News from the Frontiers of Cosmology: A companion to the book The Edge of Physics

It's the magnets, stupid

Soon the Large Hadron Collider (LHC) will attempt to reach collision energies of 7 teraelectronvolts (TeV). So, despite the early setbacks of 2008, the LHC is marching on.

It’s worth thinking about how far physics would have come had the Superconducting Super Collider (SSC) been completed. It’s even more important to think about why the SSC never got built. Ironically, it was because the SSC’s designers were not ambitious enough, at least in certain aspects of accelerator design.

The Superconducting Super Collider (SSC) was designed to achieve energies of about 40 TeV, and the tunnel to house it was to be 87 kilometers in circumference. The site chosen was near Waxahachie, in northeast Texas, about 30 miles south of Dallas. The ambitious plan was approved in 1987 by President Ronald Reagan, who used a popular sports metaphor from American football to rally the physicists (a tribe that really needs no such encouragement). “Throw deep,” he said.

So, where was the lack of ambition? Surely, 40 TeV energies and an 87-kilometre-long tunnel were ambitious enough.

To make sense of why the SSC lacked ambition, we need to look at a key aspect of particle accelerators: the magnets that create the magnetic fields to keep the particles (such as electrons and protons) confined to the beam pipe. The particles that are being accelerated want to go straight, and their trajectories have to be bent precisely by the magnetic fields, so that the particles can go round and round the tunnel. Steering a beam is not unlike driving a Formula One racing car. The faster the car, the harder it is for the driver to tackle tight bends. F1 drivers train hard to build up their arm and (especially) neck muscles to handle the turns. Magnets are the muscles of the collider world. The tighter the curve of the tunnel, the stronger the magnetic muscles need to be.

The SSC played safe when it came to the magnets. Although they chose superconducting magnets, the technology was already well-tested and not innovative. Had they designed magnets with fields that were twice as strong, they could have halved the circumference of the tunnel.

The other mistake they made was in designing a separate set of magnets for each beam, one going clockwise and the other counter-clockwise. This meant that the tunnel’s bore had to be correspondingly large, at a staggering 4.25 metres. Ultimately, it wasn’t any fancy technology that proved the SSC’s undoing. It was the cost of the civil engineering. By 1993, the cost estimates for building the SSC had ballooned from $4.4 billion to more than $12 billion. The U.S. Congress canned the project, leaving behind a 22.5 kilometer stretch of completed tunnel that now lies derelict.

So, what did the folks at CERN do when it came to the LHC? They decided to reuse the 27-kilometre-long tunnel that housed the Large Electron Positron (LEP) machine. The LEP, at its peak, achieved energies of 200 GeV. The LHC was being designed for 14 TeV collisions. How could such energetic particles be confined to such a tight orbit around the old tunnel? It all comes down to magnets. Of the more than 9,000 superconducting magnets inside the LHC tunnel, 1,232 need special mention. These are dipole magnets, and they are the machine’s neck muscles. Each weighs 35 tons, and the entire lot has to be cooled down to 1.9ºK, the temperature of superfluid liquid helium (the SSC, in contrast, used simple liquid helium at 4.5 K). It’s the immense magnetic fields created by these giant magnets at the LHC that keeps the protons confined to the beam pipe.

There was another innovation. The LEP tunnel was only 3.8-metres wide. The LHC could not afford to use two sets of cryogenically-cooled superconducting magnets — they wouldn’t fit inside the tunnel. So, the magnets for LHC were designed such that the same cryostat could house two magnets, one for the clockwise beam and the other for the counter-clockwise beam. It was a tight fit, but it worked.

Over the last two years, the FERMI and PAMELA satellites and the ATIC balloon-borne experiment have all tantalised us with hints of dark matter in our galactic neighbourhood. But how do we know that what they are seeing is not being produced by astrophysical sources such as pulsars? Well, a new paper suggests that advanced X-ray telescopes of the future could solve the mystery.

In August 2008, there was much hullabaloo about PAMELA and ATIC having seen an excess of positrons over the expected background of such particles in space. This excess could be coming from the mutual annihilation of dark matter particles. Even NASA’s FERMI satellite has seen such an excess. But, unfortunately, this does not constitute proof of the existence of dark matter particles in our galaxy. Such an excess can also be caused by nearby pulsars.

