What is 1 major discovery in cosmology that relies on improvement in existing apparatus?

The most dramatic astronomical development of the century thus far is the detection of gravitational waves from merging black holes at a distance of 400 Mpc, during the first science run of the advanced Laser Interferometer Gravitational-Wave Observatory.

The telescope was also very important. Galileo Galilei was the first person to use a telescope to look at celestial bodies (though he did not invent the telescope) and discovered the four brightest moons of Jupiter, proving that there are things in the Solar System that don't revolve around the Sun.

Physical cosmology is the branch of physics and astrophysics that deals with the study of the physical origins and evolution of the Universe. It also includes the study of the nature of the Universe on a large scale. In its earliest form, it was what is now known as "celestial mechanics", the study of the heavens.

Hope this helps you :)