CERN’s July 4 declaration of victory in the quest to find the Higgs particle (or something very much like it) is a many-splendored triumph. It confirms, as it completes, the Standard Model of fundamental physics. It hints at the splendid new prospect of supersymmetry while debunking rival speculations. Most fundamentally, it reaffirms our scientific faith that nature works according to precise yet humanly comprehensible laws—and, importantly, rewards our moral commitment to testing that faith rigorously.
Inside the tunnel of the Large Hadron Collider, particles speed through a 27-kilometer ring of superconducting magnets. Credit: David Parker/Photo Researchers, Inc.
A few months ago, when the evidence was suggestive but not yet conclusive, I discussed here
the nature of the Higgs particle, and what its discovery would mean for the enterprise of physics. Now I will supplement that discussion, focusing on what it took
to win the victory.
Physicists had to overcome three challenges to discover the Higgs particle: producing it, detecting it, and proving that they really had produced and detected it.
To put these challenges in context, let me introduce another perspective on what the Higgs particle is: The Higgs particle is The Quantum of Ubiquitous Resistance. I’m referring here to a universe-filling medium that offers resistance to the motion of many elementary particles, thus producing what we commonly think of as their mass.
The Standard Model of physics—our best-yet model of the matter and forces that make our universe—requires, for consistency of its equations, that many of its ingredients are particles with zero mass. These particles should travel at the speed of light in empty space, but in reality, some of them—like quarks, leptons, and W and Z bosons—travel more slowly. What is slowing them down?
Our Standard Model comes equipped with a Standard Reconciliation: Space is never empty! Space is filled with a material that resists the motion of those particles. Over the past decades, physicists have deduced many of the properties of the Ubiquitous Resistance by observing its effects on the forms of matter we can see. They even gave it a name: the Higgs field. But none of the known particles had the right properties to build up the Ubiquitous Resistance. So theorists drew up the specifications for a particle that would do the job. They called it the Higgs particle.
But wishing doesn’t make it so. Only experiments can grant (or deny) theorists’ wishes. With that in mind, let us consider the three challenges facing experimental observation of the Higgs particle.
Any physical material, hit hard enough, is bound to break. The smallest possible shard reveals the most basic unit of the material: its “quantum.” For the Ubiquitous Resistance, that quantum is the Higgs particle.
To break off a piece of the Ubiquitous Resistance, though, requires producing disturbances of unprecedented intensity, albeit confined to tiny volumes of space for tiny intervals of time. That is what the Large Hadron Collider (LHC) is all about. By accelerating beams of protons to extremely high energy, and bringing them into collision, the LHC creates “Little Bangs” systematically.
Once you’ve produced a Higgs particle, the next challenge is to detect it. This isn’t as easy as it sounds, as the Higgs rapidly decays into other particles. We can look for those secondary particles, but most of them are useless for detection because they are produced more abundantly by other processes. The Higgs’ tiny signal competes with a cacophony of noise. The most likely mode of Higgs decay, into a bottom quarks and its antiparticle, in particular, is diluted by garden-variety strong interaction processes which produce those particles in droves.
So detection requires cunning.
Some decay processes that we might be able to detect are sketched below. Each has its own advantages and limitations, and each adds information, so experimenters have pursued them all. (For more information on the characters you’ll encounter below—W bosons, Z bosons, and the rest of the particle zoo, this is a good starting point.)
#1: Photon pairs
After a Higgs particle is created, quantum fluctuations convert it into a particle-antiparticle pair, which recombines into two photons.
The observable signal, in this case, is the pair of photons emerging from the decay. From the energy and momentum of the two photons, one can reconstruct the mass of the Higgs particle. This is significant because there are many other ways to make photons in collisions at the LHC that don’t require the production and decay of Higgs particles. The Higgs signal would be swamped, if not for the redeeming feature that randomly produced photons will “add up” to indicate random masses for their hypothetical progenitors, and only by rare accident land on the Higgs particle mass, whatever it happens to be. The signature of the Higgs, then, is an excess of photon pairs in a very narrow mass range. The mass where there’s an excess is fingered as the Higgs particle mass. Since the energy and momentum of photons can be measured accurately, this method gives an excellent measurement of the Higgs particle mass.
The main limitation of this technique, besides the unavoidable background “noise,” is the fact that this decay process is quite rare compared to other possibilities.
#2: W boson+ (Higgs -> bottom-antibottom)
Here is one of those other possibilities: In this case, the Higgs particle is produced as a byproduct of the creation of a W boson. The W boson itself decays, but in ways that experimentalists are thoroughly familiar with, and can often identify with confidence. The presence of the W boson, itself a relatively rare occurrence, helps this class of event to stand out above the strong interaction background. Thus the most common Higgs decay, into bottom-antibottom pairs, becomes discernable when you demand an accompanying W.
There are two more possibilities:
#3: Higgs -> WW -> lepton + antilepton + neutrino + antineutrino
#4: H -> ZZ -> 2 leptons + 2 antileptons
In Processes 3 and 4, the observed particles are leptons (l), which is just another way of saying that they might be either electrons or muons, and their antiparticles; the ghostly neutrinos escape detection. The Higgs boson barely interacts with those light particles, but it can communicate with them indirectly, through fluctuations in the W and Z boson fields (a.k.a. “virtual particles”). Process 4 is special, in that it is the only case where the background is so small that individual events, as opposed to enhanced probabilities, can be ascribed with confidence to Higgs particles.
