From NOVA: “Dark Matter’s Invisible Hand” 



13 Apr 2016
Charles Q. Choi

Dark matter is currently one of the greatest mysteries in the universe.

Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al
Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

It’s thought to be an invisible substance that makes up roughly five-sixths of all matter in the cosmos, a dark fog suffusing the universe that rarely interacts with ordinary matter. But when it does, according to an unexpected finding by theoretical physicist Lisa Randall, the consequences could be momentous.

Astronomers first detected dark matter through its gravitational pull, which apparently keeps the Milky Way and other galaxies from ripping themselves apart given the speeds at which they spin.

Scientists have mostly ruled out all known ordinary materials as candidates for dark matter. The current consensus is that dark matter lies outside the Standard Model of particle physics, currently the best description of how all known subatomic particles behave.

The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.
The Standard Model of elementary particles , with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

Specifically, physicists have suggested that dark matter is composed of new kinds of particles that have very weak interactions—not just with ordinary matter but also with themselves.

Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions
Gamma rays from the Fermi Gamma-ray Space Telescope, could be produced by proposed dark matter interactions.

NASA/Fermi Gamma Ray Telescope
NASA/Fermi Gamma Ray Telescope

However, Randall and other scientists have suggested that dark matter might interact more strongly with itself than we suspect, experiencing as-yet undetected “dark forces” that would influence dark matter particles alone. Just as electromagnetism can make particles of ordinary matter attract or repel each other and emit and absorb light, so too might “dark electromagnetism” cause similar interactions between dark matter particles and cause them to emit “dark light” that’s invisible to ordinary matter.

Differentiated Dark Matter

The evidence for this theory can be seen in potential discrepancies between predictions and observations of the way matter is distributed in the universe on relatively modest scales, such as that of dwarf galaxies, says Randall, a professor at Harvard University.

Dwarf Galaxies with Messier 101  Allison Merritt  Dragonfly Telephoto Array
Dwarf Galaxies with Messier 101 Allison Merritt Dragonfly Telephoto Array

For example, repulsive interactions between dark matter particles might keep these particles apart and reduce their overall density, explaining why current estimates of the density of the innermost portion of galaxies are higher than what is actually seen.

Most dark matter models suggest that dark matter particles are all of one type—they either all interact with each other or they all do not. However, Randall and her colleagues propose a more complex version that they call “partially-interacting dark matter,” where dark matter has both a non-interacting component and a self-interacting one. A similar example in real particles can be seen with protons, electrons, and neutrons—positively charged protons and negatively charged electrons attract one another, while neutrally charged neutrons are not attracted to either protons or electrons.

“There’s no reason to think that dark matter is composed of all the same type of particle,” Randall says. “We certainly see a diversity of particles in the one sector of matter we do know about, ordinary matter. Why shouldn’t we think the same of dark matter?”

In this model, Randall and her colleagues suggest that only a small portion of dark matter—maybe about 5%—experiences interactions reminiscent of those seen in ordinary matter. However, this fraction of dark matter could influence not only the evolution of the Milky Way, but of life on Earth as well, an idea Randall explores in her latest book, Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe.

Standard dark matter models predict that dwarf galaxies orbiting larger galaxies should be scattered in spherical patterns around their parents. However, astronomical data suggest that many dwarf galaxies orbiting the Milky Way and Andromeda lie roughly in the same plane as each other. Randall and her colleagues suggest that if dark matter particles can interact with each other, they can shed energy, potentially creating a structure that could not only solve this dwarf galaxy mystery, but also have triggered the cosmic disruption that doomed the dinosaurs.

Dark Disks

In the partially interacting dark matter scenario, the non-interacting component would still form spherical clouds around galaxies, consistent with what astronomers know of their general structure. However, self-interacting dark matter particles would lose energy and cool as they jostled with each other. Cooling would slow these particles down, and gravity would make them cluster together. If these clouds were relatively immobile, they would simply shrink into smaller balls.

However, since they likely rotate—just like the rest of the matter in their galaxies—this rotation would make these clouds of self-interacting dark matter collapse into flat disks, in much the same way as spherical clouds of ordinary matter collapsed to form the spiral disks of the Milky Way and many other galaxies. Conservation of angular momentum causes these would-be spheres to flatten out. While cooling would still cause them to collapse vertically, they would not collapse along the same plane as their rotation.

