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An Atlas of the Universe

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Comet
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« Reply #15 on: May 19, 2012, 08:29:05 pm »

(3) Is the Earth expanding with the universe?

The Earth is not expanding and neither is the solar system, nor the Milky Way galaxy. These objects formed under the influence of gravity and stopped moving apart. Gravity also holds galaxies together into groups and clusters. It is mainly the groups and clusters of galaxies that are moving apart in the universe.
(4) What exists outside the universe?

Space was created in the Big Bang. Our universe has no edge or boundary - there is no outside of our universe (see question 1). It is possible that our universe is part of an infinity of universes (see question 5), but these universes do not necessarily need a space to exist in.
(5) What existed before the Big Bang?

Time was created in the Big Bang - we do not know if it existed before the Big Bang. This question is therefore hard to answer. Some theories suggest that our universe is part of an infinity of universes (called a multiverse) which are being continuously created. This is possible but very hard to prove.
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Comet
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« Reply #16 on: May 19, 2012, 08:29:33 pm »

(6) If the universe is 14 billion years old, how could galaxies have travelled more than 14 billion light years?

It is possible that our universe is infinite and has been filled with matter everywhere since the Big Bang (see question 2). But there is also nothing stopping the universe expanding faster than the speed of light. Although at any local point within the universe, nothing can travel faster than the speed of light, this is not true for the entire universe. There is no limit on how fast space can expand.

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« Reply #17 on: May 19, 2012, 08:29:57 pm »

We can imagine galaxies are like balls sitting on a rubber sheet which represents space. If we stretch the sheet, the balls move apart. Balls which are close together will only move apart slowly. Balls which are widely separated will seem to move apart very quickly.

People living on any one of the balls will see their own ball as stationary. They will see nearby balls moving away slowly and they will see distant balls moving away quickly. Very distant balls (beyond the horizon) can be moving away faster than the speed of light, but the people cannot see them - locally in their own part of the universe nothing is travelling faster than the speed of light.

http://www.atlasoftheuniverse.com/bigbang.html
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Comet
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« Reply #18 on: May 19, 2012, 08:30:28 pm »

The Distance Scale of the Universe

Because the universe is expanding, the question of the distance to a very distant galaxy is hard to answer. It all depends on your point of view.
The separation of galaxies





This is the problem of defining a distance in an expanding universe: Two galaxies are near to each other when the universe is only 1 billion years old. The first galaxy emits a pulse of light. The second galaxy does not receive the pulse until the universe is 14 billion years old. By this time, the galaxies are separated by about 26 billion light years; the pulse of light has been travelling for 13 billion years; and the view the people receive in the second galaxy is an image of the first galaxy when it was only 1 billion years old and when it was only about 2 billion light years away.

There are four different distance scales commonly found in cosmology:
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Comet
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« Reply #19 on: May 19, 2012, 08:30:46 pm »

1) Luminosity Distance - DL
    In an expanding universe, distant galaxies are much dimmer than you would normally expect because the photons of light become stretched and spread out over a wide area. This is why enormous telescopes are required to see very distant galaxies. The most distant galaxies visible with the Hubble Space Telescope are so dim that they appear as if they are about 350 billion light years away even though they are much closer.

    Luminosity Distance is not a realistic distance scale but it is useful for determining how faint very distant galaxies appear to us.
(2) Angular Diameter Distance - DA
    In an expanding universe, we see the galaxies near the edge of the visible universe when they were very young nearly 14 billion years ago because it has taken the light nearly 14 billion years to reach us. However, the galaxies were not only young but they were also at that time much closer to us.

    The faintest galaxies visible with the Hubble Space Telescope were only a few billion light years from us when they emitted their light. This means that very distant galaxies look much larger than you would normally expect as if they were only about 2 or 3 billion light years from us (although they are also very very faint - see Luminosity Distance).

    Angular Diameter Distance is a good indication (especially in a flat universe like ours) of how near the galaxy was to us when it emitted the light that we now see.
(3) Comoving Distance - DC
    The Comoving Distance is the distance scale that expands with the universe. It tells us where the galaxies are now even though our view of the distant universe is when it was much younger and smaller. On this scale the very edge of the visible universe is now about 47 billion light years from us although the most distant galaxies visible in the Hubble Space Telescope will now be about 32 billion light years from us.

    Comoving Distance is the opposite of the Angular Diameter Distance - it tells us where galaxies are now rather than where they were when they emitted the light that we now see.
(4) Light Travel Time Distance - DT
    The Light Travel Time Distance represents the time taken for the light from distant galaxies to reach us. This is what is meant when it is said that the visible universe has a radius of 14 billion light years - it is simply a statement that the universe is about 14 billion years old and the light from more distant sources has not had time to reach us.

    Light Travel Time Distance is as much a measure of time as a measure of distance. It is useful mainly because it tells us how old the view of the galaxy is that we are seeing.

For small distances (below about 2 billion light years) all four distance scales converge and become the same, so it is much easier to define distances to galaxies in the local universe around us.
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« Reply #20 on: May 19, 2012, 08:31:28 pm »

Below - all four distance scales plotted against redshift. Redshift is a measure of the stretching of light caused by the expansion of the universe - a galaxy with a large redshift is further away than a galaxy with a small redshift. The most distant galaxies visible with the Hubble Space telescope are at redshift 10, whereas the most distant protogalaxies in the universe are probably at about redshift 15. The edge of the visible universe is at redshift infinity. A typical portable telescope, by contrast, can not see very much beyond redshift 0.1 (about 1.3 billion light years).




Distance Scales in the Universe

The Luminosity Distance (DL) shows why distant galaxies are so hard to see - a very young and distant galaxy at redshift 15 would appear to be about 560 billion light years from us although the Angular Diameter Distance (DA) suggests that it was actually about 2.2 billion light years from us when it emitted the light that we now see. The Light Travel Time Distance (DT) tells us that the light from this galaxy has travelled for 13.6 billion years between the time that the light was emitted and today. The Comoving Distance (DC) tells us that this same galaxy today, if we could see it, would be about 35 billion light years from us.

If anyone wants a copy of the computer code (written in C) which I wrote to calculate all these distances, it is available here.


http://www.atlasoftheuniverse.com/redshift.html
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Comet
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« Reply #21 on: May 19, 2012, 08:31:48 pm »

http://www.atlasoftheuniverse.com/redshift.html
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« Reply #22 on: May 19, 2012, 08:32:20 pm »

The Size of the Universe

The visible universe appears to have a radius of 14 billion light years because the universe is about 14 billion years old. The light from more distant objects simply has not had time to reach us. For this reason everybody in the universe will find themselves at the middle of their own visible universe. The precise scale of the universe is complicated by the fact that the universe is expanding. Galaxies we see near the edge of the visible universe emitted their light when they were much closer to us, and they will now be much further away.

The true size of the universe is probably much larger than the visible universe. The geometry of the universe suggests that it may have an infinite size and that it will expand forever. Even if the universe is not infinite, our visible universe must be a minute speck in a much larger totality.
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