A simple model with only six parameters (the age of the universe, the density of atoms, the density of matter, the amplitude of the initial fluctuations, the scale dependence of this amplitude, and the epoch of first star formation) fits all of our cosmological data . Although simple, this standard model is strange. The model implies that most of the matter in our Galaxy is in the form of “dark matter,” a new type of particle not yet detected in the laboratory, and most of the energy in the universe is in the form of “dark energy,” energy associated with empty space. Both dark matter and dark energy require extensions to our current understanding of particle physics or point toward a breakdown of general relativity on cosmological scales…
…Read more at http://www.sciencemag.org/content/347/6226/1100.full
Sure, the Universe is expanding, and that expansion is accelerating. But beyond simply calling the cause “dark energy,” what do we know about it?
“I must choose between despair and Energy —— I choose the latter.” -John Keats
All week long, some of you have been racking your brains to come up with the deepest, most mysterious questions about the Universe to highlight for our Ask Ethan column. We’ve gotten some outstanding questions and suggestions that you’ve sent in, and while it’s a pity I can only choose one, this week’s honor goes to Piyush Gupta, who asks:
… and dark matter
Wolfgang P. Schleich, Ernst Rasel – http://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  [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.The energy wave function of a quantum particle in a linear potential , corresponding, for example, to the gravitational field close to the surface of the Earth, has a continuous energy spectrum . However, when a quantum particle such as a neutron is also restricted in its motion by two potential walls, the resulting spectrum is discrete.
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
Lisa 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
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/1401.0046 – http://arxiv.org/pdf/1401.0046v1.pdf