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Transcript
PH507 Astrophysics Dr Mark Price 1 Lecture #5: Gravity waves, neutrinos, ground and space-based observatories. Gravity wave detection. A gravity wave (or gravitational wave as it should correctly be referred to) is a ripple in the curvature of the space-time continuum (the enmeshed combination of our three perceived physical dimensions, plus time) created by the movement of matter. Long thought to exist, although never yet detected, gravity waves were first hypothesized in Albert Einstein's general theory of relativity, which predicted that an accelerating mass would radiate gravitational waves as it lost energy. For example, it would be expected that two pulsars (celestial bodies that emit radiation in regular pulses) in orbit around each other should emanate gravity waves as their orbits decay. In accordance with the First Law of Thermodynamics, which states that neither matter nor energy can be created or destroyed - although either may be transformed - the energy loss associated with the orbit's decay is radiated as gravitational waves. According to theory, gravity waves propagate at approximately the speed of light and pass through matter unchanged, alternately stretching and shrinking distances on an infinitesimal scale. Their strength decreases as a function of distance from their source. The study of gravitational waves could yield an incredible amount of information about the universe and lead to many practical applications. For example, their ability to pass through matter unaltered could enable the transmission of a signal over vast distances in space. Gravitational Wave Detection Around the world, several countries are constructing gravity wave detectors, highly sensitive instruments that are expected to be able to detect gravity waves and identify their sources. Once the projects are in operation, it is expected that the detectors will work collaboratively. In the United States, the detector project is called LIGO (for Laser Interferometer Gravitational-Wave Observatory). LIGO researchers hope to establish the existence of gravitational waves and prove whether or not they actually propagate at the speed of light and cause the displacement of matter that they pass through. Among other anticipated outcomes are confirmation of the existence of black holes, and an enhanced ability to study them and other cosmic phenomena. The LIGO system consists of suspended weights with mirrored surfaces that can move freely horizontally. If a gravitational wave were to pass through, the distance between the weights (which is measured by a laser beam moving back and forth between the mirrors and then recombined at a photodetector) would be altered. Researchers from the California Institute of Technology (Caltech) and the Massachusetts Institute of Technology (MIT) have developed a LIGO prototype sensitive enough to detect a tiny movement (many times smaller than the diameter of a single hair) in a test weight 40 meters away. Worldwide, the other gravitational wave detection projects include a collaborative effort by France and Italy called VIRGO, another by Germany and Great Britain called GEO 600, a project in Japan called TAMA 300, one in Australia called ACIGA, and NASA's LISA project. In early 2005, the LIGO project announced a plan to use a million volunteered personal computers to search for a gravity wave source. PH507 Astrophysics Dr Mark Price 2 First generation gravitational wave detector: a Weber bar. Uses a very large, very cold mass. Gravitational waves passing through the mass will distort it slightly and change the acoustic signal being passed (via the transducer) into the bar. Would only be sensitive enough to very large gravitational waves, such as those produced by a supernova within ~a few hundred light years. However the next generation of gravity wave detectors use laser interferometry, such as LIGO in the US. These should be sensitive enough to measure deflections of the order of one part in 10-21, required to realistically measure graviational waves. PH507 Astrophysics Dr Mark Price 3 The AIGO, an hour’s drive from the city of Perth in Western Australia, works in conjunction with four other gravitational wave detectors located in Italy, Japan and the United States. The U.S. detector, LIGO (Laser Interferometer Gravitational wave Observatory), is run by the California Institute of Technology and Massachusetts Institute of Technology and