Now, Antoine Calvez of UCLA and colleagues are suggesting that we look at the dwarf spheroidal galaxies that hang around the Milky Way. These dwarfs should have abundant dark matter, but a paucity of pulsars. So, if dark matter is annihilating in such galaxies, then the high-energy electrons and positrons produced by the process should up-scatter – or bump up in energy – the photons of the cosmic microwave background into the X-ray energy band. So, if we see such X-rays, then it’ll constitute solid evidence that dark matter particles are creating the electrons and positrons and not pulsars.

The fly in the ointment is that today’s X-ray telescopes are nowhere near as sensitive as would be required for such observations. But, the researchers hope that the next generation of X-ray telescopes could do the trick.

women at cern. image courtesy cern

On Monday, 8 March, CERN, the particle physics laboratory near Geneva, Switzerland — the home of the Large Hadron Collider – will be handing over controls of the facility to women to mark International Women’s Day.

According to CERN’s Pauline Gagnon, all the control rooms for accelerators and experiments, including those of the Large Hadron Collider (LHC), the ATLAS and CMS detectors, will primarily be staffed by woman.

Gagnon came up with the idea to highlight the fact that women have claimed their share of the space in physics, contrary to conventional wisdom.

On the day, live video will be available at http://cern.ch/womensday.

Last year, Fabiola Gianotti took over as spokesperson for the ATLAS experiment, which is looking for the Higgs boson among other things. Gianotti is one of the physicists featured in The Edge of Physics.  Some excerpts from Chapter 9: The Heart of the Matter:

• Physics wasn’t Gianotti’s first love. “I came to physics from very far away,” she told me.  “When I was a young girl, I loved art and music. I had been studying piano quite seriously at a conservatory and had taken courses in high school targeted towards literature, languages like ancient Greek and Latin, philosophy, and history of art. I loved these subjects, but I was also a very curious little girl. I was fascinated by the big questions. Why are things the way they are? This possibility of answering fundamental questions has always attracted me—my mind, my spirit, everything.”

• She stumbled upon physics soon afterward. “I discovered that physics is really interested in the most fundamental questions,” she said.

• More than philosophy?

• “Even more,” she said, speaking slowly to emphasize each syllable. “Because experimental physics is based on facts. It is answering fundamental questions—not just giving an answer to your question by inventing something, but proving it. This is very, very nice.”

• This was no theorist talking. Here was someone who got down-and-dirty with instruments. These concepts—supersymmetry, dark matter, the Higgs, extra dimensions—were not mere equations to her but ideas that left traces in her instruments, whether in the form of streaking jets of particles or in some anomalous measurement of momentum or energy.

• The LHC and ATLAS could uncover some deep truths about the universe. Gianotti confessed to “feelings of excitement and the awareness of being close to something very important and great for humankind.” She quoted the thirteenth-century Italian poet Dante Alighieri: Fatto non foste a viver come bruti ma per seguir virtute et conoscenza (“We were created not to live as animals but to pursue virtue and knowledge.”) “As human beings, the pursuit of fundamental research and knowledge is a need for us, which separates us from animals or vegetables. It is like the need for art,” said Gianotti.

IT WAS TWENTY YEARS AGO TODAY…

Apologies to the Beatles, but it was twenty years ago in January 1990, that Herb Gush, a physicist at the University of British Columbia, performed a landmark measurement of the cosmic microwave background using a rocket-based experiment. Had fate sided with him and had he launched the rocket a few months earlier, Gush would have won the Nobel Prize for accurately measuring the spectrum of the CMB. Instead, he became the first to independently confirm the measurements made by NASA’s Cosmic Background Explorer (COBE) satellite, for which John Mather and George Smoot won the Nobel in 2006.

The same month that Gush launched his rocket from White Sands, New Mexico, John Mather received a standing ovation at the meeting of the American Astronomical Society in Crystal City, Virginia. His experiment on COBE had shown that the radiation leftover from the big bang had exactly the spectrum expected of black-body radiation. It was a stunning confirmation of the big bang theory.

Gush almost beat Mather to the first indisputable measurement of the CMB spectrum (after the initial discovery by Penzias and Wilson in 1965). Gush had been using rockets to launch spectrometers hundreds of kilometers into space since the 1970s. But his earlier attempts with prototype spectrometers were unsuccessful as the payload failed to stay clear of the rocket’s exhaust, messing up the measurements.

Then , in the late 1980s, Gush and graduate students Ed Wishnow and Mark Halpern were ready with a sophisticated instrument that compared the CMB spectrum with the spectrum of an on-board blackbody radiator. But in the fall of 1989, the device was damaged by the malfunctioning of a vibrator in a vibrator test before launch.

