Neutrons Knock at the Cosmic Door

Figure 1 (Left) Neutron mirror apparatus. An ultracold neutron (UCN) enters a space between two mirrors that act as potential wells, giving rise to a discrete energy spectrum. A detector measures neutrons exiting the cavity formed by the mirrors. The bottom mirror sits upon a nanopositioning table that induces a vertical oscillation that produces dips in the neutron transmission at the resonances. (Right) Energy-level diagram for the neutrons in a gravitational field caught between the walls, which oscillate owing to the mirror motion (horizontal direction here is vertical in the apparatus). This, in turn, causes the neutrons to move up and down energy levels. A measurement of the energy-level spacing yields constraints on parameters of scenarios describing dark energy and dark matter, which would slightly shift the levels as indicated by the dashed lines.

Figure 1 (Left) Neutron mirror apparatus. An ultracold neutron (UCN) enters a space between two mirrors that act as potential wells, giving rise to a discrete energy spectrum. A detector measures neutrons exiting the cavity formed by the mirrors. The bottom mirror sits upon a nanopositioning table that induces a vertical oscillation that produces dips in the neutron transmission at the resonances. (Right) Energy-level diagram for the neutrons in a gravitational field caught between the walls, which oscillate owing to the mirror motion (horizontal direction here is vertical in the apparatus). This, in turn, causes the neutrons to move up and down energy levels. A measurement of the energy-level spacing yields constraints on parameters of scenarios describing dark energy and dark matter, which would slightly shift the levels as indicated by the dashed lines.

Wolfgang P. Schleich, Ernst Raselhttp://physics.aps.org/articles/v7/39

The quantum behavior of a neutron bouncing in the gravitational field of the Earth can improve what we know about dark energy and dark matter.

Spectroscopy has always set the pace of physics. Indeed, the observation of the Balmer series of the hydrogen atom led to the Bohr-Sommerfeld model about 100 years ago. A little later the discreteness of the spectrum moved Werner Heisenberg to develop matrix mechanics and Erwin Schrödinger to formulate wave mechanics. In 1947, the observation of a level shift in hydrogen by Willis E. Lamb ushered in quantum electrodynamics.

Now, a group led by Hartmut Abele of the Technical University of Vienna, Austria, reports, in Physical Review Letters [1] [http://arxiv-web3.library.cornell.edu/abs/1404.4099], experiments that once more take advantage of the unique features of spectroscopy to put constraints on dark energy and dark matter scenarios. However, this time it is not a “real atom” (consisting of an electron bound to a proton) that provides the insight. Instead, the research team observes an “artificial atom”—a neutron bouncing up and down in the attractive gravitational field of the Earth (Fig. 1). This motion is quantized, and the measurement of the separation of the corresponding energy levels allows these authors to make conclusions about Newton’s inverse square law of gravity at short distances.

Setup and results for the employed gravity resonance spectroscopy: Left: The lowest eigenstates and eigenenergies with conning mirrors at bottom and top separated by 30.1 µm. The observed transitions are marked by arrows. Center: The transmission curve determined from the neutron count rate behind the mirrors as a function of oscillation frequency shows dips corresponding to the transitions shown on the left. Right: Upon resonance at 280 Hz the transmission decreases with the oscillation amplitude in contrast to the detuned 160 Hz. Because of the damping no revival occurs. All plotted errors correspond to a standard deviation around the statistical mean. [http://arxiv-web3.library.cornell.edu/abs/1404.4099]

Setup and results for the employed gravity resonance spectroscopy: Left: The lowest eigenstates and eigenenergies with conning mirrors at bottom and top separated by 30.1 µm. The observed transitions are marked by arrows. Center: The transmission curve determined from the neutron count rate behind the mirrors as a function of oscillation frequency shows dips corresponding to the transitions shown on the left. Right: Upon resonance at 280 Hz the transmission decreases with the oscillation amplitude in contrast to the detuned 160 Hz. Because of the damping no revival occurs. [arxiv]

The energy wave function of a quantum particle in a linear potential [2], corresponding, for example, to the gravitational field close to the surface of the Earth, has a continuous energy spectrum [3]. However, when a quantum particle such as a neutron is also restricted in its motion by two potential walls, the resulting spectrum is discrete.

