First things first, we owe you an apology here at we are all in the gutter. Well, Stuart and I do, for not having followed up with our posts Wednesday and Thursday. We promised to tell you a little bit more about dark matter before the announcement today and alas, we sold you short. We still intend to do so, but in the meantime Friday caught up with us and the Cryogenic Dark Matter Search (CDMS) experiment have announced their results right on the mark and we felt we should tell you what they are.
They’ve published a really good summary here , which I’ll now take the liberty to quote because it’s rather clear.
First, a little bit about the experiment itself:
The Cryogenic Dark Matter Search (CDMS) experiment, located a half-mile underground at the Soudan mine in northern Minnesota, uses 30 detectors made of germanium and silicon in an attempt to detect such WIMP scatters. The detectors are cooled to temperatures very near absolute zero. Particle interactions in the crystalline detectors deposit energy in the form of heat,
and in the form of charges that move in an applied electric field. Special sensors detect these signals, which are then amplified and recorded in computers for later study. A comparison of the size and relative timing of these two signals can allow the experimenters to distinguish whether the particle that interacted in the crystal was a WIMP or one of the numerous known particles that come from radioactive decays, or from space in the form of cosmic rays. These background particles must be highly suppressed if we are to see a WIMP signal. Layers of shielding materials, as well as the half-mile of rock above the experiment, are used to provide such suppression.
The CDMS experiment has been searching for dark matter at Soudan since 2003. Previous data have not yielded evidence for WIMPs, but have provided assurance that the backgrounds have been suppressed to the level where as few as 1 WIMP interaction per year could have been detected.
We are now reporting on a new data set taken in 2007- 2008, which approximately doubles the sum of all past data sets.
One of the hardest things about such an experiment, is to ensure that you know your kit well enough that you can tell the real signal from background noise that come from other sources (i.e., not dark matter):
With each new data set, we must carefully evaluate the performance of each of the detectors, excluding periods when they were not operating properly. Detector operation is assessed by frequent exposure to sources of two types of radiation: gamma rays and neutrons. Gamma rays are the principal source of normal matter background in the experiment. Neutrons are the only type of normal matter particles that will interact with germanium nuclei in the billiard ball style that WIMPs would, although neutrons frequently scatter in more than one of our detectors. This calibration data is carefully studied to see how well a WIMP-like signal (produced by neutrons) can be seen over a background (produced by gamma rays). The expectation is that no more than 1 background event would be expected to be visible in the region of the data where WIMPs should appear. Since background and signal regions overlap somewhat, achievement of this background level required us to throw out roughly 2/3 of the data that might contain WIMPs, because these data would contain too many background events.
A particularly interesting aspect of the data analysis, commonly used for this type of experiment, is that it is blind. The CDMS team explains:
All of the data analysis is done without looking at the data region that might contain WIMP events. This standard scientific technique, sometimes referred to as ‘blinding’, is used to avoid the unintentional bias that might lead one to keep events having some of the characteristics of WIMP interactions but that are really from background sources. After all of the data selection criteria have been completed, and detailed estimates of background ‘leakage’ into the WIMP signal region are made, we ‘open the box’ and see if there are any WIMP events present.
And so, what did they find? In short, they found two events that fit the bill. If these are indeed real events, this has been the first direct detection of Dark Matter in our scientific history. But alas, it’s never that simple. There’s a non-negligible chance, of around 23%, that these two events were created by background sources. I.e., that the signal hitting the detectors was a result of an interaction of particles which have nothing to do with dark matter. Of course, this means there’s a 77% chance that these two events were real, but 77% is generally not considered high enough to make a detection statistical significant in scientific terms. This is what the CDMS team explains:
In this new data set there are indeed 2 events seen with characteristics consistent with those expected from WIMPs. However, there is also a chance that both events could be due to background particles. Scientists have a strict set of criteria for determining whether a new discovery has been made, in essence that the ratio of signal to background events must be large enough that there is no reasonable doubt. Typically there must be less than one chance in a thousand of the signal being due to background. In this case, a signal of about 5 events would have met those criteria. We estimate that there is about a one in four chance to have seen two backgrounds events, so we can make no claim to have discovered WIMPs. Instead we say that the rate of WIMP interactions with nuclei must be less than a particular value that depends on the mass of the WIMP. The numerical values obtained for these interaction rates from this data set are more stringent than those obtained from previous data for most WIMP masses predicted by theories. Such upper limits are still quite valuable in eliminating a number of theories that might explain dark matter.
