Koch Foundation

Draper

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>

 We think of gravity as weighty—its omnipresent grasp pulling us down to the ground. Try to lift a piano up a flight of stairs and you can feel gravity’s resistance. (
 <a href="http://www.youtube.com/watch?v=UWm0nXJYLmk">
 Laurel and Hardy
 </a>
 showed this best!) Yet in a match with the other fundamental forces of nature—electromagnetism, the weak force and the strong force—gravity gets pummeled.
 

 You can see gravity’s relative weakness simply by using an ordinary bar magnet to pick up paper clips from a desk. Battling the gravitational pull of all of Earth, the tiny magnet wins! In fact, gravity is a staggering 10
 
 40
 
 times weaker than electromagnetism. But why, among the fundamental forces, is gravity the runt of the litter? Explaining gravity’s relative feebleness is a profound challenge for physics, and an essential milestone on the road to a unified theory of all four forces.
 

 Uniting the four fundamental forces into a single unified theory is a longstanding scientific dream. On the face of it, these forces are very different. They each operate across different distances, with different strengths, determined by the properties of a special class of particles, called “exchange particles,” whose job it is to convey the forces.
 

 Exchange particles are like Frisbees thrown between particle players; the process of tossing the Frisbee brings ordinary particles together—or in some cases pushes them apart. In electromagnetism, for example, electrons interact by exchanging photons. Because the photons have no rest mass, they travel through the vacuum at the speed of light, making electromagnetism a long-range force. Long-range means that it can operate over great distances. For example, terrestrial receivers can pick up radio signals (a type of electromagnetic radiation) transmitted by Voyager 1, situated at the edge of the solar system more than 11 billion miles away. The weak force, in contrast, is conveyed by massive exchange particles called the W+, W- and Z bosons. Because these exchange particles are so heavy, particles feel the weak force only on very short ranges—within atomic nuclei. The strong force, too, operates only on short ranges. But gravity, which theorists believe is carried by particles called gravitons traveling at the speed of light, is a long-range force.
 

 Physicists have made great strides toward unifying electromagnetism with the weak interaction and, to some extent, with the strong interaction, by looking at how the forces behave at very high energies—energies that existed just moments after the Big Bang. Theoretical and experimental discoveries of the past few decades suggest that at these energies—above about 100 GeV (gigaelectronvolts)—the weak and electromagnetic forces behave as a single type of interaction, called electroweak, with identical range and strength. During this brief, hot period, the “Frisbees” that convey the weak force had no mass at all. But as the universe cooled, interactions with the Higgs field caused the W+, W- and Z bosons to acquire mass, differentiating them from massless photons and splitting what was once one force, the electroweak force, into two, electromagnetism and the weak force.
 

 Although physicists have yet to develop a “grand unified” model that includes the strong force, they believe that, at even higher energies, it too may merge with the weak and electromagnetic forces.
 

 But what about gravity? If you could draw the evolutionary family tree of the physical forces, you might see the split between electromagnetism and the weak force as something like the division of primates into various species. Gravity, though, represents a far more radical diversification. Reconciling gravity along with the other forces is something like showing how viruses and whales have common ancestry. Spanning the differences is possible, but tricky.
 

 How far up the energy scale must we climb to find a point at which gravity could be unified with the other forces? The answer is a number called the Planck energy: around 10
 
 28
 
 eV. That’s about 10
 
 17
 
 times greater than the energy of electroweak unification—an enormous difference. One would need a machine almost a quintillion times more powerful than the Large Hadron Collider or Fermilab’s (retired) Tevatron to probe the Planck energy, putting experimental tests of this kind of unification well out of reach for the conceivable future! The sheer magnitude of the Planck-to-electroweak energy ratio, related to the stark weakness of gravity, is called the hierarchy problem.
 

 String theorists have proposed an innovative solution to the hierarchy problem: Perhaps gravity is weakened through its ability to travel across an extra dimension. What sounds like science fiction has become an active branch of scientific inquiry called the “braneworld hypothesis.” The braneworld hypothesis suggests that the observable universe lives within a four-dimensional (three dimensions of space and one dimension of time) membrane, or “brane.” Beyond this brane, extended along a fifth dimension, is a region called the “bulk.” Unique among exchange particles, gravitons are free to wander the bulk; everything else is stuck on our brane. So while most particles are like ants confined to the surface of a wooden picnic table (the brane), gravitons are like termites that can bore within (and visit) the bulk. (In the language of string theory, the difference is that gravitons are represented by closed strings; other particles are open strings and have ends that stick to our brane like a curved handle attached to a door.) Because photons cannot enter the bulk, it remains invisible.
 

 An advantage of explaining gravity’s weakness through its dilution into the bulk and making the brane the venue for the other forces is that unification can take place at energies not much higher than the electroweak scale, rather than at the Planck scale, rendering the search for a unified theory much easier.
 

 There are a number of variations of the braneworld idea.
 <a href="http://www.sciencedirect.com/science/article/pii/S0370269398004663">
 One model
 </a>
 , proposed in 1998 by physicists Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali envisions a “large extra dimension” of about one millimeter throughout which gravity is spread. (String theorists are typically concerned with such tiny size scales that one millimeter does indeed qualify as “large.”) A second brane, parallel to ours, would constitute the opposite boundary of the bulk, confining gravitons to the region between the two limits. You can imagine this second brane as something like the underside of the picnic table. The extra dimension would then comprise the thickness of the table—the distance between its top and bottom—limiting the graviton “termites” to travel through a finite amount of wood.
 

 However, this proposal modifies the law of gravity in measurable ways, which have since been ruled out experimentally.
 <a href="http://prl.aps.org/abstract/PRL/v83/i23/p4690_1">
 Another version
 </a>
 , proposed by physicists Lisa Randall and Raman Sundrum, includes only one brane and posits that the warping of the structure of the bulk, along the direction of the extra dimension, would be enough to confine gravity to a limited region and dilute its strength. The theory predicts a measurable leakage of gravitons from our brane into the warped bulk, which could potentially be detected in collisions at the Large Hadron Collider through unexpected energy loss that finds no other explanation. Researchers have sought such telltale clues to test the Randall-Sundrum idea and other braneworld approaches. The tricky part is using statistical models to rule out more mundane effects that could mimic the leakage of energy (for example, the release of neutrinos). While LHC results have not yet confirmed the braneworld idea, the jury is still out as to whether or not gravity’s weakness is a result of its slick ability to leave the visible universe and travel through a higher dimension.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


 CERN Courier:
 <a href="http://cerncourier.com/cws/article/cern/41013">
 The Nobel path to a unified electroweak theory
 </a>
 
 In this article, CERN marks the anniversary of the 1979 Nobel Prize in physics, which honored the foundations of electroweak unification.
 

