synonymObservable universe (observable universe) generally refers to the Hubble volume
Hubble volume (also known as the observable universe, English:ObserverThe sphere space as the center is small enough for the observer to observe the objects within the range, that is, the light emitted by the object has enough time to reach the observer.Hubble volume radius is about46 billionLight years.
In other words, the observable size of the universe is also called the Hubble volume.
Theoretically, since the universe was born in the big bang 10 to 20 billion years ago, space expanded from the "origin of the universe" at the speed of lightray radiationIt spreads in the form of a sphere, so the radius scale of the universe should be 10-20 billion light years.The longest distance that can be observed since the Big Bangray radiationThe longest distance traveled is about 12 to 15 billion light years, that is, 26 to 27 square meters of 10. (Note: this is a dynamic concept, which is constantly expanding every second) Take this distance as the radius (that is:Hubble radius, note nothubble space telescope The observation radius of.)The sphere of is just the size of the observable 'event horizon', or simply, the size of the universe. If you know the radius, you can certainly calculate the volume, so its volume is also called the Hubble volume or Hubble space.Please note that as long as the speed of light is limited（red shiftThe "observation sphere" (the observed universe) is always smaller than the "horizon" (the observable universe, that is, Hubble Space)
Hubble volume is an astronomical term. The following explanations all come fromWikipediaEnglish version.There are a lot of astronomy in the articleProper nounAnd multiplecosmological models Don't repeat and answer them one by one.stayBig Bang The observable universe includes all galaxies and other materials that can be observed on the earth today.This is because after the expansion of the universe began, light and other signals must take a long time to be accepted by us.If the universe is isotropic (the same in all directions), then the boundary of the universe is generally the same in all directions - meaning that the observable universe isObserverbySpherical centerThe sphere of.Regardless of the actual shape of the universe, each point of the universe has its own observable universe, which may or may not coincide with the observable universe on the earth.The meaning of the term "observable" means that it is not dependent on modern technologyDetection capabilityIt only represents theoretically the possibility of light or other signals from objects to observers.In fact, we can only observe（Big BangPhotons at the Time of Recombinationdecoupling (Photon escape) At that time, particles can emit photons that are not reabsorbed by other particles for the first time.Before that, the universe was a pair of photonsOpaqueOfPlasma。At this moment, there is just enough distance between particles, so photons can be emitted from the "final scattering surface" and can be accepted by us today.And it forms the cosmic microwave background radiation that we can receive.However, if we can observe the "neutron background" or deeper“Gravitational wave”, then we may get a farther distance than the current observable universe, and even include the information of the time of the Big Bang.The CMBR co motion distance represents the radius of the universe, which is calculated to be 14 billion seconds (about 45.7 billion light years), while the boundary of the observable universe is calculated to be 14.3 billion seconds (about 46.6 billion light years), which is nearly 2% larger.The age of the universe is 13.75 billion years. However, due to the expansion of the universe, we observed some celestial bodies that were very close at the beginning but were thought to be far more distant than 13.75 billion light years (according to the inherent distance of the universe, they are equivalent at the same time and the co motion distance).The diameter of the observable universe is about 28 billion seconds, about 93 billion light years, and the radius of the observable universe is about 460 to 47 billion light years away.Normally, people often regard 13.7 billion light years as the size of the universe. People take it for granted that since there is no matter faster than light in the universe, it is reasonable to regard 13.7 billion light years as the size of the universe.However, this fails to take into account that the universe is not smooth, static, and consistent withMinkowski spacetimeOf special relativity.In fact, space and time in the universe are bent due to expansion. As revealed by Hubble's law, the speed of light times the universetime intervalIn fact, there is no realPhysical meaning。
The universe and Hubble volume
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stayBig BangLater, some parts of the universe may be too far away from the Earth, which has led to the fact that its light has not reached the Earth so far, so this part of the universe may now also be outside the observable universe.In the future, far awayGalaxyThe light emitted will have a longer timeTravel through time and spaceTherefore, part of the universe that we cannot observe now will be observed in the future.However, due toHubble's lawSome regions of the universe far enough away from us will be far away from us at a speed exceeding the speed of light（Special relativityIt prevents adjacent objects from moving at the speed of light relative to another object in the same local area, but there is no such restriction for distant objects in expanding space-time), andDark energyIt leads to the accelerated expansion of the universe. Assuming that the dark energy remains unchanged, thenCosmic expansionIf the speed continues to increase, there will be a "future visible limit". Objects beyond this limit will never be observed, because the light emitted by this object is beyond this limit.