Postdoc in the Astrophysics group at Bristol working with Professor Mark Birkinshaw, world expert in our field Various projects, OCRA, AMiBA Previously – PhD in Cambridge, working on the VSA MSci in Bristol (many moons ago!) About Me…..
Talk Structure: 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 Zeldovich effect Two new SZ experiments –OCRA –AMiBA
My Work: COSMOLOGY from: –The Cosmic Microwave Background Radiation (CMB) –The interaction of the CMB with Galaxy Clusters via the Sunyaev Zeldovich Effect OBSERVATIONAL - ie obtaining data, data processing, extracting science Tenerife, Poland, Hawaii, Taiwan….. Very hot topics in Astrophysics at the moment!
OBSERVATIONAL Observe celestial bodies (stars, galaxies etc) at various wavelengths Fit theoretical models to data to choose the most appropriate THEORETICAL Simulate celestial bodies (stellar evolution, galaxy formation etc) Create models of possible physical processes Astronomy Research: How it Works
Onto the specifics: What are we trying to achieve in Cosmology today?
Zwicky 1933: Galaxy clusters contain Dark Matter
1998: Supernovae suggest Universe is accelerating
Big questions in cosmology Will the Universe expand forever? –Depends on the mean density –We can constrain this using the CMB What is the Universe made from? –Normal stuff plus Dark Matter –What is Dark Matter? Particle physicists working on it! Why does it appear to be accelerating? –It is being pushed by Dark Energy –We can constrain this using the CMB Critical density: Universe expands forever Less dense: Expansion rate increases More dense: Universe will collapse Accelerating: Dark energy???
But what on earth is it?? The Cosmic Microwave Background is central to our cosmological understanding
Observing the galaxy, detected annoying level of static in all directions Pigeon poo? Aliens?? No! At the same time, Dicke at Princeton predicted the existence of relic radiation from the big bang, ie the CMB Nobel Prize, 1978 Penzias and Wilson, 1965
The sky is BRIGHT at radio frequencies. If we observe the sky with a radio telescope, in between the stars and galaxies, it is NOT DARK. Visualising the CMB…..
But where does it come from? It all started with: The Big Bang
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)
Universe much cooler, atoms start to form….. Hydrogen, Helium, normal stuff 300,000 years later……
Universe now neutral, Photons escape These photons, viewed today, form the Cosmic Microwave Background Radiation
Summary: Formation of the CMB The Universe started with the Big Bang It was initially hot, dense and ionised Photons 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
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
The CMB today Can observe the CMB today, 13.7 billion years after the Big Bang Radiation is much cooled: 2.73 K (-270.42°C) Conclusive evidence for the Big Bang theory - proves Universe was once in thermal equilibrium So..... what does it look like?
Observe blank sky with a radio telescope. Rather than darkness, see Uniform, high-energy glow Turn up the resolution......
Tiny temperature differences (microK) When the CMB photons escaped, structures were starting to form These structures have now become galaxies The structure formation processes have affected the CMB and we see the imprint as hot and cold spots Very difficult to measure!
What does the CMB tell us? Measure the strength of the temperature differences on different scales, eg COBE 1992:
A plethora of other experiments followed this up….until….
What does the CMB tell us? Measure the strength of the temperature differences on different scales, eg WMAP 2003:
What does the CMB tell us? In practice, we need information from a wide range of resolutions, or scales Measure the strength of the temperature differences on different scales –Low resolution (eg COBE) –Higher resolution (eg WMAP) Theorists: come up with a model (function, like straight line y=mx +c but more complex!) including all of the physics of CMB/structure formation Observers: fit the model to real observations of the CMB (like drawing a line of best fit), tweaking the values of each parameter
What does this tell us? The function on the previous slide is complex and involves many terms including: –Density of Universe in ORDINARY MATTER –Density of Universe in DARK MATTER –Density 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
Current best model The Universe appears to be flat (critical) –Will just expand forever But measurements suggest that only 30% of this density can come from matter –Contributions from ordinary and dark matter This points towards the existence of something else which we call Dark Energy –Dark energy is believed to be pushing the Universe outwards, i.e. accelerating the expansion
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)
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
More recent imprints on the CMB Lets forget the tiny temperature fluctuations for now! Majority of CMB photons have travelled through the Universe unimpeded But some have interacted with ionised material on the way Main contributor: Galaxy clusters
Rich Clusters - congregations of hundreds or even thousands of galaxies See cluster galaxies and lensing arcs in the optical But only around 5% of a clusters mass is in galaxies (Most of the mass is in Dark Matter) But a sizeable fraction is found in hot gas......
