Universe
For other uses, see Universe (disambiguation).
The Universe is all of spacetime and everything that exists therein, including all planets, stars, galaxies, the contents of intergalactic space, the smallestsubatomic particles, and all matter andenergy.[1][2][3][4][5][6] Similar terms include the cosmos, the world, reality, and nature.
The estimated diameter of theobservable universe is about 93 billion light years or 28 billion parsecs.[7]Scientific observation of the Universe has led to inferences of its earlier stages. These observations suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe, which is calculated to have begun 13.798 ± 0.037 billion years ago.[8][9] Observations of supernovaehave shown that the Universe is expanding at an accelerating rate.[10]
There are many competing theories about the ultimate fate of the universe. Physicists remain unsure about what, if anything, preceded the Big Bang. Many refuse to speculate, doubting that any information from any such prior state could ever be accessible.[citation needed]There are various multiversehypotheses, in which some physicists have suggested that the Universe might be one among many, or even an infinite number, of universes that likewise exist.[11][12]
Historical observation
XDF image shows fully mature galaxies in the foreground plane – nearly mature galaxies from 5 to 9 billion years ago – protogalaxies, blazing with young stars, beyond 9 billion years.
Throughout recorded history, severalcosmologies and cosmogonies have been proposed to account for observations of the Universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers and Indian philosophers.[14][15] Over the centuries, more precise observations and improved theories of gravity led toCopernicus's heliocentric model and theNewtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxycomposed of billions of stars, the Milky Way, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much ofmodern cosmology. Discovery of thered shift and cosmic microwave background radiation suggested that the Universe is expanding and had a beginning.[16]
History
Main article: Chronology of the universe
According to the prevailing scientific model of the Universe, known as the Big Bang, the Universe expanded from an extremely hot, dense phase called thePlanck epoch, in which all the matter and energy of the observable universewas concentrated. Since the Planck epoch, the Universe has beenexpanding to its present form, possibly with a brief period (less than 10−32seconds) of cosmic inflation. Several independent experimental measurements support this theoreticalexpansion and, more generally, the Big Bang theory. The universe is composed of ordinary matter (4.9%) including atoms, stars, and galaxies, dark matter(26.8%) which is a hypothetical particle that has not yet been detected, and dark energy (68.3%), which is a kind of energy density that seemingly exists even in completely empty space.[17]Recent observations indicate that this expansion is accelerating because of dark energy, and that most of the matter in the Universe may be in a form which cannot be detected by present instruments, called dark matter.[18] The common use of the "dark matter" and "dark energy" placeholder names for the unknown entities (purported to account for about 95% of the mass-energy density of the Universe) demonstrates the present observational and conceptual shortcomings and uncertainties concerning the nature andultimate fate of the Universe.[19]
On 21 March 2013, the European research team behind the Planck cosmology probe released the mission's all-sky map of the cosmic microwave background.[20][21][22][23][24]The map suggests the universe is slightly older than thought. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about 370,000 years old. The imprint reflects ripples that arose as early, in the existence of the universe, as the first nonillionth (10−30) of a second. Apparently, these ripples gave rise to the present vast cosmic web of galaxy clusters and dark matter. According to the team, the universe is 13.798 ± 0.037 billion years old,[9][25] and contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy. Also, the Hubble constant was measured to be 67.80 ± 0.77 (km/s)/Mpc.[20][21][22][24][25]
An earlier interpretation of astronomical observations indicated that the age of the Universe was 13.772 ± 0.059 billionyears,[26] and that the diameter of theobservable universe is at least 93 billionlight years or 8.80×1026 meters.[27]According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the Universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the Universe is finite or infinite.
Etymology, synonyms and definitions
The word Universe derives from the Old French word Univers, which in turn derives from the Latin worduniversum.[28] The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.[29] The Latin word derives from the poetic contraction Unvorsum — first used byLucretius in Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining form of unus, or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").[29]
An alternative interpretation ofunvorsum is "everything rotated as one" or "everything rotated by one". In this sense, it may be considered a translation of an earlier Greek word for the Universe, περιφορά, (periforá, "circumambulation"), originally used to describe a course of a meal, the food being carried around the circle of dinner guests.[30] This Greek word refers tocelestial spheres, an early Greek model of the Universe. Regarding Plato'sMetaphor of the sun, Aristotle suggests that the rotation of the sphere of fixed stars inspired by the prime mover, motivates, in turn, terrestrial change via the Sun. Careful astronomical and physical measurements (such as theFoucault pendulum) are required to prove the Earth rotates on its axis.
