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ASTRONOMY (Herschel)The New Cosmology: History from the Ancient Greeks to 1970Dr Robert Massey Royal Observatory Greenwich 5 November 2004 Greek Cosmology Anaximander - circle of fire Aristarchus - heliocentric concept - determined the relative distances of the Sun and Moon Eratosthenes - measured the circumference of the Earth Aristotle and Ptolemy - Earth spherical, stationary and centre of the Universe. This geocentric Universe was not very big - in the order of 10 million km. Medieval Cosmology: Ibn Rashid of Cordoba in 12th century believed that Ptolemy’s Universe was incompatible with Aristotle’s science and estimated that the ‘inner firmament’ was of the order 120 million km in radius. Copernicus - introduced the heliocentric Universe as one which made calculations of the planetary motions easier. Galileo’s observations discredited the geocentric model. Kepler - refined the heliocentric model with his three laws of planetary motion. Modern Cosmology: Newton - unified Kepler’s laws with the laws of motion - thus providing a physical explanation of the Universe. However, he believed that ‘Providence’ kept the stars motionless and therefore his Universe was a static one. Kant - proposed the idea that the Galaxy was disc-shaped. William Herschel - analysed the distribution of the stars, explained the rift in the Milky Way and determined the direction in which the Solar System was moving. Olbers - reasoned that the blackness of the sky showed that the stars did not go on for ever. Einstein - introduce the concept of a four-dimensional Universe of space-time and reasoned that the mass of the Universe results in curvature in this space-time. However, he introduced a cosmological constant in his equations to explain what was believed to be a static Universe. Henrietta Leavitt - discovered the relationship between the period of ‘cepheid variables’ (stars which vary in brightness according to the pattern of the star Delta Cephei) and their intrinsic brightness (i.e. their comparative brightness if they were all at the same distance from us). This enables the distance of galaxies to be measured. However, her original observations included very similar ‘RR Lyrae’ type variable which do not conform to the relationship - leading to a reappraisal of her results and a resulting change in the galactic distances thus determined. Slipher - discovered in 1912 that the spectrum of the Great Andromeda Galaxy (M31) was shifted and this ‘Doppler shift’ enabled the radial speed of galaxies to be measured. By the 1920s it was apparent that most galaxies were moving away from us. Hubble - discovered in 1929 the relationship between the speed of recession of galaxies and their distance (speed = H x distance), implying that the Universe was expanding and therefore Einstein’s static Universe was wrong. This offered a method of determining the distances of galaxies too distant to be measured by observing cepheid variables. However, earlier determination of the Hubble Constant H led to an underestimate of the size of the Universe by a factor of 10. Attempts to measure H between 1975 and 1995 resulted in widely varying values between 45 and 100 km per second per megaparsec (= 3.26 million lightyears) but this has now converged to 65 kms-1Mpc-1. By running the clock back, all the galaxies would be together, in other words we can estimate the age of the Universe. (i.e. galaxies travelling apart at 1/4615th the speed of light, will separate 3.26 million light years distant from each other in 4615 x 3.26 million = 15 billion years approx.). Gamow - postulated that the Universe started with a primal explosion in which the Universe (which includes space-time itself) came into existence 15 billion years ago and expanded ever since. This would imply that the matter in the Universe is dispersing and its density is decreasing. Hoyle - postulated a ‘steady-state’ theory, ridiculing Gamow’s theory as the ‘Big Bang’ theory. He maintained that the creation of one hydrogen atom per cubic metre every 10 billion years would form galaxies and maintain the density of the Universe. This theory predicted that a gradual increase radio sources with distance should be detected. Ryle - observed a rapid increase in radio sources with distance in the 1950s which implied that there were many more sources in the past (the finite speed of light means that we are observing events in the past) and this implied that the Universe is evolving. This strong evidence for the ‘Big Bang’ theory challenged Hoyle’s ‘Steady State’ theory. Penzias and Wilson - detected microwaves coming equally from all directions from space. This was interpreted as radiation from the residual heat of the Big-Bang. It has a black-body frequency distribution which peaks at about 180 GHz which is equivalent to a temperature of 2.73K (approx -270°C). This was almost incontrovertible evidence for the Big Bang (though Hoyle maintained that it can be explained from the standpoint of the Steady State theory).
