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Observation: galaxies are rotating faster than their visible matter can reasonably explain — they should be flying apart. Explanation: there is some unseen kind of matter (dark matter) that holds galaxies together against their own rotation.This may help clarify things:
Observation: The universe's overall expansion is accelerating — technically, the second derivative of position (acceleration), which should be negative (arising from gravitation), is instead positive at great distances. Explanation: dark energy, a universal repulsive force that works differently than gravitation (having no inverse square law, for one thing).
p(t) = position, a function that provides locations of stars and galaxies with respect to time.In this simple Calculus scheme, velocity (p'(t)) is the time integral of acceleration (p''(t)), and position (p(t)) is the time integral of velocity (p'(t)). This scheme is called a "differential equation", something at the heart of mathematical physics.
p'(t) = first derivative of position, or velocity.
p''(t) = second derivative of position, or acceleration.
p''(t) —> p'(t) —> p(t) Acceleration Velocity PositionIn this scheme, gravitation and dark energy (with opposite signs) give rise to acceleration, acceleration changes velocity, and velocity changes position. All these values are signed numbers — they can be positive or negative. A ball accelerates while being thrown (positive sign), to later decelerate as a result of gravitation (negative sign). ... will have a cooling effect on the universe ... Not if matter is being pushed out against gravitation by an energy source. This would prevent any overall cooling. The cooling one would expect to see from the expansion is to some degree countered by the energy contributed by the expansion force (dark energy).
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