By measuring the rates of all of these processes, one can determine how powerfully the Higgs communicates with many different things: two gluons, two photons, two Z bosons, two W bosons, and bottom-antibottom pairs. Their different rates are logically independent, of course, but theory connects them.
This is the final challenge. Finding the Higgs boson depends on assuming that the Standard Model is reliable, so we can work around the “background noise”. Here years of hard bread-and-butter work at earlier accelerators—especially the Large Electron-Positron Collider (LEP), which previously occupied the same CERN tunnel in which the LHC resides today, and the Tevatron at Fermilab, as well as at the LHC itself—pays off big. Over the years, many thousands of quantitative predictions of the Standard Model have been tested and verified. Its record is impeccable; it has earned our trust.
The next step is to search for data that the Standard Model can’t explain, like excesses of the decay products discussed earlier, and compare them against our predictions for yields from a hypothetical Higgs boson. Insofar as these quantitative predictions match the observations, which they do, one can speak of proof.
Future observations may reveal new effects, or small quantitative discrepancies in the effects already observed. (I’ll be surprised if they don’t!) But the original, simplest sketch of what The Quantum of Ubiquitous Resistance could possibly be resembles reality enough to pass muster, at least as its first draft.
Finally, I’d like to reprise the conclusion of my earlier piece, in which I considered what might happen if the hints of the Higgs did not pan out:
And if not?
I’ll be heartbroken. Mother Nature will have shown that Her taste is very different from mine. I don’t doubt that it’s superior, but I’ll have to struggle to understand it.
This week, we’ve come one step closer to understanding the rules that govern the universe. Two days ago, my colleagues at Fermilab announced our final results in a search for the answer to a mystery nearly 50 years old. In an intellectual tour de force, the CDF and my own DZero experiments analyzed a decade of data, combining dozens of hints that together tell an interesting tale. This announcement was an aperitif for an even more dramatic statement made today.
The construction of the CMS detector at the LHC. CMS is one of the detectors involved in the hunt for the Higgs. Credit: Mark Thiessen/National Geographic Society/Corbis
As physicists gathered in Melbourne, Australia, for the International Conference on High Energy Physics, one of the most anticipated conferences of the year, the two large collaborations at CERN made an extraordinary announcement. In back-to-back seminars held at CERN and simulcast to the conference (and the world), the leaders of two different experiments, CMS and ATLAS, gave strong evidence that we found something that can’t be explained by well-understood physics—something which could (and it’s worth emphasizing the “could”) be the Higgs boson.
The Higgs boson is the missing piece in the current best model of the universe, the Standard Model. In the Standard Model, building blocks called quarks and leptons are held together by the four known forces: gravity, electromagnetism and the strong and weak nuclear forces. Using these basic ideas, physicists can explain most of the measurements we have made. But one thing we have not been able to explain is one of the most fundamental and vexing questions in physics: Why do those building blocks have mass?
In 1964, Peter Higgs took some ideas that were floating around at the time, added a few of his own, and proposed a solution to this conundrum, which included a new particle that we now call the Higgs boson. The search for the Higgs boson is an energetic activity, directly involving as many as six thousand physicists—myself included—and the most powerful particle collider on Earth, the Large Hadron Collider (LHC) at CERN.
One of the fantastic benefits of being a physicist doing research at CERN and Fermilab is that I have been privileged to see this discovery evolve with an insider’s perspective in more than one world-class experiment and in collaboration with some of the finest minds on the planet. Over the past few years, we have searched through the data at both laboratories. Our measurements so far have shown where the Higgs boson isn’t. The results released today may finally show where it is.
The first tantalizing suggestions of the Higgs came in December of 2011, when scientists working with CMS and ATLAS announced that their data contained hints that the Higgs boson might be starting to show its face, and that it could have a mass about 125 times heavier than a proton. However, neither experiment had enough data to claim a discovery—or even to be certain that they were seeing anything at all.
In March, the search picked up again. This time, though, the LHC’s energy level and beam intensity were dialed up. If the LHC had been making Higgs bosons before, it would be making even more of them now—about 25% more, depending on the boson’s mass. The CERN management made their plans for 2012 so that both CMS and ATLAS would have enough “beam time” to independently discover the Higgs boson—if, that is, our hypotheses about its mass and other properties were correct. However, given the intellect and work ethic of the scientists involved, nobody really thought it would take the whole year to see a signal that “looked like” a Higgs boson, although proving anything we found was the actual Higgs boson predicted by the Standard Model could well take the entire years’ worth of data.
By June of this year, both LHC experiments had already recorded as much data in 2012 as in all of 2011. The accelerator and its detectors were performing superbly. Now the race was on to be the first to finish the job and find—or rule out—the Higgs boson.
ATLAS and CMS won’t find the Higgs itself, though; it disappears too quickly, decaying into other subatomic particles. It’s those particles that we’re looking for in the ATLAS and CMS data. Depending on the true mass of the Higgs boson, it could decay in several different ways. Seeing an excess of these decay products is an indication that we might have discovered the Higgs.
And that’s what we found! In the shrapnel of the LHC’s powerful collisions, the CMS experiment detected more pairs of photons and Z bosons than we can explain without some new kind of physics appearing. CMS also looked for supporting evidence in predicted decays to bottom quarks, W bosons and tau leptons. The ATLAS experiment also found an excess of events decaying into two photons and two Z bosons, but the ATLAS did not announce the results of their investigations into other decay modes.
