Dark Matter in 3D

Have a look at this:

hubble 3d dark matter

What is it? It is an image of part of the three dimensional (see below) distribution of clumps of dark matter in our universe, produced by an extensive survey using the Hubble telescope. How did they produce it, given that dark matter is -by definition- not visible? They deduced the presence of the chunks of dark matter by looking at the gravitational lensing it produces on visible matter. Gravity bends light, recall, and so if you have some light coming from an object, such as a star, or a whole galaxy (typically), when it passes by something massive (such as a clump of dark matter) its path will get bent on its way to you. This is of the same thing that a lens (such as perhaps in your glasses or contact lenses, and of course in your eye) does to light coming from objects you’re looking at. The way the light from an object is bent results in a pattern in the final image that tells you a lot about the object that bent the light – its mass, location, and shape.

hubble gravitational lensThere’s a picture of a lensing example to the right, the lens in that case being a visible dense cluster of galaxies. The smeared-out streaks are secondary images of the galaxies that are behind the cluster. The gravity from the concentrated mass of the cluster bent the light from them, you see. More on that example here. A clump of matter that is massive and concentrated – as the cluster is in the example – but made of dark matter (so not visible) will do a similar thing to the light from distant galaxies, and so leaves evidence of itself despite being invisible. Most dark matter clumps are not going to be as dramatic lenses as the above, but the idea is the same. With enough experience with lens images, even more slight effects (that just show up as slight distortions in the shapes of visible galaxies) can give clues to the presence of invisible mass – dark matter.

hubble graphic dark matterNow imagine If you have enough dark matter lensing examples to observe (perhaps from a big survey of the sky with a powerful telescope like Hubble!), you can put together a map of the dark matter’s distribution! This is what the scientists behind the new research did! When you look up at the sky, you see a two dimensional image. The depth (a third dimension) achieved by looking at further away objects on the galactic (and beyond) scales we’re talking about involves looking at light that was emitted much earlier. So the three dimensionality in the image at the top should really be thought of as having time running along the longest dimension. You’re seeing the evolution of the dark matter. It is growing clumpier toward the left, under the influence of gravity. Toward the right is earlier, reaching back in time more than 6.5 billion years – almost half the age of the universe. You can read the full story on NASA’s site here, and a story on the BBC here.

Dark matter, which makes up about 83% of all the matter in the universe, is crucial to our understanding of the life and evolution of the universe. The clumping together that we observe for visible matter -galaxies, stars, us- is believed to be connected to the clumping of the dark matter and this survey confirms that, more or less. Here is a picture of visible and dark matter distributions, side by side, showing similar clustering:

matter visible and invisible

From the NASA site:

This is consistent with conventional theories of how structure formed in our evolving universe, which has transitioned from a smooth distribution of matter at the time of the Big Bang.

The researchers used data from Hubble Space Telescope’s largest survey to date of the universe, the Cosmic Evolution Survey (“COSMOS”). The COSMOS field covers a sufficiently wide area of sky – eight times the area of the full Moon – for the large-scale filamentary structure of dark matter to be clearly evident. To add 3-D distance information, the Hubble observations were combined with data from Europe’s Very Large Telescope in Chile, Japan’s Subaru Telescope in Hawaii, the U.S.’s Very Large Array radio telescope in New Mexico, as well as the European Space Agency’s orbiting XMM-Newton X-ray Observatory.

The dark matter map was constructed by measuring the shapes of half a million faraway galaxies.

More information about the clumping gives us more information about the formation of structure in the universe – a large part of the the story of how we came to be. There’s a lot of exciting work to do using this new window on our universe. Some of the work will involve refining this work quite a bit – there are some puzzling features (i.e., some “naked” dark matter clumps -ones that are not associated to clumps of visible matter) that may be anomalies in the data analysis, or new and interesting physics. Time and further work will tell.


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