AAS #12: Einstein’s Double Bulls-eye

By Phil Plait | January 10, 2008 9:03 am

The picture above shows a cosmic bulls-eye of epic alignment. But before I can tell you about it, I have to tell you about how the dart got thrown.

One of the more amazing aspects of looking into deep, deep space is that the path there is tortured and twisted. Space itself can be distorted by mass; it gets bent, like a road curves as it goes around a hill. And like a truck that must follow that road and steer around the hill, a photon must follow the curve of space.

Imagine a distant galaxy, billions of light years away. It emits light in all directions. One particular photon happens to be emitted almost — but not quite — in our direction. Left on its own, we’d never see it because it would miss the Earth by thousands or millions of light years.

But on its travels, it passes by another massive galaxy. This galaxy warps space, and the photon does what it must do: it follows that curve in pace, and changes direction… and it just so happens that the curve is just right to send it our way.

The intervening galaxy is essentially acting like a lens, bending the light. If the more distant galaxy is exactly behind the lensing galaxy, we see the light from that more distant galaxy distorted into a perfect ring, a circle of light surrounding the lens. We call this an Einstein Ring. If the farther galaxy is off to the side a bit, we see an arc instead of a complete ring. Gravitationally lensed arcs and rings are seen all over the sky, and they can be used to determine the mass of the intervening galaxy! The more mass, the more distorted the light from the farther galaxy. So the Universe has given us a nice method to let us weigh it.

In a surprising twist, astronomers have found a new type of lensed galaxy: a double ring! In a rare alignment, there are two distant galaxies aligned behind an intervening lensing galaxy. They’re like beads on a wire, lined up just right such that both more distant galaxies are lensed by the nearer one. In this case, the lens is about 3 billion light years away, and the other two are 6 and 11 billion light years away, an incredible distance.

This image is amazing, but it is also a powerful scientific tool. It allows us to measure not just the mass of the lensing galaxy, but also the amount of mysterious dark matter nearby. We cannot see the dark matter, but it too bends light, and contributes to the lensings. By observing lenses like this, we can take a sample of dark matter in the Universe, and that’s a crucial first step in understanding it. Even better, these double rings allows us to measure the amount of total mass not just in the nearest galaxy, as is usual, but also in the middle galaxy as well, since it distorts the light from the galaxy behind it (turns out it’s a rather lightweight one billion solar masses; our own Galaxy has more than 100 times that mass, so the middle galaxy is considered a dwarf).

This is a beautiful happenstance; it gives us a measure of the Universe at two points, with one being for free. In fact, Tommaso Treu, the astronomer at U.C. Santa Barbara who investigated this lens, points out that if we can find as few as 50 of these double rings, we can get a much better idea of the distribution of not just dark matter, but also the even more mysterious dark energy in the Universe. That’s one of the biggest goals of modern astronomy… and we may get a handle on it due to a coincidental ring toss.

Comments (19)

  1. tacitus

    Very cool, but I have a question. Is the warping of the light done in enough of a predictable way that an image of the distant galaxies can be reconstituted from rings? I would guess not, but given the amazing stuff we can do with warped images these days (i.e. Hubble’s mirror problem) I was just wondering if anything can be done at all.

  2. Carey

    That’s like, pretty awesome and stuff.

  3. Gary Ansorge

    I guess that would also mean, the galaxies beyond the nearer galaxy can see us too??

    ACK! MAn all battle stations. The Aliens can see us!!!

    Cool Pics!

    GAry 7

  4. Dutch

    Wouldn’t it be interesting if we found out that the light that we are seeing from the most distant galaxy is actually from our own.

  5. Does dark matter have the same gravitational properties as non-dark matter?

    Is dark matter just regular matter that is not very reflective and we can’t see it from here, or is it a wholly different kind of matter that is invisible altogether?

  6. Dutch posts:

    [[Wouldn’t it be interesting if we found out that the light that we are seeing from the most distant galaxy is actually from our own.]]

    It would mean that many observations of the past decade or so were egregiously wrong. That effect only happens if the universe is “closed,” density-wise, and observations indicate that it is not.

    PsyberDave posts:

    [[Is dark matter just regular matter that is not very reflective and we can’t see it from here, or is it a wholly different kind of matter that is invisible altogether?]]

    No one is sure yet, but there are some abstruse physics reasons for thinking dark matter is non-baryonic (i.e., not what we’re used to calling matter). The likely contenders, if I’m not obsolete here, are mass-possessing neutrinos and exotic particles called “axions.”

  7. tacitus: “given the amazing stuff we can do with warped images these days (i.e. Hubble’s mirror problem) I was just wondering if anything can be done at all.”

    Just to be pedantic, Hubble’s mirror was not warped. It was ground to the wrong figure. It was actually very, very precisely ground wrong; it just focused at the wrong spot. That’s why they were able to fix it by placing a corrective mirror in the optical path, exactly the way human vision problems are corrected.

    - Jack

  8. Jeffersonian

    [retrieves skullcap from across room]

  9. blizno

    Gary Ansorgeon 10 Jan 2008 at 10:46 am

    “I guess that would also mean, the galaxies beyond the nearer galaxy can see us too??

    ACK! MAn all battle stations. The Aliens can see us!!!”

    They are right now seeing us as we were 6+ billion years ago. I doubt that those distant aliens are alarmed enough to launch a faster-than-light attack against the dusty disk forming around proto-Sol.
    It’s only the aliens a few hundred light years away from us who might be alarmed by our harnessing the horse and eventually…the steam engine.

  10. Gary Ansorge

    ACK! The steam engine. It’ll put out everybody out of work,,,

    ,,,or not,,,

    GAry 7

  11. Kaptain K

    “…if we can find as few as 50 of these double rings…”

    Now there’s an optimist for you! We finally find ONE and he says “if we can find as few as 50″! :0

  12. YinYang0564

    I think I have just read the best description of gravitational lensing I have seen. Thank you, Phil.

  13. Allen Thomson

    Sort of an idle wonderment, but I wonder what astronomers would have made of such distorted images if GTR hadn’t been invented.

  14. Rob

    Sort of an idle wonderment, but I wonder what astronomers would have made of such distorted images if GTR hadn’t been invented.

    Maybe that they’d discovered the Ringworld. :)

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