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Author Topic: How the expansion of the universe is determined  (Read 2967 times)

Offline engrByDayPianstByNight

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How the expansion of the universe is determined
« on: 13/08/2007 14:40:29 »
Hello,

    I posted this question a couple days ago in an older thread in the General Sciences section.

    From our observation of the light we know the universe is expanding (due to the so-called redshift in the observed wavelength). Presumably, the light that we've observed was originally emitted some billions of years ago, right? If that's the case, we can claim that billions of years ago, the universe was in an expansion mode. How then can we say it's still expanding today? Why could it not be in the contracting mode now?

    If, on the other hand, the light that we observed and used to determine the universe is expanding emitted only a few hundred years ago, that implies that the source of the light is only a few hundred lightyears away from Earth, which is quite a local area on the cosmological scale. Then we could only claim that this area of the universe is expanding currently. How then can we apply this claim to the universe as a whole?

     I'm sure I must have missed something here. Can someone fill me in on this? How was measuring the expansion of the universe actually carried out? Thanks.


 

Offline syhprum

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How the expansion of the universe is determined
« Reply #1 on: 13/08/2007 21:54:33 »
This is rather a puzzle as there is no independent way of determining the distance of remote objects other than by their red shift.
If the universe had moved from expansion to contraction we should see a red shift in the most remote objects, no shift in ones at an intermediate distance and a blue shift in closer ones.
Perhaps a measure of distance could be obtained by the brightness of supernovae but whether this could be used for the most remote I don't know.   

PS
I feel I may be talking nonsense!
« Last Edit: 13/08/2007 22:05:43 by syhprum »
 

Offline Soul Surfer

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How the expansion of the universe is determined
« Reply #2 on: 13/08/2007 22:46:06 »
The red shift was first observed by looking at nearer galaxies and intermediate distance galaxies before very remote ones were observed. It can be observed at distances from about 20 million light years to more than a billion light years closer than 20million light years the random motions of the galaxies themselves dominate the picture.

The observation of type 1a surpernovae does act as an independant check of the expansion of the universe and it tallies quite well with the red shift data over a lot of the distance range but there is a discrepancy that suggests that the universe is expanding faster now than it did over the first half of the period since the big bang.  this is causing a lot if interest among the theoreticians.
 

Offline engrByDayPianstByNight

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How the expansion of the universe is determined
« Reply #3 on: 14/08/2007 14:00:46 »
The red shift was first observed by looking at nearer galaxies and intermediate distance galaxies before very remote ones were observed. It can be observed at distances from about 20 million light years to more than a billion light years closer than 20million light years the random motions of the galaxies themselves dominate the picture.

The observation of type 1a surpernovae does act as an independent check of the expansion of the universe and it tallies quite well with the red shift data over a lot of the distance range but there is a discrepancy that suggests that the universe is expanding faster now than it did over the first half of the period since the big bang.  this is causing a lot if interest among the theoreticians.


So, just to make sure I understand you right, we measured the light wavelength emitted from a multitude of distances (from 20 million to 1 billion light years) and they all exhibited a redshift, and it was from these observations that we concluded, with some degree of confidence, that the universe has been expanding since the big bang? That sounds like a plausible explanation to me.

Also, Soul Surfer, could you explain a little on this Type 1A supernova? I'm not familiar with it, and why is it a good independent check of the expansion? Thanks.

 

Offline syhprum

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How the expansion of the universe is determined
« Reply #4 on: 14/08/2007 21:11:22 »
A good discussion of the complexities of this matter can be found on http://www.astrophysicsspectator.com/topics/cosmology/Type1aObservations.html.
also details of the mechanism of type 1A supernovae explosions
« Last Edit: 15/08/2007 05:48:51 by syhprum »
 

Offline Soul Surfer

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How the expansion of the universe is determined
« Reply #5 on: 14/08/2007 23:44:33 »
The link above does not seem to work for me so here's a brief description.

A white dwarf star is basically the degenerate helium core of an old medium sized main sequence star like the sun after it has blown of the last vestiges of its outer layers in the aymptotic red giant phase.  It has no source of fusion energy and is about the size of the earth but very dense but nothing like as densae as a neutron star.  it os just very slowly cooling down if left on its own

A type 1a supernova is when a white dwarf star that is part of a binary system gets extra mass dumped on it by its companion star getting into the red giant phase the mass of the white dwarf increases to a critical point where high order nuclear fusion starts in its core The star then basically blows itself apart as an immense fusion bomb.  These explosions occur at a very precise mass and in a very similar way and so should be of a uniform brightness 'they also have a very precise deacay pattern associated with the decay of a particular isotope of (i think) nickel with a known half life.   Because of there extreme brightness these supernovae can be seen in calaxies at extremely great distances because at the ir brightest the supernovae are much brighter than the galxaies that contain them.
« Last Edit: 14/08/2007 23:51:57 by Soul Surfer »
 

Offline engrByDayPianstByNight

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How the expansion of the universe is determined
« Reply #6 on: 15/08/2007 02:50:42 »
Perhaps syhprum meant

http://www.astrophysicsspectator.com/topics/cosmology/Type1aObservations.html

There's a "a" between 1 and Observations.

Thanks to you both for the explanation and reference.
 

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How the expansion of the universe is determined
« Reply #6 on: 15/08/2007 02:50:42 »

 

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