 The Particle Adventure:
 <a href="http://particleadventure.org/unseen.html">
 What holds it together?
 </a>
 
 Explore the physics of force-carrying particles on this interactive web site.
 

<a href="http://www.amazon.com/Warped-Passages-Unraveling-Mysteries-Dimensions/dp/0060531088">
 Warped Passages: Unraveling the Mysteries of the Universe’s Hidden Dimensions
 </a>
 
 In this book, physicist Lisa Randall explores extra dimensions, braneworlds, and the bulk.
 
</section>
</div>

Why Is Gravity Such a Weakling?

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>

 This is how it all could end: heat death, the moment when the universe expends its last drop of usable energy and settles into lifelessness. Like the ultimate junkyard, only relics of once-burning stars and their planetary companions remain. Wait long enough and even black holes vanish through the slow trickle of Hawking radiation.
 

 That is a bleak and humbling thought. But might nature enjoy an encore or even a revival? A second act for the universe, even if it were many billions of years from now, would offer a source of comfort. We’d know that even if our own civilization—and indeed all possible living worlds in space—blew out like a flame in the wind, flickers of new life could eventually relight.
 

 <figure class="richtext-image full-width"><img alt="merrygoround_620" class="richtext-image full-width" height="348" width="620"
 src="/wgbh/nova/media/images/merrygoround_620.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 


 In the olden days of cosmology—that is, before the 1998 discovery that
 <a href="http://imagine.gsfc.nasa.gov/docs/features/exhibit/tenyear/darkenergy.html">
 the universe’s expansion is accelerating
 </a>
 —there was hope that the universe might bounce back from a “
 <a href="http://map.gsfc.nasa.gov/universe/uni_fate.html">
 Big Crunch
 </a>
 .” In that scenario, the universe’s sheer heft would eventually cause it to stop expanding, switch gears and shift into reverse. Like a film of the Big Bang played backward, all of space would collapse to a single point. Cosmologist Stephen Hawking once speculated that time itself would switch directions during the contraction phase. (Later, after
 <a href="http://www.theglobeandmail.com/life/facts-and-arguments/my-time-with-stephen-hawking/article4085631/">
 discussions with his assistant Raymond Laflamme
 </a>
 , he changed his mind.) Today, with no end in sight for cosmic expansion, the “Big Crunch” is out of fashion. But new theories are offering some hope that the heat death may not be terminal.
 

 Cosmic cycles have a long tradition in philosophy and religion. From the ancient Chinese to the Mayans, numerous cultures embraced the idea of time as an ever-turning wheel. Many ancient peoples imagined that a cosmic “spring” would follow each cosmic “winter” in a perpetual sequence of phases, just like the seasons on Earth. Scientific cosmology followed suit. Shortly after Edwin Hubble discovered that galaxies were receding from each other—an observation that beautifully matched Belgian scientist and cleric Georges Lemaître’s hypothesis that the universe is expanding—physicist Richard Tolman began to investigate the possibility that the cosmos was eternally oscillating through cycles of creation and destruction. The idea was that each Big Crunch would be followed by a new Big Bang, tracing an
 <a href="http://teachastronomy.com/astropedia/article/Alternatives-to-the-Big-Bang-Model">
 endless accordion of expansions and contractions
 </a>
 . Unfortunately, Tolman later realized that the second law of thermodynamics, which states that entropy in a closed process can’t decrease, would force each cycle to be longer with less usable energy, a scenario that seems less like renewal and more like modulated decay.
 

 But it’s not all bad news for cosmic cycles. Believers in eternal renewal can take heart that researchers have advanced several cyclic cosmologies that are consistent with the latest discoveries about cosmic acceleration and dark energy.
 

 One such model is the cyclic universe (originally and more poetically known as the
 <a href="http://www.physics.princeton.edu/~steinh/vaasrev.pdf">
 Ekpyrotic Universe
 </a>
 , after the ancient Greek name for the fiery interludes between cycles) first proposed by astrophysicists Paul Steinhardt and Neil Turok and string theorist Burt Ovrut, later joined by Steinhardt’s student Justin Khoury. The trio (sans Khoury) hatched the idea on a train ride back to London from a conference they were attending together in Cambridge, while discussing the cosmological implications of collisions between hypothetical entities called D-branes, or branes for short. In string theory, the universe is composed of strings—energetic strands whose vibrations are hypothesized to produce particle properties—and branes, pulsating surfaces to which open strings can attach like floppy spaghetti. That special relationship has suggested the concept of braneworlds, models of the universe in which all particles in the Standard Model, from electrons and quarks to photons and gluons, are represented by open strings, gravitons (the carriers of gravity) by closed strings, and the observable universe by a region on a brane.
 

 The researchers imagined an oscillatory cosmos in which our brane repeatedly collides with a neighboring one along a higher dimension. This model explains how some particular features of our cosmos, like the formation, distribution, and recession of galaxies, could be produced by repeated infusions of energy from inter-dimensional smash-ups. Each time the branes collide, everything from the previous cycle is wiped out and new structures form. Part of the collision energy becomes what we know as dark energy, driving the acceleration of three-dimensional space. Ultimately matter and energy spread out until the universe is increasingly dilute. The branes collide once more and the cycle renews. A solitary Big Bang becomes replaced with
 <a href="http://www.physics.princeton.edu/~steinh/brane3.html">
 repeated Big Bounces
 </a>
 .
 

 One can think of the cyclic cosmology as something akin to a desert plain that experiences a tremendous burst of rain once a year. During the deluge, an enormous pool of water starts to spread across the dry land, continuing to dissipate even after the rain stops. Gradually the water seeps completely into the soil until the land becomes parched once more. Eventually a fresh torrent heralds the start of the next cycle. Similarly, while the cause of cosmic cycles hits us from beyond our brane, the resulting expansion takes place within it. (Unlike a growing pool of rainwater, though, space does not expand into anything; rather, locales such as galaxies move farther and farther away from each other.)
 

