User:WillowW/Universe notes

The universe consists of three elements: space and time, collectively known as space-time; matter and various forms of energy; and the physical laws that govern the first two.

Nomenclature and etymologies

cosmos, everything to pan, apanta ta onta

discern cosmogony from cosmology

Size and composition

Physical laws

generally assumed to be uniform across space and time standard model quantum field theory; problem of renormalization general relativity physical constants conservation laws and their relation to symmetries (but not always!)

Forms of matter and energy

three generations of leptons and quarks gauge fields and their bosons

Space and time

interconvertible within limits, same nature c as a dimensional constant symmetries and the related conservation laws 3+1 dimensionality curvature unknown topology (flat need not = plane; could be cylinder or torus)

Observational data

Scientific experiments have yielded general observations on the universe.

The universe is very large and possibly infinite in volume. The observable matter is spread over a space at least 93 billion light years across; 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.

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", i.e., to cluster hierarchically; most atoms are condensed into stars, most stars into galaxies, most galaxies into galactic clusters, superclusters and, finally, large-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 2.725 Kelvin. The hypothesis, now apparently confirmed, that the large-scale universe is homogeneous and isotropic is known as the cosmological principle.

The present overall density of the universe is very low, roughly 9.9 × 10^-30 grams per cubic centimetre. 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 four cubic meters of volume.^[1] The nature of dark energy and dark matter are presently unknown.

The universe is old and evolving. Various data suggest that the universe is at least 10 billion years old; the best current estimate is 13.7±0.2 billion years old. The universe was not the same at all times; the relative populations of cosmological objects such as quasars and galaxies has changed and space itself appears to be expanding. This expansion accounts for how two galaxies can be 90 billion light years apart, even if they have traveled for only 13.7 billion years at speeds less than the speed of light; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; 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.

The relative fractions of different chemical elements — particularly the lightest atoms such as hydrogen, deuterium and helium — seem to be identical throughout the universe and throughout its observable history. The universe seems to have much more matter than antimatter, an asymmetry possibly related to the observations of CP violation. The universe appears to have no net electric charge, and therefore gravity appears to be the dominant interaction on cosmological length scales. The universe appears to have no net momentum and angular momentum. The absence of net charge and momentum would follow from accepted physical laws (Gauss's law and the non-divergence of the stress-energy-momentum pseudotensor, respectively), if the universe were finite.

Finally, the universe appears to have a smooth spacetime continuum consisting of three spatial dimensions and one temporal (time) dimension. The spacetime appears to have a simply connected topology, at least on the length-scale of the observable universe. However, present observations cannot exclude the possibilities that the universe has more dimensions and that its spacetime may have a multiply connected global topology, in analogy with the cylindrical or toroidal topologies of two-dimensional spaces.

The universe appears to be governed throughout by the same physical laws and physical constants. According to the prevailing Standard Model of physics, all matter is composed of three generations of leptons and quarks, both of which are fermions. These elementary particles interact via at most three fundamental interactions: the electroweak interaction which includes electromagnetism and the weak nuclear force; the strong nuclear force described by quantum chromodynamics; and gravity, which is best described at present by general relativity. The first two interactions can be described by renormalized quantum field theory, and are mediated by gauge bosons that correspond to a particular type of gauge symmetry. A renormalized quantum field thoery theory of general relativity has not yet been achieved, although various forms of string theory seem promising. The theory of special relativity is believed to hold throughout the universe, provided that the spatial and temporal length scales are sufficiently short; otherwise, the more general theory of general relativity must be applied. There is no explanation for the particular values that physical constants appear to have throughout our universe, such as Planck's constant h or the gravitational constant G, although Paul Dirac conjectured that they might be coupled to the size of the universe.

Classes of creation myths:

spoken command (Ptah, Yahweh)

fertility, seed (Kalevala, Hesiod)

world egg, cosmic egg, Hiranyagarbha, brahmanda

craftman (Marduk, Wotan)

others?

Historical development

causal models

materialists (sort of): Thales, Anaimander, Anaximenes; Parmenides, Pythagoras how to get something from nothing, and diversity from unity? spheres: Eudoxos (great name!), Kallippos, Aristotle, Ptolemy

heliocentricity: Copenicus, beautiful lamp quote removal of need for rotating sphere of stars; spread uniformly (Digges) Newtonian models problems with Newtonian models (energy conservation, Olbers paradox, gravitational instability, Jeans criterion)