
[[Image:Universe.jpg|thumb|right|The deepest visible-light image of the cosmos. Hubble Ultra Deep Field. Image Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF team.]] In the first half of the 20th century, the word ''universe'' was used to mean the whole spacetime continuum in which we exist, together with all the energy and matter within it. Attempts to understand the universe in this sense, on the largest possible scales, are made in cosmology, a science that has grown from physics and astronomy. During the second half of the 20th century, the development of observational cosmology, also called physical cosmology, led to a split in the meaning of the word ''universe'', between observational cosmologists and theoretical cosmology; where the former (usually) abandon the hope of observing the whole spacetime continuum, the latter retain this hope, attempting to find the most reasonable speculations for modeling the whole of spacetime, despite the extreme difficulty in imagining any empirical constraints on these speculations and the risk of declining into metaphysics. The terms known universe, observable universe, or visible universe are often used to describe the part of the Universe that we can see or otherwise observe. Those who believe it is impossible to observe the whole continuum may use our universe, referring only to that knowable by human beings in particular. == Expansion and age, and the Big Bang theory== The most important result of cosmology, that the Universe is expanding, is derived from redshift observations and quantified by Hubble's Law. Extrapolating this expansion back in time, one approaches a gravitational singularity, a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang theory, the dominant model in cosmology today. The age of the Universe was estimated to be about 13.7 1 E9 () years, with a margin of error of about 1 percent (± 200 million years), according to NASA's Wilkinson Microwave Anisotropy Probe (WMAP). However, this is based on the assumption that the underlying model used for data analysis is correct. Other methods of estimating the age of the universe give different ages. A fundamental aspect of the Big Bang can be seen today in the observation that the farther away from us galaxy are, the faster they move away from us. It can also be seen in the cosmic microwave background radiation which is the much-attenuated radiation that originated soon after the Big Bang. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an initial period of rapid Cosmic inflation following the Big Bang. === Size of Universe and observable universe === There is disagreement over whether the Universe is finite or infinity in spatial extent and volume. However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years distant. The ''present distance'' (comoving distance) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 78 billion light years (km). This would make the comoving volume, of the known universe, equal to cubic light years (assuming this region is perfectly sphere). The observable universe contains about stars, organized in about 10 billion galaxy, which themselves form groups and clusters of galaxies and superclusters. The number of galaxies may be even larger, based on the Hubble Deep Field observed with the Hubble Space Telescope. Both popular and professional research articles in cosmology often use the term "Universe" when they really mean "observable universe". This is because unobservable physical phenomena are scientifically irrelevant; that is, they cannot affect any events that we can perceive, and therefore, it is argued, effectively do not exist. See also Causality (physics). We live in the centre of the universe that we observe, in apparent contradiction to the Copernican principle which says that the Universe is more or less uniform and it has no distinguished centre. This is simply because light does not travel infinitely fast, and we make observations of the past. As we look further and further away, we see things from epochs (times) closer and closer to the limit of time=zero of the Big bang model. And since light travels at the same speed in any direction towards us, we live at the centre of our observable universe. == Shape of the Universe == An important open question of cosmology is the shape of the universe. Firstly, whether or not the Universe is ''flat'', i.e. whether the rules of Euclidean geometry are valid on the largest scales, is unknown. Currently, most cosmologists believe that the observable universe is (nearly) flat, with local wrinkles where massive objects distort spacetime, just as a lake is (nearly) flat. This opinion was strengthened by the latest data from WMAP, looking at "acoustic oscillations" in the cosmic background radiation temperature variations. Secondly, whether or not the Universe is multiply connected, is unknown. The Universe has no spatial boundary according to the standard Big bang model, but nevertheless may be spatially finite. This can be understood using a two-dimensional analogy: the surface of a sphere has no edge, but nonetheless has a finite area (). It is a two-dimensional surface with constant curvature in a third dimension. A three-dimensional equivalent is the unbounded "spherical space" discovered by Georg Friedrich Bernhard Riemann, which has a finite volume (). In it, all three dimensions are constantly curved in a fourth. (Other possibilities include a similar "elliptical space", and a "cylindrical space", where, in conflict with ordinary geometry, the two ends of the cylinder are joined together, but without bending the cylinder. These, also, are two-dimensional spaces with finite areas; innumerable others exist. However, the sphere has the unique and, perhaps, more aesthetically pleasing property that all points on it are geometrically similar.) If the universe is indeed unbounded yet spatially finite, as described, then traveling in a "straight" line, in any given direction, would theoretically cause one to eventually arrive back at the starting point after traveling a distance equal to the "circumference" of the universe (which is impossible to our current understanding of the Universe, as its size is much greater than the size of the observable universe). Strictly speaking, we should call the stars and galaxies "views" of stars and galaxies, since it is possible that the Universe is multiply-connected and sufficiently small (and of an appropriate, perhaps complex, shape) that we can see once or several times around it in various, and perhaps all, directions. (Think of a house of mirrors.) If so, the actual number of physically distinct stars and galaxies would be smaller than currently accounted. Although this possibility has not been ruled out, the results of latest cosmic microwave background (CMB) research make this very unlikely. == Fate of the Universe == Depending on the average density of matter and energy in the Universe, it will either keep on expanding forever or it will be gravitationally slowed and will eventually collapse back on itself in a "Big Crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for the whole of eternity (see accelerating universe). For a more detailed discussion of other theories, see the ultimate fate of the Universe. == Multiverse == There is some speculation that multiple universes exist in a higher-level multiverse (also known as a megaverse), our Universe being one of those universes (lower case). For example, matter that falls into a black hole in our Universe could emerge