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  • richardmitnick 3:16 pm on December 15, 2014 Permalink | Reply
    Tags: , , , Dark Energy/Dark Matter, , ,   

    From SPACE.com: “Will We Ever Find Dark Matter?” Previously Covered Elsewhere, But K.T. is an Excellent Exponent of her Material 

    space-dot-com logo


    December 11, 2014
    Kelen Tuttle, The Kavli Foundation

    Scientists have long known about dark matter, a mysterious substance that neither emits nor absorbs light. But despite decades of searching, they have not yet detected dark matter particles.

    With ten times the sensitivity of previous detectors, three recently funded dark matter experiments — the Axion Dark Matter eXperimen Gen 2, LUX-ZEPLIN and the Super Cryogenic Dark Matter Search at the underground laboratory SNOLAB — have scientists crossing their fingers that they may finally glimpse these long-sought particles.

    University of Washington physicists Gray Rybka (right) and Leslie Rosenberg examine the primary components of the ADMX detector.
    Credit: Mary Levin, University of Washington

    LUX Dark matter

    Super Cryogenic Dark Matter Search

    Late last month, The Kavli Foundation hosted a Google Hangout so that scientists on each of those experiments could discuss just how close we are to identifying dark matter. In the conversation below are three of the leading scientists in the dark matter hunt:

    Enectali Figueroa-Feliciano: Figueroa-Feliciano is a member of the SuperCDMS collaboration and an associate professor of physics at the MIT Kavli Institute for Astrophysics and Space Research.

    Harry Nelson: Nelson is the science lead for the LUX-ZEPLIN experiment and is a professor of physics at the University of California, Santa Barbara.

    Gray Rybka: Rybka leads the ADMX Gen 2 experiment as a co-spokesperson and is a research assistant professor of physics at the University of Washington.

    The SuperCDMS experiment at the Soudan Underground Laboratory uses five towers like the one shown here to search for WIMP dark matter particles.
    Credit: Reidar Hahn, Fermilab

    Below is a modified transcript of the discussion. Edits and changes have been made by the participants to clarify spoken comments recorded during the live webcast. To view and listen to the discussion with unmodified remarks, you can watch the original video.

    The Kavli Foundation: Let’s start with a very basic, yet far from simple question. One of our viewers asks how do we know for sure that dark matter even exists. Enectali, I’m hoping you can start us off. How do you know that there’s something out there for you to find?

    E.F.F.: The primary evidence telling us dark matter is out there is from astronomical observations. In the 1930s, evidence first came in the observations of the velocities of galaxies inside galaxy clusters. Then, in the 1970s, it came in the velocities of stars inside galaxies. One way to explain this is if you imagine tying a string around a rock and twirling it around. The faster you twirl the rock on the string, the more force you have to use to hold onto that string. When people looked at the velocity rotations of galaxies, they noticed that stars were moving way too fast around the center of the galaxy to be explained from the force you could see due to gravity from the mass that we knew was there from our observations. The implication was that if the stars are moving faster than gravity could hold them together, there must be more matter than we can see holding everything in place.

    Today, many different types of observations have been done at the very largest scales, using clusters of galaxies and what’s called the cosmic microwave background. Even when we look at the small scale of particle physics, we know that there are things about the Standard Model that aren’t quite right. We’re trying to find out what’s missing. That’s part of what’s being done at the Large Hadron Collider
    at CERN and other collider experiments. Some of the theories predict particles that would be good candidates for dark matter. So from the largest cosmic scales to the smallest particle physics scales there are reasons to believe that dark matter is there and there are candidates for what that dark matter can be.

    Cosmic Microwave Background  Planck
    CMB erp ESA/Planck

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    CERN LHC Map
    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    TKF: Harry, I’m hoping that you can follow up on that a little bit. Your experiment and the one Enectali works on both look for the most promising type of theoretical particle, one that interacts so weakly with the matter in our world that it’s called the WIMP. In fact there are more than thirty dark matter experiments that are currently planned or underway, and the great majority of them search for this same type of particle. Why do all these experiments focus on the WIMP?

    H.N.: First I want to emphasize that WIMP is an acronym, W-I-M-P, which stands for weakly interacting massive particle. “Massive” means a mass that’s anywhere from a little smaller than the mass of a proton up to many times the mass of a proton. The WIMP is so popular in part because it’s easy to fit into descriptions of the Big Bang — maybe the easiest to fit. The concept to understand here is called thermal equilibrium, and that’s just when you put something in the refrigerator it ends up at the same temperature as the refrigerator. I had a leftover sandwich last night from when I went out to dinner and I put in my refrigerator, and now it’s cold. In much the same way, with WIMPs we hypothesize that dark matter in the early universe was in thermal equilibrium with our matter. But after the Big Bang, the universe gradually cooled down and our matter fell out of equilibrium with the dark matter. Then the dark matter keeps finding itself and, through a process called annihilation, turning into our matter. But the reverse process can no longer go on because our matter doesn’t have enough thermal energy.

    To explain the current abundance of dark matter, the interaction between dark matter and us numerically must be about the same as the weak interaction. That’s W-I in WIMP: weakly interacting. It implies a numerical strength that is consistent with beta decay in radioactivity or, for example, the production of the Higgs particle at the Large Hadron Collider. That the weak interaction appears from this idea about the Big Bang appeals to people. Occam’s Razor, the idea that you don’t want to make things any more complicated than they need to be, makes it attractive. But it doesn’t prove it.

    There are at least two other ways to detect the WIMP. One is at the Large Hadron Collider, as Enectali mentioned, and another is in these WIMP annihilations, where the WIMPs find each other and turn into our matter in certain places in the universe such as the center of stars or the center of our galaxy. If we get lucky, we could hit a trifecta; we could see the same particle with experiments like mine — LUX/LZ — or Enectali’s SuperCDMS, we could see it in the Large Hadron Collider, and we could also see it astrophysically. That would be the trifecta.

    Of course there’s a second reason why so many people are building these WIMP experiments, and that’s that we’ve made a lot of progress in how to build them. There’s been a lot of creativity using many techniques to look for these things. I will say that’s partly true because they’re a little easier to look for than the axion, for which you need really talented and expert people like Gray and Leslie Rosenberg.

    TKF: That’s a nice segue into Gray’s experiment. Gray, you don’t look for the WIMP; instead, you look for something called the axion. It’s a very lightweight particle, with no electric charge and no spin that interacts with our world very rarely. Can you tell us a little bit more about your experiment and why you look for the axion?

    G.R.: I look for the axion because if I looked for WIMPs then I would have to compete with very smart people like Harry and Enectali! But there are other really good reasons as well. The axion is a very good dark matter candidate. We think it may exist because of how physics works inside nuclei. It’s different from the WIMP in that it’s extremely light and you look for it by coupling to photons or, say a radio frequency kind of energy. I got involved in this because I was looking at dark matter and saw that there are a lot of people looking for WIMPs and not many people looking for axions. It’s difficult to look for, but there have been some technical breakthroughs that help. For example, just about everyone has cell phones now and so a lot of work has been done at those frequencies — which just happen to be the right frequencies to use when looking for axions. Meanwhile, there’s a lot of work on quantum computers, which means that there’s also a lot of really nice low temperature radio frequency amplification. That too helps with these experiments. So the time is right be looking for axions.

    TKF: Besides the WIMP and the axion, there are a lot of other theorized particles out there. One of our viewers wrote in and would like to know how likely is it that dark matter is in fact neither of the particles that your experiments look for but rather is composed of super heavy particles called WIMPzillas.

    H.N.: The WIMPzilla has WIMP in its name, so that means it’s weakly interacting, and the zilla part is that it’s just as massive as Godzilla. The way it works is that all of our astrophysical measurements tell us how much mass there is per unit volume – essentially, the cumulative total mass per unit volume of dark matter. But these measurements don’t tell us how to apportion that mass. Are there a great many light particles or just a few really heavy particles? We can’t tell from astrophysical data. So it could be that the dark matter consists of just a few super duper heavy things, like WIMPzillas. But because there wouldn’t be many of them out there, to detect them you’d have to build it a gigantic detector. What we run into there is that nobody wants to give us billions of dollars to build that gigantic detector. It’s just too much money. I think that’s what keeps us from making progress on the idea of the WIMPzilla.

    LUX Dark matter
    The LUX detector before its large tank was filled with more than 70,000 gallons of ultra-pure water. The water shields the detector from background radiation.
    Credit: Matt Kapust, Sanford Underground Research Facility

    E.F.F.: There are many theoretical dark matter particles. We have to pick a combination of what we can look for with the experiments that we can build and what theory and our current understanding suggests are the best places to look. Now, not all of the theories have as good a foundation as others. Some would work but have different types of assumptions built into them and so we need to make a value judgment as experimentalists. We go to the “theory café” and choose which are the best courses on the menu, then we trim the list down to those that are the most feasible to detect, and then we look at which of those we can afford. That convolution of parameters is what prompts us to look for particular candidates. And if we don’t find dark matter in those places, we will look for them elsewhere. And of course there’s no reason why dark matter has to be one thing; it might be composed of several different particles. We might find WIMPs and axions and other things we don’t know of yet.

    TKF: One of our viewers points us to a press release issued last week by Case Western University that describes a theory in which dark matter is made up of macroscopic objects. This viewer would like to know whether there’s any reason why dark matter would be more likely to be made up of the individual exotic particles that you look for than it is to be made up of macroscopic particles.

    H.N.: Papers like that are one of the reasons this field is so exciting. There are just so many different ideas out there and there’s this big discussion going on all the time. New ideas come in, we discuss them and think about them, and sometimes the new idea is inconsistent but other times people say, “Wow we have no idea that could be great.”

    This concept that the dark matter might consist of particles that coalesce into solid or massive objects has been around for a long time. In fact, there was a search 20 or so years ago where they looked for large objects in our galaxy that were creating gravitational lensing. When you look at stars out in our galaxy, if they suddenly become brighter that’s evidence of a massive object moving in front of them. You might wonder how an object moving in front of something would give it more light, but that’s the beauty of gravitational lensing — the light focuses around the object. So this idea has been out there, and this paper looks to be a very careful reanalysis.

    Another example is an idea that’s been around for a long time that maybe there is a different kind of nuclear matter out there. Our nuclear matter is made of up and down quarks and maybe there’s another type of nuclear matter that involves the strange quark. People have been searching for that for 30 or 40 years, but we’ve never been able to find it. Maybe it exists and maybe it’s the dark matter we’re searching for. I would say that in some estimate of probabilities it’s less likely, but we could be wrong. What’s great is to have the scientific discussion always going because the probabilities get reassessed all the time.

