Advances in Astronomy

The following sections are included:

• Foreword

• Preface

• Contents

We take a high-speed tour of the approximately thirteen billion-year history of our universe focusing on unsolved mysteries and the key events that have sculpted and shaped it – from inflation in the first split second to the dark energy which is currently causing the expansion of the cosmos to accelerate.

The standard model of cosmology is based on the hot Big Bang theory and the inflationary paradigm. Recent precise observations of the temperature and polarization anisotropies in the cosmic microwave background and the matter distribution in large scale structures like galaxies and clusters confirm the general paradigm and put severe constrains on variations of this simple idea. In this essay I will discuss the episte-mological foundations of such a paradigm and speculate on its possible realisation within a more fundamental theory.

The idea of possible time or space variations of the ‘fundamental’ constants of nature, although not new, is only now beginning to be actively considered by large numbers of researchers in the particle physics, cosmology and astrophysics communities. This revival is mostly due to the claims of possible detection of such variations, in various different contexts and by several groups. Here, I present the current theoretical motivations and expectations for such variations, review the current observational status, and discuss the impact of a possible confirmation of these results in our views of cosmology and physics as a whole.

One of the theoretical cornerstones of modern cosmology is the concept of cold dark matter. This theory supposes that almost 90% of the mass in the Universe is made up of some unknown, invisible (hence “dark”) particle which interacts with the rest of the Universe only through gravity and which is cold in the sense that the particles move slowly (like molecules in a cold gas). One of the grand achievements of observational cosmology in recent years has been to uncover strong evidence in favour of this idea, through means as diverse as gravitational lensing, surveys of millions of galaxies and studies of the cosmic microwave background. This observational evidence however, applies almost entirely on rather large scales (comparable to the sizes of galaxy clusters and larger). On smaller scales there is an uncomfortable disagreement between the theory and observations. Many galaxies seem to contain too little dark matter in their centres, while galaxies such as the Milky Way ought to have hundreds of small companion galaxies, but apparently have only a handful of them. I will describe these recent confirmations and refutations of the cold dark matter hypothesis and discuss their possible consequences. In particular, I will highlight some of the theoretical work which has been prompted by this discussion and describe how a debate over dark matter is leading to improvements in our understanding of how galaxies form.

Violent activity in the nuclei of galaxies has long been considered a curiosity in its own right; manifestations of this phenomenon include distant quasars in the early Universe and nearby Seyfert galaxies, both thought to be powered by the release of gravitational potential energy from material being accreted by a central supermassive black hole (SMBH). The study of galaxy formation, structure and evolution has largely excluded active galactic nuclei (AGN), but recently this situation has changed, with the realisation that the growth of SMBHs, the origin and development of galaxies and nuclear activity at different epochs in the Universe may be intimately related. The era of greatest quasar activity seems to coincide with turbulent dynamics at the epoch of galaxy formation in the young, gas-rich Universe; ubiquitous black holes are then a legacy of this violent era. Closer to home, a fraction of ordinary galaxies have re-ignited their central engines. These galaxies are more established than their distant cousins, so their activity is more puzzling. I review the evidence for causal links between SMBHs, nuclear activity and the formation and evolution of galaxies, and describe opportunities for testing these relationships using future astronomical facilities.

The Wilkinson Microwave Anisotropy Probe (WMAP) is currently mapping the temperature and polarization anisotropies of the cosmic microwave background (CMB) radiation on the full sky at five microwave frequencies. We summarize the major scientific results from the analysis of the first-year WMAP data, released on February 11, 2003: (1) the WMAP data alone fits a standard cosmological model, giving precise determinations of its six parameters; (2) the universe was reion-ized very early on in its history; (3) a joint analysis of WMAP data with smaller scale ground-based CMB experiments and large-scale structure data yields improved constraints on the geometry of the universe, the dark energy equation of state, the energy density in stable neutrino species, and the Inflationary paradigm.

Nearly a century has passed since gravitational waves were first conceived by Poisson in 1908 and shown by Einstein to be an essential consequence of general relativity in 1916. The quest for gravitational waves began with the famous Weber's bar experiment. To date we have no conclusive detection of these elusive waves but many believe we are at the verge of a breakthrough. In this article I will highlight the worldwide quest for gravitational waves and what fundamental physics, astrophysics and cosmology we might learn by opening this new window of observation.

