The BICEP2 Results and What They Mean: The First Observation of Gravitational Waves from the Early Universe
In Plain English:
The guys who came up with gravitational wave theory explain the gravitational wave story that’s been blowing up everybody’s Facebook feed in terms undergrads can understand
What it covered:
When the BICEP2 team announced that they had found b-mode-style (aka “swirly pattern”) gravitational waves that confirmed inflation model of universe-formation, the internet exploded. The video feed for the press conference crashed. When the team posted their paper on arXiv, the it got 3.5 million hits in the first 11 hours.
That’s for the formal academic research write-up (and the average academic research write-up is lucky if 35 people read it all the way through). In the first 11 hours.
That constitutes a media feeding frenzy of epic proportions, even by MIT Cosmology department standards, so MIT decided to go all out: They threw a panel featuring three of their top cosmologists (including the guy who came up with repulsive gravity theory) and the BICEP2’s primary investigator, where each physicist would give a short undergrad-friendly talk about what the BICEP2 results meant and why we should care. It was a two-and-a-half hour panel, but everyone was so excited about the science being presented on stage that it felt much shorter. Still, this is going to be a long recap, so I’m going to chunk-ify a bit more than usual.
Part I: Alan Guth
First up was Dr. Alan Guth, one of the earliest and most frequently-cited inflationary theorists. A lot of people would have played up the “I came up with this theory when I was a young physicist with no way to prove it” angle, but Guth gave us a very straightforward and elegantly simple overview.
We’ve known that gravity pulls objects toward each other since Newton, but for a long time, physicists assumed that all matter is gravitationally attracted to all other matter. In 1917, Einstein appended the idea of a cosmological constant to the theory of relativity. He posited that vacuum space must somehow push back against gravity, because otherwise we would expect the universe would shrink over time. However, when the Hubble telescope found that distant galaxies were actually accelerating away from us, Einstein disowned the cosmological constant theory.
And for a long time, physicists were really confused.
Until 1979, when Guth had a breakthrough. He realized that it was possible that the cosmological constant could actually be negative, that there could actually be a special kind of matter called “repulsive gravity material” that repels matter. If its repulsive force was strong enough, then this mysterious repulsive gravity material could account for the fact that galaxies are moving away from each other faster and faster. None of the matter on Earth behaves that way, but Guth reasoned, “Why couldn’t a repulsive material gravity material exist out in the vacuum of space where we can’t see it?”
Since The Big Bang Theory holds that the universe began in a hot, dense state (I will wait for the song to finish playing in your head.), the initial amount of space taken up by repulsive gravity material would have been tiny. Guth guesstimated that it would have been about one trillionth the size of a single proton. (It turned out, his guesstimate was actually in the ball park.)
However, once it began to repel the conventional matter away, the repulsive gravity material would become diluted by its own expansion. If we tried to look for repulsive gravity material in the modern universe, we wouldn’t be able to see it because its signal would be so faint.
If we were ever going to be able to see the repulsive gravity material, we were going to see it in extremely distant signals from the very early universe. But in 1979, there was no way to observe signals that faint, so for a long time, inflation theory survived on the basis of “We can’t find the substance that causes it, but it explains the phenomena we see better than any of the other theories.”
For a long time, the most substantial pieces of evidence for inflationary theory were the fact that the universe is fairly uniform (which would make sense it was the product of a compact, fairly uniform plasma that has since expanded) but has small pockets of nonuniformity (which would make sense if said plasma had small pockets of repulsive gravity material, which have driven expansion of said universe).
Dr. Guth endured decades of astronomers making fun of him at dinner parties until 1998, when the Planck Satelite team discovered the existence of dark energy. Dark energy, or dark matter, is very similar to repulsive gravity material, except it doesn’t repel quite as much as the material that created the universe.
If the repulsive gravity material did exist in the early universe, we would expect it to cause disruptions in the polarized plasma. However, as radio signals from the early universe travel across space toward us, they get stretched out until they look basically like completely straight lines. (And they’d be basically indistinguishable from magnetic e-modes.) However, radio patterns from the very early universe would have a swirly-pattern, because in the first few hundred thousand years after the big bang, the waves would be compressed enough to actually look like swirly-waves, as opposed to the gridlike e-modes.
We just found the swirly pattern.
Dr. Guth concluded by saying that the discovery of gravity waves was actually reassuring (as well as exciting) because these gravitational waves are the first conclusive evidence for quantum gravity. In quantum theories, everything is probablistic, meaning that we can’t define both the position and the momentum of a subatomic particle at the same time. We can only measure the probability of a particle being any at a particular position with a particular momentum. Nothing is definite. All the quarks are always spazzing. Since we’ve pretty definitively determined that the other three fundamental forces (electromagnetism, strong nuclear force, and weak nuclear force) all behave probabilistically, physicists had been puzzled by the apparent lack of quanta in gravity. But these swirly patterns indicate that gravity is also probabilistic…No one knows what that means in practical terms yet.
