Reconstructing ancient history is not easy to do. Just ask a paleontologist: No matter how many dinosaur skeletons or Neanderthal skulls scientists dig up, they still can tell only a small part of the story of what life on Earth was like millions, or even thousands, of years ago.
Which makes it particularly amazing that over the last half-century, cosmologists (and I’m happy to count myself among them) have reconstructed the history of the entire universe all the way back to seconds after the Big Bang that sparked it into existence 13.7 billion years ago. And it is not just a rough reconstruction. We know exactly what the infant universe was made of and what it looked like in those earliest moments.
That’s pretty impressive detective work, but we are still not satisfied. Now the push is on to peer back even farther, to a tiny fraction of a second after the Big Bang. This will help us address the deepest questions about our place in the cosmos: How did it all begin? Is our universe the only one? And if not, why this universe rather than some other one?
Distant Light of the Past
Studying the universe’s past presents a challenge similar to studying the Earth of long ago: Over time, things change. For living things, flesh decomposes and bone turns into fossil. For cosmic things, many particles that came out of the Big Bang—such as electrons, protons, and neutrons—have been processed in the cores of stars. The trick to understanding the past is finding artifacts that have remained largely intact over vast spans of time. In this area, cosmologists are much more fortunate than paleontologists, because the universe harbors many particles whose identities have remained unchanged for billions of years.
The most obvious of these relics are particles of light, or photons. When we view an image of a stunning galaxy from the Hubble Space Telescope, we are actually looking at a snapshot of history. If the galaxy is 2 million light-years away, then we are viewing it as it was 2 million years ago because that is how long the light traveled, undisturbed through vast stretches of empty space, before it reached us. Over the last few years, Hubble has given us views of infant galaxies as they were just 500 million years after the Big Bang, allowing cosmologists to see how quickly the raw materials from the newborn universe coalesced into stars and then galaxies and then clusters of galaxies.
The most valuable photons are even older, dating back to only 380,000 years after the Big Bang. Before that time, the universe was an opaque fog, so hot and dense that photons could not travel very far before bumping into other particles and changing direction. But then the universe cooled sufficiently for electrons to stick to nuclei and form stable atoms. The resulting gas—almost all hydrogen and helium—was transparent, allowing photons to zip freely through space at last.
Many of those photons have traveled undisturbed ever since, and in 1964 a bunch of them landed on a radio antenna set up by Arno Penzias and Robert Wilson at Bell Labs in New Jersey. They had accidentally discovered the cosmic microwave background, the afterglow of the Big Bang.
In the years since, satellites such as Planck and the Wilkinson Microwave Anisotropy Probe have mapped these photons and provided a fantastic view of the 380,000-year-old universe. By studying subtle fluctuations in the temperature of the cosmic microwave background, cosmologists have determined the total amount of energy in the universe and how the form it takes has changed over time. Matter (both ordinary atoms and the invisible stuff called dark matter) once dominated the universe, but today it constitutes only a quarter of the content of the cosmos. The rest is a strange, anti-gravity substance known simply as dark energy.
Probing the Nuclear Inferno
The cosmic microwave background is a powerful tool, but cosmologists can call on even older relics, ones that penetrate the opaque, photon-trapping fog and bring us all the way back to the first seconds of the universe’s history. Those relics are atomic nuclei, forged in the primordial fires of the Big Bang.
In 1948, George Washington University graduate student Ralph Alpher and his adviser, physicist George Gamow, theorized that over the course of its first few minutes, the universe was so hot and dense that it behaved like a nuclear fusion reactor, cooking the primordial soup of protons and neutrons into heavier atomic nuclei: deuterium or “heavy hydrogen” (one proton and one neutron), helium (two and two), and lithium (three and four). Their theory, known as Big Bang Nucleosynthesis, included detailed predictions of how much of each element would have been produced in the roughly three minutes of nuclear reactions.
Amazingly, we can test the Big Bang Nucleosynthesis theory by finding the primordial deuterium, helium, and lithium that remain today. Just as paleontologists hunt for fossils in isolated caves and dry rift valleys, cosmologists have identified relatively untarnished parts of the universe where atomic nuclei have remained largely undisturbed since the earliest times. Important targets are dwarf galaxies like I Zwicky 18, where stars did not ignite until recently, leaving most of the galaxy’s material intact. Deuterium, helium, and lithium nuclei each absorb and emit light in a unique way, allowing scientists to point telescopes at I Zwicky 18 and determine the abundances of ancient nuclei very accurately. The observed amounts of these elements are just what Alpher and Gamow’s theory predicts.
Think about what that means: Sitting here on Earth, cosmologists extrapolated our understanding back 13.7 billion years, to a few seconds after the universe began. We used that understanding to make predictions about the current universe—and we were right. We may not know for sure whether it will rain tomorrow, but we do know exactly how protons and neutrons bounced around like Super Balls in the nuclear inferno of the Big Bang. This will surely go down as one of the most impressive accomplishments of the human intellect.
And yet cosmologists want to do better still. The goal is to discover relics that predate even Big Bang Nucleosynthesis. At the moment that’s not quite possible, but there is one promising candidate: dark matter, the dense but unseen stuff that holds galaxies together.
Artifact or Worthless WIMP?
At first, dark matter may seem a strange choice. We have never directly detected it, and we do not know what it is made of. But we do know that it doesn’t seem to interact very much with anything—which, for the cosmic paleontologist, is a great asset. (The lack of interaction is why dark matter is dark: Light has no effect on it.) According to leading theoretical models, dark matter stopped interacting with the rest of the primordial particle soup very early on, about 1/10,000 of a second after the Big Bang, when the temperature of the universe was over 100 trillion degrees Fahrenheit (today it averages –455°F).
Theorists’ leading candidate for dark matter is the weakly interacting massive particle, or WIMP. Experiments in deep underground facilities, like the Soudan mine in Minnesota and the Gran Sasso laboratory in Italy, are searching carefully for wimps. At the same time, physicists are trying to create wimps directly at particle accelerators like the Large Hadron Collider near Geneva.
If these efforts succeed, we can measure the properties of wimps and then play the Big Bang Nucleosynthesis game all over again, this time with dark matter. We could predict precisely how much dark matter would be left over from the early universe and compare it with the amount we measure today. Then there are two possibilities: Either the prediction matches reality, and we can rightfully claim to understand what the universe was doing a scant fraction of second after it began; or the prediction fails, and we have to develop new, deeper theories to address the error.
Even if dark matter fulfills cosmologists’ wildest dreams, our quest will be far from over. It may sound good enough to get to within 1/10,000 of a second after the Big Bang, but theorists believe a lot of interesting things happened before then, most notably a rapid expansion of the universe, called inflation, and of course the instant of the Big Bang itself—the equivalent of tracing evolution all the way back to the origin of life.
The closer we get to that point, the better we will understand how our universe came to be, and whether other universes could have formed in the same manner. One way or another, we will keep coming closer to understanding the very beginning of time.
Sean Carroll is a theoretical physicist at Caltech and a DISCOVER blogger. His book on the Higgs Boson is