Today’s physics post is about the book “Echoes of the Big Bang” by Michael Lemonick. It is a story about the history and consequences of the discovery of the Cosmic Microwave Background (CMB). Enjoy!
Michael Lemonick’s Echo of the Big Bang provides the story of how the metaphorical “smoke” from the gunshot that was the start of the universe was discovered—a story that sucks the reader into the historical richness and technical wonder of the human journey to understand how the universe came to be as it is now. Lemonick provides enough detail about the theoretical implications of the experiments involved to satisfy readers who are more familiar with Physics, as well as paints enough of a personal story of the dedicated scientists of the projects to make any reader feel as though he is witnessing discovery alongside them. Lemonick’s book is enjoyable, to say the least.
The Cosmic Microwave Background, or the so-called CMB, has an intricate and thorny history as far as its detection is concerned—as evidenced by timeline of experiments in the appendix—but its prediction came about naturally enough. The simple observation that the farther away a galaxy is from Earth, the faster the galaxy seems to be moving away implies that space itself is expanding in all directions. This expansion has always been happening, so the universe must have been much smaller in the past. Theoretically, if one were to rewind the cosmic clock, the universe used to be a single point, otherwise called a singularity. The universe probably was not a singularity, since a singularity doesn’t make any physical sense, but the model works for the time being. Playing the cosmic clock in the forward direction from the time of that singularity, one notices that the entire universe is very dense, and thus very hot. Theoreticians like Gamow, Alpher, and Herman noticed that a hot, dense universe might emit a Blackbody spectrum, but the dense soup of other particles would block the emission until the universe were sparse enough to allow atoms to form—which turned out to be about 380,000 after the singularity began expanding. As the universe expanded, the Blackbody emission would be redshifted. Gamow calculated that the emission should be detectible as microwaves. Furthermore, since the universe was uniformly hot in those early times, these microwave emissions should be uniform throughout the sky. So, detection of this Cosmic Microwave Background is vital to understanding the history of the universe. There may be various ways to explain the observation that more distant galaxies move faster, but the existence of a ubiquitous shower of microwaves coming from all directions from a distance that corresponds to the time that the Blackbody emissions supposedly broke free would confirm beyond a doubt that the universe began quite small and expanded to become the current universe. More subtly, the small perturbations in a mostly uniform CMB also give hints as to the physics of the very early universe. These small perturbations are important since they shaped how the galaxies formed and determined how matter is currently distributed throughout the cosmos. A completely homogeneous CMB would lend itself to a uniform distribution of galaxies, but our universe has an intricate web-like structure that is well explained by the aforementioned small anisotropies in the CMB. Some additional pieces of critical information can be gleaned from the so-called “acoustic peaks” observed in the CMB. These peaks give information as to the composition of the universe—the proportions of dark matter and baryonic matter—as well as its curvature. In these ways, understanding the CMB is vital in understanding the cosmos.
Tapping into the rich well of information provided by the analysis of the CMB requires extremely sensitive measurements that account for every possible source of error. This provides a colossal engineering project. The WMAP team that set out to make the most precise measurements of the CMB to date faced many engineering difficulties—among them being the problem of accounting for all the extraneous noise that could interfere with the CMB measurements, deciding what sort of detectors to use, deciding how the satellite should spin, the excruciating process of constructing the detectors, processing the data from over three million patches of sky, fitting all of the parts into a specified volume, and responding to manufacturing errors and faulty parts. Although each of these problems were significant, the three that stand out the most as vital to conceptual mission success seem to be deciding what detectors to use, accounting for extraneous noise, and processing the vast amounts of data from three million patches of the sky. Granted, actually building the damn detectors was a feat of its own, but choosing the best detectors in the first place is at least more conceptually important than constructing them. In other words, assuming that the quality assurance of detectors is independent of the choice of detector type, mission success is greatly determined by the choice of detector.
There were two kinds of electronic detectors that were in the front running to be placed in the radiometers of the WMAP satellite that would go on to measure the remnants of the early cosmos. These two kinds of detectors were bolometers, devices that heat up by a precise and perceptible amount when hit by microwaves, and HEMTs, or high electron mobility transistors, which serve to amplify the desired signal. The main advantage of the bolometer is that it has higher sensitivity than the HEMT. However, since bolometers rely on heat for detection, they must be cooled with a cumbersome cooling system that utilizes liquid helium. The use of liquid helium makes bolometers failure prone, and the cooling system would also make testing and design much more expensive— with money being a limiting factor in the WMAP project. This is without mentioning the extra weigh brought on by the cooling system, which is a vital parameter in space missions. HEMTs, on the other hand, are more durable and do not require such a cooling system. WMAP needed detectors that were not only sensitive, but also reliable beyond compare in order to minimize the chance for mission failure on an especially difficult and lengthy mission. So, HEMTs were chosen over bolometers. After the detectors were chosen, it was crucial that the team used those detectors as effectively as possible by minimizing every possible source of error.
