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Huge Lenses To Observe Dark Energy
Posted by
timothy
on Thu Jul 03, 2008 11:50 PM
from the that's-what-my-huge-lenses-do-too dept.
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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Oymoron anyone? (Score:3, Insightful)
If it is detectable in any way, it's not "dark" anymore!
Wouldn't it be better to call it an effort to "define" dark energy?
Re:Oymoron anyone? (Score:5, Funny)
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Re:Oymoron anyone? (Score:5, Funny)
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Re:Oymoron anyone? (Score:5, Interesting)
When "96% of the universe" is only detectable by how your model fails with the visible stuff alone, measurements of the visible stuff become useless. It isn't even science at that point.
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Re:Oymoron anyone? (Score:5, Insightful)
When "96% of the universe" is only detectable by how your model fails with the visible stuff alone, measurements of the visible stuff become useless. It isn't even science at that point.
I'd say it is. I'm under the impression, that dark matter & energy hypothesis (I think it's fair to call it a hypothesis at this point) is the simplest explanation for all the observations that we have. So I'd say it's very much science, as good science as we're capable of.
Feel free to provide a nicer model that still explains the current observations, though. I'm sure the Nobel Foundation will reward you for your efforts if you get it right enough!
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Re: (Score:2)
If you are looking for a nicer model, how about this one.,
http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics [wikipedia.org]
Can you tell me, does this explain/produce the wobble observed in this [slashdot.org] experiment, for example? 'Cos if it doesn't, then there's some work to be done with it still, before it's worth considering as a valid alternative...
Re: (Score:2)
I think the wiki made pretty clear that it's not a new theory. It intends to be a kind of "relativity-lite" in the sense that relativity proved that the existence of an aether was not necessary to explain the wave-like effects, MOND demonstrates that the existence of "dark" matter might not be necessary to explain astronomical observations.
Surely you can agree that postulating the existence of matter that doesn't behave anything like anything we can observe in the lab, except gravitational interaction, AND
Re: (Score:3, Insightful)
How can observing the only stuff we can possibly observe be useless and not even science?
Are you proposing we start looking at all of the things we can't see with the technology we don't have because that would be better somehow? Is it more efficient to skip straight into the stiff you can't even conceive of yet?
We can try
Re:Oymoron anyone? (Score:5, Informative)
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Re: (Score:3, Funny)
I don't think Nibbler's gonna be too happy with someone sticking a huge lens up there...
Re: (Score:2)
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! And while you may argue that something dark is invisible on a black background you have to remember that there's hardly any sort of truly black background in the sky.
Re: (Score:3, Informative)
No, really. He's right.
From Wiki [wikipedia.org]:
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 h
dark energy? (Score:5, Insightful)
Re:dark energy? (Score:5, Informative)
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.
Parent
Not exactly (Score:5, Informative)
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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Re: (Score:2)
There is _something_ that pushes stuff away from the centre, and it's not like any force we already know.
Nitpick: there's no center, so stuff isn't pushed away from center. All stuff is pushed away from all other stuff at large scales, and that's what we call "dark energy".
Nice post otherwise!
Re:Not exactly (Score:5, Informative)
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.
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Re: (Score:2)
[...] 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.
Actually, neutrinos interact through the "weak" nuclear force, which is why they're usually described as "weakly interacting" - which is an unfortunately uninformative way to put it. This means that they have a specific, well defined, but rare interaction with ordinary matter that lets neutrino detectors find them.
What are called "forces" are actually types of fields generated by the presence of certain properties, such as mass or electric charge. The "weak" force is a field which allows certain types of
Re: (Score:2)
Hmm, well, _some_ neutrinos have been detected, so _some_ dark matter has been detected and is known to exist. I'm guessing you mean more like "Dark Energy", which is the one conjectured based on red-shift.
According
Re: (Score:2, Insightful)
The stupid reporter doesn't know the difference between dark energy and dark matter.
As for dark energy, which I guess is what they're going for since it's called the Dark Energy Survey, to paraphrase Einstein, "It's dark, like fudge."
His Dark Energy (Score:2)
Easy. All you need is an Amber Spyglass.
Re: (Score:2)
Dark matter = crap needed by the auditors to explain galaxy rotation etc. (Bookkeeping in other words).
Dark energy = Einstein's big mistake aka the cosmological constant. The big proble
Re: (Score:2)
Not an astronomer and deeply unhappy with this stuff which strikes this ex chemist as being very phlogiston or caloric or aether.
