|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| {{about|the branch of astronomy||Cosmology}}
| |
| {{Blacklisted-links|1=
| |
| *http://preposterousuniverse.com/writings/cosmologyprimer/
| |
| *:''Triggered by <code>\bpreposterousuniverse\.com\b</code> on the local blacklist''|bot=Cyberbot II}}
| |
| {{Use dmy dates|date=July 2012}}
| |
| {{Refimprove|date=February 2009}}
| |
| {{Cosmology}}
| |
|
| |
|
| '''Physical cosmology''' is the study of the largest-scale structures and dynamics of the [[Universe]] and is concerned with fundamental questions about its formation, evolution, and ultimate fate.<ref name=GFR_Ellis>For an overview, see {{cite book |chapter=Issues in the Philosophy of Cosmology |pages=1183''ff''|year=2006 |author=George FR Ellis |arxiv=astro-ph/0602280 |isbn=0-444-51560-7 |publisher=North Holland |title=Philosophy of Physics (Handbook of the Philosophy of Science) 3 volume set |editor=Jeremy Butterfield & John Earman}}</ref> For most of human history, it was a branch of [[metaphysics]] and [[religion]]. [[Cosmology]] as a [[science]] originated with the [[Copernican principle]], which implies that celestial bodies obey identical [[physical law]]s to those on Earth, and [[Newtonian mechanics]], which first allowed us to understand those laws.
| |
|
| |
|
| Physical cosmology, as it is now understood, began with the 20th century development of [[Albert Einstein]]'s [[general relativity|general theory of relativity]], and better [[astronomy|astronomical]] observations of extremely distant objects. These advances made it possible to speculate about the [[origin of the Universe]], and allowed the establishment of the [[Big Bang]] Theory, by Fr. [[Georges Lemaitre]], as the leading cosmological model. Some researchers still advocate a handful of [[Non-standard cosmology|alternative cosmologies]];<ref>[http://www.cosmologystatement.org/ An Open Letter to the Scientific Community as published in ''New Scientist'', May 22, 2004]</ref> however, most cosmologists agree that the Big Bang theory best explains observations.
| | What do you know about this Korean type of martial art? In Korea, it is practiced as the national sport, but it gives a lot more than entertainment for these who find out it. Tae Kwon Do is used as a type of self-defense and workout. Competitors come collectively in matches, somewhat like boxing, to fight, or spar, with one yet another. A lot training and practice takes spot just before official sparring matches are held, as the method is complicated, and competitors need to be conscious of what sorts of hits (strikes) are legal and illegal, and how factors are awarded.<br><br>Tae Kwon Do competitors are necessary to wear the correct protective gear, and to abide by the rules of the referee who is present during the sparring. There are three rounds in a match. The rounds final for 3 minutes, and there is a 1-minute break among rounds. Browsing To [http://syu-mai.info/2014/08/19/whats-a-criminal-defense-lawyer-8/ What"s a Criminal Defense Lawyer | Syu Mai] likely provides warnings you could give to your aunt. If, throughout a round, a competitor is knocked down and is unable to rise ahead of the referee counts to eight, the competitor loses that round, as it counts as a knock-out.<br><br>In order to score a point, a competitor ought to strike his opponent with sufficient force to abruptly move either his head or his physique from exactly where it was prior to the strike. There are some locations which are considered out of bounds for hits. These include any area under the waistline, and the back of the head and physique. The front of the head, the torso and chest are all legal strike zones, and protective gear is worn in these regions to protect the competitors from serious injury. Strikes are delivered both as punches and kicks, with the purpose being to knock the [https://www.Google.com/search?hl=en&gl=us&tbm=nws&q=opponent&btnI=lucky opponent] out of spot or to the ground.<br><br>Each energy and manage are important to Tae Kwon Do sparring, due to the force needed to move an opponent, as effectively as the specific regions permitted for striking. Be taught additional information on this related site - Visit this webpage: [http://xtlm.info/2014/08/18/four-methods-to-appear-for-lemon-law-lawyers-los-angeles/ patent pending]. The competitor need to be able to deliver his strike as powerfully and accurately as attainable. A lot training ought to take place ahead of the Tae Kwon Do competitor is capable to spar with strength and accuracy, and to defend himself from the blows of his opponent..<br><br>If you have any thoughts about exactly where and how to use [http://royaldeity3921.sosblogs.com pet health], you can speak to us at the page. |
| | |
| Cosmology draws heavily on the work of many disparate areas of research in theoretical and applied [[physics]]. Areas relevant to cosmology include [[particle physics experiments]] and [[particle physics phenomenology|theory]], theoretical and observational [[astrophysics]], [[general relativity]], [[quantum mechanics]], and [[plasma physics]].
