Once Before Time - Martin Bojowald [76]
Postulating new forms of matter to explain otherwise irreconcilable data is audacious, but nothing new in the history of physics. Wolfgang Pauli, for instance, in 1930 proposed to relate an apparent energy loss in certain radioactive decays to the existence of a particle unknown at that time. The particle was supposed to interact only weakly so as to elude direct measurements and be perceptible only by the energy it steals away. Everything was cooked up very nicely: The postulate of a new particle conveniently combined with a mechanism to make it unobservable can hardly be ruled out, or so it seemed. But not even Pauli liked this; although the energy loss was confirmed by more sensitive methods, he himself considered this explanation a last resort—an emergency treatment to save the law of energy conservation so important for all of physics. But Pauli was right: The particle was finally detected in 1956 by Frederick Reines and Clyde Cowan; Reines was awarded a share of the 1995 Nobel Prize in Physics for his part in the discovery (Cowan died in 1974). Despite the weakness of interactions, the detection was made possible by the strong source of a nuclear reactor;4 given sufficiently many elusive particles, not all of them can escape unseen. We have already encountered this particle: it is the neutrino, whose name was introduced in 1934 by Enrico Fermi. It plays an important role in modern astrophysics and cosmology.
NEGATIVE PRESSURE:
ELUSIVE REPULSION
Will cosmology be granted a similar success: a direct and unquestionable proof of a new enigmatic energy form it suggested? Before this can become reality, theorists first have to hunt for possible explanations in the zoo of particle physics or the menagerie of gravitational phenomena. Without any such indication, an experimental search would resemble that for the proverbial needle in the haystack. Many exotic candidates are currently proposed as possible sources, all providing negative pressure. The nature of some of them will eventually bring us back to the topic of quantum gravity.
DARK ENERGY: THE UNKNOWN 70 PERCENT
Nature tends to hide herself.
—HERACLITUS “THE DARK,” Fragment
Even imaginative theory has much to grapple with in dark energy, whose properties are far from common. After all, this energy form must cause accelerated expansion, and thus repel not only the remaining matter in the universe but even space and time. Such a behavior is highly unexpected for matter subject to gravity, which normally causes masses and energy distributions to attract each other—implying for instance the serious singularity problem from unchecked collapse.
Quantum gravity can lead to repulsive forces, but it is usually significant only on the tiny distance scales of the Planck length, at which energy can easily behave in unexpected ways. With dark energy, however, we see repulsion on the gigantic scales of the entire current universe. To explain this with quantum gravity is conceivable but extremely difficult. It can happen only if small quantum gravity effects in tiny spatial regions add up in just the right way for the observed behavior to occur. Concrete calculations, let alone possible direct detections of this new energy form, remain elusive. We must postpone quantum gravity a little longer.
Another possible solution is to doubt some of the cosmological premises. When evaluating data, one naturally assumes that a spatially homogeneous model with uniform density everywhere can be used. As shown by the galaxy maps, this is indeed a reasonable assumption. But the accelerated expansion began relatively recently, after the phase of the universe shown in the majority of the galaxy maps. Could it be that gravitational attraction, having worked ceaselessly in the meantime, has led to stronger matter clumping, such that the homogeneity assumption is no longer correct?
This proposal, developed particularly by Thomas Buchert, has the advantage that the validity of general relativity need not be doubted. Accelerated expansion would occur only