The Cosmic Background Microwave Radiation is the oldest light in the universe.
Light travels at a finite speed. So, believe it or not, when you turn on a light in a dark room it takes a (very tiny) amount of time for the photons (light particles) produced by the bulb to fill the room. It's just that this happens so quickly, we can't see it.
The Cosmic Background Microwave Radiation is an echo of when the first photons could travel freely, shortly after the universe's creation (about 380,000 years, which isn't very long in a universe's lifetime).
This radiation marks the moment where the lights went on in the universe. It's just that the universe is a lot bigger than your living room, so this light has taken a lot longer to reach us.
Why is it important and what is it?
The Cosmic Background Microwave Radiation is the furthest back in time we can explore using light. It's a snapshot in time and can reveal a lot about the initial conditions for the evolution of the universe.
For example, it gives us hints as to how and why the stars and galaxies formed and was also provides one of the key pieces of evidence that the universe started with a Big Bang.
As the universe expanded, the wavelengths of the Cosmic Microwave Background photons have grown (or have been ‘redshifted’) to 1mm. Their effective temperature has also decreased to just 2.7 Kelvin, or around -270ºC, just above absolute zero.
And these photons fill the Universe today. There are roughly 400 in every cubic centimetre of space.
The Cosmic Background Microwave Radiation creates a background glow that can be detected by far-infrared and radio telescopes.
The Joker and Batman are doing a spot of cloud gazing. They can only see the surface of the clouds where the light was last scattered from the cloud surface. They can't see beyond it. It a similar manner, the Cosmic Microwave Background Radiation is the surface of the last scatter of photons in our universe. It's like a cloud we cannot see past in the visible spectrum.
How was it discovered?
The Cosmic Background Microwave Radiation was discovered completely by mistake in 1964. Arno Penzias and Robert Wilson were using a large radio antenna in New Jersey when they picked up an odd buzzing sound coming from all parts of the sky.
They tried to remove the source of this (assumed) interference, and even removed some pigeons that were nesting in the antennae!
"When we first heard that inexplicable 'hum,' we didn’t understand its significance, and we never dreamed it would be connected to the origins of the universe," Penzias said in a statement. "It wasn’t until we exhausted every possible explanation for the sound's origin that we realized we had stumbled upon something big."
They were awarded the Nobel Prize in Physics in 1978 for the discovery.
The European Space Agency (ESA) is investigating the Cosmic Microwave Background Radiation with the Planck spacecraft to study this ancient light in more detail than ever before. NASA's Wilkinson Microwave Anisotropy Probe (WMAP), which launched in 2001 and stopped gathering data in 2010, and its COBE (COsmic Background Explorer) craft both provided early images of the Cosmic Background Radiation.
The Cosmic Microwave Background Radiation also provided evidence of primordial gravitational waves - which is the "smoking gun" to support the theory that the universe expanded in a cosmic inflation.
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