Cosmic Microwave Background

FAS Astronomers Blog, Volume 32, Number 2.

Just recently, it was reported in the news that Arno Penzias had passed away at the age of 90. Penzias, along with his colleague Robert Wilson, discovered the birth of the universe. 

In the 1940s, There wasn’t any direct evidence of the so-called Big Bang that many astronomers believed was the beginning of the universe. Starting in 1948, George Gamow along with Ralph Alpher and Robert Herman theorized that there might be a faint afterglow from this Big Bang. They thought that it should be observable with a temperature of around 5 K. However, the technology (and the interest) just wasn’t there to pursue it. 

In the mid-1960s, there was a group of scientists over at Princeton University led by Robert Dicke (along with James Peebles, Peter Roll, and David Wilkinson). They were looking for the faint glow from the Big Bang that Gamow, Alpher, and Herman predicted – although they didn’t know about the earlier work by Gamow and others. In fact, they were very close to finishing a device that would detect this “background radiation.”

At the same time, there were two astronomers, Arno Penzias and Robert Wilson, a little over 30 miles from Princeton in Holmdel, N.J. They were measuring microwaves coming from different parts of the sky using a horn antenna at Bell Labs on Crawford Hill. But they had a problem. There was a faint “hum” that they couldn’t explain, and it appeared to be coming from everywhere. They pointed their antenna toward New York City, and it was there. They pointed it away, and it was there. They pointed it up, and it was still there. They spent about a year trying to figure it out, including kicking the nesting pigeons out of the telescope’s antenna and, of course, cleaning up after them. 

Eventually, through various people, Penzias found out about the folks at Princeton and what they were trying to do. The story is described in several of the references listed below (Kragh, Lemonick, Panek, Weinberg, Shoenstein, Evans, Chown, and Peebles). In mid-February 1965, James Peebles gave a talk at Johns Hopkins about the possibility of background radiation left over from the Big Bang. Ken Turner attended that talk. Turner discussed Peebles’ talk with Bernard Burke. Burke received a preprint of an article describing the possible afterglow from Peebles. Burke had previously crossed paths with Arno Penzias in December 1964 on an airplane flight. Penzias placed a phone call to Burke in February 1965 and discussed several subjects including the mysterious noise they were getting. Burke suggested that Penzias give Dicke a call, which he did. Allegedly, after Dicke hung up the phone, he turned to his team and said, “Well boys, we’ve been scooped.” 

After that initial phone call, the two groups got together to figure out what the ”hum” was. The two teams published companion articles in the July 1965 issue of the Astrophysical Journal. Penzias and Wilson described their observations and included this one-line comment, “A possible explanation for the observed excess noise temperature is the one given by Dicke, Peebles, Roll, and Wilkinson (1965) in a companion letter in this issue.” Dicke et al. went on to explain the possible explanation for the noise. 

Neither of the papers acknowledge the work of Gamow and his colleagues. According to Helen Kragh, Gamow, after receiving a copy of the Penzias and Wilson paper, sent a summary of their work to Penzias with the statement: “Thus, you see the world did not start with almighty Dicke”. Kragh reports that both groups eventually acknowledged this omission.

It turns out that Penzias and Wilson found the faint glow from the first light 380,000 years after the Big Bang. Because of the expansion of the Universe, this light has expanded and cooled to a temperature of around 2.7 K. It is now known as the Cosmic Microwave Background (CMB), and it is everywhere.

The news of the discovery spread quickly. It made it into a May 1965 article in The New York Times. Others, including the Princeton group, repeated the Penzias and Wilson observations and got similar results. 

There is more to the story. Richard Panek (The 4 Percent Universe) reported that unbeknown to Penzias and Wilson, some folks anticipated their work. In 1961, a note was published in a Bell Labs technical journal mentioning a 3 K noise found in the Holmdel antenna, but no one followed up on it. In 1964, two Russians published a paper about the potential background radiation and noted that the horn antenna in Holmdel would be an ideal tool to find it.

On the technical side, the CMB represents something called blackbody radiation. A blackbody is a theoretical object that absorbs 100% of the light it receives. It then reemits light with a specific spectrum that depends only on the temperature of the blackbody. The CMB shines with a blackbody pattern of 2.7 K. 

To Penzias and Wilson and other Astronomers at the time, the background radiation looked uniform. It was the same no matter where they looked. This left a big question, if the big bang was uniform, why do we see a lumpy universe filled with galaxies, galactic clusters, and voids? 

The answer was that CMB does vary slightly in temperature. This was discovered by three satellites launched over the last 25 years: COBE (1989), WMAP (2001), and Planck (2009). Each provided a more detailed view of the CMB than the previous, slowly bringing the CMB into focus. Today, astronomers think the yellow/orange hot areas became galaxies and galactic clusters, while the blue cold areas became voids. Although, the difference in temperature is minute at only +/-200 mK (microkelvins, 10^-6 kelvins). If the shape of the images looks a little strange, it is a Mollweide projection of the Universe onto a flat surface. See Mapping the World for more about how flat maps are created from curved surfaces.

After the discovery of the cosmic microwave background, the Big Bang model of the expanding universe became well accepted. Over the years there have been modifications to the theory including Inflation, Dark Matter, and Dark Energy. But the overall premise of a hot initial state followed by a long expansion remains. 

In 1978, Penzias and Wilson received the Nobel Prize for their discovery, while Dicke and his group were passed over. No one knows exactly why Dicke’s group was overlooked. It may be that they reinvented the idea originally developed by Gamow and his colleagues, or that there were simply too many involved, and the Nobel Prize is awarded to at most three individuals in a single year. In any event, Peebles was awarded the Nobel Prize in 2019 for his many decades of theoretical work on the history of the universe.