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Garth is offline
Dec17-05, 03:17 AM
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Quote Quote by pivoxa15
The exstistence of dark matter promotes the cosmoloigcal constant again.
How does that make sense?
No - it is Dark Energy that may be identified as the cosmological constant.
Static universe: need cosmological constant
Expanding universe with dark matter: need cosmological constant
Or is it the case of adding the constant and the latter case minusing the constant
It is the accelerating universe that needs a cosmological constant.

Counter-intuitively adding pressure to the universe increases the deceleration of its expansion rate. This is because pressure adds a form of energy and energy is equivalent to mass that adds gravitation. It is the self gravitation of the mass-energy within the universe that should cause its expansion to decelerate.

The cosmological constant may be interpreted as a form of negative pressure, which Einstein originally used to counter this self-attraction. He wanted to prevent the universe either expanding or collapsing to yield a static universe, the CC balanced the gravitational forces on a large scale within the universe.

When observations of distant Type Ia super novae were interpreted to indicate that the unverse was actually accelerating, not decelerating as previously expected, then a negative pressure was invoked to deliver this.
The negative pressure is a property of something called Dark Energy, and about 73% of the universe's mass is required to be in this form to make the standard LCDM model work. It is anybody's guess as to what Dark Energy actually is and something can be learned from the equation of state it must have to 'save the appearances' of that model.

The simplest suggestion and the one with an equation of state that seems to work ([itex]p = -\rho c^2[/itex]) is the Cosmological Constant.

I hope this helps,