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ASTROWATCH

Compiled by MarcTurler X-ray constraints on The existence and nature of dark energy has This study also suggests that the dark-energy been probed using images of clusters density does not change quickly with and obtained by NASA's Chandra X-ray may even be constant, consistent with Albert Observatory. These results are completely Einstein's "" concept. independent from the previous supernova The best estimation of equation-of-state para­ results and confirm the effect of dark energy meter "w", which describes the nature of dark on the acceleration of the . energy, is, however, found to be slightly below The first evidence for dark energy came in -1. This implies that the universe might end in 1998 with the observation that distant type la a "Big Rip", where dark energy increases until supernovae are dimmer than expected. This , stars, planets and eventually suggested that the expansion of the universe is are torn apart (CERN Courier May 2003 pl3). not decelerating under its own gravity but However, it is important not to over-interpret accelerating through the effect of some myste­ the data by forgetting that they rely on some rious dark energy (CERN Courier September prior assumptions. For example, another 2003 p23). The first results of the Wilkinson recent study, also based on the X-ray emission Microwave Anisotropy Probe (WMAP) con­ of galaxy clusters, yielded the opposite result firmed in 2003 that dark energy is the main by showing that the dependence of the contributor to the -energy content of the number counts of galaxy clusters is nearly an universe (CERN Courier April 2003 pll). Along order of magnitude below the expectations of with additional observational constraints, a Galaxy-cluster images from NASA's Chandrath e "concordance model", while being in cosmological "concordance model" could be X-ray Observatory are providing a new remarkable agreement with those of a matter- derived in which 4% of the present universe is method for detecting and probing dark dominated universe (Vauclair etai. 2003). made of baryons, 20-25% is non-baryonic energy. This image is an optical (blue) andX-Such a classical Einstein-de-Sitter universe of unknown nature, and dark ray(red) composite of the galaxy-cluster Abell could even account for the WMAP results if the energy accounts for the remaining 70-75%. 2029. The giant elliptical galaxy at the centre Hubble constant has the rather low value of The new study led by Steve Allen of the is surrounded by smaller galaxies forming the 46 km/s/Mpc (Blanchard etal. 2003). The Institute of Astronomy at Cambridge University cluster. Their combined gravity traps the X- controversy over the existence and moreover is based on the X-ray emission of hot gas in 26 ray emitting gas heated to about 100 million the nature of dark energy is clearly not yet over. galaxy clusters (CERN Courier July/ August degrees. (Optical: NOAO/Kitt Peak/J Uson, 2003 pl3). Using X-ray measurements from D Dale;X-ray: NASA/CXC/loA/S Allen et al .J Further reading Chandra, the authors derive for each cluster the S W Allen etai. 2004 http://amv.org/ mass ratio of the hot gas to dark matter. Under a significance of >3a (Allen etal. 2004). It thus abs/astro-ph/0405340. the assumption that this gas fraction should be offers the first clear confirmation of the results A Blanchard etal. 2003 Astron. Astrophys. constant with redshift, they then adjust the from type la supernovae by detecting the effect 41235. distance scale to give a best fit to the data. This of dark energy on distance measurements for a S C Vauclair et al. 2003 Astron. Astrophys. work suggests the existence of dark energy with different class of astronomical objects. 412 L37.

Picture of the month

Is this the remnant of a gamma-ray burst? of the surrounding gas was pulled into the If the answer is yes, then it would be the black hole, but some of it was probably first one to be discovered in our Miiky Way flung away in oppositely directed jets, as galaxy. This composite image of the witnessed by the remaining bar-shaped supernova remnant W49B shows bright region of X-ray emitting gas. Those jets infrared rings (red and green) on both sides travelling near the speed of light could have of a glowing bar of intense X-ray radiation produced a gamma-ray burst that might be (blue). The infrared rings are likely to have observed, in a few billion years, by an been expelled during the short life of a observer in a remote galaxy located along massive star. A few thousand years ago, the jet axis. (Credit: NASA/CXC/SSC/ when the star exhausted its nuclear fuel, its J Keohane etal.; Infrared: Caltech/ core collapsed to form a black hole. Much Palomar/J Keohane etal.)

12 CERN Courier July/August 2004