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The development of cosmology will no doubt be seen as one of the scientific triumphs of the twentieth century. At its beginning, cosmology hardly existed as a scientific discipline. By its end, the Hot Big Bang cosmology stood secure as the accepted description of the universe as a whole. Telescopes such as the Hubble and James Webb Space Telescopes are capable of seeing light from galaxies so distant that the light has been traveling toward us for most of the lifetime of the universe. The cosmic microwave background, a fossil relic of a time when the universe was both denser and hotter, is routinely detected and its properties examined. That our universe is presently expanding is established without doubt, and its material composition is accurately determined. We are now in an era where understanding of cosmology has shifted from the qualitative to the quantitative. The turn of the millennium saw the establishment of what has come to be known as the Standard Cosmological Model, representing an almost universal consensus among cosmologists as to the best description of our universe. Nevertheless, it is a model with a major surprise – the belief that our universe is presently experiencing accelerated expansion. Add to that ongoing mysteries such as the properties of the so-called dark matter, which is believed to be the dominant form of matter in the universe, and it is clear that we have some way to go before we can say that a full picture of the physics of the universe is in our grasp. The first decades of the twenty-first century have seen dramatic leaps in the precision of cosmological probes, but this accuracy has yet to yield any substantial improvements to our overall understanding. Such a bold endeavor as cosmology easily captures the imagination, and there has been increasing demand for cosmology to be taught at university in an accessible manner. Traditionally, cosmology was taught as the tail end of a general relativity course, with a derivation of the metric for an expanding universe and a few solutions. Such a course fails to capture the flavor of modern cosmology, which takes classic physical sciences like thermodynamics, atomic physics, and gravitation and apply them on a grand scale. In fact, introductory modern cosmology can be tackled in a different way, by avoiding general relativity altogether. By a lucky chance and a subtle bit of cheating, the correct equations describing an expanding universe can be obtained from Newtonian gravity. From this basis, one can study all the triumphs of the Hot Big Bang cosmology – the expansion of the Universe, the prediction of its age, the existence of the cosmic microwave background, and the abundances of light elements such as helium and deuterium – and even go on to discuss more speculative ideas such as the inflationary cosmology. The origin of this book, first published in 1998, was a short lecture course at the University of Sussex, around 20 lectures, taught by Andrew to students in the final year of a bachelor’s degree or the penultimate year of a master’s degree. The prerequisites are all very standard physics, and the emphasis is aimed at physical intuition rather than mathematical rigor. It has been further developed through his experiences of teaching cosmology at Imperial College London and at the University of Edinburgh, plus much useful feedback from readers. The structure of the book is a central “spine,” the main chapters from 1 to 15, which provide a self-contained introduction to modern cosmology. In addition, there are six Advanced Topic chapters, each with prerequisites, which can be added to extend the course as desired. Ordinarily, the best time to tackle those Advanced Topics is immediately after their prerequisites have been attained, though they could also be included at any later stage. Cosmology is an interesting course to teach, as it is not like most of the other subjects taught in undergraduate physics courses. There is no perceived wisdom, built up over a century or more, which provides an unquestionable foundation, as in thermodynamics, electromagnetism, and even quantum mechanics and general relativity. Each successive edition of this book has introduced new discoveries, and lecturers can expect opportunities to crop up during their course to discuss new results which impact on cosmologists’ views of the universe. You can follow our own evolving views by checking out this book’s WWW Home Page. There you may find some updates on observations, a detailed solution set for instructors, and also a list of any errors in the book that we are aware of. If you are confident you’ve found one yourself, and it’s not on the list, we’d be very pleased to hear of it.