New theories of the enduring dark matter mystery range from planet-sized particles to dark-matter life. One of the primary qualities of dark matter –emitting neither light nor any other known kind of radiation–is that it only interacts with other matter via gravity: it carries no electromagnetic charge. Dark matter is also “dark” because it is mysterious: not composed of atoms or their usual constituents like electrons and protons.
Particle physicists have imagined new kinds of matter, consistent with the known laws of the universe, but so far none has been detected or its existence confirmed. The Large Hadron Collider’s discovery of the Higgs boson in 2012 prompted a burst of optimism that dark matter particles would soon be discovered, but so far none has been seen and previously promising classes of particles now seem to be long-shots.
“The nature of dark matter is one of the biggest mysteries in science and we need to use any related new data to tackle it,” says Harvard’s Avi Loeb of the CfA.
Astronomers realize that dark matter is the dominant component of matter in the universe. Whatever its nature, it profoundly influenced the evolution of galactic structures and the distribution of the cosmic microwave background radiation (CMBR).
The remarkable agreement between the values of key cosmic parameters (like the rate of expansion) derived from observations of two completely different kinds of large-scale cosmic structures, galaxies and the CMBR lend credence to inflationary big bang models that include the role dark matter.
Current models of dark matter presume it is “cold,” that is, that it does not interact with any other kinds of matter or radiation—or even with itself—beyond the influences of gravity. This version of cosmology is called the cold dark matter scenario. But cosmologists wonder whether more precise observations might be able to exclude even small levels of interactions.
Warm Dark Matter
Astronomer Sownak Bose with the Institute for Theory and Computation at Harvard-Smithsonian Center for Astrophysics led a team of colleagues in a study of one very popular (if speculative) “dark matter” particle, one that has some ability to interact with very light particles that move close to the speed of light. This version forms one of several possible warm dark matter (perhaps more accurately called interacting dark matter) scenarios. In particular, the hypothetical particles are allowed to interact with neutrinos –neutrinos are expected to be extremely abundant in the hot early universe.
The scientists used state-of-the-art cosmological simulations of galaxy formation to a model universe with this kind of warm dark matter. They find that for many observations the effects are too small to be noticeable. However, the signature of this warm dark matter is present in some distinct ways, and in particular in the way distant galaxies are distributed in space, something that can be tested by mapping galaxies by looking at their hydrogen gas.
The authors conclude that future, highly sensitive observations should be able to make these tests. Detailed new maps of the distribution of hydrogen gas absorption could be used to support—or exclude—this warm dark matter possibility (see the figure), and shed light on this mysterious cosmic component.
The Symmetron Field
On July 19, The Galaxy posted “Quintessence” –‘Theory About a Great Unknown of the Universe’ reporting on several radical conjectures about both dark matter and dark energy, including a symmetron field could pervade space much like the Higgs Field, and a theory that suggests both dark matter and dark energy can be unified into a fluid which possesses a type of ‘negative gravity’, repelling all other material around them.
Galaxy-Sized Dark Matter Particles
“We tend to think about particles as being tiny but, theoretically, there is no reason they can’t be as big as a galaxy,” says Asimina Arvanitaki, The Aristarchus chair in theoretical physics at the Perimeter Institute for Theoretical Physics, about solving the mystery of dark matter.
“At first, we thought it was absurd,” Arvanitaki told New Scientist’s Daniel Cossins . “I’m not surprised. How else could you respond to the idea that black holes (Milky Way’s supermassive black hole above) generate swirling clouds of planet-sized particles that could be the dark matter thought to hold galaxies together?”
Another new theory posits that both dark matter and dark energy can be unified into a fluid which possesses “a type of ‘negative gravity’, repelling all other material around them,” says Jamie Farnes from the Oxford University e-Research Center. “The outcome seems rather beautiful: dark energy and dark matter can be unified into a single substance, with both effects being simply explainable as positive mass matter surfing on a sea of negative masses.”
A more speculative candidate for what dark matter might be are massive particles that only rarely interact with normal matter. These particles could be their own antiparticles, meaning they annihilate each other when they meet, releasing energy. These invisible particles could get captured by a planet’s gravity and unleash energy that could warm that world, according to physicist Dan Hooper and astrophysicist Jason Steffen at the Fermi National Accelerator Laboratory.
In 2012, Hooper and Steffen’s proposed that rocky “super-Earths” in regions with high densities of slow-moving dark matter could be warmed enough to keep liquid water on their surfaces, even in the absence of additional energy from starlight or other sources. The density of dark matter is expected to be hundreds to thousands of times greater in the innermost regions of the Milky Way and in the cores of dwarf spheroidal galaxies than it is in our solar system.
The scientists concluded that on planets in dense “dark-matter” regions, it may be dark matter rather than light that creates the basic elements you need for organic life without a star.”
Dark matter, the team believes, could keep the surfaces of such warm for trillions of years, outliving all regular stars and may ultimately prove to be the “dark” bastion of life in our universe.
“I imagine 10 trillion years in the future, when the universe has expanded beyond recognition and all the stars in our galaxy have long since burnt out, the only planets with any heat are these here, and I could imagine that any civilization that survived over this huge stretch of time would start moving to these dark-matter-fueled planets,” Hooper said in an interview with space.com.
The Hubble Space Telescope composite image at the top of the page shows a ghostly “ring” of dark matter in the galaxy cluster Cl 0024+17. The ring-like structure is evident in the blue map of the cluster’s dark matter distribution. The map is superimposed on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence tfor its existence.
The Daily Galaxy, Max Goldberg, via Harvard-Smithsonian Center for Astrophysics