The time it took for repairs meant that the rocket launch was delayed until late January 1990. When it was finally sent up, the experiment was a success. “It was immediately clear that the spectrum was near Planckian with a temperature near 2.7K,” said Gush in an email to me in 2007.

But as luck would have it, COBE had already made the measurement. If the roles had been reversed, COBE would have confirmed Gush’s data and not the other way around. Of course, this doesn’t take anything away from COBE, which was an exquisite experiment.

Just goes to show how small the margin can be between being the first to a discovery and the second.

Gush, for his part, retired and took up farming near Palermo, Italy.

When I met cosmologist James Peebles of Princeton in 2007 for my book The Edge of Physics, Peebles was still a bit miffed that Gush didn’t share the Nobel with Mather and Smoot. “There should be a list of great measurements that were underappreciated,” he told me. “Gush was working on that experiment for more than 15 years. COBE was under development for the same length of time, and they got first data within 2 months of each other. Mather in his book is very explicit – Gush could have scooped us, and would have been famous. Instead young people don’t even know his [Gush’s] name.”

Well, here’s to Gush and his brilliant experiment.

Ice Fishing for neutrinos on a frozen Lake Baikal

WE HAVE ALL HEARD OF HOW NASA spend millions (or is it billions) on developing a pen that works in zero gravity, while the Russians used a pencil. A classic case of Russian ingenuity, it seemed, until it was exposed as an urban legend.

Russian ingenuity, however, is not a myth. I got to experience it first-hand while writing The Edge of Physics. One of the many trips I made for the book was to see the Lake Baikal Neutrino Telescope near Irkutsk, in Southern Siberia. The telescope is essentially long “strings” of photomultiplier tubes (PMTs) that are submerged more than a kilometer beneath the surface of the lake. PMTs can be thought of as the opposite of television tubes. A TV tube generates photons from electrical signals, while a PMT generates electrical signals from photons that hit its surface. The PMTs deep in the waters of Lake Baikal are looking for the blue Cherenkov light that is emitted when a neutrino hits a molecule of water.

So, where does Russian ingenuity come in? Well, for starters, they have figured out a way of deploying these detectors without the use of expensive ships and submersibles (as they do for the neutrino telescopes being built in the Mediterranean Sea). The Russians wait for Lake Baikal to freeze over, and then during the peak of the Siberian Winter, they establish an ice camp on top of the frozen lake. They bring their cranes and winches and the like, haul out their telescope from the depths of Lake Baikal, do the necessary maintenance and repairs, and get out of there before the ice melts.

Using this unusual and extremely hazardous mode of operation, they have managed to build the world’s first underwater neutrino telescope and run it for twenty years with only about $20 million. The other neutrino detectors, either underwater or embedded in the ice (such as at the South Pole), are costing hundreds of millions of dollars.

But the most telling illustration of Russian ingenuity – of course, necessitated by lack of resources sometimes, but worth appreciating regardless – was to do with the retrieval of a string that got cut one winter and sank to the bottom of the lake.  Here’s a description of it from The Edge of Physics:

• The Lake Baikal neutrino telescope is made of eleven strings of photomultiplier tubes—each with a large buoy at the top and a counterweight at the bottom—that float nearly 1.1 kilometers below the surface (the water here is a staggering 1.4 kilometers deep, enough for a building three times as tall as New York’s Empire State to sink without a trace). Smaller buoys attached to the strings float about 10 meters below the surface. All year round, a total of 228 PMTs watch for the Cherenkov light created by neutrinos, monitoring 40 megatons of water. Each winter, once the ice camp has been set up, the team has to locate the telescope, the upper part of which drifts slightly over the course of the year. A diver plunges into the ice-cold water to locate the small buoy fixed to the center of the telescope. Then the researchers cut holes in the ice above each string (whose positions they know relative to the center) and attach a winch to the small buoys to haul up the strings. The team has two months to carry out any routine maintenance, put the strings back in the water, and get out before the ice cracks. They have perfected their technique; only once in two decades of operation did they have a problem retrieving a string. In 1994, a rusty metal cable broke, severing the buoy from its string, causing the string to sink to the bottom.

