Will the BICEP2 Results Hold Up?

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 Two months ago, astronomers announced that they had detected gravitational waves from the dawn of time, the strongest evidence yet that the universe experienced a titanic growth spurt in the first moments after the Big Bang, a phenomenon called cosmic inflation. Scientists and the media—including
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/05/bicep2/">
 this blog
 </a>
 —buzzed with the news. However, a controversy that began with online rumors now might call these findings into question.
 
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<img alt="BICEP2 Twilight" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/bicep2-twilight__620.width-990_FXcXWAg.jpg"/>
<figcaption>The BICEP2 telescope at twilight. Credit: Steffen Richter, Harvard University
 
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 The signal the astronomers detected was a subtle one. Using a telescope at the South Pole called
 <a href="http://www.cfa.harvard.edu/CMB/bicep2/">
 BICEP2
 </a>
 , they picked up swirls in the polarization, or alignment, of the cosmic microwave background (CMB) radiation, the afterglow of the Big Bang. This pattern, they argued, resembled what one would expect from gravitational waves produced by inflation. These findings not only seemed to be the first direct evidence for inflation, but also were an image of the earliest gravitational waves, the ripples in spacetime predicted by Einstein’s theory of general relativity.
 

 “Seeing evidence for such a phase of rapid expansion would be extremely exciting, and it is possible that that’s what BICEP2 is seeing,” said Raphael Flauger, a theoretical physicist at New York University and the Institute for Advanced Study in Princeton, New Jersey, who did not take part in BICEP2’s research. However, a controversy that began with rumors reported on a scientist’s blog now hints the swirls the BICEP2 group thought it saw in the CMB may have actually emerged from within the Milky Way.
 

 The kerfuffle began on May 12 with a post from particle physicist
 <a href="http://www.fuw.edu.pl/~afalkows/Work/work.html">
 Adam Falkowski
 </a>
 at CERN on his blog
 <a href="http://resonaances.blogspot.com/2014/05/is-bicep-wrong.html">
 Résonaances
 </a>
 . To uncover the particular polarization pattern they detected, the BICEP2 group had to account for the “foreground” of polarized microwaves generated relatively nearby within the Milky Way, which could generate similar patterns. It’s like hearing a low rumble: before you can be convinced that it’s distant thunder, you have to make sure that it isn’t just trucks clattering down your street.
 

 “The two most important foreground components are synchrotron emission from charged particles that are deflected by the magnetic field of the galaxy, and emission from dust,” Flauger said.
 

 To account for the synchrotron radiation, scientists used data from the
 <a href="http://map.gsfc.nasa.gov/">
 WMAP
 </a>
 satellite. The controversy hinges on their handling of the dust emission, which relied on a preliminary map based on about 15 months of data from the European Space Agency’s
 <a href="http://sci.esa.int/planck/">
 Planck
 </a>
 spacecraft. Falkowski noted the BICEP2 group might have misinterpreted the Planck data, thinking that it only contained emissions from the Milky Way when it also included unpolarized emissions from other galaxies. If the BICEP2 team did not account for this fact, they might have underestimated how polarized the foreground from the Milky Way actually was. This could mean the inflation signal the group thought it saw might only be a spurious result from Milky Way emissions.
 

 The BICEP2 team stands by their results, and strongly denied the rumor that it admitted to any mistake. However,
 <a href="http://www.kaltura.com/index.php/extwidget/preview/partner_id/1449362/uiconf_id/14292362/entry_id/1_i6by6dqf/embed/auto?">
 in a seminar
 </a>
 given on May 15, Flauger suggested the BICEP2 team’s analysis of dust emissions may not be reliable.
 

 The area of the sky the BICEP2 team analyzed is not very dusty; in fact, they intentionally chose a relatively clear patch of sky to reduce the confusion with dust. However, in doing so, they may have introduced additional uncertainty into their results. “In regions that don’t contain much dust, it is hard to measure how polarized it is,” Flauger said. This means the uncertainty regarding their interpretation “seems to be relatively large,” he explained.
 

 An end to this controversy may come in October, when the Planck team intends to release its complete map of the polarization of the CMB. “It will help us understand the foregrounds from our galaxy better, and will hopefully tell us if the signal is evidence for inflation or just dust,” Flauger said. “Planck may even be able to do more than that and see hints of the inflationary signal directly.”
 

 Other research projects able to spot ancient gravitational waves could also help clear up the controversy, such as the POLARBEAR experiment in Chile, the South Pole Telescope, or the Keck Array in Antarctica, which is led by the scientists behind BICEP2, said cosmologist Julian Borrill at Lawrence Berkeley National Laboratory and the University of California at Berkeley.
 

 “It is possible that the signal of the gravitational waves is there, and it will survive in a more complete analysis,” Falkowski said. “Everyone will be immensely happy if it does.”
 

 As to where the rumors came from and whether they are true, “I think we’re already past the rumors stage and we have entered into an experts’ discussion,” Falkowski said. “I don’t think that discussing the origin and the content of the rumors adds anything important at this point.”
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 New Scientist:
 <a href="http://www.newscientist.com/article/dn25558-rumours-swirl-over-credibility-of-big-bang-ripple-find.html#.U39p88epooa">
 Rumors swirl over credibility of Big Bang ripple find
 </a>
 
 Science writer Lisa Grossman reports from the BICEP2 rumor mill.
 

 Universe Today:
 <a href="http://www.universetoday.com/112008/are-the-bicep2-results-invalid-probably-not/">
 Are the BICEP2 Results Wrong? Probably Not
 </a>
 
 “BICEP2 has made a bold claim, and now everyone gets to whack at them like a piñata,” writes Brian Koberlein.
 

