A recent issue of great interest to the scientific community has been cosmological expansion and what it implies about the matter that comprises the vast extent of our universe. Two teams of researchers have been using supernovae to study the mystery of the expanding universe and to challenge conventional astronomical theories. By tracking arriving light from these distant supernovae, modern scientists have been able to discern the general shape of the universe and its rate of expansion. Surprisingly, their research suggests that the universe is “emptier” than expected and that its rate of expansion is increasing instead of slowing down. Current attempts to explain these newly discovered properties of the universe involve an unusual type of matter than counteracts the effects of gravity. Thus modern supernova research has compelled scientists to reevaluate some of the fundamental laws that describe our universe.

HISTORY OF COSMIC EXPANSION

The evolution of cosmological expansion theory has proven to be very useful. For example, researching supernovae in order to study cosmic expansion has given scientists insight into the components of our universe (matter, light and other forms of energy). This knowledge is invaluable because matter dictates the geometry, nature and behavior of our universe.

One of the first people to theorize about the connections between matter, light and energy was Albert Einstein with his general theory of relativity. Interestingly, he was surprised and skeptical when his equations disputed the common belief that the universe remained constant in size, instead implying that it was dynamic. In order for Einstein’s equations to accurately describe a static universe, a cosmological constant was necessary. Having doctored his equations to do just that, Einstein didn’t know that this constant could represent what would later be termed “dark energy” (the matter than now supports the feasibility of the accelerating cosmic expansion model).

The idea of an expanding universe was later substantiated by the research of Edwin P. Hubble, which involved measuring galactic motion. By determining the outward velocity of galaxies through observation of redshifts in their visible spectra, Hubble’s team demonstrated that faint, distant galaxies were moving outward at a faster rate than those bright ones nearby. Such redshifts indicated that the radiation emitted by supernovae was being “stretched” by outward motion, just as the theory of an expanding universe suggested.

The most conspicuous flaw in Hubble’s research was the way in which his team measured distance. They estimated the distance to various galaxies by assuming that they all had the same intrinsic brightness. However, because of the varying brightness of supernovae, it is inherently incorrect to suppose that bright galaxies are nearby and faint ones are far away. Additionally, if a galaxy is extremely far away, its light might take so long to reach the Earth that the galaxy actually appears as it was billions of years ago, when it had potentially very different properties.

For these reasons, astronomers have searched for astronomical bodies with more consistent characteristics in order to minimize discrepancies and accurately study the expansion of the universe. In the 1970’s, scientists were tempted to use quasars due to their intense brightness - in spite of that, their diversity made them even more useless than galaxies. Finally, it was discovered that supernovae might be a suitable standard to use in cosmological exploration. The regularity of the intrinsic brightness of type 1a supernovae made these stars the best tools for surveying space-time.

SUPERNOVAE

A type 1a supernova is the final stage in the life cycle of an ordinary star. While most white dwarfs cool off and leave behind a burned-out core, some end their lives with a massive thermonuclear explosion. If a white dwarf is part of a close, semi-detached binary system it is possible for mass to transfer from the giant companion star onto the cooling core of the white dwarf. If the white dwarf acquires enough of the giant star’s mass to exceed to Chandrasekhar limit, then the increased gravitational pressure exerted on the white dwarf’s core causes carbon to fuse at an amazing rate. This is turn causes the white dwarf to explode, becoming a type 1a supernova.

Generally, supernovae occur within a single galaxy once every 300 years. Throughout a few thousand galaxies, supernovae occur almost monthly. Thus, although the universe is immense and scientists can only examine a portion of it at a time, it is possible to observe enough supernovae to make them useful tools in studying the universe. Furthermore, since variations in supernova correspond with the mass of the star, astronomers are able to use the duration of a supernova to deduce its intrinsic brightness the within 12%.

Consequently, supernovae are the best “standard candles” currently known because of the relative frequency with which they occur and the fact that astronomers are able to infer their intrinsic brightness.