• Physicist Nikolai Budnev retrieved it. Diving that deep was out of the question, but Budnev knew that the string—though its counterweight was on the lake bed—would still be vertical because of the buoyancy of the PMTs. What he did next was ingenious. He fashioned a propeller and tied it to the end of a long rope, dropping the propeller into the water. The angle of the blades was such that as the propeller sank it started rotating, making huge circles. Budnev used this simple tool to sweep the waters below. Soon, the propeller snagged the errant string, and the team pulled it up.

I can confidently say that the Russians (and the Germans who worked alongside them) at Lake Baikal are amongst the toughest bunch of physicists I have encountered.

Here are some pictures of my trip to Lake Baikal.

How giant telescopes in Chile are protected against earthquakes

The latest earthquake to hit Chile was located near Concepcion, just south of Santiago.

Sadly, early reports of fatalities are emerging. In addition to the unfortunate loss of lives and livelihoods, Chile has to worry about something that is of interest to the astronomical community: telescopes.

Given that Chile is one of the most seismically active in the world, it’s a wonder that the country hosts some of the most powerful telescopes on the planet. These include the European Southern Observatory’s Very Large Telescope (VLT) in Paranal, Chile. It’s high in the Chilean Andes, about 1600 kilometres from Santiago. Even the planned 42-metre European Extremely Large Telescope is most likely going to be built in Chile.

Eyewitness reports on the BBC say that the Concepcion quake was felt as far away as Pergamino, Argentina, 200 kilometres north of Buenos Aires and about 1800 kilometres from quake’s epicentre.

So, most likely, ESO’s facilities on Mount Paranal would have felt the quake.

The four 8.2-metre telescopes that make up the VLT are well equipped to deal with earthquakes. Here’s a paragraph from The Edge of Physics that describes how the primary mirrors of the VLT are protected when the Earth shakes:

• The primary mirror is 18 centimeters thick. Because of its weight, the mirror’s precise shape can warp when it is tilted, so 150 actuators, upon which the mirror rests, continually push and pull at least once a minute to ensure that the optimal curvature is maintained. More impressive than the actuators are the clamps around the edges of the mirror, which can, at a moment’s notice, lift the entire mirror, all 23 tons of it, off the actuators and secure it to the telescope’s support structure in case of an earthquake (moderate quakes, of less than 7.75 Richter, are not uncommon here, thanks to the ongoing collision of the Nazca and South American plates). The entire telescope is designed to swing during an earthquake, and securing the primary mirror prevents it from rattling against the metal tubes that surround it.

To see the 8.2-metre mirror and the actuators, click here for pictures (the mirror is the seventh in the slideshow).

The last blog post, Why Astronomy Matters, listed key astronomical observations that have fundamentally changed our understanding of the universe, and more importantly, our understanding of our place in the universe. We went from a scenario in which the Earth was at the centre to a sun-centred solar system, to an expanding universe that began in a big bang. The most recent set of observations in the late 1990s led to the discovery of dark energy, the energy that permeates the very fabric of spacetime and is causing the expansion of the universe to accelerate.

In a recent paper, Lawrence Krauss analyses the consequences of a universe dominated by dark energy whose density does not change with time (often referred to as the cosmological constant). One of the staggering implications of the cosmological constant is that in about 1-10 trillion years (which is comparable to the lifetimes of the longest-lived stars), all the astronomical evidence that led to the theory of the big bang will have vanished. Dark energy would have caused the expansion of spacetime to accelerate so much that all but the gravitationally-bound local cluster of galaxies (of which the Milky Way is a member) would have disappeared from sight. Given that we inferred the expansion of the universe by studying how distant galaxies are racing away from us, such a scenario would leave astronomers with little evidence of an expanding universe.

Krauss also points out that the cosmic microwave background (CMB) – the radiation leftover from the big bang, and another key piece of evidence for the big bang theory – would have been redshifted to such an extent as to make it practically unobservable. The wavelength of the CMB that would have the peak intensity would be larger than the universe’s light horizon.

So what would the universe look like to astronomers in such a universe? Krauss harkens back to a time when even Einstein thought that the universe was static, unchanging and eternal, a time before we knew of an expanding universe and a big bang. “Poetically their picture of the universe will not be significantly different than that which Einstein had when he developed general relativity: A static universe in which our galaxy was surrounded by eternal empty space, with which cosmology at the turn of the last century began, will have returned with a vengeance,” writes Krauss.

I don’t know about you, but that sends a chill up my spine.

