Saturday, November 24, 2007

Has mankind reduced the life-expectancy of the universe?

Hot on the heels of my post concerning the destruction of the universe, cosmologist Lawrence Krauss has suggested that mankind's detection of dark energy in 1998 may reduce the lifetime of our universe!

The idea, once again, is that the false vacuum energy of the scalar field responsible for inflation, (the hypothetical exponential expansion of the very early universe), may not have decayed to zero, and, since the time of inflation, may have been residing in another false vacuum state of much lower, but non-zero energy. It is suggested by Krauss and James Dent that the dark energy detected by cosmologists in the past decade, may simply be this residual false vacuum energy.

A false vacuum state is 'metastable' in the sense that it is a state of temporary stability, but one which is prone to decay, much like the nucleus of a radioactive atom. Krauss and Dent point out that in quantum mechanics, the survival probability of a metastable state will decrease exponentially until a critical cut-off time, at which point the survival probability will only decrease according to a power law. Given that a false vacuum expands exponentially, there will be a net increase in the volume of space in a false vacuum state after this cut-off time. Krauss argues that whilst the metastable residual false vacuum of our universe may have reached this critical cut-off, the act of observing the dark energy may have reset the decay according to the 'quantum zeno effect'. The consequence is that mankind's actions may have significantly increased the probability that the residual false vacuum will decay to the true vacuum, destroying all the structure in our universe.

With an irony that may have been lost on some readers, The Daily Telegraph's Roger Highfield refers to these as "damaging allegations."

There are also at least two reasons why the claims shouldn't be taken seriously. Firstly, as Max Tegmark points out in New Scientist, the quantum zeno effect does not require humans to make observations: "Galaxies have 'observed' the dark energy long before we evolved. When we humans in turn observe the light from these galaxies, it changes nothing except our own knowledge." Secondly, Krauss and Dent's assumption that dark energy can be equated with the energy density of a scalar field is inconsistent with the current observational evidence, which suggests that the dark energy is a cosmological constant. The energy density due to a cosmological constant is a property of space, not a property of any matter field in space. The energy density due to a cosmological constant is literally constant in space and time, unlike that attributable to the scalar fields postulated to explain dark energy.


Susan's Husband said...

One might also note that the real import is that we should observe dark energy as frequently as possible, thereby using the quantum zeno effect to prevent decay.

Gordon McCabe said...

Indeed, good point. It's a wonder that High Purity Germanium gamma spectrometers ever detect the photons from decaying radioactive nuclei. Maybe the nuclei only decay in the 'dead time' of the detectors...

Grinnyguy said...

My sister asked me about Quantum mechanics and how observation is important as an integral part of the theory itself. Recalling my first year university physics from a few years ago and trying to pass it onto my sister - a french and history student - was a bit of a stretch, I must admit. I barely understood it at the time, and the philosophy much less than the mathematics. Glad to see it is still stirring debate, quantum mechanics requires such a huge leap away from what we see happening in the world, it's nice to know that other people sruggle with it.