Which observational tool is primary for investigating the universe's large-scale structure and shape?

Cosmic Microwave Background (CMB)

The primary method employed by cosmologists to determine the global geometry of the universe is the detailed analysis of the Cosmic Microwave Background (CMB). The CMB represents the relic radiation from the early universe, approximately 380,000 years after the Big Bang, and it carries crucial information about the initial conditions and large-scale structure. Scientists examine the angular size and pattern of the inherent temperature fluctuations, often referred to as acoustic peaks, within the CMB map. These spots act as natural cosmic yardsticks of known physical size from the early universe. The observed angular size of these spots on the sky reveals the overall curvature: if they appear magnified, the universe is closed; if they appear smaller, it is open; and if they match theoretical predictions for flat space, the universe is deemed flat. This technique provides the most precise measurements constraining the universe's shape.