Perhaps best known in the field of particle physics as the co-author of the Higgs Hunter’s Guide, Jack Gunion has been in the theoretical trenches of the search for the Higgs boson for several decades now. He is a senior professor and leader of the theoretical particle physics group at UC Davis, where he has been a member of the faculty for over 25 years. Here is a guest post from him on today’s big news from CERN.

Tuesday December 13 has been a very exciting day for particle physics. The ATLAS and CMS experiments at the Large Hadron Collider (LHC) announced today that they are both seeing hints of a Higgs boson with properties that are close to those expected for the Standard Model (SM) Higgs boson as originally proposed by Peter Higgs and others. While the “significance” of the signals has not yet reached “discovery level” (5 sigma in technical language) the two experiments both see signals that exceed 2 sigma so that there is less than a 5% chance that they are simply statistical fluctuations. Most persuasively, the signals in the channels with excellent mass determination (the photon-photon final decay state and the 4-lepton final state) are all consistent with a a Higgs boson mass of around 125 GeV IN BOTH EXPERIMENTS. This coincidence in mass between two totally independent experiments (as well as independent final states) is persuasive evidence that the photon-photon and 4-lepton excesses seen near 125 GeV are not mere statistical fluctuations.

Observation of the Higgs with approximately the SM-like rate suggests that to first approximation the Higgs is being produced as expected in the SM and that it also decays as predicted in the SM. Many theorists, including myself, have suggested that a Higgs might be produced as in the SM but might have extra decays that would have decreased the photon-photon and 4-lepton decay frequencies to an unobservable level, making the Higgs boson much harder to detect at the LHC. The level of the observed excesses argues against such extra decays being very important. The photon-photon and 4-lepton detection modes were originally proposed and shown to be viable for a SM-like Higgs boson by myself and collaborators (in particular, Gordy Kane and Jose Wudka) way back in 1986-1987. It has taken a long time (25 years) for the technology and funding to reach the point where these detection modes could be examined. I often joked that I was personally responsible for forcing each of the LHC collaborations to spend the 30 million dollars or so needed to build a photon detector with the energy resolution required. Fortunately, it seems that the money was well-spent and the ATLAS and CMS detectors both found ways to build the needed detectors, a real triumph of international collaboration and technical expertise. Also key is the very successful operation of the LHC that has produced the enormously large number of collision events needed to dig out the Higgs signal from uninteresting ‘background’ events. Until this summer produced the first very weak signs of the Higgs, I was beginning to wonder if the Higgs would be discovered during my lifetime. Fortunately, simplicity (i.e. a very conventional SM-like Higgs boson) seems to have prevailed and ended my wait.

(more…)

Perhaps you’ve heard of the Higgs boson. Perhaps you’ve heard the phrase “desperately seeking” in this context. We need it, but so far we can’t find it. This all might change soon — there are seminars scheduled at CERN by both of the big LHC collaborations, to update us on their progress in looking for the Higgs, and there are rumors they might even bring us good news. You know what they say about rumors: sometimes they’re true, and sometimes they’re false.

So we’re very happy to welcome a guest post by Matt Strassler, who is an expert particle theorist, to help explain what’s at stake and where the search for the Higgs might lead. Matt has made numerous important contributions, from phenomenology to string theory, and has recently launched the website Of Particular Significance, aimed at making modern particle physics accessible to a wide audience. Go there for a treasure trove of explanatory articles, growing at an impressive pace.

———————–

After this year’s very successful run of the Large Hadron Collider (LHC), the world’s most powerful particle accelerator, a sense of great excitement is beginning to pervade the high-energy particle physics community. The search for the Higgs particle… or particles… or whatever appears in its place… has entered a crucial stage.

We’re now deep into Phase 1 of this search, in which the LHC experiments ATLAS and CMS are looking for the simplest possible Higgs particle. This unadorned version of the Higgs particle is usually called the Standard Model Higgs, or “SM Higgs” for short. The end of Phase 1 looks to be at most a year away, and possibly much sooner. Within that time, either the SM Higgs will show up, or it will be ruled out once and for all, forcing an experimental search for more exotic types of Higgs particles. Either way, it’s a turning point in the history of our efforts to understand nature’s elementary laws.