as a "Big Bang," starting another universe. However, all such ideas are currently untestable and cannot be regarded as anything more than speculation. == Other terms == Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world". Although words like world and its equivalents in other languages now almost always refer to the planet Earth, they previously referred to everything that exists—see Copernicus, for example—and still sometimes do (as in "the whole wide world"). ==See also== *Cosmos *Cosmic latte - the color of the Universe *Parallel universe == References == * Albert Einstein (1952). ''Relativity: The Special and the General Theory (Fifteenth Edition)''. ISBN 0-517-88441-0 == External links == * Richard Powell: ''An Atlas of the Universe'', http://www.anzwers.org/free/universe/index.html. A series of images at various scales, with explanations. * [http://www.space.com/scienceastronomy/age_universe_030103.html Age of the Universe at Space.Com] * [http://slate.msn.com/id/2087206/ My So-Called Universe] by Jim Holt, on various arguments for and against an infinite Universe and parallel universes. * [http://www.hep.upenn.edu/~max/multiverse1.html Parallel Universes] by Max Tegmark. * [http://www.hedweb.com/nihilism/nihilfil.htm Why Does Anything Exist?] Large-scale structure of the cosmos ms:alam_Semesta lv:Visums simple:Universe
==Contradiction== I removed the paragraph But some of the objects outside of the observable universe can, in principle, be observed indirectly. For example, it is theoretically possible to meet an observer located near the end of ''our'' observable universe, who in his past has observed some galaxies that left our observable universe because of expansion. I'm a mathematician not a physicist but this seems to contradict the paragraph above about Causality. ==Meaning of ''Universe''== ''The Universe is the whole spacetime continuum in which we find ourselves, together with all the matter and energy within it.'' Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world". For a large fraction of the twentieth century, the word Universe, with an upper case "U", was used to mean the whole spacetime continuum in which we find ourselves, together with all the matter and energy within it. However, since the standard Big bang model has become well established observationally during the last few decades of the twentieth century, theoretical cosmologists have come up with new ideas of "the whole spacetime continuum" which are much, much larger than the "Universe" corresponding to the Big bang model. For this reason, the word universe can now be used in the plural and with a lower case "u" when discussing theories about all of space-time. There is no clear consensus on what new word to use for the whole spacetime continuum (though some like the term multiverse), and as long as there is no conceivable method of measuring anything beyond the observable horizon (formally speaking, the particle horizon), this is a seen as a moot point (irrelevant) for empirical scientists, and is of interest only to philosophy. ==infinite universe?== ''It is not known whether the Universe is finite or infinite in spatial extent and volume, although current theories favor an infinite Universe.'' although the majority of theorists presently favor an infinite Universe. :Rubbish. The majority of theorists currently favour an infinite Universe, but this is a matter of personal taste and sociology and historical fashion and nothing to do with physics. ::I certainly also sense theorists favoring an infinite Universe. I disagree that it is a matter of taste. :: http://www.sciam.com/article.cfm?articleID=000F1EDD-B48A-1E90-8EA5809EC5880000 "Parallel Universes" Not just a staple of science fiction, other universes are a direct implication of cosmological observations. By Max Tegmark. :: User:Nealmcb 17:46, 2004 May 4 (UTC) ==size of universe== ''48 billion (48 ? 109) light years.'' 50x10^9 light-yr = 5x 10^10 *0.3 pc = 1.7x 10^10 pc = 17 Gpc = 12 /h Gpc (where h=0.7 is the Hubble constant) OK :), this is approximately correct for Omega_m=0.3, Omega_Lambda =0.7, though 10/h Gpc is closer. In any case, i'll round to 50 since it's no more than 10% precise. I'm glad the way this is improving - and i love the desire to avoid ambiguity. Wikipedia is definitely a good tool for spreading relatively wellunderstood information to outsiders without expecting them to waste hours and hours to sort through ambiguous jargon. --boud ---- I see various estimates of the number of particles in the observable universe, e.g. the claim in Wikipedia talk:Size comparisons that ''It is accepted by astrophysicists that the number of particles in the observable universe? is currently in the 1085 range''. This seems like the place to document that. Does anyone have some good references? User:Nealmcb 18:27, 2004 May 4 (UTC) ---------- "...it is estimated to be about 78 billion light years (7.4 × 1023 km)." Interesting....just based on what did this 'estimated value' came from? At least a note should be provided for this kind of 'data'. User:LegolasGreenleaf 11:26, Nov 7, 2004 (UTC) ----- "...billion ..." What kind of billion?, I meen, 10^9 or 10^12? I know that wikipedia uses 10^9 "if don't state otherwise" but I think that talking about science and to avoid ambiguity, is better 10^9 or the prefixes "giga" or "tera" [http://en.wikipedia.org/wiki/Long_scale#Alternative_approaches see wikipedia, long scale alternative approaches] ---- ==Seeing around the universe== For the time being, I am removing the following statement from the article, because I don't see how it can be true: ''"Therefore, strictly speaking, we should call the stars and galaxies "images" of stars and galaxies, since it is possible that the Universe is finite and so small that we can see once or several times around it, and the real number of physically distinct stars and galaxies could be a little smaller. There are observations underway to determine whether this is true."'' If the universe were so small that one could see even once around it, wouldn't the night (and day) sky be completely bright? If the universe were unbounded yet sufficiently finite, wouldn't the "image" of the sun exist in every direction that one looked?User:Johnstone 13:31, 8 May 2004 (UTC) :I'd put it back in. You bring up Olbers'_paradox. See that article for reasons why the sky isn't all bright. But that doesn't depend on whether the universe is bent or flat. It's more about whether you can see an infinite distance. User:Nealmcb 16:16, 2004 May 8 (UTC) ::I was aware of that paradox, though I didn't know it had a name. It was in the back of my mind when I decided to question the material I removed. As you say, that paradox is not about the idea of seeing "around" the universe, so it does not necessarily relate to my question. However, I was thinking of the universe as a "spherical space"; I have re-read the sentences in question, and now I realize that my objections do not apply for most non-spherical geometries. I've tweaked a few words to (hopefully) clarify their meaning. User:Johnstone 22:51, 10 May 2004 (UTC) ==universe (fiction)== It might be nice to have an article on "Universe (fiction)" or something -- the sense of the word where somebody says "Many of the Marvel Comics series take place in the same universe" or "The SERRAted Edge novels are set in the same universe as the Bedlam Bards novels". User:Cwitty http://www.cs.appstate.edu/~sjg/class/1010/wc/geom/finitespace.html "Is Space