A new era in fundamental physics began when pulsars were discovered in 1967. They are useful tools for a wide variety of physical and astrophysical problems, in particular for the study of theories of relativistic gravity. Being precise cosmic clocks, pulsars take us beyond the weak-field limit of the solar-system. Their contribution is crucial as no test can be considered to be complete without probing the strong-field realm of gravitational physics by finding and timing pulsars. In this review, I will explain some of the most important applications of millisecond pulsar clocks in the study of gravity. Recent discoveries such as the double pulsar, and prospects of finding a pulsar-black hole system are discussed.

Gamma-ray bursts are immensely bright and therefore have great potential as probes of the universe. They reside in small, distant galaxies, and thus provide a route to investigating the properties of such galaxies, which are themselves too faint for detailed study by more direct means. The use of GRBs as cosmic beacons is still in its infancy, but will become much more important in the future as we find many more bursts at very great distances. Ultimately we hope GRBs will help us investigate the star formation rate, and the state of intergalactic matter through the history of the universe. They may even help us determine the large scale geometry of the universe and the microscopic structure of space-time.

There are many natural sources of radio waves in the cosmos, from magnetised plasmas to molecular gas. These have two things in common; their emission can penetrate dust and clouds which block out optical light, and it can be studied using interferometry giving unparallelled angular resolution. This is invaluable for the study of star formation in our own and other galaxies and its place in stellar and cosmological evolution. Interferometric data comes at a price of complex data processing and very high data rates. The volume of radio astronomy data will increase over a thousandfold in the next few years as optical fibres are used to link ambitious new arrays. Fortunately the results will become more accessible thanks to the development of Virtual Observatories giving remote access not only to data but to data handling tools. Internationally agreed standards for describing images, spectra, catalogues and other data products allow astronomers to extract and compare results from the radio to the X-ray domain and beyond, without leaving their desks. These developments are illustrated by application to two problems; classification of young stellar objects in the Milky Way and the relationship between active galactic nuclei and starburst activity at high redshift.

Gamma-Ray Bursts have been puzzling astronomers and astrophysicists for more than 30 years. In the late nineties, the discovery of afterglow emission created a quantum jump in their understanding. We now know that they are extremely bright events taking place at cosmological distances, that they involve the fastest expansion velocities in the known universe and that their explosion is probably associated with the death of a massive rotating star. Yet, many key issues are still calling for an explanation, and new more powerful experiments are being designed and built to deepen our insight into these extraordinary phenomena. I will describe our understanding of Gamma-Ray Burst physics and the observational basis that led to the development of the present model, with particular emphasis on the last few years of research. I will then discuss the open issues and how future generation satellites, such as Swift, will help us to clarify them.

I consider the ‘life-cycle’ of astrophysical dust, from its formation and journey through the interstellar medium of the Galaxy, to its destruction. Likely structures for dust grains are examined, and the processes which control the composition of dust grains. I discuss the importance of astrophysical grain surfaces as sites of interstellar chemistry, and the effects of dust on the radiation which we observe from the Universe.

Today the sun is in a relatively uncrowded place. The distance between it and the nearest other star is relatively large (about 200,000 times the Earth–sun distance!). This is beneficial to life on Earth; a close encounter with another star is extremely unlikely. Such encounters would either remove the Earth from its orbit around the sun, or leave it on an eccentric orbit similar to a comet's. But the sun was not formed in isolation. It was born within a more-crowded cluster of perhaps a few hundred stars. As the surrounding gas evaporated away, the cluster itself evaporated too, dispersing its stars into the Galaxy. Virtually all stars in the Galaxy share this history, and in this review I will describe the role of “clusterness” in a star's life. Stars are often formed in larger stellar clusters (known as open and globular clusters) some of which are still around today. I will focus on stars in globlular clusters and describe how the interactions between stars in these clusters may explain the zoo of stellar exotica which have recently been observed with instruments such as the Hubble Space Telescope, and the X-ray Telescopes XMM–Newton and Chandra. In recent years, a myriad of planets orbiting stars other than the sun — the so-called extrasolar planets — have been discovered. I will describe how a crowded environment will affect such planetary systems and may infact explain some of their mysterious properties.

In this chapter I will describe aspects of the chemistry occurring in the transition of interstellar material from molecular clouds to planetary systems, and what we can hope to learn about this transition by studying chemistry.

