 There are two main ways in which cosmological observations support perturbed FLRW models. First, cosmologists use matter and radiation in the universe to probe the background spacetime geometry and its evolution. The universe appears to be isotropic at sufficiently large scales, as indicated by background radiation (most notably the cosmic microwave background radiation (CMB), discussed below) and discrete sources (e.g., galaxies). Isotropy observed along a single worldline is, however, not sufficient to establish the universe is well described by an FLRW geometry. A further assumption that our worldline is not the only vantage point from which the universe appears isotropic, often called the Copernican principle, is needed. Granting this principle, there are theorems establishing that observations of almost isotropic background radiation implies that the spacetime geometry is almost  FLRW.[10]  The principle itself cannot be established directly via observations (see  §2).  Given that we live in an almost FLRW models, we need to determine its parameters such as the Hubble constant \(H_0\) and the deceleration parameter \(q_0 \coloneqq {-}\ddot{R}/(RH_0^2)\), which measures how the rate of expansion is changing, and the normalized density parameters \(\Omega_m\coloneqq \rho_m/(3H_0^2)\) for each matter or energy density component \(m\). There are a variety of ways to determine the accuracy of the background evolution described by the FLRW models, which depends on these parameters. For this purpose, cosmologists seek effective standard candles and standard rulers—objects with a known intrinsic luminosity and length, respectively, which can then be used to measure the expansion history of the universe.