This moment has been a long time coming. I’ve been working as a scientist for over twenty years, and for a third decade before that I was reading layperson’s articles about particle physics, and attending public lectures by my predecessors. Even then, the Higgs particle was a profound mystery. Within the Standard Model (the equations that used at the LHC to describe all the particles and forces of nature we know about so far, along with the SM Higgs field and particle) it stood out as a bit different, a bit ad hoc, something not quite like the others. It has always been widely suspected that the full story might be more complicated. Already in the 1970s and 1980s there were speculative variants of the Standard Model’s equations containing several types of Higgs particles, and other versions with a more complicated Higgs field and no Higgs particle — with a key role of the Higgs particle being played by other new particles and forces.

But everyone also knew this: you could not simply take the equations of the Standard Model, strip the Higgs particle out, and put nothing back in its place. The resulting equations would not form a complete theory; they would be self-inconsistent. (more…)

The question of the day seems to be, “Is the wave function real/physical, or is it merely a way to calculate probabilities?” This issue plays a big role in Tom Banks’s guest post (he’s on the “useful but not real” side), and there is an interesting new paper by Pusey, Barrett, and Rudolph that claims to demonstrate that you can’t simply treat the quantum state as a probability calculator. I haven’t gone through the paper yet, but it’s getting positive reviews. I’m a “realist” myself, as I think the best definition of “real” is “plays a crucial role in a successful model of reality,” and the quantum wave function certainly qualifies.

To help understand the lay of the land, we’re very happy to host this guest post by David Wallace, a philosopher of science at Oxford. David has been one of the leaders in trying to make sense of the many-worlds interpretation of quantum mechanics, in particular the knotty problem of how to get the Born rule (“the wave function squared is the probability”) out of the this formalism. He was also a participant at our recent time conference, and the co-star of one of the videos I posted. He’s a very clear writer, and I think interested parties will get a lot out of reading this.

———————————-

Why the quantum state isn’t (straightforwardly) probabilistic

In quantum mechanics, we routinely talk about so-called “superposition states” – both at the microscopic level (“the state of the electron is a superposition of spin-up and spin-down”) and, at least in foundations of physics, at the macroscopic level (“the state of Schrodinger’s cat is a superposition of alive and dead”). Rather a large fraction of the “problem of measurement” is the problem of making sense of these superposition states, and there are basically two views. On the first (“state as physical”), the state of a physical system tells us what that system is actually, physically, like, and from that point of view, Schrodinger’s cat is seriously weird. What does it even mean to say that the cat is both alive and dead? And, if cats can be alive and dead at the same time, how come when we look at them we only see definitely-alive cats or definitely-dead cats? We can try to answer the second question by invoking some mysterious new dynamical process – a “collapse of the wave function” whereby the act of looking at half-alive, half-dead cats magically causes them to jump into alive-cat or dead-cat states – but a physical process which depends for its action on “observations”, “measurements”, even “consciousness”, doesn’t seem scientifically reputable. So people who accept the “state-as-physical” view are generally led either to try to make sense of quantum theory without collapses (that leads you to something like Everett’s many-worlds theory), or to modify or augment quantum theory so as to replace it with something scientifically less problematic.

On the second view, (“state as probability”), Schrodinger’s cat is totally unmysterious. When we say “the state of the cat is half alive, half dead”, on this view we just mean “it has a 50% probability of being alive and a 50% probability of being dead”. And the so-called collapse of the wavefunction just corresponds to us looking and finding out which it is. From this point of view, to say that the cat is in a superposition of alive and dead is no more mysterious than to say that Sean is 50% likely to be in his office and 50% likely to be at a conference.