    G.R.: These massive objects have a very amusing acronym. They’re massive compact halo objects, MACHOs. So for a while it was MACHOs versus WIMPs.

    E.F.F.: One thing that I would add is that this paper and this whole idea of the variety of models really highlights how diverse the possibilities for looking for dark matter are. In that paper, they looked into mica samples that had been buried for many, many years, looking for tracks. When you have a candidate, the theoretical community starts scanning every possibility of a signal that might have been left — not just in our detectors, but also in the atmosphere, in meteorites, in stars and in the structure that we see in the universe. There are other detectors out there that are more indirect than the ones we’ve specifically designed for dark matter. That’s one of the things that makes it exciting: maybe we find dark matter in our detectors and we might also find traces of it in other things that we haven’t even thought about yet.

    TKF: In the history of particle physics, there have been a number of particles that we knew existed long before we were able to detect them — and in a lot of cases, we knew a lot about these particles’ characteristics before we found them. This seems very different from where we are now with dark matter. Why is that? What is fundamentally different here?

    G.R.: We know about dark matter from gravitational interactions, and we have a hard time fitting gravity in with the fundamental particles to begin with. I think that’s a big part of it. Would you all agree?

    H.N.: There are some analogues, but you have to go back in time quite a bit. One of the famous analogues is the discovery of the neutron. The proton was discovered in a fantastic series of experiments during World War I by [Ernest] Rutherford, but he had good intuition and thought there should be another particle that’s like the proton that is neutral, which they called the neutron. Even though they had a pretty good idea what it should be like, it took 12 or 15 years for them to detect one because it was just difficult. Then there was an experiment done by Frederic and Irene Joliot-Curie and their group in France and they interpreted the results in a very strange way. But a guy named James Chadwick looked at their data and said, “My God that’s it!” He repeated the experiment and proved the existence of the neutron.

    That story is so important because the neutron is the key to most uses of nuclear energy. I suspect with dark matter we’ll have some sort of rerun of that. We’re all looking and somewhere, maybe even now, there’s a little bit of data that will cause someone to have an “Ah ha!” moment.

    E.F.F.: We also have this nice framework of the Standard Model, but right now we don’t really have one single theory of what should come after it. The most popular possibility is supersymmetry, which is one of the things that a large number of physicists at the Large Hadron Collider are trying to find. But it’s not at all clear that this is the solution of what lies beyond the Standard Model. That ambiguity leads to a plethora of dark matter models because dark matter lies outside of the framework of the Standard Model and we don’t know in which direction this model will grow or how it will change. Physicists are looking at all the possibilities, many of which have good dark matter candidates. There’s this chasm between where we are now and where the light of understanding is, and we don’t yet know which direction to go to find it. So people are looking in all possible directions generating a lot of great ideas.

    Supersymmetry standard model
    Standard Model of Supersymmetry

    TKF: It seems that the results of your experiments will direct the search in one way or another. One of our viewers would like to know a little bit more about how you go about detecting dark matter in your experiments. Since dark matter really doesn’t interact with us very much, how do you go about seeing it?

    G.R.: Our experiments use very different techniques. My experiment looks for axions that every once in a while couple to photons. They do so in a way that the photons produced are of microwave frequencies. This is quite literally the frequency used by your cell phone or in your microwave oven. So we look for a very occasional transmutation of an axion from the dark matter around us into a microwave photon. We also help this process along using a strong magnetic field. Because the frequency of the photon coming from the axion is very specific, this ends up being a scanning experiment. It’s almost like tuning an AM radio; you know there’s a signal out there at a certain frequency, but you don’t know what the frequency is so you tune around, listening to hear a station. Only we’re looking for a signal that’s coming from dark matter turning into photons.

    E.F.F.: Both Harry and I look for similar particles, these WIMPs. My experiment is particularly good at looking for WIMPs that are about the mass of a proton or a couple times heavier than that, while Harry’s experiment is better at looking for particles that are maybe a hundred to several hundred times heavier than the proton. But the idea is the same. As Harry mentioned before, we know the density of dark matter particles in our region of space in the galaxy, so we can calculate how many of these dark matter particles should be going through me, through you, through your room right now.

    If you stick out your hand and you assume that WIMPs are maybe sixty times the mass of the proton — I’m just picking a number here — you calculate that there should be about 20 million WIMPs going through your hand every second. Now these dark matter particles go straight through your hand and straight through the Earth, but perhaps very occasionally they interact with one of the atoms in the matter that the Earth is made of. So we build detectors that hope to catch some of those very, very rare interactions.

    My experiment uses a crystal made out of germanium or silicon that we cool down to milli-kelvin temperatures: almost at absolute zero. If you remember your high school physics, atoms stop vibrating when they get very, very cold. So the atoms in this crystal are not vibrating much at all. If a dark matter particle interacts with one of the atoms in the crystal, the whole crystal starts vibrating and those vibrations are sensed by little microphones that we call phonon sensors. They also release charge and we measure that charge as well. Both of those help us to determine not only the energy that was imparted to the target but what type of interaction it was: Was it an interaction like the one you would expect from a photon or an electron, or was it an interaction you would expect from a WIMP or perhaps a neutron? That helps us to distinguish a dark matter signal from backgrounds coming from radioactivity in our environment. That’s very important when you’re looking for a very elusive signal.

    TKF: In fact you even go to the extent of working far underground to reduce this background noise, is that right?

    E.F.F.: That’s right. And I’ll actually let Harry take it from here.

    H.N.: Our experiments are going to be in two different mines. Ours is about a mile underground in western South Dakota in the Black Hills — the same black hills mentioned in the Beatles song Rocky Raccoon. Meanwhile, Enectali is up in Sudbury, Ontario, where there’s a heavy metal mine.

    One analogy I wanted to bring up is that what Enectali and I do is a microscopic version of billiards. The targets — in my case are xenon and in his case germanium and silicon — are like the colored balls on a pool table, and what we’re trying to detect is the cue ball — the dark matter particle we can’t see. But if the cue ball collides with the colored balls, they suddenly move. That’s what we detect.

    As Enectali said, the reason we go deep in a mine and the reason we build elaborate shields around these things is so that we aren’t fooled by radioactivity or neutrons or neutrinos moving the billiard balls. And there are a lot fewer of these fakes when we go deep. Plus, it’s an awful lot of fun to go in these mines. I’ve been working in them for ten or fifteen years now and it’s great to go a mile underground.

    TKF: If one of your experiments is successful in seeing dark matter, Enectali you said in a previous conversation that the next steps would be to study the dark matter particle’s characteristics and use that knowledge to better understand the particle’s role in the universe. I’m hoping you can explain that last bit a little bit further. Just how far-reaching would such a discovery be?

    E.F.F.: We’re very lucky in that we get to ask these really big questions about what the universe is made of. We know that dark matter makes up about 25 or 26 percent of the universe, and through direct detection we’re trying to figure out what that is exactly.

    But even once we know the mass of the dark matter particle, we still need to understand a lot of other things: whether it has spin, whether it is its own anti-particle, all kinds of properties of the particle itself. But that’s not all that there is to it. This particle was produced some time ago. We want to know how it was produced, when it was produced, what did that do to the universe and to the formation of the universe. There’s a very complicated history of what happened in the universe between the Big Bang and today, and dark matter has a big role to play.

    Dark matter is the glue that holds all the galaxies, all the clusters of galaxies and all the super clusters together. So without dark matter, the universe would not look like it does today. The type of dark matter could change the way that structure formed. So that’s one very important thing that we would like to understand. Another thing is that we don’t really know how dark matter behaves here in our galaxy today. We know its density, but we don’t really know how it’s moving. We have some assumptions, but it will be very interesting to really understand the motion of dark matter – whether it’s clumpy, whether it has structures or streams, whether some of it is in a flat disk. The answers to these questions will have implications for the stars in our galaxy and beyond. All those things will be the next step in what we would love to be doing, which is dark matter astronomy.

    TKF: We have one last question from a viewer who identifies herself as “an interested artist.” Her question is: If you find dark matter, what are you going to call it? It won’t be dark anymore.

    G.R.: I can start with a bad idea. It was called dark matter originally because when you look up at the sky, there are things that produce light — like stars — and there are things that we know are out there because they interact gravitationally but they’re not producing light. They’re dark. But that name kind of implies that they absorb or block light, when in fact dark matter doesn’t. Light goes right through it. So you can call it clear matter, but dark matter at least sounds mysterious. Clear matter sounds rather boring.

    H.N.: I hope you get people better at language than physicists to answer this! If it’s physicists who name it, we’ll end up with a name like gluon. I’d prefer to have a better name than that. Since this viewer is an artist, I’ll point out a sculpture at the Tate in London by Cornelia Parker called Cold Dark Matter: An Exploded View. This idea that there’s something out there that we can’t sense yet is one of those things that sends chills down my spine. I think that scientists share that feeling of wonderment with artists.

    E.F.F.: I’d love to have a naming contest for this 20-some-odd percent of the universe. I think it would produce much better names than we would come up with on our own.

    See the full article here.

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  • richardmitnick 8:53 am on December 13, 2014 Permalink | Reply
    Tags: , , , , Dark Energy/Dark Matter,   

    Important From Ethan Siegel: “Did we just find dark matter?” 

    Starts with a bang
    Starts with a Bang

    Dec 12, 2014
    Ethan Siegel

    Not a chance. What we’ve found may be a mystery, but it’s definitely not our Universe’s missing mass

    “Time takes it all whether you want it to or not, time takes it all. Time bares it away, and in the end there is only darkness.” -Stephen King

    But we are not quite at the end of time yet! It’s only the end of the week, which means it’s time for another Ask Ethan, and to give away another 2015 Year In Space Calendar! After another great week of questions and suggestions (and there were many good ones), congratulations are in order for last-minute submitter Joe Latone, who asks about a newly released story:

    I’m seeing a lot of physics headlines like this over the past day, Researchers detect possible signal from dark matter. As you so eloquently do, would you explain a bit of the background and then distill this recent news for us?

    Let’s give you exactly what you want and need, Joe!

    Image credit: Dean Rowe of http://deanrowe.net/astro, via http://apod.nasa.gov/apod/ap100502.html.