More than one hundred planets are known outside our solar system. These extrasolar planets have been found in orbit around stars ranging from spectral type F to M with semi-major axes ranging from 0.02 to 6 au (1 au = Earth-Sun distance). Their masses range from around 0.1 to 10 Jupiter masses, though mass values are usually somewhat ambiguous because of the unknown inclination of their orbits. Migration theory suggests that planets of Jupiter-like mass are to be found at Jupiter-like distances, though so far we have primarily detected an important minority at smaller radii. The extrasolar planets discovered thus far are found around primary stars with a particularly high metal content, that may increase towards shorter periods and thus correlate with greater migration. One of the most enigmatic results is the high eccentricities of extrasolar planets in comparison with the Solar System. As time goes by our searches become ever more sensitive to a wider range of primaries with lower mass and longer period extrasolar planets. We are now at the beginning of a new adventure: the search for planetary systems that are more like our own.

Up until the dark ages, humankind knew of six planets including our own. The invention of the telescope, and the beginnings of scientific thought on orbits and planetary motion, were in the seventeenth century. The next three centuries added Uranus, Neptune and Pluto to the known list as well as the many moons, asteroids and comets that we know today. It is only in the latter part of the 20th century that we have been privileged to carry out in-situ exploration of the planets, comets and the solar wind's realm and to begin to understand the special conditions on Earth which meant that life started here. This is leading to a detailed view of the processes which have shaped our solar system.

Here, we briefly review our current knowledge of the solar system we inhabit. We discuss the current picture of how the solar system began. Important processes at work, such as collisions and volcanism, and atmospheric evolution, are discussed. The planets, comets and asteroids are all discussed in general terms, together with the important discoveries from space missions which have led to our current views. For each of the bodies we present the current understanding of the physical properties and interrelationships and present questions for further study. The significance of recent results, such as proof that there were one standing bodies of water on Mars, and the discovery of what appears to be an Oort cloud comet, are put into context.

What is in store for planetary exploration and discoveries in the future? Already a sequence of Mars exploration missions, a landing on a comet, further exploration of Saturn and the Jovian system and the first flyby of Pluto are planned. We examine the major scientific questions to be answered. We also discuss the prospects for finding other Earth-like planets elsewhere, and for finding extraterrestrial life both within and beyond our own solar system.

Earth and most planets in our solar system are surrounded by permanent atmospheres. Their outermost layers, the thermospheres, ionospheres and exospheres, are regions which couple the atmospheres to space, the Sun and solar wind. Furthermore, most planets possess a magnetosphere, which extends into space considerably further than the atmosphere, but through magnetosphere-ionosphere coupling processes closely interacts with it. Auroral emissions, found on Earth and other planets, are manifestations of this coupling and a mapping of distant regions in the magnetosphere into the upper atmosphere along magnetic field lines. This article compares planetary upper atmospheres in our solar system and attempts to explain their differences via fundamental properties such as atmospheric gas composition, magnetosphere structure and distance from Sun. Understanding the space environment of Earth and its coupling to the Sun, and attempting to predict its behaviour (“Space Weather”) plays an important practical role in protecting satellites, upon which many aspects of todays civilisation rely. By comparing our own space environment to that of other planets we gain a deeper understanding of its physical processes and uniqueness. Increasingly, we apply our knowledge also to atmospheres of extrasolar system planets, which will help assessing the possibility of life elsewhere in the Universe.

In this article I shall review the fundamentals of solar dynamo theory. I shall describe both historical and contemporary observations of the solar magnetic field before outlining why it is believed that the solar field is maintained by a hydromagnetic dynamo. Having explained the basic dynamo process and applications of the theory to the Sun, I shall conclude by speculating on future directions for the theory.

I will describe the most dynamic and highly energetic phenomena in the Solar System - these are the eruptions and flaring that occur on the Sun. They can release as much energy as 10 million volcanoes, and throw out material into the solar system with similar mass to Mount Everest! The theories of what can produce such an explosion are based around the magnetic field that confines the gas. These events can produce emission right across the electromagnetic spectrum. The status of our ability to predict these events is discussed.

One of the enduring puzzles of atmospheric physics is the extent to which changes in the Sun can influence the behaviour of the climate system. Whilst solar–flux changes tend to be relatively modest, a number of observations of atmospheric parameters indicate a disproportionately large response. Global–scale models of the coupled middle and upper atmosphere have provided new insights into some of the mechanisms that may be responsible for the amplification of the solar signal. In particular, modification of the transport of heat and chemicals, such as ozone, by waves during periods of solar activity has been shown to make an important contribution to the climate of the stratosphere and mesosphere. In this paper, a review of some of the recent advances in understanding the coupling between atmospheric layers and how this work relates to Sun–weather relations and climate change will be presented, along with a discussion of some of the challenges that remain.

The following sections are included:

• INDEX