FINDING SUPERNOVAE

To locate supernovae, researchers use large electron telescopes to take digital pictures of certain areas of the sky. By repeating this process over a period of time and from different observation points (both on Earth and in space), it is possible to observe changes in galaxies that might denote supernovae. Correcting for atmospheric interference and digitally enhancing the images (for example, an image of one part of the sky is subtracted from a similar image taken at a different time to yield the difference in light between the two) allow researchers to study variable features in distant galaxies. To view an animated plot of the light emitted by a supernova as a function of time, go to the following website:

SIGNIFICANCE OF SUPERNOVAE BRIGHTNESS

When studying distant supernovae, one observation pointedly stood out: supernovae were, on average, 25% dimmer than would be expected if the universe were slowing under the influence of gravity. With widely accepted cosmological theories being challenged, astronomers looked to possible ordinary explanations for this occurrence.

One was that cosmic dust might be filtering out some of the light. This has been discounted because the reddening in supernova color that would consequently be expected has not been observed (this would be due to the tendency of dust particles to preferentially screen out blue light). Also, there has not been the variation in measurements that would be the result of naturally uneven concentrations of cosmic dust.

A second hypothesis suggested that galaxies were bending light as it passed by, an effect called gravitational lensing. It has been determined that this is not a factor because it would only affect supernovae that are located beyond those that we are currently studying.

Lastly, it is conceivable that there are variations in distant supernovae (that formed earlier) as opposed to those supernovae that are closer (and formed later). Nevertheless, this should have a minimal effect because researchers already attempt to correct for such variations when doing their calculations.

Having set aside these explanations with reasonable conviction, scientists turned their attention to the properties of space-time. One possibility is that our universe might be negatively curved. A two dimensional circle with a fixed radius has a circumference of 2pr, whereas a circle that is curved has a circumference of slightly more than 2pr. If our universe exhibited such negative curvature, then the sphere of radiation emitted by a supernova would have a larger area than if our universe was flat. This difference in area would cause the supernova to appear fainter than predicted. However, current evidence indicates that our universe is flat rather than curved.

Besides the possible curvature of the universe, the faintness of supernovae might occur because they are farther away than implied by their redshifts. It is theorized that this divergence indicates that the universe expanded more slowly in the past than anticipated, stretching light less and hence causing less extreme redshifts.

RATE OF COSMIC EXPANSION

In the early studies of the cosmos, the universe was considered static. This theory was later revised to suggest a dynamic universe that was expanding at a slowly decelerating rate. Recently, our description of the universe has evolved yet again to reflect an accelerating rate of cosmic expansion. This modification necessarily affects our assumptions about the matter of our universe. If the universe consisted of normal matter, it would be subject to typical gravitational attraction. We would thus expect the universe to be expanding, but at a gradually slowing rate. Since this is not how the universe behaves, it is logical to think that the universe contains a different type of matter that counteracts the effects of traditional gravity. This new matter, called “dark energy” or cosmological antigravity, exerts a force opposite that of normal matter. This repulsive gravitational pressure is in fact associated with Einstein’s cosmological constant. It is thought that up until 5 billion years ago attractive gravitational force was stronger than that of dark energy, thus causing the expansion of the universe to decelerate. However, one of our fundamental understandings of gravity asserts that it weakens over distance. Thus, as the universe slowly spread out and its self-gravity weakened, the repulsive force of dark energy (which is a property of space itself, getting stronger as space expands ) was able to overcome it and accelerate the expansion of the universe.

This intriguing new form of energy is one of that factors that caused the accelerating universe to be named “Breakthrough of the Year” in the 1998 issue of Science magazine. The new technology that is allowing researchers to study supernovae in the far reaches of the cosmos also gives us the insight to perfect our model of the expanding universe. We have continually revised our views of the geometry and behavior of the universe, having come full circle from the initial belief that it was static to the modern hypothesis about the nature of it’s expansion. As technology continues to evolve and new minds enter the field, the future of cosmological study definitely holds the promise of either reinforcing our current notions of the universe or causing us to alter our opinion once again.