The Universe is huge and possibly infinite in volume. The matter which can be seen is spread over a space at least 93 billion light years across. Typical galaxies range from dwarf galaxies with as few as ten million  10 7 stars up to giants with one trillion  10 12 stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one sextillion 10 21 stars in the observable universe; though a study by Australian National University astronomers resulted in a figure of 70 sextillion 7 x 10 The matter that can be seen is spread throughout the universe, when averaged over distances longer than million light-years.
The present overall density of the Universe is very low, roughly 9. The density of atoms is about a single hydrogen atom for every four cubic meters of volume. Dark matter slows the expansion of the Universe. Dark energy makes its expansion faster. The Universe is old, and changing. The best good guess of the Universe's age is The universe has not been the same at all times in its history. This getting bigger accounts for how Earth-bound people can see 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 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 other data. Gravity is the dominant interaction at cosmological distances. The Universe also seems to have no net momentum or angular momentum.
The absence of net charge and momentum is expected if the universe is finite. The Universe appears to have a smooth space-time continuum made of three spatial dimensions and one temporal time dimension. On the average, space is very nearly flat close to zero curvature , meaning that Euclidean geometry is experimentally true with high accuracy throughout most of the Universe. The Universe has the same physical laws and physical constants throughout.
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. Special relativity holds in all the universe in local space and time. Otherwise, general relativity holds. 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.
Several conservation laws have been identified, such as the conservation of charge , conservation of momentum , conservation of angular momentum and conservation of energy. 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, which has passed all experimental tests so far. However, since rigorous experiments have not been carried out on cosmological length scales, general relativity could conceivably be inaccurate.
Nevertheless, its predictions appear to be consistent with observations, so there is no reason to adopt another theory. General relativity provides of a set of ten nonlinear partial differential equations for the spacetime metric Einstein's field equations that must be solved from the distribution of mass-energy and momentum throughout the universe.
Since these are unknown in exact detail, cosmological models have been based on the cosmological principle , which states that the universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the universe are equivalent to those of a fine dust distributed uniformly throughout the universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the universe on cosmological time scales.
The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space ; as the space itself expands, the wavelength of a photon traveling through space likewise increases, decreasing its energy.
The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental physical cosmology. Other experimental observations can be explained by combining the overall expansion of space with nuclear physics and atomic physics.
As the universe expands, the energy density of the electromagnetic radiation decreases more quickly than does that of matter , since the energy of a photon decreases with its wavelength. Thus, although the energy density of the universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was light. As the universe expanded, its energy density decreased and it became cooler; as it did so, the elementary particles of matter could associate stably into ever larger combinations.
Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. At this stage, the matter in the universe was mainly a hot, dense plasma of negative electrons , neutral neutrinos and positive nuclei. Nuclear reactions among the nuclei led to the present abundances of the lighter nuclei, particularly hydrogen , deuterium , and helium.
Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.
Other observations are not clearly answered by known physics. According to the prevailing theory, a slight imbalance of matter over antimatter was present in the universe's creation, or developed very shortly thereafter. Although the matter and antimatter mostly annihilated one another, producing photons , a small residue of matter survived, giving the present matter-dominated universe.
They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the universe; by contrast, dark energy serves to accelerate the universe's expansion. Some people think that there is more than one Universe. They think that there is a set of universes called the multiverse. By definition, there is no way for anything in one universe to affect something in another. The multiverse is not yet a scientific idea because there is no way to test it.
An idea that cannot be tested is not science. The future of the universe is a mystery.
However, there has a couple of theories based on the possible shapes of the universe: . From Wikipedia, the free encyclopedia. Nature timeline. Matter-dominated era. Single-celled life. Multicellular life. Dark Ages.
Earliest stars. Earliest galaxy. Omega Centauri.
- The Solar System!
- Essay on Our Universe: Definition, Stars and Solar System.
- Our Universe for Kids - Universe is everything!?
- The Milky Way.
- Essay on Our Universe: Definition, Stars and Solar System!
Andromeda Galaxy. Milky Way spirals. Alpha Centauri. Earliest life. Earliest oxygen. Atmospheric oxygen. Sexual reproduction. Earliest plants.
Stars—facts and information
Cambrian explosion. Earliest mammals. Earliest apes. See also: Human timeline and Life timeline.
The Origin of the Universe
See also: Reality and Physics. Main article: General relativity. Main articles: Big Bang and Nucleosynthesis. Main article: Multiverse. The New York Times. Retrieved Quantum physics and path integrals.
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- Essay: Will Dark Energy Please Come to Light? | AMNH!
New York: McGraw—Hill. Zinn Justin J Path integrals in quantum mechanics. Oxford University Press. The Oxford companion to cosmology.