| |
| | |
| ==History of study==
| |
| {{see also|Timeline of cosmology|List of cosmologists}}
| |
| | |
| Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory of [[general relativity]], which provided a unified description of gravity as a geometric property of space and time.<ref>{{cite web|title=Nobel Prize Biography|url=http://nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-bio.html|work=Nobel Prize Biography|publisher=Nobel Prize|accessdate=25 February 2011}}</ref> At the time, Einstein believed in a [[static universe]], but found that his original formulation of the theory did not permit it.<ref name="Liddle, A. 51">{{cite book | author = Liddle, A. | title = An Introduction to Modern Cosmology | publisher = Wiley | page=51 | isbn =0-470-84835-9 }}</ref> This is because masses distributed throughout the Universe gravitationally attract, and move toward each other over time.<ref>{{cite book | last = Vilenkin | first = Alex | title = Many worlds in one : the search for other universes | publisher = Hill and Wang, A division of Farrar, Straus and Giroux | location = New York | year = 2007 | isbn = 978-0-8090-6722-0 | page=19}}</ref> However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this ''[[cosmological constant]]'' to his field equations in order to force them to model a static universe.<ref>{{cite book |last1=Jones |first1=Mark |last2=Lambourne |first2=Robert | title = An introduction to galaxies and cosmology | publisher = Open University Cambridge University Press | location = Milton Keynes Cambridge, UK New York | year = 2004 | isbn = 0-521-54623-0 | page= 228}}</ref> However, this so-called Einstein model is unstable to small perturbations—it will eventually start to [[metric expansion of space|expand]] or contract.<ref name="Liddle, A. 51"/> The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the cosmological principle. The cosmological solutions of general relativity were found by [[Alexander Friedmann]] in the early 1920s.<ref>{{cite book |last1=Jones |first1=Mark |last2=Lambourne |first2=Robert | title = An introduction to galaxies and cosmology | publisher = Open University Cambridge University Press | location = Milton Keynes Cambridge, UK New York | year = 2004 | isbn = 0-521-54623-0 |page=232 }}</ref> His equations describe the [[Friedmann-Lemaître-Robertson-Walker]] universe, which may expand or contract, and whose geometry may be open, flat, or closed.
| |
| | |
| In the 1910s, [[Vesto Melvin Slipher|Vesto Slipher]] (and later [[Carl Wilhelm Wirtz]]) interpreted the [[red shift]] of [[nebula|spiral nebulae]] as a [[Doppler shift]] that indicated they were receding from [[Earth]]. However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its [[angular size]], but a physical size must be assumed to do this. Another method is to measure the [[brightness]] of an object and assume an intrinsic [[luminosity]], from which the distance may be determined using the [[inverse square law]]. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own [[Milky Way]], nor did they speculate about the cosmological implications. In 1927, the [[Belgium|Belgian]] [[Roman Catholic]] [[priest]] [[Georges Lemaître]] independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the Universe began with the "explosion" of a "primeval [[atom]]"—which was later called the [[Big Bang]]. In 1929, [[Edwin Hubble]] provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of [[Cepheid variable]] stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance. This fact is now known as [[Hubble's law]], though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables.
| |
| | |
| Given the [[cosmological principle]], Hubble's law suggested that the Universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other explanation was [[Fred Hoyle]]'s [[steady state model]] in which new matter is created as the galaxies move away from each other. In this model, the Universe is roughly the same at any point in time.
