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Universe, Human Immortality and Future Human Evaluation
By Alexander Bolonkin
ELSEVIERCopyright © 2012 Elsevier Inc.
All right reserved.
Chapter OneMacro World
The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. The term universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or nature.
Observations of earlier stages in the development of the universe, which can be seen at great distances, suggest that the universe has been governed by the same physical laws. The solar system is embedded in a galaxy composed of billions of stars, the Milky Way, and other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led the modern cosmology. Discovery of the red shift and cosmic microwave background (CMB) radiation revealed that the universe is expanding and apparently had a beginning. This high-resolution image of the Hubble ultradeep field shows a diverse range of galaxies, each consisting of billions of stars.
According to the prevailing scientific model of the universe, known as the Big Bang, the universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the universe has been expanding to its present form, possibly with a brief period (10-32s, 15 billions of years) of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of dark energy, and that most of the matter in the universe may be in a form that cannot be detected by present instruments, and so is not accounted for in the present models of the universe; this has been termed dark matter. The imprecision of current observations has hindered predictions of the ultimate fate of the universe.
Current interpretations of astronomical observations indicate that the age of the universe is 13.75 ± 0.17 billion years, and that the diameter of the observable universe is at least 93 billion light-years, or 8.80 × 1026 m. According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Because we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.
More customarily, the universe is defined as everything that exists, has existed, and will exist. According to this definition and our present understanding, the universe consists of three elements: space and time, collectively known as space-time or the vacuum; matter and various forms of energy and momentum occupying space-time; and the physical laws that govern the first two. A related definition of the term universe is everything that exists at a single moment of cosmological time, such as the present, as in the sentence "The universe is now bathed uniformly in microwave radiation."
The universe is very large and possibly infinite in volume. The region visible from Earth (the observable universe) is about 92 billion light-years across, based on where the expansion of space has taken the most distant objects observed. For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky Way Galaxy is roughly 100,000 light-years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light-years away. There are probably more than 100 billion (1011) galaxies in the observable universe. Typical galaxies range from dwarfs with as few as 10 million (107) stars up to giants with 1 trillion (1012) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around 1 sextillion (1021) stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 × 1022) (Figures 1.1 and 1.2).
The universe is believed to be mostly composed of dark energy and dark matter, both of which are poorly understood at present. Less than 5% of the universe is ordinary matter, a relatively small contribution (Figure 1.3).
The observable matter is spread uniformly (homogeneously) throughout the universe, when averaged over distances longer than 300 million light-years. However, on smaller length scales, matter is observed to form "clumps," that is, to cluster hierarchically; many atoms are condensed into stars, most stars into galaxies, most galaxies into clusters, superclusters, and finally, the largest-scale structures such as the Great Wall of galaxies. The observable matter of the universe is also spread isotropically, meaning that no direction of observation seems different from any other; each region of the sky has roughly the same content. The universe is also bathed in a highly isotropic microwave radiation that corresponds to a thermal equilibrium blackbody spectrum of roughly 2.725K (kelvin). The hypothesis that the large-scale universe is homogeneous and isotropic is known as the cosmological principle, which is supported by astronomical observations.
The present overall density of the universe is very low, roughly 9.9 × 10-30 g/cm3. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter, and 4% ordinary matter. Thus the density of atoms is on the order of a single hydrogen atom for every 4m3 of volume. The properties of dark energy and dark matter are largely unknown. Dark matter gravitates as ordinary matter and thus works to slow the expansion of the universe; by contrast, dark energy accelerates its expansion (Figure 1.3).
The most precise estimate of the universe's age is about 13.73 ± 0.12 billion years old, based on observations of the CMB radiation. This expansion accounts for how Earth-bound scientists can observe the light from a galaxy 30 billion light-years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been red-shifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating, based on studies of Type Ia supernovae and corroborated by other data.
As such, the conditional probability of observing a universe that is fine-tuned to support intelligent life is 1. This observation is known as the anthropic principle and is particularly relevant if the creation of the universe was probabilistic or if multiple universes with a variety of properties exist (Figure 1.4).
Of the four fundamental interactions, gravitation is dominant at cosmological length scales; that is, the other three forces are believed to play a negligible role in determining structures at the level of planets, stars, galaxies, and larger-scale structures. Given gravitation's predominance in shaping cosmological structures, accurate predictions of the universe's past and future require an accurate theory of gravitation. The best theory available is Albert Einstein's general theory of relativity.
That leads to a single form for the metric tensor, called the Friedmann–Lemaître–Robertson–Walker metric. This metric has only two undetermined parameters: an overall length scale R that can vary with time, and a curvature index k that can be only 0, 1, or 1, corresponding to flat Euclidean geometry, or spaces of positive or negative curvature. In cosmology, solving for the history of the universe is done by calculating R as a function of time, given k and the value of the cosmological constant Λ, which is a (small) parameter in Einstein's field equations. The equation describing how R varies with time is known as the Friedmann equation, after its inventor, Alexander Friedmann (Figure 1.5).
The solutions for R(t) depend on k and Λ, but some qualitative features of such solutions are general. First and most importantly, the length scale R of the universe can remain constant only if the universe is perfectly isotropic with positive curvature (k = 1) and has one precise value of density everywhere, as first noted by Albert Einstein. However, this equilibrium is unstable and because the universe is known to be inhomogeneous on smaller scales, R must change, according to general relativity. Einstein's field equations include a cosmological constant (Λ) that corresponds to an energy density of empty space.
Russian physicist Zel'dovich suggested that Λ is a measure of the zero-point energy associated with virtual particles of quantum field theory, a pervasive vacuum energy that exists everywhere, even in empty space. Evidence for such zero-point energy is observed in the Casimir effect.
The ultimate fate of the universe is still unknown, because it depends critically on the curvature index k and the cosmological constant Λ. If the universe is sufficiently dense, k equals +1, meaning that its average curvature throughout is positive and the universe will eventually re-collapse in a Big Crunch, possibly starting a new universe in a Big Bounce. Conversely, if the universe is insufficiently dense, k equals 0 or -1 and the universe will expand forever, cooling off, and eventually becoming inhospitable for all life, as the stars die and all matter coalesces into black holes (the Big Freeze and the heat death of the universe). As noted above, recent data suggest that the expansion speed of the universe is not decreasing as originally expected, but increasing; if this continues indefinitely, the universe will eventually rip itself to shreds (the Big Rip). Experimentally, the universe has an overall density that is very close to the critical value between re-collapse and eternal expansion; more careful astronomical observations are needed to resolve the question (Figure 1.6).
1.2.1 Conventional Stars
A star is a massive, luminous ball of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth.
For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen in its core, releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium were created by stars, either via stellar nucleosynthesis during their lifetimes or via supernova nucleosynthesis when stars explode. Astronomers can determine the mass, age, chemical composition, and many other properties of a star by observing its spectrum, luminosity, and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement, and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung–Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.
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