    First off, there’s the problem of dark matter. When we think about a cluster of galaxies — like the Coma Cluster, above — we have two ways of measuring the stuff that’s in it:

    We can look at the full spectrum of signals from the electromagnetic spectrum coming from it, including not only the light-emitting stars but also light emitted and absorbed from other parts of the spectrum. These give us windows into the amount of gas, dust, plasma, neutron stars, black holes, dwarf stars and even planets present inside.
    We can look at the motion of the objects within the cluster — in this case, the individual galaxies — and use what we know about the laws of gravitation to deduce what the total amount of mass within is.

    By comparing those two numbers, we can see whether all of the mass is accounted for by normal matter, or whether there needs to be something else that isn’t made of protons, neutrons and electrons.

    Image credit: Multiwavelength images of M31, via the Planck mission team; ESA / NASA.

    ESA Planck
    ESA Planck schematic

    We can do the same thing for individual galaxies as well. Again, it’s easy to look at all the different, multiwavelength components of the galaxy. For both individual galaxies as well as clusters, we find a certain amount of mass in the form of stars, about five-to-eight times as much in the form of neutral gas, very little in the form of plasma (although there’s plenty of plasma in the intergalactic medium), and only a fraction of what’s present in stars in the form of all the other types of mass, combined. On average, there’s about seven times as much total normal matter in addition to the stars we see in all the large galaxies and clusters we look at.

    But when it comes to the total amount of mass that we infer from gravitation, we find something surprising. Rather than needing about eight times as much total matter to account for the gravitational effects we see, which are the rotational speeds of galaxies at different distances in individual spirals and the speeds of the individual galaxies relative to the cluster center in clusters, we need something like fifty times as much!

    Image credit: European Space Agency, NASA and Jean-Paul Kneib (Observatoire Midi-Pyrénées, France/Caltech, USA), via http://www.spacetelescope.org/images/heic0309a/.

    This discrepancy, or the fact that we need about a total of five times as much matter in addition to the amount of normal matter that exists in our Universe, is known as the dark matter problem. There are many good sets of observations — including from distance/redshift measurements of standard astronomical candles, from giant surveys of the large-scale structure in our Universe, from observations of colliding galaxy clusters and from precision measurements of the Cosmic Microwave Background (the leftover glow from the Big Bang) — that show this is not a problem with the theory of gravity itself, but is rather due to the fact that there is a new type of matter in our Universe that exists in about five times the abundance of normal, atomic matter.

    And this new form of matter — dark matter — among other things, does not interact with either matter or radiation through the electromagnetic force.

    Image credit: The Particle Adventure / DoE / NSF / LBNL, original from CPEP via http://cpepweb.org/.

    It’s also been established that whatever this dark matter is, it isn’t any of the conventional particles in the standard model. It isn’t a quark, it isn’t a boson, and it isn’t even a neutrino. Whatever it is, it’s got to be an entirely new type of particle, one that hasn’t been discovered yet.

    The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Based on the gravitational properties that it’s required to have, as well, it’s expected to cluster in a giant halo, both around galaxies individually and around huge clusters in even larger, more diffuse spheroids.

    Image credit: the cluster mass profile of galaxy cluster Cl 0024
    John Kormendy of the halo around NGC 4216.

    For most models of dark matter, there’s one more property that’s expected of them: they ought to be their own antiparticle. Therefore, where the dark matter density is densest (at the centers of galaxies and clusters), there’s a chance they can annihilate. And if they do, the two annihilating dark matter particles will produce two photons, where the energy of each photon (to conserve energy and momentum) will correspond to the dark matter particle’s rest mass.

    Image credit: Particle-antiparticle annihilation (Upper), where each photon has the mass of the initial particle; particle decay into two photons (Lower), where each photon has half the particle’s initial mass.

    Sounds great, then, doesn’t it? All we have to do is point our high-energy telescopes — our X-ray and gamma ray observatories — at the centers of galaxies and clusters, and look for signals of this annihilation. This means looking for spectral “lines” of energy that don’t correspond to any known particles.

    Piece o’ cake, right?

    Image credit: K. Matsushita, from Galaxies in the Universe: An Introduction (Sparke & Gallagher).

    Not so fast. You see, one of the problems with our Universe is that there are all sorts of high-energy phenomena that are not well-understood here on Earth! Why? Because we don’t have the ability to recreate all the oddball phenomena that are out there in space, and we don’t know what causes many (or even most) of the conventional X-ray and gamma ray backgrounds we see.

    In other words, there are plenty of X-ray and gamma ray sources out there that we already know we don’t understand all that well.

    Well, as Joe points out, there was a discovery earlier this year of a new X-ray line — an energy source of about 3.5 keV — at the core of both the Andromeda galaxy and the Perseus galaxy cluster.

    Is this due to something “mundane,” like from particles being accelerated around a supermassive black hole?

    Or is this due to a new particle — like a sterile neutrino, for example — that’s responsible for the dark matter, annihilating and revealing its rest mass to be the (via E = mc^2) equivalent of 3.5 keV? (Or double that — at 7.0 keV — if this is a decaying particle instead.)

    Image credit: Alexey Boyarsky, Oleg Ruchayskiy, Dmytro Iakubovskyi, Jeroen Franse, screenshot via the full paper available at http://arxiv.org/abs/1402.4119.

    The news would love you to believe that the second possibility is worth considering, because, well, how awesome would it be to find dark matter? But not only is the evidence that this is even a real signal not at all compelling (under a 4σ significant detection even for the combined data set, when 5σ is the “gold standard” for discovery), but there is no way this could account for the dark matter in our Universe!

    Why not? You see, this is a picture of the overdensities and underdensities in our Universe just 380,000 years after the Big Bang: from the Cosmic Microwave Background itself.

    Image credit: ESA and the Planck collaboration.

    While it’s easy to think about the Universe as denser and younger during this time, it’s easy to forget that it was also hotter. This doesn’t just mean that the radiation was hotter, although that was true, but that the matter within it is also moving around at much greater speeds. This applies not only to normal matter, like atoms, but dark matter as well.

    Why is this important? Because in order to clump together, and in order to support the formation of structure due to gravitational collapse, matter needs to be moving slowly enough or that collapse won’t occur. And if dark matter is too light, structure won’t form early enough to agree with our observations!

    Image credit: V. Springel at Max-Planck-Institute at Garching.

    So what do we use to constrain this? Our best measurements come from something called the Lyman-alpha forest, which is a measure of how deep the gravitational potential wells of loosely-held-together gas clouds are dating back to when the Universe was very young. Sure, the densest objects will form stars, galaxies and even quasars early on, but there are going to be neutral gas clouds intervening, and they’re going to absorb some of that light at characteristic frequencies.

    Images credit: Michael Murphy, Swinburne U.; HUDF: NASA, ESA, S. Beckwith (STScI) et al.

    By looking at how deep these “forest lines” are, especially early on, we can constrain how light dark matter is allowed to be. Even under the most liberal of circumstances, we can see that the absorption lines are incredibly strong — consistent with dark matter being incredibly cold — which means it has to be at least above a certain mass threshold.

    Image credit: Bob Carswell, of the Lyman-alpha forest for nearby and distant galaxies.

    Well, what is that threshold? It has to be, at this point in time, heavier than about 10 keV, based on the strength of the observed absorption lines. In other words, about a factor of 3 heavier (or 50% heavier, for a decaying particle) that this supposed “dark matter signal” is!

    Don’t get me wrong, the discovery of a potential new X-ray line is very interesting, and could be a window either into new astrophysics or, potentially (if a bit fantastically and unlikely), a new type of particle. It’s just that even if it turns out to be a new particle, that particle cannot be the dark matter, since it would screw up structure formation in the Universe (particularly on small scales), and our observations of those structures simply rules that scenario out.

    Image credit: Benedetta Ciardi.

    So it’s still interesting, but could it be the dark matter? Not a chance, not unless we’ve got something woefully wrong in multiple departments here.

    Thanks for a great question, Joe, and send me your email address and I’ll make your 2015 Year In Space Calendar happen! We have two weeks left of winners and two more calendars to give away, so for your chance to win, send in your questions and suggestions here.

    See the full article here.

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    Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible.

  • richardmitnick 3:47 pm on December 12, 2014 Permalink | Reply
    Tags: , , , , Dark Energy/Dark Matter,   

    From SPACE.com: “Cosmic Mystery Solved? Possible Dark Matter Signal Spotted” 

    space-dot-com logo


    December 11, 2014
    Mike Wall

    Astronomers may finally have detected a signal of dark matter, the mysterious and elusive stuff thought to make up most of the material universe.

    While poring over data collected by the European Space Agency’s XMM-Newton spacecraft, a team of researchers spotted an odd spike in X-ray emissions coming from two different celestial objects — the Andromeda galaxy and the Perseus galaxy cluster.

    Andromeda galaxy

    Perseus galaxy cluster

    ESA XMM Newton

    The signal corresponds to no known particle or atom and thus may have been produced by dark matter, researchers said.

    “The signal’s distribution within the galaxy corresponds exactly to what we were expecting with dark matter — that is, concentrated and intense in the center of objects and weaker and diffuse on the edges,” study co-author Oleg Ruchayskiy, of the École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, said in a statement.

    “With the goal of verifying our findings, we then looked at data from our own galaxy, the Milky Way, and made the same observations,” added lead author Alexey Boyarsky, of EPFL and Leiden University in the Netherlands.

    Dark matter is so named because it neither absorbs nor emits light and therefore cannot be directly observed. But astronomers know dark matter exists because it interacts gravitationally with the “normal” matter we can see and touch.

    And there is apparently a lot of dark matter out there: Observations of star motion and galaxy dynamics suggest that about 80 percent of all matter in the universe is “dark,” exerting a gravitational force but not interacting with light.

    The Bullet Cluster
    Hot gas in this collision of galaxy clusters is seen as two pink clumps that contain most of the normal matter. The bullet-shaped clump on the right is hot gas from one cluster, which passed through the hot gas from the other larger cluster. Other telescopes were used to detect the bulk of the matter in the clusters, which turns out to be dark matter (highlighted in blue)
    Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

    Hubble Reveals Ghostly Ring of Dark Matter
    Credit: NASA/ESA Hubble
    A ghostly ring of dark matter floating in the galaxy cluster ZwCl0024+1652, one of the strongest pieces of evidence to date for the existence of dark matter. Astronomers think the dark-matter ring was produced from a collision between two gigantic clusters.
    Hot gas in this collision of galaxy clusters is seen as two pink clumps that contain most of the normal matter. The bullet-shaped clump on the right is hot gas from one cluster, which passed through the hot gas from the other larger cluster. Other telescopes were used to detect the bulk of the matter in the clusters, which turns out to be dark matter (highlighted in blue).