Now, to be sure, probability is a bit philosophically mysterious. (more…)

The lure of blogging is strong. Having guest-posted about problems with eternal inflation, Tom Banks couldn’t resist coming back for more punishment. Here he tackles a venerable problem: the interpretation of quantum mechanics. Tom argues that the measurement problem in QM becomes a lot easier to understand once we appreciate that even classical mechanics allows for non-commuting observables. In that sense, quantum mechanics is “inevitable”; it’s actually classical physics that is somewhat unusual. If we just take QM seriously as a theory that predicts the probability of different measurement outcomes, all is well.

Tom’s last post was “technical” in the sense that it dug deeply into speculative ideas at the cutting edge of research. This one is technical in a different sense: the concepts are presented at a level that second-year undergraduate physics majors should have no trouble following, but there are explicit equations that might make it rough going for anyone without at least that much background. The translation from LaTeX to WordPress is a bit kludgy; here is a more elegant-looking pdf version if you’d prefer to read that.

—————————————-

Rabbi Eliezer ben Yaakov of Nahariya said in the 6th century, “He who has not said three things to his students, has not conveyed the true essence of quantum mechanics. And these are Probability, Intrinsic Probability, and Peculiar Probability”.

Probability first entered the teachings of men through the work of that dissolute gambler Pascal, who was willing to make a bet on his salvation. It was a way of quantifying our risk of uncertainty. Implicit in Pascal’s thinking, and all who came after him was the idea that there was a certainty, even a predictability, but that we fallible humans may not always have enough data to make the correct predictions. This implicit assumption is completely unnecessary and the mathematical theory of probability makes use of it only through one crucial assumption, which turns out to be wrong in principle but right in practice for many actual events in the real world.

For simplicity, assume that there are only a finite number of things that one can measure, in order to avoid too much math. List the possible measurements as a sequence

The a_N are the quantities being measured and each could have a finite number of values. Then a probability distribution assigns a number P(A) between zero and one to each possible outcome. The sum of the numbers has to add up to one. The so called frequentist interpretation of these numbers is that if we did the same measurement a large number of times, then the fraction of times or frequency with which we’d find a particular result would approach the probability of that result in the limit of an infinite number of trials. It is mathematically rigorous, but only a fantasy in the real world, where we have no idea whether we have an infinite amount of time to do the experiments. The other interpretation, often called Bayesian, is that probability gives a best guess at what the answer will be in any given trial. It tells you how to bet. This is how the concept is used by most working scientists. You do a few experiments and see how the finite distribution of results compares to the probabilities, and then assign a confidence level to the conclusion that a particular theory of the data is correct. Even in flipping a completely fair coin, it’s possible to get a million heads in a row. If that happens, you’re pretty sure the coin is weighted but you can’t know for sure.

Physical theories are often couched in the form of equations for the time evolution of the probability distribution, even in classical physics. One introduces “random forces” into Newton’s equations to “approximate the effect of the deterministic motion of parts of the system we don’t observe”. The classic example is the Brownian motion of particles we see under the microscopic, where we think of the random forces in the equations as coming from collisions with the atoms in the fluid in which the particles are suspended. However, there’s no a priori reason why these equations couldn’t be the fundamental laws of nature. Determinism is a philosophical stance, an hypothesis about the way the world works, which has to be subjected to experiment just like anything else. Anyone who’s listened to a geiger counter will recognize that the microscopic process of decay of radioactive nuclei doesn’t seem very deterministic. (more…)

Following the guest post from Tom Banks on challenges to eternal inflation, we’re happy to post a follow-up to this discussion by Don Page. Don was a graduate student of Stephen Hawking’s, and is now a professor at the University of Alberta. We have even collaborated in the past, but don’t hold that against him.

Don’s reply focuses less on details of eternal inflation and more on the general issue of how we should think about quantum gravity in a cosmological context, especially when it comes to counting the number of states. Don is (as he mentions below) an Evangelical Christian, but by no means a Young Earth Creationist!

Same rules apply as before: this is a technical discussion, which you are welcome to skip if it’s not your cup of tea.