To be certain that we didn’t adjust our analysis techniques to produce a preconceived result, we did the searches “blind,” meaning that we designed the analysis before we looked at the relevant data. This was especially important given that we saw hints in December 2011. We didn’t want that information to bias our searches in any way. That way, if the 2012 data told the same story as that of 2011, it would tell us something about the universe and not ourselves.
When all of our results are combined, CMS claims to have found a new boson with a mass of 125 GeV (or about 133 times heavier than a proton) and a statistical significance of about five sigma (which means that this result could happen 1 time in 3.5 million by accident), while ATLAS’ measurement indicates the existence of a particle with about the same mass (126 GeV) and the same statistical significance. While both experiments’ results are significant individually, the fact that both experiments are announcing similar observations and the 2011 and 2012 measurements are compatible lends tremendous credence to today’s announcement.
It is very important to stress that neither experiment team has claimed to have observed the Higgs boson. They have observed something without a doubt, but the Standard Model Higgs boson is a very specific thing. To be sure we’re seeing the Higgs boson and not a lookalike, we need to see it in all of the predicted decay modes.
For instance, the Higgs theory makes specific predictions about the relative probabilities of the Higgs decaying into pairs of bottom quarks, tau leptons and a whole myriad of possibilities. If all of the predicted possibilities aren’t seen, or aren’t seen in the right ratio, it might be that what we’re observing isn’t the Higgs boson after all. Furthermore, the Higgs boson is predicted to have exactly zero quantum mechanical spin. Until those and other properties are confirmed, it is possible that the experiments might be picking up traces of something entirely different. So, although what has been observed is consistent with being a Higgs boson, these measurements cannot rule out some other possibilities. In fact, this announcement is not the end of the story but rather the very beginning.
Watch this space the week of July 2, 2012 for a series of live webcasts from Fermilab and CERN on the latest results in the search for the Higgs boson.
Wednesday, July 4: CERN
Come back at 3 am ET on July 4, 2012 for a live webcast from CERN revealing the latest results in the search for the Higgs boson. A scientific seminar will begin at 3 am ET followed by a press conference at 5 am ET. Stay tuned!
Latest update in the search for the Higgs boson ©CERN
Press Conference: Update on the search for the Higgs boson at CERN on 4 July 2012 ©CERN
Monday, July 2: Fermilab
Tune in at 10 am ET on July 2, 2012 for a live webcast from Fermilab revealing the latest results from the Tevatron's CDF and DZero experiments in the search for the Higgs boson.
Higgs week is here!
This week, the search for the Higgs boson—the elusive subatomic particle that is a critical piece of the Standard Model of physics—may reach its climax when, on Wednesday, two research teams announce the results of their work at the Large Hadron Collider (LHC) at CERN.
But before there was the LHC, there was the Tevatron, a particle accelerator at Fermilab. And before the LHC’s big announcement, there was a not-quite-so-big announcement from the Tevatron teams as they gathered with colleagues this morning to announce the results of the most detailed analysis so far of ten years'-worth of their Higgs search data.
The Tevatron at Fermilab. Image courtesy of Fermilab.
The Tevatron shut down last year, passing the baton to the newer, more powerful LHC. But the scientists working on two of the Tevatron’s detectors, CDF and DZero, haven’t given up searching for traces of the Higgs in their own data. Using ever-smarter computer algorithms, they aim to wring as much information as they can out of the data they’ve accumulated. As Wade Fisher, the Michigan State University scientist representing DZero at this morning’s conference, put it: “We’re still working, we’re not stopping….There’s still gas in the tank.”
What they’ve found so far is suggestive of the Higgs, but doesn’t rise to the level of discovery. Combining data from both CDF and DZero, they’ve eked out a signal that might be due to the Higgs, but there is also a one-in-550 chance that it is down to random fluctuations.
To claim a discovery, the physicists need to whittle that random-chance number down to one in three and a half million—“five sigma,” in stat-speak.
That’s what the physics world will be holding its breath for on Wednesday, when two LHC collaborations release their results.
Will they confirm the hints that the Tevatron has seen? Or will these inklings—and our hopes of completing the Standard Model of physics--evaporate into the mist of random fluctuations?
As Fermilab’s Eric James put it this morning: “We’re likely, after all this time, to find something out one way or the other.”
Editor's note: An earlier version of this article originally appeared here on December 15, 2011. We are featuring it again, updated for context, in anticipation of the July 4, 2012 announcement on the latest results from the ATLAS and CMS instruments.
What is all the buzz about the Higgs boson, aka the "God particle"?
The construction of the ATLAS detector at the LHC. ATLAS is one of the detectors involved in the hunt for the Higgs. Credit: Martial Trezzini/epa/Corbis
“Higgs” is Peter Higgs, a professor at Edinburgh, who made some interesting suggestions along the lines I’ll discuss below in 1964. The name “Higgs particle,” though standard, is not entirely fair, for several reasons: the basic idea has a significant pre-history; what’s original with Higgs has co-claimants; and the modern, mature version of the theory involves many ideas that were not anticipated in 1964. I’ll leave those issues for historians of science and the Swedish Academy to sort out.
God on the other hand deserves full credit, or blame.
Herewith a brief introduction, in question and answer format, for the buzz-curious.
What’s the basic idea?