    Dark Matter’s Link to Brilliant Galaxies Confirmed
    Credit: Paul Bode and Yue Shen, Princeton University
    The illustration shows the distribution of dark matter, massive halos, and luminous quasars in a simulation of the early universe, shown 1.6 billion years after the Big Bang. Gray-colored filamentary structure shows the distribution of dark matter; small white circles mark concentrated “halos” of dark matter more massive than 3 trillion times the mass of the sun; larger, blue circles mark the most massive halos, more than 7 trillion times of the sun, which host the most luminous quasars. The strong clustering of the quasars in the SDSS sample demonstrates that they reside in these rare, very massive halos. The box shown is 360 million light years across.

    Researchers have proposed a number of different exotic particles as the constituents of dark matter, including weakly interacting massive particles (WIMPs), axions and sterile neutrinos, hypothetical cousins of “ordinary” neutrinos (confirmed particles that resemble electrons but lack an electrical charge).

    The decay of sterile neutrinos is thought to produce X-rays, so the research team suspects these may be the dark matter particles responsible for the mysterious signal coming from Andromeda and the Perseus cluster.

    If the results — which will be published next week in the journal Physical Review Letters — hold up, they could usher in a new era in astronomy, study team members said.

    “Confirmation of this discovery may lead to construction of new telescopes specially designed for studying the signals from dark matter particles,” Boyarsky said. “We will know where to look in order to trace dark structures in space and will be able to reconstruct how the universe has formed.”

    You can read the paper at the online preprint site arXiv: http://arxiv.org/pdf/1402.4119v1.pdf

    See the full article here.

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  • richardmitnick 3:02 pm on December 12, 2014 Permalink | Reply
    Tags: , Dark Energy/Dark Matter,   

    From Kavli: “Is an Understanding of Dark Matter around the Corner? Experimentalists Unsure” 


    The Kavli Foundation

    December 12, 2014

    Media Contact

    James Cohen
    Director of Communications
    The Kavli Foundation
    (805) 278-7495

    Scientists have long known that dark matter is out there, silently orchestrating the universe’s movement and structure. But what exactly is dark matter made of? And what does a dark matter particle look like? That remains a mystery, with experiment after experiment coming up empty handed in the quest to detect these elusive particles.

    With some luck, that may be about to change. With ten times the sensitivity of previous detectors, three recently funded dark matter experiments have scientists crossing their fingers that they may finally glimpse these long-sought particles. In recent conversations with The Kavli Foundation, scientists working on these new experiments expressed hope that they would catch dark matter, but also agreed that, in the end, their success or failure is up to nature to decide.

    “Nature is being coy,” said Enectali Figueroa-Feliciano, an associate professor of physics at the MIT Kavli Institute for Astrophysics and Space Research who works on one of the three new experiments. “There’s something we just don’t understand about the internal structure of how the universe works. When theorists write down all the ways dark matter might interact with our particles, they find, for the simplest models, that we should have seen it already. So even though we haven’t found it yet, there’s a message there, one that we’re trying to decode now.”

    The first of the new experiments, called the Axion Dark Matter eXperiment, searches for a theoretical type of dark matter particle called the axion. ADMX seeks evidence of this extremely lightweight particle converting into a photon in the experiment’s high magnetic field. By slowly varying the magnetic field, the detector hunts for one axion mass at a time.

    ADMX Axion Dark Matter Experiment
    ADMX at U Washington

    “We’ve demonstrated that we have the tools necessary to see axions,” said Gray Rybka, research assistant professor of physics at the University of Washington who co-leads the ADMX Gen 2 experiment. “With Gen2, we’re buying a very, very powerful refrigerator that will arrive very shortly. Once it arrives, we’ll be able to scan very, very quickly and we feel we’ll have a much better chance of finding axions – if they’re out there.”

    The two other new experiments look for a different type of theoretical dark matter called the WIMP. Short for Weakly Interacting Massive Particle, the WIMP interacts with our world very weakly and very rarely. The Large Underground Xenon, or LUX, experiment, which began in 2009, is now getting an upgrade to increase its sensitivity to heavier WIMPs. Meanwhile, the Super Cryogenic Dark Matter Search collaboration, which has looked for the signal of a lightweight WIMP barreling through its detector since 2013, is in the process of finalizing the design for a new experiment to be located in Canada.

    LUX Dark matter

    LBL SuperCDMS
    Super Cryogenic Dark Matter Search

    “In a way it’s like looking for gold,” said Figueroa-Feliciano, a member of the SuperCDMS experiment. “Harry has his pan and he’s looking for gold in a deep pond, and we’re looking in a slightly shallower pond, and Gray’s a little upstream, looking in his own spot. We don’t know who’s going to find gold because we don’t know where it is.”

    Rybka agreed, but added the more optimistic perspective that it’s also possible that all three experiments will find dark matter. “There’s nothing that would require dark matter to be made of just one type of particle except us hoping that it’s that simple,” he said. “Dark matter could be one-third axions, one-third heavy WIMPs and one-third light WIMPs. That would be perfectly allowable from everything we’ve seen.”

    Yet the nugget of gold for which all three experiments search is a very valuable one. And even though the search is difficult, all three scientists agreed that it’s worthwhile because glimpsing dark matter would reveal insight into a large portion of the universe.

    “We’re all looking and somewhere, maybe even now, there’s a little bit of data that will cause someone to have an ‘Ah ha!’ moment,” said Harry Nelson, professor of physics at the University of California, Santa Barbara and science lead for the LUX upgrade, called LUX-ZEPLIN. “This idea that there’s something out there that we can’t sense yet is one of those things that sends chills down my spine.”

    More about the hunt for dark matter is available at:

    New Dark Matter Experiments Prepare to Hunt the Unknown: A Conversation with Enectali Figueroa-Feliciano, Harry Nelson and Gray Rybka
    Spotlight Live: Dark Matter at Long Last? Three New Experiments Ramp Up (Transcript)

    See the full article here.

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    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

    The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

  • richardmitnick 9:05 pm on December 3, 2014 Permalink | Reply
    Tags: , Dark Energy/Dark Matter, ,   

    From Symmetry: “Searching for a dark light” 


    December 03, 2014
    Manuel Gnida

    A new experiment at Jefferson Lab is on the hunt for dark photons, hypothetical messengers of an invisible universe.

    The matter we know accounts for less than 5 percent of the universe; the rest is filled with invisible dark matter and dark energy. Scientists working on a new experiment to be conducted at Thomas Jefferson National Accelerator Facility in Virginia hope to shed light on some of those cosmic unknowns.

    According to certain theories known as hidden-sector models, dark matter is thought to consist of particles that interact with regular matter through gravitation (which is why we know about it) but not through the electromagnetic, strong and weak fundamental forces (which is why it is hard to detect). Such dark matter would interact with regular matter and with itself through yet-to-be-discovered hidden-sector forces. Scientists believe that heavy photons—also called dark photons—might be mediators of such a dark force, just as regular photons are carriers of the electromagnetic force between normal charged particles.

    The Heavy Photon Search at Jefferson Lab will hunt for these dark, more massive cousins of light.

    “The heavy photon could be the key to a whole rich world with many new dark particles and forces,” says Rouven Essig, a Stony Brook University theoretical physicist who in recent years helped develop the theory for heavy-photon searches.

    Although the idea of heavy photons has been around for almost 30 years, it gained new interest just a few years ago when theorists suggested that it could explain why several experiments detected more high-energy positrons—the antimatter partners of electrons—than scientists had expected in the cosmic radiation of space. Data from the PAMELA satellite experiment; the AMS instrument aboard the International Space Station; the LAT experiment of the Fermi Gamma-ray Space Telescope and others have all reported finding an excess of positrons.


    AMS 02

    NASA Fermi LAT
    NASA/Fermi LAT

    NASA Fermi Telescope
    NASA/Fermi spacecraft

    “The positron excess could potentially stem from dark matter particles that annihilate each other,” Essig says. “However, the data suggest a new force between dark matter particles, with the heavy photon as its carrier.”

    Creating particles of dark light

    If heavy photons exist, researchers want to create them in the lab.

    Theoretically, a heavy photon can transform into what is known as a virtual photon—a short-lived fluctuation of electromagnetic energy with mass—and vice versa. This should happen only very rarely and for a very short time, but it still means that experiments that produce virtual photons could in principle also generate heavy photons. Producing enormous numbers of virtual photons may create detectable amounts of heavy ones.

    At Jefferson Lab’s Continuous Electron Beam Accelerator Facility, CEBAF, scientists will catapult electrons into a tungsten target, which will generate large numbers of virtual photons—and perhaps some heavy photons, too.

    Jlab CEBAF

    “CEBAF provides a very stable, highly intense electron beam that is almost continuous,” says Jefferson Lab’s Stepan Stepanyan, one of three spokespersons for the international HPS collaboration, which includes more than 70 scientists. “It is a unique place for performing this experiment.”

    The virtual photons and potential heavy photons produced at CEBAF will go on to decay into pairs of electrons and positrons. A silicon detector placed right behind the target will then track the pairs’ flight paths, and an electromagnetic calorimeter will measure their energies. Researchers will use this information to reconstruct the exact location in which the electron-positron pair was produced and to determine the mass of the original photon that created the pair. Both are important data points for picking the heavy photons out of the bunch.

    The photon mass measured in the experiment matters because a heavy photon has a unique mass, whereas virtual photons appear with a broad range of masses. “The heavy photon would reveal itself as a sharp bump on top of a smooth background from the virtual photon decays,” says SLAC National Accelerator Laboratory’s John Jaros, another HPS spokesperson.

    The location in which the electron-positron pair was produced also matters because virtual photons decay almost instantaneously within the target, says Timothy Nelson, project lead for the silicon detector, which is being built at SLAC. Heavy photons could decay more slowly, after traveling beyond the target. So photons that decay outside the target can only be heavy ones. The HPS silicon detector’s unique ability to identify outside-of-target decays sets it apart from other experiments currently participating in a worldwide hunt for heavy photons.

    The HPS calorimeter, whose construction was led by researchers from the French Institut de Physique Nucléaire, the Italian Istituto Nazionale di Fisica Nucleare and Jefferson Lab, is currently being tested at Jefferson Lab, while scientists at SLAC plan to ship their detector early next year. The experiment is scheduled to begin in the spring of 2015.