———————-

I tend to agree with Tom’s point that “it is extremely plausible, given the Bekenstein Hawking entropy formula for black holes, that the quantum theory of a space-time , which is dS in both the remote past and remote future, has a finite dimensional Hilbert space,” at least for four-dimensional spacetimes (excluding issues raised by Raphael Bousso, Oliver DeWolfe, and Robert Myers for higher dimensions in Unbounded entropy in space-times with positive cosmological constant) if the cosmological constant has a fixed finite value, or if there are a finite number of possible values that are all positive. The “conceptual error … that de Sitter (dS) space is a system with an ever increasing number of quantum degrees of freedom” seems to me to arise from considering perturbations of de Sitter when it is large (on a large compact Cauchy surface) that would evolve to a big bang or big crunch when the Cauchy surface gets small and hence would prevent the spacetime from having both a remote past and a remote future. As Tom nicely puts it, “In the remote past or future we can look at small amplitude wave packets. However, as we approach the neck of dS space, the wave packets are pushed together. If we put too much information into the space in the remote past, then the packets will collide and form a black hole whose horizon is larger than the neck. The actual solution is singular and does not resemble dS space in the future.”

So it seems to me that, for fixed positive cosmological constant, we can have an arbitrarily large number of quantum states if we allow big bangs or big crunches, but if we restrict to nonsingular spacetimes that expand forever in both the past and future, then the number of states may be limited by the value of the cosmological constant.

This reminds me of the 1995 paper by Gary Horowitz and Robert Myers, The value of singularities, which argued that the timelike naked singularity of the negative-mass Schwarzschild solution is important to be excluded in order to eliminate such states which would lead to energy unbounded below and instabilities from the presumably possible production (conserving energy) of arbitrarily many possible combinations of positive and negative energy. Perhaps in a similar way, big bang and big crunch singularities are important to be excluded, as they also would seem to allow infinitely many states with positive cosmological constant.

Now presumably we would want quantum gravity states to include the formation and evaporation of black holes (or of what phenomenologically appear similar to black holes, whether or not they actually have the causal structure of classical black holes), which in a classical approximation have singularities inside them, so presumably such `singularities’ should be allowed, even if timelike naked singularities and, I would suggest, big bang and big crunch singularities should be excluded. (more…)

Now that we’ve softened you up by explaining a bit about eternal inflation and its puzzles, we’re very happy to host a guest post by Tom Banks in which he really hits on some of these problems hard.

Tom is a professor at Rutgers and UC Santa Cruz, an extremely accomplished researcher in field theory and string theory, and the author of a textbook on quantum field theory. In collaboration with Fischler, Shenker, and Susskind, he proposed the (M)atrix Theory non-perturbative formulation of string theory. Most recently, he (often working with Willy Fischler) has been exploring the connections between holography and cosmology, developing a detailed model of the evolution of the universe that is compatible with the holographic principle. Here is video of a lecture Tom recently gave on holographic cosmology.

This post is at a more technical level than most of our entries here at CV, and we’re going to try to keep the discussion useful for workers in the field. Sincere questions are welcome, but we’ll be deleting any unproductive philosophical gripes or advertisements for anyone’s personal outsider theories.

——————————————————

Why I Don’t Believe in Eternal Inflation

A lot of research in high energy theory has been devoted to the topic of eternal inflation. More and more, over the last few years, I’ve come to regard this as an enormous waste of intellectual resources and I’ve chosen Cosmic Variance as a very public way to make my objections to this theoretical mistake clear. The theory was developed in the 1980s, when it seemed plausible that quantum field theory in curved space-time was a good approximation to a real theory of quantum gravity whenever the energy densities and curvatures of the background geometry were small in Planck units. This idea is simply wrong. The fact that its falsification came through a back door, the rather philosophical discussion of whether black hole evaporation violates the rules of quantum mechanics, has led to a widespread but unfortunate tendency to ignore this FACT.