During the writing of The Edge of Physics, I was struck by the role of astronomy in changing fundamental perceptions about our universe, ourselves and our place in the universe. In fact, each such epochal moment can be traced to a unique (set of) astronomical observation(s).

The Copernican Revolution, which helped fuel the Scientific Revolution, caused us to change our entire world view. Earth was no longer the centre of the solar system. Instead, Copernicus argued that the motion of planets was best explained if the planets revolved around the sun.

Fast forward to the early 1900s, when our entire universe was the Milky Way. Even Einstein thought so, which led to what he called his greatest blunder. When his own equations of general relativity showed that the universe had to be either contracting or expanding, Einstein introduced a “cosmological constant”—a fix that made the universe static. Then, Edwin Hubble and Milton Humason made a series of observations using the 100-inch telescope atop Mount Wilson which showed that the universe consisted of much more than the Milky Way. More importantly, almost all of these galaxies were moving away from us (astronomer Vesto Melvin Slipher should be credited with measuring the redshift of these galaxies before Hubble, but it was Hubble who showed that these galaxies lay beyond the confines of our galaxy).

With Hubble’s work (and the work of theoreticians), the idea of a Big Bang began to take hold. Our universe, it seemed, had begun in a fireball. But the theory had to share the stage with the steady state model of the universe. The Big Bang model was confirmed in the 1960s with the accidental yet monumental detection of the cosmic microwave background (CMB) – a radiation leftover from the hot early universe. While many, many more experiments had to be done to nail down the properties of the CMB, we think of the 1965 discovery by Penzias and Wilson as the beginning of the big bang-era.

Also in the 1960s came the discovery of dark matter – based on observations of how fast the stars and gas are moving in the Andromeda Galaxy. With Vera Rubin’s pioneering astronomy, our universe went from being made of matter to one in which nearly 90 per cent of the matter was made of unknown stuff called dark matter.

Nothing dramatic happened for a few decades. Then in the late 1990s came the discovery of dark energy. Again, it was a set of astronomical observations of supernovae that showed that the expansion of the universe was accelerating. The favoured explanation is this: the fabric of spacetime has inherent energy, which is causing the accelerated expansion. Significantly, our understanding of the composition of the universe changed beyond belief. We now think it is made of mostly mysterious stuff: 73 % dark energy, 23 % dark matter, and only 4 % normal matter.

Hidden in the discovery of dark energy is the potential for a revolution just as (if not more) significant than the Copernican Revolution. Physicists are struggling to explain why dark energy has the value it does. Nothing in known physics can help. It’s value seems to be just right for galaxies, stars, planets and hence life to form, and produce physicists who are asking the question: why is our universe the way it is? One idea – controversial, but by no means lacking support among big-name physicists – is that our universe is part of a multiverse, which is the name cosmologists give to an ensemble of universes (could even be an infinity of universes). In a multiverse, dark energy takes a random value in each universe, and we just happen to be living in which its value is conducive to the emergence of life.

How on earth do we ever detect other universes? Well, if we had a theory that predicted a multiverse, and we verified many, many elements of that theory in our universe, then we’d have to – however reluctantly – accept the existence of a multiverse. Here’s Steven Weinberg (quoted in The Edge of Physics): “The important thing is not whether you can observe every ingredient in a theory, the important thing is whether you can observe enough of the consequences of the theory to test it and confirm that the theory is right.”

If the existence of a multiverse is somehow verified by experiments, then the discovery of dark energy may become the astronomical observation that we will look back on as the one that begat a multiverse.

Photo: The DAMA/Libra Project

The controversial DAMA/Libra experiment announced this week that the experiment has further strengthened the case for the existence of dark matter in the galaxy. The problem is that no other direct detection experiment – and there are many of them – has been able to confirm the results.

For more than a decade now, the DAMA collaboration, which runs the experiment in an underground laboratory inside the Gran Sasso Mountain in Italy, has claimed that their experiment has seen evidence of dark matter. Most direct detection experiments look for signs of a dark matter particle hitting one of their sensitive detectors (and none have been found with statistical certainty yet). DAMA on the other hand is not looking for a single particle. Rather, it is looking for a change in the number of particles that hit its detectors on an annual basis.

