In 1964, Princeton physicists Dave Wilkinson, Jim Peebles, and Robert Dicke began conducting experiments to measure the cosmic microwave background (CMB), a thermal radiation that resulted from the Big Bang. Peebles, the theorist in charge of conducting CMB calculations, mentioned this research during a speech he gave at Johns Hopkins University. Peeble’s friend,
Kim Turner, who attended his lecture, also happened to share a room with MIT’s Professor Bernie Burke. Turner mentioned Peebles’ research to Burke, who visited Bell Labs researchers Arno Penzias and Robert Wilson the following day.
Recently, Penzias and Wilson had detected a residual noise in the Bell Labs radio antenna. Regardless of when or where they pointed the antenna, they were able to detect it. Penzias and Wilson concluded that the source of the noise came from outside the Earth’s galaxy. When Burke told Penzias about Peebles’ unfinished paper on radiation, he realized that this noise was caused by the CMB. Less than 40 miles from Princeton’s campus, Penzias and Wilson proved the predictions of the University’s researchers. Physics Department Chair Lyman Page remembers that Dicke was in a meeting at Princeton when he received a phone call from Penzias and Wilson. He announced to his colleagues, “Boys, we’ve been scooped!” The two research teams ultimately published their results together. One article in the Astrophysical Journal Letter belonged to Dicke, Peebles, and their Princeton colleagues, while the other article belonged to Penzias and Wilson.
DEFINING THE COSMIC MICROWAVE BACKGROUND
The successful measurement of the CMB signified a turning point in the field of experimental cosmology. It granted physicists a powerful tool with which they could study the universe. Even today, much of the research in experimental cosmology relies on measurements of the radiation.
The CMB is the thermal afterglow of the Big Bang. The “hot” Big Bang model of the universe predicted the existence of such a radiation as a remnant of the initial state of the universe. It completely fills the universe and is largely isotropic. The phrase, “Cosmic Microwave Background,” refers to the radiation’s property of constancy, which explains why Penzias and Wilson detected it at all times and from any given direction.
The present temperature of the CMB is about 2.7 Kelvin, or about –455 degrees Fahrenheit. It may seem strikingly cold, but this temperature is consistent with the model predictions. As the universe expands, radiation drops in temperature. However, the expansion of the universe is not an outward extension. Rather, scientists describe it as a case of space intervals becoming larger. In a sense, it expands from within. The discovery of the CMB provided conclusive support for a key prediction of the Big Bang model as an initial hot dense state of the universe.
REVELATIONS FROM PAST CMB RESEARCH: PICTURES OF A YOUNG UNIVERSE
The CMB’s temperature provides scientists with clues about the formation of the universe’s structure. Physicists have related the CMB’s temperature variations with variations in gravity’s strength, which in turn affects the universe’s structural formation. In this context, “structure” refers to the way in which matter emerges and exists in the universe. For instance, a cluster of galaxies may be categorized as a structure. This relationship provides researchers with a picture of the early universe and helps to explain how the universe developed into its current state.
To understand how scientists observe this extraordinary relationship between temperature and structure through the CMB, imagine the CMB as a large sphere surrounding the universe, emitting radiation. However, the radiation emitted is actually from a much earlier era, as it takes time for radiation to travel such great distances. Because of this, the radiation reveals information about a younger universe, in which its structure, as it is understood today, had not yet fully formed. As previously stated, physicists discovered a relationship between slight variations in the CMB’s temperature and the universe’s gravitational potential (the potential energy per mass unit). Force is greater in regions with greater gravitational potential. Since matter accumulates where force is strongest, gravitational potential affects how the universe structuralizes. After studying such temperature variations in the CMB, physicists created the Standard Model of Cosmology. This model is built upon six physical parameters and explains the universe’s features, including the distribution of galaxies and its increasing rate of expansion.
By studying the CMB, experimental cosmology has also developed new research areas. Many experiments now focus on the polarization of the CMB. Polarization refers to the orientation of a light wave’s magnetic and electric fields, with respect to its direction of transmission. The polarization of the CMB is particularly important to the theory of cosmic inflation. This theory proposes that immediately following the Big Bang, the universe expanded at an exponential rate. Physicists have not yet found physical evidence to support such an inflation, but polarization could be the answer to this unresolved problem. The inflation of the universe could have theoretically created the gravitational waves, or ripples in spacetime, that scientists detected as recently as 2015.
These gravitational waves theoretically created by the Big Bang, termed the “Cosmic Gravitational Wave Background,” or CGWB, would have a signature on the CMB called the B Mode signature. If it was the only signature present on the CMB, the B Mode signature would be considered “pure.” However, the gravitational wave signature on the CMB is likely not pure and is difficult to detect. Research is currently underway to try and measure the CGWB’s signature.
PRINCETON COSMOLOGY TODAY
Despite all the progress made possible by the measurement of the CMB, there remains much to be learned about the genesis of the universe. The CMB remains a valuable resource in this quest, and Princeton’s initial involvement in its discovery has contributed immensely to the progress the field has made. Fittingly, the CMB is still central to much of the work that Princeton’s experimental cosmologists do today. Two projects currently underway at Princeton focus on the structure of the universe, attempting to find empirical support for the theory of inflation. The first project is called the SPIDER, a balloon-borne experiment led by Professor William Jones. In order to help cosmologists better understand the development of the early universe, the SPIDER balloon will hopefully take sensitive measurements of the polarization in the CMB.
The second project is the Atacama Cosmology Telescope (ACT), currently overseen by physics Professor Suzanne Staggs. The ACT recently finished collecting data for the Atacama B-Mode Search, a study aiming to find the unique signature of the cosmic gravitational waves on the CMB. Researchers expect to publish results soon. Additionally, the ACT has verified that only 5% of the universe actually consists of atoms, with dark energy and dark matter accounting for the rest. However, very little is known about dark energy and dark matter, so both may be the next areas of growth in physics research.
Special thanks for this article are due to Professor Lyman Page, Chair of the U. Physics Department, who was kind enough to talk with me about the history of cosmology at Princeton, as well as his ongoing research. The other major sources for this article include the webpage of the Princeton Cosmology Group, which provided the details of current advances in the discipline, and the Physics Department website.