There are two other psychological reasons for the widespread interest in Eternal Inflation, which I will discuss below. They have led even the inventors of the resolution of the black hole information paradox through the notion of holography, to try to find a sensible holographic theory which incorporates the notion of EI. While this attempt itself is subject to a number of objections, I will not go into them here. Instead, I’ll concentrate on evidence from the seminal Coleman-De Luccia (CDL) theory of tunneling in quantum gravity, which is one of the two biggest clues to what the theory of quantum gravity really is.

There are, in my opinion, two serious conceptual errors behind the theory of EI. The first is the notion that space-time geometry is a fluctuating quantum variable. The second is that de Sitter (dS) space is a system with an ever increasing number of quantum degrees of freedom. The increase is supposed to take place as the global dS time coordinate, or the time coordinate in flat coordinates, goes to future infinity. I’ll end this post with a brief discussion of the formalism of Holographic Space Time (HST), in which both of these ideas are seen to be false, in a very explicit manner. The fact that the HST formalism is able to give an approximate description of particle physics in a curved space-time background is by itself enough to falsify any claim that the semi-classical ideas that lead to EI are inevitable consequences of ANY sensible theory of quantum gravity. For this purpose, it’s not even necessary that HST be right, only that it have a limit in which it reduces to QFT in curved space-time.

There are two flavors of EI. (more…)

Lisa Randall is a friend and collaborator, as well as a science superstar. She is one of the most highly cited physicists of all time, for a variety of contributions to field theory and particle physics, especially her work with Raman Sundrum on warped extra dimensions. Her first book, Warped Passages, was a major success, which naturally raises the question of what one does next. (Besides writing papers, I mean.)

So we’re very happy to welcome Lisa aboard to guest blog about her new book, just out today: Knocking on Heaven’s Door: How Physics and Scientific Thinking Illuminate the Universe and the Modern World. (Among other virtues, this book has the single most impressive collection of blurbers of any book ever written, from Bill Clinton to Carlton Cuse.) From personal experience I can verify that writing a book doesn’t just happen; it’s a tremendous commitment over an extended period of time, and once it’s done there’s not much chance to go back and change it. So deciding to write a book at all, and more importantly how exactly to target the writing, is a delicate and critical process.

While Lisa hasn’t yet become a regular blogger, she is active on Twitter, where you can follow her at @lirarandall.

—————————————-

In conjunction with the publication of Knocking on Heaven’s Door, I thought I’d take advantage of Sean’s kind invitation to post on Cosmic Variance to explain my motivations in writing my book. I haven’t done a lot of blogging myself but I am impressed at the care and interest that go into science blogs. They are a way of sharing developments as they happen and an opportunity to have meaningful discussion of results.

I talk about a lot of science in my book. So I thought rather than summarizing it all—at least in this post—I’d focus on the question of why I wrote this particular book. I waited several years before even considering embarking on a second book project. I certainly didn’t want to simply repeat the content of my previous book, and my own personal goal is always to branch out into new arenas—in this case into new types of writing–while still remaining true to my physics roots. I didn’t know the exact book I was after but I did know some of the topics I considered important and timely.

These topics fell into several categories. First, I wanted to give an accurate picture of what is happening in particle physics and cosmology today—both with experiments and with theory. Particle physicists know this to be the era of the Large Hadron Collider (LHC), the machine that is colliding together protons at unprecedented energies to test the nature of matter and forces at smaller distances than ever explored. The interactions between theorists and experimenters is more intense than it has been during the time I’ve been actively pursuing physics. That is because everyone realizes this interactions are essential with these challenging experiments to get to the right answers. I wanted to convey the excitement and implications of the research taking place there, so when discoveries are made, anyone interested can understand what was found and what it could mean.

Cosmologists too find this is an important time and I wanted to share some of the interest in that major topic as well. One arena that both particle physicists and cosmologists are excited about are experimental studies of the nature of dark matter. Many find this topic perplexing, whereas even if difficult to tackle experimentally, the underlying idea really is not. I wanted to explain a bit how I think about dark matter and how experiments are searching for its feeble and elusive effects.