Read also: “With neutrons, scientists can now look for dark energy in the lab

This elementary problem of nonrelativistic quantum mechanics is a slight generalization of the familiar “particle in a box” where the bottom of the box, which usually corresponds to a constant potential, is replaced by a linear one representing the gravitational field. Continue reading Neutrons Knock at the Cosmic Door

Dark Matter as a Trigger for Periodic Comet Impacts

sun's positionLisa Randall and Matthew Reece
Although statistical evidence is not overwhelming, possible support for an approximately 35 million year periodicity in the crater record on Earth could indicate a nonrandom underlying enhancement of meteorite impacts at regular intervals.
A proposed explanation in terms of tidal effects on Oort cloud comet perturbations as the Solar System passes through the galactic midplane is hampered by lack of an underlying cause for sufficiently enhanced gravitational effects over a sufficiently short time interval and by the time frame between such possible enhancements.
We show that a smooth dark disk in the galactic midplane would address both these issues and create a periodic enhancement of the sort that has potentially been observed.
Such a disk is motivated by a novel dark matter component with dissipative cooling that we considered in earlier work.
We show how to evaluate the statistical evidence for periodicity by input of appropriate measured priors from the galactic model, justifying or ruling out periodic cratering with more confidence than by evaluating the data without an underlying model. We find that, marginalizing over astrophysical uncertainties, the likelihood ratio for such a model relative to one with a constant cratering rate is 3.0, which moderately favors the dark disk model.
Our analysis furthermore yields a posterior distribution that, based on current crater data, singles out a dark matter disk surface density of approximately 10 solar masses per square parsec.
The geological record thereby motivates a particular model of dark matter that will be probed in the near future ….
…. Read more at http://arxiv.org/pdf/1403.0576v1.pdf

Dark Energy: A Short Review

data combination
Michael J. Mortonson, David H. Weinberg, Martin White
The accelerating expansion of the universe is the most surprising cosmological discovery in many decades.
In this short review, we briefly summarize theories for the origin of cosmic acceleration and the observational methods being used to test these theories.
We then discuss the current observational state of the field, with constraints from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), Type Ia supernovae (SN), direct measurements of the Hubble constant (H0), and measurements of galaxy and matter clustering.
Assuming a flat universe and dark energy with a constant equation-of-state parameter w=P/ρ, the combination of Planck CMB temperature anisotropies, WMAP CMB polarization, the Union2.1 SN compilation, and a compilation of BAO measurements yields ….
… Read more at http://arxiv.org/abs/1401.0046http://arxiv.org/pdf/1401.0046v1.pdf

Leading Dark Energy Theory Incompatible with New Measurement

The latest observations of exploding stars could call into question the cosmological constant explanation of dark energy
darkenergy3
By Clara Moskowitz
How much to read into the calculation depends on how uncertain it is, and whether systematic errors associated with the telescope and the analysis skewed the result. “It’s generally accepted that telescope calibration, supernova physics and galaxy properties are big sources of uncertainties, so everyone’s trying to figure these out in different ways,” says Daniel Scolnic of Johns Hopkins University, who led an accompanying paper estimating the data’s uncertainties. “I think that Dan did an excellent job characterizing their systematics,” says Alexander Conley of the University of Colorado at Boulder who worked on a different supernova study called the Supernova Legacy Survey that found similar results. “They still have a lot of work to do to improve the characterization for future papers, but they know that and are working on it.” However, another survey researcher, Julien Guy of University Pierre and Marie Curie in Paris, says the team may have underestimated their systematic error by ignoring an extra source of uncertainty from supernova light-curve models. He’s been in touch with the Pan-STARRS researchers, who are looking into that factor. Ultimately, most experts say the new results are tantalizing, but don’t prove the existence of new physics. “The Pan-STARRS paper presents a very thorough, careful analysis and a solid result, but it doesn’t qualitatively change our view of the cosmological parameters,” says Joshua Frieman, an astrophysicist at Fermilab in Batavia, Ill., who was not involved in the research….
Read more at www.scientificamerican.com and http://arxiv.org/abs/1310.3828

