INFO 03-1999: ISO Observations of Normal Matter Distribution
3 March 1999
The fate of the Universe depends on the total amount of existing matter.New clues on this value have been obtained by an international team ofastronomers using the European Infrared Space Observatory, ISO, bymeasuring for the first time the abundance of a particular chemicalelement, deuterium, in a very active star-forming region in the Orionnebula. Their result confirms that the total amount of normal matter isnot enough to stop the expansion of the Universe and cause it to collapseinto a Big Crunch in the future.Deuterium, an isotope of hydrogen, is a key element for astronomers studying the origin of the Universe. All the deuterium that can be detected today was produced a few minutes after the Big Bang, during a process called primordial nucleosynthesis - in which a few other elements, like hydrogen and helium, were also made. Apart from the Big Bang, there are no other known sources of deuterium in the Universe. Thus, astronomers regard deuterium as a 'fossil element' that holds a lot of cosmological information.
Specifically, the abundance of deuterium in the new-born Universe is directly proportional to the total amount of normal matter (baryonic matter) in the universe today. According to the most accepted theories, if the total amount of matter exceeds a certain number, known as the critical density, in the far future the Universe will stop expanding and will collapse into a Big Crunch; otherwise, the expansion will continue forever.
But measuring the abundance of primordial deuterium is not easy. It started off at one value at the time of the Big Bang (10 to 15 billion years ago) but has since diminished significantly because it is destroyed by nuclear reactions in the interior of the stars. Astronomers, therefore, have measured different values for deuterium in different places and are trying to extrapolate back in time to the deuterium abundance at the Big Bang.
The new result obtained by the international team led by Chris Wright and Ewine van Dishoeck (Leiden University Observatory, The Netherlands), using the Long Wavelength Spectrometer (LWS) on board ISO, allows for the first time to infer the deuterium abundance in an actively star forming region, the Orion Bar in the Orion nebula, about 1500 light-years away. The team will explain their work in a future issue of Astrophysical Journal Letters.
The deuterium they measured was locked up in the primary deuterium bearing molecule in the Universe, hydrogen deuteride (HD). They found in the Orion Bar one deuterium atom for every 100 000 hydrogen atoms, which is consistent with other measurements in other places indicating that the amount of normal matter in the Universe is not enough to cause a Big Crunch.
"This is one step along the road to determine the deuterium abundance at the time of the Big Bang, the value that everybody is trying to zoom in on!", says Wright."And it confirms that the density of normal (or Baryonic) matter is less than that required to close the Universe."
The new result demonstrates also the feasibility of a new technique for measuring the deuterium abundance in active-star forming regions, a useful legacy for future infrared space telescopes such as ESA's Herschel mission, due for launch in 2007.
The scientific community is now expecting a stream of new discoveries from ISO, since the ISO Archive, a 'goldmine' filled with nearly 30 000 scientific ISO observations, has recently been opened to astronomers all over the world. It is located at the ISO Data Centre, at Villafranca, Spain, and can be accessed for free via the Internet.