A term for "Universe" in ancient Greece was τὸ πᾶν (tò pán, The All, Pan (mythology)). Related terms were matter, (τὸ ὅλον, tò ólon, see also Hyle, lit. wood) and place (τὸ κενόν, tò kenón).[31][32] Other synonyms for the Universe among the ancient Greek philosophers included κόσμος(cosmos) and φύσις (meaning Nature, from which we derive the wordphysics).[33] The same synonyms are found in Latin authors (totum, mundus,natura)[34] and survive in modern languages, e.g., the German words Das All, Weltall, and Natur for Universe. The same synonyms are found in English, such as everything (as in the theory of everything), the cosmos (as incosmology), the world (as in the many-worlds interpretation), and Nature (as innatural laws or natural philosophy).[35]
Broadest definition: reality and probability
The broadest definition of the Universe is found in De divisione naturae by themedieval philosopher and theologianJohannes Scotus Eriugena, who defined it as simply everything: everything that is created and everything that is not created.
Definition as reality
More customarily, the Universe is defined as everything that exists, (has existed, and will exist).[36] According to our current understanding, the Universe consists of three principles: spacetime, forms of energy, including momentumand matter, and the physical laws that relate them.
Definition as connected space-time
See also: Eternal inflation
It is possible to conceive of disconnected space-times, each existing but unable to interact with one another. An easily visualized metaphor is a group of separate soap bubbles, in which observers living on one soap bubble cannot interact with those on other soap bubbles, even in principle. According to one common terminology, each "soap bubble" of space-time is denoted as a universe, whereas our particular space-time is denoted as the Universe, just as we call our moon theMoon. The entire collection of these separate space-times is denoted as themultiverse.[37] In principle, the other unconnected universes may have different dimensionalities andtopologies of space-time, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are purely speculative.
Definition as observable reality
See also: Observable universe andObservational cosmology
According to a still-more-restrictive definition, the Universe is everything within our connected space-time that could have a chance to interact with us and vice versa.[38] According to thegeneral theory of relativity, some regions of space may never interact with ours even in the lifetime of the Universe due to the finite speed of lightand the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the Universe would live forever: space may expand faster than light can traverse it.
Distant regions of space are taken to exist and be part of reality as much as we are, yet we can never interact with them. The spatial region within which we can affect and be affected is theobservable universe. The observable Universe depends on the location of the observer. By traveling, an observer can come into contact with a greater region of space-time than an observer who remains still. Nevertheless, even the most rapid traveler will not be able to interact with all of space. Typically, the observable Universe is taken to mean the Universe observable from our vantage point in the Milky Way Galaxy.
Size, age, contents, structure, and laws
Historical models
Theoretical models
Shape of the Universe
Main article: Shape of the Universe
The shape or geometry of the Universe includes both local geometry in theobservable Universe and global geometry, which we may or may not be able to measure. Shape can refer to curvature and topology. More formally, the subject in practice investigates which 3-manifold corresponds to the spatial section in comoving coordinatesof the four-dimensional space-time of the Universe. Cosmologists normally work with a given space-like slice of spacetime called the comoving coordinates. In terms of observation, the section of spacetime that can be observed is the backward light cone(points within the cosmic light horizon, given time to reach a given observer). If the observable Universe is smaller than the entire Universe (in some models it is many orders of magnitude smaller), one cannot determine the global structure by observation: one is limited to a small patch.
Among the Friedmann–Lemaître–Robertson–Walker (FLRW) models, the presently most popular shape of the Universe found to fit observational data according to cosmologists is the infinite flat model,[98] while other FLRW models include the Poincaré dodecahedral space[99][100] and the Picard horn.[101]The data fit by these FLRW models of space especially include the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck maps of cosmic background radiation. NASA released the first WMAP cosmic background radiation data in February 2003, while a higher resolution map regarding Planck data was released by ESA in March 2013. Both probes have found almost perfect agreement with inflationary models and the standard model of cosmology, describing a flat, homogeneous universe dominated by dark matter and dark energy.[9][102]
No comments:
Post a Comment