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 increase of all distances does not mean that all atoms, planets, suns, and galaxies are expanding equally. We would not even be able to detect such an expansion, since with it all standards of measurement would expand as well. (Such an effort would be like trying to determine if everything is expanding by repeatedly measuring things with a ruler; the fact that a given item consistently measures at exactly five inches by the ruler would not count for much, since if everything is expanding, the ruler is, too.) However, the sizes of ►atoms and their distances from one another are fixed by natural constants. Consequently, they themselves do not grow with the universe and so cannot cite dark energy as an excuse for their increasing abdominal girth. Galaxies, moreover, are held together by gravity and therefore resist expansion. With galaxy clusters, however, it is a different matter; being bound by gravity only to a weak extent, they are indeed undergoing the expansion effect even now. The expansion leads to a constant reduction in the density of matter in the universe. At present the universe contains, on average, one atom per cubic meter.

►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 even more mysterious dark energy was postulated to explain the Euclidian geometry of the universe and at the same time its accelerated expansion. This dark energy makes up almost three quarters — approximately 73% — of the universe. Unlike dark matter, dark energy has negative gravity. Alternatively, it is also conceivable that for great distances a still unknown law of nature turns gravity into a negative force

The universe is discrete. By this we do not refer to its commitment to secrecy, but rather to the non-continuity of particle properties such as energy and rotational momentum. Quantum theory tells us that these properties do not take on random values, but change their values only in discrete steps. That is why the number of possible ways a given region of the universe may look and develop, though large, is not infinite. The number of possibilities depends on the diameter and temperature of the region in question. Within the observable part of the universe, there are about 2 to the power of 10115 different possible states for a maximum temperature of 108 degrees Celsius. For an infinite — or at least sufficiently large — universe, this strongly indicates the existence of parallel worlds.

The universe is flat. Space is not curved, but conforms to Euclidian geometry:** Parallels intersect only in infinity. The latter can be derived from periodic irregularities — the so-called multipole moments — in the background radiation, which were caused by vibrations in the ylem, the postulated "original substance", shortly after the big bang. The gradient below represents the radiation's multipole moment as measured by different methods. Within the precision of measurement, the location of the first maximum corresponds to a flat universe.

Multipole moment of vibrations in the ylem:
Proof of the infinitude of the universe (Spektrum 11/08)

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.
      The radius of the universe would amount to 13.7 giga-lightyears if it had begun as a single point and then constantly expanded at its margins at the speed of light. Neither, however, is the case: the universe did not begin as a single point, nor is the expansion of space restricted by the speed of light. 46 giga-lightyears, meanwhile, is merely the radius of the Hubble volume, the part of the universe that is potentially observable. This can be ascertained from the background radiation's ►redshift. The part of the universe that is not observable for us in principle is beyond the Hubble volume.
     If the universe is flat, unbounded, and unconnected to itself (in a way to be explained below), it is infinitely large.*** Currently all observations point to this. However, at the current state of our knowledge we cannot yet rule out two possible alternatives. The first is that the universe may have an extremely weak curvature — so weak that it is not noticeable within the observable part of our universe. In this case the universe, though extremely large, would not be infinite.
    The second possible alternative rests on the assumption that the universe has a flat, cyclical topology roughly like a donut (mathematically, a torus). A torus surface is geometrically flat — parallels in it do not intersect — and yet connected to itself on all sides. It is a bit like the computer game "Asteroids", in which a spacecraft leaving the screen on one side enters again from the opposite side. If the universe were indeed connected to itself like this, and smaller than a Hubble volume, we would see the same heavenly objects as "mirror images" when looking in different directions. However, we would see them from different angles and, due to the finite speed of light, at different stages of their development. Therefore, we would not immediately realize that they are the same objects.
      Astronomers have indeed searched the sky in various directions for such "mirror galaxies" in recent years, but have not yet found any. That is why the currently predominant hypothesis is that the universe is disconnected from itself. Still, we cannot entirely rule out the opposite possibility. At least in a temporal sense the universe is not cyclical either, for the negative gravity stemming from dark energy makes space constantly drift apart. But we still do not know enough about this energy, so that any claims about the future development of the universe should be taken with a grain of salt.

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.
    Dark energy makes the universe drift apart at an ever faster pace so that an increasing number of galaxies will disappear from our Hubble volume. In 10 trillion years only stars in our local galaxy group will be visible in the sky from our viewpoint.**** That does not need to be of too much concern to us, though, for in about 100 trillion years all of the hydrogen in the universe will be used up anyway. The last vestiges of starlight quietly flicker out; an absolute, unrelieved darkness sets in.

Age of the Dark Stars. The remnants of extinct stars — neutron stars and dwarf stars — gradually adopt the temperature of the background radiation, which by that point has itself cooled down to a fraction of a degree above absolute zero. In the long run, galaxies dissolve into single dark stars and black holes floating unhindered through the unilluminated universe.

End of Matter. Atomic processes gradually turn the extinct remnants of stars and planets into iron, the longest-lasting element. But even iron does not last forever. Due to proton decay, the iron atoms gradually dissolve into individual elementary particles. By 1037 years, matter as we know it will have ceased to exist.****

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.

Links Related to the Topic

■ The almost infinite universe. Boundary issues of cosmology.


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