A Beginning for the Universe

It was only in the most recent seconds of human existence, com­paratively speaking, that the universe began to take shape in the human imagination. For the vast majority of the 200,000 years since the emergence of Homo sapiens, the reaches of the depths of space were unfathomable. In just the last hundred years humans began to discover the universe and develop a basic understanding of its mass and age. One key to unfolding our current view was detection and mapping of the cosmic microwave background, the remnant light and heat of the big bang. The temperature of the vacuum of space is 2.725 K, a trace above absolute cold, exactly what the universe should have cooled to if it had expanded to its current size from a hot and dense initial state 13.8 billion years ago. NASA’s COBE and WMAP spacecraft have mapped this sig­nature of the moment when the abyss of space and everything in it came into being.

Prior to and throughout the first third of the twentieth century, most people, even most astronomers, simply assumed that the uni­verse always existed. Science historian Helge Kragh comments, “The notion of a universe of finite age was rarely considered and never seriously advocated.”3 Astronomers knew very little about the depths of space before the 100-inch telescope at Mount Wil­son Observatory near Pasadena became operational. At that time, they intensely debated whether the Milky Way comprised the en­tire universe, or whether the nebulae might lie far beyond our gal-

axy. On an October night in 1923, American astronomer Edwin Hubble, working with the 100-inch, then the largest telescope in the world, observed a variable star in M31, the Andromeda Neb­ula, which ultimately confirmed that it was millions of light-years beyond the Milky Way. The announcement of Hubble’s result in 1925 radically altered the scientific and public understanding of our place in space. Just a few years later, in 1929, Hubble and his assistant Milton Humason again rocked the world by reporting that remote galaxies were racing away from the Milky Way at 700 miles per second or faster. Their observations indicated that the universe was expanding, a seemingly preposterous idea that Albert Einstein himself initially refused to believe.

Given that relativity theory recognized space and time as insep­arable, the Belgian astronomer and Jesuit priest Georges Lemai- tre interpreted Hubble and Humason’s findings of the “runaway” galaxies as meaning only one thing. The universe itself was ex­panding, which in turn suggested that the universe must have been smaller, denser, and hotter in the distant past. Among Lemaitre’s many contributions to cosmology, three of his most simple and yet profound ideas were that the universe had a beginning, that both relativity and quantum theory were needed to explain this origin in terms of expanding space-time, and that Edwin Hubble’s reced­ing galaxies were evidence of this cosmic expansion.

Lemaitre was the first to suggest that Hubble and Humason’s redshifted nebulae indicated the expansion of space-time itself.4 In 1931, in the journal Nature, Lemaitre offered a short proposal for what English astronomer Fred Hoyle later derogatorily dubbed the big bang. In that article, Lemaitre postulated “the beginning of the universe in the form of a unique atom, the atomic weight of which is the total mass of the universe.”5 He theorized inflation of the cosmos from a “primeval nebula” or “primeval atom” of dense matter. Working from Einstein’s theory of general relativity as well as emerging theory in quantum mechanics of elementary atomic particles, Lemaitre proposed that the universe inflated from a dense, highly compacted soup of subatomic particles that at a moment of quantum instability resulted in the unfolding of space. “What’s remarkable about his Nature letter,” writes John Farrell, “is that—apart from discussing the idea of a temporal beginning of the cosmos—it marks the first time that a physicist directly tied the notion of the origin of the cosmos to quantum processes.”6 Even before the neutron had been discovered, Lemaitre understood that the beginnings of the universe could be explained in part via quan­tum theory and argued that “all the energy of the universe [was] packed in a few or even in a unique quantum.” By Lemaitre’s es­timation space and time or space-time could “only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the [universe] happened a little before the begin­ning of space and time.”7 Lemaitre depicted the early universe as analogous to a “conic cup,” the bottom of which represents “the first instant at the bottom of space-time, the now which has no yesterday because, yesterday, there was no space.”8

In 1934, in “The Evolution of the Universe,” Lemaitre outlined his “fireworks theory of evolution” in which the stars and galax­ies, having evolved over billions of years, were merely “ashes and smoke of bright but very rapid fireworks.”9 He described our situ­ated view from Earth, scanning the night skies, as we look back toward the primordial past: “Standing on a well-chilled cinder, we see the slow fading of the suns, and try to recall the vanished bril­liance of the origin of the worlds.”10 Lemaitre additionally intuited that a fossil light would be the signature of the universe’s begin­ning. As James Peebles, Lyman Page, and Bruce Partridge point out: “One learns from fossils what the world used to be like. The fossil microwave background radiation is no exception.”11 Lemai – tre expected evidence of a fossil radiation from the early stages of the universe could be detected. Thinking in 1945 that cosmic rays were the signatures of this fossil light, Lemaitre supposed that these “ultra-penetrating rays” would reveal the “primeval activity of the cosmos” and were “evidence of the super-radioactive age, indeed they are a sort of fossil rays which tell us what happened when the stars first appeared.”12 Just weeks before his death in 1966, Lemaitre celebrated learning of the discovery of the cos­mic microwave background, the fossil light he had anticipated. Throughout his career, he debated with Einstein whether or not his cosmological constant was a repulsive force that “could be un­derstood as a vacuum energy density.”13 Cosmologists now regard Einstein’s cosmological constant as indicative of the effects of dark energy contributing to the universe’s expansion (figure 12.1).