(A subtle situation is that due toHubble parameterAs time goes by, there will be a scene where a galaxy that is just a little faster than light and far away from us may just be able to transmit light to us).The calculation of "future visible limit" gives a total moving distance of 19 billionSecond gap(About 62 billion light years), which means that the number of galaxies we can see in the endless future can only be 2.36 times more than those we see now.(except for some reasonsred shiftAnd cannot be observed, as described in the following paragraph).Although in principle, more galaxies will be observed in the future, in fact, more galaxies will redshift too much due to continuous expansion so that they will appear to disappear in the field of vision and cannot be observed.One of the subtleties is that if we can identify a galaxy in the observable universe at a given co motion distance, we must require that it can emit light that we can receive in the past.(For example, an early galaxy formed 500 million years after the Big Bang), but due to the expansion of the universe, the signals sent by the galaxy in the next possible historical period cannot reach us and be received in the infinite future.Although it is still at the same co motion distance, it is shorter than the co motion distance of the observable universe.This phenomenon can be used to define a universe whose distance changes with timeEvent horizonFor example, the current horizon is only 16 billion light years, which means that an event occurring within 16 billion light years can be accepted by us in the future, but events beyond 16 billion light years will never be known if they occur now.Whether popular or professionalcosmologyField research literature often uses "universe" instead of "observable universe", which has a fundamental and sufficient reason: we may never passDirect experimentHow to get information about parts of the universe that we can't contact.Although quite a number of feasible theories require an observable universe many times larger than the observable universe.There is no evidence to explain that the boundary of the observable universe is the factual boundary of the whole universe, and there is no mainstreamcosmological models First, suppose that the universe has a boundary of recognized physical form. Although some models predict that the universe is finite but unbounded, as shown inthree-dimensional spaceIt's a two-dimensional sphere with a finite area but no boundary.It seems likely that galaxies in the observable universe represent only a tiny part of galaxies in the universe.according toCosmic expansion theoryAnd his founder Alan Guth, if we assume that expansion occurs 10 seconds after the birth of the universe, then according to this seemingly reasonable assumption, the size of the universe in this period is as big as the speed of light times its age.This means that the size of the entire universe may be 10 times larger than the observable universe.If the universe is finite but unbounded, it means that the universe is smaller than the size of the observable universe.In this case, the galaxies that seem far away from us may be the illusion of neighboring galaxies, which are caused by the light circling the universe.This hypothesis is very difficult to detect because galaxies are different at different ages, or even completely different.A 2004 document believed that the size of the universe is only 78 billion light years, which may be the smallest cosmic scale, even lower than the size of the observable universe.This result is due to theWMAP(A man-madeAstronomical satellite）The results of data processing in this area are fiercely debated.
The size of the Hubble volume
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Generally, people think that the diameter of the universe is 29 billion seconds, about 93 billion light years. Assuming that the universe is smooth, about 4.1 × 10 cubic light years.staycosmologyIn time, these data are the current distance, not the distance at the time of light emission.For example, cosmic microwaveBackground radiationIssueddecouplingPhotons were emitted 380000 years after the Big Bang, about 13.7 billion years ago. These radiations were emitted by some materials that later formed most galaxies, and these galaxies are now 46 billion light-years away from us.In order to estimate the distance of light rays to us at the time of their emission.(The following detailed derivation is brief) Through the redshift and cosmological formula, although they are 46 billion light years away from us now, they were only 42 million light years away from us at that time.(Additional content: The reason why astronomers can conclude that the radius of the universe is 47 billion light years is due to the expansion of space-time. The universe is expanding, and this happens in every corner of the universe. Like an expanding balloon surface, the distance between any two points is increasing, so the distance between substances is increasing, and at different timesIn the meantime, the speed of change is not the same. This is because as the distance increases, the distance between the two becomes faster and faster.Although it takes 13.7 billion years for light to spread to us, the light source that emits light at the same time has gone farther away from us.If we want to calculate the distance between the light source and us at this time, we need some astronomical formulas and integral calculations. A simple approximation can be used to think that the distance D=3ct for distant objects, so that we can roughly get the result that D=13.7 billion years * 3 * c is about 50 billion light years.)
