Talk Poster & Background Text(PID:44276011501) Source
posted by Bob Fosbury alias The^Bob on Sunday 26th of August 2018 11:18:17 AM
Design and layout, courtesy John Law: by appointment, BRLSI, The Lunaticks. The following is my background text which gives general subject matter to be addressed. The Formation of the Universe, the Chemical Elements and Life A story told in colour Bob Fosbury European Space Agency (ESA) European Southern Observatory (ESO) Institute of Advanced Study, University of Durham Institute of Ophthalmology, University College London BRLSI: 25 October 2018 This is a broad-brush history of the Universe — from the Big Bang until the present epoch — using colour as a guide to the narrative. Why give such an important role to colour? The answer lies in the richness of the physical relationship between matter and radiation. Light carries the story of this relationship from near to far, indeed from all the way back to the time when the Universe was just a few hundred thousand years old — an infant in its nearly 14,000,000,000 year history. This light-matter interaction provides us with a most powerful window onto the physics, chemistry and, ultimately, the biology of the events that mark the evolution of the Universe through its transitions towards increasing richness and complexity. The Big Bang There is a large and a diverse array of observational evidence, much of it assembled over the last few decades, that the Universe started in a very hot, dense state. For the first tiny fraction of a second [10^(-32)s] it expanded extremely rapidly in an early 'inflationary' phase after which it slowed and proceeded at a much more pedestrian pace continuing for the next 13.8 billion years to where we are now. After the first minute or two, the atomic nuclei of the first two chemical elements, hydrogen and helium had emerged. This process has given rise to everything we are and know, and we are now piecing the story together using the curiosity-driven, evidence-based method of science. Using a wide variety of instruments on the Earth and in space, we have access to light coming from right back until the expanding fireball of the Big Bang became transparent after the first 380,000 years. Before this point, the nascent Universe was packed full of light which could only travel minute distances before it was absorbed by or scattered from an elementary particle. The initial temperature was extremely high but continuously decreased as the expansion proceeded until the temperature was low enough for the hydrogen nuclei (protons) to find electrons and combine with them to become neutral hydrogen atoms. Unlike the lone electrons, these atoms interact only weakly with light. This event happened at a temperature very close to 3000 Kelvin (see below for a definition of the Kelvin temperature scale) when the light emitted from the gas could neither be absorbed nor easily scattered. Freed from its ties to the matter, it set off on its very long journey through the increasingly tenuous universal void. These photons (particles or quanta of light) are still by far the predominant form of radiation in the Universe. Can we detect this light now and can we deduce what it was like when it was emitted? The answer is yes to both and it has been provided by an increasingly sophisticated and precise series of observations carried out since the mid-1960s, supported by theoretical models which were already being constructed before the discovery of the radiation was announced in 1966. The Nobel Prize for physics in 1978 was awarded to Arno Penzias and Robert Wilson "for their discovery of cosmic microwave background radiation". Searching for the source of excess noise in their microwave communications equipment at the Bell Telephone Laboratories in the USA, Penzias and Wilson eventually realised that they were detecting a source of high frequency radio radiation emitted uniformly from the entire sphere of the sky. This was identified as the "echo of the Big Bang" that had been predicted initially by George Gamow and his associates in the 1940s, emitted as a consequence of an early, hot phase of the Universe. Since its discovery, many observations of what became known as the Cosmic Microwave Background Radiation (CMBR) have been made from the ground and from space, most comprehensively from a series of dedicated satellites, two launched by NASA into Earth orbit (COBE–1989 — Nobel Prize to Mather & Smoot in 2006; and WMAP–2001), and the most recent by ESA into a more distant orbit at the second Lagrangian Point of the Earth–Moon system (Planck–2009). These measurements have together mapped out the early history of the Universe with exquisite precision. When the radiation was emitted, the Universe was in a state of almost perfect thermodynamic equilibrium [constant temperature everywhere] which results in a form of radiation spectrum [variation of radiation intensity with wavelength] first successfully described mathematically in 1900 by Max Planck (Nobel Prize 1918) and was soon to become one of the foundations of Quantum Physics. Such equilibrium radiation has a form that depends only on temperature, usually measured in Kelvin rather than °C [Zero Kelvin = –273°.15 Centigrade], and is commonly known as black-body (a term introduced by Gustav Kirchhoff in 1860) or cavity radiation as described by Planck's law. The cumulative measurements of all these observations have homed in on the temperature of the CMBR now at 2.72548±0.00057 K which is the most precise