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Space Science

Huge Lenses To Observe Dark Energy 121

Posted by timothy
from the that's-what-my-huge-lenses-do-too dept.
Iddo Genuth writes "UK astronomers, as a part of the Dark Energy Survey collaboration, have reached a milestone in the construction of one of the largest ever cameras to detect dark energy by completing the shipment of the glass required for the five special lenses. Each step in the process of completing this sophisticated camera brings scientists closer to detecting the invisible matter that cosmologists estimate makes up around 75% of our universe."
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Huge Lenses To Observe Dark Energy

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  • Re:dark energy? (Score:5, Informative)

    by FooAtWFU (699187) on Friday July 04, 2008 @01:06AM (#24055263) Homepage

    If you can get good enough optics, you can watch regular light and detect when it's being affected somehow by dark matter.

    Confusing enough summary though.

  • by tirerim (1108567) on Friday July 04, 2008 @02:12AM (#24055607)
    Not really. In our own solar system, all the planets combined are less than a thousandth the mass of the sun. It's pretty much impossible for planets to make up a significant fraction of the mass of a stellar system -- if they did, they would have wound up as a star.

    MACHOs [wikipedia.org] may still make up some fraction of dark matter, but the idea that they could make up most of it has been largely disproved, and they're not really planets, either. It's fairly certain at this point that most dark matter is non-baryonic.
  • by Jane Q. Public (1010737) on Friday July 04, 2008 @02:44AM (#24055735)
    There are explanations other than "dark matter" and "dark energy" that can explain the observations we see. MoND, for example (Modifien Newtonian Dynamics) is a quite popular theory among physicists, and it does not require that we believe that most of our universe is basically undetectable by humans.

    Occam's Razor works strongly in favor of MoND over such hypotheses as dark matter... only time will tell.
  • by MichaelCrawford (610140) on Friday July 04, 2008 @03:10AM (#24055893) Homepage Journal
    The biggest lens in the world currently is the 40-inch Yerkes Observatory refactor, followed closely by the 36-inch Lick at Mt. Hamilton, overlooking Silicon Valley.

    But for many years the biggest mirror was the 200-inch Hale Telescope at Palomar Mountain near San Diego. Nowadays there are several monolithic 8-metre mirrors, and the two 10-meter Keck telescopes atop Mauna Kea, Hawaii; they are composed of carefully aligned hexagonal subsections.

    Why the big disparity?

    With a lens, you have to grind and polish both sides, and what's worse, a single lens won't do because all glass refracts different colors differently, giving rise to chromatic aberration. A minimum of two lenses is required, for four surfaces to fabricate.

    For both lenses and mirrors, the tolerance of the surface is a small fraction of a wavelength of light across the whole surface. But for lenses, all the surfaces must also be very accurately parallel.

    But really the worst problem is that with a lens, the light goes through the thickness of the glass. The glass must therefore be very uniform and free of internal stresses that could alter the index of refraction in different places.

    Such glass is very difficult to make; no doubt these lenses are only possible because of recent advances in optical glass manufacture.

    That's not a problem for mirrors; observatory telescopes use "first-surface" mirrors, which are aluminized on the front, so the light doesn't go through the glass. Mirror glass therefore doesn't need such careful tolerances.

    But my guess is that they are using lenses because they have a much wider field of view; it's quite easy to make a lens with a sixty degree field of view, but with a mirror the field of view is typically a fraction of a degree. With small amateur scopes, the maximum field is about a degree, twice that of the full moon.

    That seems clear from the photo, because of the steep curvature of the glass; wide-angle lenses usually have very strong curves.

    And yes, I know what I'm talking about - I'm an avid amateur telescope maker [geometricvisions.com], and at one time was a Caltech astronomy student. I've published in the Astrophysical Journal, and have done observing runs at the Palomar 60 and 200 inch telescopes.

  • Re:Oymoron anyone? (Score:5, Informative)

    by dargaud (518470) <slashdot2@gdar g a u d . n et> on Friday July 04, 2008 @03:14AM (#24055929) Homepage
    Well, it doesn't mean that the lens sees it, but that the lens can see the effect it has on the things you _can_ see. For instance you look at a galaxy field and you notice that some are distorted in certain ways, you can infer that there's a hidden mass between you and those galaxies. The LSST [lsst.org] project on which I work has a similar goal.
  • by Have Brain Will Rent (1031664) on Friday July 04, 2008 @03:34AM (#24056027)
    As for the other, Occam's Razor has little to do with logic per se. Rather, it implies that the simplest solution (i.e., the one that introduces fewest complications) is the most likely to be correct.

    Actually it says that the solution requiring the fewest assumptions is the one most likely to be correct. The correct solution could still be more complex (which is my assumption on what you mean by complications) than other proposed solutions. When the number of assumptions is equal then the solution with the lower complexity is favoured.
  • Not exactly (Score:5, Informative)

    by Moraelin (679338) on Friday July 04, 2008 @04:40AM (#24056371) Journal

    That's somewhat incorrect, and makes a hash of two unrelated things too:

    1. Dark matter. Unlike what its name might imply, it isn't dark as in "light absorbing". It's dark as in, it doesn't interact with light at all, except through gravity.

    It's only "dark" in the same way as a sheet of glass is dark against the night sky.

    But even that metaphor is misleading. "Dark matter" is just a name for a lot of mass that should be there according to calculations (or our understanding of gravity is completely broken at large scales), but hadn't been observed. It's just a funky name. It doesn't mean it's actually dark in any form or shape.

    The best example of a scale where this is visible is inside a galaxy. With just gravity determining the speed of rotation around the centre, the stars closer to the centre should rotate faster than those on the edges. (In the same way as Mercury rotates around the sun once every 0.24 years, Earth in a year, and Pluto in 248 years.) But galaxies don't seem to rotate that way. They rotate more like a solid texture, so to speak. So there must be some mass distributed through the disc, in addition to what we see.