 Washington Post:
 <a href="http://www.washingtonpost.com/national/health-science/big-bang-backlash-bicep2-discovery-of-gravity-waves-questioned-by-cosmologists/2014/05/16/e575b2fc-db07-11e3-bda1-9b46b2066796_story.html">
 Big Bang Backlash: BICEP2 discovery of gravity waves questioned by cosmologists
 </a>
 
 Joel Achenbach reports on the controversy swirling around the BICEP2 announcement.
 
</section>

The discovery of gravitational waves from cosmic inflation is coming under scrutiny.

Prize Without Proof? Yuri Milner’s Fundamental Physics Prize

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 The Nobel Prize may be the most prestigious award in science, but the new Fundamental Physics Prize is by far the world’s most lucrative scientific award, instantly making its first winners this August multimillionaires. But the size of the payout isn’t the only difference between the two prizes: Unlike the Nobel, the Fundamental Physics Prize can be awarded for research that has not yet been verified by experiment. Is it foolhardy to extol work later findings might prove wrong?
 

 The Fundamental Physics Prize is the brainchild of Russian tycoon
 <a href="http://www.forbes.com/profile/yuri-milner/">
 Yuri Milner
 </a>
 , a one-time physics graduate student turned billionaire investor in internet companies such as Facebook, Twitter, Groupon and Zynga. Milner personally selected the inaugural class of nine winners, each of whom received $3 million, roughly three times as much as a Nobel grant.
 

 Although some of the work recognized by this year’s awards has been experimentally verified (for example, the principles of
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/02/herding-schrodingers-cats/">
 quantum computers
 </a>
 are firmly grounded in experiment), others, like string theory, which compares elementary particles to loops of vibrating string, and the
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/holograms-black-holes-and-the-nature-of-the-universe/">
 holographic principle
 </a>
 , which suggests that our three-dimensional reality is a projection of information stored on a far-off two-dimensional surface, live further out on a theoretical limb.
 

 Ideas like these may not get experimental verification any time soon, either. Take string theory. As Fundamental Physics Prize winner Ashoke Sen, a string theorist at the Harish-Chandra Research Institute in India, explains, “Unfortunately the strings are so small that the energy required for seeing these structures is huge, much larger that what we have achieved in the present day accelerators.”
 

 Yet many physicists (including, unsurprisingly, some Milner honorees) argue that unverified ideas, and even ideas that are
 
 unverifiable
 
 with today’s technology, are prizeworthy—even if future tests should prove them wrong.
 

 “Many of the most important developments in physics involve subjects for which there is little hope of experimental verification anytime soon,” argued cosmologist Alan Guth at the Massachusetts Institute of Technology, one of the winners of the Milner prize. Guth invented the theory of cosmological inflation, which suggests our universe expanded staggeringly just a sliver of a second after it was born. This rapid expansion that would help explain, among other things, why the cosmos is so extraordinarily uniform on large scales, with only very tiny variations in the distribution of matter and energy.
 

 In fact, two of the greatest theoretical breakthroughs of the 20th century—Albert Einstein’s theories of special and general relativity—were never honored by a Nobel Prize, said Stanford cosmologist Andrei Linde, another winner of the Fundamental Physics Prize. These ideas changed the world by showing that mass and energy are equivalent and that gravity is a result of mass curving the fabric of space and time. Although Einstein was awarded the Nobel Prize in 1921, he was given the prize not for relativity but for describing how light was composed of discrete packets of energy now called photons, because the Nobel committee felt that relativity had not at the time been verified.
 

 “More recently, we have the case of the
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/06/the-higgs-boson-explained/">
 Higgs particle
 </a>
 , with almost 50 years between the theoretical advance and the experimental verification,” Guth added. “If this kind of theoretical work is not respected, then progress in fundamental physics would suffer tremendously.”
 

 Even if theoretical research does lead to a dead end, it can help inform what ultimately prove to be successful ideas, said theoretical physicist Nima Arkani-Hamed at the Institute for Advanced Study in Princeton, New Jersey, a winner of the new prize who has investigated ideas such as extra dimensions of reality and new theories regarding the Higgs particle.
 

 “One of the great developments in physics in the 20th century was the Standard Model of particle physics, which explains particles such as electrons and quarks and gluons,” Arkani-Hamed said. “But before the Standard Model was known to work, there were people exploring lots of other theoretical possibilities that might be consistent with our world. Even if they didn’t pan out, collecting ideas that have a chance of working may help lead to developments like the Standard Model.”
 

<a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/12/beautiful-losers/">
 “Wrong” ideas can advance science
 </a>
 in other ways, too. “We should keep in mind that Newtonian mechanics was ultimately found to be incorrect, but it nonetheless was a momentous force in driving science forward,” Guth said. “Today there are many developments in physics that are recognized by the community as being important, even though we cannot prove that they are correct.”
 

 As to how the prize-winners might spend their gains, other than paying taxes and mortgages, they often said they were still in shock over the award. “I continue to remind my students that they should not go into physics for the money,” Guth said.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


<a href="http://www.fundamentalphysicsprize.org/">
 Fundamental Physics Prize
 </a>
 
 The official web site of the Fundamental Physics Prize Foundation.
 

 Nature:
 <a href="http://www.nature.com/news/theoretical-physicists-win-massive-awards-1.11094">
 Theoretical physicists win massive awards
 </a>
 
 Geoff Brumfield talks with winners—and critics—of the new prize.
 

