Last month, astrophysicists announced a groundbreaking discovery: compelling evidence for gravitational waves had been found and the source of these waves might be the inflationary period just after the Big Bang.
Compelling the evidence may be, but could there be another explanation?
In a paper submitted to the arXiv preprint archive last week, a trio of theoretical physicists pushed back on the historic Cosmic Extragalactic Polarization 2 (BICEP2) finding, suggesting that there may be another source of the gravitational waves: What if they weren’t generated during the rapid period of inflation?
First, let’s rewind a little. What is inflation and how are gravitational waves strong evidence of its occurrence?
In a nutshell, when the BICEP2 researchers detected polarized ripples in the cosmic microwave background (CMB) radiation, the signal strongly matched theoretical predictions as to what gravitational waves should look like. Until now, gravitational waves — that Einstein theorized during the formulation of his bedrock theory of general relativity nearly 100 years ago — have been maddeningly difficult to detect. So, at first glance, the BICEP2 finding is a historic one; this is strong observational evidence for gravitational waves, a fact that no scientist is disputing.
But how did these gravitational waves end up being etched into the CMB? This is the point of contention.
The CMB exists right at the very limit of our observational capabilities. Widely regarded as the “echo” of the Big Bang, which occurred nearly 14 billion years ago, we can analyze the very slight temperature fluctuations in the CMB (known as “anisotropies”) to gain an insight to the structure of the Universe just after the Big Bang.
For the Universe to exist in its current scale and for it to have been spawned from a point, however, cosmologists believe the Big Bang had to have been followed by an intense period of acceleration. This faster-than-light expansion occurred a billionth of a billionth of a billionth of a millionth of a second after the Big Bang.
For our current understanding of the Universe to hold true, inflation had to occur. This is where the BICEP2 results comes in.
As interpreted by the BICEP2 team, the gravitational wave polarization signature embedded in the CMB suggests that the gravitational waves themselves have inflationary origin. Gravitational waves are generated by the most energetic events in the Universe — from black holes colliding to stars exploding — like ripples traveling across the surface of a pond, these waves travel through spacetime at the speed of light.
Inflation theory researchers also believe that primordial gravitational waves may be generated during the most violent event our Universe has ever seen: inflation. And the BICEP2 results certainly point to strong evidence of an inflationary (and quantum) origin of these waves.
However, theoretical physicists James B. Dent (University of Louisiana at Lafayette), Lawrence M. Krauss (Arizona State University, Tempe) and Harsh Mathur (Case Western Reserve University, Cleveland, Ohio) think that the BICEP2 researchers may have overlooked an alternate source of these waves.
“While the Inflationary signal remains the best motivated source (of the gravitational wave signal), the current measurement unfortunately still allows for the possibility that a comparable gravitational wave background might result from a self ordering scalar field transition that takes place later at somewhat lower energy,” the physicists write.
So what does this mean?
After the Big Bang, the Universe was a seething chaotic mess of energy. As the Universe cooled, this energy slowly condensed — like raindrops forming from vapor in clouds — to create the fundamental particles and forces we know and love in our modern epoch. Each particle and force came into existence after each successive “phase change” — but these phase changes weren’t created equal and didn’t occur at the same time across the entire Universe; phase changes occurred in localized pockets.
But at a critical point, when the Universe was cool enough, these pockets are thought to have aligned all at once, “snapping” into place.
Although this critical point phase change across the entire Universe was of a lower energy than the inflationary period that came before it, Dent and co. theorize that it would have created a violent ripple that could have spawned the gravitational waves that BICEP2 is now observing in the CMB.
As pointed out by the arXiv Physics Blog on Medium.com, the theorists are not debating whether or not inflation occurred, they are highlighting another possible source for the gravitational waves.
When it comes to unraveling the mysteries of the dawn of the Cosmos, you can’t pick one theory that “proves” one mechanism — it’s a process of elimination where competing theories are tested against observational evidence. Although the BICEP2 researchers have found compelling evidence of gravitational waves with inflationary origin, this new paper shows that theoretical physicists are far from finding a consensus.
Now science will do its thing — whether these gravitational waves have inflationary or phase-change origins will continue to be tested until one theory is left standing.
Publication: Killing the Straw Man: Does BICEP Prove Inflation? Dent et al., 2014.(Apr 1, 2014 03:18 PM ET // by Ian O'Neill)