Dark matter mapped - First three-dimensional picture of elusive matter throws up mystery.

news@nature.com ^ | 7 January 2007 | Katharine Sanderson

[neverdem]

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Published online: 7 January 2007; | doi:10.1038/news070101-7

Dark matter mapped

First three-dimensional picture of elusive matter throws up mystery.

Katharine Sanderson

Concentrations of dark matter (mapped in contours above) usually - but not always - match up with normal matter (coloured).

Hot on the heels of evidence last year that dark matter really does exist (see 'Dark matter spied in galactic collision'), the same technique has been used to map this uncharacterized mass across half a million distant galaxies.

The map shows that, as predicted, the mysterious dark matter that makes up a quarter of the Universe forms a filamentous 'skeleton' upon which visible matter congregates, eventually producing stars. This is the first time such a large-scale three-dimensional picture of dark matter has been produced, and it will allow cosmologists to probe deeper into the nature of this elusive matter.

But the map, published in Nature¹, also has a few puzzles within it. Some areas show clumps of dark matter that aren't accompanied by the bright features associated with conventional, visible material (made of baryonic matter), and vice versa.

"On the large scale the general picture is as expected, but there are some small-scale discrepancies," says Richard Massey at the California Institute of Technology, Pasadena, and one of the team members who pieced together the map from hundreds of slightly overlapping images from the Hubble Space Telescope's Cosmic Evolution Survey (COSMOS).

The existence of large clumps of isolated dark matter and visible matter flies in the face of everything we know, says cosmologist Carlos Frenk of the University of Durham, UK.

The discrepancies could be a simple error resulting from the way the observations were made. But if they are real, says Massey, they will bring a huge shock. "Baryonic structures are expected to form only inside the dark-matter scaffold," he says. "There will need to be a lot of follow-up work before we really believe any individual discrepancies."

A light pull

Massey used a technique called gravitational lensing, whereby the pull from dark matter caught in between a star and the observing telescope alters the path of the light, and allows the presence of dark matter to be inferred.

Eric Linder of the University of California, Berkeley, who was not involved with the work, agrees that the map backs up the favoured theory that dark matter forms a scaffold on which galaxies form.

He suggests possibilities for the more unusual spots in the map: one is that galaxies made of dark matter (dark galaxies) exist, but he thinks this is unlikely. Another possibility is that the discrepancies are errors in the data — which seem almost inevitable given that mapping the dark matter required a very sensitive measurement of an incredibly small signal. "Right now the discrepancies are curiosities rather than items of concern," Linder says.

Massey is also confident in the robustness of his map on the whole. "A couple of individual discrepancies in the map are not a huge surprise," he says. "The technique is intrinsically more noisy, and more prone to systematic errors, near the edges of the map." That is where most of the discrepancies are seen.

Blown away

There are plausible explanations for small areas of dark matter and visible matter existing in isolation.

Dark matter, if the clump is small enough, could have any accumulating visible matter blown out of it by a high-energy phenomenon such as a quasar or a supernova, for example. The collision of two galaxies could also blow an amount of visible matter out as a faint satellite galaxy that has no associated dark matter, suggests Frenk.

But these theories can't explain the large features visible on the COSMOS map, he adds.

Like Massey and Linder, Frenk also suspects that the discrepancies are due to errors: "We know too much about the Universe," he says, to have completely missed this phenomenon up till now.

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References

1. Massey R., et al. Nature, advance online publication, doi:10.1038/nature05497 (2007).

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[Ernest_at_the_Beach]

BBC:

Hubble makes 3D dark matter map

[neverdem]

Thanks for the link.

[Moonman62]

You're being silly. There is no point of origin. Space is expanding. The effects of dark matter have been observed for several decades including the collision of two galaxies mentioned in the article.

[gonzo]

"...dark matter really does exist..."

Damn! I just finished a book showing conclusively that it DOESN'T exist! Now what?

Think I'll stick with FR as my hobby until the deluge ................. FRegards

[woofie]

watch out for the shadowy matter

[Chena]

I do not believe in dark matter.

Then it doesn't matter. ;)

[Southack]

"There is no point of origin. Space is expanding. The effects of dark matter have been observed for several decades including the collision of two galaxies mentioned in the article."

Utter nonsense. If "space" was expanding then molecules would be growing larger.

If dark matter existed, then its gravity (that's why scientists want to pretend it exists, remember) would pull matter toward it, taking into account the "dark matter" map that you see in the article for this thread, which isn't uniform.

The truth is that dark matter is fiction.

[true_blue_texican]

Unless you are a cosmologist working with valid data,

Well, now, there's a non-sequitir. Dark Matter is the cosmologist's equivalent to the Immaculate Conception. As soon as first principles cause a contradiction, new doctrine is created to explain away the contradiction. Like the scholastics before them, modern cosmologists never consider that the problem is in their first principles.

[Physicist]

I think calling an element of the universe we simply do not have the eyes or the technology to directly detect as "dark" gives a negative, pejorative and subjective value to something that might be real but lie just beyond our present level of ignorance.

It's not pejorative. It's literally true. Whatever it is, it does not couple to electromagnetism; it cannot radiate or absorb light. It's not just a matter of a higher or lower frequency. The only handle we have on it (indeed, the only one we can have) is gravity. That's how it was first inferred, and that's how this map was made.