consists of two sites in Louisiana and Washington. The US sites are about 3500km apart but effectively operate as a single detector. Scientists hope that this worldwide network of laser interferometers will catch the waves predicted to be produced by cataclysmic events, such as collapsing stars and colliding black holes. The LIGO scientists are confident they'll be able to detect a variety of cataclysms, each taking place upwards of a hundred million light years away, including colliding neutron stars, coalescing black holes, and a collapsing star. Shame nothing has been detected so far! Next generation detectors: LISA LISA (Laser Interferometer Space Antenna). Planned for launch in 2015 plans to use three spacecraft as a giant floating interferometer, with each of the spacecraft a distance of 5 million kilometres apart. Expected to be a million times more sensitive that current ground based instruments. Unfortunately, as of February this year, NASA changed the status of the mission to “delayed indefinitely”! PH507 Astrophysics Dr Mark Price 4 Neutrino Detectors. The first neutrino detector built was in 1968, and basically consisted of a very large tank containing 600 tonnes of bleach buried 1.5 km down a mine in South Dakota. The detector works by observing the interaction of a neutrino with a chlorine atom (hence the bleach) via the reaction: 37 37 17 Cl 18 Ar e i.e. a neutrino interacts with a chlorine atom to produce a radioactive isotope of Argon and an electron. The tank is left for between 50-100 days, during which time the radioactive Argon atoms have accumulate within the tank. The argon is then removed, and the amount counted. This gives us the number of neutrino reactions that have occurred within the tank. This gave the first direct evidence of the existence of neutrinos from the Sun, and had an incidence rate of one neutrino detected every 48 hours! This early detector could only detect one type of neutrino, the so-called ‘electron neutrino’. This led to the so-called ‘Solar neutrino problem’, as the detection rate was only one third of that predicted. The second generation of neutrino detectors were water based (utilising 3000-8000 tonnes of ultra-pure water), although they were originally constructed to measure proton decay. Which they never did. PH507 Astrophysics Dr Mark Price 5 These photomultipliers on the outside of the tank record the Cerenkov radiation emitted from super-luminal particles that are emitted when the incident neutrino interacts with a proton. This type of detector is sensitive to all three neutrino types: the electron neutrino, the mu neutrino and the tau neutrino. This then solved the Solar neutrino problem, as it became clear that the neutrinos could ‘oscillate’ between one type and another. Current state of the art detectors are based in Antarctica and use strings of photomultipliers buried in the deep, transparent Antarctic ice. These detect the blue Cerenkov radiation emitted when neutrinos interact within the ice in the polar ice cap. IceCube ( http://icecube.wisc.edu) is currently undergoing initial testing, and when fully operational will detect interactions within a volume of 1 km3 of ice (equivalent to 1 million tons of water!). PH507 Astrophysics Dr Mark Price 6 Ground-based optical, infrared and submm observatories. JCMT (James Clerk Maxwell Telescope) Currently the world’s largest submillimetre telescope. Briefly covered in a previous lecture. UKIRT (United Kingdom InfraRed Telescope). The world’s largest, dedicated infrared telescope facility. Built on the top of the dormant volcano in Hawaii, at an altitude of almost 4000 metres. Incorporates a 3.8 metre optical mirror (which weighs over 6 tonnes). Main instruments include: WFCAM UIST: UFTI: CGS4: IRPOL: Michelle: IRCAM/TUFTI: Wide field 0.8-2.5 micron camera covering a 0.75 square degree tile in 4 pointings. 1-5um imaging and long-slit grism spectroscopy with R~1500-3500. Also integral-field spectroscopy and imaging- and spectro-polarimetry. 1-2.5um camera with 1024x1024 pixels; pixel scale 0.09". Imaging polarimetry and K-band 400 km/s FP also available. 1-5um long-slit grating spectrometer with R ~ 400-40,000. UKIRT's polarimetry module for use with all instruments except Michelle, which has its own waveplates. 