-time repels rather than absorbs matter energy, indeed easily make a universe of small sizes expand in an accelerated way. No specially selected matter under negative pressure is necessary. Acceleration instead happens as a consequence of a new behavior of gravitation, a changed force that at high energies and small extensions of the universe deviates from general relativity. If the gravitational force is no longer purely attractive but has a repulsive component, even conventional matter can be in agreement with accelerated expansion.
So far, however, it remains unclear whether the degree of acceleration is sufficient. In any case, the horizon problem does not arise in the absence of a starting point such as the big bang singularity; but as in inflationary models based on negative pressure, one needs expansion by a sufficiently large factor to ensure a homogeneous matter distribution. How this is possible in general will be revealed by investigations currently in progress by several groups in the UK, for instance Ed Copeland in a fundamental study together with David Mulryne, Nelson Nunes, and Maryam Shaeri, as well as independently by Aurélien Barrau and Julien Grain in France, Shinji Tsujikawa in Japan, Gianluca Calcagni, and others.
These cosmological investigations rely on substantial derivations of equations to describe the evolution of small disturbances in the matter distribution. A latticelike structure of space-time leads to deviations from the continuum equations of general relativity, which can be evaluated and eventually compared with observations. Whether the involved calculations required could lead to success remained unclear for a long time. In general relativity, one is dealing with an overdetermined set of equations: There are more equations to be solved than free variables, a fact related to the main difficulty in finding a quantum theory of gravity. In general relativity, the set of equations is consistent, but if extra terms or effective forces are included as they may result from quantum gravity, this consistency is not easy to maintain. If two of the equations give different solutions for one and the same variable,