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Post by abacus9900 on Sept 20, 2010 18:49:35 GMT 1
What is the 'Hubble Constant?'
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Post by eamonnshute on Sept 20, 2010 19:02:35 GMT 1
It is a measure of the rate of expansion of the universe. It has a value of approximately 70 km/sec per Megaparsec (a parsec is 3.26 light years). That means that galaxies at a distance of 1 megaparsec are travelling away from us at about 70 km/sec (on average).
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Post by abacus9900 on Sept 20, 2010 19:14:52 GMT 1
Thanks eamonn, so the further away galaxies are, the faster they are travelling away from us?
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Post by eamonnshute on Sept 20, 2010 20:07:10 GMT 1
Yes, as long as we are not being too pedantic. It is more accurate to say that the space in between is expanding, but it is pretty much the same thing.
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Post by abacus9900 on Sept 20, 2010 20:39:49 GMT 1
Yes, as long as we are not being too pedantic. It is more accurate to say that the space in between is expanding, but it is pretty much the same thing. Ok, but I find it a bit puzzling. Why is it that space that is further away from us is expanding at a greater rate? Presumably, any observer at any other point in the universe will experience the same thing. |
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Post by eamonnshute on Sept 20, 2010 21:02:44 GMT 1
The expansion is the same everywhere. Think of a piece of elastic with a bead every metre. If we stretch it by 100% then the distance between any two adjacent beads will increase to 2m. But for two beads 10m apart the distance will increase to 20m.
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Post by abacus9900 on Sept 20, 2010 21:14:56 GMT 1
The expansion is the same everywhere. Think of a piece of elastic with a bead every metre. If we stretch it by 100% then the distance between any two adjacent beads will increase to 2m. But for two beads 10m apart the distance will increase to 20m.  Allow me to think about that for a bit. |
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Post by abacus9900 on Sept 20, 2010 21:51:51 GMT 1
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Post by StuartG on Sept 20, 2010 22:07:00 GMT 1
Wasn't there a theory [proposed by one scientist] that given enough time our solar system will be so far away from everything else, we will be on our own. [ie. beyond the light horizon], but if this were so then some newly formed systems would fill the void?
StuartG
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Post by speakertoanimals on Sept 21, 2010 13:37:42 GMT 1
Roughly, except:
It is really the space in between us that is expanding, not quite the same thing as all the galaxies are running away from us. Which is why we can have galaxies where the apparent speed is greater than lightspeed, without violating special relativity.
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Post by principled on Sept 21, 2010 19:18:48 GMT 1
So, the space expands between us. But what about gravity? Is the space between us and the sun increasing by the Hubble constant? If gravity does affect this expansion, then one would assume that expansion will not be the same everywhere in every direction. Is this so? Can we measure this distortion? And as gravitational force drops as distance increases, one would assume that different parts of the universe will move away from each other at different rates and this rate would rise as gravitational forces weaken. mmmm P |
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Post by speakertoanimals on Sept 21, 2010 20:43:25 GMT 1
So, the space expands between us. But what about gravity? Is the space between us and the sun increasing by the Hubble constant? If gravity does affect this expansion, then one would assume that expansion will not be the same everywhere in every direction. Is this so? Can we measure this distortion? And as gravitational force drops as distance increases, one would assume that different parts of the universe will move away from each other at different rates and this rate would rise as gravitational forces weaken. mmmm P Gravity -- there is no such thing, just curved spacetime. Expanding is another part of the same theory that says that gravity is curved spacetime. So, being curved is kind of the small-scale answer (what spacetime looks like near the sun or near the earth) whereas expanding is what spacetime looks like on much larger scales. Expansion effects the solar system, but since the effects increase with distance, the size of the effects on the scale of the solar system, and over the lifetime of the solar system, make such a small change to planetary orbits, that you might just as well ignore it. And we are talking really tiny here, one part in a septillion over the age of the solar system. The point is 'gravity' isn't really just gravity, but is actually curved, expanding spacetime. On small scales, matter is lumpy, so you get local 'gravitational attraction' effects (solar systems, galaxies, groups of galaxies), whereas on the larger scale, we just get the overall expansion. Rather than varying expansion, what we in effect have is gravitational attraction wins over expansion on small scales, and expansion wins in large scales. If the expansion had been much bigger, then gravitational attraction would never have had a chance, and everything would have been expanded apart before any clumps could form, to become galaxies and clusters of galaxies. And if the expansion is (and keeps on) accelerating, then finally galaxies, solar systems, planets, and finally even atoms and subatomic particles would be torn apart by the expansion, in the big rip. |
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Post by typobrane on Oct 5, 2010 16:03:03 GMT 1
The visible universe, that is the area of the universe that we can see is increasing. A measured line of sight to the CMB will be getting longer by 186,000 miles every second. In the distant past this meant that more and more of the stars etc. of the universe would become visible as time went on. But now because the recessional velocity of the most distant parts of our visible universe are in effect moving away from us faster than the speed of light the amount of visible material like stars, galaxies and quasars of the universe will become less.
The furthest thing on our visible horizon will always be the CMB and has always been so. The interesting thing here is “how will we see it disappear”? Ole Romer first demonstrated that light travelled at a finite speed by observing the movement of Io a moon that orbits Jupiter. The Earth orbits the Sun faster than that of Jupiter and as our obit takes us towards Jupiter, Io appears to orbit Jupiter faster and as our obit takes us away from Jupiter, Io appears to slow down. Either Io gets excited and moves faster as Earth approaches or else it is an effect of light traveling at a finite speed. This gives us a clue to what we shall see, the greater the distance the faster the recessional velocity the slower things will appear to be moving. When we look at distant objects that have a recessional velocity equal to the speed of light they will appear to have no movement. Then from that distance onwards back to the CMB any movements will appear in reverse order and although our telescopes are not capable of seeing such events yet, one day we will see distant quasars explode into behemoth primordial stars and these stars will the accelerate away from us and morph into the CMB.
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Post by principled on Oct 5, 2010 17:02:08 GMT 1
typobra
Thanks.
An interesting post
P
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Post by abacus9900 on Oct 5, 2010 18:34:08 GMT 1
typobra, is it not possible that the observable universe is actually smaller than other models due to the possibility that the light we observe has come from much closer galaxies and circumnavigated the universe more than once?
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Allow me to think about that for a bit.