The idea is that as Earth moves around the Sun, it should encounter a larger flux of dark matter particles on approximately 2 June relative to 2 December. The Sun is moving towards the star Vega in our galaxy. As the Earth revolves around the Sun, in June it encounters dark matter particles with a velocity that is the sum of its speed around the Sun and the speed of the solar system towards Vega. In December, the velocities are subtracted. Assuming that the galaxy is filled with a halo of dark matter particles, this difference in velocities is expected to lead to the change in the flux of dark matter particles encountered by detectors on Earth on 2 June relative to 2 December.

In April 2008, the DAMA/Libra collaboration reported that they had evidence of this phenomenon after 11 annual cycles of observation. Now, they have announced that 2 more additional years of observation have confirmed the same, one year of which was with an upgraded detector.

Much of the dark matter community has steadfastly ignored the DAMA/Libra data, mainly because it has proved impossible to confirm independently, and they think that the annual modulation signal is just some systematic noise (which the DAMA/Libra collaboration denies, saying it has accounted for all systematic noise).

However, there are growing attempts to start reconciling the two sets of results. If the DAMA/Libra experiment is indeed seeing dark matter, then it has to be squared against the negative results of other extremely sensitive experiments like CDMS-II and Xenon10. One candidate is so-called inelastic dark matter. See this paper for details.

A STORY this week in New Scientist (which I wrote) looks at the likelihood of the European Space Agency’s Planck satellite detecting gravitational waves generated during inflation. Stephen Hawking’s wager with Neil Turok adds a touch of spice to the tale. At a meeting in Cambridge last August on primordial gravitational waves, Hawking reiterated his 2002 bet with Turok, arguing “that primordial gravitational waves will be observed, with a ratio of tensors to scalars, of above 5%.”

Basically, Hawking is betting that inflation would have generated gravitational waves strong enough to be detected by Planck (not directly, but as imprints in the cosmic microwave background). Inflation is that episode in the history of the universe – just a fraction of a second after the big bang – when the universe expanded exponentially. Inflation is needed to explain why the universe is flat and homogeneous, and is thought to have occurred when a field called the inflaton suffused spacetime and changed very slowly (physicists talk of the inflaton rolling down a gently-sloping potential hill). This caused spacetime to literally blow up.  It was a very violent period, leading to the roiling of spacetime and the generation of gravitational waves.

Turok, on the other hand, argues that inflation never occurred. He and Paul Steinhardt explain the observed properties of the cosmos using their cyclic universe model, which posits that the universe cycles through a series of big bangs and big crunches. In their model, the gravitational waves generated in the early universe would not have been strong enough to leave a detectable imprint on the CMB.

But it’s possible that inflation could have occurred, and yet the gravitation waves would not have been strong enough for Planck to see. So, Hawking could still lose the bet, without Turok and Steinhardt being right about the cyclic universe model.

How could that happen? Well, the New Scientist story explains one physicist’s take on it. Qaisar Shafi of the University of Delaware worked out models of inflation in which the inflaton field rolls down a potential that’s similar to the potential of the Higgs field. Add to that the fact that at the end of inflation, the inflaton field has to interact with standard model fields to reheat the universe and give rise to the standard model particles. One consequence of this is that the inflation-generated gravitational waves would not be strong enough for Planck to detect.

The strength of gravitational waves could also be lower if the universe is supersymmetric, an idea that says that there is a super-partner particle for every known standard model particle (see New Scientist feature on supersymmetry). Say, the Large Hadron Collider detects supersymmetry at the teraelectronvolt (TeV) scale. What this would mean is that the gravitino (the super-partner of the graviton) is only about a thousand times heavier than a proton. And this has implications for inflation: a gravitino with such a mass means that the energy scale of the universe at the end of inflation was also small, and as such, any inflation-generated gravitational waves would not be within reach of Planck.

I suspect most physicists would rather find evidence for supersymmetry at the LHC and forego the detection of inflation-generated gravitational waves until a future, more sensitive experiment is ready.

Why? Well, supersymmetry is the most likely candidate for physics beyond the standard model (and we know there has to be physics beyond the standard model). Finding evidence of supersymmetry would help physicists chart a course towards a theory of quantum gravity.

Finding evidence of inflation-generated gravitational waves, while extremely exciting and genuinely gratifying, would probably not help physics in the same earth-shaking manner as the discovery of supersymmetry. It would help fix the energy scale of inflation and maybe even provide clues to whether models of inflation built using string theory are on the right track. But it would not have the same potential for steering physics in a new direction as the sighting of a supersymmetric particle at the LHC.

That’s a roundabout way of saying that it’s better for physics that Hawking loses his bet and the LHC finds supersymmetry.