 New York Times:
 <a href="http://www.nytimes.com/2012/07/31/science/9-scientists-win-yuri-milners-fundamental-physics-prize.html">
 9 Scientists Receive a New Physics Prize
 </a>
 
 Kenneth Chang reports on the announcement of the Fundamental Physics Prize winners.
 
</section>

Could this extreme magnetic shield reveal new physics?

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 Next to a nuclear reactor in Germany is a four ton, ten cubic meter metal box surrounded by lasers. The box is the most magnetically-shielded room ever built: the magnetic field inside is just a tenth of a billionth of a tesla, weaker than anywhere else in the solar system. This sanctuary could help physicists probe infinitesimal effects that may have profoundly influenced the newborn universe, and help solve one of the greatest secrets of the cosmos: why the universe has more matter than antimatter.
 
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<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/shield_620.width-990_G7u7clR.jpg"/>
<figcaption>This image shows the inner three layers of the magnetic shield (also known as the insert), and the cylindrical layer with 780 copper rods. The cylindrical layer is used to generate a very uniform magnetic field, which is necessary for EDM experiments. Also shown are two measurement devices: A mercury magnetometer is placed between the wooden support structure, and a high precision pendulum device, which is used for the absolute alignment of conventional magnetic field probes, is on top of the support table. Credit: Technische Universität Müchen
 
 </figcaption>
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 The answer to this mystery could come from a decades-old quest exploring the structure of the neutron. Although neutrons are electrically neutral, they are made of particles known as quarks that are positively or negatively charged. Overall, these electric charges cancel out in neutrons, but there is the possibility they are not equally distributed—for instance, perhaps the north poles of neutrons are slightly positive, while the south poles are negative, explains Tim Chupp, a physicist at the University of Michigan who helped test the new shield and design experiments for it. The distribution of charge within the neutron is called its electric dipole moment.
 

 If there is a tiny imbalance of electric charge within the neutron, this could be evidence of a special kind of asymmetry, called a charge parity (or “CP”) violation, that explains why there is more matter than antimatter in the universe. “This asymmetry would have had a strong effect in the early universe, shaping how the universe evolved a microsecond to a nanosecond after the Big Bang,” says physicist
 <a href="http://www.universe-cluster.de/fierlinger/group.html">
 Peter Fierlinger
 </a>
 of the Technical University of Munich, who led the overall effort that developed the new shield. But to detect it, physicists must work with neutrons that are shielded from the natural magnetic field of the Earth and the artificial magnetic fields of motors, electronics, and all the ubiquitous devices that produce their own magnetic fields.
 

 Experiments conducted since the 1950s have not found a neutron electric dipole moment, but the new shield could improve the sensitivity of such measurements a hundredfold, down to the theoretically predicted scale of this phenomenon.
 

 These exquisitely precise measurements are made possible by a magnetic shield that is ten times better than previous state-of-the-art shields. Previous research typically used a trial-and-error approach to find designs that magnetically shielded their interiors. Instead, Fierlinger and his colleagues used computer models of how magnetic fields interact with matter to optimize elements of their room’s design, including the spacing and thickness of the layers making up the walls.
 

 The result is shield constructed like a Russian nesting doll, made of four one-millimeter-thick sheets of a soft nickel-iron alloy that are extraordinarily easy to magnetize and demagnetize, and act like a sponge to absorb outside magnetic fields. Within a one-cubic-meter space inside this haven, items would experience as little as a tenth of a billionth of a tesla—about 500,000 times less than the magnetic field typically felt at the surface of the Earth, and some ten times less than the magnetic field that suffuses “empty” space in the solar system. The scientists detailed their research May 12 in the Journal of Applied Physics.
 

 The neutron is slightly magnetic, and so experiences a torque in the presence of an external magnetic field. If the neutron has an electric dipole moment, the amount of torque it experiences from an external magnetic field will change if an electric field is also applied on it. The lasers surrounding the shield are part of instruments known as optical atomic magnetometers that help measure and correct for the effects of extraneous magnetic fields.
 

 “This magnetic shield will help us perform tests of the Standard Model with unprecedented precision,” says physicist Wolfgang Korsch at the University of Kentucky, who did not take part in this research. “Any deviation from the predictions of the Standard Model will clearly hint at new physics.”
 

 “These low-energy precision measurements are very complementary with the experiments conducted at high-energy atom smashers such as the LHC,” says theoretical physicist Michael Ramsey-Musolf at the University of Massachusetts at Amherst, who did not participate in this study. “They’re both ways to solve the mystery of physics beyond the Standard Model.”
 

 There are many other discoveries this magnetic shield could lead to, the researchers add. For example, it could help detect magnetic monopoles, long-theorized, hitherto-unseen particles that each only possess one magnetic pole, either north or south, unlike every known particle, which each have two. Magnetic sensors inside the shield could detect the faint, unique magnetic signals from any monopoles zipping through the room.
 

 “The symmetries in nature may make beautiful patterns, but what are really interesting are the imperfections or breaking of symmetries—that’s where interesting physics lies,” says Chupp.
 

 The shield may also open up unexpected avenues of research. “It could help detect the magnetic fields generated by electric currents in the brain,” Korsch says. “If they could actually put a person in there to do those measurements, I’d find that quite exciting.”
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 PhysicsWorld:
 <a href="http://physicsworld.com/cws/article/news/2015/may/20/extraordinary-magnetic-shield-could-reveal-neutrons-electric-dipole-moment">
 Extraordinary magnetic shield could reveal neutron’s electric dipole moment
 </a>
 
 A profile of the new magnetic shield and its scientific potential.
 

