Dr. N.L. Kachhara
The term universe has a variety of meanings based on the context in which it is described. In materialist philosophical terms, the universe is summation of all matter that exists and the space in which all events occurs, which an equivalent idea amongst some theoretical scientists, is known as total universe. In cosmological terms, the universe is thought to be finite or infinite space time continuum in which all matter and energy and the physical laws and constants that govern them exist. In a well defined, mathematical sense, the universe can even be said to contain that which does not exist: according to the path integral formulation of quantum mechanics, even unrealized possibilities contribute to the probability aptitudes of events in the universe. The terms known universe, observable universe, or visible universe are often used to describe the part of the universe that can be seen or otherwise observed by humanity. Due to the fact that cosmic inflation removes vast parts of the total universe from our observable horizon, most cosmologists currently accept that it is impossible to observe the whole continuum and may use our universe, referring only to that knowable by human beings in particular.
The Doppler shift measures the change in frequency (or sound of the pitch) as the source moves towards or away from the observer. A source of light that is approaching the viewer will be seen to the viewer to have a higher frequency than a source of light that is receding from the viewer. In 1929, observations from distant galaxies made by Edwin Hubble showed that light from those galaxies behaved (red shifted) as if they were going away from us. If all the distant galaxies are receding from us, on the average, that means that the universe as a whole could be expanding. Extrapolating this expansion back in time, one approaches a gravitational singularity, a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang theory, the dominant model in cosmology today. The age of the universe from this time of the Big Bang, was estimated to be about 13.7 billion years, with a margin of error of about one percent (+ 200 million years), according to NASA's WMAP (Wilkinson Microwave Anisotropy Probe). However, this is based on the assumption that the underlying model used for data analysis is correct. Other methods of estimating the age of the universe give different ages.
There is disagreement over whether the universe is indeed finite or infinite in spatial extent and volume. Many astronomers and cosmologists believe the universe is infinite due to recent findings in NASA's WMAP project supporting a flat (therefore infinite) universe. However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years distant. The present distance (commoving distance) to the edge of the observable universe is larger, due to the ever-increasing rate at which the universe has been expanding; it is estimated to be about 93 billion light years. This would make the commoving volume, of the known universe, equal to 1.9x1033 cubic light years (assuming this region is perfectly spherical). The observable universe contains about 7x1022 stars, organized in about 100/140 billion galaxies, which themselves form clusters and super clusters. The number of galaxies may be even larger, based on the Hubble Deep Field observed with the Hubble Space Telescope. The Hubble Space Telescope discovered galaxies, which are over 13 billion light years from Earth.
There are four types of red shifts Doppler red shift, Realistic Doppler, Cosmological red shift and Gravitational red shift. The consensus among astronomers is that the red shifts they observe are due to some combination of the three established forms of Doppler like red shifts. The most distant objects exhibit larger red shifts. For galaxies more distant than the Local Group and the nearby Virgo Cluster, but within a thousand mega parsecs or so, the red shift is approximately proportional to the galaxy's distance. This is known as Hubble's law. In the widely accepted cosmological model based on general relatively, red shift is mainly a result of the expansion of space: This means that the farther away a galaxy is from us, the more the space has expanded in time since the light left that galaxy, so the more the light has been stretched (that is photons emitted have been stretched to longer wavelengths and lower frequency during their journey) the more red shifted the light is, and so the faster it appears to be moving away from us.
Recent observations have suggested the expansion of the Universe is not slowing down, as expected from the first point, but accelerating. It is widely, though not quite universally, believed that this is because there is form of dark energy dominating the evolution of the universe. The universe consists mainly of matter, rather than antimatter. Only 4% of the matter and energy in the universe is luminous, that is directly observable from its emitted electromagnetic radiation ("light" in its most general sense); the remainder consists of dark energy (73%) and dark matter (23%). The nature and composition of dark energy and dark matter are not known. The luminous matter within the universe is sparse and consists principally of galaxies, which are distributed uniformly when averaged over length-scales longer than 300 million light years; on smaller length scales galaxies tend to clump into clusters, super clusters and even larger structures. The universe is bathed in a microwave radiation that is highly isotropic (uniform across different directions) and corresponds to a black body spectrum of roughly 2.7 Kelvin. The relative percentages of the lighter chemical elements especially hydrogen, deuterium and helium are apparently the same throughout the universe. The universe has at least three spatial dimensions and one temporal (time) dimension, although extremely small additional dimensions cannot be ruled out experimentally; space-time appears to be smoothly and simply connected, with very small curvature so that Euclidean geometry is accurate on the average throughout the universe. The universe appears to be governed by the same physical laws and constants throughout its extent and history.