If dark matter in large galaxies was concentrated in disks, it’s likely that at least some of the orbiting dwarf galaxies would be concentrated in flat planes because of the gravitational pull of dark matter on the dwarf galaxies, Randall and her colleagues say. The researchers suggest these “dark disks” should be embedded in the visible disk of larger galaxies.

But here’s where dark matter begins to exert its influence. The relationship between the dark disk and the stars in the galaxy is not entirely stable. The Sun, for example, completes a circuit around the Milky Way’s core roughly every 240 million years. During its orbit, it bobs up and down in a wavy motion through the galactic plane about every 32 million years. Coincidentally, some researchers previously suggested that meteor impacts on Earth rise and fall in cycles about 30 million to 35 million years long, leading to regular mass extinctions.

Earlier researchers proposed a cosmic trigger for this deadly cycle, such as a potential companion star for the Sun dubbed “Nemesis” that would ensnare meteoroids and send them hurling toward Earth. Instead, Randall and her colleagues suggest that the Sun’s regular passage through the Milky Way’s dark disk might have warped the orbits of comets in the outer solar system, flinging them inward. Such disruption may have then led to disastrous cosmic impacts on Earth, including the collision about 67 million years ago that likely caused the Cretaceous-Tertiary extinction event, the most recent and most familiar mass extinction which killed off all dinosaurs (except those which would evolve into birds).

The main suspect behind this disaster is an impact from an asteroid or comet that left behind a gargantuan crater more than 110 miles wide near the town of Chicxulub in Mexico. The collision, likely caused by a meteor about 6 miles across, would have released as much energy as 100 trillion tons of TNT, more than a billion times more than the atomic bombs that destroyed Hiroshima and Nagasaki, killing off at least 75% of life on Earth.

In research* detailed in 2014 in the journal Physical Review Letters, Randall and her colleague Matthew Reece analyzed craters that are more than 12.4 miles in diameter and were created in the past 250 million years. When they compared the ages of these craters against the 35-million-year cycle they proposed, they discovered that it was three times more likely that the craters matched the dark matter cycle instead of simply occurring randomly.

“I want to be clear that I did not set out to explain the extinction of the dinosaurs,” Randall says. “This work was about exploring the story of how our universe came about, to explore one possible connection between many different levels of the universe, from the universe down to the Milky Way, the solar system, Earth, and life on Earth.”

Geologist Michael Rampino at New York University, who did not participate in this study, finds a potential link between a dark disk and mass extinctions “an interesting idea,” he says. “If true, it ties together events that happened on Earth to large-scale cycles in the rest of the solar system and even the galaxy in general.”

Dark Life?

However, not everyone agrees that Randall and her colleagues present a convincing case. “They tie mass extinctions to the cratering record, but there are all kinds of estimates one can make with the cratering record depending on what craters one think makes the cut—do you accept all craters above a certain size or craters out to a certain age?” says astrobiophysicist Adrian Melott at the University of Kansas, who did not take part in this research. “You can get all different kind of answers, from cycles 20 million to 37 million years long.”

Moreover, other research suggests that the cycle of mass extinctions on Earth is actually roughly 27 million years long, Melott says. “That’s way too short a duration for motion oscillating back and forth through the disk.”

Randall notes that data from the European Space Agency’s satellite Hipparcos, launched in 1989 to precisely measure the positions and velocities of stars, allowed for the theoretical existence of a dark disk. She adds that ESA’s Gaia mission, launched in 2013 to create a precise 3D map of matter throughout the Milky Way, could reveal or refute the dark disk’s existence.

ESA/Gaia satellite
ESA/Gaia satellite

One intriguing possibility raised by interacting dark matter models is the existence of dark atoms that might have given rise to dark life, neither of which would be easily detected, Randall says. Although she admits that the concept of dark life might be far-fetched, “life is complicated, and we have yet to understand life and what’s necessary for it.”

*Science paper:
Dark Matter as a Trigger for Periodic Comet Impacts

Science team:
Lisa Randall and Matthew Reece

Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA

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