 Symmetry:
 <a href="http://www.symmetrymagazine.org/article/october-2005/explain-it-in-60-seconds">
 Explain it in 60 seconds: CP violation
 </a>
 
 A concise explanation of CP violation from Yosef Nir of the Weizmann Institute of Science.
 

 Wikipedia:
 <a href="http://en.wikipedia.org/wiki/Neutron_electric_dipole_moment#Experimental_technique">
 Neutron Electric Dipole Moment
 </a>
 
 An overview of the theoretical implications of the neutron electric dipole moment and the experimental searches for its value.
 
</section>

Experiments using a new magnetic shield could reveal why the universe has more matter than antimatter.

In the Beginning

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 Did the universe have a beginning? What, if anything, came before the Big Bang?
 

 Today, we see galaxies rushing away from us in every direction, suggesting that, if you could press the rewind button on the entire universe, the whole thing would screech to a halt at a moment about 13.7 billion years in the past, when the entire cosmos was apparently compressed into a singularity—an infinitely small, dense point.
 

 “How does the universe begin from such a state?” asks Alexander Vilenkin, a theoretical physicist at Tufts University. Indeed, the laws of physics as we know them break down around singularities, so physicists have devised a number of ways to sidestep the singularity problem.
 

 One possibility is that the universe is cyclic: Every Big Bang expansion is followed by a contraction, ending in a “Big Crunch” from which a new Big Bang emerges, and so on and so on in an infinite series that extends eternally into the past and future. The idea was first proposed centuries ago, but received a
 <a href="http://www.sciencemag.org/content/296/5572/1436">
 fresh take
 </a>
 from the physicists Paul Steinhardt and Neil Turok in 2002. There is a problem with this elegant idea, though: the
 <a href="http://en.wikipedia.org/wiki/Second_law_of_thermodynamics">
 second law of thermodynamics
 </a>
 , which states that the total amount of disorder or entropy in a system increases over time—the party-pooper law that prevents the existence of perpetual motion machines. A universe that experienced repeated cycles of expansion and contraction would have get more and more disordered over time until it began completely disordered, something we do not see in our universe. One way to avoid such increasing entropy would be for the volume of the cosmos to increase with each cycle. However, if one ran this scenario backward in time, one would still be forced to conclude the universe began with a singularity.
 

 If our Big Bang wasn’t preceded by a Big Crunch, perhaps our universe instead existed as kind of dormant seed—“like a cosmic egg,” says Vilenkin—before suddenly breaking open in the Big Bang. But here, too, there is a problem: In the uncertain world of quantum physics, the “egg” couldn’t stay stable forever. It would have expanded and contracted and could have even collapsed into nothingness. “This means it couldn’t have existed forever in the past,” Vilenkin said, findings he and his student Audrey Mithani detailed in the
 <a href="http://iopscience.iop.org/1475-7516/2012/01/028">
 January issue
 </a>
 of the Journal of Cosmology and Astroparticle Physics.
 

 But the same quantum fluctuations that could have cracked the cosmic egg could be birthing new universes as you read this, says Vilenkin. This idea, called eternal inflation, suggests that our universe is just one bubble within a
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/a-tour-of-the-multiverse/">
 larger multiverse
 </a>
 which is perpetually popping out new bubble universes. Although inflation may have stopped in bubbles such as ours, new instances of inflation occur in the multiverse forever into the future, keeping the idea of eternal inflation true to its name. But what about the past? If one assumes that the multiverse is expanding and not contracting, then it had to have expanded from a certain point in time, Vilenkin explains. Even eternal inflation must have a beginning.
 

 Even if the universe did have a beginning, it likely occurred so very far in the past that
 <a href="http://arxiv.org/abs/1205.0589">
 the cosmos might as well appear as if began an eternity ago
 </a>
 , says theoretical physicist Leonard Susskind at Stanford University in California.”We’re talking about the beginning potentially occurring at time scales vastly, vastly larger than the age of our universe, longer than any time that you can name,” Susskind explains. “Statistically, given this extremely long amount of time, we probably occurred very, very late in history, making us very far from the beginning, so most of the information about the beginning would be lost to us. I think we’re really in the dark about what it would’ve been like.”
 

 Still, Vilenkin is hopeful that it might be possible to observe evidence of the beginning. In some versions of the eternal inflation model, bubbles
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2011/11/when-universes-collide/">
 occasionally collide
 </a>
 , which we might detect as distortions in the cosmic microwave background radiation that pervades all of space. If there are a number of collisions between bubbles that are clumped together in one direction more than another, “that might be linked with the beginning of the universe,” he said.
 

 Vilenkin has no problem with the universe having a beginning. “I think it’s possible for the universe to spontaneously appear from nothing in a natural way,” he said. The key there lies again in quantum physics—even nothingness fluctuates, a fact seen with so-called virtual particles that scientists have seen pop in and out of existence, and the birth of the universe may have occurred in a similar manner.
 

 “Of course, maybe someone will come up with another model of an eternal universe, and we’ll have to start thinking about it all over again,” Vilenkin said.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 arXiv:
 <a href="http://arxiv.org/abs/hep-th/0702178">
 Eternal Inflation and Its Implications
 </a>
 
 Alan Guth, the physicist who originated the inflation hypothesis, summarizes the arguments for eternal inflation.
 

 Edge:
 <a href="http://edge.org/conversation/the-cyclic-universe">
 The Cyclic Universe
 </a>
 
 Neil Turok on the past and present of the cyclic universe model.
 

 FQXi:
 <a href="http://fqxi.org/community/podcast/2012.05.30">
 Did the Universe Have a Beginning?
 </a>
 
 In this podcast, Alexander Vilenkin asks whether the universe could have existed forever into the past.
 