Suppose that a species of fish evolved to the point that some of them became physicists, and began to ponder how things move. At first the fish-physicists would, by observation and measurement, derive very complicated laws. But eventually a fish-genius would imagine a different, ideal world ruled by much simpler laws of motion–the laws we humans call Newton’s laws. The great new idea would be that motion looks complicated, in the everyday fish-world, because there’s an all-pervasive medium–water!–that complicates how things move.
Modern physics proposes something very similar for our world. We can use much nicer equations if we’re ready to assume that the “space” of our everyday perception is actually a medium whose influence complicates how matter is observed to move.
Are there precedents for such an outrageous dodge?
Yes. In fact it’s a time-honored, successful strategy.
For example: In its basic equations, Newtonian mechanics postulates complete symmetry among the three dimensions of space. Yet in everyday experience there’s a big difference between motion in vertical, as opposed to horizontal, directions. The difference is ascribed to a medium: a pervasive gravitational field.
A much more modern example occurs in quantum chromodynamics (QCD), our fundamental theory of the strong force between quarks and gluons. There we discover that the universe is filled with a medium, the sigma (σ) field, that forms a sort of cosmic molasses for protons and neutrons. The σ field slows protons and neutrons down. Allowing a bit of poetic license, we can say that the σ field gives protons and neutrons mass. Many consequences of the σ field have been calculated and successfully observed, so that to modern physicists it is now every bit as real as Earth’s gravity field. But the σ field exists everywhere and everywhen; it is not tied to Earth.
What’s the new idea, then?
In the theory of the weak force, we need to do a similar trick for less familiar particles, the W and Z bosons. We could have beautiful equations for those particles if their masses were zero; but their masses are observed not to be zero. So we postulate the existence of a new all-pervasive field, the so-called Higgs condensate, which slows them down. This proposal, which here I’ve described only loosely and in words, comes embodied in specific equations and leads to many testable predictions. This proposal has been resoundingly successful.
What is the Higgs particle, conceptually?
Trouble is, no known form of matter has the right properties to make the Higgs condensate. In order to build that medium, we need to add to our inventory of world-ingredients. The simplest, “minimal” implementation introduces exactly one new elementary particle: the Higgs particle.
What is the Higgs particle, specifically?
There’s a quotation I love from Heinrich Hertz, about Maxwell’s equations, that’s relevant here.
To the question: "What is Maxwell’s theory?" I know of no shorter or more definite answer than the following: "Maxwell’s theory is Maxwell’s system of equations."
Similarly, Higgs particles are the entities that obey the equations of Higgs particle theory. Those equations prescribe everything about how Higgs particles move, interact with other particles, and decay—with just one, albeit glaring, exception: The equations do not determine the mass of the Higgs particle. The theory can accommodate a wide range of values for that mass.
What is a Higgs particle, operationally?
A Higgs particle is a highly unstable particle, visible only through its decay products. It has zero electric charge, and—unlike all other known elementary particles—no intrinsic rotation, or “spin.” These null properties reflect the fact that many Higgs particles, uniformly distributed through space, build up the Higgs condensate, which we sense as emptiness or pure vacuum. (Although individual Higgs particles are highly unstable, a uniform distribution of them is stabilized through their mutual interactions. Visible Higgs particles are disturbances above that uniform background.)
As mentioned before, theory does not predict what mass a Higgs particle should have. Masses anywhere from 10 Giga-electron Volts (GeV) to 800 GeV might be accommodated, though problems start to emerge near either extreme. (Physicists commonly use GeV as the unit of mass for elementary particles. One GeV is close to, but slightly more than, the mass of one proton.)
Because Higgs particles are unstable, to study them one must produce them. That requires concentrating lots of energy into a very small space to create enormous energy density. The required concentration of energy is achieved at particle colliders. At the LHC, two counter-rotating beams of high energy protons are made to pass through one another, or cross, at a few points. At each crossing some fraction of the protons, which are moving in opposite directions at very close to the speed of light, collide. The collisions produce fireballs that explode into tens or hundreds of stable or near-stable particles including electrons and positrons, pi mesons, photons, protons and antiprotons, and several other possibilities.
Known physical processes account for the vast majority of this debris. Production and decay of Higgs particles, if they exist, will produce some additional debris. To get evidence for the existence of Higgs particles, therefore, one must identify some distinctive patterns in the observed debris that could result from Higgs particle decays but which are difficult to produce with conventional processes.
Putting it another way: If you’re looking for needles in a haystack, you’d better have a really good grip on what hay can look like—and it helps to look for needles that are hard to mistake!
Several patterns play an important role in the analysis, but I’ll discuss just one—a crucial one—to give a flavor of what’s involved. One process of Higgs particle production and decay is depicted in this sketch:
The sequence of events in the sketch above unfolds reading upwards. Gluons inside the fast-moving protons convert, by quantum fluctuations, into a “virtual” top quark and its antiparticle. The virtual top quark and antiquark swiftly recombine into a Higgs particle. Then the Higgs particle decays by a similar mechanism: quantum fluctuations convert it into a particle-antiparticle pair, which recombine into two photons. At the end of the day, it is those two photons that are observed. (I’m particularly fond of this exotically beautiful quantum process, which I discovered theoretically in 1977.) The point is that more conventional processes, i.e. processes that don’t involve Higgs particles, but which produce two energetic photons are fairly rare. Thus the calculated contribution from Higgs particles, should they exist, can be discerned above the background.
What did we know about the Higgs before July 4, 2012?
Prior to the July 4 announcement, we already knew that a very large range of potential mass-values had been ruled out. Only a small window in the range between 115 and 127 GeV remains viable.
