Universe (from Latin universus "whole"; derived from unus and versus "turned into one"), also called cosmos, world, and outer space: a) the totality of all things, b) the space in which all material things exist.
►Astronomy is the scientific discipline that explores and describes the universe. What is beyond the universe — indeed, whether there is anything "beyond" it — cannot be decided by empirical observation alone. We can, however, empirically investigate how the universe has developed and how it will develop in the future. We obtain relevant information by observing the heavenly bodies and the cosmic ►background radiation, microwave radiation caused by the ►big bang that evenly fills the entire universe. Our observations indicate the following:
►The universe expands at an ever-increasing speed. The redshift of starlight can only be explained by the hypothesis that the distance between all galaxies constantly increases. This does not occur due to the galaxies' drifting apart since the time of the big bang, as we initially believed. Its explanation is, rather, the so-called ►dark energy — an item in the ►distance formula whose physical nature still has not been clarified — that spreads the spatial structure itself apart, and does at increasing speed.
►The universe is 13.7 billion years old. We obtain this numerical value by at least three different measurement methods. If we calculate back in time from the current redshift of distant galaxies, we reach a point in time at which the universe's density must have been at its highest point. Shortly after that, the first heavy elements must have evolved, elements whose ratio of isotopes has since undergone changes due to radioactive decay and which can also assist us in determining the age of the universe. The current temperature of the background radiation also fits into this picture: The ►big bang, the birth of the universe, took place approximately 13.7 billion years ago.*
►The universe is homogeneous and isotropic. This is a sophisticated way of saying that all places and directions look more or less the same. The Earth does not have any special status. At least, this is indicated by all observations of the distribution of matter in the universe. Doubling the distance resolution of a telescope lets you see eight times more galaxies — just what would be expected if the galaxies are evenly distributed in space. Even the cosmic ►background radiation is extremely uniform. This uniformity can only be explained by an extremely fast expansion of space — the so-called inflation — that must have occurred shortly after the ►big bang.
►The universe largely consists of unknown and invisible matter. Normal matter in solar systems, cosmic dust, and interstellar gas together contribute only 4% to the mass of the universe. More than a quarter of the universe consists of a still-mysterious ►dark matter. This dark matter is noticeable only because of its gravity; it was discovered by measuring the rotational speeds of stars in rotating galaxies. Stars rotate much faster around their galactic centers than would be possible on the basis of the galaxies' normal matter alone.
►The universe is probably infinitely large. Some popular scientific books give either 13.7 or 46 giga-lightyears as the universe's radius. Both numbers are nonsense. The first of them stems from mistaking the age of the universe for its size; the second stems from mistaking the observable part of the universe for the universe in its entirety.
Only one thing is certain: The universe will cease to exist at some point.
Age and End of the Universe
Don't worry, the universe is still in its youth. Nevertheless, this youth, like any other, will eventually give way to different, less vital ages. According to what we know today, the history of the universe will proceed as follows:
► Stellar Age. This is the age in which we are today. The universe is filled with hydrogen gas stemming from the ►big bang.
The gas constantly produces new stars. A star's lifespan can be up to 100 billion years if the star is light enough; however, most stars, including our sun, are heavier than that and will therefore end sooner. In five billion years our sun will have used up all of its hydrogen and will swell up as a red giant star. Among other things, this will burn up the Earth.
► Age of the Black Holes. The last remaining objects in the universe will be the ►black holes that evolved out of massive stars. Even they are subject to decay, however; due to the Hawking radiation, they constantly shed mass. The more massive a black hole is, the longer this takes. Still, in 10100 years even the largest and last black hole will be gone.
► Obliteration of the Past. After the black holes disappear there will be nothing left in the universe but radiation particles from the Hawking radiation and a thin, cold neutrino gas. The temperature further declines while the universe keeps expanding. The gas is completely homogeneous everywhere. There are no traces left of any of the things that once existed in the universe. The ►entropy of the universe has reached its maximum value. The past has been forever obliterated.
► The End of Time. Eventually the universe has expanded to such an extent that all particles are separated from one another by ►event horizons. All interaction has ceased. Each particle has reached the lowest energy value covered by quantum theory. From now on, nothing happens anymore. It is no longer possible to distinguish one moment from another. The past having long since been erased, there is now no longer any future either; only the present remains. The end of ►time has come.
Read more under: ►Big Bang
* Some textbooks cite still other values for the age of the universe, such as 14.5 billion years. This is not a "popular mistake" but simply a result of different bases for calculation. The numerical value of 13.7 billion years is based on the background radiation's redshift and on distant supernovae. The value of 14.5 billion years, by contrast, is based on the mass ratio of uranium isotopes, although with an error margin of approximately 15%. Once a survey of the background radiation has been performed with the help of the European space probe Planck, which is scheduled to be launched in Spring 2009, we will hopefully know more about the age of the universe.
** To be more precise, a so-called Robertson-Walker metric without curvature. The metric is a measure for ►distances in space and can be derived from the field equations of ►relativity theory. It describes the spatial and temporal structure of the universe. Formulated in 1935 and 1936 by the two physicists mentioned in its name, this metric describes a non-rotating universe.
*** In online forums you will frequently encounter the claim that the universe must be finite if it is expanding. The argument given for this is that if something is already infinitely large, there could be no further space into which it could expand. However, the universe's expansion is entirely compatible with its infinitude. Imagine space were completely filled up with infinitely many small rubber cubes, all of which expand uniformly. Then space itself will expand, too, even though it is already infinitely large.
**** In April 2007 the US physicists Lawrence Krauss and Robert Scherrer published a relevant calculation that takes the impact of ►dark energy on the expansion of the future universe into account.
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