</section>

Who's Afraid of the Dark? Alternatives to Dark Energy

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 Last week,
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/04/whos-afraid-of-the-dark-alternatives-to-dark-matter/">
 we asked whether astronomers could be wrong about dark matter
 </a>
 , the invisible stuff that seems to help hold galaxies together. Is it possible that
 <a href="http://www.pbs.org/wgbh/nova/physics/dark-matter.html">
 dark matter
 </a>
 doesn’t really exist?
 

 This week, we’ll investigate whether there are viable alternatives to the idea of
 <a href="http://www.pbs.org/wgbh/nova/insidenova/2011/10/where-were-you-when-you-found-out-about-dark-energy.html">
 dark energy
 </a>
 , the mysterious stuff that astrophysicists believe is pushing our universe
 
 apart
 
 .
 

 In every direction we look, galaxies are hurtling away from us. That isn’t surprising in itself—after all, the Big Bang sent space and everything in it flying apart. One would expect that the gravitational pull of all the “stuff” in the cosmos would gradually slow down this expansion, bringing it to a dead stop or even collapsing everything back together in a “
 <a href="http://en.wikipedia.org/wiki/Big_crunch">
 Big Crunch
 </a>
 .” Yet instead, astronomers see that the galaxies in our universe are rushing apart
 <a href="http://www.pbs.org/wgbh/nova/space/how-big-universe.html">
 faster and faster
 </a>
 .
 

 What could be causing this acceleration? Physicists call it dark energy, and it could make up more than 70 percent of the cosmos. But so much remains unknown about dark energy that some scientists are asking whether it exists at all.
 

 What if, instead of a mysterious unseen energy, “there is something wrong with gravity?” asks Sean Carroll, a theoretical physicist at the California Institute of Technology.
 

 Einstein’s theory of general relativity represents gravity as the curvature of space and time. Perhaps this idea “is still right, but we’re not solving the equations correctly,” suggests Carroll. “We’re used to thinking of the universe expanding perfectly smoothly, and we know it isn’t, and maybe these deviations are important.” If we accounted for how the universe is clumpy instead of smooth, it might turn out that the gravitational pull of clusters of galaxies and other large agglomerations of matter alter spacetime more than previously appreciated. Distant objects would thus appear to be farther away than they actually are, leading to the false conclusion that the universe’s expansion is accelerating.
 

 The problem with this kind of model, Carroll says, is that while it suggests that these clumped-up astronomical bodies might distort our view of the universe more than suspected, gravity still remains the weakest of the known
 <a href="http://en.wikipedia.org/wiki/Fundamental_force">
 fundamental forces
 </a>
 of nature. Also, these astronomical clumps would evolve in size and gravitational strength over time. In contrast, the mysteries that dark energy was invoked to solve require something with a lot of energy that changes less over time.
 

 Another approach is to modify the laws of gravity to do away with dark energy. This tack suggests that “the laws of gravity as we know them work better on relatively small scales such as our solar system,” says Carroll, but perhaps they need “tweaks” to work on cosmic scales. Carroll and other theorists have developed alternative descriptions of gravity that could explain why the universe evolved as it did. One set of scenarios suggests that the strength of gravity increases over time and has different values depending on the distances involved. But critics argue that, to avoid contradicting well-established features of general relativity, these models are unacceptably contrived.
 

 Another family of alternative gravity models analyzes how gravity behaves if there are extra dimensions of reality, as suggested by string theory. But this approach has problems of its own: It leads to empty space “decaying” into particles in potentially detectable ways, Carroll says.
 

 To avoid modifying gravity, some theorists have suggested that our galaxy and its neighborhood might lie within a
 <a href="http://www.scientificamerican.com/article.cfm?id=does-dark-energy-exist">
 giant void
 </a>
 , an emptier-than-average region of space roughly 8 billion light years across. With so little matter to slow down its expansion, the void would expand faster than the rest of the universe. If we lived near the heart of this void, our observations of accelerating cosmic expansion would be an illusion.
 

 “The advantage of giant void models is that they don’t require any new physics to explain the apparent acceleration of the universe, like the existence of some weird dark energy or a modified theory of gravity,” says theoretical cosmologist Phil Bull at the University of Oxford.
 

 Still, “there are lots and lots of problems with void scenarios,” says theoretical physicist Malcolm Fairbairn at King’s College London. “It’s very difficult to get them to fit existing data—for instance, the cosmic microwave background (CMB) radiation usually gets distorted in these models compared to what we actually see.” For the void model to match observations of CMB radiation, we would need to be very close to the center of the void, to within one part in 100 million. That “seems like an unacceptable ‘fine-tuning’ to some people,” says Bull. “Why should we find ourselves so close to the center?”
 

 In addition, astronomers using NASA’s Hubble Space Telescope recently found
 <a href="http://www.nasa.gov/mission_pages/hubble/science/cosmic-expansion.html">
 evidence against the existence of such a void
 </a>
 . After refining their measurements of the rate at which the universe is expanding, they all but ruled out the possibility that the accelerating expansion is an illusion created by a void. In addition, if we are living inside a void, Bull and his colleagues argue, we should see very strong fluctuations of cosmic microwave background radiation reflected off hot gas in the clusters of galaxies surrounding the void. Yet we do not see any reflections that strong. “This was pretty much the final nail in the coffin for void models,” Bull says.
 