    See the full article here.

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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 4:26 pm on November 25, 2014 Permalink | Reply
    Tags: , , , , Dark Energy/Dark Matter,   

    From Starts With a Bang: “Dark Matter and the Origin of Life” 

    Starts with a bang
    Starts with a Bang

    November 25, 2014

    James Bullock, UC Irvine

    How material we’d never notice if we kept our eyes on Earth alone helped give rise to all that we are.

    The origin of life is one of the great mysteries of science. Even the definition of life is widely debated. But one thing that’s agreed upon is this: complex molecules are required. This is because life does amazingly elaborate things: it extracts energy from its environment, it replicates, and it evolves by natural selection. Intricate biochemical machinery like this requires a set of intricate building blocks.

    Nothing of the kind emerged from the Big Bang. It was just too hot, too dense, and expanding too fast for anything complicated to form. So what allowed the Universe to go from this simple beginning to something as inticate as life? Well, initially at least, it was dark matter. If you took our Universe, kept the overall geometry and initial conditions the same, and just removed the dark matter, it’s hard to understand how anything as complex as life could have developed. As mysterious and removed as it seems, dark matter, according to standard cosmology, has been absolutely essential for life.

    Let’s start with the Big Bang. It served up a pretty bland primordial soup as far as the matter we’re used to is concerned: a smooth ionized gas consisting of hydrogen and helium. No big atoms, certainly no molecules or planets. If our story ended here then we’d have the most boring universe imaginable. Hydrogen and helium don’t allow much in the way of complexity. You can’t build a self-replicating cell out of hydrogen and helium, much less a dinosaur or an upright ape. But flash forward 13.8 billion years and we’ve got complex structure everywhere, a Galaxy filled with planets, and at least one place where life has blossomed on the back of carbon-based chemistry.

    The 6 most important elements for life on Earth. Only one (H) was given to us in the Big Bang.(No image credit)

    None of this would have happened if the Big Bang had emerged with only hydrogen and helium. Thankfully our Universe was also born with an additional kind of matter, one that does not appear on the periodic table: dark matter. The dark matter is important because of its gravity. While it doesn’t reflect light or interact strongly with normal matter, it does have mass. Importantly, there is about five times as much dark matter as normal matter. Without the extra gravitational tug from dark matter we would not exist.

    While stars are ultimately responsible for forging elements heavier than hydrogen and helium, dark matter has been the prime factor allowing stars to form in the first place. On top of this, the dark matter that surrounds galaxies today is also essential for recapturing elements that get blown out by supernovae into intergalactic space. Dark matter allows these elements to get recycled back into galaxies, where they can be put to good use making new stars, planets, and (in a few cases) astronomers. Let’s dig a little deeper to see how this actually works.

    All sky view of our home galaxy, the Milky Way. Brought to you by dark matter. http://apod.nasa.gov/apod/image/1105/3000_CC_BY-NC.jpg
    In a big picture sense, modern cosmology tells us that our Universe is governed by two mysterious substances — dark matter and dark energy — locked in an epic battle to shape the character of our cosmos.

    The Ancient and Medieval cosmos as depicted in Peter Apian’s Cosmographia (Antwerp, 1539).

    Schema huius præmiʃʃæ diuiʃionis Sphærarum.
    The scheme of the aforementioned division of spheres. · The empyrean (fiery) heaven, dwelling of God and of all the selected · 10 Tenth heaven, first cause · 9 Ninth heaven, crystalline · 8 Eighth heaven of the firmament · 7 Heaven of Saturn · 6 Jupiter · 5 Mars · 4 Sun · 3 Venus · 2 Mercury · 1 Moon
    28 December 2005
    from Edward Grant, “Celestial Orbs in the Latin Middle Ages”, Isis, Vol. 78, No. 2. (Jun., 1987), pp. 152-173.

    Dark matter plays the role of Creator: its gravity is pulling sections of the Universe to buckle back on itself, forming galaxies along the way. Dark energy is doing just the opposite. It’s fighting the collapse by propelling the universe to expand at an ever faster rate. Luckily for us, dark matter has been winning for most of cosmic time, particularly in the all-important early stages. Our Galaxy, the Milky Way, would have never collapsed out of the expanding rush of the Big Bang without the aid of dark matter’s pull. That means no Sun, no Earth, and no you.

    About 14 billion years ago, when that soup of hydrogen, helium, and dark matter emerged from the Big Bang, everything was expanding. This isn’t the best situation for building complexity. No prokaryote is going to spontaneously emerge in a Universe consisting of hydrogen atoms flying away from each other in an expanding horde.

    A cosmological simulation of dark matter growing clumpier over time. Image credit: Andrey Kravtsov

    But not every part of the Universe kept expanding. Though born smoother than the calmest sea, our Universe was not perfectly flawless from the beginning. There were tiny irregularities — 0.001% in density — that began to grow over time because of gravity. Areas with more matter attracted even more over time. The dark matter played a key role: it provided extra mass and made structure grow much faster than it would have otherwise. The dark matter also remained much clumpier in the beginning than the normal matter for another reason: it doesn’t interact with light. Early on, the blindingly bright ambient photons left over from the big bang scattered off of protons, smoothing out the distribution. This process (called Silk Damping) took an already smooth distribution of normal matter and made it even smoother. Light can’t scatter off of dark matter, so the dark matter remained relatively clumpy on the length scales that would eventually grow into galaxies.

    Cosmic Microwave Background  Planck
    Cosmic Microwave Background per ESA/Planck

    Had there been no dark matter in the beginning, there would have been a much lower level of primordial structure and much less gravity to make those tiny imperfections grow. The resulting universe today would be unrecognizable. Virtually nothing akin to the galaxies we know would exist and dark energy would have won out long ago to prevent new structure formation. Instead, as time went on, gravity dragged more and more dark matter into the places that started off mildly over-dense. Eventually, those pockets of extra mass broke away from the general expansion, funneling gas and dark matter to collapse back in on itself. Galaxies began to form within those pockets of dark matter while the space in between galaxies kept right on expanding.

    The original primordial soup was pretty bland. Modified from an image taken from http://www.mbio.ncsu.edu/jwb/soup.html

    These pockets of collapsed dark matter — dark matter halos — govern where galaxies form. Stars in galaxies convert hydrogen and helium into ever heavier elements, including the carbon, nitrogen, and oxygen that are essential for life on Earth. But when massive stars explode as supernovae they expel key elements for life back out into space. They are launched with immense speeds, hundreds of kilometers per second, and have so much energy that they would be blown out of their host galaxies forever without the huge gravitational cocoon of their dark matter halos to trap them.

    Dark matter halos provide gravitational cocoons around galaxies. They capture heavy elements blown out of supernovae allowing them to fall back in, building ever richer reservoirs of the heavy elements essential for life. Credit: STScI

    Dark matter around galaxies allows much of the ejecta from supernovae to recycle back into the next generation of stars and planets rather than escape into intergalactic space. Galaxies become steadily enriched with heavy elements over time and develop a sort of galactic ecosystem that contains heavy atoms and even complex organic molecules. Our home planet — with its rocky surface and liquid water — would likely never have taken shape without the Milky Way’s dark matter halo to trap escaping material.

    Over the last 14 billion years or so, dark matter has been driving the Universe to ever increasing levels of complexity. Early on, it easily won its battle with dark energy in this regard. But if indications are correct, all of this is about to change. Relative to dark matter, the effect of dark energy is growing stronger with time. Dark matter is doomed to lose its cosmic arm wrestling match in a big way. When the Universe is a few times its current age, virtually all new galaxy formation will cease.

    Because of dark energy’s inevitable triumph, our Universe is approaching a pinnacle in complexity. At some point in the future, fresh fuel for star formation will stop falling into galaxies and no new stars will be able to form. This will represent a high-point for the likelihood of life in our Universe as well. Just before the rate of star formation drops away to a negligible rate, the Universe will be flush with heavy atoms and complex molecules. It will be a last triumphant opportunity for our Universe to produce life, and maybe even a few critters to look up at the stars and wonder how they came to be.

    See the full article here.

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    Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible.

  • richardmitnick 1:46 pm on November 21, 2014 Permalink | Reply
    Tags: , Dark Energy/Dark Matter, Fermilab SIDET   

    From FNAL- “Frontier Science Result: Dark Matter R&D Getting to know dark matter’s traces” 

    Fermilab is an enduring source of strength for the US contribution to scientific research world wide.

    Friday, Nov. 21, 2014
    Federico Izraelevitch, Fermilab, and Marco A. Reyes, University of Guanajuato

    When an energetic particle — perhaps a dark matter particle — interacts with the nucleus of an atom, the nucleus can recoil. Some fraction of the energy transferred to the recoiling nucleus disturbs electrons in adjacent atoms, producing free electric charge. This fraction is called ionization efficiency. The bigger this number, the larger the signal in the detector and the easier it is to detect nuclear recoils.

    Ionization efficiency measurements at low energies are important to calibrate the energy measurement of silicon detectors used in dark matter direct-detection experiments. The calibration will also help experiments trying to observe coherent neutrino scattering, such as CONNIE, which is at a nuclear power plant in Angra dos Reis, Brazil.

    At low energies, the current best measurements of the ionization efficiency in silicon have considerable uncertainty.

    Scientific charge-coupled devices (better known as CCDs) made of silicon are now able to detect a few electronvolts of ionization energy. These detectors can detect low-energy nuclear recoils where the ionization efficiency has never been measured. A test beam apparatus, shown in the picture below, will provide a measurement of the ionization efficiency in silicon for low recoil energies, in the range of 1 to 30 kiloelectronvolts, or keV.

    Scientists working on today’s result stand next to the experiment’s scintillator bar array, located at SiDet at Fermilab. From left: Andrew Lathrop (Fermilab), Federico Izraelevitch (Fermilab), Marco Reyes (University of Guanajuato), Gustavo Cancelo (Fermilab), Gaston Gutierrez (Fermilab), Junhui Liao (University of Zurich) and Juan Estrada (Fermilab). Inset, from left: Javier Tiffenberg (Fermilab), Vic Scarpine (Fermilab), Leonel Villanueva-Rios (University of Guanajuato), Jorge Molina (National University of Asuncion), Alex Kavner (University of Michigan) and Dante Amidei (University of Michigan).


    The test beam, located at the Institute for Structure and Nuclear Astrophysics at the University of Notre Dame, provides 30- to 600-keV neutrons. The neutrons scatter off a silicon detector and are measured by an array of plastic scintillators and devices called photomultipliers. Scientists will use this apparatus to determine how the ionization efficiency changes with the lower nuclear recoil energy.

