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So, the universe is expanding, and, from what I've heard, that expansion is accelerating. However, I'm unclear as to the specific mechanism involved, particularly as would be described in a Loop Quantum Gravity model. Many of the popular books I've read describe the situation in terms of a General Relativity based rising loaf of raisin bread, in which the raisins (galaxies and galaxy clusters) themselves are not moving, but rather, in which the bread (space) between them is expanding.

So, if this expansion of spacetime is translated from General Relativity to Loop Quantum Gravity, would it be more accurate to think of the number of "atoms" of spacetime increasing or to think of the individual units of spacetime becoming larger? Or is my sign flipped and the units of spacetime would actually be getting smaller, thus making the cosmic distances appear to increase?

If, at its most fundamental level, the speed of light is equal to the Planck length / the Planck time, then how would either the addition of extra units of spacetime or the expansion (or the contraction) of those basic units effect the ratio of length to time that is the universal constant of the speed of light? Must the clock rate of the universe shift to compensate for the expansion of the universe?

Or am I way off base and there's really no way to conceptualize of LQG Spin Foams in these terms?

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  • $\begingroup$ Since the "atoms" of spacetime are the intersection of loops and these intersections are often referred to as events, the the number of events would grow with "time" and so there would be an increase in the number of intersections/events/atoms? So, the number increases, not the size, right? $\endgroup$ – Thor Jan 6 '19 at 18:27
  • $\begingroup$ Duplicate of physics.stackexchange.com/questions/18840/… $\endgroup$ – Mitchell Porter Feb 9 '19 at 23:50
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From my understanding, loop quantum gravity says that space is made out of a lattice of connected nodes. The connection between two nodes defines a Planck Length, and thus defines the speed of light between the two nodes. It doesn't make any sense to ask how long these connections are, because they do not exist within space; space is made out of them. Therefore, the only way that space can expand is if new nodes and new connections are created between the existing nodes.

I am not an expert on LQG, so take this with a grain of salt.

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