| |
| | |
| For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the Universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model, and since the precise measurements of the cosmic microwave background by the [[Cosmic Background Explorer]] in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos. One consequence of this is that in standard general relativity, the Universe began with a [[Gravitational singularity|singularity]], as demonstrated by [[Roger Penrose]] and [[Stephen Hawking]] in the 1960s.
| |
| | |
| ==Energy of the cosmos==
| |
| Light [[chemical elements]], primarily [[hydrogen]] and [[helium]], were created in the [[Big Bang]] process (see [[Nucleosynthesis]]). The small atomic nuclei combined into larger atomic nuclei to form heavier elements such as [[iron]] and [[nickel]], which are more stable (see [[Nuclear fusion]]). This caused a ''later energy release''. Such reactions of nuclear particles inside [[star]]s continue to contribute to ''sudden energy releases'', such as in [[nova]] stars. Gravitational collapse of matter into [[black hole]]s is also thought to power the most energetic processes, generally seen at the centers of galaxies (see [[Quasar]] and [[Active galaxy]]).
| |
| | |
| Cosmologists cannot explain all cosmic phenomena exactly, such as those related to the [[Accelerating universe|accelerating expansion of the Universe]], using conventional [[Energy forms|forms of energy]]. Instead, cosmologists propose a new form of energy called [[dark energy]] that permeates all space.<ref>[http://www.sciencemag.org/cgi/content/abstract/300/5627/1914?siteid=sci&ijkey=eJV6VmToFgZIE&keytype=ref Science 20 June 2003:Vol. 300. no. 5627, pp. 1914 - 1918] Throwing Light on Dark Energy, Robert P. Kirshner. Retrieved December 2006</ref> One hypothesis is that dark energy is the energy of [[virtual particle]]s, which are believed to exist in a vacuum due to the [[uncertainty principle]].
| |
| | |
| There is no clear way to define the total energy in the Universe using the most widely accepted theory of gravity, [[general relativity]]. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each [[photon]] that travels through intergalactic space loses energy due to the [[redshift]] effect. This energy is not obviously transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of [[conservation of energy]].<ref>e.g. {{cite book | author = Liddle, A. | title = An Introduction to Modern Cosmology | publisher = Wiley | isbn =0-470-84835-9 }} This argues cogently "Energy is always, always, always conserved."</ref>
| |
| | |
| [[Thermodynamics of the universe]] is a field of study that explores which form of energy dominates the cosmos - [[relativistic particle]]s which are referred to as [[radiation]], or non-relativistic particles referred to as matter. Relativistic particles are particles whose [[rest mass]] is zero or negligible compared to their [[kinetic energy]], and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light.
| |
| | |
| As the Universe expands, both matter and radiation in it become diluted. However, the Universe also cools down, meaning that the average energy per particle gets smaller. Therefore radiation becomes weaker, and dilutes faster{{why|date=April 2013}} than matter. Thus with the expansion of the Universe, radiation becomes less dominant than matter. The very early Universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per [[photon]] becomes roughly 10 [[electronvolt|eV]] and lower, matter dictates the rate of deceleration and the Universe is said to be 'matter dominated'. The intermediate case is not treated well [[analytic solution|analytically]]. As the expansion of the universe continues, matter dilutes even further and the [[cosmological constant]] becomes dominant, leading to an acceleration in the universe's expansion.
| |
| | |
| ==History of the universe==
| |
| | |
| {{see also|Timeline of the Big Bang}}
| |
| The history of the Universe is a central issue in cosmology. The history of the Universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the [[Lambda-CDM model]].
| |
| | |
| ===Equations of motion===
| |
| {{main|Friedmann-Lemaître-Robertson-Walker metric}}
| |
| The [[equations of motion]] governing the Universe as a whole are derived from [[general relativity]] with a small, positive [[cosmological constant]].<ref>
| |
| {{cite journal
| |
| | title = Supersymmetry of FRW barotropic cosmologies
| |
| | author = P. Ojeda and H. Rosu
| |
| | journal = Internat. J. Theoret. Phys.