 To support the existence of dark energy—or vindicate one of these alternatives—we need giant sky surveys which will clock the speeds of even more galaxies, Fairbairn says. The colorful scenarios that theorists are dreaming up “ultimately show what an interesting and weird universe we live in,” Carroll says. “It’s one where we must keep an open mind as to what the answers may be.”
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 COSMOS:
 <a href="http://www.cosmosmagazine.com/node/2289/full">
 Doubts Over Dark Energy
 </a>
 
 Reexamining the evidence for dark energy.
 

 New York Times Magazine:
 <a href="http://www.nytimes.com/2007/03/11/magazine/11dark.t.html?pagewanted=all">
 Out There
 </a>
 
 In this article, Richard Panek explores the evidence for dark energy.
 

 NPR’s 13.7:
 <a href="http://www.npr.org/blogs/13.7/2012/03/20/148985296/dark-energy-and-the-joy-of-being-wrong">
 Dark Energy and the Joy of Being Wrong
 </a>
 
 In this blog post, Adam Frank recounts the history of the discovery of dark energy.
 
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Do Computers Dream of Electric People?

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 Are we living in someone else’s fantasy?
 

 The Chinese philosopher
 <a href="http://en.wikipedia.org/wiki/Zhuangzi">
 Zhuangzi
 </a>
 posed this question more than two thousand years ago when he recalled waking from
 <a href="http://en.wikipedia.org/wiki/Zhuangzi#The_butterfly_dream">
 a dream
 </a>
 unsure whether he was a man who dreamed he was a butterfly or a butterfly dreaming that he was a man. Today, with the advent of computers that can simulate cells, cities, and even solar systems, philosophers and scientists are asking this ancient question in a new way: Are we living in a computer simulation?
 

 This question is more than just the premise of “
 <a href="http://www.imdb.com/title/tt0133093/">
 The Matrix
 </a>
 .” It’s a conjecture that lives at the intersection of humanity and technology—and though it might seem like philosophy, it spurs ambitious new questions about what computers are capable of and about the nature of reality itself. As theorists begin to think of our universe as nothing more than a vast collection of information, can we ever truly know whether our reality is as “real” as we think it is?
 

 The philosopher
 <a href="http://www.nickbostrom.com/">
 Nick Bostrom
 </a>
 , director of the
 <a href="http://www.scientificamerican.com/article.cfm?id=the-future-of-humankind">
 Future of Humanity Institute
 </a>
 at the University of Oxford, posed the latest iteration of this ancient question in a
 <a href="http://www.simulation-argument.com/simulation.html">
 2003 paper
 </a>
 . His “simulation argument” begins with the observation that modern computers have
 <a href="http://en.wikipedia.org/wiki/Moore's_law">
 improved at an exponential rate
 </a>
 since their invention. If computing power continues to grow at this pace, advanced civilizations will one day be able to build titanic, densely-packed supercomputers capable of doing everything from beating the stock market to predicting the weather months or years in advance. “Post-human” programmers might even use these machines to simulate entire civilizations, vast electronic worlds that would put today’s computer games to shame.
 

 What would it take to create this kind of simulation?
 

 When it comes to simulating a person, scientists estimate it might take 10
 
 17
 
 operations per second—that’s one followed by 17 zeroes—
 <a href="http://www.nickbostrom.com/superintelligence.html">
 to simulate a human brain
 </a>
 , based on the number of neurons in the brain and rate of which those neurons “talk” to each other. Assuming that simulating the sensory events a person experiences—every taste, sound, smell, touch and sight that is coded in our neurons—takes about 100 million bits per second, and that approximately 100 billion humans have lived on Earth to date, Bostrom estimates it might take 10
 
 36
 
 calculations in total to create a simulation of the whole of human history that is indistinguishable from reality.
 

 That’s just to simulate the parts of the universe that humans can sense. What about the microscopic structure of the Earth’s interior or the subtle features of distant stars? These little details could be safely omitted until a simulated person needed to observe them. In addition, to save computing power, maybe not every person in a simulation would be fully simulated. Perhaps some of the characters in the simulation would be “zombies or ‘shadow-people’—humans simulated at a level sufficient for the fully simulated people to not notice anything suspicious,” Bostrom writes in his paper.
 

 So how close are we to achieving this dream (or nightmare)? Today’s most powerful supercomputers are capable of operating at roughly 10 petaflops per second—that is, 10
 
 16
 
 calculations per second. A planet-sized computer based on current electronics might carry out 10
 
 42
 
 operations per second. Bostrom also notes that quantum physicist Seth Lloyd of MIT has calculated that a 1-kilogram “ultimate laptop” that operates at the known limits of physics might be capable of
 <a href="http://arxiv.org/pdf/quant-ph/9908043.pdf">
 5 × 10
 
 50
 
 operations per second
 </a>
 . So, the planet-sized computer might be able to simulate all of human history in a millionth of a second; the ultimate laptop, a hundredth of a billionth of a second.
 

 Given that fully simulating every person who has ever lived might only take a tiny fraction of an advanced civilization’s resources, Bostrom reasons that the number of computer-generated minds buzzing away inside simulations could vastly outnumber the total sum of real minds that have ever lived. If that is true, the odds are that we are simulated, not real. It may even be possible that our simulators are themselves simulated, and their simulators are simulated, and so on. “Reality may thus contain many levels,” Bostrom says.
 

