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Peebles Maps Road to the Big Bang

by Louise Good

Jim Peebles
Jim Peebles

Pioneering cosmologist and the Albert Einstein Professor of Science Emeritus at Princeton University P. J. E. (Jim) Peebles delivered a UH Distinguished Lecture, "Discovering the Big Bang," at the Campus Center Ballroom on November 15.

"Science is a series of successive approximations," stated Peebles. In his talk, he recounted some of the steps, or approximations, that have led to our current model of the Universe: It began with the Big Bang about 13.7 billion years ago, and is now made up mostly of dark matter, which we can't see, and "dark energy," which is the force causing the Universe to expand at an ever-increasing rate. Since the 1960s, Peebles has played a significant role in developing this model.

In 1912, Vesto Slipher was the first to measure the shift of spectral lines of what we now know are galaxies. This Doppler effect applied to light is known as the redshift, since as objects recede from us, the wavelengths of the light they emit become longer. In the 1920s, Edwin Hubble used these measurements as well as other observations to prove that the Universe was more than just the Milky Way galaxy—that the so-called "nebulae" were actually other galaxies. He formulated what is now called Hubble's law: the velocities at which galaxies move away are proportional to their distance from the observer.

Georges Lemaître, a Belgian Roman Catholic priest, was the first to propose what later became known as the Big Bang theory in papers published in 1927–1933. His ideas were theoretical, based on mathematics rather than observations, and because he published his papers in obscure journals, it took many years before his ideas became well-known.

In 1948 George Gamow and Ralph Alpher first predicted the existence of the cosmic microwave background (CMB), a faint glow of radio emission from the hot Big Bang. In the 1960s, Peebles was part of a group headed by his Princeton colleague Robert Dicke that set out to measure the CMB. What they discovered was that Arno Penzias and Robert Wilson at nearby Bell Laboratories had already found the CMB inadvertently while doing other experiments. Penzias and Wilson were experimenting with a very sensitive horn-shaped antenna built to detect radio waves bounced off satellites. To measure faint radio waves, they had to eliminate all recognizable interference from their receiver. After they removed all other interference, they found an ever-present noise that was evenly spread over the sky. A meeting between the Bell Labs and Princeton groups determined that this was indeed the CMB.

The CMB has since been measured with greater precision by the satellites Cosmic Background Explorer (COBE), launched in 1989, and the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001. Peebles praised the head of the COBE team, John Mather, for his courage in dedicating many years of his career to this one project, because had it failed, his career would have also failed. Fortunately, COBE successfully mapped the CMB, thereby providing definitive evidence of the Big Bang, and Mather shared the 2006 Nobel Prize in physics.

Fritz Zwicky was the first to deduce the existence of what we now call "dark matter" in 1933. He applied the virial theorem—a way of estimating the total mass of an object such as a group of galaxies from the movement of its individual members—to obtain evidence of unseen mass in the Coma cluster of galaxies and to explain why the galaxies remain in the cluster.

Peebles concluded his history of modern cosmology by discussing dark energy, a force that explains why the Universe appears to be expanding at an accelerating rate and accounts for over 70 percent of the total mass-energy of the Universe.

During this visit to Hawaii, Peebles also gave an IfA colloquium entitled "Galaxy Formation: Successes and Challenges for the Standard Cosmology." He asked his fellow scientists if the standard Big Bang cosmology is an adequate model for "observational astronomy of the 21st century, or only the simplest approximation we can get away with at the present level of the evidence?" Clearly, Jim Peebles is not finished building a model that approximates the Universe.