3 About Me…..‘Postdoc’ in the Astrophysics group at Bristol working with Professor Mark Birkinshaw, world expert in our fieldVarious projects, OCRA, AMiBAPreviously – PhD in Cambridge, working on the VSAMSci in Bristol (many moons ago!)
4 Talk Structure:Some key concepts in astrophysics, and what we spend our time doing!The point of all this – what are we trying to achieve in the field of Cosmology?The Cosmic Microwave Background (relic radiation from the Big Bang)Galaxy clusters and the Sunyaev Zel’dovich effect
5 Before we go any further…. some things you need to know.
10 ‘Photon’ – a PARTICLE of light ‘Photon’ – a PARTICLE of light. Remember this, or please ask if you forget!
11 Astrophysics: ‘That branch of astronomy which treats of the physical or chemical properties of the celestial bodies. Hence astrophysicist, a student of astronomical physics.’
12 Topics in Astrophysics….. Solar System: planets, the sunStars: stellar composition, stellar evolution, star formation, supernovae, extra-solar planetsGalaxies: structure, properties, stellar velocities (dark matter), formation, evolution, clustering…Active galaxies: mechanisms, power sources (black holes)High-energy phenomena: Gamma ray burstsGalaxy clusters: galaxy properties, gas properties, lensing (dark matter), super clustering….Large scale structure, structure formation theoriesCosmology: properties of the Universe as a whole, formation (the Big Bang), fate??
13 Cosmology: ‘The science or theory of the Universe as an ordered whole, and of the general laws which govern it. Also, a particular account or system of the universe and its laws.’
16 Obtain dataGo to telescopeDownload from archiveProcess dataWork out what it tells us!Publish in journal
17 In practice, need 2 approaches OBSERVATIONALObserve celestial bodies (stars, galaxies etc) at various wavelengthsFit theoretical models to data to choose the most appropriateTHEORETICALSimulate celestial bodies (stellar evolution, galaxy formation etc)Create models of possible physical processes
19 My Work: Very hot topics in Astrophysics at the moment! COSMOLOGY from:The ‘Cosmic Microwave Background Radiation (CMB)’The interaction of the CMB with ‘Galaxy Clusters’ via the ‘Sunyaev Zel’dovich Effect’OBSERVATIONAL - ie obtaining data, data processing, extracting scienceTenerife, Poland, Hawaii, Taiwan…..Very hot topics inAstrophysicsat the moment!
20 Onto the specifics: What are we trying to achieve in Cosmology today?
22 Zwicky 1933: Galaxy clusters contain Dark Matter
23 1998: Supernovae suggest Universe is accelerating
24 Big questions in cosmology Critical density: Universe expands foreverLess dense: Expansion rate increasesMore dense: Universe will collapseAccelerating: Dark energy???Big questions in cosmologyWill the Universe expand forever?Depends on the mean densityWe can constrain this using the CMBWhat is the Universe made from?‘Normal’ stuff plus Dark MatterWhat is Dark Matter? Particle physicists working on it!Why does it appear to be accelerating?It is being ‘pushed’ by Dark Energy
25 The Cosmic Microwave Background is central to our cosmological understanding But what on earth is it??
26 Penzias and Wilson, 1965Observing the galaxy, detected ‘annoying level of static’ in all directionsPigeon poo? Aliens??No!At the same time, Dicke at Princeton predicted the existence of ‘relic radiation from the big bang’, ie the CMBNobel Prize, 1978
27 Visualising the CMB…..The sky is BRIGHT at radio frequencies. If we observe the sky with a radio telescope, inbetween the stars and galaxies, it is NOT DARK.
28 But where does it come from? It all started with: The Big Bang
33 The Big Bang Not really an ‘explosion’ Universe expanded rapidly as a whole and is still expanding today as a result of the Big Bang (Hubble)Matter was created in the form of tiny particles (protons, neutrons, electrons)Too hot for normal ‘stuff’ to form (eg atoms, molecules)Photons scatter off charged particles – like a ‘fog’ (Thomson scattering)
34 300,000 years later…… Universe much cooler, atoms start to form….. Hydrogen, Helium, normal ‘stuff’
36 Universe now neutral, Photons escape These photons, viewed today, form the Cosmic Microwave Background Radiation
37 Summary: Formation of the CMB The Universe started with the Big BangIt was initially hot, dense and ionisedPhotons were continually scattered from charged particles until….….temperature decreased and atoms formed (neutral particles)Photons (light) ‘escaped’ and became able to stream freely through the Universe.Observe the same photons today, much cooled, as the Cosmic Microwave Background
38 An important aside – formation of structures At the same time as all this was going on, structures were starting to form out of the cosmic ‘soup’
42 The CMB todayCan observe the CMB today, 13.7 billion years after the Big BangRadiation is much cooled: 2.73 K ( °C)Conclusive evidence for the Big Bang theory - proves Universe was once in thermal equilibriumSo what does it look like?
43 Observe ‘blank’ sky with a radio telescope. Rather than darkness, see Uniform, high-energy glowTurn up the resolution......
45 Tiny temperature differences (microK) When the CMB photons ‘escaped’, structures were starting to formThese structures have now become galaxiesThe structure formation processes have affected the CMB and we see the imprint as ‘hot’ and ‘cold’ spotsVery difficult to measure!