Part II: Scott Hughes
Next up was Dr. Scott Hughes, a theoretical physicist who specializes in gravitational wave sources. His job was to explain why these gravitational waves could only be caused by the repulsive gravity material from the time of The Big Bang.
He started with a basic explanation of how electromagnetic radiation travels in waves of varying width. Visible light is one small chunk of the electromagnetic spectrum. Electromagnetic waves are created by electromagnetic fields, due to movement of charged particles (usually electrons), and we can detect patterns called field lines that allow us to “see” the shape of the electromagnetic field.
“Field lines always want to point back toward the charge,” he explained, “But the information about where the charge is is limited by the speed of light.”
Then he balled his hands into fists and raised them above his head. “You in the back of the room! You are attracted to my fists!” he declared. “You are drawn to them. You can’t help it. But if I go like this (he jiggles his fists around a little bit), you won’t know exactly where my fists are at this precise moment. You can only get that information after the amount of time it takes the light to reach you.”
Since electrons are always moving, electromagnetic field lines are always “wiggling”, based on their “best guess” of where the charge is. Those wiggles manifest as waves.
Gravitational waves behave a lot like electromagnetic waves. They produced by protons “roiling” or creating gravity ripples as they move through vacuum fields, something that would only happen in the hot, dense state that preceded The Big Bang or in a situation where neutron stars were orbiting each other at near light speeds (something that neutron stars don’t do).
The upshot of this is that since gravity fields have detectable field lines, and since the inflation model is pretty much the only event that would produce “swirly-pattern” gravitational waves, we can be pretty sure that the waves detected by BICEP2 came from an early inflationary universe.
Part III: John Kovac
Up next was the man of the hour, John Kovac who is the primary investigator on the BICEP2 project and the Keck Array project which is collecting data that will be used to develop a “less fuzzy” picture of the b-modes. All of the data collection from BICEP2 was completed over a year and a half ago, but analyzing astronomical data and condensing it into research paper format is a long and arduous process. A 1-2 year lag between the end of data collection and a paper’s debut is not unusual at all.
This panel took place less than two weeks after the initial press conference, and almost everyone was on the edge of their seat, waiting to hear about the BICEP2 results in detail. But Kovac actually seemed more excited about the Keck Array. Which would totally make sense. The project is underway, and he had just gotten back from an on-site visit at the South Pole.
Yes, the literal South Pole. The BICEP2 telescope operated out of the Amundsen-Scott South Pole Station, less than ¾ of a mile from the geographic south pole of the Earth. “I can confirm that Santa Claus definitely does not live there,” he quipped.
The station is so remote that technicians who stay there to monitor the telescope over the winter (known as the “winter overs”) have to stay there for nine solid months. During the winter, temperatures at the South Pole drop to -103 degrees Farenheit (-75 degrees Celsius), so cold that the planes can’t be trusted to operate safely. There is no fresh food. There is little-to-no daylight (depending on the month). However, they do have extremely clear and dry air, which is exactly what you want for a cosmic microwave background tracking telescope and a spectacular view of the southern Aurora, which is “very good for the mental health of winter-overs.”
Dr. Kovac prefaced his talk with a remembrance of his colleague Dr. Andrew Lange, the Caltech astrophysicist who helped bring the BICEP2 team together. His work was integral to getting the project off the ground, but he died in 2010 before seeing the results.
Then we got into the science. Dr. Kovac explained that there’s nothing particularly special about the lenses of BICEP2; the thing that makes gravitational waves the fact that their peaks are so wide and stretched out due to inflation. Keep in mind that the waves began as “wake-trails” from protons shifting around in a space with an unimaginably small radius and that in the first 38,000 years after the “big bang”, inflation caused them to stretch out so much that they now spread across light-years.
In order to make sense of such long-wavelength waves, you need a really, really long exposure camera. That’s what BICEP2 is. If you tried to take a snapshot of the gravity waves, it wouldn’t work because in a snap shot, the camera’s shutter is only open for miliseconds, just long enough to record a specific pattern of light waves; but in that short an interval of time, you would see only a small snippet of a gravity wave. Not enough to make sense of what it looks like as a wave.
The BICEP2 images are actually thousands of images taken over the course of years that have been integrated into one image. Our eyes can’t process anything with as long a wavelength as a gravity wave, but when thousands of gravitational wave snippets are displayed simultaneously in one image, we can see the pattern. (And that is why BICEP2 is kind of like a Tralfamadorian.)
