How Dark Matter Came to Matter

Jaco de Swart, Gianfranco Bertone, Jeroen van Dongen
The history of the dark matter problem can be traced back to at least the 1930s, but it was not until the early 1970s that the issue of ‘missing matter’ was widely recognized as problematic. In the latter period, previously separate issues involving missing mass were brought together in a single anomaly. We argue that reference to a straightforward ‘accumulation of evidence’ alone is inadequate to comprehend this episode. Rather, the rise of cosmological research, the accompanying renewed interest in the theory of relativity and changes in the manpower division of astronomy in the 1960s are key to understanding how dark matter came to matter. At the same time, this story may also enlighten us on the methodological dimensions of past practices of physics and cosmology.


Emergent Gravity and the Dark Universe

new_theory_of_gravity_954x716Recent theoretical progress indicates that spacetime and gravity emerge \break together from the entanglement structure of an underlying microscopic theory. These~ideas are best understood in Anti-de Sitter space, where they rely~on~the area law for entanglement entropy. The extension to de Sitter space requires taking into account the entropy and temperature associated with the cosmological horizon. Using insights from string theory, black hole physics and quantum information theory we argue that the positive dark energy leads to a thermal volume law contribution to the entropy that overtakes the area law precisely at the cosmological horizon. Due to the competition between area and volume law entanglement the microscopic de Sitter states do not thermalise at sub-Hubble scales: they exhibit memory effects in the form of an entropy displacement caused by matter. The emergent laws of gravity contain an additional `dark’ gravitational force describing the `elastic’ response due to the entropy displacement.
We derive an estimate of the strength of this extra force in terms of the baryonic mass, Newton’s constant and the Hubble acceleration scale α0=cH0, and provide evidence for the fact that this additional `dark gravity~force’ explains the observed phenomena in galaxies and clusters currently attributed to dark~matter.

Read also: New theory of gravity might explain dark matter

Dark Matter Grows ‘Hair’ Around Stars And Planets

Dark matter may make up 27% of the Universe’s energy density, compared to just 5% of normal (atomic) matter, but in our Solar System, it’s notoriously sparse. In particular, there’s just a nanogram’s worth per cubic kilometer, which makes the fact that we’ve never directly detected it seem inevitable.
But recent work has demonstrated that Earth and all the planets leave a ‘wake’ of dark matter where the density is enhanced by a billion times or more. Time to go put those dark matter detectors where they belong: in the path of these dark matter hairs….


Dark matter might cause fundamental constants to change over time

dark matter_1The fundamental constants of nature—such as the speed of light, Planck’s constant, and Newton’s gravitational constant—are thought to be constant in time, as their name suggests. But scientists have questioned this assumption as far back as 1937, when Paul Dirac hypothesized that Newton’s gravitational constant might decrease over time.

Now in a new paper published in Physical Review Letters, Yevgeny V. Stadnik and Victor V. Flambaum at the University of New South Wales in Sydney, Australia, have theoretically shown that dark matter can cause the fundamental constants of nature to slowly evolve as well as oscillate due to oscillations in the dark matter field. This idea requires that the weakly interacting dark matter particles be able to interact a small amount with standard model particles, which the scientists show is possible.
In their paper, the scientists considered a model in which dark matter is made of weakly interacting, low-mass particles. In the early Universe, according to the model, large numbers of such dark matter particles formed an oscillating field. Because these particles interact so weakly with standard model particles, they could have survived for billions of years and still exist today, forming what we know as dark matter.
Although these low-mass dark matter particles are weakly interacting, they are thought to still interact with standard model particles to some extent, but it’s unclear exactly how much. By using data from experiments that have measured the amount of helium produced during big bang nucleosynthesis, as well as measurements of the rare element dysprosium and the cosmic microwave background, Stadnik and Flambaum have derived the most stringent limits to date on how strongly such dark matter particles interact with photons, electrons, and light quarks, improving on existing constraints by up to 15 orders of magnitude.
The new limits on the dark matter interaction strength allow for the possibility that an oscillating, low-mass dark matter field coupled to standard model particles causes variations in the fundamental constants. As the scientists explain, this could have important implications for understanding life’s origins.
“The fundamental constants are ‘fine-tuned’ to be consistent with the existence of life in the Universe,” Stadnik told “If the physical constants were even slightly different, life could not have appeared. The discovery of varying fundamental ‘constants’ may help shed important light on how the physical constants came to have their life-sustaining values today. We simply appeared in an area of the Universe where they are consistent with our existence.”
Whether or not the fundamental constants actually do vary due to dark matter is still an open question, but the scientists hope that future experiments with atomic clocks, laser interferometers, and other devices may help test out the new idea.
“We have shown that linking dark matter and variation of the fundamental constants of Nature leads to a major breakthrough in the sensitivity of dark matter searches,” Flambaum said. “We plan to continue searching for other novel signatures of dark matter that may lead to the direct detection of dark matter for the first time.”

