Relevant notes and citations provided to TED by Allan Adams.
First, a little background. On the morning of Monday, March 17, 2014, a small group of scientists announced a gigantic discovery. As it happened, that Monday was also the first day of the TED2014 conference, and news of the discovery raced around the crowd. On Tuesday, TED's curator, Chris Anderson, invited me to spend four minutes on stage explaining the result in the afternoon session. I asked Randall Munroe of xkcd fame (also speaking at TED) to help with illustrations, which got turned into slides in the nick of time for this totally improvised talk. A very fun day!
A beautiful example of what we see when we gaze deep into the night sky is the Hubble Deep Field, which reveals the thousands of galaxies that fill our view whenever we look beyond the stars and dust of our own Milky Way. This NASA image is just crazy full of magic. Oddly, descriptions of science often feel like sermons at a wake — like lessons about life, with an inevitable arc — a story completed, closed, finished. But just look at the Hubble Deep Field! It looks like how doing science feels — glorious, vertiginous, impossibly grand, an adventure waiting to be written.
Here I'm talking about the cosmic microwave background, or CMB. The term "Big Bang" is a great example of a terrible name that stuck — indeed, it was coined by astronomer Fred Hoyle to mock the absurdity of the theory! However absurd the Big Bang sounded to Hoyle, the idea turned out to be basically correct, and the name stuck.
The list of people who mapped the CMB is long and august, but perhaps the most notable teams are the Cosmic Background Explorer (COBE), the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck spacecraft, which have mapped the CMB across the bulk of the sky to an accuracy of about two parts in a million. The list of lessons extracted from these maps is jaw-dropping: The maps have played an important role in determining the age of the universe (more precisely, the time elapsed since the Big Bang), the total amount of visible matter in the universe (not very much), the total amount of invisible (or dark) matter in the observable universe (a lot more), and even the weight of empty space (spoiler: it's not zero, but it's really, really, really tiny).
An edit to what I say here: "14 billion years" and "13 billion years" both refer to the time elapsed since the CMB was emitted. I bizarrely rounded differently in my head on the fly. In fact, the Big Bang took place about 13.798 +/- 0.037 billion years ago.
(Aside: That error margin should knock your socks off — determining the age of the cosmos with that accuracy by just looking at the night sky today is like you looking at me in this video and deducing my age in the video to within a couple of hours — insane! Sherlock Holmes has nothing on cosmologists.)
The afterglow we see (that is, the CMB) was emitted about 377,730 +/- 3,200 years later. Before that date, everything was so hot that the universe, like a thunderbolt, was simultaneously glowing and opaque, with light absorbed as quickly as it was emitted. But by its 377,730th birthday, the universe cooled enough to become transparent. The light emitted just at that moment was thus not absorbed, and has been plying its way across the cosmos ever since, cooling as the universe expanded. That's what the CMB is — a fossil of light.
Here I make a cognitive slip that's all too common, and I could kick myself for perpetuating it: The "size" of the observed universe — the distance to the stuff that emitted the currently visible CMB 13.8 billion years ago — is not in fact 13.8 billion light-years. It's actually much larger — about 46 billion light-years — because the universe has continued to expand since the CMB was emitted. The stuff that emitted the CMB light we are just now seeing is thus much farther away.
More precisely, 2.72548 +/- 0.00057 Kelvin. If the thermostat in my room had that kind of precision, the rotary dial would have to be roughly 2.5 kilometers across. Do not mess with precision cosmologists.
"BICEP2 Press Conference", Harvard-Smithsonian Center for Astrophysics, March 17, 2014
Something delicious about this announcement was the way the data came out. It was released as two papers freely available on the web, so that from the moment of the announcement, anyone anywhere in the world with an Internet connection and some curiosity could access it. Interestingly, the scientists had not even submitted the papers to a journal, much less waited for them to be reviewed and published! They did their analysis, they checked carefully, and they put their results out there for everyone to see, check, learn from, question and try to shoot down for themselves. This reflects a sea-change in attitudes about the communication of scientific results in which free, immediate and open-access are the organizing principles, with traditional journals relegated, quite literally, to an afterthought. The epicenter for this transformation in our corner of the scientific universe is the arXiv, probably the single most important conduit for communication in the mathematical and physical sciences — but that is another story.
Illustrations by the brilliant Randall Munroe of xkcd fame.
More precisely: What the BICEP2 team appears to have found is direct evidence for quantum fluctuations in the shape of space-time, which was stretched by inflation to significant fractions of the size of the observable universe. What I didn't say explicitly, but which lies at the heart of the spectacle of this discovery, is that this is the first direct evidence of a place and time in which quantum mechanics and gravity both mattered. If the evidence holds up — and all indications so far are very good — this would be the first detection of effects involving quantum gravity — something I did not expect to see in my lifetime. As someone who has spent much of his youth (and increasingly, middle age) pondering the mysteries of quantum gravity, this is like being visited by an angel. Nature didn't have to be so kind. It's all too easy to imagine a perfectly sensible universe in which all such effects are far too small to measure with anything like the currently conceivable technology. Nature owed us nothing. This was a gift.
