Tag Archives: Scale of the Universe

Welcome to an interactive Flash Animation of the Scale of the Universe! Please click PLAY and then move the slider zoom in or out. You can also use arrow keys if the scroll bar is too sensitive.

From the smallest possible unit of distance (known as the Planck Length) to the other reaches of space and the universe and everything inbetween, this interactive Flash Animation gives you an idea of the incredible scale of the universe. Fascinating for any biologist, chemist, physicist, atronomer, cosmologist, science student or simply anyone who marvels at our insignificance in the grand scale of things. Scale of the Universe animation created by Cary and Michael Huang (HTwins.net).

Scale of the Universe

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Size and Scale of the universe: The universe versus the observable universe
Some parts of the universe may simply be too far away for the light emitted from there at any moment since the Big Bang to have had enough time to reach Earth at present: they would currently lie outside the observable universe. In the future the light from distant galaxies will have had more time to travel: some regions not currently observable will become observable in the future. However, due to Hubble’s law, regions sufficiently distant from us are expanding away from us much faster than the speed of light, and the expansion rate appears to be accelerating due to dark energy. Assuming dark energy remains constant (an unchanging cosmological constant), so that the expansion rate of the universe continues to accelerate, there is a “future visibility limit” beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit can never reach points that are expanding away from us at less than the speed of light. A subtlety here is that because the Hubble parameter is decreasing with time, there can be cases where a galaxy that is receding from us just a bit faster than light does manage to emit a signal which reaches us eventually). This future visibility limit is calculated to be at a comoving distance of 19 billion parsecs (62 billion light years), which implies the number of galaxies that we can ever theoretically observe in the infinite future (leaving aside the issue that some may be impossible to observe in practice due to redshift, as discussed in the following paragraph) is only larger than the number currently observable by a factor of 2.36.

Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. Also, a galaxy at a given comoving distance is defined to lie within the “observable universe” if we can receive signals emitted by the galaxy at any age in its past history (say, a signal sent from the galaxy only 500 million years after the Big Bang), but because of the universe’s expansion, there may be some later age at which a signal sent from the same galaxy will never be able to reach us at any point in the infinite future (so for example we might never see what the galaxy looked like 10 billion years after the Big Bang), even though it remains at the same comoving distance (comoving distance is defined to be constant with time, unlike proper distance which is used to define recession velocity due to the expansion of space) which is less than the comoving radius of the observable universe. This fact can be used to define a type of cosmic event horizon whose distance from us changes over time; for example, the current distance to this horizon is about 16 billion light years, meaning that a signal from an event happening at present would eventually be able to reach us in the future if the event was less than 16 billion light years away, but the signal would never reach us if the event was more than 16 billion light years away.

Both popular and professional research articles in cosmology often use the term “universe” to mean “observable universe”. This can be justified on the grounds that we can never know anything by direct experimentation about any part of the universe that is causally disconnected from us, although many credible theories require a total universe much larger than the observable universe. No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place, though some models propose it could be finite but unbounded, like a higher-dimensional analogue of the 2D surface of a sphere which is finite in area but has no edge. It is plausible that the galaxies within our observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of cosmic inflation and its founder, Alan Guth, if it is assumed that inflation began about 10−37 seconds after the Big Bang, then with the plausible assumption that the size of the universe at this time was approximately equal to the speed of light times its age, that would suggest that at present the entire universe’s size is at least 1023 times larger than the size of the observable universe.

If the universe is finite but unbounded, it is also possible that the universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. A 2004 paper claims to establish a lower bound of 24 gigaparsecs (78 billion light-years) on the diameter of the whole universe, meaning the smallest possible diameter for the whole universe would be only slightly smaller than the observable universe (and this is only a lower bound, so the whole universe could be much larger, even infinite). This value is based on matching-circle analysis of the WMAP data; this approach has been disputed.

A Bayesian analysis of the seven-year WMAP data yields that the most likely topology of the universe is T2 × R1 and the size L of the compact direction is 1.9 times the distance to the last scattering surface (the Cosmic Microwave Background: 14.4 Gpc). Therefore L is roughly 90 billion light-years. (Source: Wikipedia)