For scientists, the really huge, cymbals-crashing discoveries are hard to come by these days.
A breakthrough can take years of labor, and the final result might be a graph of data in which the signal is barely discernible amid the noise. Dramatic findings are invariably questioned by other scientists. Doubt lingers. Science isn’t a tall stack of hard facts; it’s a difficult and deeply human process that lurches toward an approximation of the truth.
This is the current situation for cosmology, the study of the origin and structure of the universe.
Cosmologists may be on the verge of confirming a mind-boggling, 35-year-old theory called “inflation.” The theory holds that the very young, hot, dense universe underwent a brief inflationary eruption — stretching itself in a violent yawn at the dawn of time — before calming down and expanding in a more civilized fashion to become the twinkling cosmos we see today.
The best evidence for inflation seemed to have arrived on March 17, when the leaders of an experiment called BICEP2 held a news conference at Harvard University. They said their South Pole telescope had detected squiggles in ancient cosmic radiation that were probably caused by gravitational waves generated when the universe underwent inflation.
But then other scientists threw dust in the team’s face. They said the BICEP2 scientists underestimated the effects of foreground dust that contaminates the view. Dust can mimic the effects of gravitational waves.
Now everyone is waiting for more information. Scientists working with data from the European Space Agency’s Planck space telescope recently decided to collaborate with the BICEP2 team to come up with a map of galactic dust. That map could potentially obliterate the BICEP2 discovery — or validate the original interpretation.
The debate about what, exactly, the BICEP2 telescope detected demonstrates the difficult and often fractious nature of modern science. Big-money prizes and lofty honors can hinge on intrinsically ambiguous experimental results. When does data condense into a “discovery”?
“The overarching theme here is that, out there at the frontiers of discovery, it’s very foggy,” said Michael Turner, a cosmologist at the University of Chicago.
Leonard Susskind, a Stanford University cosmologist, said: “The whole field is facing a kind of crisis. All the easy experiments are done. In 1900 you could do an experiment on a tabletop that would convey deep and important information about the way the world works.”
Now, astronomers go to mountaintops in the Chilean desert and to the bottom of the world to set up their telescopes, and everyone seems to agree that this is heroic science.
But the origin of the universe is a tough nut to crack.
Cosmic evolution takes place beyond the human time scale. The ancient shepherds saw the same stars in virtually the same positions as we do today. All of human history fits into a slice of cosmic space-time that’s pastrami-thin.
At issue here is not the big-bang theory itself, which is supported by multiple lines of observational evidence. This is about how the big bang unfolded. If the universe underwent inflation, then the stuff we see in our telescopes is a tiny fraction of what’s actually out there. There’s a cosmic horizon beyond which the great bulk of the universe will forever be hidden.
Fifty years ago, scientists at Bell Labs in New Jersey chanced upon cosmic microwave background (CMB) radiation, the afterglow of the big bang. That radiation dates to the time when the cooling, expanding universe first became transparent to light.
Now scientists are picking through the patterns in the ancient light, looking for the polarization that, according to inflation theory, should have been created by primordial gravitational waves.
This would be the first observed evidence of the nature of the universe when it was extremely small, dense and hot. That polarized light would not merely be evidence for inflation; it would be a measure of it — actual data that describes the intensity of the event.
There is a saying that the big bang is the poor man’s particle accelerator. Scientists these days have to build multibillion-dollar particle colliders to study what kind of stuff emerges in high-energy environments. But this would be data just sitting in the sky, waiting to be harvested by the right kind of (comparatively inexpensive) telescope.
But if it is just dust that the BICEP2 scientists detected, it could be an embarrassment for the field. They could have been more cautious when they announced their results. They held the news conference before their scientific paper had been peer-reviewed and published in a journal.
“We felt it was imperative for us to report our results, and we did it as faithfully and honestly as we could given the information we had at the time,” said BICEP2 principal investigator John Kovac of Harvard.
The team’s initial paper, as submitted to the journal Physical Review Letters, ended with a barely hedged statement of triumph: “The long search for tensor B-modes is apparently over, and a new era of B-mode cosmology has begun.”
In the final version published in June, the authors continued to claim that the evidence points to a dawn-of-time signal, but they acknowledged they could not rule out the possibility that this signal may come entirely from galactic dust.
Katherine Freese, a University of Michigan cosmologist who has devised one of the models for inflation, was ecstatic about the BICEP2 announcement, particularly when she realized that the value for the gravitational effect was consistent with her own inflation model. But the scientists should have hedged their language, she said.