For much more thorough discussions of the physics behind this discovery, and for an explanation of why primordial gravitational waves left these "twists" in the CMB, see:
"Ripples from the Big Bang"
The New York Times, March 24, 2014
"How to Train Your Universe"
Excursionset.com, March 17, 2014
"Gravitational Waves in the Cosmic Microwave Background"
Preposterous Universe, March 16, 2014
"A Primer on Today's Events"
Of Particular Significance, March 17, 2014
"If It Holds Up, What Might BICEP2's Discovery Mean?"
Of Particular Significance, March 18, 2014
"Inflation on the Back of an Envelope"
Quantum Frontiers, March 23, 2014
Here I am skipping an enormous chunk of physics and logic to cram the end of the story into my allotted four minutes. As explained more patiently and thoroughly in the links above, the discovery of primordial gravitational waves via "twists" in the CMB is a smoking gun for the idea of cosmic inflation, which posits that the universe went through an epoch of rapid expansion that terminated in the Big Bang. Inflation solved a host of conceptual problems with the standard Big Bang model but had one annoying property: The same mechanism that solved various problems also erased most evidence of the inflationary epoch — with the exception of primordial gravitational waves, which, if produced with sufficient strength during inflation, would leave their signature in twists in the CMB. By early 2014, however, a long list of circumstantial evidence, together with a murky set of theoretical intuitions, led many of us scientists to believe that the production of inflationary gravitational waves was just too weak to be detectable. This left the theory of inflation in a sort of purgatory — too successful to be entirely wrong, but too elusive to confirm directly.
The BICEP2 results, if they hold up, appear to directly confirm the inflationary picture, in particular some of the simplest and most theoretically natural models of inflation. There are many theories of cosmology that do not use inflation to explain what we see in the CMB, and yet, to a good approximation, every model that accounts for the BICEP2 results ends up looking an awful lot like inflation. Future experiments will be the final arbiter, but from where I stand, the BICEP2 results have inflation tattooed all over them.
I also skipped over this story of inflation to the more dramatic picture that inflation suggests for the large-scale structure of the universe beyond our observable bubble, a picture called the "multiverse." This picture of our observable world as just one little neighborhood in a much larger universe — as one cosmological bubble in an inflating universe of other bubbles — is not conclusively indicated by the BICEP2 discovery. Rather, given everything we know about inflation and the way it fits into the physics of quantum mechanics and gravity, the BICEP2 results make this picture very natural. To give you a sense of how seriously people took this picture even before the staggeringly awesome BICEP2 results, check out the last paragraph of a paper called "Living in the Multiverse" by the great theorist Steven Weinberg.
"About the multiverse, it is appropriate to keep an open mind, and opinions among scientists differ widely. In the Austin airport on the way to this meeting, I noticed for sale the October issue of a magazine called Astronomy, having on the cover the headline “Why You Live in Multiple Universes.” Inside I found a report of a discussion at a conference at Stanford, at which Martin Rees said that he was sufficiently confident about the multiverse to bet his dog’s life on it, while Andrei Linde said he would bet his own life. As for me, I have just enough confidence about the multiverse to bet the lives of both Andrei Linde and Martin Rees’s dog."
BICEP2 didn't prove the existence of the multiverse — but I'm pretty sure that Sir Martin Rees' dog is resting a lot easier tonight.
Randall Munroe loves drawing lots and lots of bubbles. Here he does a beautiful job communicating an important feature of the multiverse: Inside each bubble, the local laws of physics can be slightly different. For example, in this bubble, there are electrons; in that bubble, there aren't. Here, people; there, octo-plants. In this bubble, the signature of inflationary gravitational waves is not just detectable, but downright gigantic; in that one, no one will ever know about inflation — not for sure, anyway. If this isn't the best of all possible worlds, well, it's still pretty spectacular.
Two last important points:
First, gorgeous as the results look at first, it is important to maintain a healthy dose of skepticism until further data corroborates — or contradicts — the discovery announced by the BICEP2 team. Time, and a lot of hard work, will tell whether the BICEP2 result is as robust as it seems. For now, though, it sure looks beautiful.
Second, it's easy to look at this discovery and miss the magnitude of it. Look back at the image of the Hubble Deep Field. Our entire cosmos, everything we can see, everything we will ever see, is almost certainly just a tiny pocket in a vastly, unimaginably larger universe of endless possibility. That's not just titillating metaphysics or the ravings of a punch-drunk theorist — it's the most parsimonious explanation we have for the things we see. So the next time you look up into the night sky, remember BICEP2. And imagine the universe ringing like a bell announcing our place in the family of things.