10-20um imaging and long-slit grating spectroscopy. Echelle spectroscopy and imaging/spectro-polarimetry also available. Instrument currently at Gemini 1-5um camera with 256x256 pixels; pixel scale 0.08". L-band imaging polarimetry also available. IRCAM has been decomissioned. PH507 Astrophysics Dr Mark Price 7 Gemini. A rather unique telescope, actually consisting of two telescopes (hence the name), called ‘Gemini North’ and ‘Gemini South’ respectively. Gemini North is (again) located on the top ot Mauna Kea in Hawaii, and Gemini South is located in the Chilean Andes mountains. Both telescopes are nominally identical, and have 8-metre single surface mirrors (as opposed to the Keck telescopes mutli-segmented mirror). A picture taken from inside the Gemini North dome (above). Gemini South, showing the south celestial pole. PH507 Astrophysics Dr Mark Price 8 Gemini North - list of instruments Altair (formerly known as GAOS; facility AO system) GCAL (facility calibration unit) GMOS-North (optical multi-object, long-slit and IFU spectrograph and imager) Michelle (mid-IR imager/spectrometer) NIFS (near-IR integral field spectrograph) NIRI (near-IR imager with grism spectroscopy) TEXES (high resolution mid-IR spectrograph) Visible GMOS (multiobject, long-slit and IFU spectrograph and imager) Near-IR Mid-IR Other facilities NIRI (imager with Michelle (imager/ grism spectroscopy) spectrometer) Altair (formerly known as GAOS; facility AO system) NIFS (integral field spectrograph) GCAL (facility calibration unit) TEXES (high resolution spectrograph) A picture showing the 8-metre Gemini North mirror. PH507 Astrophysics Dr Mark Price 9 Gemini South - list of instruments Acquisition Camera (optical imaging) bHROS (bench-mounted high-resolution optical spectrograph) FLAMINGOS-2 (near-IR multi-object spectrograph) GCAL (facility calibration unit) GMOS-South (optical multi-object, long-slit and IFU spectrometer and imager) GNIRS (near-IR spectrograph) GSAOI (near-IR imager for use with adaptive optics) NICI (near-IR coronagraphic imager) Phoenix (high resolution near-IR spectrometer) T-ReCS (formerly known as MIRI; mid-IR imager and spectrometer) Gemini South - table of instruments Visible Near-IR GMOS (multiobject, long-slit and IFU spectrograph and imager) Phoenix (high resolution spectrometer) bHROS (highresolution spectrograph) GNIRS (long-slit, cross-dispersed and IFU spectrograph) Acquisition Camera (optical imaging) NICI (coronagraphic imager) FLAMINGOS-2 (multi-object spectrograph) GSAOI (highresolution imager for use with MultiConjugate Adaptive Optics) Mid-IR T-ReCS (formerly known as MIRI; imager and spectrometer) Other facilities GCAL (facility calibration unit) PH507 Astrophysics Dr Mark Price 10 Atacama Large Millimetre Array: ALMA. If successfully completed, will revolutionise infrared/submillimetre astronomy. Currently being constructed in the Atacama desert in Chile. A multi-million dollar venture funded between the US, Europe (through ESO) and Japan. About sixty-four 12-metre antennae located at an elevation of 16,400 feet in Llano de Chajnantor, Chile. Imaging instrument in all atmospheric windows between 10 mm and 350 microns. Array configurations from approximately 150 meters to 10 km. Spatial resolution of 10 milliarcseconds, 10 times better than the VLA and the Hubble Space Telescope. Able to image sources arcminutes to degrees across at one arcsecond resolution. Velocity resolution under 0.05 km/s. Faster and more flexible imaging instrument than the VLA. Largest and most sensitive instrument in the world at millimetre and submillimetre wavelength. Point source detection sensitivity 20 times better than the VLA. One of the driest sites in the world, at an altitude of 5000 metres. Oxygen levels here are only 60% that at sea level. First telescope expected to be operational in 2007. Whole array ‘first-light’ in 2015. PH507 Astrophysics Dr Mark Price 11 Space-based observatories. HST: Hubble space telescope. Probably the most famous space-based telescope. Lauched from the space shuttle shuttle Discovery in April 1990, Hubble is rapidily approaching the end of its proposed life cycle. In fact, it was originally envisaged to be in operation for 15 years. Incorporates a 2.4 metre (now perfect) mirror and currently has the following instruments: 1) Wide Field Planetary Camera 2. Consists of 4 separate CCD cameras. Three wide field cameras, and a narrow field camera for planetary observations. 2) Space Telescope Imaging Spectrograph.A low resolution spectrometer sensitive to radiation from 115 nm (ultra-violet) – 1000 nm (near infra-red). 