But I wanted to do more than just summarize the physics. (more…)

Jim Kakalios of the University of Minnesota has achieved internet demi-fame — he has a YouTube video with over a million and a half views. It’s on the science of Watchmen, the movie based on Alan Moore’s graphic novel. Jim got that sweet gig because he wrote a great book called The Science of Superheroes — what better credentials could you ask for?

More recently Jim has written another book, The Amazing Story of Quantum Mechanics. But even without superheroes in the title, everything Jim thinks about ends up being relevant to movies before too long. The new movie Source Code features a twist at the end that involves — you guessed it — quantum mechanics. Jim has applied his physicist super-powers to unraveling what it all means, and was kind enough to share his thoughts with us in this guest post.

——————————————————————-

There is an interesting discussion taking place on the internets concerning the ending of the newly released film SOURCE CODE, that suggests that the film concludes with a paradox. I believe that any such paradox can be resolved – with Physics!

This entire post is one big honkin’ SPOILER, so if you want to read about the final twist ending of a film without having seen said film – by all means, read on, MacDuff!

In SOURCE CODE, Jake Gyllenhaal plays US helicopter pilot Colter Stevens, whose consciousness is inserted into another man’s body (Sean Fentress, a school teacher in Chicago) through a procedure that requires a miracle exception from the laws of nature (involving quantum mechanics and “parabolic calculus” – by the way, there is no such thing as parabolic calculus). Thanks to some technobabble (or as Q-Bert on Futurama would describe it – weapons grade bolognium) Colter’s mind can only enter Sean’s body in the last eight minutes of Sean’s life. As Sean is sitting on a city bound Chicago commuter train, on which a bomb will explode at 7:58 AM, killing everyone aboard, the goal is for Colter to ascertain who planted the bomb. He cannot stop it from exploding, he is told, because that has already happened. He cannot affect the past, but he can bring information obtained in the past back to his present time. Learning the identity of the bomber would enable the authorities to prevent the detonation of a threatened second “dirty atomic” bomb is downtown Chicago.

While the above can be discerned from the movie trailer, what I am going to discuss next involves the actual ending of the film, and if you do not want this ending spoiled, you should stop reading now. (more…)

I tell everyone I meet that we are at the dawn of the Dark Matter Decade. Usually they slowly back away, but I’m pretty persistent. Our technology has reached the point that we have an excellent chance of actually detecting most of the matter in the universe for the first time.

We’re very happy to have a guest post from Neal Weiner, one of the leading theorists working in the fast-moving area. (Don’t forget our previous guest post from one of the leading experimentalists.) Neal is responsible for some of the most imaginative models for what’s going on in the dark sector, and is excited about the upcoming experimental prospects. If you want to know what particle physicists are thinking about dark matter these days, you’ve come to the right place.

For anyone in the New York area, Neal is giving a public lecture on dark matter at AMNH on Friday the 4th (tomorrow). If you have a chance to go, I’d recommend not missing it.

—————————————————————–

The Era of Dark Matter Direct Detection

Commonly, when I speak to my friends who don’t spend their time obsessing about the prospects for dark matter discovery, I am confronted by indifference, or worse, pessimism, when I mention the next few years of dark matter experiment. The history of dark matter direct detection has largely been a string of experiments, increasingly able to better find nothing, interrupted by occasional unverified claims, they point out. Why should this era be any different?

In contrast, I remain incredibly optimistic about the coming era. I feel this level of sensitivity is special, and that if we are to discover WIMP scattering, it should be in the next few years.

Why am I so optimistic?