On the other hand, an excess of events, above expectations from known processes, had been observed in the two-photon channel mentioned above and (less clearly) in several others. The excesses are compatible with, and could be explained by, the existence of Higgs particles with mass close to 125 GeV.
The observed excess might also be compatible with a statistical fluctuation in the background processes—e.g., an improbable run of normal processes leading to photon pairs, comparable to rolling four consecutive sixes at dice.
What will it mean if we find the Higgs?
First of all, it will be a dazzling triumph for theoretical physics. Physicists will have used intricate equations and difficult calculations to predict not only the mere existence of the Higgs particle, but also (given its mass) its rate of production in the complex, extreme conditions of ultra high energy proton-proton collisions. Those equations will also have accurately rendered the relative rates at which the Higgs particle decays in different ways. Yet the most challenging task of all may be computing the much larger, competing background “noise” from known processes, in order to successfully contrast the Higgs’ “signal.” Virtually every aspect of our current understanding of fundamental physics comes into play, and gets a stringent workout, in crafting these predictions.
The animating spirit of research in fundamental physics, captured in the maxim “Today’s sensation is tomorrow’s calibration,” will not rest in that triumph, however. A Higgs particle at mass 125 GeV would portend a new level of fundamental understanding and discovery. Let me explain why.
Within our current theories of the fundamental interactions, embodied in the so-called Standard Model, the Higgs particle mass might, as previously mentioned, have any value within a wide range. Yet there are good reasons to suspect that despite its many virtues, the Standard Model is incomplete. Notably, its equations postulate four different forces (strong, weak, electromagnetic and gravitational) and six different materials they act on. It would be prettier to have a more coherent, unified theory. And in fact there are beautiful, concrete proposals for unified field theories, within which we have just one force and just one kind of material. But to make the unified theory work quantitatively, in detail, we need to expand the equations of the Standard Model so that they integrate a concept called supersymmetry.
Supersymmetry has many aspects and ramifications, but two are most relevant here. First, supersymmetry (for experts: more specifically, focus point supersymmetry) predicts that the Higgs particle mass should lie in the range 120-130 GeV. Finding Higgs particles with mass in that range would give strong circumstantial evidence both for supersymmetry and for the unification that supersymmetry enables.
Second, supersymmetry predicts the existence of many additional new fundamental particles, besides the Higgs particle, that should be accessible to the LHC. So if supersymmetry is right, the LHC will have many more years of brilliant discovery in front of it.
And if not?
I’ll be heartbroken. Mother Nature will have shown that Her taste is very different from mine. I don’t doubt that it’s superior, but I’ll have to struggle to understand it.
We know dark matter is out there—but what is it?
An invisible army of black holes? A cosmic graveyard of burned-out stars? A swarm of rogue planets that roam the depths of interstellar space? While examples of objects like these have been observed, we now know that they can’t account for the enormous mass of dark matter required to explain why galaxies rotate so fast. Following Sherlock Holmes’ dictum that once you have ruled out the impossible, whatever remains, however improbable, is the answer, scientists have been forced to conclude that dark matter is an entirely new form of matter, never before observed.
Here is what we think: Every galaxy is engulfed by a cloud of dark matter particles that extends far beyond that galaxy’s visible edge. Each dark matter particle is electrically neutral and has a mass tens or thousands of times that of the familiar proton. Finally, there is a lot of this dark matter. Our best estimate is that there is about five times as much dark matter as there is luminous matter, making our visible universe a thin frosting on a dark matter cake.
But physicists will need to observe dark matter first-hand before anyone should believe it is real. Our search for dark matter takes three distinct approaches: direct, indirect, and production—that is, actually making our own dark matter particles.
The search for dark matter rests on a three-legged stool, with direct, indirect and collider experiments all promising approaches to find it. Credit: Don Lincoln/Fermilab
The direct approach starts with a detector cooled to more than 459 degrees below zero Fahrenehit, so close to absolute zero that the atoms that make up the detector are nearly stationary. The detector is buried as much as a mile underground to protect it from ordinary cosmic rays, high-energy particles that are constantly bombarding the Earth. Though these detectors can’t actually “capture” a dark matter particle, should one happen to pass through and collide with the nucleus of an atom inside the detector, the detector will ring like a bell and the passage of the dark matter particle will be observed.
There are dozens of experiments underway using this approach, including one, called the DAMA (DArk MAtter) experiment, that has made a provocative finding. Scientists think that dark matter flows past the solar system like a wind, so DAMA uses the motion of the Earth around the Sun to winnow out a dark matter signal. For half a year, the Earth is moving into the dark matter wind, and for the other half, it is moving with the wind. Therefore, we expect to see an annual variation in the number of dark matter hits. This is exactly what DAMA has seen for many years now.
The problem is that other experiments which are nominally more sensitive don’t see this annual variation. This has led to considerable confusion and it will take additional work to understand if DAMA has seen the first hints of dark matter or merely an unexplained measurement artifact.
Indirect searches exploit the notion that dark matter might consist of both a matter and antimatter component. If so, occasionally a pair of matter and antimatter dark matter particles might meet and annihilate each other in a flash of gamma rays or matter/antimatter pairs that can be observed by satellites that are designed to detect gamma rays or antimatter in the cosmos. In fact, two such experiments, PAMELA and GLAST, have observed signals that could be the signature of dark matter, but could also have more prosaic explanations. Meanwhile, other experiments see no such signals.