Finite?" by Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks ==Shape of the Universe== Hi, I'd like to question the analogue of the shape of the Universe and the shape of the Earth. There are no current experimental hints, that the Universe is not flat. This is stark contrast to our experience on Earth, where it is rather easy to find that it is not flat (ships under horizon etc.) We should change/improve that sentence. User:Awolf002 23:17, 10 May 2004 (UTC) == At least 156 billion light-years? == According to http://www.space.com/scienceastronomy/mystery_monday_040524.html , the universe is at least 156 billion light-years across. :I changed the article accordingly. If someone thinks it was too early and we should wait until the new estimate becomes widely accepted, change it back.User:Paranoid 12:50, 28 May 2004 (UTC) ::while changing from 50 to 78 Bly in radius, it looks like you left unchanged the language ''The observable universe contains about 7 × 1022 stars, organized in about 1010 galaxies,''. Anyone know where that estimate came from? Based on applying the local density to the total volume? Based on observations of deep fields? Something else? If based on density, these numbers would also need to be adjusted based on the new volume. And this all relates to my question above about the number of particles in the universe. --User:Nealmcb 00:04, 2004 May 29 (UTC) == Age - forever uncertain == According to http://www.sciencenews.org/articles/20040522/fob1ref.asp , physicists from [http://www.lngs.infn.it/ National Laboratories of Gran Sasso] found the age to be 14.7, not 13.7. : Well, kind of... You need to understand, that the age of the Universe can (and should) be obtained by many ''independent'' methods. Each of these methods has its own assumptions and systematic errors. So it is very unlikely you get the exact same numbers from all of these measurements. If the theory about the Universe (on which these numbers are based on) is correct, then these numbers should agree within their uncertainties! :The article seems inconsistent. We have one paragraph stating the age is 13.7 (or 14.7) +-.2, and another saying it is 15.556 +- 0.024 billon years. These numbers aren't agreeing within their uncertainties. We need to square this somehow, at least we should point out the inconsistency. User:Zeimusu 16:17, 2004 Jun 15 (UTC) ::I suspect the 15.556 one is less reliable. If we knew the age with such precision, nobody would be discussing it, doing research and publishing paper about it.User:Paranoid 18:00, 15 Jun 2004 (UTC) Does anyone know who wrote that stuff about "the age is the inverse square of the temperature"? I know quite a bit about cosmology, and never heard of such a formula - and actually I think that this claim makes no sense. WRT the 14.7 billion years, I think that was misreported in the popular science media. What the people actually showed that the age of stars and globular clusters is 1 billion years more than previously thought - but adding that 1 billion years simply to the WMAP result makes no sense, since the WMAP result does not depend in any way on the age of the stars. There could possibly be problems now that the age of some stars seems to be older than the age of the universe, but AFAIK, this is not the case, despite this age correction for the stars. :Accordingly, I have deleted this paragraph. If anyone feels it must be reinstated, lets discuss it here first. User:Zeimusu 12:27, 2004 Jul 23 (UTC) well, I can claim that the universe is 20 billion years old and you CANNOT prove me wrong...=) while we may have a vague figure of the ''visible'' universe, what is beyond what our vision could reach is anyone's guess...who's to say that the next second the Hubble telescope will not see a 'boundary' somewhere beyond the deep field... I think i'm a lil bit drunk... but this stuff is fun to think about — User:LegolasGreenleaf 11:34, Nov 7, 2004 (UTC) == Age of Universe - Observation and Theory == The often quoted age of 13.7+/-0.2 Gyr for the age of the universe comes from the first year WMAP results: This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a pretty good age for the universe. Assuming all the various models used are valid in getting to this number, the accuracy of actual data allows a margin of error around 1%. However, this age is only accurate if the assumptions built into the various models being used are also accurate. This is referred to as “strong priors” and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. Although this is not a totally invalid procedure in certain contexts, it should be noted that the caveat, “based on the fact we have assumed the underlying model we used is correct”, then the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model). The age of the universe based on the “best fit” to WMAP data “only” is 13.4+/-0.3 Gyr (the slightly higher number of 13.7 includes some other data mixed in). This number represents the first accurate “direct” measurement of the age of the universe (other methods typically involve Hubble law and maximum age of stars, etc). There is a sense of triumphantism in the scientific community surrounding results like this, and therefore a more careful analysis of the methods and assumptions used, tend to be overlooked. This, of course, is a classic example of how different methods for determining the same parameter (in this case – the age of the universe) can give different answers with no overlap in the “errors”. It is quite common to see two sets of uncertainties, one related to the measurement and other the related to the systematic errors of the model. In some cases, this can not be done (in theoretical a prediction), but it is not evident why WMAP were not able to do this? Worth checking out is Science 299 (2003) 1532-1533, available here http://arxiv.org/abs/astro-ph/0303180 There is a purely theoretical approach to calculating the age of the universe which I can outline in more detail here. This comes from a very recent development and hasn't been published yet. Even after publication, it can take some years before a new result like this makes its way into the mainstream (so don't be surprised if you have not heard about this yet.) It is probably best to leave this development out of the main page until such time as it gains greater acceptance. For now this discussion forum should suffice for a preview however: The redshift of an object in a dynamic universe is related to a scale factor of that universe by the relation R=Ro/(1+z). Where R represents the “scale” of the universe as seen at the redshift z, where the current scale is Ro. The “scale” is just a device to measure the size of the universe, it can be thought of as the radius, but most people use the “scale factor” a=R/Ro, which would be dimensionless regardless of how you represented R. The temperature of the universe is inversely proportional to its scale; somewhat analogous to a gas that would cool down if expanded, or heat up if compressed, the temperature of the universe is thus related to redshift as T=To(1+z). We can do a quick test by using the current temperature of 2.7K and the redshift of CMB as 1089 to calculate the temperature of the decoupling surface T= 2.7*1090 = 2943K (this is the temperature of the universe when the CMB was emitted - around the dull red glow of a hot poker.) One of the most important cosmological models, is based on the Friedmann equations. This allows you to describe how the universe has evolved over time using an equation like this: t=to(1+z)^-3(1+w)/2. As