Astronomers have mapped a giant structure made of dark matter
It extends almost 60 million light-years out
from a huge galaxy cluster, and is our best glimpse so far
of the scaffolding that gives the Universe its large-scale structure
Hubblecast episode 58: Caught in the cosmic web
Presented by Dr J, aka Dr Joe Liske
Hello and welcome to the Hubblecast
In its very early years, the Universe was extremely smooth and uniform
There were no stars, and no galaxies, and all of the matter was distributed
throughout the Universe very, very evenly – almost perfectly so
But not quite.
Even back then there were tiny variations
in the density from place to place.
And over time, these tiny fluctuations grew and condensed
into a web-like pattern that pervades
and gives structure to the Universe of today
Formed of vast filaments and sheets of dark matter
this structure is known to astronomers as the cosmic web
Now the trouble is, because it’s made of dark matter
you can’t actually see it when you look up into the sky
You can model it using computer simulations, but until very recently
it had only ever been observed indirectly
Scientists know that less than a quarter of the matter in the Universe
is actually visible, in the form of stars, nebulae, gas and so on
The majority is dark matter, which cannot be seen directly
Astronomers have had compelling evidence for the cosmic web for years
thanks to galaxies clustering along it
But directly observing the dark-matter web itself has been far more difficult.
In fact, the very first successful observations of a
filament of the web were only made earlier this year
Now, a team of scientists has used Hubble to take this a step further
making detailed observations of a dark matter filament
measuring its length, shape and density
Finding one of these filaments is no easy task
First, you need to look where you are likely to find one
Theories say galaxy clusters form where filaments of the cosmic web meet
with the filaments gradually funneling galaxies and dark matter into the clusters
So the team focused Hubble on the galaxy cluster MACS J0717
which is known to still be growing
And then you need Albert Einstein
As predicted by the theory of general relativity
a beam of light is bent as it passes near an object with a large mass
So although you can’t directly see a dark matter filament
its mass should nevertheless
bend the light coming from galaxies behind it, subtly distorting their shaopes
Next, you have to combine Einstein and Hubble
The space telescope can make very detailed observations
of galaxy clusters, perfect for spotting these tiny distortions
And if you map them all out, the hidden filament of dark matter is revealed
And then finally, you take this two dimensional map
and you extend it into three dimensions
The astronomers used data from the Keck-II
and Subaru telescopes in Hawaii among others
to measure distances to the galaxies within
the filament mapped by Hubble, and to trace their motions
They used this to make the first ever three-dimensional reconstruction
of a filament and of how it is funneling matter into a massive cluster of galaxies
The filament extends across at least 60 million light-years of space
From our perspective, we see it gently curving towards us
then continuing almost along our line of sight
before it plunges into the back of the galaxy cluster
Measurements of hundreds of galaxies within the filament
show them moving along the filament and into cluster MACS J0717
Observing and reconstructing the cosmic web is quite a big deal
because it tells us about the underlying structure of the cosmos
Now in the future we’ll be seeing more research like this
The NASA/ESA/CSA James Webb Space Telescope
scheduled to be launched later this decade
will be an excellent tool for studying the cosmic web
because of its greatly improved sensitivity compared to Hubble
This is Dr J signing off for the Hubblecast
Once again, nature has surprised us beyond our wildest imagination
Hubblecast is produced by ESA/Hubble at the European Southern Observatory
Transcribed by ESA/Hubble
www.spacetelescope.org