It should, but so what? Any physicist will tell you that dark matter and dark energy are just placeholders for things that they don't understand. Just because we know phlogiston, aether, and caloric don't exist now doesn't mean the people who came up with them were doing bad science.
Re: (Score:3, Interesting)
"If it's "dark" that means it's undetectable by normal means like giant lenses for instance. How could you just see dark energy? "
I would imagine it's like 'seeing' wind because trees sway.
Dark Energy Transport (Score:4, Funny)
[Presses Big Red Button]
(((((((((((((((BOOOOOOOOOOOOOOM))))))))))))
Narrative: Somewhere on a distant planet
Astronomer "Did you see that flash, hah proof dark energy doesn't exist"
What about planets (Score:4, Insightful)
Re:What about planets (Score:5, Informative)
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.
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History of Dark Energy (Score:2)
Dark energy = false vacuum? (Score:5, Interesting)
Dark energy may be an indicator that we live in a false vacuum [wikipedia.org]. If this is the case and the true vacuum is speeding towards us with the speed of light then we are doomed. So, add another doomsday scenario to your list.
Re: (Score:2)
Yeah. And when that happens, we are supposed to cheer for a bunch of post-human smut assholes that thing destroying the universe is a nice thing to observe without interference just because something will be there afterwards, too.
True vacuum? (Score:2)
Is that what you get in a Dyson sphere? [wikipedia.org]
P.S. re: your sig, I'd say Dick Cheney is living proof that government can make man richer, provided we're talking about the right man...
That Wikipedia article is deeply flawed (Score:2)
Memo to self: must get out more. Also, must try harder not to trivialise serious subjects.
not necessarily (Score:2)
huge lenses (Score:5, Funny)
scientists needing huge optics 8-) whood a thought it
Pardon The Pun, But... (Score:4, Funny)
I don't think it exists. (Score:3, Informative)
Occam's Razor works strongly in favor of MoND over such hypotheses as dark matter... only time will tell.
Re: (Score:2)
Non of them are as consistent.
And there is nothing strange about a large part of the universe being quasi-undetectable. (which would be dark matter).
Dark _energy_ otoh isnt that illogical in terms of occams razor. Its as simple as the thought "the universe being here cannot be energy neutral as a whole".
Re: (Score:2)
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.
Logical or not, requiring that people ASSUME that most of the universe is made of barely- or non-detectable matter or energy is introducing outside complications of huge proportions. In contrast, MoND requires nothing of the sort. Ergo
Re:I don't think it exists. (Score:4, Informative)
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.
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Re:I don't think it exists. (Score:5, Interesting)
LCDM accounts for this. Heck, you don't think that we scientists got together one day and said "I know, lets make up some goofy theory and then fudge the data to fit it!" do you? You do realize multiple theories were purposed, predictions were created, new data was taken, and conclusions drawn about which theories were supported by the new evidence, right? And that LCDM is the one that survived all the vetting?
Just checking... See, that's sort of how science is supposed (and did in this case) work.
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Re: (Score:2)
For evidence of 'dark matter', you just need to look in the space between Bush's ears...
You don't always need to see everything. (Score:2, Funny)
If you smash your car into an invisible wall on a freeway, I'm sure you would be surprised since you didn't expect it. On the other hand those who are looking for invisible walls on freeways would be able to spot your lone smashed car on the freeway with their massive telescope and take a closer look.
They are trying to find that lone smashed up car in space.
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Why Lenses and Not Mirrors? (Score:5, Informative)
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.
There are wide-field mirror/lens hybrids (Score:2)
You can't really make very big maksutovs. You can make big schmidts - there is a 48-inch one at Palomar that is used for all-sky surveys, but they have a very practical problem that the focal surface is a very strongly curved sphere!
It's very easy to make a lens that has a wider field than a schmidt
Re: (Score:2, Informative)
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 diffic
Three answers: (Score:5, Informative)
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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This will happen... (Score:2, Funny)
"Damn, I dropped my contacts again. Oh wait, here they are. Hmmmm, they seam bigger for some reason...."
Hrm (Score:3, Interesting)
You can't observe dark energy or dark matter. They are fill in terms for unobserved matter and energy that must exist based off our limited observations, but we can't see.
I'm of the mind that neither exist and are kludges to stop the leaky pipes of modern science from falling apart, because key parts of our understanding of Cosmology and Astronomy are just plain WRONG.
"Huge Lenses To Observe Dark Energy" (Score:2)
The title of this story sounds like a description of a nerd leaning into his monitor to stare at goatse...