Physics

Stanford study suggests dark matter is lighter and more elusive than previously thought

Stanford study suggests dark m...
A simulation of the dark matter structure around the Milky Way galaxy – ironically, the brighter spots indicate more dark matter
A simulation of the dark matter structure around the Milky Way galaxy – ironically, the brighter spots indicate more dark matter
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A simulation of the dark matter structure around the Milky Way galaxy – ironically, the brighter spots indicate more dark matter
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A simulation of the dark matter structure around the Milky Way galaxy – ironically, the brighter spots indicate more dark matter

Although it makes up about 85 percent of all matter in the universe, dark matter is frustratingly hard to pin down. In order to figure out what it is, much of the search is about ruling out what dark matter isn't, and now physicists at Stanford and the SLAC National Accelerator Laboratory have narrowed it down further. Using observations of galaxies orbiting the Milky Way, the team found that dark matter is likely lighter than previously thought – and interacts even less with normal matter.

Dark matter is so called because it doesn't emit or react to light in any way, and rarely interacts with regular matter, making it all but invisible to us. The only way we know anything is there at all is because of its gravitational effects on the stuff we can see.

What dark matter actually is and what its properties are remains open for debate, but scientists have developed a few models. One of the most widely-accepted is known as the Lambda cold dark matter (Lambda CDM) model, and it assumes a few things about the mysterious stuff: it's "cold" meaning it moves much slower than the speed of light, and it's "collisionless," so it very rarely bumps into regular matter.

The Lambda CDM model also tells us that dark matter played a vital role in the formation of the universe as we know it. In the early days, the cosmos was basically flat and uniform, but because it moved slower, dark matter tended to clump together in "haloes." Eventually the gravity drew regular matter into those clumps, causing galaxies to form in structures that look like giant sparkling spider webs.

While this model has worked well to explain these large-scale structures, it tends to fall apart when applied to smaller scales, like individual galaxies. For example, the model predicts that our own Milky Way should be orbited by thousands of smaller satellite galaxies, but as of right now only 59 have been discovered.

The researchers on the new study used this as a starting point to tweak the model. They ran simulations of how the universe looked when filled with dark matter of varying properties, and then overlaid the resulting dark matter haloes with the known structure of the Milky way's satellite galaxies, to see how well it all fit.

They found that for everything to fit together neatly, dark matter must have a lighter mass and must be "warmer" (i.e. moves a bit faster) than previously assumed. It also seems to interact even less often with regular matter – about a thousand times more weakly than the previous limit. That might explain why none of the many experiments designed to detect those interactions have registered any signals yet.

"What's really exciting is that our study nicely bridges experimental observations of faint galaxies today with theories of dark matter and its behavior in the early universe," says Ethan Nadler, lead author of the study. "It connects a lot of pieces, and by doing so it tells us something very profound about dark matter. Although we still don't know what dark matter is made of, our results are a step forward that sets tighter limits on what it actually can be."

Of course, the dark matter debate is far from over. Other studies suggest the mysterious stuff may be in fuzzy and excited states, made of electrically charged particles, gather in "hairs" around planets, or exist as a kind of "dark fluid" with negative mass.

The research was published in the Astrophysical Journal Letters. The simulated evolution of dark matter can be seen in the video below.

Source: SLAC

Simulation of the formation of dark matter structure around the Milky Way

6 comments
augustus
There must be dark matter as our models do not make sense without it - and our models are infallible of course.
bwana4swahili
Simply tweaking the gravity 'constant' for large separations yields similar results and is a lot simpler AND the universe loves the KISS principle.
Rotogizmag
Simply anything probably won't work, unless there are reasons to believe the simply something. I suspect the simply somethings such as MOND and other non-Einstein/Newtonian gravity assumptions are not correct. Without laboratory measurements that try local scale confirmation or insightful new ideas, not just an assumed simple scaling factor like MOND, I highly doubt these models. The fact is we just don't know anything about dark matter. Assumptions are fine, but not compelling without depth, cogent reasoning, and evidence from multiple means of measurement and comparatives to multiple assumed theories. If MOND were real there should be some small scale effects as well and certainly some logical reason for such simple deviation in the strength law that applies to virtually all fields.
highlandboy
Another story about the possible properties of a so far imaginary substance. This is a theory that has stuck around that people are starting to believe. The reasoning is the longer the theory exists the more likely it is to be true. Yet so far very little is known (if it does exist), so it is very difficult to disprove. This is not science. In fact it is not a theory- that would require more facts. This is hypothetical reasoning based on “my current theory doesn’t quite work”.
amazed W1
Dark matter is an intellectual construct, so there is no reason "per se" that it should exist. This absence of mass might be explained by forces from other sources, ranging from "quantum entanglement" to Scott's forces induced by relative movements of plasma. Perhaps we are too conditioned by searches for holy grails and life on exoplanets to let the concept be filed as a probable/possible rather than as a definite.
Nobody
Many un-provable theories have been developed to prop up a big bang at about 13 billion years ago. If the big bang did occur it was likely 50-100 billion years ago. With the longer timeline we don't need the inflation theory. This longer timeline would also allow for more generations of stars to create the heavier elements. And if only 80% of each early generation went nova and formed the next generation that would leave 20% of each generation time to become cold unobservable cinders in space but their gravity would be observable. Also after a few generations about 90% of the total ordinary matter would become cold and unseen. Now we don't need the exotic dark matter theory to explain this. This also brings up the possibility of the bulk of the unseen mass of the universe being along the outer expansion ring and drawing us toward it. This would then replace the dark energy and expansion theory. ALL OF THIS could simply be explained by a far longer timeline than just 13 billion years. Consider this. Much of what we observe in the distant universe isn't there anymore and with gravitational lensing everything is further distorted. It's like looking at everything through a kaleidoscope and trying to make the math match what you think you see. By the way, I have several science degrees including physics and chemistry and have spent years working with mathematical models used for spectral analysis. I will be the first to say that math doesn't purify bad data. It can be twisted to fit ANY preconceived theory. I could be totally wrong but so could they.