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How long since the Big Bang ? Jeremy Mould National Optical Astronomy Observatory Introduction • Subject today is the Expanding Universe • The Expanding Universe model describes • the motions of galaxies Just like the model of Copernicus describes the motions of the planets Outline • Hubble’s evidence for an expanding Universe • How fast is the Universe expanding now ? • • (the Hubble Constant) Variation of the expansion rate over time How long is it since the Big Bang ? Edwin Hubble 1889 1953 Hubble’s evidence Overview on the Hubble Constant • • • • • • what is redshift ? measuring distances geometrically we start with the Large Magellanic Cloud the Hubble Space Telescope Key Project Cepheids as standard candles Supernovae as standard candles redshift • • 1 parsec is 3 x 1013 km if we can measure the angle, we can get the distance Supernova 1987A • • • • a massive star exploded in the LMC February 1987 the LMC is our nearest neighbor galaxy in fact, it’s a satellite SN1987A • the ring lit up 250 days after the supernova • radius known • angle known • => distance SN1987A Schematic of SN1987A The Hubble Constant Key Project Goal: Measure how fast the Universe is expanding to 10% accuracy The HST Key Project Jeremy Mould Robert Kennicutt, U.Ariz Wendy Freedman, CIW Shaun Hughes, RGO Barry Madore Nancy Silbermann Shoko Sakai Randy Phelps Robert Hill Abi Saha, NOAO Peter Stetson, DAO Brad Gibson Laura Ferrarese Holland Ford Garth Illingworth Dan Kelson John Graham John Hoessell Ming Sheng Han John Huchra Anne Turner Paul Harding Fabio Bresolin servicing mission The first servicing mission replaced the WFPC with WFPC2 in December 1993, thus remedying spherical aberration The power of the Hubble Space Telescope • From the ground we can resolve galaxies up to 2 or 3 Mpc away • HST was designed to have ten times the resolution of ground based telescopes • The project to find Cepheids up to 20 Mpc away was designated a Key Project for HST The Cepheid period luminosity relation M100 M100 Cepheid light curves in NGC 1365 N1365 PL top: visual The PL relation in NGC 1365 bottom: infrared • • • • • • Beyond the Cepheids Beyond 20 Mpc even HST has difficulty resolving Cepheids We use four other standard candles to measure distances a further factor of ten The Tully Fisher relation for spiral galaxies Supernovae of type Ia Surface brightness fluctuations = resolvability The fundamental plane for elliptical galaxies Tully Fisher relation • • big galaxies rotate faster a galaxy with a given rotation speed is a standard candle The Hubble Constant • • All 4 standard candles agree H lies in the range 65 to 77 km/s/Mpc How standard are standard candles ? • most stars have a chemical composition like that of the sun • but there are some variations.... • Cepheids with different chemistry pulsate differently • accounting for this changes our distances a few percent How the expansion slows Expansion rate in the past divided by current rate Big Bang low density universe Now What is the density of the Universe ? 2dF map of the 3d density of the local few billion light years Colless and team Dark energy 2dF found that the density of the Universe was only 25% of the critical density But Boomerang found that the curvature of the Universe was nonetheless zero. Rediscovery of L See www.mso.anu.edu.au/~brian Measuring the age Scale factor Size now 13.5 a time in billions of years Summary • we start with the distance of the LMC • a Cepheid of period P is a standard candle of luminosity L • HST maps the Cepheids out to 20 Mpc • Four other standard candles map the expansion out to 200 Mpc • H0 = 72 +/- 7 km/sec/Mpc • Universe is 13.5 +/- 1.5 Gyrs old Global parameters: the Hubble Constant Is H0 > 60 km/sec/Mpc ? Jim Peebles’ ruling SIM is the Space Interferometry Mission SIM is proceeding with Phase B development and on track for a launch in 2009. http://sim.jpl.nasa.gov What SIM will do for cosmology ? Oldest stars • globular star clusters • parallaxes with SIM or GAIA • measure ages to half a billion years LSST, a digital survey of the sky each week • • • • Weak lensing mass map of the Universe 100,000 supernovae per year z e (1,2) Earth crossing asteroids 10,000 primordial transNeptunian objects View of the LSST telescope structure in the Steward Observatory straw man design. The primary mirror is at the center, and the secondary and tertiary are almost equidistant from the primary. The detectors are just ahead of the central hole in the primary (see: http://dmtelescope.org/ design.html) Where to get more information • ‘Measuring the Universe’ by Stephen Webb • www.stsci.edu (Hubble Space Telescope) http://oposite.stsci.edu/pubinfo/1999.html • www.noao.edu/noao/staff/jrm PHOTO NO.: STScI-PRC99-25 MAGNIFICENT DETAILS IN A DUSTY SPIRAL GALAXY In 1995, the majestic spiral galaxy NGC 4414 was imaged by the Hubble Space Telescope as part of the HST Key Project on the Extragalactic Distance Scale. An international team of astronomers, led by Dr. Wendy Freedman of the Observatories of the Carnegie Institution of Washington, observed this galaxy on 13 different occasions over the course of two months. Images were obtained with Hubble's Wide Field Planetary Camera 2 (WFPC2) through three different color filters. Based on their discovery and careful brightness measurements of variable stars in NGC 4414, the Key Project astronomers were able to make an accurate determination of the distance to the galaxy. The resulting distance to NGC 4414, 19.1 megaparsecs or about 60 million light-years, along with similarly determined distances to other nearby galaxies, contributes to astronomers' overall knowledge of the rate of expansion of the universe. The Hubble constant (H0) is the ratio of how fast galaxies are moving away from us to their distance from us. This astronomical value is used to determine distances, sizes, and the intrinsic luminosities for many objects in our universe, and the age of the universe itself. Due to the large size of the galaxy compared to the WFPC2 detectors, only half of the galaxy observed was visible in the datasets collected by the Key Project astronomers in 1995. In 1999, the Hubble Heritage Team revisited NGC 4414 and completed its portrait by observing the other half with the same filters as were used in 1995. The end result is a stunning full-color look at the entire dusty spiral galaxy. The new Hubble picture shows that the central regions of this galaxy, as is typical of most spirals, contain primarily older, yellow and red stars. The outer spiral arms are considerably bluer due to ongoing formation of young, blue stars, the brightest of which can be seen individually at the high resolution provided by the Hubble camera. The arms are also very rich in clouds of interstellar dust, seen as dark patches and streaks silhouetted against the starlight. Image Credit: Hubble Heritage Team (AURA/STScI/NASA)