1)This level of sensitivity is special

When we talk about discovering dark matter through direct detection, we are typically referring to discovering WIMPs, or Weakly Interacting Massive Particles (although a variety of searches for axions are ongoing). These are particles with masses ranging from roughly the proton mass, to 1000 x the proton mass. The hope is that by putting large (~100 kg or larger) experiments underground, where cosmic rays are shielded, experiments can detect the rare scattering of one of these WIMPs as they pass through the detector. (Estimates of the local density suggest that for WIMPs 300 x the proton mass, there should be about 1000 of them in a cubic meter of space near Earth.)

For dark matter to scatter off of the nucleus, it must interact with it. In the standard model, there are only a limited number of possibilities, and for “renormalizable” interactions, there are only two. It can scatter by exchanging a Z-boson, or by exchanging a Higgs boson.

If the interaction is through a Z-boson, the strength is completely calculable. While a “weak” interaction, the Z-boson provides a relatively strong interaction as far as weak interactions go. Indeed, a WIMP exchanging a Z-boson to elastically scatter off a nucleus would have been seen already about a decade ago, and is excluded by about four orders of magnitude by present experiments (i.e., current experiments would have seen roughly 10^4 events, instead of few or none).

However there is a second possibility – that the WIMP interacts through a Higgs boson. The coupling of the Higgs to ordinary matter is orders of magnitude weaker, with a strength 10 – 100 times weaker than the current generation of experiments, but within reach of the next decade’s experiments. This is not something just pointed out now – Burgess, Pospelov and ter Veldhuis pointed this out a decade ago.

While other force carriers appear in new physics models, such as supersymmetry, even there, the Higgs is often the dominant one. Thus, if you had asked me twenty years ago* what the most interesting levels of sensitivity to think about were, I’d have told you to look for the Z and the Higgs exchange. We know it’s not the Z, and we’re about to know about the Higgs.

*OK, twenty years ago I’d actually have said “huh?”, but that misses the point.

2) If anomalies mean anything, we should find out soon

A great deal of thinking and excitement on the theoretical side has come from considering dark matter anomalies. The DAMA collaboration has reported an annual modulation in the flashes of light in a NaI(Tl) experiment for a decade. This modulation signature was pointed out by Drukier, Freese and Spergel in 1986. When the Earth orbits the sun, sometimes we move with the galactic rotation and sometimes we move against it, consequently the flux of WIMPs should change seasonally, and events in the detector should as well. This is precisely what the DAMA collaboration has observed.

Competing experiments, such as XENON, CDMS, Edelweiss, ZEPLIN and others have seen no such evidence, however, excluding the most conventional scenarios. This has prompted a variety of new ideas: light dark matter, inelastic dark matter, resonant dark matter, luminous dark matter… All of these allow a signal at DAMA consistent with other searches. When compelled by a novel result, theorists begin to see a wider range of possibilities. But even these possibilities make predictions.

More recently, the CoGeNT experiment has seen event rates in their detector above what is expected from background. While no claim has been made of discovery, it is in a range where light dark matter should be expected to be found. XENON and CDMS (and in particular a recent low-energy analysis of the CDMS data, who use the same target) do not see what would have been expected, but a clear background explanation is lacking.

These may be signs of dark matter, and they may not be. If they are, we may already have guessed the correct model, or we may not have, but enough upcoming experiments have sensitivity that almost any scenario should be tested.

What should we be looking for this year?

• CoGeNT will update its data: with more exposure time, their radioactive backgrounds should decay, allowing the signal to be extracted more clearly. Does it modulate as expected? If so, theorists will have to go back to the drawing board.
• KIMS should report soon: the KIMS experiment (Korea Invisible Mass Search) is a CsI(Tl) experiment, with a 100kg target. DAMA began as a 100kg, and grew to 250 kg target of NaI(Tl). KIMS will not test WIMP-sodium scattering explanations of DAMA, but will test WIMP-iodine explanations, and even scenarios where the tiny amount of thallium is what the dark matter interacts with.
• COUPP: the Chicagoland Observatory for Underground Particle Physics is now operating a 4kg target of CF3I at SNOLAB in Canada. With both fluorine (which is light) and iodine (which is heavy and present in DAMA), it should have the ability to test most interpretations of DAMA as well as CoGeNT.
• XENON100: the gorilla in the room is the XENON100 experiment. With already a large exposure on a 30kg target of XENON recorded, the community is eagerly awaiting their results. They could come early in 2011 and may shake up the field.