measurement ever made of a black-body. The spectrum emitted at this temperature lies within the radio microwave region and so, with a peak intensity close to a wavelength of 1mm, is entirely invisible to our eyes, but can be marginally detected as noise in an old de-tuned TV-set. [This temperature decreases with the age of the Universe and has been measured at many lookback times [time measured backwards from now] using astrophysical measurements.] The mathematical models of the expansion of the Universe tell us that, when it was emitted as the Universe became transparent, the CMBR had a temperature of almost exactly 3000K. This is the temperature of a cool M-star (half the temperature of the Sun and a more orange-tinted colour) and also of the type of quartz-halogen filament lamp bulb commonly used in scientific and medical instruments. Such a lamp emits a good, but not perfect, black-body spectrum. What would it actually look like should you be able to transport yourself back to then in a very well-insulated time machine? Imagine covering the entire sphere of the sky around you with edge-to-edge tungsten lamp filaments with no gaps between the glowing metal. It would be blindingly bright and hot and it would appear white to your eyes (which have an automatic 'white balance' setting) rather than the orange tint it would have if compared directly with the Sun. So, the first colour to appear in the Universe would have been very close to that of a simple light bulb! [Show demo.] But you could not have been around to actually see it, not least because the chemical elements you are made of had not yet been synthesised. This brings us neatly to the next topic. The Formation and Distribution of the Chemical Elements During the expansion of the cosmic fireball after the Big Bang, the first of the chemical elements appeared after about one minute. These were hydrogen, helium and a little lithium and even less beryllium: the first four elements in the Periodic Table. During the next minute, the Universe had expanded and cooled beyond the conditions that would enable any further nuclear reactions to occur: it became too cool and too tenuous for protons and neutrons to assemble themselves into heavier elements. There was also the problem that there were no stable nuclei with masses of 5 or 8, a gap that had to be jumped over in order to make carbon. It meant a wait of some few hundreds of million of years — called the Cosmic Dark Ages before the expanding mass of hydrogen and helium was able to fragment into clouds which could collapse under their own gravity to form the first generation of stars. It was in the cores of these stars that the temperature and density again reached sufficiently high values for the transmutation of the lighter elements into heavier ones, a fusion process which releases huge amounts of energy that eventually escapes from the surface of the star in the form of starlight. In order to construct the Universe we see around us today, new elements must not only be made, they must also be distributed in such a way that they can form new generations of stars and, ultimately, planets like the Earth and animals like us. The formation and the distribution of new elements (lazy astronomers tend to call elements other than hydrogen and helium metals) happens differently in stars of different total mass. Relatively lightweight stars like our Sun evolve quite slowly and reach an age comparable to almost the current age of the Universe before their nuclear fuel runs out. Very massive stars however shine extremely brightly and age much more rapidly, the most massive reaching only a few million years before exploding with a violence that, for a few months, will outshine a whole galaxy of 100 billion stars. It is the relatively massive stars, more than ten or so times the mass of our Sun, that manage to both make and export large yields of metals. Lower mass stars do however make significant amounts of carbon, nitrogen, oxygen, magnesium, silicon and iron. Understanding the processes by which this nucleosynthesis occurs started in 1920 when Arthur Eddington proposed that stars obtained their energy by fusing hydrogen into helium. It was not until the 1930s that the nuclear reactions that achieved this were first elaborated, but it was during the Second World War — the Manhattan Project — that the interest in nuclear physics became intense and meaningful calculations became possible. It was Fred Hoyle in 1946 who first considered the synthesis of elements beyond helium. In 1953, Hoyle made a prediction that the carbon nucleus must have a particular property that would allow there to be as much carbon in stars as is observed — perhaps the first anthropic argument: we could not exist without carbon. Such a resonance in the carbon nucleus had not been observed at that time and Hoyle had to pester the Caltech nuclear physicist William Fowler and his team to reassemble a heavy piece of equipment in his lab to make a measurement. Fowler’s colleagues strenuously resisted but they were eventually worn down by this persistent Yorkshireman and did it. They were was astonished to find that Hoyle was exactly right and he was thereafter considered to be a “clever chap”. Hoyle, Fowler and Margaret and Geoffrey Burbidge subsequently wrote one of the most remarkable articles in the history of science when, in a 107-page paper in the Reviews of Modern Physics in 1957, they set out a detailed scheme describing the formation of essentially the entire