According to Big Bang theories, every thing in the universe, all forms of matter and energy, and even space-time itself came into being at a single event, a gravitational singularity; as space expanded with time, the matter and energy cooled sufficiently to allow the stable condensation of elementary particles into primordial nuclei and atoms. Once atoms formed, matter became transparent to most wavelengths of electromagnetic radiation; the ambient microwave radiation observed today is the residual radiation that decoupled from the matter.
According to the prevailing scientific models, the Universe is governed by the Standard Model of physics (which governs various forms of matter and fields), as well as special and general relativity (which govern space-time and its interaction with matter and fields). The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological scales. The universe appears to have no net momentum and angular momentum. Hence the theory of general relativity (the most accurate description of gravity presently available) offers the best predictions for the overall development of the universe, including its origin, expansion (which mainly accounts for the observed red shift), large scale structures and ultimate fate, According to the theory of general relativity, some regions of space may never interact with ours even in lifetime of the universe, due to the finite speed of light and the expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the universe lives forever; space may expand faster than light can cover it. It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with then. Strictly speaking, the observable universe depends on the observer. By traveling, an observer can come into contact with a greater region of space time than an observer who remains still, so that the observable universe for the former is larger than for the latter; nevertheless, even the most rapid traveler may not be able to interact with all of space. Typically, the observable universe is taken to mean the universe observable from a stationary observer on Earth.
Despite its experimental verification, some scientists find the theory of general relativity implausible and have suggested alternatives. Such theories can only be considered scientific if they offer predictions that differ from those of general relativity. The main scientific alternative is Brans Dicke theory, which augments general relativity with a scalar field that determines the local value of the gravitational constant G. Other, more radical suggestions include the variable G cosmologies (in which the universe's physical constants vary with the age or size of the universe), the tired light hypothesis of Fritz Zwicky and the plasma cosmology theory. The validity of most such theories seems unlikely, given the available data.
There is some speculation that multiple universes in a higher-level multiverse (also known as a megaverse) exist, our universe being one of those universes. For example, matter that falls into a black hole in our universe could emerge as a Big Bang, starting another universe. However, all such ideas are currently not testable and cannot be regarded as anything more than speculation. The concept of parallel universes is understood only when related to string theory.
The obvious question that could be asked to challenge or define the boundaries between physics and metaphysics is: what came before the Big Bang? Physicists define the boundaries of physics by trying to describe them theoretically and then testing that description against observations. Our observed expanding universe is very well described by flat space, with critical density supplied mainly by dark matter and a cosmological constant that should expand forever.
It we follow this model backward in time to when the universe was very hot and dense, and dominated by radiation, and then we have to understand the particle physics that happens at such high densities of energy. The experimental understanding of particle physics starts to pop out after the energy scale of electroweak unification, the theoretical physicists have to reach for models of particle physics beyond the Standard Model, to Grand Unified Theories, supper symmetry, string theory and quantum cosmology.