 The cyclic scenario avoids the entropy problem that plagued Tolman’s model, Steinhardt explains, because in the cyclic cosmos, “entropy density does not accumulate. What’s happening in ours is that you are producing entropy, but our three dimensions don’t re-collapse, so it just remains thinned out after the period of expansion. Now when you collide you produce new entropy at very high density. The old stuff is of such low density that it’s just irrelevant at the end of the next cycle. So you can go right into the next cycle with essentially the same density you’ve had before. The total entropy has increased, but the entropy density goes to negligibly small each cycle.”
 

 One weakness in the cyclic scenario is that there is no experimental evidence for extra dimensions beyond our perceived space and time. Fortunately, when the Large Hadron Collider
 <a href="http://home.web.cern.ch/about/updates/2014/06/cern-announces-lhc-restart-schedule">
 reboots in early 2015
 </a>
 , we will be able to test some models that predict extra, unseen dimensions.
 

 Another cyclic approach eschews higher dimensions in favor of a topological twist. Known as
 <a href="https://accelconf.web.cern.ch/accelconf/e06/PAPERS/THESPA01.PDF">
 conformal cyclic cosmology
 </a>
 , it was proposed by Oxford mathematician Roger Penrose, based on the work of his former student K. Paul Tod, as a way of explaining why entropy is so low at the dawn of time and so high at its dusk—which is another way of explaining why eggs splat when they fall out of the carton but never spontaneously reassemble themselves, and, indeed, why time only goes one direction. Penrose postulated that the geometry of space itself could be changing over time, becoming ever more tangled, like an unkempt, snarled head of hair. In conformal cyclic cosmology, this tangliness is described by a mathematical object called the Weyl curvature tensor, which starts at zero at the beginning of time and grows larger and larger over the eons.
 

 The cyclic part enters via a mathematical sleight-of-hand called conformal invariance. Conformal invariance is a symmetry between geometries of different scales, transforming size but keeping shape intact. It acts as a kind of mathematical magnifying glass. Penrose noted that many billions of years from now—after all the stars in the universe are extinguished, all black holes are drained of their energy through Hawking radiation, and all matter completely decayed—space would be perfectly uniform – a reservoir of useless energy. Thus, except for scale and degraded (high entropy) energy, it would be identical to the cosmic beginning. Suppose, then, that a conformal transformation connects the emptiness at the end of time with the emptiness at the beginning, like a strip of paper glued into a ring. Degraded particles would become vibrant (zero entropy) once more. The demise of the universe would become a
 <a href="https://www.youtube.com/watch?v=sM47acQ7pEQ">
 new start
 </a>
 .
 

 Only a few years ago, many researchers thought that cycles were so over, banished by the fall of the Big Crunch scenario. But like retro fashions, they keep returning, brought back by savvy designers who tailor them to the needs of contemporary cosmological conditions—and the cycle continues.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


<a href="http://www.randomhouse.com/book/129417/cycles-of-time-by-roger-penrose">
 Cycles of Time: An Extraordinary New View of the Universe
 </a>
 
 Roger Penrose’s book explores the theory of conformal cyclic cosmology.
 

<a href="http://www.physics.princeton.edu/~steinh/endlessuniverse/index2.html">
 Endless Universe: Beyond the Big Bang
 </a>
 
 Paul Steinhardt and Neil Turok make the case for cyclic cosmology in this popular 2007 book.
 

 The Nature of Reality:
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2012/06/in-the-beginning/">
 In the Beginning
 </a>
 
 Science writer Charles Choi on the cosmologists who dare to ask what happened before the Big Bang.
 
</section>
</div>

Discover why some theorists think our universe could one day be reborn, in an endless cycle of creation and destruction.

When the End is Just the Beginning: Exploring Cosmic Cycles

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>


 Adapted from
 
<a href="https://medium.com/starts-with-a-bang/battle-of-the-nobel-laureates-712d444610d3">
 Einstein’s Dice and Schrödinger’s Cat
 </a>

 by Paul Halpern. Courtesy Basic Books, the Perseus Books Group.
 



 Who will be the next Einstein? Will his ingenious contributions ever be surpassed? Is there anyone brilliant enough to complete his dream of a unified theory of nature? Despite being an accomplished physicist, Nobel laureate, and Renaissance man, Einstein’s sometime ally, sometime adversary Erwin Schrödinger never came close to Einstein’s fame internationally. If anything, his cat has taken all the glory—at least as a cultural meme.
 

 <figure class="richtext-image full-width"><img alt="einstein_620" class="richtext-image full-width" height="348" width="620"
 src="/wgbh/nova/media/images/einstein_620.width-800.jpg"/><figcaption>
 Albert Einstein in 1921, adapted from a photograph by Ferdinand Schmutzer. Public domain
 <div class="credit"></div></figcaption>
 </figure>
 


 Though Einstein was his collaborator and mentor, Schrödinger briefly imagined himself successor to the throne when, in January 1947, he thought he had found a theory of everything. Jumping the gun, he announced to the Royal Irish Academy that his new General Unitary Theory superseded Einstein’s general theory of relativity. The international press picked up on Schrödinger’s startling announcement, asking if he might have fulfilled Einstein’s quest. However, Schrödinger’s unification attempt turned out, like so many others, to be a false start, with no experimental evidence to support it.
 

 Schrödinger certainly hasn’t been the only one to try to fill Einstein’s shoes. Since 1919, when the public first tasted the theory of relativity through the announcement of the solar eclipse measurements, it has had an insatiable appetite for news about Einstein and possible successors. While he was alive, as we’ve seen, the press trumpeted every unified field theory he proposed as if it were a major breakthrough. After his death, stories about brilliant individuals tantalizingly close to completing his mission have continued to make headlines. All in all, Einstein, his unfinished quest, and the question of who might inherit his throne have served as touchstones for almost a century.
 

 Research scientists know that progress in any field is usually incremental, taking place over years or even decades. Groundbreaking discoveries are few and far between. Often a scientist needs to be lucky enough to be in the right place at the right time to make a mark. Most scientific research today is completed by large teams, rather than by single individuals.
 