[silverleaf]

Looks like blood and other cells floating in a bloodstream, doesn't it?

[silverleaf]

Many cosmologists discuss their beliefs.

For some, ongoing discoveries affirm their beliefs. For others, ongoing discoveries change their beliefs.

Some (most?) of history's greatest scientist- at least before the past 50 years' aggressive attempts to secularize all public discussions - have accepted that there is room in the discussion tent for science and philosphy to co-reside.

[varon]

bttt

[Moonman62]

Utter nonsense. If "space" was expanding then molecules would be growing larger.

Actually, new space is created at an exponential rate. Space has physical properties. It is more than mere nothingness. An analogy would be the water in the ocean increasing in volume. The things in the ocean would not get any larger.

If dark matter existed, then its gravity (that's why scientists want to pretend it exists, remember) would pull matter toward it, taking into account the "dark matter" map that you see in the article for this thread, which isn't uniform.

That's exactly what astronomers see. This study has anomalies because of errors. That anomalies increase toward the edges of the field confirms it.

The truth is that dark matter is fiction.

The truth is you are going to great lengths to be willfully ignorant.

[RadioAstronomer]

Utter nonsense. If "space" was expanding then molecules would be growing larger.

Flapdoodle.

Local spacetime is bound by gravity. There is enough mass in your average galaxy to create a gravity well that prevents expansion. Molecules are even more absurd.

[THBlue50]

Yep. Looks like somebody threw up.

[Tenacious 1]

[And, by the same token, "beliefs" such as the' YEC' misinterpretation of Scripture are embarassingly irrelevant to the study or discussion of cosmology.]

Maybe not so irrelevant. Whether you be a religious person or not, theories start to mesh where hypothetical meets theoretical in the metaphysical conundrum of the unknown. Dark matter organized in ladder fashion that seems to hold the universe together? Sounds pretty organized to me for big bang chaos.

[GOPJ]

Thanks for the ping.

[Wuli]

"It's not pejorative. It's literally true. Whatever it is, it does not couple to electromagnetism; it cannot radiate or absorb light. It's not just a matter of a higher or lower frequency. The only handle we have on it (indeed, the only one we can have) is gravity. That's how it was first inferred, and that's how this map was made."

It is pejorative, because "light" and "dark" carry, in all human cultures, weighted values that goe beyond, and even long precede the mundane scientific knowledge and understanding of the properties of phontonic and other energy wavelengths.

"Dark" while scientifically meaning that our human photonic receptors do not receive photon input from something, carries a deeper meaning, culturally as "dark" places are dangerous or, at a minimum at least pose some level of risk and at a maximum are "evil", bad and everything opposite of "good" (which is associated with light). Thus "dark" always carries a subjectively pejoritive value.

To say that "dark" matter does not "absorb" "light" is an error, for we do not really know that. In fact it may "absorb" light so thoroughly, as part of the attributes whereby it seems to not radiate or reflect light.

It would be more accurate and less pejoritive if the missing matter was referred to simply as "hidden"; hidden from we who do not yet have the eyes, technical or otherwise, to "see" it.

[Physicist]

Thus "dark" always carries a subjectively pejoritive value.

And "heavy" means "fat" to many laymen, so that's pejorative, too; perhaps we should say the top quark is "big boned". What are you trying to protect against? Scientists know what is meant by "dark". If some laymen think it means "evil", what of it?

To say that "dark" matter does not "absorb" "light" is an error, for we do not really know that.

We absolutely do know that. Absorption is as easy to detect as emission. Simply measure the spectrum of light from more distant sources, and see what's missing. (Google the term "Lyman Alpha Forest" for a wealth of details.)

Indeed, our ability to measure precisely all of the intervening baryonic matter (i.e., matter made of protons and neutrons) provided one of the clues that revealed the existence of dark matter in the first place: there was nowhere near enough absorption to account for the observed gravitational lensing effects.

In fact it may "absorb" light so thoroughly, as part of the attributes whereby it seems to not radiate or reflect light.

So it absorbs energy but it doesn't radiate it? Where does the energy go? Think about it.

It would be more accurate and less pejoritive if the missing matter was referred to simply as "hidden";

"Hidden" carries its own set of cultural baggage. It implies that there's nothing different about it, except that we haven't looked in the right place. In fact, we know enough about dark matter to say that it's physically unlike anything we've found on Earth. Moreover, we now know where it is.

hidden from we who do not yet have the eyes, technical or otherwise, to "see" it.

We've ruled out seeing it by the strong, weak, or electromagnetic means. That leaves only gravity (which we are using as best we can, indirectly) and some undiscovered, untheorized, unanticipated force, for which there is otherwise no theoretical or experimental motivation to expect.

[Southack]

"Actually, new space is created at an exponential rate. Space has physical properties. It is more than mere nothingness. An analogy would be the water in the ocean increasing in volume. The things in the ocean would not get any larger."

Creating new space is *entirely* different from that of space itself "expanding."

A more appropriate way to look at it is that if an ocean freezes, the ice expands and takes up more space...the water literally expands.

Is space literally expanding, or is new space being created?