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As I understand it, gravity cannot be attenuated by any medium (in the way that EM radiation can be, for instance).

Does this, then, not make it a source (theoretically, I am not talking of practicalities) of infinite energy - if we assume the universe itself is infinite and looks the same everywhere (ie there are objects with mass everywhere)?

Or is it more correct to say that gravity has a net contribution of nothing to the universe's energy density because the attractional energy is balanced by a negative potential energy?

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Gravity travels at light speed (or less, possibly), so even in an infinite non-expanding universe of finite age you'd only be gravitationally interacting with a finite mass in a finite volume. Our universe is observed to be expanding, further inhibiting us from coming into contact with new objects.

Furthermore, a common assumption of cosmological models is that the universe is homogeneous and isotropic on large scales. Meaning it is pretty much identical in all directions. Observations support this assumption. One net effect of this is that the gravitational potential energy you experience from objects outside (roughly) your supercluster is zero (and really close to it from things outside your own galaxy, actually). They just balance each other out.

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Gravity plus dark energy reportedly can combine to do awful things to the amount of energy in the universe:

If dark energy does exist, then it ultimately causes the expansion of the Universe to accelerate. On their journey from the CMB to the telescopes like WMAP, photons (the basic particles of electromagnetic radiation including light and radio waves) travel through giant superclusters of galaxies. Normally a CMB photon is first blueshifted (its peak shifts towards the blue end of the spectrum) when it enters the supercluster and then redshifted as it leaves, so that the two effects cancel. However, if the supercluster galaxies are accelerating away from each other because of dark energy, the cancellation is not exact, so photons stay slightly blueshifted after their passage.

A blueshifted photon is a more energetic photon, and nothing seems to be losing energy to make up the difference. Article is from 2010 though. This may all be sorted out by now.

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