| |
| | volume = 45
| |
| | pages = 1191–1196
| |
| |date=Jun 2006
| |
| | doi = 10.1007/s10773-006-9123-2
| |
| | publisher = Springer
| |
| |arxiv = gr-qc/0510004 |bibcode = 2006IJTP...45.1152R
| |
| | issue = 6 }}
| |
| </ref> The solution is an expanding universe; due to this expansion, the radiation and matter in the Universe cool down and become diluted. At first, the expansion is slowed down by [[gravitation]] attracting the [[radiation]] and matter in the Universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the Universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago. | |
| | |
| ===Particle physics in cosmology===
| |
| {{main|Particle physics in cosmology}}
| |
| [[Particle physics]] is important to the behavior of the early Universe, because the early Universe was so hot that the average energy density was very high. Because of this, [[scattering]] processes and [[particle decay|decay]] of unstable particles are important in cosmology.
| |
| | |
| As a rule of thumb, a scattering or a decay process is cosmologically important in a certain cosmological epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the Universe. The time scale that describes the expansion of the Universe is <math>1/H</math> with <math>H</math> being the [[Hubble constant]], which itself actually varies with time. The expansion timescale <math>1/H</math> is roughly equal to the age of the Universe at that time.
| |
| | |
| ===Timeline of the Big Bang===
| |
| {{main|Timeline of the Big Bang}}
| |
| Observations suggest that the Universe began around 13.8 billion years ago.<ref>{{cite web
| |
| |last =
| |
| |first =
| |
| |title = Cosmic Detectives
| |
| |url=http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
| |
| |authorlink =
| |
| |coauthors =
| |
| |work =
| |
| |publisher = The European Space Agency (ESA)
| |
| |date = 2013-04-02
| |
| |doi =
| |
| |accessdate = 2013-04-25}}
| |
| </ref> Since then, the evolution of the Universe has passed through three phases. The very early Universe, which is still poorly understood, was the split second in which the Universe was so hot that [[subatomic particle|particles]] had energies higher than those currently accessible in [[particle accelerator]]s on Earth. Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses.
| |
| Following this, in the early Universe, the evolution of the Universe proceeded according to known [[high energy physics]]. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the [[cosmic microwave background]] was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first [[star]]s and [[quasar]]s, and ultimately galaxies, [[clusters of galaxies]] and [[supercluster]]s formed. The future of the Universe is not yet firmly known, but according to the [[ΛCDM]] model it will continue expanding forever.
| |
| | |
| ==Areas of study==
| |
| Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of the Big Bang cosmology, which is presented in [[Timeline of the Big Bang]].
| |
| | |
| ===Very early Universe===
| |
| The early, hot Universe appears to be well explained by the Big Bang from roughly 10<sup>−33</sup> seconds onwards. But there are several [[Big Bang#Standard Problems|problems]]. One is that there is no compelling reason, using current particle physics, for the Universe to be [[shape of the universe|flat]], homogeneous, and [[isotropic]] (see the [[cosmological principle]]). Moreover, [[grand unified theory|grand unified theories]] of particle physics suggest that there should be [[magnetic monopole]]s in the Universe, which have not been found. These problems are resolved by a brief period of [[cosmic inflation]], which drives the Universe to [[Flatness (cosmology)|flatness]], smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles. The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and [[quantum field theory]]. Some cosmologists think that [[string theory]] and [[brane cosmology]] will provide an alternative to inflation.
| |
| | |
| Another major problem in cosmology is what caused the Universe to contain more particles than [[antimatter|antiparticles]]. Cosmologists can observationally deduce that the Universe is not split into regions of matter and antimatter. If it were, there would be [[X-ray]]s and [[gamma ray]]s produced as a result of [[annihilation]], but this is not observed. This problem is called the baryon asymmetry, and the theory to describe the resolution is called [[baryogenesis]]. The theory of baryogenesis was worked out by [[Andrei Sakharov]] in 1967, and requires a violation of the particle physics [[Symmetry#In physics|symmetry]], called [[CP-symmetry]], between matter and antimatter. However, particle accelerators measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists look for additional violations of the CP-symmetry in the early Universe that might account for the baryon asymmetry.