    A preliminary, proof-of-concept run of seven hours using only two scintillator bars generated the result shown in red in the [below] plot. A total of 69 scattering neutron events were used in the measurement. Scientists compared the data with simulations using a theoretical model developed by Lindhard, et al, in 1963. The measurement produced the preliminary result shown by the red solid line in the plot.

    The ionization efficiency for silicon is plotted as a function of nuclear recoil energy. The black line and dots with error bars show the best measurements to date. The solid red line shows our fit to preliminary new data, from 2 to 20 keV. The dashed lines display the 1 sigma error bands of a single parameter χ2 fit to the model (developed by Lindhard, et al, in 1963). In our next run we expect these errors, for points every 1 keV, to shrink to the yellow band. The recoil energy range will cover from 1 to 30 keV.

    The team will soon run for two weeks, with a full setup of 21 scintillator bars. Calculations and simulations predict a collection of about 1,000 neutron events. With these statistics the errors bars will be reduced from the red dashed lines to the yellow band shown in the plot.

    See the full article here.

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  • richardmitnick 4:45 pm on November 13, 2014 Permalink | Reply
    Tags: , Dark Energy/Dark Matter, DM-Ice, , WIPAC   

    From IceCube: “DM-Ice collaborators discuss dark matter search” 

    IceCube South Pole Neutrino Observatory

    Deep in the ice at the South Pole, the IceCube Neutrino Observatory sits and waits for high-energy particles to pass in its midst. However, another detector, DM-Ice, is situated among IceCube’s strings, partnering with its technology for a different purpose: the search for dark matter. Currently, the only detector to make a strong claim to have seen a dark matter signal is DAMA, at the Gran Sasso National Laboratory in Italy. DM-Ice aims to carry out a definitive test of DAMA’s claim.

    DM-Ice at IceCube

    DAMA at Gran Sasso
    DAMA II at Gran Sasso
    DAMA at Gran Sasso

    Among the established WIPAC community are two enthusiastic physicists who can reveal some of the mystery behind the search for dark matter. Reina Maruyama of Yale University has been the principal investigator of DM-Ice since its inception in 2010, during her time as a WIPAC researcher. Among her team of collaborators is Matt Kauer, a postdoc at WIPAC, who is currently working to advance the development of the detector to its full scale.

    Matt Kauer and Reina Maruyama at WIPAC during a meeting of the DM-Ice Collaboration.

    Q: Can you explain to us what dark matter is?

    Matt (M): Dark matter makes up approximately 27% of all matter and energy in our universe right now. But, in fact, no one really knows what this matter is, where it comes from, or what it interacts with. There are a handful of theories explaining what dark matter could be, but we have yet to confirm its origin and nature.

    Reina (R): From the luminosity of stars, we can infer their mass. From measuring the speed of rotation of stars, we can also infer their mass and the mass of objects that they rotate around. The second measurement gives us masses much higher than the first, which leads us to believe that there must be much more mass out there than we can see. We call this dark matter. There are other observations that point toward the existence of this invisible mass, like seeing light from distant stars bent around invisible objects. The question is, what is this matter. One of our favorite hypotheses is the so-called WIMP (Weakly interacting massive particles) model. If the dark matter we see out there is made of WIMPs, we might be able to see their interaction with ordinary matter, even if this happens very occasionally.

    DM-Ice detector and dark matter modulation explained. Graphic: Jamie Yang/WIPAC.

    Q: What is the goal of DM-Ice?

    R: DM-Ice really started with a request from the dark matter community. For the last 15 years, the DAMA collaboration has claimed that they have observed dark matter. Their signature is coming from an annual modulation in the number of observed dark matter induced interactions in the DAMA detector, due to the orbit of the Earth around the Sun. The flux of dark matter from the galactic halo on Earth should be higher in early June, when the rotational speed of our planet is added to that of the Sun with respect to the galaxy. In early December, when these two velocities are in the opposite directions, the dark matter signature should be smaller. Since DAMA announced these results, there have been ongoing discussions in the community about whether what DAMA is seeing is really a dark matter signature or just some background fluctuation.

    M: DAMA is located in Italy, under the Gran Sasso Mountain, while DM-Ice is buried in the Antarctic ice at the South Pole. From our location, we have a reversed phase of environmental backgrounds with respect to the Northern Hemisphere while the dark matter signature is the same in both hemispheres. Thus, seeing an annual modulation with DM-Ice that’s consistent with DAMA’s dark matter signature would be a smoking-gun confirmation.

    R: Right. DAMA’s results have been out there for a very long time, and there are many concerns that the dark matter community has expressed about them. Although the DAMA collaboration has tried to address every concern that people have raised, the truth is that there are many things that can vary annually. We’re trying to look for a very, very small signal, and there are many possibilities that could mimic the signature that DAMA sees.

    Q: A deployment at the South Pole is never a simple task. What’s the story behind DM-Ice?

    R: The idea came when IceCube was being deployed, back in 2009. Having a detector in the Southern Hemisphere is a great choice, since it allows a cross check of systematics. Francis Halzen (the IceCube PI) and I talked a lot about it, and finally I agreed to at least take a look and assess if it was feasible. I first thought it was the craziest idea, but then I went to the Pole for work related to IceCube and saw what it’s like to work there. And I saw how fantastic this team was, and I came back thinking that this was actually possible. And that’s what we did; we put together a prototype for DM-Ice, starting from scratch.

    M: Quickly obtaining NaI crystals for the detector seemed challenging, but there was an elegant solution. The NAIAD experiment was an old dark matter experiment from the early 2000’s in the Boulby mine in the U.K. The experiment had been decommissioned but the crystals were still in storage at the mine, so we talked with them, and they shipped us two of their crystals for DM-Ice. Those are the crystals now taking data at the South Pole.

    Q: And all this happened very fast, didn’t it?

    R: Yes. DM-Ice was designed and built in nine months and we deployed the prototype during the next polar season at the end of 2010, the final IceCube construction season. The result of this intense year of work is what’s operating at the South Pole now.

    M: It’s pretty amazing. The teams at PSL (Physical Sciences Laboratory), WIPAC, and in general the IceCube community, made this possible. They supported us with the design and manufacturing of the material components and electronics. The logistics of getting an experimental apparatus to the South Pole requires a lot of coordination. We work with IceCube and WIPAC to maintain the data acquisition electronics at the South Pole.

    Q: When we read about DM-Ice we learn that it’s a sodium iodide detector. How exactly does a sodium iodide detector work?

    R: Sodium iodide detectors have existed for the last 50 or more years. This crystal is transparent, dense and has low backgrounds, all of which are important properties when you are trying to look for interactions of yet-to-be-observed particles that very rarely interact. And when they do interact, they could look like interactions induced by well-known particles.

    The detection principle is quite simple. We measure dark matter interactions by recording the recoil of target nuclei scattered by a WIMP. When the sodium iodide nuclei get a kick from scattered WIMPs, they would essentially excite electrons in the detector. As the electrons decay back down to their ground state they emit light. Then we collect that light using photomultiplier tubes (PMTs), just like in IceCube, and depending on how many photons come out, it could tell us the energy of that interaction.

    Q: What is the difference between a dark matter reaction and just another particle reaction?

    M: The amplitude and shape of the interaction are the relevant parameters. With a typical dark matter interaction in DM-Ice, we expect on the order of 100 to 200 photons to be emitted during the nuclear relaxation. This translates to a very small energy range we’re interested in. The shape of the signal, or the time-scale over which the photons are emitted, also provides information about the type of particle interaction being observed.

    R: Detecting this collision with a very distinct energy signature would be an indication of dark matter, but on top of that, if we can observe the annual modulation we have mentioned, with the correct phase and correct rate, then we have an additional signature for dark matter.

    Maruyama at South Pole for DM-Ice deployment. Image: DM-Ice Collaboration.

    Q: So, what is the detector’s current status as of 2014?

    M: DM-Ice 17 is taking data right now in the ice at the South Pole, mainly as a proof of concept for a full-scale deployment in the ice. We now have 17.5 kilograms of target material from the crystals we inherited from NAIAD, but these crystals are a little too high in internal backgrounds for a competitive analysis. We are currently collaborating with vendors to develop much cleaner crystals for use in the full-scale detector.

    R: As Matt says, our prototype is too small and too high in background to really be able to test DAMA, but we have proved that we can deploy and operate a dark matter detector at the Pole. The challenge is now to build the full-scale detector, which would be sensitive enough to see what DAMA sees. We have good teams at Yale, WIPAC, and other places in the U.S., Canada, and the U.K. contributing to our efforts.

    M: Here at WIPAC, the DM-Ice team consists of six people, contributing through different analysis and R&D projects geared toward the full-scale 250kg detector. We are, for example, working with two prototype crystals that we have underground at FNAL in Chicago and measuring the potassium backgrounds in those crystals.

    Q: What is the near future for DM-Ice?

    R: Our job is to be ready when IceCube is ready to drill again at the Pole, hopefully deploying the planned detector extensions. When IceCube drills again, we will have improved DM-Ice detectors that can go in the ice as well. In the meantime, we will run a similar sort of DM-Ice detector in the Northern Hemisphere. It would be a test to reproduce what DAMA found with an independent detector. However, we might just find the same result that DAMA did, without really learning much more about its origin.

    The original idea was to put this detector at the South Pole because it is really the ultimate test. If we see the same signature as DAMA, it would be very difficult to attribute it to the seasons. If we don’t see the same annual modulation that DAMA sees, then the scenario of it being dark matter can be ruled out, even if we don’t know the origin of that signal. Basically, we would be able to confirm or rule out DAMA’s claims of a dark matter observation.

    Q: Can you tell us more about what we can learn from a northern detector?

    R: There are different scenarios that could come from a northern deployment. You see no annual modulation, or you see the same signature as DAMA. If the signature is there, we might be able to test some background hypotheses to figure out what is there aside from dark matter. But we might also end up with a dark matter-only possible scenario, as DAMA did. I think we still have to bring this detector to some other location to verify that the dark matter signature phase stays the same everywhere on Earth to confirm that it’s due to dark matter. In summary, we might learn a few things from a northern run or we might not, but if we go straight to the Southern Hemisphere, then it’s one shot and we would have a definitive answer.

    Q: Will WIPAC be an important partner for a northern detector as well?

