ASTROWATCH Astronomy sets new targets This month Astrowatch travels to Manchester for the meeting of the International Astronomical Union. Emma Sanders reports on two weeks of discussion and discoveries, from the distant universe to new planets on our doorstep. A cosmic milestone: the Hubble Space Telescope detects Cepheid variable stars in the spiral galaxy NGC 4603. (NASA/ESA.) Recent years have seen a transformation of underlined the new science that can now be Data and the'Value of Fundamental many areas of astronomy. New instrumen­ done. New precision measurements are set­ Parameters. tation makes arcsecond resolution almost tling old problems - how galaxies formed, how His remark provoked a response from commonplace across the whole electromag­ the universe will end - and providing new tests Wendy Freedman of Carnegie Observatories in netic spectrum, where previously it was the for fundamental physics, such as general the conference newsletter. She pointed out domain of just radio and optical observations. relativity and magnetic field theory. the need to control systematic effects and to New satellites have opened up windows on target high accuracy as well as precision. She the universe in wavelengths such as ultra­ Cosmological parameters noted welcome progress with new experi­ violet, X-ray and infrared, where little was "We are entering a new era of precision cos­ ments designed to reduce the systematic previously known.The International mology," said Malcolm Longair of Cambridge, errors that have historically dogged all mea­ Astronomical Union conference in Manchester kicking off the session on New Cosmological surements of cosmological parameters. "My > CERN Courier October 2000 15 ASTROWATCH Meeting highlights "For me the highlight has to be the convergence of different cosmological parameters: microwave background measurements, supernovae studies, large- scale structure and gravitational lensing...all the numbers agree!" Virginia Trimble, chair of the Division of Astrophysics of the American Physical Society, works at California, Irvine "I'm very excited about the discovery of a planet around Epsilon Eri, because it's an old friend -1 have studied it in the past." Indeed, the discoveries of no fewer than nine new planets were announced in Manchester. Epsilon Eriidani is particularly interesting as it has a size and orbit similar The Antarctic launch site of the Boomerang balloon-borne mission. to those of Jupiter and the observations suggest that there may also be an Earth- believe we have entered the realm where 10% scatters the cosmic microwave background sized planet orbiting the central star. measurements are possible and factors of two (CMB) radiation. Values for the constant of 75 are behind us." and 65 were reported with uncertainties of Carole Jordan, University of Oxford The Hubble constant This sector is an around 15%. The accuracy of this technique excellent example of such new optimism. will improve as the resolution of X-ray tele­ Hubble's law states that the further away a scopes increases. galaxy, the faster it appears to recede.The The problem of the oldest stars appearing constant of proportionality - the Hubble con­ older than the universe (the "old wine in new stant - fixes the age of the universe. bottles" dilemma) seems to have been solved After years of conflict between those who following adjustments of the local distance believed the Hubble constant to be 50 and scale after the Hipparcos mission. those who found 100, different determi­ Calculations now place the oldest stars at 9 X-ray image. Radio image. nations of this fundamental distance measure 11-12 x 10 years old, consistent with the (Chandra/NASA.) (VLA/NRAO.) are now converging on a value of around 75. latest value for the age of the universe. Freedman used a new calibration of the The microwave background Observations "With the first results from Chandra and period-luminosity relation for Cepheid variable of the CMB are the closest that astronomers XMM, it is the first time there is comparable stars and got a value of 75 ± 10 using data can get to the beginning of the universe.The resolution between X-ray and radio from the Hubble SpaceTelescope.The big CMB dates from 300 000 years after the Big observations. One thing I learned this uncertainty now comes in the local calibration Bang, when radiation uncoupled from matter. morning was that new X-ray maps of galaxy of the Cepheids: if we knew the exact distance Experimental advances in CMB astronomy clusters show the holes where the hot X-ray to our nearest neighbour galaxy (the Large mean that high-resolution maps of the sky are emitting gas of 100 million degrees has Magellanic Cloud), the uncertainty in the becoming available for the first time. The been blown out by the relativistic plasma in Hubble constant would be less than 5%. conference was excited about results from the the radio jets...they match perfectly with However, a word of warning came from Tom