| |
| | |
| Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and [[particle accelerator|experiment]], rather than through observations of the Universe.
| |
| | |
| ===Big bang nucleosynthesis===
| |
| {{Main|Big bang nucleosynthesis}}
| |
| | |
| Big Bang nucleosynthesis is the theory of the formation of the elements in the early Universe. It finished when the Universe was about three minutes old and its [[temperature]] dropped below that at which [[nuclear fusion]] could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from [[hydrogen]] [[ion]]s ([[proton]]s), it principally produced [[deuterium]], [[helium|helium-4]], and [[lithium]]. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by [[George Gamow]], [[Ralph Asher Alpher]], and [[Robert Herman]]. It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early Universe. Specifically, it can be used to test the [[equivalence principle]], to probe [[dark matter]], and test [[neutrino]] physics. Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.
| |
| | |
| ===Cosmic microwave background=== | |
| {{Main|Cosmic microwave background}}
| |
| | |
| The cosmic microwave background is radiation left over from [[decoupling]] after the epoch of [[recombination (cosmology)|recombination]] when neutral [[atoms]] first formed. At this point, radiation produced in the Big Bang stopped [[Thomson scattering]] from charged ions. The radiation, first observed in 1965 by [[Arno Penzias]] and [[Robert Woodrow Wilson]], has a perfect thermal [[black body|black-body]] spectrum. It has a temperature of 2.7 [[kelvin]]s today and is isotropic to one part in 10<sup>5</sup>. [[Cosmological perturbation theory]], which describes the evolution of slight inhomogeneities in the early Universe, has allowed cosmologists to precisely calculate the angular [[power spectrum]] of the radiation, and it has been measured by the recent satellite experiments ([[Cosmic Background Explorer|COBE]] and [[WMAP]]) and many ground and balloon-based experiments (such as [[Degree Angular Scale Interferometer]], [[Cosmic Background Imager]], and [[BOOMERanG experiment|Boomerang]]). One of the goals of these efforts is to measure the basic parameters of the [[Lambda-CDM model]] with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The recent measurements made by WMAP, for example, have placed limits on the neutrino masses.
| |
| | |
| Newer experiments, such as [[QUIET]] and the [[Atacama Cosmology Telescope]], are trying to measure the [[polarization (waves)|polarization]] of the cosmic microwave background. These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies, such as the [[Sunyaev-Zel'dovich effect]] and [[Sachs-Wolfe effect]], which are caused by interaction between [[galaxy|galaxies]] and [[galaxy cluster|clusters]] with the cosmic microwave background.
| |
| | |
| ===Formation and evolution of large-scale structure===
| |
| {{Main|Large-scale structure of the cosmos|Structure formation|Galaxy formation and evolution}}
| |
| | |
| Understanding the formation and evolution of the largest and earliest structures (i.e., [[quasar]]s, [[galaxy|galaxies]], [[galaxy groups and clusters|clusters]] and [[supercluster]]s) is one of the largest efforts in cosmology. Cosmologists study a model of '''hierarchical structure formation''' in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling. One way to study structure in the Universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the Universe and measure the matter [[power spectrum]]. This is the approach of the [[Sloan Digital Sky Survey]] and the [[2dF Galaxy Redshift Survey]].
| |
| | |
| Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the Universe, as it clusters into [[Galaxy filament|filaments]], superclusters and [[void (astronomy)|voids]]. Most simulations contain only non-baryonic [[cold dark matter]], which should suffice to understand the Universe on the largest scales, as there is much more dark matter in the Universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.
| |
| | |
| Other, complementary observations to measure the distribution of matter in the distant universe and to probe [[reionization]] include:
| |
| *The [[Lyman-alpha forest]], which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early Universe, by measuring the absorption of light from distant quasars by the gas.
| |
| *The 21 centimeter [[Absorption (electromagnetic radiation)|absorption]] line of neutral atomic hydrogen also provides a sensitive test of cosmology
| |
| *[[Weak lensing]], the distortion of a distant image by [[gravitational lensing]] due to dark matter.
