17 April 2009

Physics: Dark energy - does it really exist?

The April 2009 edition of Scientific American contains an article by Oxford cosmologists Timothy Clifton and Pedro G. Ferreira that addresses the question of whether “dark energy” exists.

The question of whether this baffling form of energy exists arose in the wake of the discovery 11 years ago, due to measured anomalies in the red shifts of light from supernova explosions, that the expansion of the universe is accelerating. This flies in the face of our expectation that the gravitational pull of all the matter in the universe should be causing the expansion of the universe following the “Big Bang” to slow, leading ultimately to a massive collapse.

The fact that there is something that we cannot detect (dark energy) pushing the universe along rather than causing it to slow is so puzzling that some cosmologists are revisiting the fundamental postulates that led them to deduce its existence in the first place.

Of these the most fundamental is the Copernican assumption that we live in an ordinary neighbourhood of space, that there is nothing special about our part of the universe. This revolutionary (in the 16th century) principle, so seemingly mundane, is a very powerful one for cosmologists: it means that they can work on the basis that the universe has a uniform density and looks the same in every direction. We can therefore extrapolate from what we see in our own region to the universe at large. There is a huge amount of evidence to support this idea.

A possible alternative explanation which would not require the existence of dark energy involves postulating that we live in a massive region in which the density of the universe is only half or a third of the density elsewhere. With that assumption it is possible to construct a chain of reasoning that says that the anomalous red shifts of supernova explosions that make it look as though the expansion of the universe is accelerating is in fact due to the fact that the light we see from the explosion travels at different speeds as it passes through more and less dense regions.

Whichever of these explanations turns out to be the more supportable, this conundrum is a huge challenge for modern physics. Both explanations strike cosmologists as extremely unlikely. Fortunately the cosmic void idea does not exactly mimic dark energy, so there are some ideas for very sophisticated observations that would help us to distinguish between the two.

Read the full article here.

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