So why is everyone so excited, if the significance is not enough to break out the champagne? Simply put, because even though none of us is going to take this as scientific proof of dark matter detection, most of us also knows that real detections are often preceded by marginal detections. And that’s exciting. There’s a feeling that we’re really not that far, and 77% can feel very encouraging. In the words of my office mate – 77% has never been more exciting.
So what could all this missing stuff be? To put it simply the two original competing theories were those of MACHOS and WIMPS. MACHOS are Massive Astrophysical Compact Halo Objects, put simply big things in our Galaxy that we can’t see directly. They could be brown dwarfs, black holes or neutron stars. While these don’t give out much optical radiation they can be detected by a novel technique called gravitational microlensing. In this a MACHO briefly gets in-front of a background star and it’s gravity focuses the star’s light towards Earth. This leads to the background star appearing slightly brighter. Studies of this effect such as the OGLE programme, seem to indicate that such objects could only make up a small fraction of dark matter in our Galaxy. Additionally, theorists can use the events shortly after the Big Bang and the observed abundances of light elements like Helium to set limits on how much baryonic (matter made out of protons, neutrons etc.) there is out there. It turns out that it’s only a small proportion of the implied dark matter density, so something else must be at work. By the way, the OGLE project didn’t just not detect a load of black holes, it’s also been very successful in finding planets around other stars.
So the main candidates left are the Weakly Interacting Massive Particles or WIMPS. If these are the main component of dark matter then we can constrain what their properties are without knowing exactly what they are. Firstly we don’t detect loads of interactions between these dark matter particles and ordinary baryonic matter, so they must interact weakly. Secondly we can tell something about how fast they move from how clumpy the Universe is. If the dark matter is slow-moving (aka Cold Dark Matter), then it will form lots of small clumps which can merge together to form galaxies. This implies there should be a lot of small galaxies. If the dark matter moves fast (aka Hot Dark Matter), close to the speed of light, then it should have a smooth distribution with very few clumps, this would lead to very few small galaxies. Scientists can do simulations of the evolution big chunks the universe, starting with some initial conditions and a particular type of dark matter. They let these run and see how the outputs compare with the distribution of matter we see in the Universe. These simulations indicate that Cold Dark Matter is a much better fit than Hot Dark Matter (although there are some details that still don’t quite fit).
So it looks like dark matter is made out of some sort of massive particle, that doesn’t move close to the speed of light and doesn’t interact with normal baryonic matter much.
PS The title of this post isn’t quite right, but I couldn’t forgive Emma for passing up an obvious title for yesterday’s post.
So you’ve just discovered a spiral galaxy with your brand new telescope. Well, it wasn’t you exactly that discovered it, but you have found a pretty picture the Hubble Space Telescope took of it on the internet. Here it is (it’s called M101):
Isn’t that nice?
Now galaxies are pretty similar to celebrities (bear with me, I’ve got a point, really) as once you’ve taken a picture of one, the next thing you want to know is how much mass it has (and whether it’s ever had an affair with Tiger Woods). Easy you think – just count up the mass from all the stars and you’re done. Except, like most things in life, it’s not that simple…
The problem is shown by something called a Rotation Curve – this plots the velocity that stars at different distances from the galaxy’s centre are orbiting round it. Since the majority of the stars are in the central bulge (look at the picture above – it’s brightest in the centre), then you’d expect that the orbital velocities would decrease as you got further out. (This happens in the Solar System – Earth is going round the Sun faster than things further out, like Neptune.) However, when the first Rotation Curves were plotted they were flat, implying that the orbital velocities are the same in the outer regions as they are closer in. This is neatly summed up by this illustration I found at AstronomyOnline:
This suggests that there must be a lot more mass in the galaxy, invisible mass that’s not in the form of stars or clouds of gas or dust, which extends far beyond its visible edges, and can only be detected by its gravitational effect on the ‘normal’ matter its enveloping. This is dark matter, and it looks like it makes up the majority of the mass of the Universe. Which is embarrassing – no scientist likes to admit that they don’t really know what most of the Universe is made of.
Stop by tomorrow to find out what we think we know about this mysterious stuff and what it could be.
Last week a rumour spread round the internet that an experiment based down a mine in the USA (the Cryogenic Dark Matter Search (CDMS), run by the University of California) was about to announce the first direct detection of dark matter, when they present their new datasets this Thursday in fact. Unfortunately it looks like this is probably not true (see the updates on the original post here) but by that point we’d already had the idea to make this ‘Dark Matter Week’ here in the gutter, so we’re going ahead with it anyway! Hopefully over the next few days we’ll explain what we know about dark matter, how we know that it exists at all, whether we’ll ever see it and we’ll then end with whatever announcement CDMS makes.
Or at least that’s the plan.
To kick off things off, here’s one explanation for the existence of dark matter…..
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