“If this kind of thing happened all the time, it would give us a bad name,” she said.
She pointed out, however, that inflation theory makes other predictions that are consistent with observations in recent years. For example, the CMB radiation is not perfectly smooth but rather has slight density fluctuations that are consistent with the inflation theory.
“There’s a bunch of generic predictions of inflation. They all came out right. If gravitational waves are there, too — dammit, there’s something right there,” she said. “That’s where the Nobel Prize comes in.”
One of the founders of inflation theory, and the man who named it, is Alan Guth of MIT. When he first dreamed up key elements of the theory in December 1979, he jotted in a notebook, “SPECTACULAR REALIZATION.”
Inflation theory, at first glance, seems a little bit like cheating: The universe erupts seemingly out of nowhere and nothing. It quickly gets big, simultaneously filling with energy. It’s like a self-filling bucket that grows larger at bewildering speed.
How is that possible?
“The simple idea that encapsulates inflation,” says Guth, trying to sum up his complex theory in words that could fit into a tweet, “is that the propulsion mechanism that drove the big bang was a repulsive form of gravity.”
Guth recognized that gravity resides in the physics equations as a form of negative energy. Gravity exactly counterbalances all the positive energy (including energy that has condensed into matter) in the universe.
“The total energy of our universe is, as far as we can tell, consistent with being zero,” Guth says.
Unfortunately, some features of the theory appear to be intrinsically unprovable. Most models of inflation make sense only if our universe is a mere bubble in a “multiverse” that has an infinite array of universes popping up all over the place. Those other universes will always be too far away to be seen.
Philosophy question: Can an undetectable multiverse be considered a truly scientific idea?
Despite these limitations, inflation caught fire quickly among theorists because it offered a nifty explanation for puzzling characteristics of the cosmos. The universe isn’t just big; it’s remarkably uniform. Look one direction into deep space and the view is the same as what you see in precisely the opposite direction. Theorists talk about the “flatness” of the universe.
Inflation says the universe looks flat only because, after all that inflation, we’re seeing only a tiny fragment of what really exists — the same way that an Iowa cornfield looks flat even though it represents a slightly curved section of a spherical planet.
The viability of the inflation theory, Guth said, does not pivot on the outcome of the BICEP2 controversy.
“If it does all go away, it will be a big disappointment, and in terms of public relations it would look bad for cosmology and for inflation in particular,” Guth said. “Scientifically, it does not constitute any evidence against inflation.”
Under the strict rules of the Nobel Prize, a maximum of three people can be honored for a discovery. At least half a dozen people — including Guth and the Russian American theorist Andrei Linde — contributed significantly to inflation theory.
Linde, now a professor at Stanford, acknowledges that Guth has scientific priority for the basic idea of inflation. But in the early 1980s, Linde came up with the first mathematically complete, “working” version that has been the basis of many inflation theories in the years since.
“It’s not one thing,” Linde said. “It’s a class of theories. Inflation is a principle.”
Guth and Linde, along with another early inflation theorist, Alexei Starobinsky, recently shared the $1 million Kavli Prize for astrophysics for the inflation theory.
Perhaps the most vocal critic of the theory is Paul Steinhardt, who helped devise one of the first working versions of the theory right about the time that Linde did. Guth, Linde and Steinhardt shared the 2002 Dirac Medal from the International Center for Theoretical Physics for their work on inflation.
Steinhardt, a professor at Princeton University, turned against the theory because he says it can be tinkered with to fit any possible experimental observation.
In the multiverse scenario, all possible universes exist, with random laws of physics. We find ourselves, not surprisingly, in a universe that has the physical laws that enable the appearance of life, the evolution of complex organisms and the eventual appearance of cosmologists making PowerPoint presentations.
Inflation theory and the multiverse cannot be proved wrong, and Steinhardt says that is a deal-breaker for him.
“It makes the theory a nonscientific theory,” Steinhardt said. “For the last 400 years, most people would say the key thing that distinguishes science from non-science is that scientific ideas have to be subject to tests. Some people are nowadays thinking, no, that doesn’t necessarily have to be the case. That’s a mega-issue.”
Steinhardt affects no pain at being bypassed recently by the Kavli Prize.
“I think I would be uncomfortable to receive an award for a theory in which I no longer have confidence,” he said.
Our own cosmic bubble will be around for an extremely long time, but perhaps not forever, or not in a form congenial to life. The search for answers to these big questions carries with it a humbling realization: The universe is what it is.
We don’t get to decide. The best we can hope for is to discover, perhaps only approximately, what’s going on.