On the Trail of Dark Energy: Physicists Propose Higgs Boson ‘Portal’

One of the biggest mysteries in contemporary particle physics and cosmology is why dark energy, which is observed to dominate energy density of the universe, has a remarkably small (but not zero) value. This value is so small, it is perhaps 120 orders of magnitude less than would be expected based on fundamental physics.

Illustration of Standard Model particles. (Credit: Image courtesy of DOE/Fermi National Accelerator Laboratory)

Illustration of Standard Model particles. (Credit: Image courtesy of DOE/Fermi National Accelerator Laboratory)

Resolving this problem, often called the cosmological constant problem, has so far eluded theorists.
Now, two physicists — Lawrence Krauss of Arizona State University and James Dent of the University of Louisiana-Lafayette — suggest that the recently discovered Higgs boson could provide a possible “portal” to physics that could help explain some of the attributes of the enigmatic dark energy, and help resolve the cosmological constant problem.
In their paper, “Higgs Seesaw Mechanism as a Source for Dark Energy,” Krauss and Dent explore how a possible small coupling between the Higgs particle, and possible new particles likely to be associated with what is conventionally called the Grand Unified Scale — a scale perhaps 16 orders of magnitude smaller than the size of a proton, at which the three known non-gravitational forces in nature might converge into a single theory — could result in the existence of another background field in nature in addition to the Higgs field, which would contribute an energy density to empty space of precisely the correct scale to correspond to the observed energy density.
The paper was published online, Aug. 9, in Physical Review Letters.
Current observations of the universe show it is expanding at an accelerated rate. But this acceleration cannot be accounted for on the basis of matter alone. Putting energy in empty space produces a repulsive gravitational force opposing the attractive force produced by matter, including the dark matter that is inferred to dominate the mass of essentially all galaxies, but which doesn’t interact directly with light and, therefore, can only be estimated by its gravitational influence.
Because of this phenomenon and because of what is observed in the universe, it is thought that such ‘dark energy’ contributes up to 70 percent of the total energy density in the universe, while observable matter contributes only 2 to 5 percent, with the remaining 25 percent or so coming from dark matter.
The source of this dark energy and the reason its magnitude matches the inferred magnitude of the energy in empty space is not currently understood, making it one of the leading outstanding problems in particle physics today.
“Our paper makes progress in one aspect of this problem,” said Krauss, a Foundation Professor in ASU’s School of Earth and Space Exploration and Physics, and the director of the Origins Project at ASU. “Now that the Higgs boson has been discovered, it provides a possible ‘portal’ to physics at much higher energy scales through very small possible mixings and couplings to new scalar fields which may operate at these scales.”….

Read more at http://www.sciencedaily.com/releases/2013/08/130810063645.htm

Why is the Universe Accelerating?

sean_carrolSEAN M. CARROLL
The universe appears to be accelerating, but the reason why is a complete mystery.
The simplest explanation, a small vacuum energy (cosmological constant), raises three difficult issues: why the vacuum energy is so small, why it is not quite zero, and why it is comparable to the matter density today.
I discuss these mysteries, some of their possible resolutions, and some issues confronting future observations….
… Read more at http://www.astro.caltech.edu/~george/ay21/readings/carroll.pdf

Physics of the Dark Universe

Some Highlights of Volume 2:

Read more at http://www.journals.elsevier.com/physics-of-the-dark-universe/news/volume-2-now-online/