Horatio:
O day and night, but this is wondrous strange!
Tonight‘s video: Shedding Light on the Dark Universe with Dr. Patricia Burchat presented at TED2008.
Patricia Burchat sheds light on dark matter
A transcript of the video available on the TED page here if you do not have the bandwidth to view it. Below the squiggle, if you‘re interested, I expand a bit, adding some historical background and a touch of mathematics (I promise, no integrals or differential equations :). I hope you all enjoy and tune in for the next exciting episode, Shedding Light on the Dark Universe: Part 2—Dark Energy and the Ever Expanding Waistline Universe
There are a lot of videos posted from various sources here on dKos, music videos to nourish our souls, comedy and political satire that tickle and tackle the irony of our lives, and video news to keep the fire of change burning in our psyche. A few videos, too few in my humble opinion, are about science. I thought our community might appreciate a series presenting interesting science videos from Ted, YouTube, wherever.
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For my good friend twigg, who had no idea what he was suggesting:)
Can you be convinced that something you can‘t see actually exists?
Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the anisotropies observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only "dark" but also, by definition, utterly transparent.Wiki
In 1933 Fritz Zwicky, someone I‘ve mentioned before, first showed that there was more mass associated with the Coma cluster of galaxies (mentioned in the video) than could be accounted for by observable evidence and by applying the virial theorem argued that the galaxies on the perimeter of the cluster traveled too fast to account for the enclosed visible mass of the system. Since then astronomers and astro-physicists have studied the dynamics galaxy clusters and found the data to be consistent with Zwicky‘s hypothesis.
It took some 40 years, however, for Zwicky‘s hypothesis to be corroborated via an alternative technique. Vera Rubin (quite an interesting scientist, by the way), using a new and highly sensitive spectrometer developed by Kent Ford, studied the rotational velocities of stars orbiting a galaxy and published the startling results.
In 1974 the astronomer Vera Rubin, was working on a project investigating stars at the outer edges of galaxies. What she discovered was quite a surprise.
Shortly after the apple fell on his head, Newton famously declared that gravity was 'universal'. An apple falling on Earth obeys the same mathematical rules as an apple falling on the other side of the Universe. In the same way that the Sun controls the orbiting planets by exerting gravity on them, a spiral galaxy must be controlled by the gravity-giving black hole at its centre.
It has long been known that Pluto, at the edge of our solar system, travels much slower than Mercury, close to the Sun. In fact observations like these allowed Newton to pin down his laws in the 17th century. When Vera Rubin did her work on galaxies she expected to find that as you reach the edge of a galaxy the stars would be moving much slower than those close to the centre. But it didn't work out like that at all.BBC
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Dr. Rubin found that stars on the periphery of galaxies were moving at the same orbital velocity as those closer in. The graph on the left (also shown in the video) is a typical representation of the phenomena. In science when predictions go that wrong something‘s got to change (unlike Republican economic theories). Either Newton‘s inverse square law isn‘t correct (see MOND) or there must exist a lot more mass/energy (gravitational pot- ential) associated with the galaxy than can be detected. Dark Matter Physics succinctly states the alternative hypothesis...
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Rotation curve of a typical spiral galaxy: predicted (A) and observed (B). Dark matter can explain the velocity curve having a 'flat' appearance out to a large radiusWiki.
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Now that there were two independent sets of empirical results, astronomers and astro-physicists began a search for other ways to test the dark matter hypothesis. Foremost among possible tests is a concept known as strong gravitational lensing. This is an idea that Einstein proposed as a fundamental test of his theory—a gravitational field will bend the path of an electromagnetic radiation (the equivalence principal). As a matter of fact, one of the first confirmations of General Relativity was made by the observation of the deflection of light during the full solar eclipse of May 29th, 1919 by Arthur S. Eddington and Frank W. Dyson (also mentioned in passing here). Simply put, a sufficiently strong gravitational field can act as a lens—a gravitational lens—and, as Dr. Burchat demonstrates with the base of her wine glass, can create an Einstein ring.