Periodic Table of the elements using a tour de force of nuclear physics. Although this work has subsequently been greatly extended and refined, it stands as one of the keystones of astrophysics and has become widely known as B-squared-FH after the authors' names. The fact that Hoyle's contribution was never as widely recognised as it deserved to be (he did not share the Nobel Prize in 1983 with Willy Fowler) has been attributed to a poor knowledge amongst astrophysicists of his earlier (1954) paper in which a fundamental equation representing his original ideas was described in words rather than in mathematical symbols and so somehow missed by most of the eager young theorists. The detailed picture is complex but but basically consists of three different mechanisms: 1. The gradual fusion of hydrogen into helium (associated with a very large release of energy, essentially a continuous, diffuse, slow burning hydrogen bomb!) followed by a series of further fusion stages: helium -> carbon and oxygen and then on to neon, magnesium, silicon, sulphur to iron. Once a core of iron has been created, there is no way to continue generating energy from fusion reactions: iron has the most firmly bound nucleus containing 26 protons and, most commonly, 30 neutrons and there is no more energy to be found from fusion by trying to add protons or neutrons to it. Iron (actually 56Ni and 56Co which are radioactive and decay within weeks to 56Fe, heating the debris as they do) is therefore the heaviest element that can be made during this slow, gentle nuclear fusion in the cores of stars [although some heavier elements can still be made through slow neutron capture, or s-process reactions, see below]. In stars like the Sun, this chain does not proceed very far and few of the products get exported to the interstellar medium. It is the more massive stars that generate the bulk of the export yield, partly because of their ability to reach the high core temperatures needed to drive the nuclear reactions at the end of the chain and partly due to the nature of their export mechanism — namely a violent explosion! 2. During the gradual, sequential process of fusion reactions over millions or billions of years, free neutrons can attach themselves to atomic nuclei to build heavier and heavier elements. The time gaps between these neutron captures can be much longer than the time taken for a neutron-rich nucleus to undergo a radioactive decay to a more stable element 'isotope'. In this way, called slow neutron capture or s-process, gaps in the series of elements produced by fusion reactions can be filled in and some of the heavier elements produced. 3. Where then do the heavier elements such as gold, platinum, uranium etc. come from? At the end of its life, one of these massive stars will catastrophically disintegrate when the fall in energy production from its iron-rich core leads to failure of pressure support for the star against the inexorable inward pull of gravity [there is also an increasing leakage of energy from the iron core by neutrino emission which further reduces the supporting pressure]. The core suddenly collapses and this results in an enormous explosion, called a supernova. It can expel a lot of the processed stellar material into interstellar space at high speeds: this can mix with and sweep up surrounding interstellar gas to form the raw material, enriched in heavier elements, for the next generation of stars and planets. In the very early stages of this explosion there is first of all a region (a radial wave) of very high temperature sweeping through the star produced by the shock of collapsing material bouncing from an unyielding core. This sudden elevation in temperature results in a dramatic but momentary increase in the rate of fusion reactions, called explosive nucleosynthesis. This is followed by another very short period during which additional neutrons can be captured by the existing heavy nuclei to build unstable isotopes of all the heavy elements on a timescale so fast that there is no time for any of them to undergo radioactive decay during the accumulation. This last process is called rapid neutron capture or r-process and it produces the material that subsequently radioactively decays to form the more stable elements of the upper part of the Periodic Table, including the gold, platinum and uranium! Hints of the existence of this process came from studies of the heavy elements produced by H-bombs in the post-war nuclear testing programmes. How do we know that this happens? The debris from the exploding supernova is initially, for a few months or so, made to glow by the radioactive elements so recently made. As this light fades, the expanding gas clouds will plough into any surrounding interstellar gas and be heated by the shock waves generated by the supersonically high-speed collisions between the gas clouds. Each chemical element in the mixture will emit light in many colours that can be identified with our spectrometers. By using physical (mathematical) models of these hot gas clouds, we can make a quantitative estimate of the amounts of each element, including the newly minted ones coming from the supernova. These multi-coloured wisps and filaments of gas are quite remarkably beautiful in appearance and contain a wealth of information about the gradual increase in the 'metal' content of the Universe as it ages. It is these structures that once contained much of the material that made our planet and the things on and in