The Holmdel Horn Antenna, located at Bell Telephone Laboratories in Holmdel, New Jersey, USA. In 1965 while using this antenna, American radio astronomers Arno Penzias and Robert Wilson stumbled on the cosmic microwave background radiation that permeates the universe. This was one of the most important discoveries in cosmology in the 20th century, providing confirmation of the Big Bang theory of the creation of the universe. Penzias and Wilson received the 1978 Nobel Prize in Physics for this discovery, and in 1989 the Holmdel horn antenna was designated a National Historic Landmark.
This type of antenna is called a Hogg horn antenna, invented by D. C. Hogg at Bell Labs in 1961. It consists of a flaring metal horn, with a reflector mounted in the mouth at a 45° angle. The reflector is a segment of a parabolic reflector, so the antenna is really a parabolic antenna fed off-axis. This type of antenna has several features that make it a good radio telescope: it has very broad bandwidth, its aperture efficiency can be calculated accurately, and the horn shields the antenna so it picks up very little thermal radio noise from the ground.
Since it is no longer being used, the antenna has been rotated so its aperture is facing the ground, to keep rainwater out.

This is an image of a place or building that is listed on the National Register of Historic Places in the United States of America. Its reference number is 89002457.