Final answer:
The cosmic microwave background exhibits temperature fluctuations, signaling density variations in the early universe. Observations by the BOOMERANG project, and later by WMAP and Planck satellites, indicate we live in a flat, critical-density universe through the measurement of hot and cold spot sizes in the CMB.
Step-by-step explanation:
Observational evidence that density variations existed in the early universe comes from the fact that the cosmic microwave background (CMB) exhibits temperature fluctuations, which appear as hot and cold spots of varying sizes. These variations in the CMB temperature are indicative of small differences in density that were present when the universe was a few hundred thousand years old. Theoretical calculations and simulations predict that if the density of the universe is equal to the critical density, these hot and cold spots should be about one degree in size, a prediction confirmed by the WMAP and Planck satellite observations. Larger spot sizes suggest a density greater than critical, whereas smaller sizes indicate a density less than critical. The patterns observed in the CMB by instruments like BOOMERANG, and later by WMAP and Planck satellites, support the fact that we live in a flat, critical-density universe, adhering to the critical density model.