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Fr. Wayne
2006-Jan-12, 09:51 PM
In the New York Times article http://www.nytimes.com/2006/01/12/science/12cosmos.html?ex=1137733200&en=6fad4e1d8bc4e78b&ei=5059&partner=AOL There was this phrase:
"more than five billion years ago, before dark energy became a dominant force in the universe."
:question: Can anyone out there help me find a source for this 5 billion year old theory?

Bob
2006-Jan-12, 10:13 PM
http://en.wikipedia.org/wiki/Cosmic_expansion

RussT
2006-Jan-13, 12:05 AM
I believe this is what they are refering to...

Quote 'Chandra Furthers Understanding About Dark Energy
May 18, 2004 - A mysterious force, which astronomers call "Dark Energy", seems to be speeding up the expansion of the Universe. New observations from the Chandra X-Ray Observatory have independently confirmed this expansion by measuring the distances to galaxy clusters. It seems that the expansion of the Universe was slowing down after the Big Bang until 6 billion years ago; at that point the force of this dark energy took over and expansion began to speed up. The big mystery still remains... what is dark energy? ' end quote.

Fr. Wayne
2006-Jan-13, 01:26 AM
http://en.wikipedia.org/wiki/Cosmic_expansion
"Under this scenario, dark energy would ultimately tear apart all gravitationally bound structures, including galaxies and solar systems, and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a Big Rip. On the other hand, dark energy might dissipate with time, or even become attractive. Such uncertainties leave open the possibility that gravity might yet rule the day and lead to a universe that contracts in on itself in a "Big Crunch". "

Thanks for the fast reply- very enlightening. :cool:

Dragon Star
2006-Jan-13, 02:47 AM
Something is bugging me about dark energy...

Why did is suddenly kick in 6 billion years ago?

Did it change it's charge or something of the sort?

Ken G
2006-Jan-13, 04:28 AM
It didn't kick in, it was always there. But dark energy, if it is a cosmological constant, yields a constant antigravity, while attractive gravity weakens as galaxy clusters get farther and farther away from each other. Thus at some point, about 6 billion years ago, the constant antigravity began to gradually surpass the attractive gravity. That's all it is, it's like if you had a constant income from your job but you were gradually paying off your student loans, eventually you'd start getting more and more into the black.

Tim Thompson
2006-Jan-13, 04:36 AM
Why did is suddenly kick in 6 billion years ago?
It didn't "suddenly kick in". In fact, it did not change at all, but remained the same as it had always been & still is. Assuming that it is due to the cosmological constant, then it is & always has been constant. However, in an expanding universe the distance between galaxy clusters is not constant, it is constantly increasing. So the inverse-square-law attraction of gravity between galaxy clusters is constantly decreasing. In the early universe that force is too strong for the expansion pressure of the cosmological constant, and the universe slows as it expands. But it slows increasingly slowly, as the gravitational attraction between galaxy clusters (that's the slowing force) steadily decreases. Eventually a threshold is reached. The galaxy clusters get far enough from each other that the slowing force of gravity is no longer strong enough, and the constant outward force of the "dark energy" is then strong enough to overcome the braking of gravity, and the expansion stops decelerating and starts acelerating.

That story, which is the most common, is valid if & only if the "dark energy" is due to a cosmological constant. If some other force is at work, all bets are off. The "big rip" will not happen, I think, if the accelerating expansion is due to a cosmological constant, because it is too weak ever to do that. But if some other force is ar work, then whether or not there is a "big rip" depends entirely on the (currently unknown) functional form of that force.

Fr. Wayne
2006-Jan-15, 12:37 AM
Admittedly I need to brush up on my language, but it appears to me that when dark energy forces overcame gravity 5-6 billion years ago (bya) and our Solar system began about 4.5 bya, then theoretically the conditions present when the universe reached this dark-over-gravity effects could be deemed as necessary for either the stability or entropy which made our solar system viable. Is it possible that only Population I stars had all the conditions needed for life? Could this deflation of universal gravity put a limit of 5-6 bya on conditions necessary for life and a further decrease on Drake's equation for number of planets in our galaxy capable of spawning life naturally?

Tim Thompson
2006-Jan-15, 06:12 PM
I don't think it has anything to do with the development of life. The expansion, even the accelerated expansion, does not affect gravitationally bound systems. Since life develops in gravitationally bound systems (or so we assume), then the expansion does not directly affect it. But the expansion does determine cosmic history, which is the background on which life deveops, so it surely is relevant, at least indirectly.

