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

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.

In 1998, during the analysis of the brightness of Type Ia supernovae, it was discovered that the most remote explosions seemed to be too weak. Type Ia supernovae appear in binary systems. One of the stars is a white dwarf, a relic of an evolutionary cycle of stars similar to the Sun. When the second star of the system enters the red giant phase and swells up, its external layers, containing mainly hydrogen, begin to fall onto the white dwarf, which gradually grows in mass. When the white dwarf reaches 1.4 solar masses, it explodes and is completely torn apart. Since the conditions that trigger the explosion are similar every time, Type Ia supernovae always release more or less the same amount of energy. Astronomers rely on this property to measure distances in space.

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.

Examining the density of dark energy in various periods after the Big Bang can help choose the correct model. If the density remained constant, it would mean that dark energy is related to the cosmological constant, that is to say, the property of space-time. But if the acceleration of the Universe is caused by a scalar field, then, given the swelling-up of space-time, the density of dark energy should change. "This used to be a problem. In order to assess the changes in the density of dark energy immediately after the Big Bang, one needs to know how to measure the distance to very remote objects. So remote that even Type Ia supernovae connected to them are too faint to be observed," says Prof. Demiański.

The group of Polish and Italian astrophysicists suggested using gamma-ray bursts (GRBs), the most powerful explosions observed in the Universe today, to measure the largest distances in the Universe. They analyzed the so-called long bursts that probably arise during the collapse of the core of a large star. The process leads to the formation of a black hole. The gamma radiation emitted at that time is so intense that even objects that exploded 400 million years after the Big Bang can be observed.

The main problem was how to assess the total energy of a burst. To that end, the scientists analyzed databases of previous gamma explosions. It turned out that a part of the explosions occurred in galaxies the distance to which could be measured using other methods, for example, by means of Type Ia supernovae. "We focused on those instances. We knew the distance to the galaxy and we also knew how much energy of the burst reached the Earth. This allowed us to calibrate the burst, that is to say, to calculate the total energy of the explosion," explains 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 team of scientists from the universities in Warsaw and Naples, headed by Dr Ester Piedipalumbo, analyzed data gathered by astronomers. Extremely remote gamma-ray bursts are quite rare. The Amanti catalogue listed 95 such phenomena and failed to provide enough clues as to the exact nature of dark energy. "It is quite a disappointment. But what is important is the fact that we have in our hands a tool for verifying hypotheses about the structure of the Universe. All we need to do now is wait for the next cosmic fireworks," concludes Prof. Demiański.

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.