Origin and Evolution of the Universe. Группа авторов
and voted the “Best Professor” on campus a record 9 times, in 2006 he was named the Carnegie/CASE National Professor of the Year among doctoral institutions, and in 2010 he received the ASP’s Richard H. Emmons Award for undergraduate teaching. He has produced 5 astronomy video courses with The Great Courses, coauthored an award-winning astronomy textbook (5 editions), and appears in more than 100 television documentaries. In 2004, he was awarded the Carl Sagan Prize for Science Popularization. He was selected as one of only two recipients of the 2017 Caltech Distinguished Alumni Award. He makes a hobby of observing total solar eclipses throughout the globe, having seen 16 so far, all successfully.
Fred C. Adams
Born in Redwood City, California, Fred Adams received his undergraduate training in Mathematics and Physics from Iowa State University in 1983 and his PhD in Physics from the University of California, Berkeley, in 1988. For his PhD dissertation research, he received the Robert J. Trumpler Award from the Astronomical Society of the Pacific. After serving as a postdoctoral research fellow at the Harvard-Smithsonian Center for Astrophysics, he joined the faculty in the Physics Department at the University of Michigan in 1991. Adams was promoted to Associate Professor in 1996 and to Full Professor in 2001. He is the recipient of the Helen B. Warner Prize from the American Astronomical Society and the National Science Foundation Young Investigator Award. He has also been awarded both the Excellence in Education Award and the Excellence in Research Award from the College of Literature, Arts, and Sciences at the University of Michigan. In 2002, he was given The Faculty Recognition Award from the University of Michigan. In 2007, he was elected to the Michigan Society of Fellows. In 2014, was elected to be a fellow of the American Physical Society and he was named as the Ta-you Wu Collegiate Professor of Physics at the University of Michigan.
Christopher P. McKay
Chris is a senior scientist with the NASA Ames Research Center. His research focuses on life in extreme environments and the search for life on other worlds in our Solar System. He is also actively involved in planning for future Mars missions including human exploration. Chris been involved in research in Mars-like environments on Earth, traveling to ice-covered lakes in Antarctica, permafrost in the Siberian and Canadian Arctic, many deserts including the Atacama, Namib, & Sahara Deserts to study life in these extreme environments. He was a co-investigator on the Huygens probe to Saturn’s moon Titan in 2005, the Mars Phoenix lander mission in 2008, and the Mars Science Laboratory mission, in 2012.
Chapter 1
The Origin of the Universe
Edward L. Wright
Introduction
One of the most significant developments of 20th century natural philosophy has been the acquisition, through astronomical observations and theoretical physics, of a substantial understanding of the earliest moments in the history of the Universe. The best current model for the origin of the Universe is known as the inflationary scenario in the Hot Big Bang model. This chapter describes the observational underpinnings of the Big Bang theory and some theoretical models based on it. The development uses elementary mathematics because this is the simplest way to describe much of physical science.
Expansion of the Universe
The discovery by Hubble (1929) that distant galaxies are moving away from us with a velocity, V, that is proportional to their distance, D, was the first evidence for an evolving Universe. Observations show that this recessional velocity is
where the coefficient H0 is known as the Hubble constant and Vp the peculiar velocity of the galaxy, which is peculiar in the sense that it is different for each galaxy. Typical values of Vp are 500 km/s, and the recessional velocities V range to several times 100,000 km/s for the most distant visible objects.
The Hubble law does not define a center for the Universe, although all but a few of the nearest galaxies seem to be receding from our Milky Way galaxy. Observers on a different galaxy, A, would measure velocities and distances relative to themselves, so we replace V by V–VA and D by D–DA. The Hubble law for A is
which is exactly the same as the Hubble law seen from the Milky Way (except for slightly different peculiar velocities), provided that VA = H0DA. Thus, every galaxy whose velocity satisfies the Hubble law will also observe the Hubble law. An observer on any galaxy sees all other galaxies receding; thus, the Big Bang model is “omnicentric.”
The idea that the Universe looks the same from any position is codified in:
The Cosmological Principle: The Universe is homogeneous and isotropic.
Note that the Hubble law does not define a new physical interaction that leads to an expansion of the Universe. Instead, it is only an empirical statement about the observed motions of galaxies. Each individual object moves on a path dictated by its initial trajectory and the forces that act on it. Since a uniform distribution of matter produces no net force by symmetry, the only forces felt by an object are caused by the structures around it, and these forces can be computed to good accuracy using the equations of Newtonian mechanics and gravity. On small scales, local forces dominate the motions of objects. For example, the orbit of an electron in an atom is determined by electrostatic forces, and the distance between the electron and the nucleus does not increase with time. The orbits of the planets in our solar system are determined by the gravitational force of the Sun, and the distance between a planet and the Sun does not follow the Hubble law. Note that the individual galaxies in Figure 1.1 do not expand. But the motion of galaxies more distant than 30 million light years is well described by the Hubble law, and this observed fact tells us that the density distribution in the early Universe was almost uniform and that the initial peculiar velocities were small.
Figure 1.1 Schematic evolution of our expanding universe and its contents. In the earlier stage, at the top, a given volume of space has a high density of matter (yellow objects) and background photons. The latter are shown in blue because they have short wavelengths and high energies. In a later stage — middle — this volume has expanded so that its density of matter has dropped. At the same time, the density of photons has dropped, as their average wavelengths have increased (now shown in green). The photons have moved. However, the relative positions of the matter (yellow galaxies) are preserved because they follow a pure uniform Hubble expansion. In a still later stage of the expansion — bottom frame — this region has further expanded. The space density of matter is still lower, and the density of photons has dropped further. The average wavelength of the photons has increased, corresponding to a lower temperature of this background radiation, so that they are now shown in dark red.
The recessional velocity of an object is easily measured with use of the Doppler shift, which causes the length of electromagnetic waves received from a receding object to be larger than the wavelength at which they were emitted by the object. Because the long wavelength end of the visible spectrum is red light, the Doppler shift of a receding object is called its redshift. The observed wavelength λobs is larger than the emitted wavelength λem and the ratio