Tuesday, November 27, 2007

Could humanity's observation of dark energy have shortened the life ?

Could humanity's observation of dark energy have shortened the life span of the universe? The answer is "yes" according to the author of a new scientific paper that has recently come to light. Featured in the latest edition of New Scientist magazine, the subscriber-only story, "Has observing the universe hastened its end?", discusses the paper and its claims.

Now, before I go further, I must point out that this work has not yet appeared in any peer-reviewed journal. It has been submitted to Physical Review Letters, and a pre-print can be found on arXiv.org. Had Prof. Lawrence Krauss not been one of the authors, I would have completely written it off. Early on in the New Scientist article, Prof. Krauss states that "incredible as it seems, our detection of the dark energy may have reduced the life expectancy of the universe." Incredible indeed—let's examine how he and his colleague, James Dent, came to this conclusion.

Their official paper, titled "The Late Time Behavior of False Vacuum Decay: Possible Implications for Cosmology and Metastable Inflating States," is far from grandiose. It extends a body of work initiated in the 1950's by Soviet physicist L. A. Khalfin. Khalfin determined that the long time behavior of a metastable quantum state is not described by the traditional exponential decay, but rather by a power-law type decay. That determination is accurate, but has remained obscure because nearly all experimental systems will have decayed long before this transition becomes significant. Krauss and Dent attempt to extend this idea from traditional quantum mechanics to quantum field theory, and look at its implications for cosmology.

To understand the potential implications of the calculations in the paper, one must start at the beginning—the Big Bang, and even before. It is currently believed that the universe blinked into existence somewhere around 13.7 billion years ago. This event—the Big Bang—is theorized to have been precipitated when a "bubble of weird high-energy 'false vacuum' with repulsive gravity decayed into a zero-energy 'ordinary' vacuum." The energy released during this transition would have created intense heat, and all the matter we see in the universe.

This idea was challenged in the late 1990's by the discovery of dark energy. Dark energy, coupled with the fact that the expansion of the universe is accelerating, suggests that the Big Bang did not produce a zero-energy vacuum, but another metastable false vacuum. Using the analogy of a decaying radioactive atom where shifts in energy states occur at random, Professor Krauss says that "it is entirely possible [the energy state of the universe] could decay again, wiping the slate of our universe clean." In a nutshell, this would mean that we, and everything we know, would cease to exist.

How does this relate to the work in the research article? If the current false vacuum state that our universe exists in survives past a a certain point—the point where the decay switches from exponential to power-law—then it should become eternal. This is explained by the assumption that the false vacuum state will grow at an exponential rate for all time. If its decay suddenly becomes slower—as in the power law decay regime—then the false vacuum will grow faster than it could possibly decay and would never be destroyed. According to calculations contained in the paper, the closer the false vacuum energy is to zero, the less time that will need to elapse before the decay rate switches from fast (exponential) to slow (power law). Given the fact that, in our universe, the vacuum energy is just above zero (0.01 eV), we could be well past the point at where the universe has switched from the fast decay to slow decay—although without an estimate of the current decay rate, this cannot be known for sure.

However, since this is a quantum issue at its core, Krauss points out that measurements can affect the outcome of the system. He suggests that our measurements of supernovae in 1998, which detected the existence of dark energy, may have reset the false vacuum's decay clock to zero, switching it back to the fast decay regime, and greatly decreasing the universe's chance of surviving. "In short, we may have snatched away the possibility of long-term survival for our universe and made it more likely it will decay," says Krauss.

How could something like this possibly happen? In quantum mechanics, there is an effect known as the quantum Zeno effect—an oddity of the quantum world that suggests a system can be kept in an excited state simply by repeated measurements. This can be described using a quantum system initially in state 'A'. After time begins, the system wants to decay to state 'B' but, before it reaches state 'B', it will exist as a superposition of states 'A' and 'B'. If one measures the system shortly after it begins, it would have a high probability of collapsing entirely to state 'A' again, essentially resetting the system's internal quantum clock. Krauss is suggesting that, by observing the dark energy, we reset the internal quantum clock of the false vacuum universe, and that may have caused it to return to a point before it has switched from the fast decay to the slow decay—in the process greatly reducing the universe's ultimate chance of survival.

Now, as I said at the beginning, the work has yet to be published in a peer-reviewed journal. While the work in the paper seems sound to me—although I am admittedly far, far from an expert in this field—it barely touches on the larger picture that the New Scientist article focuses on. In fact, the only mention of the possibility of disrupting our universe's lifetime is written in the second to last sentence in the research article, where the authors state, "Have we ensured, by measuring the existence of dark energy in our own universe, that the quantum mechanical configuration of our own universe is such that late time decay is not relevant?"

There are problems that need to be addressed in the research paper before this work can be extend to the real world (or universe). The most obvious—and the first addressed by the authors—is that gravity was ignored in this work. They do say that in future work, they will examine whether this power-law time dependence even exists for the vacuum when the effects of a gravitational field are taken into account. Even without that omission this claim is controversial. For an opposing viewpoint, the New Scientist writer contacted Prof. Max Tegmark of MIT who states that the quantum Zeno effects is not predicated on humans doing the observations of dark energy or light. "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," says Tegmark.

While Krauss makes and interesting claim, it is still quite far out there.
Posted by Ajay :: 10:03 AM :: 0 comments

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