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FEATURE ARTICLE

Observing the Beginning of Time

New maps of the cosmic background radiation may display evidence of the quantum origin of space and time

Craig Hogan

Figure 1. Separation of light from darkness . . .Click to Enlarge Image

The sky is never really dark at night. Even in the remotest parts of the universe, the cosmic background radiation fills all of space and bathes everything with light almost evenly from all directions. This light originated in the earliest moments of the expanding universe, and it is the source of energy from which matter emerged. Since the earliest times, the light has shaped, and been shaped by, the evolution of the universe—its composition and its structure. In the early universe this radiation was the dominant form of energy. Matter was just a trace component. Even today the cosmic background radiation has more energy than all the light coming from all the stars in the universe. The reason the sky looks dark today is that the cosmic background radiation is concentrated in the millimeter-wavelength range, a cold light (scarcely three degrees above absolute zero) that is invisible to our eyes. A long time ago it was much hotter than the inside of a star.

Recent experiments—using special-purpose telescopes on satellites and balloons, and in such exotic locations as the South Pole—have revealed patterns of very slightly bright and dim patches in the cosmic radiation. These patterns of light reflect patterns of gravity that are left over from the very early universe. Close study of these anisotropies is quantitatively transforming our knowledge of the overall properties of the universe as a whole, giving us precise information on how big it is, how old it is, and what kind of matter and energy it is made of. Here I discuss another transformation, of a more conceptual nature, that has the potential to change how we think about space and time, and matter and energy—how they came to be, and how they are put together.

Figure 2. History of the universe . . .Click to Enlarge Image

Consider this seeming paradox: The biggest and smallest things in nature are the same things. At first this statement seems to make no sense, yet it is not an obscure metaphor or Zen koan. It is a profound truth about the universe, exactly and literally. It can even be represented by a picture (Figure 3). When we look at the largest structures in the cosmic background radiation—the largest and most distant things we can possibly see, stretching across the whole sky at the edge of the universe—we are looking at patterns that were imprinted in the first moments of creation, when these patterns were single quanta—the smallest amount of something (anything), according to quantum theory—far smaller than the smallest subatomic structure ever seen in the laboratory. Even though we are used to the idea that everything in the universe is connected with everything else, such a literal connection between the quantum world and the cosmic world is surprising. That is because most of the time when we look at large things—anything you can see without a microscope—they look continuous. There is no obvious sign that they are made of discrete microscopic elementary particles. So it is remarkable that when we look at the very largest things, we start seeing the quanta again. The universe expanding all around us acts like a giant microscope.

Figure 3. Tiny variations (anisotropies) . . .Click to Enlarge Image

Here is another seeming paradox about these apparently tiny, strange and exotic fluctuations: These primordial quanta are the most important organizing agent of the universe. The quantum effects from the early universe, at first so subtle, are eventually amplified, first by the inflationary universe and then by their own gravity. Ultimately they determine where all the matter ends up—where galaxies appear and when they collapse out of the expansion, whether a galaxy ends up big or small, at this place or that place. In fact, these fluctuations are responsible for the formation of everything within galaxies, including the stars and planets. The enormous complexity of a whole galaxy grew out of an almost structureless elementary particle.

Strange as these findings may seem, they essentially represent the settled opinion of the cosmological community. Theorists are now thinking about even more curious possibilities. We may soon be able to measure indirectly the quanta of gravity, the gravitons. These are literally "particles of spacetime." Perhaps when we find one we will learn something about how time is put together and the reason it behaves the way it does. We may even be able to detect signs of the quantum discreteness in the states of these particles, equivalent to a kind of "pixelation" of the spacetime continuum. In principle, we might reach the stage where the data are painting a picture not of space and time, and matter and energy as we know them, but of a deeper underlying structure in which these concepts cannot be prized apart. If we ever reach that stage, we will have a glimpse of the beginning of time as it emerged into existence as a property of a more fundamental, irreducibly discrete underlying entity.




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