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<blockquote data-quote="Umbran" data-source="post: 6132927" data-attributes="member: 177">As my professors would have said, &quot;gravity&quot;, when all is said and done, is the effect on objects.&nbsp; Something curves space-time, objects moving in space-time move in accordance to that overall curvature - that is gravity.&nbsp; So, anything that changes the curvature exerts some gravitational effect.&nbsp; It doesn't matter how many terms we are adding up to come up with that final curvature, or where they enter - gravity is what you get when you add them all up!&nbsp; Thus, the cosmological constant produces a gravitational effect.&nbsp; You go into speaking about Planck scale effects.&nbsp; I would advise against that.&nbsp; &quot;Planck scale&quot; is a quantum concept.&nbsp; The cosmological constant is a classical concept.&nbsp; We have a real b*tch of a time getting these to work together.There's an error in thinking about the rubber-band on the rubber sheet - an error of scale.&nbsp; If you are using the rubber-sheet analogy for the expanding universe, then material objects like galaxies are point objects, not extended things lying on on the surface of the sheet.&nbsp; Well, remember - space can expand without the dark energy.&nbsp; Dark energy affects the rate of change of expansion.&nbsp; So, it isn't that the neutron would be at point A with Dark energy, but point B without it.&nbsp; It is about how the position of that neutron would change over time.&nbsp; Right now, the orbit is at point A.&nbsp; At some time later, the neutron would be at Point B or Point C, depending on the rate of expansion.Freyar and I seem to differ a bit on this point - gravitationally bound systems are not a vague concept to me, but a very specific one.&nbsp; &quot;Gravitationally bound&quot; and &quot;gravitationally interacting&quot; are not equivalent.&nbsp; Bound systems don't expand, by definition.&nbsp; The cosmological constant does not change this - if the things are bound, the distance between them isn't going to increase with time.&nbsp; The thing to remember is that a bound system doesn't just have the mass-energy of the objects, and the kinetic energy of motion.&nbsp; It also has a binding energy, just as an electron in an atom has an energy of binding to the nucleus.&nbsp; That biding energy, like any energy (like, say &quot;dark energy&quot;) *changes* the spacetime metric locally, such that expansion does not occur.&nbsp; That's why I say it is part of the definition of a bound state.</blockquote>

[QUOTE="Umbran, post: 6132927, member: 177"] As my professors would have said, "gravity", when all is said and done, is the effect on objects. Something curves space-time, objects moving in space-time move in accordance to that overall curvature - that is gravity. So, anything that changes the curvature exerts some gravitational effect. It doesn't matter how many terms we are adding up to come up with that final curvature, or where they enter - gravity is what you get when you add them all up! Thus, the cosmological constant produces a gravitational effect. You go into speaking about Planck scale effects. I would advise against that. "Planck scale" is a quantum concept. The cosmological constant is a classical concept. We have a real b*tch of a time getting these to work together. There's an error in thinking about the rubber-band on the rubber sheet - an error of scale. If you are using the rubber-sheet analogy for the expanding universe, then material objects like galaxies are [I]point objects[/I], not extended things lying on on the surface of the sheet. Well, remember - space can expand without the dark energy. Dark energy affects the rate of change of expansion. So, it isn't that the neutron would be at point A with Dark energy, but point B without it. It is about how the position of that neutron would change over time. Right now, the orbit is at point A. At some time later, the neutron would be at Point B or Point C, depending on the rate of expansion. Freyar and I seem to differ a bit on this point - gravitationally bound systems are not a vague concept to me, but a very specific one. "Gravitationally bound" and "gravitationally interacting" are not equivalent. Bound systems don't expand, by definition. The cosmological constant does not change this - if the things are bound, the distance between them isn't going to increase with time. The thing to remember is that a bound system doesn't just have the mass-energy of the objects, and the kinetic energy of motion. It also has a binding energy, just as an electron in an atom has an energy of binding to the nucleus. That biding energy, like any energy (like, say "dark energy") *changes* the spacetime metric locally, such that expansion does not occur. That's why I say it is part of the definition of a bound state. [/QUOTE]

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