And then we got to the part that I actually had a really hard time understanding: One of the big challenges for the BICEP2 team was distinguishing the B-mode gravity waves from the E-modes that make up the majority of cosmic microwave background radiation (the radiation from the early universe).
However, when electrons move in electromagnetic microwaves, they tend to move in straight lines (unless they get defracted through something.) The direction in which the electrons move is the polarization. E-mode polarization is very grid-like, since the eCMBtromagnetic waves wouldn’t have had much to bump into while traveling through the vacuum of the expanding universe.
B-modes, on the other hand, are unmistakably swirly.
Of course, the fact that we’re sitting under an atmosphere that refracts or bends all kinds of radiation, including light and gravity waves was something that the researchers had to carefully control for. They also had to account for the fact that CMB could possibly be refracted through disks of dust that form around the edges of galaxies and for the fact that the telescopes lenses could get smudged (among other things).
(In fact, in the weeks since the announcement, a few challengers have come out of the woodwork, saying the BICEP2 team may not have been able to account for all of the potential sources of refraction in the night sky.)
However, the B-modes the team observed were even stronger than most inflationary models had predicted, and their results were statistically significant, out to five sigmas— meaning that the chances of BICEP2 detecting this swirly-pattern due to random variation and refraction through galaxies is pretty close to nil.
So this is a really exciting result, but Kovac emphasized that we’ll know a lot more about these B-modes after analyzing the data from the Keck Array and the BICEP3. Right now, we can say pretty safely that we found them, but we still don’t know them very well.
Part IV: Max Tegmark
After a brief “stretching” break, the final speaker of the night Dr. Max Tegmark took the stage. “I’m going to be provocative and argue that these results aren’t quite as important as we’ve been making them out to be,” he announced. “We’ve been saying that these results are important because they mean that inflation exists. But why shouldn’t we believe that inflation exists?”
For a long time, the main objection to inflation theory was How can we assume that repulsive gravity material exists we’ve never found any matter that behaves that way? Dr. Tegmark said that this might be a reasonable objection-
“except…Oops! We discovered dark energy!” That was the Planck satelite in 1998.
And then people said,Well, you’re also assuming a scalar field. What if there is no scalar field?…
“Well okay, except…Oops! We discovered the Higgs Boson!”
And then people said, Well, a theory is supposed to predict things. What does inflation even predict?!
After a brief brouhaha over being handed the wrong reading glasses (“Wait! These are not my glasses. These are your glasses. Of course. I was wondering why they looked so feminine”), Dr. Tegmark elaborated on Dr. Guth’s earlier point about inflation’s ability to explain the fact that the universe is remarkably uniform (but with a few small perturbations). The inflationary models predict the overall density of the universe accurately; non-inflationary models don’t.
Dr. Tegmark argued that in the past decade and a half, “Cosmology has been transformed from a flaky subject to one where you can rule everything else out.”
And, in fact, based on these results, the cosmologists have been able to rule out most of the inflationary models. The last one standing, is the oldest one and the simplest one. Dr. Tegmark speculated that this might be an Occam’s Comeback, because the simplest Higgs theory was also the one that best predicted the LHC results. (I was unaware that Occam’s Razor had ever been out of favor.)
“Of course there are still some objections from people who are like (i a stoned Californa-American accent), ‘What about the ‘unknown unknowns’, man?’”
But since we know that the swirly patterns we’re observing date from 380,000 years after The Big Bang (because the speed of light is constant and we can calculate distance, we can figure out how far away in time the galaxies we see are), and the only other event we can think of that would create these types of gravitational waves is one that wouldn’t have happened in the early universe (Something as massive as a neutron star had been spinning around at near-light speeds that early in our universe’s lifetime, astronomers would have noticed it. And the oldest neutron stars we know of are only 200 million years old, meaning they wouldn’t have formed until long after The Big Bang. Plus we’ve never seen them orbiting around each other at near light speeds in any context.), it’s probably pretty safe to say that inflation happened.
I was sitting there, kind of zoning out wondering what biology must have been like in the 1950s and 1960s, right around the time we figured out DNA structure and discovered retroviruses (the era when we went from being flaky to being legit), so I missed the build-up leading to what, for me, was the biggest shocker of the night:
“Inflation created the Big Bang,” Dr. Tegmark said.
(Wait! Whaat?! Dammit, why can’t I turn off the biology part of my mind for two seconds?)