 Yet the myth persists of the lone genius changing everything around us. Type “next Einstein” into any Internet search engine and expect to be bombarded with results—everything from recipes for educational success to claims made in resumes or personal ads. Here are a few assorted examples of recent media musings: Will the next Einstein be a “
 <a href="http://www.foxnews.com/story/2007/11/16/laid-back-surfer-dude-may-be-next-einstein/">
 surfer dude
 </a>
 ”? Is he a
 <a href="http://www.dailymail.co.uk/news/article-1369595/Jacob-Barnett-12-higher-IQ-Einstein-develops-theory-relativity.html">
 child prodigy
 </a>
 with an exceptional IQ? What if the next Einstein is a
 <a href="http://www.kitguru.net/channel/science/jules/will-the-next-einstein-be-a-computer/">
 computer
 </a>
 ? Could a
 <a href="http://www.techradar.com/us/news/software/operating-systems/-ubuntu-on-android-may-help-find-next-einstein—1159142">
 smartphone application
 </a>
 identify him? Or might an old-fashioned DVD designed for little ones do the trick? As a 2009 New York Times headline advised with tongue firmly in cheek, “
 <a href="http://www.nytimes.com/2009/10/24/education/24baby.html?_r=0">
 No Einstein in Your Crib? Get a Refund!
 </a>
 ”
 

 The formula that produced Einstein was a perfect match between crucial scientific problems that demanded radical solutions, exceptional insights that often overturned commonly held beliefs, an ironically photogenic visage (who knew that rumpled sweaters, a Brillo pad mustache, and a mop of unruly gray hair could be so compelling?), and the omnipresent glare of the camera. His rise to fame coincided, more or less, with the golden age of Hollywood, when cinematic newsreels projected the latest fashions, feats, and foibles of celebrities. Like Douglas Fairbanks, Mary Pickford, Charlie Chaplin, the Barrymores, and countless other stars of the cinema in the twenties, thirties, and forties, Einstein traipsed across the screens of thousands of Main Street movie houses. The public viewed him stopping on his strolls to wave to admirers, giving speeches about current affairs, headlining benefits for various charities, and occasionally reporting progress in his research. Hungry to fill their quota of human interest stories, reporters lapped up news about the German Jewish scientist like scrawny cats with spilled milk.
 

 It is not clear if that formula will ever be repeated. For one thing, there has been an explosion of publications. Many theories vie for prominence—far more than in the days of Einstein and Schrödinger. Yet the energies required to test these approaches have required increasingly expensive and time-consuming projects, such as the Large Hadron Collider (LHC) near Geneva, Switzerland. Unlike, for example, the eclipse measurements, experimental science has generally proceeded far more slowly and cautiously, requiring far greater quantities of data before announcing results. In high-energy physics, teams typically involve hundreds of researchers rather than lone pioneers. At the same time, the media have diversified, so not everyone’s eyes are fixed on the same scientific celebrities.
 

 Peter Higgs, one of the recipients of the 2013 Nobel Prize in physics, has become a rare contemporary example of a well-known, accomplished theorist. Yet his name recognition hardly rivals Einstein’s. The particle named after him, the Higgs boson, has come to be known colloquially as the “God particle.” When it was discovered in 2012 at the LHC, much of his press coverage was shared with a divine being. (To India’s dismay, its native son Satyendra Bose hardly got any mention.)
 

 As the LHC is restarted with collision energies amped up to 13 TeV, the physics community will be sifting through its collected data searching for hints of new theories. If interesting results are found, undoubtedly eager theorists will propose new unification ideas. Let’s hope that the media will look at these suggestions with a critical eye and wait for solid evidence, if it emerges, before declaring that the next Einstein has arrived.
 
</section>
</div>

Einstein’s fame arose from a perfect storm of science, culture, and personality, says physicist Paul Halpern.

The Myth of the "Next Einstein"

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>

 Bang on a drum and you create a disturbance. The drum’s vibrations set off a chain reaction: molecules move, air expands and is compressed, and a sound wave is born. It is straightforward to separate the cause—drum vibrations—and the effect—the sound. The harder you bang on the drum, the more energetic the sound wave.
 

 <figure class="richtext-image full-width"><img alt="drums_620" class="richtext-image full-width" height="348" width="620"
 src="/wgbh/nova/media/images/drums_620.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 


 Now, replace the beaten drum with a gravitational disturbance, such as the sudden collapse of a stellar core into an ultra-compact neutron star or black hole. Einstein predicted that such a collapse would create gravitational waves. But do these waves carry energy and, if so, how is that energy distributed—that is, what is its density (energy per unit volume) from point to point?
 

 Gravitational energy is notoriously hard to define. In Einstein’s equations of general relativity, celebrating their 100th anniversary this year, gravitation and energy are on opposite sides. One side of the equations—call it the geometric side—represents gravitation as distortions in the fabric of spacetime. The other—call it the material side—describes the matter and energy in each region, including all forms of non-gravitational radiation, such as light. General relativity informs us that matter and energy compel spacetime to warp, creating what we feel as gravity. For example, the Sun’s mass distorts spacetime and generates a gravitational well in its vicinity. In short, matter and energy are the
 
 cause
 
 of gravitation’s
 
 effect
 
 .
 

 Where then does gravitational energy fit in? Is it a cause, an effect, or both? Einstein’s equations of general relativity do not offer a clear answer. Gravitational energy doesn’t neatly fit on either the geometric or material side of the equations.
 

 Einstein recognized the situation early on, and developed a separate formula for measuring the energy and momentum of gravitational fields. Known as the Einstein energy-momentum complex, it determines the gravitational energy and momentum within any region of spacetime, given its geometric structure.
 

 Due to mathematical limitations of Einstein’s definition, other physicists began to develop independent energy-momentum formulas. These include formulations by French-Greek physicist Achilles Papapetrou, Russian physicists Lev Landau and Evgeny Lifshitz, American physicist Steven Weinberg, and others. Each of these complexes obeys conservation laws, meaning that energy and momentum can be transformed but not lost. For basic cases, such as determining the energy of a non-rotating black hole of mass
 
 M
 
 , they beautifully match each other in predicting an energy of
 
 E= Mc
 
 2
 

 . Thus, they conform to what might be expected for a relativistic definition of energy.
 

 Yet one prediction made by these formulas is most unsettling. In 1955,
 <a href="http://www.nytimes.com/1995/12/23/world/nathan-rosen-86-of-israel-physicist-worked-with-einstein.html">
 Nathan Rosen,
 </a>
 a former assistant of Einstein, applied several different energy-momentum complexes to a particular model of gravitational waves and calculated its energy in each case to be zero. He consequently proposed that gravitational waves don’t carry energy and thereby cannot really exist in nature. His words carried special weight, since he and Einstein had worked on that very subject. Rosen offered his hypothesis at a Bern conference celebrating the jubilee of special relativity.
 

