Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: ianhutt on 10/07/2008 21:55:42
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ian hutt asked the Naked Scientists:
Distant stars are usually measured in light years i.e. the distance that light travels in a year. However because of the expansion of the universe the star was nearer to us when the light started its journey than it is today.
Clearly the more distant the object the greater this difference will be. S do measurements of distant objects therefore take into account the expansion of the universe?
I am a long term listener via podcasts of science programmes (http://www.thenakedscientists.com/HTML/podcasts/) and regard yours and probably NPR Science Friday as by far the best available. I especially enjoy the question and answers!
Regards and best wishes
Ian Hutt (Amersham, Bucks - Pensioner (just))
What do you think?
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Ian - distances quoted are for how we see the object at the time. If, for instance, a galaxy is said to be 10 million light years away, then that is the distance it was 10 million years ago when the light we see from it now was just leaving it. Obviously, it is now 10 million years expansion time further away from us.
P.S. How is Amersham these days? I used to drink in the Iron Horse (which I believe is now closed).
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Compared with the margin of error in calculating galactic distances the increase due to expansion is negligible ...
Putting the expansion of the universe into context
It is important always to remember that it is the space itself that is expanding and not things that are moving quickly. On our scale this expansion is incredibly tiny.
Let me do some simple calculations
The Hubble constant or expansion coefficient of our universe is about 70km/sec per megaparsec in astronomers units.
Let us now put it into more familiar units and relate it to distances in our solar system. A parsec is about 3.26 light years or 206,265 times the distace of the earth from the sun.
So over a distance of 206,265,000,000 the distace of the earth from the sun the expansion velocity is 70,000,000 millimeters per second (remember there are a million millimetres in a kilometer)
So the expansion of space between the earth and the sun is 70,000,000/206,265,000,000 millimetres per second.
This is about .00034 millimetres per second in about 150 million Km
let us give this velocity a bit longer to show itself, say one year
there are 365x24x60x60 = 31,536,000 seconds in a year
so in a year the space equal to the distace from the earth to the sun the space expands by 31,536,000x 0.00034 millimetres 10,722 mm or about 10 metres.
But surely I hear you say if the earth got ten metres further away from the sun every year over many millions of years this would have a significant effect? the answer is no! because the expansion of space does not significantly push the earth away from the sun because the energy of the attraction of gravity between the earth and the sun is so much greater that it completely overwhelms this effect and no change takes place.
It is only on the vast scales of distances between clusters of remote galaxies that the effect of the expansion of space can be measured. To measure it in the laboratory would be totally impossible.
http://www.thenakedscientists.com/forum/index.php?topic=15650.msg183443#msg183443
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RD - It does become significant over very large distances. In my instance above of a galaxy 10 million light years away, that galaxy will be considerably further away by now as the space between it and us will have had 10 milion more years in which to expand.
For even more distant objects such as quasars (which are billions of light years away) it becomes even more significant. It is possible (in fact, highly likely) that many of the most distant objects we can see today will by now be so far away that their light couldn't reach us at all due to the expansion of space giving a recession speed in excess of c.
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The error margin in calculating the distance of a galaxy 10 million light years away will be greater than the increase due to expansion which has occurred during that period.
Expansion will have increased the distance to a galaxy 10 million light years by less than one percent in that period.
Distances of the order of "10 million light years" are not accurate to 1 part in a hundred.
Consider the error margin in the Hubble constant...
The exact value of the Hubble Constant is still not accurately known, but is generally believed to be between 50-100 km/sec/Mpc.
http://www.phys.unsw.edu.au/astro/wwwlabs/hdfSize/hdfSize_intro.html
But I take your point about us seeing astronomical objects where they were in the distant past because of the time their light has taken to reach Earth. e.g. some the stars we see do not presently exist: they've gone supernova.
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The error margin in calculating the distance of a galaxy 10 million light years away will be greater than the increase due to expansion which has occurred during that period.
Expansion will have increased the distance to a galaxy 10 million light years by less than one percent in that period.
Yeah, 10,000 light years is a stone's throw [:D]