Category Archives: dark matter

cosmology, dark matter, galaxy clusters

I’m Going Down an Invisible Ring of Dark Matter…

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With Apologies to the Late Johnny Cash

The galaxy cluster CL 0024+17 (ZwCl 0024+1652) as seen by Hubble Space Telescopes Advanced Camera for Surveys.
The galaxy cluster CL 0024+17 (ZwCl 0024+1652) as seen by Hubble Space Telescope's Advanced Camera for Surveys.
A ghostly ring of dark matter in the same galaxy cluster, as inferred by observations of the cluster made by Hubble Space Telescope.
A ghostly ring of dark matter in the same galaxy cluster, as inferred by observations of the cluster made by Hubble Space Telescope.

It’s out there. Dark matter. You’ve probably read about it in the papers, or heard scientists talk about it. Nobody’s quite sure exactly what it is, yet. But, we do know it’s out there, distributed in clumps throughout the galaxies in the universe.

Tracking down dark matter, which is one of the great “Holy Grails” of astronomy and astrophysics these days, is no easy matter (so to speak). Dark matter doesn’t give off light, it doesn’t reflect light. It just sits there, making its presence known by the gravitational effect its mass has on light. To find it, astronomers study how the light from more distant galaxies (beyond or “behind” the dark matter clumps) is distorted and smeared into arcs and streaks by the gravity of the dark matter. The smears are produced by a phenomenon called “gravitational lensing.”

One Ring to Rule Them All… and in the Dark Matter Bind Them

Today a team of astronomers announced that they found a RING of dark matter at the heart of a galaxy cluster. An ordinary “visible light” image of the cluster shows galaxies and some smears of light that are distorted images of galaxies behind the cluster. The smears indicate that there’s something in the cluster causing the light from the distant galaxies to bend or distort. That “something” turns out to be a ring of dark matter. Now, if much of the dark matter we’ve “seen” so far in the universe is clumpy, how does a ring of it end up at the heart of a galaxy cluster?

Good question.

According to one of the astronomers, Dr. M. James Jee of Johns Hopkins University in Baltimore, MD, the collision between two galaxy clusters created a much larger cluster, but it also formed a ripple of dark matter separate from the gas and dust components of the cluster that left distinct footprints in the shapes of the background galaxies. “It’s like looking at the pebbles on the bottom of a pond with ripples on the surface,” he said. “The pebbles’ shapes appear to change as the ripples pass over them. So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring.”

This is the first time such a distinct ring of dark matter has been seen, and at first Jee did not want to believe the ring-like structure was anything but a statistical fluke in the data analysis. But, like any good scientist does, he systematically went through the data, and eventually was convinced it was real.

“I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction,” Jee explained. “I couldn’t believe my result. But the more I tried to remove the ring, the more it showed up. It took more than a year to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this.”

It’s really quite a cosmic detective story. Dark matter is sort of like the “last frontier” of matter studies in some ways. We know it’s ubiquitous in the universe, and we’re now starting to trace out its distribution, its “shapes” (if you will). There’s more dark matter than “regular” (so-called “baryonic” matter in the universe) and it serves as a sort of gravitational glue to bind galaxies and clusters together.

If you want to read more about this story, and see some very cool animations, go to the Space Telescope Institute website and visit their news center. And stay tuned. I think there’s going to be more news about this “binding force” of dark matter as scientists do more cosmic mapping!

dark matter

It’s Dark Out There!

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Yesterday there was a news conference at NASA about something called “dark energy.” What is this stuff? Well, strictly speaking it’s not matter. It’s a force. We’re all familiar with the force of gravity, which acts to hold things together, particularly at the atomic level. Across huge distances — and I’m talking big ones, like between galaxies and clusters of galaxies (what astronomers like to refer to as “cosmic” distances”), gravity is part of a complex dance that warps galaxies that pass too close and keeps members of a cluster more or less together. Important stuff, this gravity. We all know that the universe has been expanding since the Big Bang, about 13.7 billion years ago. Everybody just assumed that gravity would have some effect on this cosmic expansion, maybe even slow it down.

Such a gravitational braking force would affect light from distant objects, and people who study shifts in the wavelengths of light from very distant stellar explosions called Type Ia supernovae though that they’d see the slowing effect of gravity in the spectra (the minute details of the light) of the supernovae. Turns out they didn’t. In fact, what they DID find is that the expansion of the universe is speeding up! Something is accelerating the expansion and this speed-up started about 5 billion years ago — roughly about the time our Sun and planets were forming.

That “push apart” force is called “dark energy.” It’s a lousy name for a factor that Albert Einstein postulated way back in the early years of the 20th century. He couldn’t believe it existed and so he discarded it. In retrospect that doesn’t look like a good move on Einstein’s part, but hey — you have to admit it does seem a little strange to have something mysterious out there pushing the galaxies apart faster than gravity can hold them together.

Now, this dark energy doesn’t act on something as small as our planet or you and me — like gravity at cosmic scales, it acts across cosmic distances. So, I wouldn’t worry too much about the solar system flying apart or the Milky Way doing something brash like sending the Orion Arm on over to Andromeda for a friendly visit. It doesn’t work that way. The full extent of dark energy’s influence will echo across time for another 30 billion years or so, if it continues pushing at its presence pace.

It’s not really a big thing to worry about personally, but as part of the puzzle that is our universe — and while we understand much about the cosmos, we certainly don’t have a handle on all of it — it certainly does pose a tantalizing new piece for scientists to chew over as they push the limits of our telescopes back to the earliest epochs of history. Keep your ears open for more on this dark energy stuff — and don’t believe anybody who tells you it’s a fantastic new source of energy for perpetual-motion machine. That’s woo-woo territory…

A short programming note: I’d like to direct your attention to the link I added for Gemini Observatory over in my links column on the left. I’ve been doing some work with the fine folks there and I thought you might like to see some of the good astronomy they’ve been doing with their telescopes at Mauna Kea and Cerro Pachón in Chile. Give ’em a visit and see what’s new at Gemini!