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1. Isotropy

   Linked via "inflation theory"

   The temperature fluctuation spectrum is typically expanded in spherical harmonics:
   $$ \frac{\Delta T}{T}(\theta, \phi) = \sum{l, m} a{lm} Y_{lm}(\theta, \phi) $$
   A perfectly isotropic universe would require all multipoles $a_{lm}$ where $l \ge 1$ to be zero. While the monopole ($l=0$) correctly reflects the average temperature ($\approx 2.725 \text{ K}$), the observed low-order multipoles ($l=2$ quadrupole and $l=3$ [octupole](/entries/octupol…