 Few physicists believed Rosen’s conjecture. All forms of radiation carry energy; why should gravity be different? As Richard Feynman argued two years later in his “
 <a href="http://www.edition-open-access.de/sources/5/34/index.html">
 sticky bead argument
 </a>
 ,” presented at a general relativity conference in Chapel Hill, a gravitational wave could jostle a bead on a stick, moving it up and down and, through friction, generate heat—a form of energy—in the process. If the gravitational wave didn’t carry the energy, he argued, where else could it have arisen? Something must have made the bead hot. Feynman did not try to explain why the energy-momentum complexes yielded a value of zero for the energy of gravity waves; presumably, he simply thought they were incomplete or wrong.
 

 Canadian physicist Fred Cooperstock, who had worked for a year with Rosen, takes the value of zero seriously. But while Rosen argued that gravitational waves don’t exist at all, Cooperstock argues that they are real, but carry no energy. Cooperstock’s
 <a href="http://www.gravityresearchfoundation.org/pdf/awarded/2015/Cooperstock_2015.pdf">
 unorthodox hypothesis
 </a>
 is that gravitational energy exists only where the material side of Einstein’s equations is non-zero; that is, in places with matter or (non-gravitational) energy in the first place. Consequently, all empty regions of spacetime have zero gravitational energy. That precludes gravitational waves carrying energy through the void. (If there is no true void, such a point may be moot.) In his view, gravitational waves convey geometric information (ripples in curvature), but not energy, from one point to another. Fluctuations ripple through spacetime, causing notable effects, while somehow carrying no energy.
 

 “I’ve never seen anyone prove that information must carry energy,” Cooperstock says. It is like an elderly woman texting her daughter to bring home a sizable bag of groceries. Even though the text message carries information, but not energy, it triggers some heavy lifting.
 

 A breakthrough came in the 1970s, when Russell Hulse and Joseph Taylor
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/press.html">
 detected and measured the properties of the first-known binary pulsar system
 </a>
 , PSR 1913+16. They demonstrated that the system’s orbital period is declining with time, matching a prediction made by Einstein about the transmission of gravitational waves between two masses. It was the first indirect indication of gravitational waves, and it won them the Nobel Prize. But skeptics like Cooperstock argue that fluctuations in the curvature of spacetime caused the results without actually conveying energy through space.
 

 Today, several laboratories around the world are racing to detect gravitational waves directly. Leading the pack is the LIGO (Laser Interferometer Gravitational-wave Observatories) project, recently upgraded to
 <a href="http://www.caltech.edu/news/dedication-advanced-ligo-46822">
 Advanced LIGO
 </a>
 , based in Hanford, Washington and Livingston, Louisiana. MiniGRAIL, a spherical gravitational wave detector based in Leiden, Holland, is trying to detect gravitational waves using an ultra-cold, 1,300 kilogram copper alloy sphere. A space-based mission called
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/11/theres-more-than-one-way-to-hunt-for-gravitational-waves/">
 LISA
 </a>
 (Laser Interferometer Space Antenna) is currently being planned.
 

 Despite numerous efforts, as we celebrate the 100th anniversary of general relativity, gravitational energy remains an elusive construct. It has become an even weightier matter than Einstein first thought. However, if astronomers discover gravitational waves and can map out their energy, the burden of proof will finally be lifted. Understanding gravitational energy would help place it on the same footing as other natural interactions, such as electromagnetism, and will bring science closer to a modern-day “holy grail”: uniting gravity with the other forces of nature.
 

 
 Go Deeper
 
 

 Editor’s picks for further reading
 

 
 Nature of Reality:
 <a href="http://www.pbs.org/wgbh/nova/blogs/physics/2014/11/theres-more-than-one-way-to-hunt-for-gravitational-waves/">
 There’s More Than One Way to Hunt for Gravitational Waves
 </a>
 
 Jennifer Ouellette explores the diverse methods with which researchers are searching for direct evidence of gravitational waves.
 

 TED:
 <a href="http://www.ted.com/talks/janna_levin_the_sound_the_universe_makes?language=en">
 The Sound the Universe Makes
 </a>
 
 In this video, astrophysicist Janna Levin explains how gravitational waves are made and LIGO’s role in searching for them.
 

 Wikipedia:
 <a href="http://en.wikipedia.org/wiki/Sticky_bead_argument">
 The Sticky Bead Argument
 </a>
 
 The history of the sticky bead argument and its influence on physicists’ thinking about gravitational waves and energy.
 
</section>
</div>

What is gravitational energy, and can new experiments shed light on its elusive nature?

The Unbearable Lightness of Gravity

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>

 First, the good news: Despite earlier doomsday prognoses, the cosmos is not fated to implode. If you stay up late at night worrying that the entirety of creation will cave in on you in a universal Big Crunch, rest assured. Astronomical evidence suggests that the Big Bang expansion will never reverse itself; the universe will not collapse back down to a point. Rather, the universe, like ever-sprawling suburbs, will burgeon forever, its galaxies receding farther and farther away from each other.
 

 Now, the bad news: Cosmic expansion,
 <a href="http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html/">
 discovered in 1998 through supernova observations
 </a>
 conducted by Nobel Prize-winning physicists Saul Perlmutter, Brian Schmidt, and Adam Riess and their research teams, is not only continuing, it is picking up its pace. Though we don’t know what is causing the acceleration, one leading idea, called “phantom energy,” has ominous implications. If phantom energy continues to drive the universe faster and faster outward, it could literally tear space into shreds—a doomsday scenario called the “Big Rip.”
 

 Phantom energy,
 <a href="http://arxiv.org/abs/astro-ph/9908168">
 proposed by Dartmouth physicist Robert Caldwell
 </a>
 in 1999, is one possible variety of the mysterious stuff researchers call dark energy. Dark energy is the catchall phrase for the hidden agent of cosmic acceleration, and encompasses many different approaches. The simplest proposal adds an extra “antigravity” term, called a “cosmological constant,” to Einstein’s equations of general relativity. Einstein himself had proposed the term to stabilize his equations, but then hastily removed it after Edwin Hubble showed in 1929 that the universe was expanding—and not stable at all. Adding the term back into the equations, while assuming that the geometry of the universe is flat, leads to a prediction that the cosmic growth rate will gradually and steadily increase.
 