 This does not prove that we live in a simulation, Bostrom emphasizes. There are a number of caveats that could stop this bizarre future before it starts. One glum possibility is that civilizations might very well go extinct or collapse—say, by annihilating themselves in a nuclear war—before they can develop supercomputers of such immense power. Another thought is that civilizations simply have no desire to commit the vast resources needed to create supercomputers. Or perhaps advanced civilizations might not indulge in such simulations—maybe they would be ethically opposed to simulating minds and their suffering, or they might prefer to entertain themselves with
 <a href="http://en.wikipedia.org/wiki/Experience_machine">
 machines that directly stimulate their brain’s pleasure centers
 </a>
 . “Personally, I assign less than 50 percent probability to the simulation hypothesis—rather something like in the 20 percent region, perhaps, maybe,”
 <a href="http://www.simulation-argument.com/faq.html">
 Bostrom writes
 </a>
 , although he describes this as a gut feeling rather than part of his logical argument.
 

 Unless the simulators decide to make themselves known, there may be no way to prove or disprove the simulation argument. Some have suggested looking for “glitches” in the simulation, but such glitches would be more plausibly explained as hallucinations, visual illusions, fraud or self-deception. Even if errors did pop up, a smart simulator could simply wipe any memory of the anomaly from our simulated brains.
 

 If we are living in a computer simulation, how should we live our lives? “The simulation hypothesis currently does not seem to have any radical implications for how one should live,” Bostrom said. Still, “it helps to shed light on, among other things, the prospects of our species.”
 

 Also, thinking of the universe as a computer may actually be a helpful approach in science. “You can start thinking about what kind of computer it is, what kind of operations can it do, what kinds of problems can it solve,” said theoretical computer scientist Scott Aaronson at MIT. “That’s an extraordinarily fruitful way of thinking about the universe that has led to the whole field of
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/02/herding-schrodingers-cats/">
 quantum computers
 </a>
 —devices based on the quantum physics that explains how the fundamental building blocks of the universe behave.”
 

 We may never know whether we are living in someone else’s fantasy; whether we’re the man or the butterfly. But if we do one day develop supercomputers capable of simulating minds and universes, perhaps our creations will be able to answer the question for us.
 
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Video: Do We Live in a Multiverse?

