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Huge Lenses To Observe Dark Energy
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."
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.
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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 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.
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.
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, 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.
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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 project on which I work has a similar goal.
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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.
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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.
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No, really. He's right.
From Wiki:
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".
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