| |
| These will help cosmologists settle the question of when and how structure formed in the Universe.
| |
| | |
| ===Dark matter===
| |
| {{Main|Dark matter}}
| |
| | |
| Evidence from [[Big Bang nucleosynthesis]], the [[cosmic microwave background]] and structure formation suggests that about 23% of the mass of the Universe consists of non-baryonic dark matter, whereas only 4% consists of visible, [[baryonic matter]]. The gravitational effects of dark matter are well understood, as it behaves like a cold, [[Radioactive decay|non-radiative]] fluid that forms [[galactic halo|haloes]] around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable [[supersymmetry|supersymmetric]] particle, a [[weakly interacting massive particle]], an [[axion]], and a [[massive compact halo object]]. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations ([[MOND]]) or an effect from [[brane cosmology]].
| |
| | |
| ===Dark energy===
| |
| {{Main|Dark energy}}
| |
| | |
| If the Universe is [[Flatness (cosmology)|flat]], there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the Universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There is strong observational evidence for dark energy, as the total energy density of the Universe is known through constraints on the flatness of the Universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the Universe has begun to gradually accelerate.
| |
| | |
| Apart from its density and its clustering properties, nothing is known about dark energy. [[Quantum field theory]] predicts a [[cosmological constant]] (CC) much like dark energy, but 120 [[orders of magnitude]] larger than that observed. [[Steven Weinberg]] and a number of string theorists (see [[string landscape]]) have invoked the 'weak [[anthropic principle]]': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in a universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant which allows for life to exist) it does not attempt to explain the context of that universe. For example, the weak anthropic principle alone does not distinguish between:
| |
| * Only one universe will ever exist and there is some underlying principle that constrains the CC to the value we observe.
| |
| * Only one universe will ever exist and although there is no underlying principle fixing the CC, we got lucky.
| |
| * Lots of universes exist (simultaneously or serially) with a range of CC values, and of course ours is one of the life-supporting ones.
| |
| | |
| Other possible explanations for dark energy include [[quintessence (physics)|quintessence]] or a modification of gravity on the largest scales. The effect on cosmology of the dark energy that these models describe is given by the dark energy's [[equation of state (cosmology)|equation of state]], which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.
| |
| | |
| A better understanding of dark energy is likely to solve the problem of the [[ultimate fate of the Universe]]. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than [[superclusters]] from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a [[big rip]], or whether it will eventually reverse.
| |
| | |
| ===Other areas of inquiry===
| |
| Cosmologists also study:
| |
| *whether [[primordial black hole]]s were formed in our universe, and what happened to them.
| |
| *the [[GZK cutoff]] for high-energy cosmic rays, and whether it signals a failure of [[special relativity]] at high energies
| |
| *the [[equivalence principle]], whether or not Einstein's [[general relativity|general theory of relativity]] is the correct theory of [[gravitation]], and if the fundamental [[laws of physics]] are the same everywhere in the Universe.
| |
| | |
| ==See also==
| |
| {{portal|Physics}}
| |
| *[[String cosmology]]
| |
| *[[Physical ontology]]
| |
| *[[List of cosmologists]]
| |
| *[[Hubble's law]]
| |
| *[[Photon]]
| |
| | |
| ==References==
| |
| {{reflist|2}}
| |
| | |
| ==Further reading==
| |
| | |
| ===Popular===
| |
| * {{cite book | author = [[Brian Greene]] | title = [[The Fabric of the Cosmos]] | publisher = Penguin Books Ltd | year = 2005 | isbn = 0-14-101111-4 }}
| |
| * {{cite book | author = [[Alan Guth]] | title = The Inflationary Universe: The Quest for a New Theory of Cosmic Origins | publisher = Random House | year = 1997 | isbn = 0-224-04448-6 }}
| |
| * {{cite book | authorlink = Stephen Hawking | last = Hawking | first = Stephen W. | title = [[A Brief History of Time|A Brief History of Time: From the Big Bang to Black Holes]] | publisher = Bantam Books, Inc | year = 1988 | isbn = 0-553-38016-8 }}
| |
| * {{cite book | authorlink = Stephen Hawking | last = Hawking | first = Stephen W. | title = [[The Universe in a Nutshell]] | publisher = Bantam Books, Inc | year = 2001 | isbn = 0-553-80202-X }}
| |
| * {{cite book | author = [[Simon Singh]] | title = [[Big Bang: the origins of the universe]] | publisher = Fourth Estate | year = 2005 | isbn = 0-00-716221-9 }}
| |
| * {{cite book | author = [[Steven Weinberg]] | title = The First Three Minutes | publisher = Basic Books | year = 1993; 1978 | isbn = 0-465-02437-8 }}
| |
| | |
| ===Textbooks===<!-- This section is linked from [[Big Bang]] -->
| |
| *{{cite book | author=Cheng, Ta-Pei | title=Relativity, Gravitation and Cosmology: a Basic Introduction | location=Oxford and New York| publisher=Oxford University Press| year=2005 | isbn=0-19-852957-0}} Introductory cosmology and general relativity without the full tensor apparatus, deferred until the last part of the book.