3) Near Infrared Camera and Multi-Object Spectrometer (NICMOS). The only cryogenically cooled instrument on HST. A cooled grating spectrometer sensitive to the near infra-red between 0.8 and 2.5 microns. 4) Advanced Camera for Surveys. A wide field of view survey camera. Again sensitive from the near infra-red through to the UV. 5) Fine Guidance Sensors. Used for sensitive astrometry (effectively: measuring the exact position of objects). Can get positional accuracy of nearby stars down to 200 micro-arcseconds. PH507 Astrophysics Dr Mark Price 12 ISO (Infrared Space Observatory) Launched in November 1995 carried four instruments working in the near infrared – submillimetre. Lasted until 1998 when it’s liquid helium ran out. 1) ISOCAM: A CCD camera sensitive to wavelengths of 2.5 – 17 microns 2) ISOPHOT. An infrared photometer sensitive to spot wavelengths between 2.5 – 240 microns. 3) Long Wavelength Spectrometer. Used an array of 10 germanium photoconductors sensitive to wavelengths from 45 microns – 196 microns. 4) Short Wavelength Spectrometer. Sensitive from 2.4 microns – 45 microns. Hugely successful, despite serious problems with ionising radiation from the Van Allen belts. Data from ISO has been used to publish over 1600 scientific papers. One of it’s most important discoveries was that water ‘vapour’ is ubiquitous in the galaxy. PH507 Astrophysics Dr Mark Price 13 Space Infrared Telescope Facility (SIRTF) (aka Spitzer). Following on from the success of ISO, NASA launched the last of it’s big observatory missions in 2003. Originally called SIRTF, the telescope was renamed to the (rather horrid) sounding “Spitzer”. Consists of a 85 cm primary mirror (ISO had a 60 cm one), and three main instrument clusters. 1) The InfraRed Array Camera (IRAC). Works at four wavelengths simaltanously in the near infra-red (3.6, 4.5, 5.8 and 8 microns). The two shorter wavelength channel are 512 x 512 pixel InSb detectors, and the two longer ones are SiAs. 2) InfraRed Spectrograph (IRS). A spectrometer covering four wavelength bands between 5.3 and 40 microns. 3) Muliband Imaging Photometer (MIPS). Comprised of InSb and Ge:Ga photoconductors spanning the wavelength band from 24 – 100 microns. Each detector subsystem is more sensitive than ISO, although the instruments are very similar. Spitzer has a nominal predicted lifetime of five years, at which point it’s liquid helium supply will run out, and the instruments will be rendered useless. PH507 Astrophysics Dr Mark Price 14 Kepler: an optical telescope to search for terrestrial sized planets. Due for launch in 2008, has a proposed four year lifetime. Scientific objectives. 1. Determine how many terrestrial and larger planets there are in or near the habitable zone of a wide variety of spectral types of stars. 2. Determine the range of sizes and shapes of the orbits of these planets. 3. Estimate the how many planets there are in multiple-star systems. 4. Determine the range of orbit size, brightness, size, mass and density of shortperiod giant planets. 5. Identify additional members of each discovered planetary system using other techniques. 6. Determine the properties of those stars that harbour planetary systems. Detects planets by looking at the minute change in a star’s luminosity when a planet transits across it. During transit the star’s luminosity dips by about 0.001% and so Kepler has 42 very sensitive CCD-based photometers which will stare at 100,000 stars simultaneously. PH507 Astrophysics Dr Mark Price 15 Schematic showing the Kepler spacecraft. Expected results from the full four year mission: Detection of about 50 planets if most have R ~ 1.0 Re About 185 planets if most have R ~ 1.3 Re About 640 planets if most have R ~ 2.2 Re Or, more likely, some combination of the above. Note, these are earth-sized planets within a star’s habitable zone, i.e. the area around a star where liquid water (and therefore life) is likely to form, and persist. PH507 Astrophysics Dr Mark Price 16 Things you should know for the exam. 1) Differences between refracting and reflecting telescopes. Advantages and disadvantages. 2) Infra-red and sub-mm detection methods. 3) Basic CCD operation. 4) How to convert from wavelength -> frequency -> eV. Resources and further reading. “Astrophysical Techniques” (4th edition) by C. R. Kitchen. “Millimetre-wave optics, devices and systems” by J. Lesurf http://www.wikipedia.co.uk/ (Beware: some info is not correct!). http://www.nasa.gov/home/ (NASA homepage – lots of links). http://sci.esa.int/ (European Space Agency homepage – lots of links).