you can see, things are starting to get a bit more tricky, but this equation simply relates the age of the universe to the redshift. This particular example has an additional term w, which comes from something called the equation of state, relating the pressure and density of the universe (p=wdc^2, where p is pressure, d is density and c^2 is the speed of light squared). In a universe like our own, most of the contents is in the form of stuff that does not exert much pressure on its surroundings (clouds of hydrogen gas, stars etc). In this model, w=0 and is known as a pressureless, or “dust” model. Here t=to(1+z)^-(3/2), and throwing in our redshift of 1089 and a current age of the universe to=13.7 Gyr gives us around 380,000 years for the age of the universe when the CMB was emitted. This may not seem so tricky after all, but unfortunately, it is not quite that simple. Embedded in these models is an assumption about time and an interpretation of metric distance which is not entirely correct. That is not to say that they are entirely wrong either: The metric distance defined between two points in an expanding universe increases over time. However, the General Theory of Relativity does not explicitly state how that change in distance should be interpreted. It is entirely valid to consider this change as a fundamental change in the underlying “concept” of distance (and the same situation would also apply to the concept of time). This type of model immediately solves an important problem relating to our CMB calculation above. If the photons in the CMB went from being hot enough to fry a burger, how come those same photons can't even defrost one today? Where did all that energy go? Of course, this comes back to our idea of the change in the distance scale: These universe expands by a change in the unit system, so the temperature likewise changes with the unit system. In this context, the temperature/scale of the universe can be thought of as being constant over the history of the universe, with no loss of energy in the CMB. Things do start to get technical here, but there is a nice confirmation of this model which actually validates it against recent observations. Coming back to the math, the change in the distance is related to time with the redshift relation t=to(1+z)^-2. However, there is an additional change in time related to redshift as t=to(1+z)^-(1/2), which (the product of both) brings us back to the original form for our “dust” w=0 universe. The idea of time-variable time probably sounds bizarre, but this is expected since there is no “absolute” concept of time in General Relativity (even though it seems people try and introduce this idea in most models.) So this was a very round about way of saying that we can relate the temperature of the universe to the age of the universe. Since we can measure the current temperature and have a model to extrapolate back, all we need to know now is the origin of the graph and read off the age. The earliest valid point in the evolution of the universe if the Planck time. At this time, the universe had the Planck temperature at a state of essentially zero entropy. The Planck temperate is the maximum attainable temperate in the universe and can be thought of as the Hawking temperature of black hole with a radius of the Planck length. The Planck temperature Tp comes out to around 4.5x10^30K, and we can state Tp=To(1+zmax), where To=2.725K and zmax=1.65x10^30 is the maximum redshift at the Planck time tp. We know that tp=to(1+zmax)^-2, so putting in the Planck time gives us a n age of the universe of 11.667 Gyr. This is not the end of the story however: If time was absolute and never changed, then this would be the correct value, but we need to take into consideration of the change in time over the age of the universe. This is a fairly simple integration and results in a age one third as much at 15.556 Gyr. The CMB temperature is known to a 2mK accuracy, and with some error in things like the Planck units (mainly from G), the accuracy of this age determination is around 24 Myr. There is a simplification where if expressed in Planck units, the temperature is equal to the inverse square. Dividing To/Tp gives the current temperature expressed in the amount of the Planck temperature 6x10^-31. Taking the inverse square gives 2.72x10^60 which is the age in Planck units. Multiplying by the Planck time gives the 11.667 Gyr again. There is mainly other simple relations like this, including the critical density as the Planck temperature raised to the forth power. In Planck units, the density is 1.3x10^-121, which multiplied by the Planck density is 3.3x10^-30 g/cm^3. This was a very stripped down and somewhat mangled explanation, but hopefully it has shed some light on the “age of the universe” question. :I notice that you not only snipped my questions and chose to ignore them; you also left our discussion in sci.astro. I count that as a defeat. You call our discussion "fruitless" - you are right there, but you might consider *who* was the one who kept ignoring questions and arguments, and thereby made the discussion fruitless...User:Bjoern 14:52, 3 Dec 2004 (UTC) ::Not so much a defeat, as a waste of time; your questions and arguments didn't warrant the time required to address them. However, if you are still interested, I will be presenting this work at the annual meeting[http://www.aaasmeeting.org] of the American Association for the Advancement of Science on February 20th 2005 (so I need to focus my attention on that presentation for the time being). :Wow, more discussion than I was expecting! I wonder if that can be incorporated into this page or another, it was worth reading. The universe page should have 13.7 billion years as its value for the age of the universe, but I'm going to copy some of your caveats to the main page. User:Zeimusu 14:41, 2004 Jul 27 (UTC) ==Capitalizaton== I asked this question on Talk:Kardashev scale but I'm still not confident. this article mizes and matches the use of Universe and universe. Is there a distinction between the two forms or is this a formatting error? Many articles have the word Universe capitalized but this article starts out by uncapitalizing it and then shifts to upper case. The title: Size of Universe and observable universe seems to be using both versions in one line. If it isn't an error there should be a note about it somewhere. Comments? BrokenSegue\">User:BrokenSegue">User:BrokenSegue|BrokenSegue\">User:BrokenSegue 03:27, 19 Nov 2004 (UTC)
