What we see in the Universe, is only 4% of the total existing mass and energy. Two new studies attempt to shed light on the remaining 96%, the mysterious dark matter and even more mysterious dark energy.
The first study attempts to determine the amount of the dark energy, a theoretical force that acts opposite the gravity and is responsible for the accelerating expansion of the Universe. In the future the collected data could confirm or disprove the prevailing view according to which dark energy suddenly appeared about 5 to 7 billion years ago.
The second study investigated dwarf galaxies orbiting the Milky Way, and concluded that the dark matter they contain can’t be composed of hypothetical particles which essentially focus the attention of astronomers. Although the study does not answer the question of the composition of dark matter, it reveals what it is not composed of.
Looking back in time
According to the prevailing theory, the expansion of the Universe was not always accelerating. Cosmologists believe that the expansion slowed up 5 to 7 billion years ago, when dark energy appeared mysteriously and began to accelerate its expansion. Today, this force is so strong that it is believed to correspond to 73% of the total mass-energy of the Universe (according to Einstein’s relativity, matter and energy are equivalent).
But if the dark matter appeared suddenly, the distance separating the galaxies before its appearance was much smaller than in the case that the dark energy would always exist.
To find out whether it was true an international research team examined galaxies at large distances, of billions light years from the Earth. Essentially, looking at these galaxies is like looking at the distant past.
The measurements came from the Sloan Digital Sky Survey, a comprehensive research effort for digital sky map. Will Percival from the University of Portsmouth, a member of the research team said: “Today we have only one third of the final survey data, and they already allow us to measure the speed of expansion of the Universe 6 billion years ago”.
The survey was based on observations of 250,000 galaxies, but many more measurements are required for confirmation or refutation of the theory.
The results were presented at the National Astronomy Conference held last week in Manchester.
The second study, published in Physical Review Letters, examined ten of the more than 20 dwarf galaxies that orbit the Milky Way. Previous measurements have shown that within the galaxies there are large quantities of dark matter.
The dark matter represents 23% of mass-energy of the Universe, i.e. five times more than normal matter. It is believed to consist of unknown fundamental particles, which do not emit or refract light, neither interact with normal matter.
Many physicists believe that the key ingredient is the WIMP (weakly interacting massive particles ), whose existence has not been confirmed. However, theoretically some WIMP annihilate each other when they meet and emit gamma ray flashes, which would be easy to record by the dwarf galaxy satellites.
However, despite these efforts the gamma-ray telescope «Fermi» failed to detect such flashes in a large energy range. And the inability to detect flashes seems to contradict the theoretical hypotheses about four types of WIMP. It means that the believed key ingredient of the dark matter probably does not exist anywhere. It is a negative result, but is an important step in solving the mystery of dark matter.
As it seems, searching for the dark components of the Universe will not end soon.