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Dark matter may power supernovae

Stellar explosions known as type Ia supernovae could be triggered by dark matter. So says a physicist in the US, who has worked out how certain burnt-out stars can explode even though they lack the mass to generate fusion reactions. According to the new research, the stars ignite because they accumulate so-called asymmetric dark matter, which, if real, could be detectable in a new generation of earthbound experiments.
Asymmetric dark matter, like familiar visible matter, would come in both matter and antimatter varieties. It was proposed on the basis that the density of dark matter in the universe today, as revealed by its gravitational interactions, is only about five times that of normal matter. In cosmological terms, the two matter densities are almost identical, and this suggests a common link between visible and dark matter. That being a very slight imbalance between matter and antimatter, which, following mutual annihilation in the early universe, resulted in the densities observed today.
This similarity does not apply to the current favourite dark-matter particles – weakly interacting massive particles (WIMPs) – which are their own antiparticles and could not have undergone a lopsided annihilation.

In the latest work [Dark matter ignition of type Ia supernovae], Joseph Bramante of the University of Notre Dame in Indiana looked for evidence of asymmetric dark matter in observations of type Ia supernovae, the “standard candles” that showed the universe’s expansion to be accelerating. Such supernovae are thought to be generated by white dwarfs, the very dense burnt-out remnants of Sun-like stars. Normally, white dwarfs are not massive enough to compress to the point where their internal temperature allows fusion reactions to take place. But astrophysicists believe they can accumulate additional mass by sucking material from nearby stars. They would eventually reach the “Chandrasekhar limit” of about 1.4 solar masses, at which point they would collapse and then blow apart as a result of an explosive burst of fusion energy.
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LHC results for dark matter from ATLAS and CMS

The ATLAS and CMS DM searches covered a huge range of final states during the first data-taking run of the LHC looking for signs of WIMP production. Although observation is consistent with SM background expectation, stringent limits have been set on different benchmark models, emphasising the complementarity of collider searches and direct detection searches. Collider searches can powerfully constraint the low DM-mass region where the direct detection experiments suffer a lack of sensitivity.
However the current benchmark models employed to describe the DM-SM interaction suffer of validity limitations in the high-energy regime. Thus a different choice will be performed for Run-II making use of simplified models which explicitly define the mediator particle, providing a more fair description of the interaction itself, and overcoming Effective Field Theory approach limitations.

XENON1T will join the hunt for dark matter this autumn

The XENON1T detector being assembled within the large tank that holds it deep underground. (Courtesy: Elena Aprile/XENON1T)

The XENON1T detector being assembled within the large tank that holds it deep underground. (Courtesy: Elena Aprile/XENON1T)

The hunt for dark matter will gain a more-than-an-order-of-magnitude boost in detection sensitivity when the next-generation XENON1T detector achieves first light this autumn. The challenges of constructing the world’s largest direct-detection dark-matter experiment and the scientific prospects for the future were presented by project spokesperson Elena Aprile of Columbia University, US, at the April Meeting of the American Physical Society in Maryland last weekend.
The XENON experiment began 10 years ago with XENON10, a 25 kg tank of liquid xenon deep under a mountain at the Gran Sasso National Laboratory in Italy. XENON100 followed in 2008 with 161 kg of liquid xenon and more than a hundred times the sensitivity of its predecessor. As the latest iteration, XENON1T is far more than a “second generation” detector – it contains 3300 kg of xenon and another hundred times the sensitivity of XENON100. Continue reading XENON1T will join the hunt for dark matter this autumn