By Joshua Winn, Princeton University
The Cosmic Microwave Background (CMB) was a profound discovery related to the cosmological redshift, a discovery that was announced in 1965. Read on to know more about how it all came to be; as well as the scientists who led this important discovery—Robert Wilson and Arno Penzias.
The Discovery
Robert Wilson and Arno Penzias were radio astronomers employed by Bell Labs, where they had access to a microwave antenna that was unusually well-shielded from terrestrial interference. But despite that shielding, when they observed a persistent source of static; they got frustrated.
After painstakingly ruling out equipment problems, they concluded that space is awash with microwaves. Over time, observations showed that his ever-present radiation amounts to 400 photons per cubic centimeter, flying every which way, with the spectrum of a nearly perfect blackbody.
CMB as a Blackbody
This cosmic microwave background radiation, or CMB, is the most perfect blackbody known. No other light source in nature, or in any laboratory, matches the Planck function so closely over such a wide range of wavelengths.
Also interesting is the implied temperature of the CMB that you obtain by fitting the data with a Planck function. It’s 2.72548 Kelvin, hovering just a few degrees above absolute zero.
This article comes directly from content in the video series Introduction to Astrophysics. Watch it now, on Wondrium.
Source of the Radiation
What produced all those photons, with such a perfect thermal spectrum? Why are they everywhere? And why is the temperature so cold? In our study of spectroscopy, we learned that blackbody radiation comes from optically thick materials in thermal equilibrium, the conditions in which the photons are constantly randomizing their energies through collisions, absorptions, and emissions by charged particles.
But the universe isn’t optically thick! The Earth isn’t suspended in a fog. The night sky is black. The universe is transparent. Photons can travel for billions of years without hitting anything, straight from some distant galaxy to our telescopes. And the universe today certainly isn’t all at the same temperature. Space is very cold, and stars are very hot.
Does CMB Support the Big Bang Theory?
To make sense of the cosmic blackbody spectrum, we are led to the conclusion that the universe used to be optically thick; it used to be much denser. The existence of the cosmic microwave background is another pillar of evidence supporting the Big Bang theory.
At early times, the universe was like the interior of a star. Just ions and electrons, everywhere, hot and dense enough to glow with blackbody radiation, like the inside of a kiln. But then over time the universe expanded, the density dropped, and at some point, the universe became transparent. All those photons were still there, but the chance of getting absorbed or scattered had become negligible.
After that, the photons kept sailing along in straight lines, and they’re still there, billions of years later. But, like all photons propagating across the expanding universe, their wavelengths have been stretched.
Temperature of the Blackbody
A blackbody spectrum has an important mathematical property. If you start with a collection of photons whose wavelengths follow a blackbody spectrum with temperature T, and then you stretch the wavelengths of all the photons by the same factor, a, then the transformed collection of photons will still have a blackbody spectrum, but with a colder temperature: T-divided-by-a.
In other words, the expansion of the universe preserves the blackbody spectrum of the photons, even after there’s no way to maintain thermal equilibrium. The photons aren’t interacting with anything, any more. But the temperature of that blackbody spectrum drops in proportion to one over a. That’s why the cosmic microwave background has such a low temperature, today.
When the universe was like the inside of a star, it glowed at visible wavelengths. But since then, the universe has expanded, stretching the wavelengths from microns to millimeters, into the microwave region of the electromagnetic spectrum. Given that the CMB temperature is 2.7 Kelvin, today, and that it varies as one over a, in general the temperature is 2.7 divided by a of t.
Irregularities in CMB
There is another profound feature in the CMB. It was said earlier that the CMB was the most perfect blackbody in existence. But it’s not completely perfect. It has slight irregularities.
An all-sky map of the CMB shows hot spots, and cold spots. The differences are exaggerated in the map. In reality, the temperatures of the hot and cold spots differ only in the 4th decimal place.
Hot and Cold Spots on CMB
The hot and cold spots represent parts of the universe that were ever-so-slightly denser, or less dense, than the average. Those density fluctuations are what eventually led to the formation of galaxies, stars, planets—everything we see—through the amplifying effect of gravity.
A region of the universe slightly denser than average exerts a stronger gravitational force on its surroundings, pulling more material toward it, and increasing its density even further. Given enough time, this amplifying process can convert a hot spot—higher in density by only one part in 100,000—into a supercluster of galaxies.
Common Questions about the Discovery of the Cosmic Microwave Background
Robert Wilson and Arno Penzias were radio astronomers employed by Bell Labs, where they had access to a microwave antenna that was unusually well-shielded from terrestrial interference. But despite that shielding, when they observed a persistent source of static; they got frustrated.
After painstakingly ruling out equipment problems, they concluded that space is awash with microwaves. This was the CMB, a profound discovery related to the cosmological redshift, announced in 1965.
The implied temperature of the CMB can be obtained by fitting the data with a Planck function. It’s 2.72548 Kelvin, hovering just a few degrees above absolute zero.
It was said earlier that the CMB was the most perfect blackbody in existence. But it’s not completely perfect. It has slight irregularities. An all-sky map of the CMB shows hot spots and cold spots. The differences are exaggerated in the map. In reality, the temperatures of the hot and cold spots differ only in the 4th decimal place.