| |
| * {{cite book | first = Scott | last = Dodelson | year = 2003 | title = Modern Cosmology | publisher = Academic Press | isbn = 0-12-219141-2 }} An introductory text, released slightly before the [[Wilkinson Microwave Anisotropy Probe|WMAP]] results.
| |
| * {{cite book | last = Grøn | first = Øyvind |authorlink=Øyvind Grøn| coauthors = Hervik, Sigbjørn | title = Einstein's General Theory of Relativity with Modern Applications in Cosmology | location = New York | publisher = Springer | year = 2007 | isbn = 978-0-387-69199-2}}
| |
| * {{cite book | last = Harrison | first = Edward | authorlink=Edward Robert Harrison | year = 2000 | title = Cosmology: the science of the universe | publisher = Cambridge University Press | isbn = 0-521-66148-X }} For undergraduates; mathematically gentle with a strong historical focus.
| |
| * {{cite book | first = Marc | last = Kutner | title= Astronomy: A Physical Perspective | publisher = Cambridge University Press | year = 2003 | isbn = 0-521-52927-1 }} An introductory astronomy text.
| |
| * {{cite book | first = Edward | last = Kolb | coauthors = Michael Turner | title = The Early Universe | publisher = Addison-Wesley | year = 1988 | isbn = 0-201-11604-9 }} The classic reference for researchers.
| |
| * {{cite book | first = Andrew | last = Liddle | title = An Introduction to Modern Cosmology | publisher = John Wiley | year = 2003 | isbn = 0-470-84835-9 }} Cosmology without general relativity.
| |
| * {{cite book | first = Andrew | last = Liddle | coauthors = David Lyth | title = Cosmological Inflation and Large-Scale Structure | publisher = Cambridge | year = 2000 | isbn = 0-521-57598-2 }} An introduction to cosmology with a thorough discussion of [[inflation]].
| |
| * {{cite book | first = Viatcheslav | last = Mukhanov | title = Physical Foundations of Cosmology | publisher = Cambridge University Press | year = 2005 | isbn = 0-521-56398-4 }}
| |
| * {{cite book | author = Padmanabhan, T. | title = Structure formation in the universe | publisher = Cambridge University Press | year = 1993 | isbn = 0-521-42486-0 }} Discusses the formation of large-scale structures in detail.
| |
| * {{cite book | first = John | last = Peacock | title = Cosmological Physics | publisher = Cambridge University Press | year = 1998 | isbn = 0-521-42270-1 }} An introduction including more on general relativity and quantum field theory than most.
| |
| * {{cite book | first = P. J. E. | last = Peebles | title = Principles of Physical Cosmology | publisher = Princeton University Press | year = 1993 | isbn = 0-691-01933-9 }} Strong historical focus.
| |
| * {{cite book | first = P. J. E. | last = Peebles | title = The Large-Scale Structure of the Universe | publisher = Princeton University Press | year = 1980 | isbn = 0-691-08240-5 }} The classic work on [[large-scale structure of the universe|large-scale structure]] and correlation functions.
