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Exploring evidence of interaction between dark energy and dark matter

One of the most important problems of theoretical physics is to explain the fact that the universe is in a phase of accelerated expansion. Since 1998 the physical origin of cosmic acceleration remains a deep mystery. According to general relativity (GR), if the universe is filled with ordinary matter or radiation, the two known constituents of the universe, gravity should slow the expansion. Since the expansion is speeding up, we are faced with two possibilities, either of which would have profound implications for our understanding of the cosmos and of the laws of physics. The first is that 75% of the energy density of the universe exists in a new form with large negative pressure, called dark energy (DE). The other possibility is that GR breaks down on cosmological scales and must be replaced with a more complete theory of gravity. In this paper we consider the first option. The cosmological constant, the simplest explanation of accelerated expansion, has a checkered history having been invoked and subsequently withdrawn several times before. In quantum field theory, we estimate the value of the cosmological constant as the zero-point energy with a short-cut scale, for example the Planck scale, which results in an excessively greater value than the observational results….

read more at https://arxiv.org/pdf/1804.03296.pdf

Beyond the WIMP: Unique Crystals Could Expand the Search for Dark Matter

Proposed detector technology would be sensitive to an unexplored range of particle masses

A computerized simulation of the large-scale distribution of dark matter in the universe. An overlay graph (in white) shows how a crystal sample intensely scintillates, or glows, when exposed to X-rays during a lab test. This and other properties could make it a good material for a dark matter detector. (Credit: Millennium Simulation, Berkeley Lab)

A new particle detector design proposed at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) could greatly broaden the search for dark matter – which makes up 85 percent of the total mass of the universe yet we don’t know what it’s made of – into an unexplored realm. Continue reading Beyond the WIMP: Unique Crystals Could Expand the Search for Dark Matter

Cosmic Instability Could Have Created Dark Matter

A proposed instability in the Higgs field could have seeded the Universe with primordial black holes that now serve as dark matter.

Our Universe may be sitting on the edge of destruction, as theory suggests that the Higgs field is in a metastable state. If this field tunneled to its “true” minimum energy state, the release of energy would be cataclysmic. The danger may be over-stated, as other physical mechanisms could have kept the Universe stable throughout its history. Nevertheless, the Higgs instability could have a major cosmological impact as the source of dark matter. According to this new scenario, dark matter consists of a large population of black holes that formed from fluctuations in the unstable Higgs field at the dawn of the Universe. These so-called primordial black holes have been proposed before, but this is the first hypothesis that doesn’t require physics beyond the standard model.

Physicists have long been aware that the Universe might rest in a “false vacuum.” This idea has recently taken on new urgency, as calculations based on the measured Higgs mass have shown that a lower energy state may exist for the Higgs field. Analyzing the implications of the Higgs instability, José Espinosa, from the Catalan Institution for Research and Advanced Studies (ICREA) in Spain, and David Racco and Antonio Riotto, both at the University of Geneva, have found that this menacing mechanism could—counterintuitively—be instrumental in creating the dark matter that makes galaxies and other life-accommodating structures possible. In their calculations, the team explored fluctuations in the Higgs field during an early expansion phase of the Universe, called inflation. Under certain assumptions, these fluctuations become seeds for microscopic black holes with masses around 1015 kg that could have a density consistent with cosmological predictions of dark matter.

Michael Schirber – https://physics.aps.org/synopsis-for/10.1103/PhysRevLett.120.121301

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.
Read more at https://arxiv.org/pdf/1703.00013.pdf

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 more at https://arxiv.org/pdf/1611.02269v1.pdf

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….

Read more at www.forbes.com

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 Phys.org. “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.”

Read more at: phys.org – arxiv.org