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The NASA/ESA Hubble Space Telescope has revealed a never-before-seen optical alignment in space: a pair of glowing rings, one nestled inside the other like a bull‘s-eye pattern. The double-ring pattern is caused by the complex bending of light from two distant galaxies strung directly behind a foreground massive galaxy, like three beads on a string.ESA
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An Einstein ring is a long known consequence of General Relativity but the first one wasn‘t observed until the Very Large Array (VLA) and Einstein ring in the radio spectrum 1987. Since that time the Hubble Telescope has found many, many more and in 2008 the first double Einstein Ring (image left) ever seen was discovered by the Hubble telescope.
The ring was found by an international team of astronomers led by Raphael Gavazzi and Tommaso Treu of the University of California, Santa Barbara. The discovery is part of the ongoing Sloan Lens Advanced Camera for Surveys (SLACS) program.HubbleSite |
Einstein rings provide astronomers a simpler way to deduce the mass associcated with an intervening field so that they need not spend hours upon hours of observation time plotting the positions of stars in a galaxy or galaxies in a cluster over time to determine their rotational velocities. They‘ve just to consider a single equation to deduce the mass of the intervening object.
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where
- G is the gravitational constant,
- M is the mass of the lens,
- c is the speed of light,
- DL is the angular diameter distance to the lens,
- DS is the angular diameter distance to the source, and
- DLS is the angular diameter distance between the lens and the source.
Note that, over cosmological distances DLS≠DS−DL in general.Wiki
Geometry of an Einstein ring.Wiki
Note that θ1 in the graphic is θE in the equation above.
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Because we don‘t know what form Dark Matter might take scientists have considered, as a sort of first cut, three possible states under which the proposed dark matter particles might exist, hot, warm, and cold.
Hot dark matter refers to particles that are relativistic, that is, they have a velocity greater than 95% of the speed of light. The neutrino would be an example of such a particle. If there were enough of these weakly interacting particles the missing mass responsible for the observed data then there would be no need to assume a some other unknown particle. The problem with this suggestion, though, is that anything traveling at near the speed of light would tend to smooth out the density fluctuations that are evident in the universe we see today—galaxies and other small scale structures would not have formed.
Hot dark matter cannot explain how individual galaxies formed from the big bang. The microwave background radiation as measured by the COBE satellite is very smooth and fast moving particles cannot clump together on this small scale from such a smooth initial clumping. To explain small scale structure in the Universe it is necessary to invoke cold or warm dark matter. Hot dark matter therefore is nowadays always discussed as part of a mixed dark matter theory.
Warm dark matter would be weakly interacting particles travelling between .3 and .95 c (speed of light). The suggested particles that make up warm dark matter are the sterile neutrinos and gravitinos (the supersymetric partner of the graviton). However, both of these proposed particles present some fundamental problems as hypothetical dark matter candidates.
Cold dark matter candidates include WIMPS (weakly interacting massive particles), Axions, or MACHOS (I want a job making up acronyms [there are, by the way, also RAMBOS] {if you‘ve got WIMPS...} :) or Massive compact halo objects. Cold dark matter is currently the concensus candidate hypothesis as it most closely fits a wider range of observations within the current model of our universe.
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Hamlet:
And therefore as a stranger give it welcome.
There are more things in heaven and earth, Horatio,
Than are dreamt of in your philosophy.
Of course, there are alternative theories to explain the gravitational effects described above. Modified Newtonian Dynamics proposed by Mordehai Milgrom and it‘s variations (TeVeS) address Dr. Rubin‘s galactic rotational velocity data. John Moffet of the the Perimeter Institute for Theoretical Physics has proposed Scalar Tensor Vector Gravity or MOG (MOdified Gravity), and Dark Fluid theory which attempts to group both Dark Matter and Dark Energy into a single construct, a kind of perfect universal gas. All of these theories and the Dark Matter hypothesis are subject to falsification by observation.
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Thank you for taking the time to read and view the various videos presented here. I hope you‘ve learned a bit and had some fun doing it. I can provide a bibliography of the 60 or so webpages I visited to research this little project if you would like. Email me at the address in my profile.
By the way, if you stand strong in the way of the geek here is a page of splendid nerdiferousness which breaks the promise I made above the fold.
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