How the development of life depends on the type of star is a much different question. Massive hot stars don't last long (relatively speaking), while low mass cool stars last a long time, but don't get very warm. So you have to find the zone of habitability (http://www.solstation.com/habitable.htm), to find out how comfortable the star will be for life (see also Planet Orbits & Zone of Habitability (http://www.grandunification.com/hypertext/Planet_Orbits.html).

Fr. Wayne
2006-Jan-15, 06:28 PM
"Don't try this at home - We're what you call "professionals" - MythBusters"

I sincerely appreciate how well you keep track of many people's inquiries and respond with both accuracy and calmness. You are a key asset to BAUT's successful peak performance since last September. Grazie!

Dragon Star
2006-Jan-15, 06:28 PM
Ok, thanks Ken and Tim, makes a lot more sense now.;)

RussT
2006-Jan-16, 12:45 AM
It didn't "suddenly kick in". In fact, it did not change at all, but remained the same as it had always been & still is. Assuming that it is due to the cosmological constant, then it is & always has been constant. However, in an expanding universe the distance between galaxy clusters is not constant, it is constantly increasing. So the inverse-square-law attraction of gravity between galaxy clusters is constantly decreasing. In the early universe that force is too strong for the expansion pressure of the cosmological constant, and the universe slows as it expands. But it slows increasingly slowly, as the gravitational attraction between galaxy clusters (that's the slowing force) steadily decreases. Eventually a threshold is reached. The galaxy clusters get far enough from each other that the slowing force of gravity is no longer strong enough, and the constant outward force of the "dark energy" is then strong enough to overcome the braking of gravity, and the expansion stops decelerating and starts acelerating.

Tim, this an excellent explanation that I have not seen before, however, it has the same flaw IMHO, as many of the Big Bang explanations have..."Assumption".
It just 'assumes' that the Comological Constant is there from the begining, without Identifying what it is or where it came from, and then just "invokes" it when the time is right.

Ken G
2006-Jan-16, 02:43 AM
It just 'assumes' that the Comological Constant is there from the begining, without Identifying what it is or where it came from, and then just "invokes" it when the time is right.
That's a little like saying the flaw in the theory of gravity is that it just "assumes" that things fall. The cosmological constant will become part of our theory of gravity when and if its presence is clearly observed. We are pretty close to that point now, but not there yet.

RussT
2006-Jan-16, 09:51 PM
That's a little like saying the flaw in the theory of gravity is that it just "assumes" that things fall. The cosmological constant will become part of our theory of gravity when and if its presence is clearly observed. We are pretty close to that point now, but not there yet.

Ken, I see what you mean when you say it like that, however, we do "know"
enough about that kind of gravity to understand why something falls...Mass.

When we are talking about the Cosmological Constant, we are talking Dark Energy, which in this case is actually Anti-Gravity. We don't even know how to classify the "Extra Gravity" that is inherent in Dark Matter/Dark Energy, let alone Anti-Gravity!

The problem with all of this as I see it, is the only way we know how to identify the "Extra Gravity" is by talking about it from a "Matter/Mass/Energy" perspective.

Fortunate
2006-Jan-16, 11:18 PM
Ken, I see what you mean when you say it like that, however, we do "know"
enough about that kind of gravity to understand why something falls...Mass.

When we are talking about the Cosmological Constant, we are talking Dark Energy, which in this case is actually Anti-Gravity. We don't even know how to classify the "Extra Gravity" that is inherent in Dark Matter/Dark Energy, let alone Anti-Gravity!

The problem with all of this as I see it, is the only way we know how to identify the "Extra Gravity" is by talking about it from a "Matter/Mass/Energy" perspective.

Russ, I feel that there are two different lines of investigation here. One is to find the form of the equations that correctly describe the "motion" of the expansion. I believe that the term "cosmological constant" is used here merely to express the possibilty that the additional factor in the expansion may be constant rather than varying - we still don't know which it is.

The second problem is to explain why the acceleration is taking place. From my meager understanding, I don't think we have made much progress on the "why" part. I think we are still trying to describe the details of the time-dependent "trajectories."

I realize that I have somewhat abused the terms "motion" and "trajectories."