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Astronomers have mapped a giant structure made of dark matter

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It extends almost 60 million light-years out 

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from a huge galaxy cluster, and is our best glimpse so far 

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of the scaffolding that gives the Universe its large-scale structure

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Hubblecast episode 58: Caught in the cosmic web

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Presented by Dr J, aka Dr Joe Liske

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Hello and welcome to the Hubblecast

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In its very early years, the Universe was extremely smooth and uniform

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There were no stars, and no galaxies, and all of the matter was distributed 

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throughout the Universe very, very evenly – almost perfectly so

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But not quite. 

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Even back then there were tiny variations 

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in the density from place to place. 

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And over time, these tiny fluctuations grew and condensed 

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into a web-like pattern that pervades 

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and gives structure to the Universe of today

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Formed of vast filaments and sheets of dark matter 

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this structure is known to astronomers as the cosmic web

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Now the trouble is, because it’s made of dark matter

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you can’t actually see it when you look up into the sky

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You can model it using computer simulations, but until very recently

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it had only ever been observed indirectly

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Scientists know that less than a quarter of the matter in the Universe 

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is actually visible, in the form of stars, nebulae, gas and so on

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The majority is dark matter, which cannot be seen directly

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Astronomers have had compelling evidence for the cosmic web for years

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thanks to galaxies clustering along it

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But directly observing the dark-matter web itself has been far more difficult.

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In fact, the very first successful observations of a 

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filament of the web were only made earlier this year

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Now, a team of scientists has used Hubble to take this a step further

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making detailed observations of a dark matter filament

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measuring its length, shape and density

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Finding one of these filaments is no easy task

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First, you need to look where you are likely to find one

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Theories say galaxy clusters form where filaments of the cosmic web meet

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with the filaments gradually funneling galaxies and dark matter into the clusters

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So the team focused Hubble on the galaxy cluster MACS J0717

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which is known to still be growing

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And then you need Albert Einstein

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As predicted by the theory of general relativity

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a beam of light is bent as it passes near an object with a large mass

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So although you can’t directly see a dark matter filament

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its mass should nevertheless 

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bend the light coming from galaxies behind it, subtly distorting their shaopes

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Next, you have to combine Einstein and Hubble

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The space telescope can make very detailed observations 

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of galaxy clusters, perfect for spotting these tiny distortions

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And if you map them all out, the hidden filament of dark matter is revealed

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And then finally, you take this two dimensional map 

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and you extend it into three dimensions

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The astronomers used data from the Keck-II 

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and Subaru telescopes in Hawaii among others

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to measure distances to the galaxies within 

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the filament mapped by Hubble, and to trace their motions

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They used this to make the first ever three-dimensional reconstruction 

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of a filament and of how it is funneling matter into a massive cluster of galaxies

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The filament extends across at least 60 million light-years of space

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From our perspective, we see it gently curving towards us

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then continuing almost along our line of sight

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before it plunges into the back of the galaxy cluster

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Measurements of hundreds of galaxies within the filament 

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show them moving along the filament and into cluster MACS J0717

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Observing and reconstructing the cosmic web is quite a big deal

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because it tells us about the underlying structure of the cosmos

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Now in the future we’ll be seeing more research like this

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The NASA/ESA/CSA James Webb Space Telescope

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scheduled to be launched later this decade

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will be an excellent tool for studying the cosmic web

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because of its greatly improved sensitivity compared to Hubble

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This is Dr J signing off for the Hubblecast

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Once again, nature has surprised us beyond our wildest imagination

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Hubblecast is produced by ESA/Hubble at the European Southern Observatory

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Transcribed by ESA/Hubble

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www.spacetelescope.org