 While a cosmological constant seems to model the accelerating universe nicely, at least according to current astronomical observations, many researchers are seeking a more tangible explanation. Instead of an extra term arbitrarily added to the gravitational equations, they are looking for an actual physical substance with repulsive properties. This substance must have a bizarre property called negative pressure. Though we never encounter negative pressure in everyday life, you can picture how it works by imagining poking a soap bubble and seeing it inflate rather than pop.
 

 Gravitational researchers model the behavior of substances using an equation, called an equation of state, that relates the material’s pressure to its density. For conventional substances such as fluids and gases, both pressure and density are positive, so their ratio, usually called “
 
 w
 
 ,” is positive as well. However, if a substance were to mimic a cosmological constant, its
 
 w
 
 ratio would be negative one. That is, if you try to squeeze it, it will get bigger, not smaller.
 

 In 1998, shortly after the discovery of cosmic acceleration, Caldwell, along with astrophysicists Rahul Dave, then at Penn, and Paul Steinhardt of Princeton,
 <a href="www.physics.princeton.edu/~steinh/Caldwell.pdf">
 argued for the greatest amount of flexibility
 </a>
 in trying to describe dark energy. Borrowing the ancient Greek term for the “fifth essence,” they dubbed it “quintessence” and asserted that its equation of state could vary over time and space. Rather than assuming that
 
 w
 
 is negative one, they urged that its actual value be pinned down through astronomical observation.
 

 This flexibility spurred Caldwell to think about the worst-case scenario imaginable for dark energy. He wondered what would happen if the amount of negative pressure exceeded the density—that is, if w was less than negative one. In our soap bubble analogy, larger negative pressure would mean that pressing on its exterior would cause it to puff up even faster. In the far future, he realized, gravity and all other forces would eventually lose potency, overwhelmed by the hulking menace of what he dubbed phantom energy. The reason is that unlike the steady density of the energy associated with a cosmological constant, the density of phantom energy would grow greater and greater, rising along with its negative pressure, building to a lethal crescendo—a Big Rip.
 

 In Caldwell’s end game scenario, our distant descendants (relocated to other planets, perhaps) will notice the first sign of trouble billions of years from now as other galaxies recede beyond detection and disappear from the sky. (That would happen under the cosmological constant picture too, but is exacerbated in the case of phantom energy.) Eventually our local group of galaxies, including Andromeda, will become cosmic hermits. Then, tens of millions of years before the ultimate doomsday, the local group and then the Milky Way itself will break up like a dropped box of peanut brittle.
 

 In the cosmic twilight times, all planetary systems will be torn apart and the stars and planets will explode. About thirty minutes later, all atoms will burst like fireworks. Then, in a mere fraction of a second, space itself will tear to shreds in the all-destroying Big Rip.
 
 1
 


 If the devouring of reality by a ravenous shredder sounds revolting, you can console yourself with an
 <a href="http://arxiv.org/abs/astro-ph/0510771">
 alternative
 </a>
 developed by physicist Pedro Gonzalez-Diaz of the Institute of Mathematics and Fundamental Physics in Madrid. He conjectures that phantom energy could feed one of the many wormholes that could exist in the fabric of spacetime. Over billions of years, the wormhole would swell up faster than the cosmic expansion rate and eventually engulf the material content of whole universe—galaxies and such—sparing it from a Big Rip. Theoretically, then, everything could pass unscathed out the other end of the wormhole, emerging in another sector of reality, wherever that would be. Not exactly a joy ride, but at least it wouldn’t leave you in tatters!
 

 The jury is still out on what causes cosmic acceleration. It could well be the case that the culprit is not phantom energy, but rather a gentler form of dark energy—such as a type of quintessence with a
 
 w
 
 value greater than or equal to negative one. If so, the expansion of space would increase more gradually, leaving the local group and the Milky Way gravitationally bound together as other galaxies eventually recede from view. While our enclave of space would be lonely, it would remain intact. In short, we’d experience a “Big Stretch” rather than a “Big Rip.”
 


 1
 

 A
 <a href="http://www.springerlink.com/content/6842285247657415/">
 recent calculation
 </a>
 by a team of researchers led by Chinese physicist XiaoDong Li uses current data to estimate that the Big Rip will take place 16.7 billion years from now. The dissolution of atoms would take place a mere 30 quintillionth of a second before the Big Rip.
 



 Go Deeper
 
 

 Editor’s picks for further reading
 


 NBCNews.com:
 <a href="http://www.msnbc.msn.com/id/3077730/ns/technology_and_science-space/t/will-universe-end-big-rip/#.UCkQKkQRYck">
 Will universe end in a ‘big rip’?
 </a>
 
 Space writer Robert Roy Britt talks with cosmologists about the possibility of a Big Rip.
 

 Open Yale Courses:
 <a href="http://www.youtube.com/watch?v=40OQdPbeM0c">
 Dark Energy, the Accelerating Universe, and the Big Rip
 
</a>
 In this video, Yale astronomer Charles Bailyn reviews the evidence for dark matter and dark energy and presents the possibility of a Big Rip.
 
</section>
</div>

The Phantom Energy Menace:  Is the Cosmos Doomed to a Big Rip?

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>

 Think of the Big Bang, and you probably imagine a moment in time when matter, energy and space itself all burst into existence at once. Yet many astrophysicists now believe that the “Big Bang” was actually two distinct events: first the inaugural instant of space and time, and second the generation of most of the “stuff” that populates that space. So, which really deserves to be called the Big Bang?
 

 <figure class="richtext-image full-width"><img alt="firework_620" class="richtext-image full-width" height="348" width="620"
 src="/wgbh/nova/media/images/firework_620.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 


 Ambiguity has plagued the expression “Big Bang” since its origin. When British astronomer Fred Hoyle coined it during a radio interview in 1948, he meant it as the ultimate put down. Hoyle refused to believe that the universe had a beginning, a first moment of time and a genesis of all matter and energy. Rather, he thought that the cosmos maintained itself in a “steady state” through a slow trickle of particles into reality. He hypothesized a “creation field” that would gradually generate new matter to fill the gaps between galaxies moving away from each other, keeping the overall density of the universe the same.
 

