Monday, September 19, 2011

Gamma-ray bursts shed light on the nature of dark energy

An artistic image of the explosion of a star leading to a gamma-ray burst
Source: FUW/Tentaris/Maciej Frołow


What is the nature of dark energy, a recently discovered dominant constituent of the Universe today? Is expansion-accelerating dark energy an intrinsic property of space-time itself or rather a field unknown to science? A new distance-measuring method developed by scientists from the Faculty of Physics, University of Warsaw (FUW) and the University of Naples Federico II can provide the answer. “We are able to determine the distance of an explosion on the basis of the properties of the radiation emitted during gamma-ray bursts. Given that some of these explosions are related to the most remote objects in space that we know about, we are able, for the first time, to assess the speed of space-time expansion even in the relatively early periods after the Big Bang,” says Prof. Marek Demiański (FUW). The method was used to verify models of the structure of the Universe containing dark energy.


The fainter brightness of remote Type Ia supernovae was a clear indication that they were even more distant than assumed. Instead of slowing down the expansion, the Universe was accelerating. A new form of mass-energy – dark energy – needed to be introduced into the theory in order to reconcile the previous models of the Universe with the observations. The calculations indicate the existence of a huge amount of dark energy, nearly 20 times greater than the amount of mass-energy related to the world accessible to human senses. “Overnight, dark energy became, quite literally, the greatest mystery of the Universe,” says Prof. Demiański.

To this day no one knows exactly what dark energy is. There are two models explaining its nature. According to the first one, dark energy is a property described by the famous cosmological constant introduced by Albert Einstein. According to the second model, the accelerated expansion is caused by some unknown scalar field. “In other words, it is either-or: either space-time expands by itself or is expanded by a scalar physical field inside it,” says Prof. Demiański.




The next step was to find statistical dependencies between various properties of the radiation emitted during a gamma-ray burst and the total energy of the explosion. Such relations were discovered. “We cannot provide a physical explanation of why certain properties of gamma-ray bursts are correlated,” points out Prof. Demiański. “But we can say that if registered radiation has such and such properties, then the burst had such and such energy. This allows us to use bursts as standard candles, to measure distances.”


The insufficient amount of observational data remains the main problem in the data analysis of gamma-ray bursts. For this reason, many groups of astronomers and astrophysicists combine their efforts in order to register them in the fastest and most accurate manner possible. One of such projects is “Pi of the Sky”, a system of robotic telescopes for real-time monitoring of large areas of the sky, co-organized by the Faculty of Physics, University of Warsaw.