    But again, the whole point is that we can't see it. If it were just a cloud of pitch-black baryonic matter, that would actually be easy and comfortable. We'd just do what you said: look at what happens to the light of stars behind it. Since it's plenty of it inside a galaxy, we have plenty of stars to look at and notice if something like that was between us and them. But all we can see is some extra gravity, with all that involves for both star movement and gravitational lensing.

    A much more accurate name would be "completely transparent matter."

    2. Dark energy.

    This is an even funnier concept. With all that mass in the universe, there's gravity all around. Duly noted, the gravity pull of a hideously distant galaxy is really tiny, but it's there. The universe expansion should slow down as gravity pulls everything towards the centre. The funny thing is: it doesn't. It's actually accelerating, and weirdly enough, the farther something is, the faster it seems to accelerate away.

    There is _something_ that pushes stuff away from the centre, and it's not like any force we already know.

    It's also something we'd be hard pressed to reproduce in a lab. Whatever it is, it's insignificantly weak at small ranges, and only starts to matter at very very very large distances. Even at galactic scales (hundreds of thousands at light years) it seems to do practically nothing at all, but move a few _billion_ light years away, and you start seeing whole galaxies accelerating away. It's not something you can reproduce in a lab.

    It's also weird in that a normal energy (e.g., the potential energy in a compressed spring) would get used up, or rather converted into work, as it pushes stuff away. So the force would logically diminish. This one only seems to grow stronger.

    So basically this big "WTF?" is what's called "dark energy". There's some energy that's pushing the universe apart, but we don't know what it is, and how to detect it.

  • Three answers: (Score:5, Informative)

    by MichaelCrawford (610140) on Friday July 04, 2008 @04:55AM (#24056445) Homepage Journal
    An uncoated mirror actually does oxidize, but it does so in a very nice way, forming an optically-homogenious layer of very tough aluminum oxide.

    In that respect it's completely different from iron oxidation.

    The other way is to overcoat it with something tough and transparent; traditionally silicon monoxide was used.

    One can both protect the aluminum and enhance its reflectivity by giving it multiple layers of tough, transparent minerals. Interference effects cause it to reflect better than aluminum would alone.

    That's how laser mirrors work - they're not aluminized. It's the same principle as antireflective coatings on camera and eyeglass lenses, but a different choice of refractive indices and thicknesses causes it to enhance rather than cancel reflections.

  • by Kentari (1265084) on Friday July 04, 2008 @06:13AM (#24056865) Homepage

    Actually, the lenses here are "just" a corrector of a 500 Megapixel camera (FTA). The light gathering is done by the Blanco 4-meter telescope. The camera will have a huge field of view (for a professional telescope) of 2.2 degrees.

    Why lenses and not mirrors as corrector? Because mirrors reflect light and you have to either work with tilted mirrors or tolerate obstruction. Tilted mirrors as corrector would require very complex surfaces (read assymmetrical, aspherical, ...) which would actually be more difficult to figure than a 1m lens (which might also have quite complex surfaces, but at least symmetrical).

    As for the maximum field of view of amateur telescopes: My 200mm f/4.5 (900mm focal length) Newtonian yields a 2.7 degree field of view with a 31mm Nagler eyepiece. A commercially available 114mm f/4.5 Newtonian would yield a 4.9 degree field of view with that eyepiece. And my little 80mm F/5.5 apochromatic refractor would yield a 5.8 degree FOV. These extremely large field of views give delicious views of the Milky Way by the way... The days of 1 degree Max FOV are long past...

  • Re:Not exactly (Score:5, Informative)

    by Moraelin (679338) on Friday July 04, 2008 @07:54AM (#24057383) Journal

    Well, that's how science works. If someone comes up with a better theory that doesn't involve "ether", we'll go with that one.

    There are several hypotheses to that effect already. One is for example the Modified Newtonian dynamics [wikipedia.org], which pretty much just messes with the F=ma to explain galaxy rotation.

    Another possibility would be to mess with gravity itself. For small distances it would be as usual inversely proportional to the square of the distance, but then it would gradually turn into just 1/r instead.

    If you want to explain away dark energy too, it gets funnier, since past a point it must actually become negative.

    That said, it's not just hypothesized "ether", though.

    We _know_ for example that any star, including our sun, produces immense quantities of neutrinos. Which are just that: totally transparent matter. They have an almost zero (not exactly zero, but very very very very close) probability to interact with ordinary matter, and a bunch of them passed right through you as you read this message. The only real interaction between neutrinos and the rest of the universe (or each other) is that they both create gravity and are subject to gravity.

    That's one kind of "dark matter" that we already know exists, and have been detected. They're not just hypothesized.

    Now whether they're _all_ the missing matter or not, that's another question.

    We also have one famous photo in which two galaxies collide, and the bulk of the gravity lensing "fields", i.e., the gravity wells, actually moved ahead of the actual galaxy. It's as if the galaxies were braked by friction with each other's dust and interstelar atoms, but whatever creates the bulk of the gravity well moved ahead.

  • Re:Oymoron anyone? (Score:3, Informative)

    by gstoddart (321705) on Friday July 04, 2008 @09:16AM (#24057947) Homepage

    If it is detectable in any way, it's not "dark" anymore!

    Hey I didn't know "dark" now meant "invisible", thanks for the update!

    No, really. He's right.

    From Wiki [wikipedia.org]:

    In physics and cosmology, dark matter is matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter.

    If we could detect it though any other mechanism than inferring it exists based on gravitational effects, it literally would cease to be dark matter -- because that's how they define "dark matter".

    Cheers

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