 Hoyle had a point: Science rightly eschews processes without clear mechanisms, and at the time, no one could explain how the whole cosmos could emerge instantly out of nothing. Nevertheless, his alternative steady state approach could not match up with mounting evidence that the observable universe was once extremely hot and dense. In 1964, Bell Laboratory researchers Arno Penzias and Robert Wilson found the smoking gun: cooled-down radiation from the early universe, known as the
 <a href="http://wmap.gsfc.nasa.gov/universe/bb_tests_cmb.html">
 cosmic microwave background
 </a>
 (CMB), which convinced almost everyone in the scientific community that the universe started with a bang.
 

 The discovery of the CMB was a victory for the Big Bang theory, yet it also presented a puzzle: The radiation was strikingly uniform across the sky. While
 <a href="http://map.gsfc.nasa.gov/universe/bb_cosmo_fluct.html">
 later studies
 </a>
 identified tiny variations of less than one part in 10,000, the standard Big Bang model couldn’t justify such uniformity. There simply wasn’t enough time in early cosmic history, when the universe was small, for energy to have traveled across space and evened out its temperature.
 

 Enter the brilliant concept of inflation, proposed in 1981 by
 <a href="http://web.mit.edu/physics/people/faculty/guth_alan.html">
 Alan Guth
 </a>
 , and later modified by Andrei Linde, Paul Steinhardt, Andreas Albrecht, and others. Guth realized that a sudden, ultra-rapid stretching of the universe could take a tiny uniform patch and expand it to a size where it ultimately would grow and become the observable universe. During the fleeting instant of inflation, any irregularities in the primordial cosmos would be propelled beyond detection, offering a kind of blank slate. It is like taking a crinkled tablecloth and stretching it out so quickly that it appears flat on a tabletop and any wrinkles left are off the table and out of view. Only tiny, jiggling quantum fluctuations would disturb the uniformity; these fluctuations would be the seeds of the galaxies and galaxy clusters we see today.
 

 Inflation solved critical problems in cosmology, but it also split the Big Bang into distinct phases: In the inflationary portrait, the creation of almost all of the matter and energy in the universe takes place at the close of the inflationary period, through a process called “reheating,” rather than before inflation. Reheating involves a massive release of energy from inflation’s driving engine: an entity called the “
 <a href="http://www.stanford.edu/~alinde/golden.gif">
 inflaton
 </a>
 ,” thought to be a fluctuating energy field that ignited ultra-rapid cosmic expansion. Theorists think that at the end of inflation, the inflaton field released an enormous reservoir of potential energy into space—which, following Einstein’s famous equivalence between energy and mass, converted into a deluge of particles. Before then, because stretching causes cooling, the universe was actually relatively cold. As the cosmos rapidly expanded, its hot initial temperature dropped by a factor of many thousand (the precise amount depends on the particular model), becoming extraordinarily hot only after reheating. If you feel that an event should be fiery if it’s going to be called the “Big Bang,” then reheating, not the cosmic dawn, was the true “bang.” (Max Tegmark has made this case in
 <a href="http://www.preposterousuniverse.com/blog/2014/04/21/guest-post-max-tegmark-on-cosmic-inflation/">
 a recent blog post
 </a>
 .)
 

 So, science is faced with a dilemma. Which moment should the term “Big Bang” refer to: cosmic genesis, when space emerged, or the time of reheating, when most of the matter and energy was created? If the Big Bang denotes post-inflationary reheating, then we’ll have to talk about pre-Big Bang cosmology, which makes cosmologists squirm. Also, reheating occurred at slightly different times in various parts of the universe, making it more of a process than a sudden burst. On the other hand, if the Big Bang refers to the first moment of time, it was not much of a “bang.” That’s because the energy fields created then wouldn’t have been very hot.
 

 Furthermore, in some versions of inflation, the seeds of rapid growth developed from the loam of an
 <a href="http://www.stanford.edu/~alinde/1032226.pdf">
 eternal cosmos
 </a>
 . In that case, there could have been
 
 no
 
 initial moment of creation.
 

 Perhaps it is time to replace the expression “Big Bang” with more precise nomenclature. Could there be a better way of distinguishing the idea of an initial cosmic moment from the concept that a field caused sudden expansion that tapered off and led to the generation of matter and structure?
 

 Unfortunately, past attempts to find a replacement term for “Big Bang” have met with failure. Most famously, in 1993 Sky and Telescope magazine sponsored a contest to identify a new name. Astronomer and science writer Timothy Ferris, who called for a better designation, served as one of the judges. Ferris said in an
 <a href="http://articles.philly.com/1993-10-03/news/25936693_1_big-bang-matter-and-energy-georges-lemaitre">
 interview
 </a>
 that the term was “inappropriately bellicose” and unsuitable for conveying “the event thought to have spawned the starry skies.”
 

 Despite more than 13,000 submissions from participants in 41 countries, including colorful suggestions such as “The Grand Expansion,” “The Hubble Bubble,” and “The First Fireball,” the contest was declared a bust. The judges did not find any of the entries as simple and evocative as “Big Bang.” Consequently, they announced that nobody had won and that
 <a href="http://community.seattletimes.nwsource.com/archive/?date=19940201&amp;slug=1892815">
 the old name would remain
 </a>
 .
 

 More than 20 years later, “Big Bang” still doesn’t seem quite right, especially as cosmology has declared the nascent history of the cosmos more complex than once believed. Perhaps as our picture of the early universe continues to become more refined, new names will emerge to mark the complexity of its initial development. Just as we talk about an ovum, zygote, embryo and fetus in the process of human development, maybe we will someday have precise terms for early phases in the emergence of space, time, energy and matter.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


<a href="http://www.amazon.com/The-Inflationary-Universe-Alan-Guth/dp/0201328402">
 The Inflationary Universe
 
</a>
 Alan Guth’s accessible “diary” of the birth of inflation theory.
 

 Wikipedia:
 <a href="http://en.wikipedia.org/wiki/Chronology_of_the_universe">
 Chronology of the universe
 </a>
 
 10
 
 150
 
 years of cosmic history, from the inflationary epoch to the heat death of the universe, all on one Wikipedia page.
 
</section>
</div>

New insights into the early history of the universe have some physicists saying that it is time to rename the Big Bang.

The Big Bang’s Identity Crisis

<div class="rich-text"><section class="entry-content instapaper_body clearfix" itemprop="articleBody">
<div class="add-container clearfix" id="add-block-placer">
</div>

 If you were born on an isolated desert island in the middle of the ocean and had no communication with the outside world, your knowledge of geography would be limited. Peering through binoculars, gazing out in any direction, your view would be bounded by the sea’s horizon. Although you might speculate about what lies beyond the edge, you’d lack tangible evidence to support your hypothesis.
 

