What is the oldest photo of the universe? The oldest pictures we have today are of our universe's teen years, except for one photo, one baby picture of the universe, the cosmic microwave background. Today, the universe is more than 13.7 billion years old. 

But the cosmic microwave background is a map of the universe when it was just 378,000 years old. The image is similar to a temperature map of the United States. But instead of showing a wide range of temperatures, the cosmic microwave background shows tiny almost unperceivable temperature differences in the newborn universe. 

Scientists Arno Penzias and Robert Wilson unexpectedly discovered the cosmic microwave background in the 1960s while using a large microwave detector to study the Milky Way. They were surprised to find a static or fuzz coming from all directions of the sky and that all of the static radiated with the same temperature, 2.73 Kelvin above absolute zero. 

The signal was so unexpected that the two scientists initially thought the noise was coming from the antenna or some other earthly source. But one by one, they methodically ruled out all other possibilities, even cleaning out pigeon droppings from inside the antenna to make sure that wasn't interfering, and found that the signal was real. It was coming from the cosmos. You may have even seen some of the static yourself. About 2% of the fuzz on an old television set comes from the cosmic microwave background. 

In the 1980s, a NASA satellite called COBE mapped the cosmic microwave background and revealed that it was not exactly the same temperature in every direction. COBE uncovered islands of relative warmth and cold. But the temperature differences COBE observed were incredibly small. The temperature variations from place to place were less than one ten-thousandth of a degree. If you wanted to change the temperature in your living room by that amount, you would need to run your heater for less than a hundredth of a second. 

Later, NASA's WMAP and the European Space Agency's Planck spacecraft gave us an even better, more detailed map of the cosmic microwave background. Amazingly, these little temperature fluctuations grew into all the structures in the universe, galaxies, stars, and planets. 

Over time, the slightly hotter areas pulled in more and more material, and the slightly colder areas grew colder and emptier. Today, hot, dense stars in galaxies are surrounded by cold, mostly empty space. Maps of the cosmic microwave background tell us about the early history of the universe when it was hotter and denser.