| |
| * {{cite book | first = Martin | last = Rees | title = New Perspectives in Astrophysical Cosmology | publisher = Cambridge University Press | year = 2002 | isbn = 0-521-64544-1 }}
| |
| * {{cite book | first = Steven | last = Weinberg | title = Gravitation and Cosmology | publisher = John Wiley | year = 1971 | isbn = 0-471-92567-5 }} A standard reference for the mathematical formalism.
| |
| * {{cite book | first = Steven | last = Weinberg | title = Cosmology | publisher = Oxford University Press | year = 2008| isbn = 0-19-852682-2 }}
| |
| * Benjamin Gal-Or, “Cosmology, Physics and Philosophy”, Springer Verlag, 1981, 1983, 1987, ISBN 0-387-90581-2, ISBN 0-387-96526-2.
| |
| | |
| ==External links==
| |
| | |
| ===From groups===
| |
| * [http://www.damtp.cam.ac.uk/user/gr/public/cos_home.html Cambridge Cosmology]- from Cambridge University (public home page)
| |
| * [http://map.gsfc.nasa.gov/m_uni.html Cosmology 101] - from the [[NASA]] [[WMAP]] group
| |
| * [http://cfcp.uchicago.edu/ Center for Cosmological Physics]. [[University of Chicago]], [[Chicago|Chicago, Illinois]].
| |
| * [http://www.pbs.org/wgbh/nova/origins/ Origins, Nova Online] - Provided by ''[[Public Broadcasting Service|PBS]]''.
| |
| | |
| ===From individuals===
| |
| * [[Sean M. Carroll|Carroll, Sean]]. "''[http://preposterousuniverse.com/writings/cosmologyprimer/ Cosmology Primer]''". California Institute of Technology.
| |
| * Gale, George, "[http://plato.stanford.edu/entries/cosmology-30s/ Cosmology: Methodological Debates in the 1930s and 1940s]", ''The Stanford Encyclopedia of Philosophy'', Edward N. Zalta (ed.)
| |
| * Madore, Barry F., "''[http://nedwww.ipac.caltech.edu/level5/ Level 5] : A Knowledgebase for Extragalactic Astronomy and Cosmology''". Caltech and Carnegie. Pasadena, California, USA.
| |
| * Tyler, Pat, and Phil Newman "''[http://universe.gsfc.nasa.gov/ Beyond Einstein]''". Laboratory for High Energy Astrophysics (LHEA) [[NASA]] [[Goddard Space Flight Center]].
| |
| * [[Edward L. Wright|Wright, Ned]]. "''[http://www.astro.ucla.edu/~wright/cosmolog.htm Cosmology tutorial and FAQ]''". Division of Astronomy & Astrophysics, UCLA.
| |
| * {{cite news | author=[[George Musser]] | title=Four Keys to Cosmology
| |
| | url=http://www.sciam.com/article.cfm?chanID=sa006&articleID=0005DCFC-253F-1FFB-A53F83414B7F0000 | work=Scientific American
| |
| | publisher=Scientific American | date=January 2004 | accessdate=27 June 2008 }}
| |
| * {{cite news | author=[[Cliff Burgess]] | coauthors=[[Fernando Quevedo]] | title=The Great Cosmic Roller-Coaster Ride | url=
| |
| | format=print | work=[[Scientific American]] | publisher=Scientific American | pages=52–59 | date=November 2007 | quote=(subtitle) Could cosmic inflation be a sign that our universe is embedded in a far vaster realm? }}
| |
| | |
| {{Astronomy navbox|state=uncollapsed}}
| |
| | |
| {{Astronomy subfields|state=uncollapsed}}
| |
| | |
| {{DEFAULTSORT:Physical Cosmology}}
| |
| [[Category:Physics]]
| |
| [[Category:Physical cosmology| ]]
| |
| [[Category:Philosophy of physics]]
| |
| [[Category:Philosophy of time]]
| |
| [[Category:Astronomical sub-disciplines]]
| |
| [[Category:Astrophysics]]
| |
| | |
| [[no:Kosmologi#Fysisk kosmologi]]
| |