Could parallel universes exist? If so, what would they look like and how would they form? Video by Greg Kestin; article by Charles Choi.Our universe may not be alone. It could just be one of multiple realms making up a “multiverse.”In fact, there are a half-dozen or so lines of reasoning that lead to this conclusion, with some pointing to the even wilder possibility that we live in a kind of multiverse-within-a-multiverse-within-a-multiverse.
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As far-fetched as the idea of a multiverse might seem, “it’s not like we come up with these crazy ideas and try to shove them into our theories,” said theoretical physicist Brian Greene at Columbia University, author of “The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos,” which explores a range of possible multiverses. “Our mathematical theories naturally yield these unfamiliar notions of other universes, and we’ve got a great deal of respect for mathematics, because over the centuries it’s proven itself a very potent guide to reality. This doesn’t mean the conclusions of mathematics are necessarily right, but they’re sufficiently compelling to make us willing to investigate them.”The inflationary multiverseWhat if our Big Bang was just one of many? Cosmologists think that, in the moments after the Big Bang, the universe experienced a rapid growth spurt called inflation. “When one explores the mathematics of inflation, one finds that in most versions of inflation, the Big Bang should not be a unique event,” said Greene. “Instead, our Big Bang should be one of a series, perhaps an infinite series, of Big Bangs.”Physicists call this scenario eternal inflation, and it suggests that our universe is just one bubble “within a grand collection of a larger multiverse” that is regularly popping out new bubble universes, Greene said. “It doesn’t take much energy to spawn a universe, because inflation has this wonderful property of mining energy from a reservoir of energy contained in the gravitational field that, in most models, one cannot ever use all up.”
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It might even be possible to test this idea by looking for evidence that such bubble universes have collided with one another. “If our universe got hit by an expanding bubble, there should be some imprint of that collision in the cosmic microwave background (CMB) radiation, the heat left over from the Big Bang,” Greene said. “People have been scouring CMB data looking for the cosmic fingerprints of such collisions, looking for telltale signs of other realms by performing observations on our own realm.”The quilted multiverseNot convinced? There’s more than one way to build a multiverse, and the quilted multiverse starts with the simple assumption that the universe goes on forever. If the universe is really infinite, then it contains infinite variations—and infinite copies—of everything. You know the one about the monkey typing Shakespeare? Given enough time, he won’t peck out just one edition of the Complete Works; he’ll bang them out infinitely. “You can imagine there are realms out there that are close copies or identical copies of ours somewhere else in the universe, or versions of ours that are modified in mind-bending ways,” Greene said. “These chunks of space would be like patches in a quilted patchwork, which is where the name for this idea comes from.”To test the quilted multiverse hypothesis, scientists have to find out if space really is infinite. “There have been suggestions that space might not go on forever, but might, for instance, have the shape of a giant donut, or torus as we call it,” Greene said. “In this configuration where the universe does not go on forever, if you look at distant stars or galaxies, the light from them can travel multiple cycles, yielding multiple images of the same object, and if one sees those, that would be pretty strong evidence the universe does not go on forever.”The quantum multiverseIn the quantum world, there is no way to predict with certainty the outcome of a measurement before it’s made. A system exists in a “superposition” of states until the moment that it is observed. So how does a single state emerge from that jumble of possibilities? Perhaps, some theorists say, each outcome really does come to be–in its own distinct universe.When quantum mechanics “came blistering onto the scene of physics in the 1920s and 1930s, it introduced a new idea into physics — that there are experiments where one can only ever predict the possibility of one outcome or another, as opposed to the older idea of Newtonian physics, which suggested one can predict what will happen with absolute certainty,” Greene said. “That raised an interesting puzzle: If a particle has a 30 percent likelihood of being here and 40 percent chance of being there and 20 percent likelihood of being way over there, when experiments find particles at one location or another, what happens to the other outcomes? In the 1950s, Hugh Everett, then a grad student at Princeton, suggested all possible outcomes do occur, each in its own universe.”There are alternative interpretations of quantum mechanics, such as Ghirardi–Rimini–Weber (or GRW) theory, named after its developers, which suggests each event can only have one possible outcome, Greene noted. Since many of these other interpretations make predictions that scientists can potentially test, maybe one day researchers can find support for them, which would weaken the case for a quantum multiverse. Until then, it’s possible that every possible timeline actually exists, including ones where fictional characters such as Sherlock Holmes actually lived.The brane multiverseString theory proposes that, underneath it all, the universe is made up of little one-dimensional threads that vibrate in different ways to make the particles with which we’re familiar. In the mid-1990s, theorists suggested that these strings could form higher-dimensional structures they dubbed membranes, or branes for short. The universe as we know it occupies one such brane.“If one can have a big 3-D structure, it could be equivalent to our universe, and theory allows multiple such 3-D objects, or multiple universes,” Greene said. “An analogy I like to use is that everything we know of our lives in essence takes place on a giant slice of bread, and there’s this larger cosmic loaf that has other slices, with each slice being a universe.”The most powerful particle accelerator on Earth, the Large Hadron Collider, might be able to test whether the brane multiverse is real. “If you collide protons on our ‘slice of bread’ and the collision is strong enough, some of the debris can be ejected off our slice of bread, and you can notice if this happened by a loss of energy in the output,” Greene said. Such tests could begin when the Large Hadron Collider restarts in 2015.The simulated multiverseOne of the wildest multiverse concepts begins with the idea that our universe is actually just a computer simulation. It sounds like the premise for “The Matrix,” but the question of whether our world is just a fantasy is ancient, dating back at least to Chinese philosopher Zhuangzi, who more than 2,000 years ago described waking from a dream unsure of whether he was a man who dreamed he was a butterfly or a butterfly dreaming that he was a man.In 2003, philosopher Nick Bostrom, director of the Future of Humanity Institute at the University of Oxford, posed a modern take on this ancient contemplation of the true nature of reality. He estimated it would take 10 36 calculations in total—a 1 with 36 zeroes behind it—to create a simulation of the whole of human history that was indistinguishable from reality, and that a planet-sized computer based on current electronics might conceivably perform 10 42 operations per second.Simulating every person who has ever lived might only take a tiny fraction of an advanced civilization’s resources, Bostrom reasoned. In that case, the number of computer-generated people and other entities within simulations could vastly outnumber all the real minds that have ever lived. By this logic, the odds are that we all are simulated, not real. In turn, it may be possible that our simulators are themselves simulated, and their simulators are simulated, and so on.Seems rock-solid, right? The problem, says Greene, “is that it’s very hard to imagine falsifying the simulated multiverse notion — even if we can find out we are simulated, the simulator can erase that memory if they wanted, so any proof that the universe is simulated would go away. There are a number of physicists who nod to this idea as a mind-bending curiosity, but it’s not going to affect our day-to-day lives or work.”Beyond reach?We may never find proof that other universes exist. But even if we could, there is virtually no known way to reach them.Scientists have speculated that wormholes, “shortcuts” in space and time, might allow one to reach other universes. However, wormholes may not actually exist, and even if they do, it might not be possible to traverse them. “It’d be lovely go get into a spaceship to go through a wormhole to travel back and forth to other universes, but it doesn’t look to me like the equations will allow us to do so,” said theoretical physicist Leonard Susskind at Stanford University.Even if we can never travel to another universe, that does not mean we are alone. “Even in our single lonely universe, there is a gargantuan number of opportunities for life to exist,” Greene said. “The many discoveries of extrasolar planets are thrilling, and that may lead one to seriously consider that there may be other life forms out there in our universe.”Go Deeper Editor’s picks for further readingFQXi: <a href="http://www.fqxi.org/data/articles/Philosophy_of_the_Multiverse__RC.pdf">Philosophy of the Multiverse</a> In this essay, discover why many theorists are drawn to the idea that our universe is just one among many.NOVA: <a href="http://www.pbs.org/wgbh/nova/manyworlds/">Parallel Worlds, Parallel Lives</a> Discover web resources associated with NOVA’s “Parallel Worlds, Parallel Lives,” a film about the life and work of Hugh Everett III.<a href="http://www.briangreene.org/?page_id=26">The Hidden Reality</a> Brian Greene’s 2011 book asks whether our universe is the only universe.Video CreditsProduced, animated, and edited by Greg KestinEditorial help from Kate Becker, Anna Rothschild, and Lauren AguirreA special thanks to Andrew FriedmanOriginal Footage © WGBH Educational Foundation 2014MEDIA RIGHTSMusic: Night Music, Air Prelude, Not as it Seems, and Comfortable Mystery 3 by Kevin MacLeod (incompetech.com) CC By 3.0Images: Bicep2: Courtesy of National Science Foundation Big Bang, Galaxies, and Cosmic Web: Courtesy of NASASCIENTIFIC NOTES<a href="http://www.wgbh.org/programs/NOVA-16/episodes/The-Fabric-of-the-Cosmos-Universe-or-Multiverse-32950">Many physicists believe there is a strong connection between the inflationary multiverse and an important feature of string theory; for more see: http://www.wgbh.org/programs/NOVA-16/episodes/The-Fabric-of-the-Cosmos-Universe-or-Multiverse-32950</a>Although there is not yet evidence favoring the quantum multiverse over several other interpretations of quantum mechanics, it is still consistent with the results of every quantum experiment ever conducted.

Charles Choi

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