Two American physicists may have identified a fundamental new characteristic of the fabric of space.
Leonard Parker and Alpan Raval of the University of Wisconsin at Milwaukee are revisting what Einstein called his "greatest blunder": the proposal that a mysterious 'cosmological constant' affects the expansion of the Universe1.
Until recently, this cosmological 'fudge factor' had seemed redundant, but recent observations have brought the Einstein's blunder back into fashion. Now, Parker and Raval suggest that we probably shouldn't be resurrecting this idea just yet.
There are two theoretical models that attempt to account for the Universe's puzzling behaviour. The alphaCDM model invokes Einstein's cosmological constant (alpha); whereas the other, the quintessence model, is a mathematical minefield. Parker and Raval suggest a third way -- an idea that dispenses with Einstein's alpha and yet avoids the numericalg horrors of quintessence.
Einstein invoked alpha in 1917 in an attempt to reconcile his equations of relativity with scientific opinion of the time. His equations predicted that the Universe should be expanding, but researchers then believed the Universe to be static, so Einstein added alpha-- which implies that a repulsive force counteracting gravitational attraction pervades space -- to his equations to make them 'work'.
But when, in 1929, Edwin Hubble found evidence that the Universe actually is expanding, there was no need for alpha. Until, that is, opinion swung again in 1998 after astronomers found that very distant supernovae are even farther away than they seem -- implying that the expansion of the Universe is accelerating. A cosmological constant with a value greater than zero could explain this, essentially giving the Universe its extra push.
If this is true, it means that even empty space (vacuum) contains energy. This is not as strange as it sounds.
The Universe may be expanding, but it is also 'flat' -- meaning that parallel light beams never converge. This is possible only if the Universe contains a certain critical amount of matter. There does not seem to be enough visible matter around, so physicists invoke 'dark matter' to make up the difference. But even then there is a shortfall, which a vacuum containing energy, and thus a non-zero alpha, could account for. This is the alphaCDM model: a non-zero alpha plus a certain amount of 'cold' dark matter (CDM).
The quintessence model does not rely on a cosmological constant, but assumes that an energy field pervades space -- not vacuum energy, but a new form of 'dark energy' that provides the pressure to accelerate the Universe's expansion. Again, there are problems with this model. First, it contains an uncomfortably large amount of arbitrary numbers and second, it does not seem to fit quantum theory.
Parker and Raval say that their new model avoids both these shortcomings. They too assume that there is a new kind of energy field pervading all space, but say that it only affects gravitational forces between objects. (Quintessence affects other kinds of interaction such as electromagnetic forces.)
The duo says that such an energy field, together with cold dark matter, can account for the observations of a flat, accelerating Universe as effectively as the other two models -- without needing a cosmological constant. But, unfortunately, observational data are currently too hazy to tell the various theories apart.
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