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[[Image:Universe.jpg|thumb|right|The deepest visible-light image of the cosmos. Hubble Ultra Deep Field. Image Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF team.]] In the first half of the 20th century, the word ''universe'' was used to mean the whole spacetime continuum in which we exist, together with all the energy and matter within it. Attempts to understand the universe in this sense, on the largest possible scales, are made in cosmology, a science that has grown from physics and astronomy. During the second half of the 20th century, the development of observational cosmology, also called physical cosmology, led to a split in the meaning of the word ''universe'', between observational cosmologists and theoretical cosmology; where the former (usually) abandon the hope of observing the whole spacetime continuum, the latter retain this hope, attempting to find the most reasonable speculations for modeling the whole of spacetime, despite the extreme difficulty in imagining any empirical constraints on these speculations and the risk of declining into metaphysics. The terms known universe, observable universe, or visible universe are often used to describe the part of the Universe that we can see or otherwise observe. Those who believe it is impossible to observe the whole continuum may use our universe, referring only to that knowable by human beings in particular. == Expansion and age, and the Big Bang theory== The most important result of cosmology, that the Universe is expanding, is derived from redshift observations and quantified by Hubble's Law. Extrapolating this expansion back in time, one approaches a gravitational singularity, a rather abstract mathematical concept, which may or may not correspond to reality. This gives rise to the Big Bang theory, the dominant model in cosmology today. The age of the Universe was estimated to be about 13.7 1 E9 () years, with a margin of error of about 1 percent (± 200 million years), according to NASA's Wilkinson Microwave Anisotropy Probe (WMAP). However, this is based on the assumption that the underlying model used for data analysis is correct. Other methods of estimating the age of the universe give different ages. A fundamental aspect of the Big Bang can be seen today in the observation that the farther away from us galaxy are, the faster they move away from us. It can also be seen in the cosmic microwave background radiation which is the much-attenuated radiation that originated soon after the Big Bang. This background radiation is remarkably uniform in all directions, which cosmologists have attempted to explain by an initial period of rapid Cosmic inflation following the Big Bang. === Size of Universe and observable universe === There is disagreement over whether the Universe is finite or infinity in spatial extent and volume. However, the observable universe, consisting of all locations that could have affected us since the Big Bang given the finite speed of light, is certainly finite. The edge of the cosmic light horizon is 13.7 billion light years distant. The ''present distance'' (comoving distance) to the edge of the observable universe is larger, since the universe has been expanding; it is estimated to be about 78 billion light years (km). This would make the comoving volume, of the known universe, equal to cubic light years (assuming this region is perfectly sphere). The observable universe contains about stars, organized in about 10 billion galaxy, which themselves form groups and clusters of galaxies and superclusters. The number of galaxies may be even larger, based on the Hubble Deep Field observed with the Hubble Space Telescope. Both popular and professional research articles in cosmology often use the term "Universe" when they really mean "observable universe". This is because unobservable physical phenomena are scientifically irrelevant; that is, they cannot affect any events that we can perceive, and therefore, it is argued, effectively do not exist. See also Causality (physics). We live in the centre of the universe that we observe, in apparent contradiction to the Copernican principle which says that the Universe is more or less uniform and it has no distinguished centre. This is simply because light does not travel infinitely fast, and we make observations of the past. As we look further and further away, we see things from epochs (times) closer and closer to the limit of time=zero of the Big bang model. And since light travels at the same speed in any direction towards us, we live at the centre of our observable universe. == Shape of the Universe == An important open question of cosmology is the shape of the universe. Firstly, whether or not the Universe is ''flat'', i.e. whether the rules of Euclidean geometry are valid on the largest scales, is unknown. Currently, most cosmologists believe that the observable universe is (nearly) flat, with local wrinkles where massive objects distort spacetime, just as a lake is (nearly) flat. This opinion was strengthened by the latest data from WMAP, looking at "acoustic oscillations" in the cosmic background radiation temperature variations. Secondly, whether or not the Universe is multiply connected, is unknown. The Universe has no spatial boundary according to the standard Big bang model, but nevertheless may be spatially finite. This can be understood using a two-dimensional analogy: the surface of a sphere has no edge, but nonetheless has a finite area (). It is a two-dimensional surface with constant curvature in a third dimension. A three-dimensional equivalent is the unbounded "spherical space" discovered by Georg Friedrich Bernhard Riemann, which has a finite volume (). In it, all three dimensions are constantly curved in a fourth. (Other possibilities include a similar "elliptical space", and a "cylindrical space", where, in conflict with ordinary geometry, the two ends of the cylinder are joined together, but without bending the cylinder. These, also, are two-dimensional spaces with finite areas; innumerable others exist. However, the sphere has the unique and, perhaps, more aesthetically pleasing property that all points on it are geometrically similar.) If the universe is indeed unbounded yet spatially finite, as described, then traveling in a "straight" line, in any given direction, would theoretically cause one to eventually arrive back at the starting point after traveling a distance equal to the "circumference" of the universe (which is impossible to our current understanding of the Universe, as its size is much greater than the size of the observable universe). Strictly speaking, we should call the stars and galaxies "views" of stars and galaxies, since it is possible that the Universe is multiply-connected and sufficiently small (and of an appropriate, perhaps complex, shape) that we can see once or several times around it in various, and perhaps all, directions. (Think of a house of mirrors.) If so, the actual number of physically distinct stars and galaxies would be smaller than currently accounted. Although this possibility has not been ruled out, the results of latest cosmic microwave background (CMB) research make this very unlikely. == Fate of the Universe == Depending on the average density of matter and energy in the Universe, it will either keep on expanding forever or it will be gravitationally slowed and will eventually collapse back on itself in a "Big Crunch". Currently the evidence suggests not only that there is insufficient mass/energy to cause a recollapse, but that the expansion of the universe seems to be accelerating and will accelerate for the whole of eternity (see accelerating universe). For a more detailed discussion of other theories, see the ultimate fate of the Universe. == Multiverse == There is some speculation that multiple universes exist in a higher-level multiverse (also known as a megaverse), our Universe being one of those universes (lower case). For example, matter that falls into a black hole in our Universe could emerge as a "Big Bang," starting another universe. However, all such ideas are currently untestable and cannot be regarded as anything more than speculation. == Other terms == Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world". Although words like world and its equivalents in other languages now almost always refer to the planet Earth, they previously referred to everything that exists—see Copernicus, for example—and still sometimes do (as in "the whole wide world"). ==See also== *Cosmos *Cosmic latte - the color of the Universe *Parallel universe == References == * Albert Einstein (1952). ''Relativity: The Special and the General Theory (Fifteenth Edition)''. ISBN 0-517-88441-0 == External links == * Richard Powell: ''An Atlas of the Universe'', http://www.anzwers.org/free/universe/index.html. A series of images at various scales, with explanations. * [http://www.space.com/scienceastronomy/age_universe_030103.html Age of the Universe at Space.Com] * [http://slate.msn.com/id/2087206/ My So-Called Universe] by Jim Holt, on various arguments for and against an infinite Universe and parallel universes. * [http://www.hep.upenn.edu/~max/multiverse1.html Parallel Universes] by Max Tegmark. * [http://www.hedweb.com/nihilism/nihilfil.htm Why Does Anything Exist?] Large-scale structure of the cosmos ms:alam_Semesta lv:Visums simple:Universe
