Open Universe?

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SiriuslyDog

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I understand that most astronomers believe that the universe is open - that it will continue to expand forever (or unitl it is dead). If the expansion of the universe is due entirely to the Big Bang, won't gravity eventually stop the expansion and ultimately close the universe? Is there some other force continuing to push galaxies/the universe apart?
 
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MeteorWayne

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A two part answer.

The expansion rate is higher than gravity would have time to pull it back together.

In addition, the recent observations that suggest an increase in expansion (dark energy) only make the situation worse, so unless something in the observations or their interpretations change, the expansion rate will only accelerate.
 
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SiriuslyDog

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Thanks, but can you explain more about dark energy expansion?
Is it some sort of electromagnetism? What's the source?

I thought that force of gravity is infinite - that it works over infinite distances. So, however weak it is, it is still a contracting force that would apply to everything in the universe. So by my reckoning, unless there is some other force pushing things apart, gravity should win out in the end.
 
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MeteorWayne

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OK 2 separate things here, let me address the second forst. Yes the range of gravity is infinite, but it only supplies so much force. Since the Universe is expanding at such a high rate, it will conti8nue to expand faster than the force of gravity will slow it down.

No the first part; dark energy....who knows. No one knows at all what it is, there are a number of possibilities. Any electromagnetic effect would not be one of the possible explanations.

What has been found over about the last 5 or 10 years, is that the rate of expansion of the Universe is now speeding up, and has been doing so for about the past 5 billion years. Since there is no explanation, it is given the temporary name "dark energy" which means "a force that is causing the rate of expansion of the Universe to speed up that we don't know what it is". We just have the observations showing this effect which does not fit any form of gravity and normal physics. So it has this temporary name.
 
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SiriuslyDog

Guest
Thanks again. I appreciate the education, but of course I have more questions...

How do we know that the speed of expansion has increased?
Is it still all based on how much the light is red-shifted that is coming from distant galaxies?

And while you are at it, how do we know how far away the galaxies are that we are viewing?
It seems to me that any measurement would have to be pretty imprecise - but I am sure I'm ignorant of a lot of the work that's been done.
 
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MeteorWayne

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Again, I''ll answer in reverse order, the distances are judged based on redshift, calibrated with other methods such as Special variable stars that have a known brightness/variable period, and certain supernova that have a known brightness. For your first question, there are multiple lines of evidence. I generally don't like using wiki, but i this case it seems a good summary, read the second section "evidence for dark energy"
http://en.wikipedia.org/wiki/Dark_energy
 
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dragon04

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SiriuslyDog said:
Thanks again. I appreciate the education, but of course I have more questions...

How do we know that the speed of expansion has increased? /quote]


As MW said, there are certain types of supernovae that we us as a standard in terms of brightness. Because we know how bright that type of Supernova will be every time one explodes, we can then fond out how far away it is based on how far red-shifted it is.

The problem (and the answer to your question) came about when at greater distances, these Supernovae were more red-shifted than the math predicted. After various ways of checking, it was determined that the only way they could be more red-shifted was if the Universe was expanding at an accelerating rate.
 
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