Cosmic background radiation spectrum measured by the Far Infrared Absolute Spectrometer (FIRAS) aboard the Cosmic Background Explorer satellite (COBE). Cosmology since 1970 Problems with the Big Bang Theory Fourier analysis applied to recent observations by BOOmerang of the detailed cellular structure of the Cosmic Background Radiation implies that the Universe is ‘flat’. The Universe is exactly at the critical density between accelerated expansion and decelerated expansion (the so-called ‘Big Crunch’ model). There is a horizon problem. At the beginning for photons did not have time to cross the early Universe and establish thermal equalisation (resulting in isotropic cosmic background radiation (CBR). The Big Bang should have created magnetic monopoles - which have not been observed. Inflation Some of these problems were overcome by the proposal by Guth (1980) that at the beginning the Universe a ‘phase change’ drove an extremely rapid expansion for about 10-32 seconds after which the expansion slowed. During this time the radius of the Universe became infinite and therefore its surface appears flat. Although the Universe is infinite in radius, the observable Universe is finite because of the finite speed of light. In fact, we cannot directly observe the beginning of the Universe because of the opacity of matter (free electrons and protons). The Universe became transparent 380,000 years after the Big Bang. COBE The Cosmic Background Explorer satellite examined the CMR at high resolution and showed anisotropic temperature variations of about 3.3mK around the average of 2.728K resulting from the Doppler shift produced by our motion round the Galaxy. At higher resolutions, variations of about 18mK were observed. These are evidence of density variations in the plasma (before 3080,00 years after the Big Bang) which would give rise to the formation of galaxy clusters. Hubble Deep Field (HDF) In the mid 1990s the Hubble Space Telescope (HST) photographed, with a very long time exposure, an ‘empty’ area the size of that of the full Moon just north of the ‘Plough’ where no stars or galaxies had appeared on earlier photographs. The field was peppered with very faint galaxies in the early stages of formation. They appeared to be very different from galaxies nearby (and hence observed long after the formations of the Universe), with a much greater proportion of irregular galaxies. Thus it is evident that the Universe is evolving. The HST similarly photographed a ‘deep field’ in the southern celestial hemisphere in 1998 which showed that the same scene appears in more than one direction. The HST has since probed deeper into space and has photographed an ultra-deep field (HUDF) showing galaxies as they appeared only 700 to 400 million years after the Big Bang! How will the Universe end? The matter in the Universe, which is revealed by electromagnetic radiation (light, infra-red, microwave radiation etc.) is insufficient to overcome the expansion of the Universe. However, there is matter which is only detectable by its gravitation effects. This ‘dark matter’ causes galaxies to spin faster than one would expect from what we can ‘see’ and in 2002 Gray and Taylor observed an excess in the deviation of background light by galactic clusters. We still do not know what constitutes this ‘dark matter’. All sorts of objects have been conjectured from black holes and Massive Compact Halo Objects (MACHOs) and Weakly Interacting Massive Particles (WIMPs) through to magnetic monopoles. Since 1998, distant Type II supernovae in the Hubble Deep Field have been measured and turn out to be brighter than expected which implies that the Universe expanded more slowly in its early history and that its expansion is increasing with time - the expansion is accelerating. What mechanism is causing this acceleration (and therefore the kinetic energy of the matter within it) is at present unknown (hence ‘dark’ - as in ‘the dark ages’) but has been misnamed ‘Dark Energy’. What is certain is that Einstein’s cosmological constant within the various cosmological models has to be reviewed and that at the Universe will become progressively less dense - empty and cold. WMAP The NASA Wilkinson Microwave Anisotropy Probe (WMAP) team has made the first detailed full-sky map of the oldest light in the Universe, 379,000 years after the Big Bang. This is the equivalent of taking a picture of an 80-year-old person on the day of their birth. Age of the Universe 13.7 billion years (give or take 1%) First stars formed 200 million years after the Big Bang. The Universe is flat (parallel rays of light will never intersect nor diverge) Hubble constant H = 71 kms-1Mpc-1. The Universe contains 4% atoms, 23% cold dark matter and 73% ‘dark energy’.
Key Epochs
Planck Era - from 0 to 10-43 seconds at the end of which Universe was 10-33 across. Inflationary Era - from 10-35 to 10-33seconds. Electroweak Era - the weak and the electro force separate between 10-38 and 10-10seconds. Proton Era - protons and neutrons form and neutrinos decouple from 10-10 and 10-4 seconds. Electron Era - 1 second after the Big Bang. 100s later, helium and deuterium nuclei form. Radiation Era - 2 minutes to 379 000 years after the Big Bang. Matter Era - 379 000 years after the Big Bang to the present. Future Research ESA's Herschel Space Observatory is to be launched in 2007. Its 3.5-metre mirror will be able to see the first galaxies and stars that ever existed, and therefore it will help to solve the question of how they formed about 13 billion years ago. Moreover, Herschel will detect a kind of light (far-infrared light) that cannot be seen from the ground due to the atmosphere, so it will reveal phenomena that have remained hidden thus far. The Planck Telescope is to be launched in 2008. With its off-axis tilted Gregorian design with a primary mirror 1.75 x 1.5 meters in size, Planck will provide a map of the Cosmic Microwave Background (CMB) field at high angular resolution, covering at least 95% of the sky over a wide frequency range. Planck has been designed to have ten times better sensitivity to temperature variations of the CMB and more than fifty times the angular resolution of the Cosmic Background Explorer (COBE) spacecraft. The James Webb Telescope is to be launched in 2011. With its 6.5-metre mirror it is designed to study the earliest galaxies and some of the first stars formed after the Big Bang. These early objects have a high redshift from our vantage-point, meaning that the best observations for these objects are available in the infrared. JWST's instruments will be designed to work primarily in the infrared range of the electromagnetic spectrum, with some capability in the visible range. Richard H Phillips
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