Universe
==Contradiction== I removed the paragraph But some of the objects outside of the observable universe can, in principle, be observed indirectly. For example, it is theoretically possible to meet an observer located near the end of ''our'' observable universe, who in his past has observed some galaxies that left our observable universe because of expansion. I'm a mathematician not a physicist but this seems to contradict the paragraph above about Causality. ==Meaning of ''Universe''== ''The Universe is the whole spacetime continuum in which we find ourselves, together with all the matter and energy within it.'' Different words have been used throughout history to denote "all of space", including the equivalents in various languages of "heavens", "cosmos" and "world". For a large fraction of the twentieth century, the word Universe, with an upper case "U", was used to mean the whole spacetime continuum in which we find ourselves, together with all the matter and energy within it. However, since the standard Big bang model has become well established observationally during the last few decades of the twentieth century, theoretical cosmologists have come up with new ideas of "the whole spacetime continuum" which are much, much larger than the "Universe" corresponding to the Big bang model. For this reason, the word universe can now be used in the plural and with a lower case "u" when discussing theories about all of space-time. There is no clear consensus on what new word to use for the whole spacetime continuum (though some like the term multiverse), and as long as there is no conceivable method of measuring anything beyond the observable horizon (formally speaking, the particle horizon), this is a seen as a moot point (irrelevant) for empirical scientists, and is of interest only to philosophy. ==infinite universe?== ''It is not known whether the Universe is finite or infinite in spatial extent and volume, although current theories favor an infinite Universe.'' although the majority of theorists presently favor an infinite Universe. :Rubbish. The majority of theorists currently favour an infinite Universe, but this is a matter of personal taste and sociology and historical fashion and nothing to do with physics. ::I certainly also sense theorists favoring an infinite Universe. I disagree that it is a matter of taste. :: http://www.sciam.com/article.cfm?articleID=000F1EDD-B48A-1E90-8EA5809EC5880000 "Parallel Universes" Not just a staple of science fiction, other universes are a direct implication of cosmological observations. By Max Tegmark. :: User:Nealmcb 17:46, 2004 May 4 (UTC) ==size of universe== ''48 billion (48 ? 109) light years.'' 50x10^9 light-yr = 5x 10^10 *0.3 pc = 1.7x 10^10 pc = 17 Gpc = 12 /h Gpc (where h=0.7 is the Hubble constant) OK :), this is approximately correct for Omega_m=0.3, Omega_Lambda =0.7, though 10/h Gpc is closer. In any case, i'll round to 50 since it's no more than 10% precise. I'm glad the way this is improving - and i love the desire to avoid ambiguity. Wikipedia is definitely a good tool for spreading relatively wellunderstood information to outsiders without expecting them to waste hours and hours to sort through ambiguous jargon. --boud ---- I see various estimates of the number of particles in the observable universe, e.g. the claim in Wikipedia talk:Size comparisons that ''It is accepted by astrophysicists that the number of particles in the observable universe? is currently in the 1085 range''. This seems like the place to document that. Does anyone have some good references? User:Nealmcb 18:27, 2004 May 4 (UTC) ---------- "...it is estimated to be about 78 billion light years (7.4 × 1023 km)." Interesting....just based on what did this 'estimated value' came from? At least a note should be provided for this kind of 'data'. User:LegolasGreenleaf 11:26, Nov 7, 2004 (UTC) ----- "...billion ..." What kind of billion?, I meen, 10^9 or 10^12? I know that wikipedia uses 10^9 "if don't state otherwise" but I think that talking about science and to avoid ambiguity, is better 10^9 or the prefixes "giga" or "tera" [http://en.wikipedia.org/wiki/Long_scale#Alternative_approaches see wikipedia, long scale alternative approaches] ---- ==Seeing around the universe== For the time being, I am removing the following statement from the article, because I don't see how it can be true: ''"Therefore, strictly speaking, we should call the stars and galaxies "images" of stars and galaxies, since it is possible that the Universe is finite and so small that we can see once or several times around it, and the real number of physically distinct stars and galaxies could be a little smaller. There are observations underway to determine whether this is true."'' If the universe were so small that one could see even once around it, wouldn't the night (and day) sky be completely bright? If the universe were unbounded yet sufficiently finite, wouldn't the "image" of the sun exist in every direction that one looked?User:Johnstone 13:31, 8 May 2004 (UTC) :I'd put it back in. You bring up Olbers'_paradox. See that article for reasons why the sky isn't all bright. But that doesn't depend on whether the universe is bent or flat. It's more about whether you can see an infinite distance. User:Nealmcb 16:16, 2004 May 8 (UTC) ::I was aware of that paradox, though I didn't know it had a name. It was in the back of my mind when I decided to question the material I removed. As you say, that paradox is not about the idea of seeing "around" the universe, so it does not necessarily relate to my question. However, I was thinking of the universe as a "spherical space"; I have re-read the sentences in question, and now I realize that my objections do not apply for most non-spherical geometries. I've tweaked a few words to (hopefully) clarify their meaning. User:Johnstone 22:51, 10 May 2004 (UTC) ==universe (fiction)== It might be nice to have an article on "Universe (fiction)" or something -- the sense of the word where somebody says "Many of the Marvel Comics series take place in the same universe" or "The SERRAted Edge novels are set in the same universe as the Bedlam Bards novels". User:Cwitty http://www.cs.appstate.edu/~sjg/class/1010/wc/geom/finitespace.html "Is Space Finite?" by Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks ==Shape of the Universe== Hi, I'd like to question the analogue of the shape of the Universe and the shape of the Earth. There are no current experimental hints, that the Universe is not flat. This is stark contrast to our experience on Earth, where it is rather easy to find that it is not flat (ships under horizon etc.) We