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The central concept behind Christof Wetterich's proposal is scale invariance: Imagine you try to measure the size of the universe with a yard stick. If you meas

by mpauly 8y ago

The central concept behind Christof Wetterich's proposal is scale invariance: Imagine you try to measure the size of the universe with a yard stick. If you measure that the universe is expanding, you can't be sure that it really is the universe expanding, or maybe just your yard stick shrinking. This is true in so-called scale invariant theories. These theories do not contain any intrinsic length scale. The moment your theory contains a fundamental length l you break scale invariance. In this case you are able to compare your yard stick to the length l to find out if your yard stick was shrinking or the universe really is expanding. Our present model of the world still contains a few fundamental length scales (such as the Planck length, or a length scale associated to the nuclear force), but people such as Christof Wetterich are building models that try to get rid of (some of) these fundamental length scales. A major approach in this direction is asymptotically safe quantum gravity, that also led Christof Wetterich and Mikhail Shaposhnikov to the most precise prediction of the Higgs mass prior to the Higgs' discovery in 2009, see https://arxiv.org/abs/0912.0208 https://arxiv.org/abs/0912.0208.

gubbrora 8y ago
But the yardstick argument assumes uniform expansion. Small scale structure like our yardstick is held together by electromagnetic forces which counteracts the expansive force. That's my understanding anyway. If we create a light year long line of yardsticks then we would expect the line to expand faster than the yardsticks that make it up. Locally this would look like growing gaps between the yardsticks. we never used the fact that the line was a light year long. So let's just put two yardsticks next to each other in a vacuum. They drift apart and eventually so far they are not part of each others' observable universes.
- Pharmakon 8y ago
  That’s accurate. Metric expansion only dominates loosely bound structures on a scale beyond individual galalxies. The yardstick, solar system, and Milky Way are all on scales too small, and are too strongly bound, for metric expansion to have an impact. Galaxy clusters and the space between galaxies however, are impacted.

by mpauly 8y ago

gubbrora 8y ago

But the yardstick argument assumes uniform expansion. Small scale structure like our yardstick is held together by electromagnetic forces which counteracts the expansive force. That's my understanding anyway. If we create a light year long line of yardsticks then we would expect the line to expand faster than the yardsticks that make it up. Locally this would look like growing gaps between the yardsticks. we never used the fact that the line was a light year long. So let's just put two yardsticks next to each other in a vacuum. They drift apart and eventually so far they are not part of each others' observable universes.

Pharmakon 8y ago
That’s accurate. Metric expansion only dominates loosely bound structures on a scale beyond individual galalxies. The yardstick, solar system, and Milky Way are all on scales too small, and are too strongly bound, for metric expansion to have an impact. Galaxy clusters and the space between galaxies however, are impacted.