Dark-matter map points to galaxy formation

So exciting – dark matter to the fore!

8 January 2007

The Universe is permeated by filaments of invisible dark matter that intersect at galaxies and other major structures. This is the conclusion of astronomers from the Cosmic Evolution Survey (COSMOS), who have created the first large-scale map of the distribution of dark matter. The apparent overlap of dark matter filaments with galaxies and other massive structures adds further weight to the theory that the Universe owes its structure to the gravitational pull of dark matter (Nature doi:10.1038/nature05497).

Dark matter is fundamentally different from normal “luminous” matter that makes up stars, planets and humans. It is invisible to modern telescopes, giving off no light or heat, and it seems to interact with normal matter only through gravity. Although dark matter has never been observed directly, most cosmologists believe dark matter plays a crucial role in how large structures such as galaxies emerged after the Big Bang.

Dark and light

The COSMOS team used the Hubble Space Telescope and several terrestrial instruments to chart the position of elusive dark matter in three dimensions. This was done by observing how light from distant galaxies is bent by the gravitational pull of dark matter in a process called gravitational lensing.

The map also reveals that regions of space containing large quantities of luminous matter almost always also contain large quantities of dark matter, which is exactly what physicists would expect to see if the gravitational collapse of dark matter was responsible for the structure in the Universe. “It’s reassuring how well our map confirms the standard theories for structure formation”, said lead researcher Richard Massey of the California Institute of Technology.

However, the survey also reveals areas with large quantities of dark matter with no corresponding luminous matter. In principle, this is also feasible because physicists believe that there is much more dark matter in the Universe than luminous matter.

While astronomers have already used gravitational lensing to map smaller regions surrounding individual galaxies, this is the first “wide-sky” survey that covers a region of the sky about the eight times the size of a full moon. The survey is also the first to look at dark matter in three dimensions – the third dimension being the distance (or time) travelled by the light after interacting with the dark matter. The distance was determined by combining observations made by Hubble and earthbound telescopes. This new ability to chart the evolution of dark matter through both space and time could also shed light on another elusive quantity – dark energy, which is believed to be accelerating the expansion of the Universe.

About the author

Hamish Johnston is editor of Physics Web

Light and dark, very Star Wars indeed!

January 8, 2007

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01:19:13 am, Categories: Physics, Space and Cosmology, 531 words

First Dark Matter, Then Dark Energy, Now a Dark Force? The main annual conference of the American Astronomical Society began this morning, and it didn’t take long to roll into action. In one of the very first sessions, Glennys Farrar of New York University described some startling hints of a fifth force of nature, on top of the Fab Four: electromagnetism, gravity, and the two forces that govern atomic nuclei. The idea of a fifth force has a checkered history, and experiments seem to rule it out. But those experiments apply only to ordinary matter. They say nothing about dark matter.

Dark matter is the unknown substance that provides the gravitational glue holding together large cosmic structures such as galaxies and clusters of galaxies. The poster child for dark matter, which got a lot of attention last summer, is the Bullet Cluster of galaxies. It is actually a pair of clusters that have rammed into each other. The center of mass of each cluster (pinpointed by seeing how the cluster affects light from bodies in back of it) is offset from the bulk of the ordinary matter — so most of the mass of the clusters must be un-ordinary.

What’s less well known is that the smaller of the two colliding clusters is a cluster in a hurry, zipping along at 4700 kilometers per second. Some researchers have argued that its speed, though eyebrow-raisingly high, is not beyond the pale. Farrar disagreed. She and her graduate student Rachael Rosen estimated a few months ago that gravity should have accelerated the cluster to maybe 3000 km/s. Even if the cluster had an improbable combination of elongated shape, high initial velocity, and special viewing geometry, it should move no faster than 3400 km/s.

Farrar concluded that some new force must be helping to accelerate it. Such a force might also explain a number of other discrepancies that astronomers have found. For instance, the universe contains more galaxy superclusters than standard dark-matter models predict. A new force would grease the wheels of supercluster formation.

It would also have telltale effects within our own galaxy. The Milky Way captures and dismembers small galaxies that stray too close, creating long, drawn-out trails of stars. If the dark matter inside those galaxies felt an extra force, the trails would get skewed. Looking for such effects will be a way to prove or disprove the new force.

What the putative force might be, no one knows, any more than anyone knows what dark matter is to begin with. Astronomically, it acts just like gravity. But its source could be vastly different — the result, perhaps, of a property akin to electric charge which only dark matter possesses. Proposed new theories of physics such as string theory predict new energy fields that might generate novel forces, but in the past physicists have generally supposed that these forces would make themselves felt only over sub-subatomic distances.

I think the most interesting thing is the cluster velocity discrepancy. It may or may not entail a new force of nature, but it is certainly something that has to be explained one way or another. As Farrar told me: “One thing you can be sure about is that there is something we don’t understand.”
Posted by George Musser

Well, “may the force be with you”?