It turns out that the hot dense state, that we think of as being “The Big Bang” state may have actually been the result of the repulsive gravity material moving around and giving off lots of energy and heat. In fact, Dr. Tegmark argued, “The Big Bang” was more like a “Cold Swoosh”. The initial expansion would have been more like an acceleration than an explosion and the temperature of the conventional matter would have dropped as the matter-particles were pushed apart by inflation.
So, The Big Bang wasn’t really the beginning of our universe, Dr. Tegmark concluded. We don’t know what was there before expansion began or where the repulsive gravity material came from, because we can’t see that era in our universe’s history.
He capped it all off by critiquing a caption that ran in the New York Times, which framed “The Big Bang” as the beginning of the universe, and echoing the age-old scientist complaint about the media being slow to assimilate new scientific concepts.
However, the “Cold Swoosh” idea doesn’t mean that “The Big Bang” didn’t matter; it was a huge turning point and it marks the beginning of the universe that we could see. But there’s still a lot more to learn about our cosmological origin story.
My Personal Take:
I saw a few of the pop science articles that came out in the first couple of days after the BICEP2 announcement, but I didn’t really pay attention to it. As a bio major who doesn’t know very much about the landscape of cosmological theory, my main response was to say “Why is everybody freaking out so much? Didn’t we already establish this theory with the whole Higgs-Boson thing back in 2012?”, shrug, and then go about my usual biology journalist business.
But when I heard about MIT’s gravitational wave panel, I thought, “You know what? It doesn’t matter than I’m bad at physics. I love space, and I’m probably not going to come across a better opportunity to learn about these ‘b-mode’ things, so I should go.”
I was a few minutes late to the party, because I’m a biology person, who doesn’t keep track of where MIT keeps its astrophysics buildings. When I got there, the auditorium was packed. I was apprehensive. Was it really going to be worth it to sit in the back of a crowded MIT amphitheater and listen to a bunch of physicists talk about gravitational waves for two-and-a-half hours.
Turns out the answer was YES.
If you’ve always had a tough time wrapping your head around a particular concept and you have the opportunity to go hear the guy who originated the idea speak about it, do it. Go listen to the scientists.
Cosmology isn’t my field , but I have read my fair share of inflation explanations in books and magazines, and Dr. Guth’s explanation was by far the most clear and concise explanation of inflation theory that I’ve ever heard.
Probably because he came up with it. Inflation is a product of his particular style of thinking, so it jives with his style of explaining. That was really nice to hear, because I think that a lot of people have a really hard time explaining physics in ways that feel intuitive.
So the bottom line there is: If you have access to MIT-caliber physicists (and MIT puts a lot of videos from its colloquia online), use it.
I say that as someone who avoided physics like the plague all through her academic career. (Despite growing up in the same town as The Spallation Neutron Source and the biggest national lab in the country, studying biology, being obsessed with star and planet formation, having a physicist for a parent, and getting as far as organic chemistry in college, I have never actually taken a physics class. Not one. Not even in high school…I apologize for any scientific screw-ups I may have made in the above summaries as a result of said physics-avoiding.)
But there really is nothing quite like sitting in a room with a couple hundred students and hearing about a revolutionary discovery straight from the mouths of the guys who will probably get a Nobel Prize for this discovery.
I was actually a little bit envious of the physics students in the audience. I mean, how awesome would it have been to be an MIT student in biology around the time Rosalind Franklin figured out DNA’s double helix structure? Or in geology around the time plate tectonics came out?
A lot of non-scientists have think word “theory” means that something is an opinion and not a fact. That if a “theory” were actually a “fact”, it would be a “law”.
This fundamental misunderstanding about what the word “theory” means to scientists is,I think, just as pernicious as the widespread denial of global warming and evolution. A “scientific law” is a statement that can be expressed as a mathematical equation. A “scientific theory” is a conceptual framework that can’t be expressed as an equation, but in order to be called a “theory”, it has to have just as much predictive power as a law. There are mathematical models we use to investigate how evolution plays out, but there is no one “evolution equation”. That is the only reason evolution is not considered a “law”.
Obviously, the transition from being a “possible explanation” to being a “theory” isn’t a seamless or instantaneous one. It all depends on how critical the scientific community is and how strong the evidence is.
I tend to think about scientific theories as belonging to one of two groups: the lower-case “theories”, which are still somewhat contended and/or may be lacking a key piece of evidence and the upper-case “The Theory of-” theories, which may be re-interpreted or elaborated upon but are unlikely to ever be empirically refuted. The “The Theories of” are an elite group: The Theory of Evolution. The Theory of General Relativity. The Theory of Plate Tectonics. The Theory of Quantum Mechanics.
