David H. Lyth - The History of the Universe
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This book gives an accessible account of the history of the Universe; not only what happened, but why it happened. An author of textbooks on the early Universe and inflation, David Lyth now explains both cosmology and the underlying physics to the general reader. The book includes a detailed account of the almost imperceptible structure in the early Universe, and its probable origin as a quantum fluctuation during an early epoch known as the epoch of inflation. It also explains how that early structure is visible now in the cosmic microwave radiation which is our main source of information about the early Universe, and how it gave rise to galaxies and stars.The main text of the book assumes no knowledge of mathematics or physics so that it is accessible to everybody, while an appendix contains more advanced material. As a result the book will be useful for a wide spectrum of readers, including high-school students, undergraduates, postgraduates and professional physicists working in areas other than cosmology. It will also serve as additional reading for university courses in general astronomy, astrophysics or cosmology itself.
David H. Lyth: author's other books
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David H. Lyth
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Springer International Publishing Switzerland 2016
David H. Lyth The History of the Universe Astronomers' Universe 10.1007/978-3-319-22744-3_11. The Universe
David H. Lyth 1
(1)
Physics, Lancaster University, Lancaster, UK
From our home on the Earth, we look out into the distances and strive to imagine the sort of world into which we were born.. The search will continue.. It is not satisfied and it will not be suppressed.
(Edwin P. Hubble in Realm of the Nebulae, 1936)
I suppose that we humans have always tried to understand whats around us. Until recently though, we didnt achieved much. As far as can be told from the written record, our ideas about the nature of physical reality were mostly way off course, and ideas about the Universe beyond the Earth were completely wrong.
The first step to understanding the Universe was not taken until the early sixteenth century, when Copernicus realized that the passage of the planets across the sky implies that both they and the Earth are going around the Sun in roughly circular orbits. Now, five centuries later, we know much more.
We know the geography of the Universe. We are on the edge of a galaxy , which contains billions of stars more or less like the Sun. The stars you see in the sky are a small sample of those stars. There are billions of other galaxies, more or less like ours, separated from each other by vast distances.
We know the history of the Universe, starting from a simple state of affairs when it was about a hundredth of a second old. Ill be calling that the known history . The history is a long one, because the Universe is now about 13.8 billion years old. Throughout the known history, the Universe has been expanding . At very early times, there were no galaxies, but just a nearly homogeneous gas. It wasnt quite homogeneous though, and that is what led to the formation of galaxies . Each galaxy began as a slightly over-dense region, which attracted more and more matter towards it through the force of gravity until it finally collapsed under its own weight .
When a galaxy first forms its just a cloud of gas. Then over-dense regions within it collapse, to form stars. There are many different types of star, a veritable zoo. The study of individual objects like stars, or even galaxies, is called astrophysics and is not regarded as part of cosmology which studies the Universe as a whole. Just to satisfy your curiosity though, Ill describe the zoo briefly in Chap.
We know, with one proviso, what the Universe is made of. There is electromagnetic radiation , which can be regarded as consisting of particles called photons , and there are also some other particles called neutrinos . Theres also ordinary matter , that you and I are made of. Finally, theres what is called Cold Dark Matter or CDM . The CDM is called Cold because its particles have hardly any random motion , corresponding to a very low temperature . Its called Dark because it doesnt emit any radiation. But it doesnt absorb any either, so it should really be called Cold Transparent Matter. Even more remarkably, there is no discernable effect when a CDM particle encounters a particle of ordinary matter, even though such encounters must be taking place all the time. As a result of all this, we know of the CDM only through its gravitational effect, and we dont know anything about the nature of the particles that its made of. (Thats the proviso.)
We know what fraction of the Universes energy is carried by each of those components, and now comes a surprise. The fractions dont add up to 1, because the vacuum itself has energy. The energy density of the vacuum is called the Cosmological Constant . As with the CDM, we know that the Cosmological Constant is there because of its gravitational effect. Its called a constant because it doesnt change with time. This is in contrast to the contribution of particles to the energy density. That contribution decreases with time as the particles move apart with the expansion.
Our biggest single source of information about cosmology came from whats called the Cosmic Microwave Background (CMB) , shown in Fig. varying only be about one part in ten thousand. That is because the cosmic gas was homogeneous to the same accuracy when the CMB was emitted. 
Fig. 1.1
The image is a map of the whole sky, and is analogous to the map of the whole Earth that one sometimes sees. The boundary of the map corresponds to a single direction in the sky, and its centre corresponds to the opposite direction. The map shows the brightness of the CMB, the dark blue regions being the brightest and the red regions being least bright. The variation in brightness is only about one part in 10,000 (Source: NASA [ www.nasa.gov ])
More information comes from the observation of galaxies. The stars we see in the sky are in our galaxy, but hundreds of thousands of other galaxies have been observed through telescopes. As shown in Fig., their distribution averaged over a large volume is nearly uniform . 
Fig. 1.2
A survey of the large galaxies, that are within two opposite wedges of the Universe around us. The survey finds more than 220,000 galaxies. The Earth is at the centre of the image, and the circle is about 2 109 light years away. Each dot is a large galaxy. The redshift of a galaxy is defined as its distance divided by what is called the Hubble distance, and the latter is approximately the distance to the present horizon (Image courtesy of M. Blanton and the Sloan Digital Sky Survey)
The distribution of galaxies can be analyzed statistically, and so can the anisotropy of the CMB . In both cases, one finds that the statistical properties are the same in every region, and that they dont pick out any particular direction. We say that the Universe is statistically homogeneous and statistically isotropic .
We dont know for sure what happened before the beginning of the known history, but it was presumably preceded by some era when the Universe was still an expanding gas of particles. The entire era when there was an expanding gas is called the Big Bang . The Big Bang was probably preceded by what is called inflation . Conditions at the end of a long era of inflation are more or less independent of what they were at the beginning. As a result, a suitably-chosen inflation scenario can provide a complete explanation of what we observe. This is both a blessing and a curse. Its a curse because it means that observation cant distinguish between different scenarios for what happened before inflation. Its a blessing, because it removes the need to think about such scenarios; for practical purposes, the history of the Universe begins with inflation!
Alternatives to inflation have been proposed, in which the very early Universe is contracting, the contraction only later giving way to expansion. There are also cyclic proposals, in which that process is repeated. The alternatives are more complicated than inflation, and less popular.
In all of this, Ive been writing Universe, but of course we dont really know about the whole Universe. We only know about a spherical region around us, called the Observable Universe . The boundary of this region is called the present horizon . The distance to the present horizon is the biggest distance that any particle can have travelled, starting at the beginning of the Big Bang and ending at the present epoch. It really does mark the limit of the observable Universe, because all of our knowledge about the Universe is obtained by observing particles that were emitted during the Big Bang.
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