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Quoting Wikipedia: It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require sp

Quoting Wikipedia: It is an intrinsic expansion whereby the scale of space itself changes. The universe does not expand "into" anything and does not require space to exist "outside" it. Technically, neither space nor objects in space move. Instead it is the metric governing the size and geometry of spacetime itself that changes in scale https://en.m.wikipedia.org/wiki/Expansion_of_the_universe https://en.m.wikipedia.org/wiki/Expansion_of_the_universe

onlyrealcuzzo 6y ago
What is the metric governing the size and geometry of spacetime? Gravity??
- ben_w 6y ago
  Apparently it can be “loosely” thought of as such, but that implies there is some interesting difference way above my level: https://en.m.wikipedia.org/wiki/Metric_tensor_(general_relativity) https://en.m.wikipedia.org/wiki/Metric_tensor_(general_relat...
- bobbylarrybobby 6y ago
  Gravity is not the metric, but it does interact with the metric (well, gravity is what we call it when mass or energy affects space time). The presence of mass/energy causes the shortest distance between two points to no longer be a straight path through space. The path an object takes through space time is always the path with the shortest space time interval among all paths (in that sense the path is “straight” the same way that in flat space, a straight line is the shortest path between two points) — this distance is given by the metric tensor — but gravity makes this path appear curved in space.
- drdeca 6y ago
  The metric of the spacetime manifold. See https://en.wikipedia.org/wiki/Metric_tensor https://en.wikipedia.org/wiki/Metric_tensor . When you have a space that isn't just normal globally euclidean space (such as: on the surface of a sphere), the idea of a vector as in like "a direction you can go in and an amount of how much or how fast or whatever" isn't something that makes sense as something independent of a base location. Instead, each point in the space has associated with it a space of "tangent vectors" at that point, and these spaces are related to each other. The metric tensor associates to each point a "bilinear form" with some properties, essentially a way of doing something like a dot product of two tangent vectors at that same point. This in turn allows for defining the notion of the length of some curve through the manifold.

onlyrealcuzzo 6y ago

What is the metric governing the size and geometry of spacetime? Gravity??

ben_w 6y ago
Apparently it can be “loosely” thought of as such, but that implies there is some interesting difference way above my level: https://en.m.wikipedia.org/wiki/Metric_tensor_(general_relativity) https://en.m.wikipedia.org/wiki/Metric_tensor_(general_relat...
bobbylarrybobby 6y ago
Gravity is not the metric, but it does interact with the metric (well, gravity is what we call it when mass or energy affects space time). The presence of mass/energy causes the shortest distance between two points to no longer be a straight path through space. The path an object takes through space time is always the path with the shortest space time interval among all paths (in that sense the path is “straight” the same way that in flat space, a straight line is the shortest path between two points) — this distance is given by the metric tensor — but gravity makes this path appear curved in space.
drdeca 6y ago
The metric of the spacetime manifold. See https://en.wikipedia.org/wiki/Metric_tensor https://en.wikipedia.org/wiki/Metric_tensor . When you have a space that isn't just normal globally euclidean space (such as: on the surface of a sphere), the idea of a vector as in like "a direction you can go in and an amount of how much or how fast or whatever" isn't something that makes sense as something independent of a base location. Instead, each point in the space has associated with it a space of "tangent vectors" at that point, and these spaces are related to each other. The metric tensor associates to each point a "bilinear form" with some properties, essentially a way of doing something like a dot product of two tangent vectors at that same point. This in turn allows for defining the notion of the length of some curve through the manifold.