    M: Oh yes! WIPAC and our collaborators at the PSL, also at UW–Madison, contribute far beyond the South Pole expertise and logistics.

    R: I would say that’s what is unique about WIPAC and the University of Wisconsin—the existence of a scientific institution coupled with a very good technical and university-oriented engineering center. Being able to build big things at a university is rare, and I think that’s why IceCube was successful and why the DM-Ice demonstrator was possible. Yale also has similar capabilities, and together there is great intellectual and technical support behind DM-Ice.

    The team of collaborators working alongside Maruyama and Kauer include distinguished WIPAC physicists Francis Halzen and Albrecht Karle, and engineers Perry Sandstrom and Jeff Cherwinka, as well as students Antonia Hubbard, Walter Pettus, Bethany Reilly, and Zack Pierpoint from the UW–Madison and Yale communities.

    See the full article here.

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    ICECUBE neutrino detector
    IceCube is a particle detector at the South Pole that records the interactions of a nearly massless sub-atomic particle called the neutrino. IceCube searches for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The IceCube telescope is a powerful tool to search for dark matter, and could reveal the new physical processes associated with the enigmatic origin of the highest energy particles in nature. In addition, exploring the background of neutrinos produced in the atmosphere, IceCube studies the neutrinos themselves; their energies far exceed those produced by accelerator beams. IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice.

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  • richardmitnick 5:20 pm on November 12, 2014 Permalink | Reply
    Tags: , , , , Dark Energy/Dark Matter,   

    From SPACE.com- “Dark Matter’s New Wrinkle: It May Behave Like Wavy Fluid” 

    space-dot-com logo


    November 12, 2014
    Charles Q. Choi

    The mysterious dark matter that makes up most of the matter in the universe may behave more like wavy fluids than solid particles, helping to explain the shapes of galaxies, a new study suggests.

    (a) This figure shows that a comparison of the distribution of matter is very similar on a large scale between wave dark matter, the focus of this research, and the usual dark matter particle. (b) This figure shows that in galaxies the structure is very different in the interpretation of the wave, which has been carried out in this research; the research predicts the soliton of dark matter in the center surrounded by an extensive halo of dark matter in the form of large “spots,” which are the slowly fluctuating density waves. This leads to many predictions and solves the problem of puzzling cores in smaller galaxies.
    Credit: Broadhurst

    Dark matter is one of the greatest mysteries in the cosmos. It is thought to be an invisible and mostly intangible substance that makes up five-sixths of all matter in the universe.

    The scientific consensus is that dark matter is composed of a new type of particle, one that interacts very weakly with all the known forces of the universe and is mostly only detectable via the gravitational pull it exerts. However, what kind of particle dark matter consists of remains unknown.

    There are two known types of particles in the universe, fermions and bosons. Fermions include particles such as protons, neutrons and electrons, while bosons include particles such as the photons that make up light.

    The mainstream focus for dark matter has been on massive fermions, said study co-author Tom Broadhurst, a cosmologist at the University of the Basque Country in Spain. However, so far these fermion candidates for dark matter have not been generated by the Large Hadron Collider (LHC), the most powerful particle accelerator on Earth, nor have any been confirmed by the Large Underground Xenon (LUX) experiment, the most sensitive dark-matter detector ever built.

    As a result, some researchers have suggested that dark matter might not be made of extremely high-mass heavy fermions, but low-mass light bosons instead. For instance, Broadhurst and his colleagues investigated the behavior of a boson with a mass of less than 10^-22 electron-volts, or less than a tenth of a billionth of a billionth of billionth the mass of an electron.

    The difference between fermions and bosons is that a fermion cannot occupy the same state at the same time as another fermion. As an analogy, a state is like a seat, and two or more fermions cannot sit in the same seat simultaneously. In contrast, two or more bosons can occupy the same state at the same time, and can therefore clump into so-called Bose-Einstein condensates that act like single blobs.

    Now, Broadhurst and his colleagues have for the first time simulated what galaxies might look like if dark matter was made of light bosons. They said their models more accurately reflect what galaxies actually look like than more conventional models where dark matter is made of fermions.

    The researchers investigated dwarf spheroidal galaxies, the smallest and most common class of galaxy, which have centers with masses equal to about 10 million suns. The basic properties of dwarf spheroidal galaxies are very difficult to explain with simulations in which dark matter is made of heavy fermions; these models suggest that much smaller galaxies should exist than what astronomers see, and that dark matter in dwarf spheroidal galaxies should be much less smoothly distributed than what is observed.

    Broadhurst and his colleagues simulated the way the gravitational pull of dark matter Bose-Einstein condensates influences the evolution of galaxies. They found these simulated blobs of dark matter led to galaxies that better matched the ones that astronomers see.

    The scientists found these dark matter Bose-Einstein condensates are full of waves. Stable waves known as soliton waves are expected in the middle of galaxies, “surrounded by extended lumpy halos of dark matter comprised of giant quantum density fluctuations that fluctuate over time,” Broadhurst said. This behavior can help explain both the size of the dwarf spheroidal galaxies seen and why dark matter is distributed relatively smoothly within them.

    Another consequence of dark matter Bose-Einstein condensates is that galaxy formation should have begun about 330 million years after the Big Bang. This is substantially delayed compared to models that envision dark matter being made of fermions, which suggest that galaxy formation should have begun about 50 million years after the Big Bang. Future observations by NASA’s Hubble Space Telescope could help determine whether dark matter consists of fermions or bosons, study team members said.

    Broadhurst and his colleagues Hsi-Yu Schive and Tzihong Chiueh detailed their findings in June in the journal Nature Physics.

    See the full article here.

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  • richardmitnick 2:23 pm on November 8, 2014 Permalink | Reply
    Tags: , Axion Dark Matter Experiment, , Dark Energy/Dark Matter, LUX-ZEPLIN, , SuperCDMS   

    From Kavli: “New Dark Matter Experiments Prepare to Hunt the Unknown” 


    The Kavli Foundation

    Kelen Tuttle (Fall 2014)

    Three astrophysicists – Enectali Figueroa-Feliciano, Harry Nelson and Gray Rybka – discuss preparations for three recently funded dark matter experiments, and the likelihood that one of them will strike gold.

    THIS MONTH, THREE NEW EXPERIMENTS take significant steps in the hunt for dark matter, the elusive substance that appears to make up more than a quarter of the universe, but interacts very rarely with the matter that makes up our world. The experiments – the Axion Dark Matter eXperiment Gen 2, LUX-ZEPLIN and the Super Cryogenic Dark Matter Search at SNOLAB – learned in July that each would receive much needed funding from the U.S. Department of Energy and the U.S. National Science Foundation. Each of these “second-generation” experiments will be at least 10 times more sensitive than today’s dark matter detectors, increasing the likelihood that they will see the small, rare interactions between dark matter and the regular matter we all interact with every day.

    As the experimental plans start to coalesce and detector equipment starts to arrive for ADMX Gen2, LZ and SuperCDMS SNOLAB, three scientists discuss the likelihood that these projects will at long last discover dark matter. The participants:

    ENECTALI FIGUEROA-FELICIANO – is a member of the SuperCDMS collaboration and an associate professor of physics at the MIT Kavli Institute for Astrophysics and Space Research.
    HARRY NELSON – is the science lead for the LUX-ZEPLIN experiment and is a professor of physics at the University of California, Santa Barbara.
    GRAY RYBKA – leads the ADMX Gen 2 experiment as a co-spokesperson and is a research assistant professor of physics at the University of Washington.

    THE KAVLI FOUNDATION: We know that dark matter is five times more prevalent than ordinary matter, and we’re able to infer that clumps of dark matter help hold together clusters of galaxies. So this substance is a huge part of what makes up our universe and an important part of why our universe looks the way it does. Why, then, haven’t we been able to observe it directly? What’s holding us back?

    HARRY NELSON: A big part of the challenge is that dark matter doesn’t interact with us very much. We know that dark matter is passing through our galaxy all the time, but it doesn’t disrupt the type of matter we’re made of.

    But more than that, dark matter doesn’t interact with itself very much either. The matter that we see around us every day interacts with itself: Atoms form molecules, the molecules form dirt, and the dirt forms planets. But that’s not the case with dark matter. Dark matter is widely dispersed, and doesn’t form dense objects like we’re used to. That, combined with the fact that it doesn’t interact with our type of matter very often, makes it hard to detect.

    ENECTALI FIGUEROA-FELICIANO: What Harry says is exactly right. In my mind, nature is being coy. There’s something we just don’t understand about the internal structure of how the universe works. When theorists write down all the ways dark matter might interact with our particles, they find, for the simplest models, that we should have seen it already. So even though we haven’t found it yet, there’s a message there, one that we’re trying to decode now.

    Gray RybkaGray Rybka leads the ADMX Gen 2 experiment as a co-spokesperson and is a research assistant professor of physics at the University of Washington.

    TKF: In fact, nature is being so coy that we don’t yet even know what dark matter particles look like. Gray, your experiment – ADMX – looks for a different particle altogether than the one that Tali and Harry look for. Why is that?


    GRAY RYBKA: As you say, my project — the Axion Dark Matter eXperiment, or ADMX —searches for a theoretical type of dark matter particle called the axion, which is extremely lightweight with neither electric charge nor spin. Harry and Tali look for a different type of dark matter called the WIMP, for Weakly Interacting Massive Particle, which describes a number of theorized particles that interact with our world very weakly and very rarely.

    Both the WIMP and the axion are really good dark matter candidates. They’re especially great because they would explain both dark matter and other mysteries of physics at the same time. I suppose I like the axion because there aren’t a lot of experiments looking for it. If I’m going to gamble and spend a lot of time making an experiment to look for something, I don’t want to look for something that everyone else is looking for.

    We’ve been updating the ADMX experiment since 2010 and have demonstrated that we have the tools necessary to see axions if they are out there. ADMX is a scanning experiment, where we scan the various masses this axion could have, one at a time. How fast we scan depends on how cold we can make the experiment. With Gen2, we’re buying a very, very powerful refrigerator that will arrive next month. Once it arrives, we’ll be able to scan very, very quickly and we feel we’ll have a much better chance of finding axions – if they’re out there.
    “In my mind, nature is being coy. There’s something we just don’t understand about the internal structure of how the universe works… there’s a message there, one that we’re trying to decode now.” —Enectali Figueroa-Feliciano

    TKF: And, Harry, why do you bet on the WIMP?

