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Well, some of this is to do with Einsteins general theory of relativity, especially the concept of the 'cosmological constant'. Einstein originally included it in his equations, but later removed it because he didn't like the way it could affect his equations. It has been coming back into favour recently, and you get very different results depending on what value you assign it.
There is a good summary at http://pancake.uchicago.edu/~carroll/encyc/ ,which is not too mathematical!
Finally think I found the answer.Actually pretty interesting also considering the theory of spontaneous generation of matter was dissmissed a century or two ago when scientists discovered that maggots were actually fly larvae,they don't just appear inside decaying carcases.Anyway here's the paragraph,but I'd recommend reading the whole thing.
One thing we know for certain, though, is that quintessence is a fundamentally fickle force. Early in the universe's history, quintessence was all-powerful, frantically driving inflation in the first moments after the Big Bang. For ages afterward, it lay quietly in space, struggling half-heartedly against gravity and allowing the universe to slowly expand. Today, however, quintessence seems to have returned to full-force, slapping gravity out of the way as it accelerates the universal expansion. As long as the density of quintessence remains constant - as long as the weight of nothing does not change - our universe will continue to expand, faster and faster, forever. Within 100 billion years, we would be able to see only a few other galaxies, perched on the rim of the observable universe. If, though, for any reason, quintessence changes again, this time to a negative value, the dark energy will yield to gravity and the universe will collapse.
Here's the link.
http://www.jyi.org/issues/currentIss...ures/cull.html
[quote above article here]
to find out if the universe is going to collapse in on itself, or expand out to temperatures of absolute zero or infinity, you have to know the exact amount of mass-energy in the universe. because the amount of matter in Einsteins equations determines the amount of space-time curvature, we must find out the average matter density of the universe in order to find out if there is enough matter and energy for gravitation to reverse the effects of the big bang: the big crunch. if the average density is less than 10^29 grams per cubic centimeter, then the universe will continue to expand forever. however, if the average significantly larger than this then there will be enough matter for the gravitational force of the universe to reverse the big bang.
at the moment, the big question mark is that:
there are different ways of getting the mass of the universe. you could go sun by sun through a galaxy and then getting averages for their masses and then getting an average over amny galaxys. and another is to measure how much time it takes the stars to move around the galaxy and then you can use Newtone equations to get the approx. mass of the galaxy. basically though, the figures obtained from these calculations are below the critical amount, but they all vary in their result.
the most likely reason?:
astromomers know that up to 90% of the mass of a galaxy is in the form of "missing mass" or "dark matter" ... because it is not luminos, but it has weight. (dust clouds, black holes, cold dwarf stars, asteroids, planets etcetra...)
but even with more approximations in place in the calculations to compensate for the "dark matter" the estimated mass of the galaxy is considerabely smaller than that predicted by Newtons equations.
the reason for this? tiny leprechauns scourging gold, of course.
so it looks like the universe is going to die an entrophic death, that is, it will expand limitlessy (increasing entrophy) until the temperature reaches almost absolute zero. to understand how entrophy death occurs you have to understand the three laws of thermodynamics, which govern all chemical and nuclear processes on earth and in (litterly) the stars.
British scientist and author C.P. Snow had an elegant way of remembering the three laws:
Quote:
1.you cannot win (that is, you cannot get something for nothing, because matter and energy are conserved)
2.you cannot break even (you cannot return to the same energy state, because there is always an increase in disorder <the universe _IS__EXPANDING_>; entrophy always increases <unless gravity wins>
3.you cannot get out of the game (because absolute zero is unatainable)
</rant>
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This is what I was referring to....Quote:
http://www.antionline.com/showthread...hreadid=223054
Anyways, I dont have a lot of time to reply to all this right now, but I will get more time to elaborate. Maybe the link will help to understand my question about the constant pull of gravity.