Going forward, improvements to established detector technologies (such as CDMS) and the maturation of the liquid nobles (such as XENON, but also LUX, DEAP/CLEAN, WARP, DarkSide and more) promise an era of rapid progress, with sensitivity improving by orders of magnitude over the next decade. If WIMPs are there, this coming era is our best opportunity to see them. When coupled with the LHC and new data from astrophysics experiments (Fermi, and PLANCK among others), our attitudes of what dark matter is – or at least what it is not – will soon be entirely different.

It’s a big universe out there — maybe bigger that we think. A lot of people these days are contemplating the possibility that the wider world isn’t just more of the same; it could be that there are regions very different from ours, even with different low-energy laws of physics, outside our observable universe. It’s an old idea, which we now label the “multiverse,” even though we’re talking about regions of space connected to ours. A lot of other people are aghast that this is considered science. Personally I think science talks about unobservable things all the time, and this question is going to be resolved by people doing hard work to make sense of multiverse scenarios rather than by pronouncements about what is or is not science.

We’re very happy to have a guest post from one of the people who is doing exactly that hard work — Matt Johnson, who guest-blogged for us before. He and his collaborators just come out two papers that examine the cosmic microwave background, looking for evidence of “bubble collisions.”

First Observational Tests of Eternal Inflation
Stephen M. Feeney (UCL), Matthew C. Johnson (Perimeter Institute), Daniel J. Mortlock (Imperial College London), Hiranya V. Peiris (UCL)
arXiv:11012.1995

First Observational Tests of Eternal Inflation: Analysis Methods and WMAP 7-Year Results
Stephen M. Feeney (UCL), Matthew C. Johnson (Perimeter Institute), Daniel J. Mortlock (Imperial College London), Hiranya V. Peiris (UCL)
arXiv:1012.3667

The hope is that these other “universes” might not be completely separate from our own — maybe we collided in the past. They’ve done a very careful job going through the data, with intriguing but inconclusive results. (See also Backreaction.)

Looking for this kind of signature in the CMB is certainly reminiscent of the concentric circles predicted by Gurzadyan and Penrose. But despite the similarities, it’s different in crucial ways — different theory, different phenomenon leading to the signal, different analysis, different conclusions. The road to sorting out this multiverse stuff is long and treacherous, but our brave cosmological explorers will eventually guide us through.

Here’s Matt.

———————————————————————–

Observing other universes: is this science fiction?

Perhaps not. Stephen Feeney, Daniel Mortlock, Hiranya Peiris and I recently performed an observational search for the signatures of colliding bubble universes in the cosmic microwave background. Before getting to our results, let me explain some of the back-story.

The idea that there might be other universes is taken quite seriously in high energy physics and cosmology these days. This is mainly due to the fact that the laws of physics, and the various “fundamental” constants appearing in them, could have been otherwise. More technically worded, there is no unique vacuum in theories of high energy physics that involve spontaneous symmetry breaking, extra dimensions, or supersymmetry. Having a bunch of vacua around is interesting, but to what extent are they actually realized in nature? Surprisingly, when a spacetime region undergoing inflation is metastable, there are cases when all of the vacua in a theory can be realized in different places and at different times. This phenomenon is known as eternal inflation. In an inflating universe, if a region is in a metastable vacuum, bubbles containing different vacua will form. These bubbles then expand, and eat into the original vacuum. However, if the space between bubbles is expanding fast enough, they never merge completely. There is always more volume to convert into different vacua through bubble formation, and the original vacuum never disappears: inflation becomes eternal. In the theory of eternal inflation, our entire observable universe resides inside one of these bubbles. Other bubbles will contain other universes. In this precise sense, many theories of high energy physics seem to predict the existence of other universes.

(more…)