Rather than waiting for dark matter to come to us, though, some physicists are hoping to make their own dark matter right here on Earth. Currently the only particle accelerator capable of making dark matter is the Large Hadron Collider at CERN. By exploiting Einstein’s famous equation E = mc2, we hope to convert the kinetic energy of the beams directly into dark matter. Because dark matter is electrically neutral, it would escape our detectors undetected, but upon adding up the energy contained in all the particles that we can detect, we would notice that the energy books are unbalanced and that some energy is missing.
The scientists working on two of the LHC’s detectors, the ATLAS (A Toroidal Large Apparatus) experiment and my own CMS (Compact Muon Solenoid), are now searching their data tirelessly, looking for collisions with these characteristics. The situation is evolving rapidly as the LHC delivers a torrent of particles to the detectors.
It’s a race between the three different approaches to see which one will be the first to observe a reliable signature of dark matter. No one should be the slightest bit convinced until at least two of the approaches begin to tell a consistent story. One thing is certain; with five times as much dark matter as ordinary matter, the race is on for discovery and Nobel Prize glory.
Editor's picks for further reading
CERN Courier: Shedding Light on Dark Matter
In this article, learn how scientists are using the CMS instrument at the LHC to look for signs of dark matter.
NOVA scienceNOW: Dark Matter Mystery
In this video, host Neil deGrasse Tyson reports from a half mile underground in an abandoned mine, where scientists are using special detectors to look for dark matter particles.
Scientific American: Instant Egghead: Dark Matter
Scientific American editor George Musser explains dark matter in 60 seconds using a coffee cup, some crumbs, and a compact disk.
Astronomers have a pretty sweet job. They are paid to stare at the heavens and wonder. Some of their observations are pretty ordinary, but some observations are revolutionary—like the measurements of galaxy rotation that convinced astronomers that our universe is studded with invisible mass called dark matter. In this pencast, I will explain how that apparently simple observation led astronomers to such an extraordinary conclusion.
When astronomers watch rotating galaxies and compare their observations with predictions based on Newton’s laws of gravity, they find something strange. Stars near the center of galaxies are well behaved and move as expected. However stars farther from the center are rebellious. They move far faster than the laws of physics predict they should; so fast, in fact, that these galaxies shouldn’t exist: They should be ripped apart. Since we know that galaxies have existed for billions of years, this is a glaring paradox.
This conundrum nagged at scientists for over half a century. Astronomers proposed many solutions, from suggestions that our understanding of inertia is wrong to new ideas of how gravity works. But the likeliest explanation is that galaxies contain more matter than we see.
When I say “see,” I don’t mean just “seeing” with our eyes or even with the familiar telescopes that are sensitive to visual light. I mean “seeing” with any and every kind of telescope in our arsenal, including the huge antennas that pick up radio emission from the vast clouds of hydrogen that typically make up most of the mass of galaxies.
To acknowledge the fact that this proposed extra matter is invisible to our ordinary methods of detection, we call it “dark matter.” We know it’s out there, but what is it? Come back next week for more about the quest to capture traces of dark matter here on Earth.
Editor's picks for further reading
American Museum of Natural History: Vera Rubin and Dark Matter
In this profile, learn how astronomer Vera Rubin's galaxy observations helped establish the presence of dark matter.
NOVA scienceNOW: The Dark Matter Mystery
In this video, explore the evidence for dark matter.
TED: Patricia Burchat sheds light on dark matter
In this talk, physicist Patricia Burchat explores dark matter and dark energy.
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 fresh take from the physicists Paul Steinhardt and Neil Turok in 2002. There is a problem with this elegant idea, though: the second law of thermodynamics, 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 January issue 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 larger multiverse 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 the cosmos might as well appear as if began an eternity ago, 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 occasionally collide, 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.
Editor's picks for further reading
arXiv: Eternal Inflation and Its Implications
Alan Guth, the physicist who originated the inflation hypothesis, summarizes the arguments for eternal inflation.
Edge: The Cyclic Universe
Neil Turok on the past and present of the cyclic universe model.
FQXi: Did the Universe Have a Beginning?
In this podcast, Alexander Vilenkin asks whether the universe could have existed forever into the past.
Are physicists victims of their own success? They have strived to find the fundamental laws of matter, and in recent years they’ve done it. The so-called standard model of physics provides us with a thorough census of the subatomic particles that combine to make everything we see, and its equations define a complete mathematical explanation of how they behave. The golden age, it seems, has come and gone.
Or has it?
A millennium from today, historians will look back at the twentieth century primarily as the age of a rich flowering of science. Within a few decades molecular biology unveiled the body and soul of the genetic code, cosmology reconstructed the history of the universe, and geophysics disclosed a home planet more dynamic than ever previously imagined. Yet the biggest revolution of all, from which all those others drew, was ironically the smallest: the conquest of microphysics.
Three icons of twentieth century science. Clockwise: The double helix structure of DNA, revealed by analysis of its x-ray diffraction pattern (credit: Richard Wheeler via Wikimedia); anisotropies in the microwave background radiation, providing a picture of the very early universe (credit: WMAP Science Team, NASA); motion of the continents, proved and then mapped by analysis of magnetic field reversals at mid-ocean ridges. In each case, the enabling sensitive instruments and technologies depend upon profound understanding of the properties of matter, based on microphysics (credit: EMAG2)
History does not come with time-stamps affixed, but two epochal experiments roughly bracket the High Golden Age of microphysics. In 1912, Ernest Rutherford decoded atoms experimentally, revealing that each has a tiny nucleus containing all its positive charge and almost all its mass. That nucleus is surrounded by electrons, which are bound to it by electric forces. Just a year later, Niels Bohr introduced strange new ideas about the laws of motion in the microworld. His breakthrough matured into quantum mechanics.