    Harry Nelson leads the LUX-ZEPLIN experiment and is a professor of physics at the University of California, Santa Barbara. (Image: Sonia Fernandez/UCSB)

    NELSON: Even though I’m betting on WIMPs, I like axions too. I even wrote some papers on axions way back when. But these days, as Gray said, I look for WIMPs. My collaboration is currently operating the Large Underground Xenon, or LUX, experiment in the famous Black Hills of South Dakota, inside a mine that was the outgrowth of the 1876 gold rush that formed the city Deadwood. This month, we start our 12-month run with LUX. We’re also now carefully developing our plans to upgrade our detector to make it more than 100 times more sensitive for the new LUX-ZEPLIN project.

    But to tell you the truth, I actually have a little bit of the attitude that all of these possibilities are unlikely. I’m not saying that hunting for them is worthless; that’s not it at all. It’s just that nature doesn’t have to respect what physicists want. We desire to better understand our own strong interaction, the mechanism responsible for the strong nuclear force which holds the atomic nucleus together. The axion would help do that.

    The WIMP is great because it’s consistent with the physics of the Big Bang in a straightforward way. A lot of science is based on what’s called Occam’s razor: We make the simplest possible assumptions and then test them very well, and only give up simplicity if we absolutely need to. I’ve always felt that the WIMP is a tiny bit simpler than the axion. Both are unlikely, but are still the best candidates we can think of. It’s probably more likely that dark matter is somewhat different than either the WIMP or the axion, but we must start somewhere and the WIMP and axion are the best starting points we can imagine.

    TKF: If you think it’s unlikely that the WIMP is out there, why do you look for it?

    NELSON: The WIMP and axion have the absolute best theoretical motivations. And so it’s great that both WIMPs and axions have really strong experiments going after them.

    Tali Figueroa-Feliciano​Enectali (Tali) Figueroa-Feliciano is a member of the SuperCDMS collaboration and an associate professor of physics at the MIT Kavli Institute for Astrophysics and Space Research.


    FIGUEROA-FELICIANO: As an experimentalist, I come at this from the point of view that theorists are very clever, and have come up with an incredible array of possible scenarios for what dark matter could be. And, as Harry said, we attempt to use Occam’s razor to try to weed out which of these things are more probable than the others. But that’s not an infallible way to go about it. Dark matter might not follow the simplest explanation possible. So we have to be a little agnostic about it.

    In a way it’s like looking for gold. Harry has his pan and he’s looking for gold in a deep pond, and we’re looking in a slightly shallower pond, and Gray’s a little upstream, looking in his own spot. We don’t know who’s going to find gold because we don’t know where it is.

    That said, I think it’s really important to stress how complementary these three searches are. Together, we look in a lot of the places where dark matter could be. But we certainly don’t cover all of the options. As Harry says, it could be that dark matter is there, but our three experiments will never see anything because we’re looking in the wrong place – it could be in another fork of the river, where we haven’t even started looking yet.

    RYBKA: I look at it a bit more optimistically. Although as Tali said all the experiments could be looking in entirely the wrong place, it’s also possible that they’ll all find dark matter. There’s nothing that would require dark matter to be made of just one type of particle except us hoping that it’s that simple. Dark matter could be one-third axions, one-third heavy WIMPs and one-third light WIMPs. That would be perfectly allowable from everything we’ve seen.

    FIGUEROA-FELICIANO: I agree. I should have said that the gold nugget we’re looking for is a very valuable one. So even though the search is hard, it’s worthwhile because we’re looking for a very valuable thing: to understand what dark matter is made of and to discover a new part of our universe. There’s a very beautiful prize at the end of this search, so it’s absolutely worthwhile.

    Dark matter in galaxy clusterThe inferred distribution of dark matter is superimposed in purple over this Hubble Space Telescope image of galaxy cluster Abell 1689. (Image: NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) & J-P. Kneib (LAM))

    TKF: Tali, tell us a little about the pond where you’re panning for that very valuable nugget of dark matter.

    FIGUEROA-FELICIANO: My experiment is currently running in Soudan, Minnesota, inside a mine that’s a bit over half a kilometer (at 2,341 feet) underground. This experiment, called SuperCDMS Soudan, was designed to demonstrate a new technology we’ve been developing that allows us to search for WIMPs that are on the lighter-mass side. It turns out that certain classes of WIMPs, ones that are lighter than Harry searches for, deposit very little energy into detectors. Our detectors are able to distinguish very small amounts of energy deposited in the detector from all the many different signals that we get from radioactive materials, cosmic rays, and all sorts of other things that stream though our detectors. Being able to make that separation is very important, both for SuperCDMS and for LZ.

    “There’s nothing that would require dark matter to be made of just one type of particle except us hoping that it’s that simple.” —Gray Rybka

    The next step for our experiment is called SuperCDMS SNOLAB. SNOLAB is a nickel mine in Canada that’s 2 kilometers (6,531 feet) deep. We’ve been approved to build a brand new experiment down there to search for these low-mass WIMPs. Also, if LUX or LZ sees a higher mass WIMP, we’ll be able to check that measurement. Right now, we’re in the process of finalizing the design and taking the first steps of putting this brand-new SNOLAB experiment together. We expect to have a first phase of detectors in the next couple of years.

    TKF: If one of your experiments finds evidence of dark matter, after the celebratory champagne, what would be the next steps?

    LUX physicist Jeremy Mock inspected the LUX detector before the large tank was filled with more than 70,000 gallons of ultra pure water. The water shields the detector from background radiation. (Image: Matt Kapust, Sanford Underground Research Facility)

    RYBKA: Bottle it and sell it, I guess! But really, I’d say that all of the experiments would need to keep going even after such a discovery, until someone could conclusively prove that the discovered dark matter makes up 100 percent of all the dark matter in the universe.

    NELSON: I would agree with that. We would also need to dig in and really try to understand what we discovered. There’s an old saying in particle physics that you haven’t discovered a particle until you know its mass, spin and parity, a property that’s important in the quantum-mechanical description of a physical system. To really discover dark matter, we’ll need to prove that it’s what we think it is, and we’ll need to learn its characteristics. After you discover a particle, everyone gets a lot smarter at what to do with it. This has been going on with the Higgs boson lately. Folks at the Large Hadron Collider are getting cleverer because now that they’ve seen the particle, they can focus on interrogating it.

    When we start to do that with dark matter, we’re going to see something new. That’s just how scientific progress works. Right now, we can’t see through the wall because we haven’t figured out what the wall is made of. But once we understand what’s in the wall – my analogy for dark matter – we’ll see through it and see to the next thing.

    FIGUEROA-FELICIANO: Let me add my two cents to that. There are three different things that I think would happen if one of our experiments saw convincing evidence for dark matter. First, we would want to confirm the discovery using a different technique. In other words, we will want as much confirmation as we can before we declare victory.

    SuperCDMS experiment at the Soudan Underground Laboratory uses five towers like the one shown here to search for WIMP dark matter particles. (Image: SuperCDMS Soudan collaboration)

    Then, people will come up with 100 different ways to test the particle’s properties, as Harry described. After that, a phase of “dark matter astronomy” will help us learn the particle’s role in the universe. We’ll want to measure how fast it’s going, how much of it there is, how it behaves in a galaxy.

    TKF: There’s clearly a lot to be done once we find even just one type of dark matter particle. But it sounds like there could be a whole new zoo of dark particles. Do you think we’re going to need a “Dark Standard Model”?

    NELSON: I’ve often had the following thought: Here we are, in our measly 15 percent of the matter in the universe, wondering what dark matter is. If dark matter is as complex as we are, it might not even know that we exist. We’re just this minority 15 percent, but somehow we think we’re so important. But experiments undertaken by dark matter might not even know that we exist because we’re a much smaller perturbation on dark matter’s world than dark matter is on us.

    The dark matter sector may be as complex – or perhaps even five times as complex – as ours. Just as we’re made mostly of atoms made up of electrons and nuclei, maybe dark matter is too. In some of the searches for WIMPs, you have to be careful about that. It may be that the way these things interact with our matter is rather different than the simplest possible case that we’re looking for.
    “Here we are, in our measly 15 percent of the matter in the universe, wondering what dark matter is. If dark matter is as complex as we are, it might not even know that we exist.” —Harry Nelson

    Gray Rybka (left) and Leslie Rosenberg examine ADMX’s primary components. (Image: Mary Levin / University of Washington)

    FIGUEROA-FELICIANO: Harry, if you were to apply Occam’s razor to our universe, how does it fare with the Standard Model?

    NELSON: Well, it doesn’t do very well. The Standard Model is a lot more complex than it needs to be. So maybe the same is true for dark matter. Maybe there are even dark photons out there. The idea is interesting. With ADMX, Gray is looking for a particle that has to do with the strong interaction. Tali and I are looking for a particle that has to do with the weak interaction. And searches for the dark photon look for a relationship between the electromagnetic interaction and the dark matter sector.

    The community really wants to figure out dark matter. There’s a feeling of urgency about it, and we’ll look for it in all the ways that we can.

    RYBKA: It’s true. With ADMX, we’re mostly focused on the axion, but we also look for dark photons at the lower masses. There are the dark matter candidates that people are really, really excited about, like axions and WIMPs. Those get experiments built that are dedicated to them. And then there are the ideas that might be good but don’t have quite as much motivation, like dark photons. People still look for ways to test those ideas, often with existing experiments.

    TKF: It’s clear there are a wide variety of places where we could find dark matter. We’re panning for this gold wherever we can, but we’re not entirely sure that it exists anywhere we’re looking. What’s it like searching for something that you might never find?

    FIGUEROA-FELICIANO: I think that the people who work on dark matter have a certain personality, a bit of a gambler’s streak. We go for the high stakes, putting all the chips in. There are other areas of physics where we would be sure to see something. Instead, we choose to look for something that we might not actually see. If we do see it, though, it’s a huge deal.

    We’re extremely lucky that we actually get paid to try to figure out what the universe is made of. That’s an incredibly wonderful thing.

    NELSON: Sometimes I think of what it must have been like to be Columbus and his crew, or the explorers who first went to the Earth’s poles. They were way out in the middle of the ocean, or in the ice, not quite sure what would come next. But they had set goals: India and China for Columbus, the poles for those explorers. We’re explorers too, we set goals for ourselves too, to seek certain pre-defined sensitivities to dark matter. We’re innovating with modern technology to reach our specific goals. And we may make it the New World or the North Pole, and that’s wonderfully exciting.

    See the full article, with other material, here.

    The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

    The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

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