Boomerang and Maxima balloon missions the radio images!" Shanks of Durham.The method of using (March pl2). Cepheid variables as standard candles is The balloon results have confirmed an Sir Martin Rees, Institute of Astronomy in highly dependent on the composition of these overall picture of a spatially flat universe, Cambridge stars: estimations of distances could be in where Q = 1 (where Q is the ratio of matter in error by as much as 25%. the universe to the critical level needed to halt The symposium also highlighted recent the universe's expansion).This is consistent own definition of precision would be restricted progress in measuring the Hubble constant with the inflation model of the Big Bang. to measurements at the few percent level, and using the Sunyaev Zel'dovich effect.This "The glorious thing is that the peak is right we are not there yet," she said. "However, I occurs when hot gas in clusters of galaxies where it should be," said Jim Peebles of 16 CERN Courier October 2000 ASTROWATCH The interacting antennae galaxies imaged by the Hubble Space Telescope. In the infrared, infrared image of the sky from the old IRAS the point between the two galaxies contains 50% of the total luminosity because a bright satellite. ISO improves resolution by a factor starburst is completely hidden by the dust. (NASA/ESA.) of100, showing the background to be composed of many discrete sources. (IRAS.) Princeton, commenting on the CMB angular constant" - a kind of vacuum energy density power spectrum.The peak shows that the that changes the acceleration of the universe. maybe what we are observing now is just a characteristic scale of coherent patches on This picture is confirmed by observations of special case. Just as the ellipse of the Earth's the CMB sky is about one degree. type la supernovae, which have always orbit changes each time, are there lots of However, there are calibration problems essentially the same luminosity and can different versions of these "fundamental" with the balloon experiments. Paul Richards therefore be used for distance calculations. parameters? He added that, in his view, there from California commented that, to obtain the Observing how their light is redshifted shows were two types of cosmologists: the theoretical fit, it was necessary to reduce the values how much the universe has expanded. Saul astrophysicists and the mud wrestlers. He found by Maxima by one standard deviation Perlmutter of Berkeley told the conference included himself in the latter category. (8%) and increase those from Boomerang by about plans for a space-based supernova one standard deviation (20%). detector called SNAP (SuperNova The formation of galaxies The next set of detectors is not far behind. Acceleration Probe), which would dramatically The optimism from cosmologists was not Three different interferometer arrays will sur­ increase both the statistics - detecting up to mirrored in the sessions on the extragalactic vey small patches of the CMB: the Cosmic 2000 supernovae per year - and the redshift infrared background. "We still don't have the Background Imager, the Degree Angular Scale of the supernovae observed. right story for how galaxies form," said Interferometer and the Very Small Array "What's amazing now is that different lines Peebles. (November 1999 pl3).These instruments are of analysis are coming together in a story that Michael Harwit of Cornell set the meeting best for the detailed study of CMB structure. agrees with what theory tied down years ago," some ambitious targets. What were the first Astronomers are eagerly awaiting the next Peebles told CERN Courier. "This is deeply galaxies and how did they evolve? When did meeting of the American Association for the impressive and has caused me to up my stars start to.form and how can we explain the Advancement of Science in February 2001, grades," he said. "A year ago I was against this distribution of the heavy elements that we when the first results are expected. convergent model; now, if I were writing a observe today? Our understanding of the An accelerating universe Large-scale report card, I'd say: lambda [cosmological evolution of galaxies and stars is far from structure surveys, such as the Anglo- constant] shows great promise, low mass complete and observations are fraught with Australian 2dF and the US Sloan Survey, are density - very good, homogeneity - difficulties. Important clues are certainly to be starting to yield their first results, giving a excellent!'" found in the cosmic infrared background, first better understanding of the evolution of large- In his summary of the symposium on funda­ detected in 1996.The infrared emission scale clustering. Astronomers do not observe mental cosmological parameters, Martin Rees comes from light reprocessed by dust. enough baryonic or dark matter in the uni­ of Cambridge brought the meeting back down Reviewing the current state of affairs, verse to account for a Q value of l.This has to the ground.