Have you ever been woken up by the late-night wail of an ambulance? Usually they dash past your window and the next minute they become quiet. To all those who listen carefully: Did you notice a change in the sound? The pitch declines with increasing distance and eventually, the sirens mingle with the blurry noises of the city.
This observation is not new; in fact, Doppler described it in the 19th century. He just did not listen to ambulance cars. When a moving body emits waves it will push the wave fronts into each other, thereby causing the wavelength to decrease in front of it and the other way around behind it. Analogously a ship creating waves will shorten the distance between the fronts since it pushes them.
Since the wave principle applies to light, too, astronomers have found a way to utilize this phenomenon to understand the universe. If a galaxy is moving away from us, the light it emits will change in wavelength as it is escaping the wave fronts. As a result, we perceive the wavelength as increased. In practice this means that the light that galaxies emit will become redder, known as the red shift, when they move away.
We can infer from the finding of red shift going in all directions that the universe is actually expanding. After this notion became obvious researchers tried eagerly to put this into mathematical equations and hoped to predict data this way. However researchers are still quarreling about the actual form of this equation.
In order to determine this, astrophysicists from a large consortium, including scientists from Hopkins, have tried a new way of collecting data to verify the most popular "equation of state" describing this phenomenon. The ESSENCE project should provide a reliable prediction of the expansion of the universe. Basically these astronomers take photographs of supernovae, or star explosions, and then determine their red shift.
However one constant of this equation remains unresolved. There is a theoretical value for this constant but the researchers aim for an empirical proof. The problem of theories concerning the expansion of the universe is that these models predict observations accurately but are forged to solve particular problems ?- they lack general predictive power. This lack of generality motivated the researchers to strike out in a new direction.
The goal is quite ambitious: "Is the dark energy of the universe consistent with a cosmological constant?" This is a hypothetical energy that is responsible for the expansion of the universe. The question the researchers are asking is if we can formalize this behavior in a simple equation and therefore make accurate and general predictions.
The novel element of the ESSENCE team's approach is that they take photographs of supernovae instead of galaxies to determine their red shift. Such an explosion can be considered a "standardized candle" that gives evidence about the speed and acceleration of the universe expands when the red shift is measured. They are especially suited for this purpose since supernovae tend to be highly energetic and luminous.
This method demands expensive resources and a large amount of data. Around 200 explosions observed over a five-year period are sufficient. Four meter telescopes in both the northern and southern hemispheres are used to obtain reconcilable data.
The image analysis can be considered a science itself. Once the photos are taken, several steps are necessary to sort the wheat from the chaff. Usually every new image is held against an older template to locate the actual supernovae. This is partially done by computers; the final say, however, is on the part of the scientist.
Besides this the capture of the images has to be in real time, which imposes heavy requirements on the equipment. Powerful computers are needed for the evaluation of the images since the errors are removed pixel by pixel.
The ESSENCE project has only begun over the last few years, and it will take many more to gather enough data to draw real conclusions. But compared to the size of the expanding universe, the wait is nothing.
