Ph222 Astrophysical Concepts PDF

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 Cosmology 1 2 Up to now we've been thinking in terms of individual 'things' - stars, dust clouds, galaxies, clusters of galaxies...

PH222 - Astrophysical Concepts Aaron Golden Centre of Astronomy, School of Natural Sciences University of Galway 1 Cosmology 1 2 Up to now we've been thinking in terms of individual 'things' - stars, dust clouds, galaxies, clusters of galaxies 3 But there is only ONE Universe How did it orginate? How did it turn out the way we observe it 'now'? How will it all end? Cosmology 4 But there is only ONE Universe How did it orginate? How did it turn out the way we observe it 'now'? How will it all end? Cosmology 5 6 The Virgo Supercluster's visible core The core here (20 Mpc distant) contains ~ 1300, covers 1/4 of the full Moon. 7 8 The Great Attractor 9 Baryon Acoustic Oscillations - fluctuations in the density of the visible baryonic matter of the universe, caused by acoustic density waves in the primordial plasma of the early universe. 10 Baryon Acoustic Oscillations... After matter and radiation decoupled, the ripple like flucations associated with BAO 'froze out' to form the density concentrations that seeded galaxy formation 11 12 Cosmology - the Biggest Picture possible... 13 14 Competing models of how the Universe both came to be and is as we see it - either it's evolving or it is in some form of equilibrium Specific predictions can be be made based on which model you want to test. 15 Two completely different cosmological models... Fundamentally, if the Universe 'evolved' it should have a start that was at a higher density than what we observe today... 16 Proof of the Big Bang the further astronomers observed galaxies, Hubble's law held up... either they were all expanding away from each other from an initial event, ot else they were expanding in a uniform way (with constant creation?) Robert Dicke's group at Princeton calculated that if there was an initial explosion, the instant photons were created would be observable, red- shifted. 1964, US physicist Arno Allan Penzias and radio-astronomer Robert Woodrow Wilson discovered the CMB, estimating its temperature as 3.5 K 17 When did this event take place? 18 Using Hubble's Law to estimate the age of the Universe is typically quoted in units of ( / )/ for convenience - but given this is / / the actual units are are... So the inverse of the Hubble constant are in units of time. = 71 = 2.3 10 = = 13.8 10 years 19 How we think it all happened... 20 The Cosmic Microwave Background The 'fingerprints' of creation... Remarkably uniform - but some evidence of structure `frozen in'... (BAO!) 21 What about 'now' - whatever that means... If the Universe is unconstrained - infinite - then it throws up some problems... 22 If there is stuff everywhere - why is it dark at night? 23 24 25 Working through Olber's Paradox Universe old & in extent uniform distribution of identical luminous objects 1. objects per unit volume, radiate units of energy/time 2. divide space into large no. of thin spherical shells centred on Earth of width 3. consider shell of inner radius & outer radius + i. if is small, shell volume =4 ii. No. of luminous objects in this volume is = =4 iii. as is small, we can say a source in this shell is at a distance from Earth iv. energy radiates spherically away from a shell source, so energy per unit area at is = v. So the total energy/area/time from all the sources in the shell is = vi. Note this is independent of a remote shell contributes just as much as a local one vii. So an infinite Universe infinite shells infinitely bright sky! 26 27 A Universe of finite size AND/OR finite age... 28 So what's the answer? All we have is our observational data - what can we derive from it? We're going to use the expansion effect to try and build a basic cosmological model 29 Let's start trying to build an (evidence based) cosmological model We base it on Hubble's Law First thing we need to do is to figure out what to measure changing with time... 30 31 R(t) - captures the relative expansion that occurs between objects over cosmological time We are going to use this expression as part of a Newtonian model 32 ( = = = gravitational force...) 33 How do derive this? =- = Tune in tomorrow... 34 Wrap-up broadened our viewpoint, from galaxies galaxy clusters superclusters Universe introduced the concept of cosmology - origin & fate of the Universe introduced a ground rule - cosmological principle the steady state vs. big bang cosmologies CMB & Hubble Time first foray into basic models, starting with Olber's Paradox the concept of a dimensionaless scale factor & a Newtonian cosmology 35