Final answer:
The important discovery made at the Horn Antenna in Holmdel was the discovery of cosmic microwave background radiation, which provided evidence for the Big Bang theory.
Step-by-step explanation:
The important discovery that was made at the Horn Antenna in Holmdel was the discovery of cosmic microwave background radiation.
Arno Penzias and Robert Wilson, scientists at AT&T's Bell Laboratories, were using the antenna to measure astronomical sources when they noticed a faint static-like noise coming from all directions.
After ruling out all possible sources of interference, they realized that the noise was leftover radiation from the Big Bang, providing evidence for the widely accepted Big Bang theory of the origin of the universe.