ROSAT image of the Coma cluster X-rays - see hot gas via Bremstrahlung 10-30% of total mass
Cluster Gas Gas stripped from galaxies and sucked in from outside Trapped in huge gravitational potential Hot, dense and energetic Ionised (charged) - may interact with incident radiation (such as the CMB) Accurately represents the characteristics of the whole Universe Clusters are Cosmic Laboratories
Sunyaev and Zeldovich, 1969 Postulated that the CMB could interact with the gas in galaxy clusters The Sunyaev Zeldovich (SZ) Effect
What is it, exactly? Low energy CMB photon collides with high energy cluster electron Photon receives energy boost Net effect: shift CMB to higher frequencies in the direction of a cluster
What is it, simply? Cluster makes partial shadow in the CMB
What is so interesting? Its INDEPENDENT of the DISTANCE of the cluster responsible The strength of the shadow tells us about the characteristics of the CLUSTER GAS Mirrors UNIVERSAL CHARACTERISTICS
What does it look like? VSA image (from earlier!)
Exciting new Science! In most branches of Astronomy, it is difficult to observe very distant objects The SZ effect is distance-independent, so in theory we can observe ALL clusters in existence Current hot topic: surveying the sky using radio telescopes to find new clusters via the SZ effect
To study Cosmology via clusters, we need lots of them A large, sensible sample of objects is usually called a catalogue
SZ Cluster surveys Cluster catalogues to date have been derived from X- ray observations –Severe limitations since the X-ray signal falls off quickly with increasing distance SZ surveys will enable us to generate catalogues of ALL clusters in existence (with a few caveats!) Cluster evolution –Study how cluster properties change as a function of distance (and hence cluster age) Evolution of the Universe –Study how the number of clusters per unit volume changes with distance: cosmology
My Work I previously worked with the Very Small Array, looking at both the CMB and the SZ effect I am now involved with two new SZ experiments, OCRA and AMiBA We are: –Studying known clusters –Performing surveys to find new ones
One Centimetre Receiver Array The other members of the OCRA team: Mark Birkinshaw, Aziz Mohammed, Peter Wilkinson, Ian Browne, Stuart Lowe, Michael Peel, Richard Davis, Richard Battye, Andrzej Kus, Marcin Gawronski, Roman Feiler, Eugeniusz Pazderski, Bogna Pazderska
OCRA: The One Centimetre Receiver Array Planned 100 pixel 30-GHz array receiver Excellent imaging and surveying capabilities Ideal for performing surveys to study populations of radio sources and SZ clusters Prototype in operation to test concept Two elements identical to those for full OCRA Science observations more than feasible All observations to date made with OCRA-p
Radio Sources Always a problem! Can completely drown out the cluster signal Need accurate 30GHz measurements of sources OVRO/BIMA @ 30GHz for some clusters Recently visited the Green Bank Telescope to observe all sources Much higher resolution than the Torun telescope
What next? OCRA-F 8 beams (4 x OCRA-p) Eventually 16 horns Under construction, will be on the telescope early next year Imaging capabilities –Resolve substructure in SZ Trial blind surveys
Array for Microwave Anisotropy Other team members: Paul T.P. Ho, Ming-Tang Chen, J.H. Proty Wu, Keiichi Umetsu, Mark Birkinshaw, Chao-Te Li, Guo-Chin Liu, Kai-Yang Lin, Patrick Koch, Yo-Wei Liao, Hiroaki Nishioka
Current status Located on Mauna Loa, Big Island, Hawaii 90GHz interferometer –Radio sources less problematic 9m platform 7 60cm dishes –CMB fluctuations contaminate! Hexapod mount system Observing nearby clusters Upgrade imminent
Summary The Big Bang left behind radiation which we can observe at radio frequencies today –The Cosmic Microwave Background The 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 whole Galaxy clusters may create shadows in the CMB –The Sunyaev Zeldovich Effect The SZ effect is distance-independent so very useful for cluster physics and also Cosmology –OCRA and AMiBA are working well!