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
Quantum Origin of Structure
Let's consider the universe on the largest scales. Recent observations support a remarkably simple model of the universe as a whole, based on Einstein's (non-quantum) theory of space and time. It appears that space as a whole is very close to uniform and is much larger than the piece of it that we can ever observe (a property we guess from the apparent Euclidean flatness right out to the edge of what we can observe). The entire three-dimensional space has been steadily expanding for about 14 billion years from a much smaller, hotter and denser initial state. (Since we are observing it from inside, we cannot see what it is expanding into!)
The cosmic background radiation has been traveling freely through this expanding space since the universe was less than a million years old, when (because of the expansion) it was a thousand times hotter than it is today. The spectrum of the radiation reproduces to extreme precision the prediction of Big Bang theory, and dates back to the first year of cosmic history. This ancient light has precisely the simplest mixture of colors—a blackbody spectrum, a phenomenon that was first described mathematically by Max Planck a century ago. (His description was the very first piece of quantum physics, but of course Planck knew nothing of the Big Bang and the cosmic background radiation.) The radiation must have been there already much earlier, because the theory also correctly predicts the composition of primordial matter—a simple mixture of hydrogen, helium and lithium isotopes created during the first few minutes of the expansion, starting when the temperature was still over a billion degrees. The accumulated, interwoven evidence is so strong, precise, varied and direct that most cosmologists now regard the Big Bang not as a theory or a model, but as a fact.
The expanding universe at the heart of this model is thought to be given its form by the action of an energy field, the inflaton. With the right properties, the inflaton's interactions can lead to repulsive gravity and create an instability that drives the original expansion of the Big Bang by making everything fly apart from everything else. We think this process of inflation is the way the universe got to be much bigger than an atom. Inflation is what made the Big Bang big.
Now shift to the universe on small scales. The basic structure of everything around us—pizzas, teenagers, bad TV shows—is determined by the way atoms behave. The set of mathematical rules called quantum mechanics, discovered during the 1920s, describes atoms with exquisite precision. Quantum physics tells us that energy always comes in discrete packets, rather than a continuum, and that the packets of energy also behave like waves. These quanta obey the Heisenberg uncertainty principle: The more definitely we know a particle's location (the smaller its wave packet) the greater its momentum tends to be. Elementary particles obey these rules spontaneously, forming identically stable structures such as the nuclei and electron orbitals of atoms with very specific patterns of behavior and structural properties that give the everyday world its regularity and shape (Figure 4). For almost 80 years, scientists have used quantum mechanics as the basis for physics, chemistry and biology because it describes the building blocks of nature. It accounts for everything that happens on the microscopic scales of elementary particles. Photons make light. Quarks, gluons and electrons make atoms. Atoms make molecules. And molecules make pizzas.
Because inflation makes small things big, it creates a new and unexpected role for quantum physics in determining the behavior of the entire universe on the very largest scales. Quantum effects lead to a subtle imprint of inflation on the present-day universe. The inflaton field was not perfectly smooth, but contained imperfections or "fluctuations." The energy that created the universe, like all energy fields, was a quantum field; it came in discrete packets of energy called inflatons, in the same way that light comes in discrete packets of energy called photons, or that atoms are made of discrete elementary particles such as electrons. Quantum fields are never completely at rest—even the emptiest vacuum state is roiling with fluctuations of "virtual particles" emerging and then disappearing back into the vacuum, whose (very real) physical effects are seen clearly in laboratory experiments sensitive to virtual photons. Thus the primordial inflaton could never be perfectly uniform in space. This means that the primordial expansion got a slightly bigger kick in some locations than in others. The effect of a single inflaton quantum was enormously inflated, in the same way that the universe itself was, so a single inflaton left its influence on an astronomically vast tract of space (Figure 5).
The inflaton fluctuations are very important. For one thing, they are the reason that the universe eventually broke up into galaxies, stars and planets. The inflaton fluctuations, frozen into the fabric of space, were converted into very slightly denser and sparser regions of matter. The dense regions eventually collapsed due to gravity. Without these perturbations, the universe would still be perfectly smooth today. Every galaxy we see (even whole clusters of galaxies) ultimately derives from about one elementary inflaton particle in the early universe.
Think about this for a minute. The standard theory now holds that our whole galaxy started its life more or less as a single elementary particle, much smaller than an atom. I used to think this was a crazy idea, but now I am persuaded by high-quality data that it is probably true.
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