BlueRose

Plenty of dark matter near the Sun.

Astronomers at the University of Zürich, the ETH Zurich, the University of Leicester and NAOC Beijing have found large amounts of invisible "dark matter" near the Sun. Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive "halo" of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations. The authors also find tantalising hints of a new dark matter component in our Galaxy. The team's results will be published in the journal Monthly Notices of the Royal Astronomical Society.


The high resolution simulation of the Milky Way used to test the mass-measuring technique. Credit: Dr A. Hobbs.

Dark matter was first proposed by the Swiss astronomer Fritz Zwicky in the 1930s. He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart. At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone. In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the Universe, but the amount of dark matter in the solar neighbourhood has remained more mysterious. For decades after Oort's measurement, studies found 3-6 times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The community was left puzzled, generally believing that the observations and analyses simply weren't sensitive enough to perform a reliable measurement.

In this latest study, the authors are much more confident in their measurement and its uncertainties. This is because they used a state-of-the-art simulation of our Galaxy to test their mass-measuring technique before applying it to real data. This threw up a number of surprises. They found that standard techniques used over the past 20 years were biased, always tending to underestimate the amount of dark matter. They then devised a new unbiased technique that recovered the correct answer from the simulated data. Applying their technique to the positions and velocities of thousands of orange K dwarf stars near the Sun, they obtained a new measure of the local dark matter density.

Lead author Silvia Garbari says: "We are 99% confident that there is dark matter near the Sun. In fact, our favoured dark matter density is a little high. There is a 10% chance that this is merely a statistical fluke. But with 90% confidence, we find more dark matter than expected. If future data confirms this high value, the implications are exciting. It could be the first evidence for a "disc" of dark matter in our Galaxy, as recently predicted by theory and numerical simulations of galaxy formation. Or it could be that the dark matter halo of our Galaxy is squashed, boosting the local dark matter density."

Many physicists are placing their bets on dark matter being a new fundamental particle that interacts only very weakly with normal matter -- but strongly enough to be detected in experiments deep underground where confusing cosmic ray events are screened by over a kilometre of solid rock.

An accurate measure of the local dark matter density is vital for such experiments as co-author Prof. George Lake explains: "If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time your finish reading this article. Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of."

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Dark matter is a hot topic. It's there but you can't see it. You can measure it because it's evident that something is there to hold galaxies together and prevent them from just flying apart. What we see in visible matter is not enough to account for the gravity forces required to keep everything together.

There are other forces we know of but can neither measure nor explain yet. Gravity is a prime one. We can see it's effects, we can calculate how much force it provides, but we find it impossible to capture it or what ever force it is to 'see it'.

Or how about dark energy. Where does it come from? Why did the value of it's push change? Dark energy is the supposed force that is causing everything to fly apart in the universe. Taken to the end of entropy, nothing will be near anything else and there will be no energy to get from one place to another, to see any star or body in the heavens nor the ability to actually travel there.

This started with the astronomer by the name of Hershel. Hershel discovered that everything seemed to be moving away from each other. He found this out by measuring what is called red shift. The easiest way to explain this is sound as an analogy. When you hear a train coming, the sound is compressed slightly and the pitch is altered just a bit higher. As it passes and leaves, the sound decompresses, changing again the pitch to a slightly lower tone. Just by hearing this sound, humans can fairly well tell which way it is going in relation to them and some general idea of how fast. Light is much the same with different properties. Blue light is the compressed light coming towards you. Red light, is the decompressed light leaving. Instead of pitch, the light is shifted towards it's value lower for red. So the further it is into ultraviolet, the faster it is going away from you. Through this measurement, it has been found that the most distant and fastest traveling galaxies are redshifted into ultraviolet, no longer in the infrared.

This led to the big debates over the big bang or at the end of it, the big crunch, when everything came back together as a single solid mass containing all mass in the universe in one pin point. So which was it to be? A never ending expansion or a return to what the conditions were, prior to the big bang?

In trying to find this out, a mathematical factor was developed called the Omega factor (Ω). The Ω is a sort of inverse ratio. Less than 1 means that everything will keep expanding. An Ω greater than one means The Big Crunch. Today's guesstimation is somewhere between .6 and .7.

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