Microwave Radiation Helps Astronomers Explore the Cosmos

cosmic microwave background
This is how the sky appears in microwave wavelengths. Those variations in color signify temperature fluctuations in the every early universe. The material in those regions (which appear as they did some 13.7 billion years ago) later went on to form the first galaxies and stars. NASA

Did you know that the same radiation your oven uses to zap your burrito for lunch helps astronomers explore the distant reaches of the universe? It's true, and microwave emissions play an important role in helping us look back into the infancy of the cosmos.

Hunting Down Microwave Emissions

The closest source of non-terrestrial microwaves is our Sun. However, the specific wavelengths of microwaves that come from it get absorbed by our atmosphere.

Water vapor in our atmosphere can interfere with the detection of microwave radiation, absorbing it and preventing it from reaching Earth's surface. So, astronomers who study microwave radiation in the cosmos put their detectors at high altitudes, or even in space. 

Microwave telescopes are inherently large so that they can detect the long wavelengths that make up microwave signals. The size of the detector needs to be many times greater than the radiation wavelength (which can be up to a meter) even to resolve an astronomical object the size of our Moon. That's another good reason for locating these observatories on mountaintops or in space. 

What Gives Off Microwaves in the Universe

Some of the most fascinating sources of microwaves lie well outside our solar system, across our galaxy and across the universe. For example, active galaxies (AGN), powered by supermassive black holes at their cores are some of the strongest microwave emitters.

Additionally, these black hole engines can create massive jets of plasma that also glow brightly in the microwave. Some of these microwave-emitting structures can be larger than the entire galaxy that contains the black hole.

The center of our own Milky Way galaxy is a microwave source, although it's not so extensive as in other, more active galaxies.

Our black hole (called Sagittarius A*) is a fairly quiet one, as these things go. It doesn't appear to have a massive jet, and only occasionally seems to feed on material that passes too close.

Pulsars (rotating neutron stars) are also strong sources of microwave radiation. These powerful, compact objects are second only to black holes in terms of ultimate density. With powerful magnetic fields and fast rotation rates broad spectrum radiation is produced, with the microwave emission being particularly strong. In fact, most pulsars are usually referred to as "radio pulsars" because of their strong radio emissions, but they can also be "microwave-bright". 

The Cosmic Microwave Background Radiation (CMB)

When a microwave telescope is pointed around the sky, it detects a faint microwave glow. The Cosmic Microwave Background Explorer (COBE) satellite made a detailed study of this cosmic microwave background (CMB) beginning in 1989. Since then, other studies made with such orbiting instruments as the Wilkinson Microwave Anisotropy Probe (WMAP) have detected this radiation. Astronomers use the minor fluctuations in the CMB to learn more about the origins and evolution of the universe.

The CMB is the afterglow of the Big Bang, the event that set our universe in motion.

The leftover "heat" from this event was spread out over the entire universe (which was small at first, but grew very quickly). As the newborn cosmos expanded the density of the heat dropped. Another way to understand this is: as the universe evolved and expanded the average temperature continued to fall.

Today, the CMB represents a temperature of about 2.7 kelvin. Astronomers "see" that diffuse temperature as microwave radiation. The CMB is uniform across the entire observable universe. Discovered in 1964 by radio astronomers Arno Penzias and Robert Wilson, a discovery for which they won the Nobel Prize in 1978, it was the first hard evidence in support of the Big Bang theory.

Tech Talk about Microwaves in the Universe

Microwaves emit at frequencies between 0.3 gigahertz (GHz) and 300 GHz. (One gigahertz is equal to 1 billion Hertz.) This range of frequencies corresponds to wavelengths between a millimeter (one thousandth of a meter) and a meter.

For reference, TV and radio emissions emit in a lower part of the spectrum, between 50 and 1000 Mhz (megahertz). A "Hertz" is used to describe how many cycles per second something emits at, with one Hertz being one cycle per second. 

Microwave radiation is often described as being an independent radiation band, but is also considered part of the science of radio astronomy. Astronomers often refer to radiation with wavelengths in the far infrared, microwave and ultra high frequency (UHF) radio bands as being part of "microwave" radiation, even though they are technically three separate energy bands.

Edited and updated by Carolyn Collins Petersen.