should change/improve that sentence. User:Awolf002 23:17, 10 May 2004 (UTC) == At least 156 billion light-years? == According to http://www.space.com/scienceastronomy/mystery_monday_040524.html , the universe is at least 156 billion light-years across. :I changed the article accordingly. If someone thinks it was too early and we should wait until the new estimate becomes widely accepted, change it back.User:Paranoid 12:50, 28 May 2004 (UTC) ::while changing from 50 to 78 Bly in radius, it looks like you left unchanged the language ''The observable universe contains about 7 × 1022 stars, organized in about 1010 galaxies,''. Anyone know where that estimate came from? Based on applying the local density to the total volume? Based on observations of deep fields? Something else? If based on density, these numbers would also need to be adjusted based on the new volume. And this all relates to my question above about the number of particles in the universe. --User:Nealmcb 00:04, 2004 May 29 (UTC) == Age - forever uncertain == According to http://www.sciencenews.org/articles/20040522/fob1ref.asp , physicists from [http://www.lngs.infn.it/ National Laboratories of Gran Sasso] found the age to be 14.7, not 13.7. : Well, kind of... You need to understand, that the age of the Universe can (and should) be obtained by many ''independent'' methods. Each of these methods has its own assumptions and systematic errors. So it is very unlikely you get the exact same numbers from all of these measurements. If the theory about the Universe (on which these numbers are based on) is correct, then these numbers should agree within their uncertainties! :The article seems inconsistent. We have one paragraph stating the age is 13.7 (or 14.7) +-.2, and another saying it is 15.556 +- 0.024 billon years. These numbers aren't agreeing within their uncertainties. We need to square this somehow, at least we should point out the inconsistency. User:Zeimusu 16:17, 2004 Jun 15 (UTC) ::I suspect the 15.556 one is less reliable. If we knew the age with such precision, nobody would be discussing it, doing research and publishing paper about it.User:Paranoid 18:00, 15 Jun 2004 (UTC) Does anyone know who wrote that stuff about "the age is the inverse square of the temperature"? I know quite a bit about cosmology, and never heard of such a formula - and actually I think that this claim makes no sense. WRT the 14.7 billion years, I think that was misreported in the popular science media. What the people actually showed that the age of stars and globular clusters is 1 billion years more than previously thought - but adding that 1 billion years simply to the WMAP result makes no sense, since the WMAP result does not depend in any way on the age of the stars. There could possibly be problems now that the age of some stars seems to be older than the age of the universe, but AFAIK, this is not the case, despite this age correction for the stars. :Accordingly, I have deleted this paragraph. If anyone feels it must be reinstated, lets discuss it here first. User:Zeimusu 12:27, 2004 Jul 23 (UTC) well, I can claim that the universe is 20 billion years old and you CANNOT prove me wrong...=) while we may have a vague figure of the ''visible'' universe, what is beyond what our vision could reach is anyone's guess...who's to say that the next second the Hubble telescope will not see a 'boundary' somewhere beyond the deep field... I think i'm a lil bit drunk... but this stuff is fun to think about — User:LegolasGreenleaf 11:34, Nov 7, 2004 (UTC) == Age of Universe - Observation and Theory == The often quoted age of 13.7+/-0.2 Gyr for the age of the universe comes from the first year WMAP results: This measurement is made by using the location of the first acoustic peak in the microwave background power spectrum to determine the size of the decoupling surface (size of universe at the time of recombination). The light travel time to this surface (depending on the geometry used) yields a pretty good age for the universe. Assuming all the various models used are valid in getting to this number, the accuracy of actual data allows a margin of error around 1%. However, this age is only accurate if the assumptions built into the various models being used are also accurate. This is referred to as “strong priors” and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. Although this is not a totally invalid procedure in certain contexts, it should be noted that the caveat, “based on the fact we have assumed the underlying model we used is correct”, then the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model). The age of the universe based on the “best fit” to WMAP data “only” is 13.4+/-0.3 Gyr (the slightly higher number of 13.7 includes some other data mixed in). This number represents the first accurate “direct” measurement of the age of the universe (other methods typically involve Hubble law and maximum age of stars, etc). There is a sense of triumphantism in the scientific community surrounding results like this, and therefore a more careful analysis of the methods and assumptions used, tend to be overlooked. This, of course, is a classic example of how different methods for determining the same parameter (in this case – the age of the universe) can give different answers with no overlap in the “errors”. It is quite common to see two sets of uncertainties, one related to the measurement and other the related to the systematic errors of the model. In some cases, this can not be done (in theoretical a prediction), but it is not evident why WMAP were not able to do this? Worth checking out is Science 299 (2003) 1532-1533, available here http://arxiv.org/abs/astro-ph/0303180 There is a purely theoretical approach to calculating the age of the universe which I can outline in more detail here. This comes from a very recent development and hasn't been published yet. Even after publication, it can take some years before a new result like this makes its way into the mainstream (so don't be surprised if you have not heard about this yet.) It is probably best to leave this development out of the main page until such time as it gains greater acceptance. For now this discussion forum should suffice for a preview however: The redshift of an object in a dynamic universe is related to a scale factor of that universe by the relation R=Ro/(1+z). Where R represents the “scale” of the universe as seen at the redshift z, where the current scale is Ro. The “scale” is just a device to measure the size of the universe, it can be thought of as the radius, but most people use the “scale factor” a=R/Ro, which would be dimensionless regardless of how you represented R. The temperature of the universe is inversely proportional to its scale; somewhat analogous to a gas that would cool down if expanded, or heat up if compressed, the temperature of the universe is thus related to redshift as T=To(1+z). We can do a quick test by using the current temperature of 2.7K and the redshift of CMB as 1089 to calculate the temperature of the decoupling surface T= 2.7*1090 = 2943K (this is the temperature of the universe when the CMB was emitted - around the dull red glow of a hot poker.) One of the most important cosmological models, is based on the Friedmann equations. This allows you to describe how the universe has evolved over time using an equation like this: t=to(1+z)^-3(1+w)/2. As you can see, things are starting to get a bit more tricky, but this equation simply relates the age of the universe to the redshift. This