Looking at the Dark Matter World

Hi readers of Physics Horizon. It has been a few months since there was the last news post! The reason for this is I have been working hard on my 4th year at University. I hope to be able to bring you more regular news in the coming weeks and I am really glad that people have been looking and using my site during the quiet time on the site.

I thought for the first post of 2015 I would give my readers an overview of what I am currently working on for my 4th year project.

The first step in doing so is to introduce you to the idea of galaxy clusters. You may have heard that the Universe has the form of a cosmic web. This can be seen visually in the Millennium computer simulation below. This web is built up of threads of galaxies separated by vast voids of empty space. At the points where the threads intersect you have nodes with a large amount of galaxies. As these are more massive they gravitationally attract galaxies in the thread regions and hence large bound clusters of galaxies can form.


So what we have at the moment is an idea of these massive structures called galaxy clusters which are continuously attracting galaxies out in the field, into their dense environments.

The next thing to remember is that although we can see stars and galaxies that observable matter only constitutes roughly a quarter of the matter in the Universe. The unseen dark matter prevails throughout and structures such as galaxies and even galaxy clusters are effectively embedded in large halos of dark matter.

Evidence for these surrounding halos of dark matter can be seen in a number of different effects. On the galactic scale, spiral galaxies should have a rotation speed which drops off with an increasing radius for their centre which is the classic Keplerian idea. In order to measure the rotation speed the redshift either side of a spiral galaxies disk can be measured. One side should be redshifted in wavelength (moving away) and the other slightly blueshifted (moving towards) and hence the speed of the rotation can be determined. When this analysis was performed it was found to everyone’s surprise that the rotation curve did not fall off with radius from the centre but in fact stayed flat! This implies that more matter than can be optically accounted for must exist throughout the galaxy.

Galaxy Rotation Curve


On the larger scale of galaxy clusters another method can be used to infer the presence of dark matter. Einstein 100 years ago published his general theory of relativity, which described the Universe as having a 4-dimensional space-time. This can be imagined for our purpose as a 2D flat sheet, but keep in mind that the surface of the sheet is actually in the real Universe 3 dimensions of space and 1 of time interwoven. He describes the orbits of planets and influence of gravity as the warping of the geometry of our space-time sheet by massive objects, opposing Newton’s explanation of gravity as a force.

The idea behind gravitational lensing is that large masses such as entire galaxy clusters can bend the space-time sheet a substantial amount. Light can normally travel in a straight line on the sheet from a source to an observer. However is you have this large mass between the two then the light has no choice but to follow the sheet and bend around the galaxy cluster. Evidence for this effect close to the dense cores of these galaxy clusters can be actually observed in nature! They are seen in the second image below as the arcs about the core of the galaxy clusters. These are actually much further away background galaxies which have been spread out, magnified and warped by the cluster lens.



Now the important point to take from the process of gravitational lensing is that if its the mass of the galaxy cluster which is causing the warping of space-time, then by measuring this effect we can directly calculating the mass of the cluster within the arc radius. By using these arcs and calculating the amount of mass within the centre of these galaxy clusters, it was found that once again the observed mass cannot account for everything!

The final piece of the project puzzle is to understand what happens when galaxies outside of these large galaxy clusters gets drawn in. As mentioned earlier both the galaxy and the cluster are embedded in dark matter halos, with the latter being much more massive. As the galaxy falls into the cluster the gravitational attraction between the dark matter halos of the galaxy and cluster becomes stronger. The cluster halos begins to strip the galaxy halo of dark matter as it gets tidally attracted, which effectively also contracts the dark matter halo of the infalling galaxy.

My project is to use analyse this technique of gravitational lensing but not upon the galaxy cluster halos but actually upon the galaxy halos. By measuring them at different distances from the cluster centre there should be a trend observed of the galaxy halos becoming less massive and more contracted the closer to the core they become.

This research is at the cutting edge of large scale astrophysics and cosmology. It offers a unique view of galaxy and structure evolution and allows one a glimpse at an unseen world. The dark world with the motion of this infamous dark matter.




One Comment Add yours

  1. tastehitch says:

    Mate, this is cool – it makes me realise how even more tiny I am when you see how tiny galaxies are on the universal scale!

    Thanks for getting going with the blogging again.

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