Cosmogony is the scientific discipline that explores the formation of the and the celestial bodies. There is agreement within that the came into being about 13-14 billion years ago with the so-called . The nature of this event is beyond any physical interpretation due to the presumably immense temperatures close to the , canceling any contemporary physical theory.

In the course of a symmetry break immediately after the big bang (BB) a very short-time explosive cosmic expansion occurred. One assumes this epoch of to last for about 10-35 to 10-33 seconds after the BB. During this short phase the universe was undergoing expansion by a factor of 1030 to 1050; the amount posited depends on the mathematical description.

Inflation is of fundamental meaning for the formation of cosmological structure. Prior to inflation, the universe as a whole was subject to the laws of quantum mechanics. In particular these say that some uncertainty underlies the posi­tion and momentum of a particle; this must not be understood as a limitation of our perception and information, but rather as a natural, inherent property of and energy. As a consequence, the quantum fields during pre-inflation cannot be distributed over space in a perfectly homogenous manner. The very early universe is, rather, infused with quantum fluctuations. While the universe expands exponentially during inflation, these quantum fluctuations are as well “inflated” to macroscopic size. The density variations arising in this way constitute the sprouts for any formations of large cosmological structures.

After the end of inflation the “normal” cosmic expansion begins, described by the general theory of relativity. For the first 10,000 years in the life of the universe, radiation energy density domi­nates over matter. This prevents any early increase in perturbations in the density of matter. The situ­ation changes when matter gets the upper hand in the cosmic energy budget after some 10,000 years; the universe changes from being radiation domi­nated to being matter dominated. The reason is that radiation loses its energy more rapidly than matter due to the additional effect of redshift in an expanding space. From this moment of matter­radiation-equality on, matter density fluctuations are able to self-gravitationally amplify.

This effect of self-amplification concerns only so-called at first. This is some form of matter, the nature of which remains unknown at the time of this writing, that interacts only via the gravitational (and maybe the weak) force—this is also the reason for being hard to detect. Numerous observations show, however, that provides more than 80% of the substantial content of the universe. Only 15%-20% consists of “normal,” so-called baryonic matter that makes up all interstellar gas and dust, the stars and planets and ourselves. Whereas dark matter can now obey the process of self-gravita­tion to develop local density peaks unhindered, baryonic matter is still prevented from doing so by the influence of the ubiquitous high-energy back­ground radiation; the temperature in the universe is still high enough to keep all baryonic matter in the state of a plasma, a hot gaseous mixture of negatively charged electrons and positive atomic nuclei. The high-energy background photons thus permanently interplay with the charge carriers. Any attempt of the baryons to accumulate within some region would be scotched by the effect of radiation pressure.

Approximately 380,000 years after the BB, the global temperature falls enough to allow for stable combinations of electrons and atomic nuclei: Neutral atoms can form. Because electromagnetic radiation is not coupling to neutral particles, the large-scale streaming of baryonic matter is no longer influenced by the background radiation. The baryons are now able to stream freely into regions of high-density dark matter established soon after the BB. In the course of time these initially moderate high-density regions gain mass via matter accretion until they start to decouple from the general flow of cosmic expansion due to their self-gravity and finally constitute inde­pendent, isolated objects, so-called dark matter halos. Baryonic matter also becomes denser and denser within the dark halos until, again, its self-gravity is powerful enough to overwhelm its intrinsic thermal pressure and thus causes the gas cloud to collapse. This, just like the dark matter decoupling from the cosmic flow, as mentioned above, is called a nonlin­ear phase of structure growth. During the collapse, shock waves arise and heat the gas to millions of degrees Kelvin. Unlike dark matter, gas can cool via several atomic processes. Thus it can further contract around the dark halo center until it becomes a rota­tion-stabilized flat disk. Local condensations occur within this gas disk, forming molecular clouds in which stars can finally emerge.

It is known from observations as well as numer­ical simulations that there are fragmentation pro­cesses occurring inside molecular clouds, producing even higher-density peaks of small local extension. These globules are the cradles of the stars. Stars form in groups rather than singly. The progres­sively rising power of self-gravitation leads to a further compression of the globules, while the tem­perature increases due to the growing pressure. The collapse ends as soon as the thermal pressure becomes as strong as the gravitational force. When the inner temperature reaches some 15 million degrees Kelvin, the fusion of hydrogen can start to produce helium nuclei. This will provide for the energy production of the star over the largest part of its lifetime. Prestellar objects with less than 8% of the solar mass (M® = 1.99 x 1030 kg) are not able to generate temperatures high enough to enable the hydrogen fusion process. These objects, much smaller than normal stars, are called brown dwarfs. On the other hand, stellar objects with more than 60 M® presumably cannot form because of strong winds being generated before the onset of the nuclear burning and blowing away a consid­erable fraction of the prestellar material.

During the star formation process, angular momentum conservation causes a gas disk, within which planets can form, to form around the pre- stellar object. Different from stars, planets form in a growth process, starting from microscopically small dust particles that stick together after encoun­tering each other and thus form continuously growing clumps. The planetesimals that build up in this way collect material by gravitation along their orbit and thus accelerate their further growth. In this manner, planets half the size of the earth can form within only some 100,000 years. Such objects, thousands of kilometers in size, can suc­cessively build up huge planets with up to 10 Earth masses. Giants like these are additionally able to accrete gas from the surrounding disk, which can finally lead to gas giants like Jupiter and Saturn.

See also Aquinas, Saint Thomas; Aristotle; Augustine of Hippo, Saint; Black Holes; Bruno, Giordano; Copernicus, Nicolaus; Cosmology, Inflationary; Demiurge; Einstein, Albert; Galilei, Galileo; Hawking, Stephen; Lucretius; Newton, Isaac; Nicholas of Cusa (Cusanus); Plato; Presocratic Age; Singularities; Stars, Evolution of; Time, Emergence of; Time, Galactic; Time, Sidereal; Universes, Baby

Further Readings

Carroll, B. W., & Ostlie, D. A. (2006). An introduction to modern astrophysics. San Francisco: Pearson, Addison-Wesley.

Coles, P. (2001). Cosmology: A very short introduction. Oxford, UK: Oxford University Press

Longair, M. S. (1998). Galaxy formation. Berlin, Heidelberg, New York: Springer.

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Nicolaus Copernicus

Nicolaus Copernicus

Cosmological Arguments

Cosmological Arguments