Pedagoguery

Since 1998, when the first evidence was found that the expansion of the universe is accelerating, scientists have wondered about the nature of the “stuff” responsible for that expansion. Labeled dark energy, for lack of a better term, it appears to be constant through space and time. Furthermore, evidence of existence, although indirect, continues to accumulate. It must be out there, but what exactly is “it?”

The evidence for the existence of dark energy is quite strong. In fact, three independent lines of evidence have arisen to support the concept. The first was the survey of high redshift supernovae. This survey was looking for a special kind of supernova, called a Type 1a supernova. In this kind of supernova, a white dwarf accumulates enough matter from a companion star to cross the Chandrasekhar limit. This is the theoretical mass limit above which a white dwarf must collapse. When this happens, the carbon and oxygen that comprise the core of the white dwarf undergoes a runaway fusion reaction, blowing the star apart. Because the mass for all of these objects is the same, 1.4 times the mass of the sun, in theory, they should all explode with the same brightness, making them a good standard candle. This allows scientists to determine how far away they are by looking at their apparent brightness and comparing it with what it should be. Using the Hubble space telescope, astronomers have been able to look back to a redshift of nearly 2, back to a time when the universe was one third of its current size, and the evidence is clear – the universe is expanding. By comparing the results to models, we come up with a universe consisting of about 5% normal matter, 25% dark matter, and 70% dark energy.

A second line of evidence comes from the Wilkinson Microwave Anisotropy Probe (WMAP). This is a satellite that observes the cosmic background radiation and looks for minute fluctuations. The microwave background radiation is what physicists call a blackbody spectrum. Essentially, any object radiates a specific spectrum based on its temperature. The characteristic temperature of the microwave background radiation is 3.7 degrees K, or 3.7 degrees above absolute zero. However, there are tiny fluctuations in the spectrum, amounting to only one part in 100,000. The size of these fluctuations can tell us some fundamental facts about the composition of our universe, and the analysis of the WMAP data gives us a universe with about 5% normal matter, 25% dark matter, and 70% dark energy.

The third line of evidence arises from studies of galaxy clustering. Scientists have run various computer models of the large scale evolution of the universe. They vary the initial conditions – proportion of normal matter to dark matter (which can be cold, hot, or warm), and dark energy. By comparing the models with what is observed, they find that the observations fit best a universe with about 5% normal matter, 25% dark matter, and 70% dark energy.

This convergence of evidence is quite remarkable. Three independent lines all arriving at nearly the same answer. Either that answer is correct, or we have some very fundamental misunderstandings in the laws of physics. Given how closely other observations fit with our understanding of physical laws the first explanation is by far the more likely one.

That still leaves us with the question: what is dark energy? Unfortunately, we still have no good answers. A number of candidates have arisen, but none of them are quite satisfactory. The first of these is something called vacuum energy. According to quantum mechanics, at very small scales, virtual pairs of particles are constantly being created and destroyed in the vacuum. They are called virtual because they cannot be directly observed – they exist for too short a time to be observable within the constraints of the Heisenberg Uncertainty Principal. However, their collective effect can be, and has been, observed, so we know this is true. The collective effect of these particle pairs can be the same as Einstein's cosmological constant, which is to say, a large scale repulsive force – sort of an anti-gravity that only works on very large scales. This is also how dark energy seems to work. So far, so good. There is one major problem, however. Simple efforts to calculate how strong vacuum energy should be come up with a figure that is 10120 times too high. This is a phenomenally large number. If the cosmological constant were that high, the universe would have exploded into a size so large so fast that atoms couldn't have formed. We certainly wouldn't be around to be observing such a universe, so clearly something is wrong. This is in fact the single most embarrassing discrepancy between theory and observation in all of physics.

There are other possibilities, but they too have their drawbacks. One possibility uses vacuum energy, but postulates that it is different in different places in the universe. This would mean that there are different domains within the universe, each expanding at different rates, and that life does not arise in domains where vacuum energy is large. However, this is in one sense, simply throwing up your hands and giving up on the problem. It gives up the possibility that the problem can be solved from first principals and just says, in effect, that it is because it is. A second possibility is that the value of dark energy is constant in space, but varies over time. Thus, it could have been much smaller or larger in the past. In once sense, the concept of inflation supports this idea. This concept is called quintessence, and careful observations are being made about the long term magnitude of dark energy to confirm or rule out this possibility. A final possibility denies the existence of dark energy at all, and instead postulates a modification to General Relativity, Einstein's theory of gravity. The problem with this is that it is very hard to modify general relativity without violating existing observations and experimental constraints.

The problem of dark energy is one of the most puzzling and intriguing problems in modern physics, and scientists are working on a number of ways to further investigate the phenomenon. It is possible that we will find an answer within our lifetimes, but it is also possible that the mystery will endure.

Next issue I will talk about an intriguing idea call dark energy stars.