The first step in minimizing the error from CMB measurements is to decide which measuring technique to use. The WMAP team decided to use differential measurements, comparing every data point to every other data point. These differential measurements were also performed at five different frequencies to ensure that the microwave measurements were actually from the CMB. To further ensure fidelity, WMAP would measure the CMB across the entire sky—significantly increasing the computational load. The most important factor in minimizing experimental error is tracking every possible source of extraneous noise that could interfere with the already delicate measurements. Noise could come from the internal electronics in the radiometers, external noise from the Sun and Earth, or other stray signals. Measuring on multiple frequencies helps, but in order to account for all the noise, the WMAP team simulated the sort of noise they were expecting by shining various sources of noise on the detectors with a control set of data and running simulations to get a sense of what sort of data was reliable.
Using differential measurements means more comparisons, and more comparisons means more computing power. Measuring at multiple frequencies further complicates the issue. Even worse, measuring the entire sky at a resolution of 0.2 degrees gives over three million patches of sky to analyze—a drastic increase over the 6,000 patches of sky analyzed in COBE. Astrophysicists at the time speculated that the computational challenge was impossible without a drastic increase in raw computing power. Luckily, Ned Wright took the challenge personally and came up with an algorithm on the 100-mile drive home from a conference on the topic that would analyze the data in a drastically reduced time. This breakthrough was essential in the success of the WMAP mission.
WMAP gives the accurate measurements necessary to probe into arguably the most fundamental question: Where did we come from? This question, among others, drives science forward, which, to borrow a phrase from Carl Sagan, “allows the universe to know itself.” It is an immeasurably tiny portion of the universe that gets the privilege to understand itself—even a little bit—and with this fact, I feel honored. For posterity, I will provide several questions that came up during reading: Why does Dicke’s model of the yo-yo universe require the times between bang and crunch to get longer and longer? Does the Ekpyrotic model of the universe also solve the flatness problem? How does inflation’s solution to the flatness problem change when one takes into account that the cosmological constant is not zero (specifically, \Lambda= 0.7)? Why does inflation theory predict background gravity waves and the Ekpyrotic model not? When using Hubble’s constant to calculate the age of the universe, do we take into account the fact that expansion is accelerating—and if so, do we assume the rate of acceleration is constant?
1939: McKeller and Adams unwittingly measured the CMB when looking at the excitation of cyanogens molecules in interstellar space.
1946: Robert Dicke almost discovered the CMB with the radiometer that he invented, if only he had read the literature by Alpher, Herman, and Gamow.
1960: Edward Ohm did indeed measured the CMB, but mistakingly attributed it to ground noise. He also should have read the literature.
1963: W. Jakes measured the CMB and listed it as an unexplained anomaly. He also should have read the literature.
1964: Arno Penzias and Robert Wilson measured the CMB and had the sense to call Dicke at Princeton in 1965, who told them what they had found.
1977: George Smoot detects the dipole in the CMB in a U-2 aircraft. The dipole unveils the lumpy structure of the universe.
1987: The Nagoya rocket measured the CMB to six frequencies, but the results did not match prediction.
1990: John Mather announces the results of the FIRAS experiment on COBE, which was a thousand times more sensitive than the Nagoya rocket. The results fit the predictions amazingly well.
1992: COBE finds the perturbations in the CMB that shows the matter distribution of the universe.
1993: Wilkinson, Page, Wollack, and Jarosik worked on the Saskatoon experiment, which found small-scale perturbations in the CMB, as well as the first acoustic peak, which determines the curvature of the universe. The results were not refined enough for a claim to discovery.
1996: Page’s TOCO experiment confirmed Euclidean geometry of the universe, but the experiment wasn’t nearly as precise as WMAP would be in determining this feature.
Late 1990’s: Many ground-based experiments like BOOMERANG, MAXIMA, and DASI were conducted, but not as reliable as the WMAP satellite would be due to ground interference.
2001: WMAP launched and proceded to put COBE to shame in terms of measurement resolution and precision.
2003: WMAP’s first observational period ends. Papers are published. The world is changed.
2010: WMAP’s last data is counted. Cosmological parameters are tightened in certainty.