Let's do the spacetime warp again

[Philip Thrift]

Sabine Hossenfelder @skdh

https://twitter.com/skdh/status/1184471550873030657

The average of a function of a variable is not the same as the function of the average of the variable. We know it’s not. But whenever we use general relativity on large scales, we assume that this is the case.

[  https://en.wikipedia.org/wiki/Jensen%27s_inequality#Statements ? ]

Dark matter nightmare: What if we are just using the wrong equations?

http://backreaction.blogspot.com/2019/10/dark-matter-nightmare-what-if-we-just.html

General relativity does not tell us what is going on.

I don’t mean that general relativity needs to be modified. I mean that we incorrectly use the equations of general relativity to begin with.

Needless to say, taking an average on one side of the equation requires that you also take an average on the other side. But since the gravitational part is non-linear, this will not give you the same equations that we use for the solar system: The average of a function of a variable is not the same as the function of the average of the variable. We know it’s not. But whenever we use general relativity on large scales, we assume that this is the case.

That it is difficult to average non-linear equations is of course not a problem specific to cosmology.

...

comment from Sabine Hossenfelder:

Folks:

Could you please stop submitting comments about your favorite explanations for dark matter or dark energy or your personal theories of something.

ha!

@philipthrift