46 What does the CMB tell us? Measure the strength of the temperature differences on different scales, eg COBE 1992:
47 A plethora of other experiments followed this up….until….
48 What does the CMB tell us? Measure the strength of the temperature differences on different scales, eg WMAP 2003:
49 What does the CMB tell us? In practice, we need information from a wide range of ‘resolutions’, or scalesMeasure the strength of the temperature differences on different scalesLow resolution (eg COBE)Higher resolution (eg WMAP)Theorists: come up with a model (function, like y=mx +c but more complex!) including all of the physics of CMB/structure formationObservers: fit the model to real observations of the CMB (like drawing a line of best fit), tweaking the values of each parameter
51 What does this tell us?The function on the previous slide is complex and involves many terms including:Density of Universe in ORDINARY MATTERDensity of Universe in DARK MATTERDensity of Universe in DARK ENERGY(The sum is the total density, and governs the fate of the Universe as discussed earlier).We can constrain some of the big questions in cosmology by observing the CMB
52 Current ‘best model’ The Universe appears to be flat (critical) Will just expand foreverBut measurements suggest that only 30% of this density can come from matterContributions from ‘ordinary’ and ‘dark’ matterThis points towards the existence of ‘something else’ which we call Dark EnergyDark energy is believed to be pushing the Universe outwards, i.e. accelerating the expansion
53 What next for CMB research? New satellite, Planck, launch date 2008?Set to solve all the mysteries…..allegedly!This, and some ground based experiments are trying to measure CMB polarisation (difficult!)Another route: look for ‘secondary’ features in the CMB (ie those that have occurred since the Big Bang)
54 Before we move on: Quick CMB revision…. The CMB is light originating from the Big Bang We can see it coming from all directions The sky ‘glows’ at radio frequencies
55 More recent imprints on the CMB Let’s forget the tiny temperature fluctuations for now!Majority of CMB photons have travelled through the Universe unimpededBut some have interacted with ionised material on the wayMain contributor: Galaxy clusters
56 Rich Clusters - congregations of hundreds or even thousands of galaxies See cluster galaxies and lensing arcs in the opticalBut only around 5% of a cluster’s mass is in galaxies (Most of the mass is in Dark Matter)But a sizeable fraction is found in hot gas......
57 X-rays - see hot gas via Bremstrahlung 10-30% of total mass Chandra Image of the Coma cluster
58 Cluster Gas Gas stripped from galaxies and sucked in from outside Trapped in huge gravitational potentialHot, dense and energeticIonised (charged) - may interact with incident radiation (such as the CMB)Accurately represents the characteristics of the whole UniverseClusters are ‘Cosmic Laboratories’
59 Sunyaev and Zel’dovich, 1969 Postulated that the CMB could interact with the gas in galaxy clustersThe ‘Sunyaev Zel’dovich (SZ) Effect’
60 What is it, exactly?Low energy CMB photon collides with high energy cluster electronPhoton receives energy boostNet effect: shift CMB to higher frequencies in the direction of a cluster
61 What is it, simply?Cluster makes partial ‘shadow’ in the CMB
62 What is so interesting?It’s INDEPENDENT of the DISTANCE of the cluster responsibleThe strength of the shadow tells us about the characteristics of the CLUSTER GASMirrors UNIVERSAL CHARACTERISTICS
63 What does it look like?VSA image(from earlier!)
64 Exciting new Science!In most branches of Astronomy, it is difficult to observe very distant objectsThe SZ effect is distance-independent, so in theory we can observe ALL clusters in existenceCurrent hot topic: surveying the sky using radio telescopes to find new clusters via the SZ effect
65 To study Cosmology via clusters, we need lots of them A large, sensible sample of objects is usually called a ‘catalogue’
66 SZ Cluster surveysCluster catalogues to date have been derived from X-ray observationsSevere limitations since the X-ray signal falls off quickly with increasing distanceSZ surveys will enable us to generate catalogues of ALL clusters in existence (with a few caveats!)Cluster evolutionStudy how cluster properties change as a function of distance (and hence cluster age)Evolution of the UniverseStudy how the number of clusters per unit volume changes with distance: cosmology
67 My WorkI previously worked with the Very Small Array, looking at both the CMB and the SZ effectI am now involved with two new SZ experiments, OCRA and AMiBAWe are:Studying known clustersPerforming surveys to find new ones
68 OCRA Prototype detector on Torun telescope, Poland 32m dish Various receivers, ours works at 30GHzSuffers from atmospheric contaminationMost useful observations are made in the winter
69 OCRA We recently published results from 4 well-known clusters Now observing larger sample, should be able to derive more science from thisFuture: array receiver, blind surveysExcellent imaging instrument
70 AMiBA Taiwanese project, based in Hawaii 90 GHz Interferometer Hexapod mountTesting observations with 7 60cm dishesHigh significance detections of well-know clusters, will be published ‘soon’!
71 AMiBA Ultimately: 19 dishes, 1.2m diameter? Potential problems with the platform flexing…Also problems with ground emissionVery powerful survey instrumentAlso – polarisation in the CMB
72 Can overcome most problems but it’s not easy! Challenges….The SZ effect is TINYGalaxy clusters contain galaxies (!), which may emit radio waves and drown the SZ signalRequire further information, or observations at multiple frequencies.Radio galaxies are less bright at higher frequencies, but higher frequency observations suffer from atmospheric contaminationRemember the fluctuations in the CMB itself? They can also contaminate!Go to higher resolutionCan overcome most problems but it’s not easy!
73 SummaryThe Big Bang left behind radiation which we can observe at radio frequencies todayThe Cosmic Microwave BackgroundThe CMB has imprints upon it caused by the formation of the structures we see today (eg galaxies)The CMB tells us much about the Universe as a wholeGalaxy clusters may create ‘shadows’ in the CMBThe Sunyaev Zel’dovich EffectThe SZ effect is distance-independent so very useful for cluster physics and also Cosmology