By Don Lincoln, Fermilab
The Cosmic microwave background radiation, which, scientists also call the CMB, is uniform and is an important signature of the big bang. When the universe expanded, it stretched the wavelength of photons that are still traveling from the moment the universe became transparent. The wavelength of those photons is now about a millimeter and the frequency is 282 GHz, the range for microwaves; hence the name cosmic microwave background radiation.
The Universe after the Big Bang
In the early moments of the universe, it was so hot that hydrogen atoms couldn’t form. The temperatures slammed together particles so hard that, if a hydrogen atom did form, it would be immediately broken apart, splitting into its constituent proton and electron.
That all changed about 400,000 years after the big bang. The expanding universe cooled to a specific temperature—about 3,000 Kelvin, or about 5,000 degrees Fahrenheit—and then an amazing thing happened. As the temperature lowered through that threshold, finally neutral atoms of hydrogen could form. Prior to that, the photons of the big bang couldn’t go very far, because they would travel just a short distance until they encountered a charged particle and got scattered. But, with all of the charged particles now tied up into hydrogen atoms, the photons could travel freely.
Basically, the universe went from a hot, glowing fog of gas, to something transparent. That’s the first light we can actually see from the early universe. Before that, we can see nothing, because the light was scattered by the hot primordial fog. And the temperature of the light at that moment was also 3,000 Kelvin, with a wavelength that reflected that high temperature.
CMB Radiation
When it comes to CMB radiation, it’s probably worth mentioning that it isn’t a single unique wavelength. Hot things like glowing steel or a fire emit light in a range of wavelengths. There are a class of objects called black bodies that emit radiation with a characteristic spectrum. Black bodies are objects which absorb all light that falls on them. Since it is impossible to escape the universe, the universe is a black body. And, at the current time, it acts as if it is glowing with a temperature of about 2.73 Kelvin or about −455 degrees Fahrenheit.
Arno Penzias and Robert Wilson first observed the CMB in 1964. One of the first things they realized was just how impressive their measurement was. The universe was an absolutely textbook example of a black body. In fact, when they plotted the spectrum they measured, with properly determined uncertainties, the error bars were smaller than the thickness of the pencil lead used to draw their graph. It was amazing.
Another thing they determined was just exactly how uniform the CMB was. Within the uncertainty of their measurement, the temperature of the universe was identical in all directions. This measurement was simply astonishing.
This article comes directly from content in the video series The Evidence for Modern Physics: How We Know What We Know. Watch it now, on Wondrium.
Nonuniformities of the CMB
In the early 1970s, a number of physicists were thinking about the conditions of the universe when it was hot enough to be opaque. That hot and dense plasma was able to transmit sound waves and it is possible to calculate both the wavelength of the sound waves and just how loud they would be. From that, researchers predicted that there should be slight nonuniformities of the CMB—no more than about one part in 10,000 or maybe one part in 100,000. And the race was on to first observe these nonuniformities, both from ground-based observations and also satellite ones.
However, the first accepted observation of nonuniformities in the CMB was made in 1992 by a group of scientists using the Cosmic Background Explorer, or COBE, satellite.
This satellite scanned the heavens, precisely measuring the CMB. Some corrections were necessary to take into account the movement of the Milky Way compared to the CMB, as well as radio emissions in the plane of the Milky Way. When those corrections were accomplished, the result was a map of the sky covered with red and blue splotches. The red splotches were a tiny bit hotter than average and the blue splotches were a tiny bit cooler. They were about one part in a 100,000 different from average. With such tiny differences, it makes perfectly good sense that Penzias and Wilson concluded that the CMB was uniform.
The Wilkinson Microwave Anisotropy Probe
It’s hard to describe just how much excitement accompanied the COBE data. It gave a first detailed look at the sound of the big bang, reverberating through the cosmos. George Smoot, one of the leaders of the COBE experiment, and a man with a flair for the dramatic, told reporters that if they were religious, it was like looking at the face of God. It certainly was an amazing thing to watch unfold.
Since the CMB measurement was made, about 30 years ago, technology has improved. In 2001, the Wilkinson Microwave Anisotropy Probe, or WMAP, was launched. It basically did the same thing as the COBE satellite, but with vastly superior precision. That particular experimental group announced their first results in 2003. It was a considerable improvement, but it told a similar story. The WMAP ceased operations in 2010.
The third-generation satellite to study the small anisotropies, which is to say nonuniformities, of the CMB was called Planck. The Planck satellite was launched in 2009 and operated until 2013.
The angular precision of the Planck satellite was even better. The dark and light blotches were much smaller and more precise. Because the size of the features on the sky were so much finer, the meaning of red and blue was refined, with the brightest red and blue spots being about 300 microkelvin different from the average. That’s about one part in 10,000 variation.
Common Questions about the Cosmic Microwave Background Radiation
About 400,000 years after the big bang, the expanding universe cooled to a temperature of about 3,000 Kelvin, or about 5,000 degrees Fahrenheit.
The COBE data gave a first detailed look at the sound of the big bang, reverberating through the cosmos.
The third-generation satellite to study the small anisotropies of the CMB was called Planck. The Planck satellite was launched in 2009 and operated until 2013.