Quantum mechanics broke the code of the microworld, but it took decades to master its text. Finally, in the “November Revolution” of 1974, two separate experimental groups announced the discovery of a striking set of new particles, the charmonium system. Their discoveries provided a brilliant confirmation of, and a fertile proving ground for, the collection of theoretical ideas we now call the Standard Model. The charm quarks (and their antiquarks) that make the charmonium particles rounded out the theory of the weak interaction, and the forces that bind them were just right for the theory of the strong interaction. Those theories tamed nuclear physics, and together with electromagnetism and gravity they complete the description of matter.
The Standard Model provides, we believe (after very thorough, rigorous, quantitative testing!), a complete mathematical explanation of how subatomic particles combine to make atoms, atoms to make molecules, and molecules to make materials, and how all this stuff interacts with light and radiation. Its equations are comprehensive yet economical, symmetrical but spiced with interesting detail, austere yet strangely beautiful. The Standard Model provides a complete, secure foundation for astrophysics, materials science, chemistry, and physical biology. Good stuff!
The Standard Model marks the ultimate triumph of reductionism. As Isaac Newton put it, we analyze matter by finding complete and simple laws governing the behavior of its elementary components, and then use those laws to synthesize the properties of macroscopic objects.
Triumph on that scale has a dark side: It’s a tough act to follow. By the late 1980s, articles and books with titles like “The End of Physics” began to appear. At the same time, “Theory of Everything” hyperbole erupted.
Neither reaction, however unseemly, was entirely baseless. The achievements of this golden age did mark the end of a certain special—and especially wonderful—kind of physics. After plumbing the bottom of ordinary matter (that is, physical material that’s reasonably accessible and usefully stable), where do you go? As physicists deciphered the atom, they revolutionized chemistry and enabled microelectronics; as they deciphered the nucleus, they revolutionized not only astrophysics and physical cosmology, but also bomb technology and medicine. There is no realistic prospect that the sort of frontier physics explored at the Large Hadron Collider, as esoteric and expensive as it is marvelous, will yield practical fruit. (This is not to say that the indirect value of this work, which serves as “the moral equivalent of war” for many talented, enthusiastic, creative young seekers, will not repay the money invested in it. It will, handsomely.) Its application in the natural world is likely to be restricted to the extremely early universe, and (maybe) a few super-extreme astrophysical situations, like Hawking’s black hole explosions.
But lamenting the passing of a golden age, or professing to reanimate it, are exercises in nostalgia. A healthier attitude, and an attitude that is truer to the unselfconscious exploratory spirit of the golden age itself, is to engage with its legacy of unanswered challenges and new opportunities. What a legacy it is, and what opportunities there are!
For the Standard Model, despite its practical success in describing ordinary matter, leaves many loose ends and unanswered questions. One of its ingredients, the Higgs particle, has not yet been observed directly. That embarrassment may soon be remedied, but other flaws run deeper. Its equations remain lopsided in peculiar ways. They beg to be embedded in a larger, still more symmetric theory. There are, in fact, excellent ideas for advancing toward such unification. Those ideas suggest new lines of experimental investigation, notably the search for proton decay and for supersymmetric particles. The other interactions, and indeed quantum mechanics itself, have not yet been organically united with gravity. String theory might help with those problems, but it’s clear that crucial ideas still await discovery.
We’d also like to understand why the laws of microphysics appear so nearly unchanged if we run time backwards. The only known explanation predicts the existence of a remarkable new class of particles called axions. These wraithlike cousins of photons, more elusive even than neutrinos, plausibly provide the astronomical dark matter. And if axions don’t—what does?
These and other unanswered challenges amply refute the notion that physicists are, in any meaningful sense, close to having a “Theory of Everything” (or that we’ve reached “The End of Physics”).
Yet the biggest challenges, I think, are of a different kind. The art of using our comprehension of microphysics is an open-ended invitation to creativity. Music-making doesn’t end when you’ve learned how your instrument works—it begins.
Can we engineer quantum computers, and through them fashion truly alien forms of intelligence? Can we tune in to the messages the universe itself broadcasts in gravity waves, in neutrinos, and in axions? Can we understand the human mind, molecule by molecule, and systematically improve it? To ask these questions is to discover, in the ripeness of one golden age, the seeds of new ones.
Modern astronomy offers a curious mixture of humility and bravado. Earth is not special, we say, as Copernicus asserted and Galileo confirmed. Rather it is, in the scheme of things, a tiny speck in an unremarkable location. Yet, from our modest perch, we make sweeping statements about the entire observable universe—from here to billions of light years away. Although we are the smallest of the small, we speak with authority about the largest of the large.
How can we be so bold? The key is to make use of our typicality to assume that the universe is homogenous—pretty much the same throughout. But now, the discovery of a phenomenon called dark flow is challenging our assumption that the universe doesn’t allow one place to be any more “special” than any other. Astronomers call this assumption the Copernican Principle. Thus when Edwin Hubble discovered in 1929 that all galaxies, except our nearest neighbors, seem to be moving away from us, astronomers used the Copernican Principle to infer that space is expanding in a uniform way—at the same rate in every cosmic locale. Big Bang growth marches at the same pace everywhere.
