Perhaps you saw the news this week about new evidence that we do, indeed, live in a multiverse. A scientist claims he's found signs in the cosmic microwave background radiation — the afterglow, so to speak, from the Big Bang — that our universe collided with another universe early in our cosmic history.
I don't mean to burst your bubble, but we haven't found definitive proof that we live in a multiverse. Don't get me wrong: hunting for signs of a multiverse (or other exotic phenomena) in the latest maps of the cosmic background radiation (CMB) is a perfectly legitimate area of research, albeit one that lurks near the fringes. But it's hella hard to spot possible signatures, and far too easy to be fooled by a false positive. Most preliminary findings — and this one is definitely preliminary — don't hold up under closer scrutiny. (See the 2014 BICEP2 announcement, which turned out to due to galactic dust, as a case study.)
Just to be clear: there are many different notions of a multiverse, ranging from bubbles that form and pinch off from our own space-time to become their own parallel worlds, to the "Many Worlds" hypothesis, dating back to the 1950s. That theory holds that different potential outcomes contained within the quantum wave function don't vanish after it collapses, but instead are realised in separate universes (which translates into a whole lotta universes).
In the current case, we're talking about the kind of multiverse predicted by the theory of eternal inflation. The notion that our universe underwent a brief, intense period of expansion after the Big Bang was introduced by MIT cosmologist Alan Guth — along with Stanford's Andrei Linde and Princeton's Paul Steinhardt — in the 1980s. "I would say most versions of inflation in fact lead to eternal inflation, producing a number of pocket universes," Guth told New Scientist. As I wrote over at Quanta last year:
Inflation holds that our universe experienced a sudden burst of rapid expansion an instant after the Big Bang, blowing up from a infinitesimally small speck to one spanning a quarter of a billion light-years in mere fractions of a second.
Yet inflation, once started, tends to never completely stop. According to the theory, once the universe starts expanding, it will end in some places, creating regions like the universe we see all around us today. But elsewhere inflation will simply keep on going eternally into the future.
This feature has led cosmologists to contemplate a scenario called eternal inflation. In this picture, individual regions of space stop inflating and become "bubble universes" like the one in which we live. But on larger scales, exponential expansion continues forever, and new bubble universes are continually being created. Each bubble is deemed a universe in its own right, despite being part of the same space-time, because an observer could not travel from one bubble to the next without moving faster than the speed of light. And each bubble may have its own distinct laws of physics.
A theory that predicts a multiverse is one thing; proving it experimentally is quite another matter. That said, there are some physicists, like Matthew Johnson of the Perimeter Institute and University of Waterloo, who have searched for signs of a collision between a bubble universe and our own in the CMB, relying on data from the Wilkinson Microwave Anisotropy Probe (WMAP). They didn't find anything, although they hope to build on that earlier analysis when the Planck satellite data becomes public.
What Ranga Chary at the Caltech data center for the Planck telescope collaboration is claiming to have found is something different. (Chary has not responded to Gizmodo's request for comment.) "They're hunting for lions, and we're hunting for polar bears," Johnson told New Scientist. As reporter Joshua Sokol writes:
Instead of looking at the CMB itself, Chary subtracted a model of the CMB from Planck's picture of the entire sky. Then he took away everything else, too: the stars, gas and dust. With our universe scrubbed away, nothing should be left except noise. But in a certain frequency range, scattered patches on the sky look far brighter than they should. If they check out, these anomalous clumps could be caused by cosmic fist-bumps: our universe colliding with another part of the multiverse.
These patches look like they come from the era a few hundred thousand years after the big bang when electrons and protons first joined forces to create hydrogen, which emits light in a limited range of colours. We can see signs of that era, called recombination, in the light from that early hydrogen. Studying the light from recombination could be a unique signature of the matter in our universe — and potentially distinguish signs from beyond.
That light is tough to spot, since it tends to blend in with the CMB. Chary found a few patches that were much brighter than expected. One possible explanation is that a collision with another universe led to extra protons and electrons in those patches. But it could also be noise that just looks like a signal.
It's not Chary's analysis that's raising questions, it's his interpretation of what those bright patches signify. David Spergel (Princeton University) told New Scientist that he thinks that dust might once again be the culprit and suggested to Sokol that physicists should seek out alternative explanation: "The dust properties are more complicated than we have been assuming, and I think this is a more plausible explanation." Johns Hopkins' Joseph Silk was harsher in his dismissal, calling Chary's claim "completely implausible."As for Johnson, he told Gizmodo that he will remain sceptical pending confirmation from independent analysis.
Even Chary cautions that his conclusion is still tentative, writing that "Unusual claims like evidence for alternate universes require a very high burden of proof." And yet somehow all anyone outside of physics is hearing is this:
Look, I love the idea of a multiverse as much as the next person; it's a fascinating research area with the potential for uncovering some very exciting new physics. But frankly, this paper doesn't warrant the amount of press it's been getting this week. It's the first draft of a paper by a scientist at the Planck data center at Caltech (as opposed to an official paper from the full Planck collaboration). It's been submitted to a reputable journal, but has yet to undergo any peer review, and while it's an intriguing analysis, Johnson, for one, thinks acceptance for publication might prove difficult.
So why are we even talking about it? It's only a story because people think the multiverse is cool. And it is. It really is. But it's a disservice to Chary and the process of science to trumpet a discovery this early in the game.
Chary, R. (2015) "Spectral variations of the sky: constraints on alternate universes," arXiv.
Wainwright, Carroll et al. (2014) "Simulating the universe(s): from cosmic bubble collisions to cosmological observables with numerical relativity," Journal of Cosmology and Astroparticle Physics 03: 030.
Wainwright, Carroll et al. (2014) "Simulating the universe(s) II: phenomenology of cosmic bubble collisions in full general relativity," Journal of Cosmology and Astroparticle Physics 10: 024.
Image courtesy of ESA and the Planck Collaboration.