particular example has an additional term w, which comes from something called the equation of state, relating the pressure and density of the universe (p=wdc^2, where p is pressure, d is density and c^2 is the speed of light squared). In a universe like our own, most of the contents is in the form of stuff that does not exert much pressure on its surroundings (clouds of hydrogen gas, stars etc). In this model, w=0 and is known as a pressureless, or “dust” model. Here t=to(1+z)^-(3/2), and throwing in our redshift of 1089 and a current age of the universe to=13.7 Gyr gives us around 380,000 years for the age of the universe when the CMB was emitted. This may not seem so tricky after all, but unfortunately, it is not quite that simple. Embedded in these models is an assumption about time and an interpretation of metric distance which is not entirely correct. That is not to say that they are entirely wrong either: The metric distance defined between two points in an expanding universe increases over time. However, the General Theory of Relativity does not explicitly state how that change in distance should be interpreted. It is entirely valid to consider this change as a fundamental change in the underlying “concept” of distance (and the same situation would also apply to the concept of time). This type of model immediately solves an important problem relating to our CMB calculation above. If the photons in the CMB went from being hot enough to fry a burger, how come those same photons can't even defrost one today? Where did all that energy go? Of course, this comes back to our idea of the change in the distance scale: These universe expands by a change in the unit system, so the temperature likewise changes with the unit system. In this context, the temperature/scale of the universe can be thought of as being constant over the history of the universe, with no loss of energy in the CMB. Things do start to get technical here, but there is a nice confirmation of this model which actually validates it against recent observations. Coming back to the math, the change in the distance is related to time with the redshift relation t=to(1+z)^-2. However, there is an additional change in time related to redshift as t=to(1+z)^-(1/2), which (the product of both) brings us back to the original form for our “dust” w=0 universe. The idea of time-variable time probably sounds bizarre, but this is expected since there is no “absolute” concept of time in General Relativity (even though it seems people try and introduce this idea in most models.) So this was a very round about way of saying that we can relate the temperature of the universe to the age of the universe. Since we can measure the current temperature and have a model to extrapolate back, all we need to know now is the origin of the graph and read off the age. The earliest valid point in the evolution of the universe if the Planck time. At this time, the universe had the Planck temperature at a state of essentially zero entropy. The Planck temperate is the maximum attainable temperate in the universe and can be thought of as the Hawking temperature of black hole with a radius of the Planck length. The Planck temperature Tp comes out to around 4.5x10^30K, and we can state Tp=To(1+zmax), where To=2.725K and zmax=1.65x10^30 is the maximum redshift at the Planck time tp. We know that tp=to(1+zmax)^-2, so putting in the Planck time gives us a n age of the universe of 11.667 Gyr. This is not the end of the story however: If time was absolute and never changed, then this would be the correct value, but we need to take into consideration of the change in time over the age of the universe. This is a fairly simple integration and results in a age one third as much at 15.556 Gyr. The CMB temperature is known to a 2mK accuracy, and with some error in things like the Planck units (mainly from G), the accuracy of this age determination is around 24 Myr. There is a simplification where if expressed in Planck units, the temperature is equal to the inverse square. Dividing To/Tp gives the current temperature expressed in the amount of the Planck temperature 6x10^-31. Taking the inverse square gives 2.72x10^60 which is the age in Planck units. Multiplying by the Planck time gives the 11.667 Gyr again. There is mainly other simple relations like this, including the critical density as the Planck temperature raised to the forth power. In Planck units, the density is 1.3x10^-121, which multiplied by the Planck density is 3.3x10^-30 g/cm^3. This was a very stripped down and somewhat mangled explanation, but hopefully it has shed some light on the “age of the universe” question. :I notice that you not only snipped my questions and chose to ignore them; you also left our discussion in sci.astro. I count that as a defeat. You call our discussion "fruitless" - you are right there, but you might consider *who* was the one who kept ignoring questions and arguments, and thereby made the discussion fruitless...User:Bjoern 14:52, 3 Dec 2004 (UTC) ::Not so much a defeat, as a waste of time; your questions and arguments didn't warrant the time required to address them. However, if you are still interested, I will be presenting this work at the annual meeting[http://www.aaasmeeting.org] of the American Association for the Advancement of Science on February 20th 2005 (so I need to focus my attention on that presentation for the time being). :Wow, more discussion than I was expecting! I wonder if that can be incorporated into this page or another, it was worth reading. The universe page should have 13.7 billion years as its value for the age of the universe, but I'm going to copy some of your caveats to the main page. User:Zeimusu 14:41, 2004 Jul 27 (UTC) ==Capitalizaton== I asked this question on Talk:Kardashev scale but I'm still not confident. this article mizes and matches the use of Universe and universe. Is there a distinction between the two forms or is this a formatting error? Many articles have the word Universe capitalized but this article starts out by uncapitalizing it and then shifts to upper case. The title: Size of Universe and observable universe seems to be using both versions in one line. If it isn't an error there should be a note about it somewhere. Comments? BrokenSegue\">User:BrokenSegue">User:BrokenSegue|BrokenSegue\">User:BrokenSegue 03:27, 19 Nov 2004 (UTC)
Universe
Welcome! Hello, and Wikipedia:Welcome, newcomers to Wikipedia. Thank you for your contributions. I hope you like the place and decide to stay. Here are a few good links for newcomers: *Wikipedia:Five pillars *Wikipedia:How to edit a page *Wikipedia:Tutorial *Wikipedia:Picture tutorial *Wikipedia:How to write a great article *Wikipedia:Naming conventions *Wikipedia:Manual of Style *If you're ready for the complete list of Wikipedia documentation, there's also Wikipedia:Topical index. I hope you enjoy editing here and being a Wikipedia:Wikipedians! By the way, you can sign your name on Talk and vote pages using three tildes, like this: ~~~. Four tildes (~~~~) produces your name and the current date. If you have any questions, see the Wikipedia:Help, add a question to the Wikipedia:village pump or ask me on my Talk page. Again, welcome! - User:UtherSRG 18:46, May 19, 2005 (UTC)
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UA | UB | UC | UD | UE | UF | UG | UH | UI | UJ | UK | UL | UM | UN | UO | UP | UR | US | UT | UW | UX | UY | UZ |Words begining with Universe:
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