it, including our bodies. Until quite recently, it was thought that these supernovae were responsible for the synthesis of most or all of these exotic, massive elements. Theoretical work, however, had suggested that there could be a further type of event, following from the massive star supernovae, where there were lots of neutrons waiting to build heavy elements via the r-process. On the 17th of August 2017, the two gravitational wave detectors, LIGO in the USA and its sister facility Virgo, near Pisa in Italy, witnessed the unmistakable signs of two neutron stars — the remnants of two separate supernovae in a binary system — spiralling towards and then smashing into each other. Within seconds, this news was distributed to astronomical observatories around the world and in space resulting in the identification of the source of the spacetime disturbance with an object 130 million light years away in the constellation of Hydra. Within the following few weeks, the aftermath of the merging event had been observed across the electromagnetic spectrum from gamma-rays to radio waves marking the birth of what has become known as "Multi-Messenger Astronomy". This discovery was spectacularly relevant to the question of the sites of origin of the heavy chemical elements since the visible and infrared measurements with the large groundbased telescopes actually showed the radioactively heated glow from the newly forged r-process elements. If you glance at the gold ring on your finger, you can now be sure that half or more of the gold atoms in it were born in the collision of a pair of neutron stars at some time in the distant past. An exotic material indeed! If any of you have some emerald jewellery, you can look at that in a new light since the beryllium in the crystal is synthesised when an energetic cosmic ray collides with the nucleus of heavy element in interstellar space and splits it into two parts, one of which is in your gem. So what is a neutron star and where do they come from? Well, it is basically the collapsed remnant of the supernova explosion of our massive star [although very massive stars can collapse directly to a black hole]. It contains a large fraction of the r-process heavy elements produced in the explosion but with the atomic nuclei totally dissembled to a mass of close-packed neutrons, a matchbox full of which would weigh some three billion tonnes: that's about the weight of half a cubic km of rock. Neutron stars are well known to us as pulsars, discovered by Dame Susan Jocelyn Bell Burnell [DBE FRS FRSE FRAS FInstP] who discovered the first radio pulsar (initially called a Little Green Man because of its extremely regular radio bleeps) in 1967. [She was Dean of Science in the University of Bath from 2001 until 2004 and has just donated her Breakthrough Prize of £2.3 million to a fellowship programme.] It now seems that some of those neutrons get converted to heavy elements by suffering the r-process for the SECOND time when two neutron stars in a binary system eventually collide! This kind of event leaves a black hole and a cloud of neutron-rich heavy elements. Having populated most* of the Periodic table with elements, the Universe is now poised to build even more exotic stuff. This starts with the building of molecules from atoms, a construction process that can happen in (cool) stars, on planets and in interstellar and interplanetary space. * The heaviest elements we have seen are so short-lived that we do not find them in nature, that have to be made in machines like particle colliders or nuclear reactors. The origin and appearance of early life There have been numerous attempts to define what life actually is. NASA, in its pragmatic way, has come up with the phrase “Life is a self-sustaining chemical system capable of Darwinian evolution”. This is a notion that helps astrobiologists to think about life elsewhere in the Universe and how to find it. A scientist with a single data point is reluctant to draw universal conclusions. With two data points, however, an extrapolation can sometimes be justified. In spite of considerable efforts, directed mostly towards the planet Mars, life beyond the Earth has not yet been found and so we still have no way of knowing whether it originated just once — choosing our fortunate planet as a location — or whether it is indeed a common phenomenon spread throughout the Universe with its hundreds of billions of galaxies. We now know enough to realise that the manifestations of life are deeply embedded in our planet and not just in a thin crust near the surface. We are also realising how profoundly the processes of life have influenced the rocks, the oceans and the atmosphere of Earth over the last four billion years. A lifeless planet would look very different. We tend to think of planets or their satellites (moons) as favoured locations for life since we know that some fraction of them could, like Earth, host liquid water on or near their surfaces. We may well discover that other, perhaps unexpected, locations host a process that we would consider to be life. Since the determination of the structure of DNA in the early 1950s, the first synthesis of this molecule has somehow become synonymous with the origin of life. But DeoxyriboNucleic Acid is, after all, just a super efficient, easily replicated data storage medium that must have developed after the self-organising process that was to result in life was already well-underway in the young Earth. Recent research on the biochemistry of