Anisotropic initial conditions - how much is too much?
Inflation is a mechanism that takes care of highly varied initial conditions, and yields the fairly isotropic, homogeneous, and flat universe we see today.
It has been assumed for a long time now, that we can start in whatever initial conditions, in terms of initial anisotropy, and inflation will just take care of everything. Moreover, since we can only collect data on the last stages of inflation, if inflation was sufficiently long, those initial conditions are effectively erased.
This was termed the ‘Cosmic no-hair theorem’, as an analogue to the ‘no-hair theorem’ that states that black holes are indistinguishable if they have the same very few quantities (mass, spin, and charge). By the same token, universes that started off at very different initial conditions, but very long ago, may be indistinguishable.
We set out to find how much of an initial anisotropy some models can take, and, after a fashion, numerically test the ‘cosmic no-hair theorem’.