Matter and radiation are gravitationally attractive, so in a maximally symmetric space-time filled with matter, the gravitational force will inevitably cause any lumpiness in the matter to grow and condense. That's how hydrogen gas turned into galaxies and stars. But vacuum energy comes with a high vacuum pressure, and that high vacuum pressure resists gravitational collapse as a kind of repulsive gravitational force. The pressure of the vacuum energy flattens out the lumpiness, and makes space get flatter, not lumpier, as it expands. So one possible solution to the flatness problem would be if our universe went through a phase where the only energy density present was a uniform vacuum energy. If this phase occurred before the radiation dominated era, then the universe could evolve to be extraordinarily flat when the radiation dominated era began, so extra ordinary flat that the lumpy evolution of the radiation and matter dominated periods would be consistent with the high degree of remaining flatness that is observed today. This type of solution to the flatness problem was proposed in the 1980s by cosmologist Alan Guth. This model is called the Inflationary universe. In the Inflation model, our universe starts out as a rapid expanding bubble of pure vacuum energy, with no matter or radiation. After a period of rapid expansion, or inflation, and rapid cooling, the potential energy in the vacuum is converted through particle physics processes into the kinetic energy of matter and radiation. The universe heats up again and we get the standard Big Bang. So an inflationary phase before the Big Bang could explain how the Big Bang started with such extraordinary spatial flatness that it is still so close to being flat today. The inflationary model also solves the horizon problem and magnetic monopole problem.
The Inflation model described above is far from an ideal theory. It's too hard to stop the inflationary phase, many of the assumptions that go into the model, such as an initial high temperature phase and a single inflating bubble have been questioned and alternative models have been developed. Today's inflation models have evolved beyond the original assumption of single inflation event giving birth to a single universe, and feature scenarios where universes nucleate and inflate out of other universe in the process called eternal inflation. There is also another attempt to solve the problems of Big Bang cosmology using a scalar field that never goes through an inflationary period at all, but evolves so slowly so that we observe it as being constant during our own era. This model is called Quintessence, after, the ancient spiritual belief in the Quinta Essentia, the spiritual matter from which the four forms of physical matter are made.
The inflation model assumes a quantum vacuum that has more energy in its nothingness than it should. Modern physical theory, specifically quantum electrodynamics, tells us that the vacuum can no longer be considered a void. This is due to the fact that, even in the absence of matter, the vacuum is neither truly particle nor field free, but is the seat of virtual particle pair (e.g. electron-positron) creation and annihilation process, as well as zero- point fluctuation (ZPF) of such fields as the vacuum electromagnetic field. The energy density associated with the vacuum electromagnetic ZPF background is considered to be infinite. Thus we see that, with its roots in relativity theory which banished the ether, QED has in some sense come full circle to provide us with a model of an energetic vacuum that once again constitutes a plenum rather than a void. The question is where the zero-point energy comes from, specifically the vacuum electromagnetic zeropoint energy. The possibility that this is due to generation by the motion of charged particles that constitutes the matter has been investigated with positive results. The picture that emerges is that the electromagnetic ZPF spectrum is generated by the motion of charged particles throughout the universe which are themselves undergoing ZPF induced motion, in a kind of self-generating grand ground state of the universe. In contrast to other particle field interactions, the ZPF interaction constitutes an underlying, stable 'bottom rung' vacuum state that decays no further but reproduces itself on a dynamic generation basis. In such terms it is possible to explicate on a rational basis the observed presence of vacuum zero-point energy.
Attempts are being made to extract the vacuum energy. Some countries, including USA and Soviet Union, have undertaken programs to explore this on an experimental basis. Noble Laureate T.D. Lee has suggested a new branch of study known as 'vacuum engineering'.
Lastly, we briefly mention about Newtonian cosmology. The Newtonian cosmology had several paradoxes that were resolved only with the development of general relativity. The first of these was that it assumed that space and time were infinite, and that the stars in the universe had existed for an infinite time; however, since stars are constantly radiating energy, a finite star seems inconsistent with the radiation of infinite energy. Secondly, Jean Philippe de Cheseaux noted that the assumption of an infinite space filled uniformly with stars lead to prediction that the nighttime sky would be as bright as the Sun itself; this became known as Olber's paradox in the 19th century. Third, Newton himself showed that an infinite space uniformly filled with matter would cause infinite forces and instabilities causing the matter to crushed inwards under its on gravity. This instability was clarified by the Jeans instability criterion. One solution to these latter two paradoxes is the Charlier universe, in which the matter is arranged hierarchically (systems of orbiting bodies that are themselves orbiting in a larger system, ad infinitum) in a fractal way such that the universe has a negligible small overall density.