the emergence of life has therefore begun to focus on the environments within which the raw materials could be concentrated and provided with the available energy necessary to drive the synthesis of the organic molecules needed for life to take off. From what is known at the moment, it seems that the most likely location for this to occur is within deep sea structures known as alkaline hydrothermal vents. These are not associated with volcanic activity and are distinct from the 'black smokers' that puff out very hot (250–400°C) black clouds of water and which are unstable and relatively short-lived (~decades). The alkaline vents are much gentler and longer living (hundreds of thousands of years) structures containing flows of mineral-rich warm (60–90°C) water, providing networks of sheltered narrow channels with plenty of time for biochemistry to operate. It is proposed that it was within these structures that the proteins were built that would evolve to become the basic toolkit from which life in all its diverse forms would be built. One of these molecules would be able to harvest electrons to fuel the process of building sugars from carbon dioxide. Being in the Stygian depths of the ocean, this molecule would get its energy from the available chemicals such as iron and sulphur. When it finally emerged into daylight, however, it was able to harvest the abundant water and sunlight to manufacture its sugars by a process called photosynthesis. We call this molecule chlorophyll and it has many relatives in nature, most of which are highly coloured. We call them porphyrins and they provide a large fraction of nature's palette of colours, especially the green of plants and the red of animal blood. It is photosynthesis that ultimately provides almost all of the food we eat and a very large fraction of the energy that our civilisation consumes (the latter we hope to continue for not much longer!). So, what colour is chlorophyll really and why is it important to know this? To normal trichromatic (ie. human) vision, chlorophyll looks green. This is because — and this is a general property of porphyrins — it absorbs very strongly in the deep blue part of the spectrum. It also strongly absorbs deep red light and it is these red photons that the molecule uses to perform its remarkable trick of splitting water to provide the electrons that kick off the chain of events resulting in the building of sugars using the carbon from carbon dioxide in the air. The free oxygen that has transformed our planet is simply a waste-product of this process and one that was extremely toxic to the life that existed when it was first produced! When you remove both the red and the blue from the visible spectrum you are left with green. Therefore the light reflected from, or transmitted through, a leaf looks green. Green is the colour that the plants don't need and cannot readily use and so they just get rid of as much of it as they can. However, when you go just a little further into the red than our eyes can see, the chlorophyll becomes almost perfectly transparent. To this far red light the leaf appears like a white skeleton of structural elements that hold the leaf together. This makes it highly transparent and it also reflects (scatters) like a thin layer of very clean snow. So, in fact, far from being just green, vegetation is extremely red as can be seen in near- infrared photographs of the landscape. It is just that the red cone detectors in our eyes narrowly avoid extending their sensitivity far enough into the red to allow us to see this. Removing this far red light energy, which is useless for photosynthesis, from the leaf is biologically valuable since it could otherwise overheat the leaf and result in a catastrophic and unnecessary loss of water, especially in hot climates. The extremely sharp jump in transmission/reflectivity in the deep red (700nm wavelength) is a very strong and unambiguous signature of chlorophyll and it can readily be detected on Earth from distant spacecraft that have been commanded to examine their home planet. It has been given the name of the Chlorophyll 'Red Edge' by astrobiologists. Since it is quite possible that photosynthesis has evolved on other suitable planets hosting liquid water, the colour of chlorophyll may serve as a key biomarker for the remote detection of life and the next generation of large astronomical telescopes in space and on the ground will open up the possibility of detecting it should it exist. The colours of living things do not only result from chemical pigments like chlorophyll and haemoglobin, the most brilliant colours — notably blues and greens — often arise form nano-scale structures that are built by both plants and animals. We call these 'structural colours' and recent detailed investigations of the intricate regularities which produce them are eagerly studied by engineers that can then reproduce the physical effects that have been optimised by a long evolutionary process. This is a process called biomimetics: a human harvest of the products of hundreds of millions of years of Darwinian evolution. This active use by intelligent life of the complex building blocks that have evolved and been optimised over a period of more than a quarter of the time since the Big Bang may well usher in the next major transition to a yet more complex phase of the Universe. Biological lifeforms as the carrier of advanced intelligence could then pass into history.
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