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Astro Bob: Celebrate Halloween with Hubble

Spooky dark matter and cosmic cobwebs highlight this creepy offering from the Hubble Space Telescope.

A Cosmic Cobweb
Hundreds of small galaxies light up this photo of the cluster Abell 611 located roughly 3.2 billion light years from Earth in the constellation Lynx. Most appear as fuzzy ovals, but a few have distinct spiral arms. The cobweb-like wisp to the left of the cluster's center is light from a distant, background galaxy bent and distorted by the gravity of the massive group.
Contributed / ESA, Hubble, NASA, P. Kelly, M. Postman, J. Richard, S. Allen
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Not even the scariest ghosts and ghouls the human imagination can conjure compare to the ghastly things that inhabit the dark corners of the universe. Looking at the Hubble image of the galaxy cluster might not send shivers down your spine, but lurking betwixt and between the bright galaxies is the spookiest substance we know next to nothing about — dark matter.

Like all galaxy clusters, Abell 611 poses a mystery to astronomers. There's not enough mass contained within its web of galaxies to prevent the cluster from flying apart. They're speeding so fast around its core that something invisible to our eyes and instruments must be keeping the whole thing together. Whatever it is we have yet to fathom its nature. The best we can do is postulate the existence of dark matter, material that doesn't show itself except through the gravity it exerts on normal matter in the vicinity.

Dark vs light matter
On the left is a visible light photo from Hubble of the massive galaxy cluster Cl 0024+17 showing a number of galaxies that are stretched and distorted by gravitational lensing. On the right, a blue shading has been added to show the location of invisible dark matter required to account for their appearance.
Contributed / NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

Although dark matter permeates the universe it primarily reveals itself on cosmic scales of tens of millions of light-years. When it comes to "small stuff" like our solar system astronomers can ignore its minute effects and calculate the motions of the planets without having to factor in dark matter's contribution.

To give you an idea of how much of this unknown matter exists, scientists estimate that 95% of the Milky Way is composed of the stuff. In fact, just under 5% of the entire material universe is made of atoms. The rest is … you guessed it.

We may clueless about dark matter's composition, but there are lots of ideas out there. Two of the most whimsically named candidates include weakly interacting massive particles (WIMPs) — hypothetical subatomic particles that don't interact with light like regular matter does — and massive astrophysical compact halo objects (MACHOs), a hypothetical slew of very massive objects made of matter we already know of but which are extremely hard to observe because they emit so little light.

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Despite astronomers' best efforts, however, no conclusive evidence has been found of WIMPs, MACHOs and any of the other possibilities including freaky axions and gravitinos .

Cobweb closeup
In this cropped view we see the "cobweb" in more detail. It's likely a galaxy, the light of which has been greatly distorted by the massive gravitational field of the foreground galaxy cluster. Extremely massive objects can warp the fabric of spacetime, creating a "lens" or primitive telescope that can reveal distant background galaxies.
Contributed / ESA, Hubble, NASA, P. Kelly, M. Postman, J. Richard, S. Allen

Fortunately, we can at least get a handle on how much dark matter resides in Abell 611 and other clusters through gravitational lensing. To the left of the cluster's glowing core, the cobwebby arc is light from a more distant source (likely a galaxy) that has been bent and distorted by Abell 611’s vast mass. According to Einstein, space and time are fused into a single entity called spacetime . Massive objects cause spacetime to curve the same way a lens bends a ray of light passing through it.

Gravitational lensing
In gravitational lensing, a massive galaxy cluster sitting between the Earth and a much more distant galaxy, acts as a natural magnifying glass and warps the fabric of space and time to amplify the remote object's light. It also distorts its appearance. Lensed galaxies look like wispy arcs.
Contributed / ESA

Matter concentrated within a massive galaxy cluster acts exactly the same way and bends, distorts and magnifies the light of distant objects located far behind the cluster. By measuring how much the light has been bent we can determine the cluster's total mass. Astronomers then compare that number with an estimate of its mass based on the visible matter (galaxies) we see.

The difference between the calculated mass and what's observed is staggering — something like 95% of Abell 611's material is hidden from view. Utterly invisible. It makes you want to hang your head. But unlike the goblins and ghouls of the season we know these dark spirits are real even if they continue to resist explanation.

Read more from Astro Bob
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Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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