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To: S. maltophilia who wrote (3171)	7/25/2012 12:59:53 PM
From: TimF	1 Recommendation Read Replies (1) \| Respond to of 3872

I think you mean "dark energy". Dark matter slows the expansion, and absent any "dark energy" might reverse it. If we have a "big rip" it would be because of dark energy forcing an ever increasing rate of expansion to the universe, and eventually even causing the parts (ever smaller and smaller parts) of even small tightly gravitationally bound objects to expand away from each other. I put dark energy in quotes above since no one really knows what it is. Not even to the extent of whether its actually some type of energy or some basic principle of our universe, a "cosmological constant". We don't directly observe dark matter either, but we can tell where it is concentrated by its effects on other things (changes in orbits, gravitational lensing of distant objects etc.), "dark energy" if its actually some sort of substance or energy, would be everywhere, but it may not be some thing (even broadly using "thing" to include energy not just physical objects), it might just be "the way the universe works" (for some unknown reason). One alternate explanation of the acceleration of the universe's expansion that could possibly (but not definitely) avoid any big rip scenarios is universetoday.com en.wikipedia.org An obvious candidate is the cosmological constant. With the supernova result it appears that it should be re-introduced to explain the data. Another possibility is that the cosmological constant is indeed zero, but that there is a particle field that through its decay acts like a cosmological constant. It has now come to be known as ?quintessence? (Ostriker and Steinhardt 2001). Independent of which explanation is correct we can designate the energy density of this ?dark energy?? as WL. Figure 4 shows the probability distribution between the matter density WM and WL as the supernova distances define them. It is obvious that models with WL=0 are excluded at the >95% level for all models with a positive matter content. This comes from the fact that an accelerating, negative-pressure component is required, which can not be achieved with matter or radiation. eso.org All of this stuff is really out there, beyond anything that anyone is really remotely sure about. We observe the galaxies around us receding at an increasing pace (or really just observe that most distant galaxies have a larger redshifts then expected and interpret that as meaning that they are moving away at an increasing rate, and label the cause of that "dark energy". Also there are some that still dispute the idea that the expansion is accelerating, but that viewpoint has fewer and fewer adherents as new evidence piles on the original 1998 observations that suggested acceleration (for example space.com )