 Confined to our planet and its environs, we face the same situation: We can see a portion of the universe, but we can only speculate about its full extent. We might surmise through its flat geometry that it continues indefinitely in all directions, like a prairie stretching out as far as the eye can see. (Flat in this context refers to a straight three-dimensional space, like an endless box.) However, our understanding of the
 
 actual
 
 universe is bounded by the edge of the
 
 observable
 
 universe. We cannot know for sure what lies beyond the enclave our instruments can detect.
 

 Accordingly, we might wonder: How large is the part of the universe we’re potentially able to observe directly? At first glance, the answer might seem like a simple calculation. The speed of light is approximately 186,282 miles per second, or about 5.9 trillion miles per year. The time that has elapsed since the Big Bang is 13.75 billion years. Multiple the two figures and—voilà—we find that over the entire history of the universe, light could have travelled 13.75 billion light-years, or 81 billion trillion miles. But, in fact, that answer would be wrong.
 

 Let’s think about when the light was produced. From the time of the Big Bang to the era of recombination (when neutral hydrogen atoms formed) some 380,000 years later, the universe was opaque to light. Photons bounced between charged particles and didn’t travel very far. The reason is that charged particles interact with photons—either absorbing or emitting them. Only after the era of recombination could light journey through space. That is because photons can pass through neutral hydrogen gas without being diverted. Therefore, any estimate of the size of the observable universe must assume that the farthest light we see was released after that pivotal era when space became transparent. (We may someday be able to detect neutrinos and other particles from before that era, pushing the timeline earlier and enlarging the realm of what is observable, but for now we are still limited.) The difference between the two times doesn’t change the calculation much, but is important to note.
 

 Another adjustment is far more important. Since the primordial burst of creation, space has been stretching as the universe expands. A galaxy’s distance from us today is far greater than it was when it released the light. We can think, by analogy, of a relay race in which a girl tosses a ball to her teammate and then runs away from him. If the coach later asks the teammate what is the farthest throw he has caught he would give a very different answer than if he is asked where is the farthest player he has caught a ball from. Similarly, the distances traveled by the photons hurled by light sources do not reflect the much greater extent of the sources’ current positions. Thus, we could potentially observe light sources that are much farther out than 13.75 billion light-years, if their light was released when they were close enough to Earth.
 

 Yet another factor that expands the limit of the observable universe is its acceleration. Not only is the universe expanding; its growth has been speeding up. Data from the Hubble Space Telescope, the WMAP (Wilkinson Microwave Anisotropy Probe) satellite and other instruments have been used to pin down the rate of acceleration, along with the current expansion rate, the age of the universe, and
 <a href="http://lambda.gsfc.nasa.gov/product/map/current/best_params.cfm">
 other important cosmological parameters
 </a>
 .
 

 Taking advantage of this wealth of information, in 2005 a team of astrophysicists led by J. Richard Gott of Princeton performed a
 <a href="http://iopscience.iop.org/0004-637X/624/2/463/">
 detailed calculation
 </a>
 of the radius of the observable universe. Their answer was 45.7 billion light-years—more than three times bigger than our first, naïve estimate! Within this sphere lie hundreds of billions of galaxies, each with hundreds of billions of stars.
 
 1
 

<div class="wp-caption alignnone" id="attachment_977" style="max-width: 510px">

 <figure class="richtext-image full-width"><img alt="How large is the observable universe?-observable_universe_500.jpg" class="richtext-image full-width" height="500" width="500"
 src="/wgbh/nova/media/images/observable_universe_500.width-800.jpg"/><figcaption><div class="credit"></div></figcaption>
 </figure>
 

 Image credit: Andrew Colvin
 
</div>

 Gott’s team calculated this radius by figuring out how far away from us a source would be today if the light we now observe from it was emitted during the recombination era. In our relay race analogy, that’s determining where someone must have stood if she threw a ball and we caught it, and then using her running speed to figure out where she must be right now.
 

 Interestingly, as the universe expands, the size of the observable portion will grow—but only up to a point. Gott and his colleagues showed that eventually there will be a limit to the observable universe’s radius: 62 billion light-years. Because of the accelerating expansion of the universe, galaxies are fleeing from us (and each other) at an ever-hastening pace. Consequently, over time, more and more galaxies will move beyond the observable horizon. Turning once again to our relay race analogy, we imagine that if the players get faster and faster as the race goes on, there will be more and more who were so far away when they first threw the ball that the light would never have had time to reach us.
 

 Naturally not everything within the observable universe has been identified. It represents the spherical realm that contains all things that could potentially be known through their light signals. Or to draw from a
 <a href="http://www.defense.gov/transcripts/transcript.aspx?transcriptid=2636">
 famous comment
 </a>
 by former Secretary of Defense Donald Rumsfeld, the observable universe contains “known unknowns,” such as dark matter, that could eventually be analyzed. Beyond the observable universe lie “unknown unknowns”: the subject of speculation rather than direct observation.
 


 1
 

 The 45.7 billion light-year radius includes only light sources. If neutrinos and other particles that could penetrate the opaque conditions of the early universe are included the value becomes 46.6 billion light-years.
 



 Go Deeper
 
 

 Editor’s picks for further reading
 


 arXiv:
 <a href="http://arxiv.org/abs/astro-ph/0107568">
 The Long-Term Future of Extragalactic Astronomy
 </a>
 
 In this article, astrophysicist Avi Loeb investigates how our view of the universe will change in the distant future.
 

<a href="http://www.amazon.com/Edge-Universe-Voyage-Cosmic-Horizon/dp/0470636246">
 Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond
 </a>
 
 In his latest book, Paul Halpern investigates what may lie beyond the boundaries of the observable universe.
 

 James Webb Space Telescope:
 <a href="http://www.jwst.nasa.gov/firstlight.html">
 The End of the Dark Ages: First Light and Reionization
 </a>
 
 Learn more about the era of recombination and observations of the very early universe in this NASA resource.
 
</section>
</div>

Articles

Why Is Gravity Such a Weakling?

When the End is Just the Beginning: Exploring Cosmic Cycles

The Myth of the "Next Einstein"

The Unbearable Lightness of Gravity

The Phantom Energy Menace: Is the Cosmos Doomed to a Big Rip?

The Big Bang’s Identity Crisis

How large is the observable universe?