In my own calculations related to cosmology, I’ve generally followed the accepted method of using the assumption of homogeneity to greatly simplify the equations. Otherwise the procedure would be much trickier—like applying a recipe to a dish that requires different ingredients for every morsel. If it does turn out that the universe has local differences, cooking up cosmological solutions will be a tall order indeed!
Astonishingly, a newly identified phenomenon called dark flow could slash through cosmic uniformity, casting the Copernican Principle into doubt. Dark flow represents the movement of hundreds of galaxy clusters at about two million miles per hour in the direction of a patch of sky between the constellations Centaurus and Vela. Like the cloaked duo of dark matter and dark energy, dark flow is another masked marauder challenging long-held cosmological assumptions. It is “dark” in the sense of having mysterious origins—origins that may lie beyond our cosmic horizon, or perhaps even in another universe.
Galaxy clusters like the one shown in the inset seem to be drifting toward the patch of sky indicated in purple on this image of the cosmic microwave background radiation. Credit: NASA/WMAP/A. Kashlinsky et al.
Discovered in 2008 by Alexander “Sasha” Kashlinsky of NASA’s Goddard Space Flight Center, dark flow is a streak of irregularity in a universe that is otherwise as uniform as a perfect, rising loaf of bread. Kashlinsky and his research team discovered dark flow by cleverly analyzing data collected by the Wilkinson Microwave Anisotropy Probe
(WMAP) satellite during the 2000s. WMAP’s main purpose was to map out the cosmic microwave background (CMB) radiation released some 380,000 years after the Big Bang, when electrons and protons first cooled down enough to form hydrogen atoms. This “baby picture of the universe” has offered cosmologists a unique look back in time and given them new insight into the universe’s birth, growth, and structure formation.
Kashlinsky’s group put the detailed map to different use, though. when they examined the motion of galaxy clusters through the background radiation. The galaxy clusters are filled with hot gas that scatters light from the background radiation, shifting the spectral lines that define the “fingerprint” of that light. Because the amount of shifting depends on the clusters’ speeds relative to the CMB, this acts as a kind of speedometer, telling us how fast they are moving.
Soon after Kashlinsky and his collaborators published their results, they were confronted by a sharp challenge to their claims. In an article posted on his website, “Dark Flow Detected – Not!”, UCLA astronomer Edward (Ned) Wright pointed out several errors and inconsistencies in their paper’s statistical analysis and argued that these placed their conclusions in doubt. Undaunted by Wright’s allegations, Kashlinsky posted a detailed rebuttal and gathered further evidence for dark flow.
In 2010, Kashlinsky and his team published a follow-up paper with results that were even more startling. Not only did they confirm dark flow, they found its parade of clusters to be far more extensive that they had previously thought. Remarkably, from a survey of more than 1000 clusters, they provided evidence that dark flow extends out as far as 2.5 billion light years away. With such a large scale, it slashes through a significant chunk of the observable universe.
In recent decades, cosmology has become an increasingly rigorous science. Claims in the field, particularly ones of such a revolutionary nature as dark flow, must be sifted by sophisticated statistical tests and verified by independent analyses. Interestingly, while no other teams have found dark flow to the extent mapped out by Kashlinsky’s group, some groups have found a less potent, but still notable, movement in roughly the same direction. For example, researchers Richard Watkins of Willamette University, Hume Feldman of the University of Kansas, and Michael Hudson of the University of Waterloo have noted a significant flow of galaxies, but at much lower rate than Kashlinsky’s team found. Kashlinsky’s next goal is to analyze data from the European Space Agency’s Planck satellite, hoping it will offer proof positive of dark flow and reveal its extent.
If dark flow were conclusively established what would it mean? The Copernican Principle, at least for the observable universe, would be cast into doubt. There would be something special about a particular segment of space. Space would have a gaping irregularity—a fissure through its firmament. How could astronomy explain such a rift?
Kashlinsky and others have speculated that the inflationary universe model could provide the answer. According to many versions of inflation, the observable universe grew from a fluctuation in a primordial energy field. Beyond our “bubble” could be countless other universes that grew from other fluctuations in the great cosmic bath called the multiverse. Perhaps dark flow represents the result of a gravitational tug from mass housed in another universe—or at least a region beyond the observable universe. This interaction would have happened very early in cosmic history, long before the universe grew to its present-day size. Nevertheless it could have left the relic of irregularity, much like geological processes long ago produced today’s mountain chains.
In coming years, we’ll see if dark flow positions itself in the pantheon of bona fide cosmic mysteries, such as dark matter and dark energy, or if further analysis will reveal dark flow to have been an illusion. If dark flow does stand up to scrutiny, though, we may have to reevaluate the assumptions we’ve made about our universe. Perhaps the universe isn’t as uniform as we thought; perhaps what we see from our perch here on Earth isn’t necessarily what you get in distant corners of the cosmos.
Editor's picks for further reading
Ars Technica: Supernova Research Challenges Cosmic "Dark Flow" Mystery
The case against dark flow.
Science Friday: Does The Universe Have a "Dark Flow?"
Host Ira Flatow talks with theoretical physicist Michael Turner about dark flow.
Scientific American: In Our Expanding Universe, Earth Is Nothing Special
John Rennie asks: What do "Fight Club" and the Copernican principal have in common?