Becoming a “The Theory of” is the closest thing to knighthood that an abstract idea can achieve. And in science, ideas earn their “theory” title by accurately predicting empirical facts and data.
For a long time, inflation was a collection of lower-case theories that could not really be proved or disproved. But sitting in that auditorium, all I could think was. “Oh my god, I’m witnessing the emergence of a new ‘The Theory of!’”
The Theory of Inflation.
Cosmology’s first knight at the Science Round Table.
If that doesn’t get you excited about physics, I don’t know what will.
Best One-Liner from an Audience Member:
(For the record, my physics-speak is nowhere near as good as my bio-speak, so I’m paraphrasing audience questions a bit more heavily than usual. But as always, tried to get the speakers’ key points down as precisely as possible.)
Audience member: Well, if the repulsive gravity matter was there before TBB, we’re going to need a new name for T=0? I think that’s the reason for some of the lingering confusion in the terminology.
Dr. Tegmark: It’s important that scientists be careful about how words are used, because we shouldn’t imply that we know more than we do. We don’t know what was there before the big bang, and we don’t even know if there was a T=0.
Same audience member: So maybe we should be like Prince. We could make a little symbol and call it “The Time formerly known as The Big Bang”.
Best Explanation of a Physics Factoid that has Always Confused Me :
Dr. Hughes: Gravity is the weakest force. (drops a pen on the floor & then picks it up.) Look! I can lift things! (Audience laughs.) We laugh, but this is actually much more profound than it might seem. If you really think about it, it is the electrical and chemical impulses in a the couple hundred grams of muscle in my arm versus all the gravity of…The Earth. And I’m winning! Sure, the pen will drop if I let go. But I’m winning.
(I am totally going to steal this explanation if I ever need to explain why gravity is the weakest force.)
Audience Question that Appealed Most to My Inner Scientist:
Audience Member: Does this result refute string theory?
Dr. Tegmark: String theories have tried to incorporate inflation but they don’t predict gravitational waves in the numbers that we’re seeing them. The string theory that predicts the most b-modes only predicts 1/3 as many waves as BICEP2 saw…So it’s actually a really exciting time for string theorists.
(Audience laughs uproariously. Because we all know that having to completely reorganize the theories you’ve built your career around is…stressful.)
Dr. Tegmark: Oh, no. I didn’t mean that in a “mean” way. I mean they can look at their theories’ predictions more precisely now…
Most Perplexing Response to an Audience Member’s Question:
Audience member: Does this mean that we now have a quantum theory of gravity?
Dr. Hughes: Safest thing we can say is that a theory exists. I don’t know what that theory is.
(This was a really shocking response to me, because when I talk to scientists, I usually talk to biologists. They are very, very empirical and don’t talk about having a theory until they’ve actually constructed the ideas we need to express the theory. But here we have a physicist talking about a quantum theory of gravity, as if it’s out there already, floating around and waiting for someone to find it. I’ve known for a long time that the physics/mathematical/computer science branch of STEM is much more rationalistic than we are, but I still felt a twinge of culture shock when I heard this response.)
Best Possible Way to End a Cosmology Forum:
Audience member: If God came to you and said you could have any experiment you wanted, no matter how impossible, what data set would you want?
Dr. Tegmark: Hmm…put a black hole at a safe distance and watch it evaporate…
Dr. Guth: A trip to the early universe so I could watch.
Dr. Tegmark You wouldn’t ask a for heat shield with that?
Dr. Guth: Well, I’d prefer it…Otherwise it would be a one way trip.
Dr. Kovac: More B-modes with smaller angular scales. (In other words, “more of what I was looking at when I made my discovery!”) We’ll be able to learn about the distribution of mass in the early universe and find out how big the pockets of repulsive gravity material were. This is actually a realistic goal.
Dr.Hughes: I second that, because if we find more B-modes, a lot of other cool stuff will come along for the ride.
- Repulsive gravity material = a substance which expands while maintaining a constant density and drove the early expansion of the universe. For a long time, we had very little evidence that repulsive gravity material ever existed, but then we found dark energy.
- Gravitational wave = a ripple in space-time caused by the movement of protons. Similar in behavior to an electromagnetic wave, but it carries gravitational energy rather than electromagnetic energy.
- B-modes = a swirly-pattern of radiation that could pretty much only be generated by the movement of matter in the early inflation of the universe
- The Big Bang = a time immediately before rapid expansion began where the universe was in a hot, dense state. We don’t know whether this was the beginning or not.
Tl;dr: The discovery of “swirly-pattern” gravitational waves has pretty much proved that the universe is expanding due to the influence of repulsive gravity. Also, MIT cosmologists have wicked senses of humor.