Physics Quick Hits

Astronomy based papers

* One of the keys to solving the mystery of dark matter/modified gravity are increasingly detailed observations of galactic clusters to which theoretical predictions can be compared.

* It is hard to devise realistic mathematical models that would produce the planet Mercury:

Of the solar system's four terrestrial planets, the origin of Mercury is perhaps the most mysterious. Modern numerical simulations designed to model the dynamics of terrestrial planet formation systematically fail to replicate Mercury; which possesses just 5% the mass of Earth and the highest orbital eccentricity and inclination among the planets. However, Mercury's large iron-rich core and low volatile inventory stand out among the inner planets, and seem to imply a violent collisional origin. . . . we analyze a large suite of terrestrial accretion models that account for the fragmentation of colliding bodies. We find that planets with core mass fractions boosted as a result of repeated hit-and-run collisions are produced in 90% of our simulations. While many of these planets are similar to Mercury in mass, they rarely lie on Mercury-like orbits. Furthermore, we perform an additional batch of simulations designed to specifically test the single giant impact origin scenario. We find less than a 1% probability of simultaneously replicating the Mercury-Venus dynamical spacing and the terrestrial system's degree of orbital excitation after such an event. While dynamical models have made great strides in understanding Mars' low mass, their inability to form accurate Mercury analogs remains a glaring problem.

Of course, in principle, there is no reason that Mercury couldn't be simply a statistical fluke.

* Can an interacting dark energy (IDE) model explain observations of the very early universe that are contrary to the standard ΛCDM model of cosmology? (The observations are a clean fit to a "no dark matter" model.)

In this paper we study implications of the possible excess of 21-cm line global signal at the epoch of cosmic dawn on the evolutions of a class of dynamically interacting dark energy (IDE) models. 

We firstly summarize two dynamical mechanisms in which different background evolutions can exert considerable effects on the 21-cm line global signal. One is the decoupling time of Compton scattering heating, the other stems from the direct change of optical depth due to the different expansion rate of the Universe. 

After that, we investigate the IDE models to illustrate the tension between the results of Experiment to Detect the Global Epoch of reionization Signature (EDGES) and other experiments. 

To apply the analyses of these two mechanisms to IDE models, we find that only the optical depth can be significantly changed. Accordingly, in order to relieve the tension by including the effects of the decoupling time of Compton scattering heating, we deduce a possible evolution form for the Hubble parameter within IDE that begins at an early stage around z∼100 and then smoothly evolves to a value at z∼17 which is smaller than that obtained in the standard paradigm. Eventually, we fulfill this scenario by adding an early dark energy dominated stage to the cosmological paradigm described by IDE models, which can alleviate the tension between EDGES and other cosmological observations but cannot completely solve it.

* The cosmic microwave background data gathered to date can't distinguish between neutrinos that behave the way we expect them to and neutrinos with a much stronger self-interaction. A new telescope should be able to tell the difference and my money is on a finding that CMB data is consistent with neutrinos with only normal self-interactions.

Of the many proposed extensions to the ΛCDM paradigm, a model in which neutrinos self-interact until close to the epoch of matter-radiation equality has been shown to provide a good fit to current cosmic microwave background (CMB) data, while at the same time alleviating tensions with late-time measurements of the expansion rate and matter fluctuation amplitude. 

Interestingly, CMB fits to this model either pick out a specific large value of the neutrino interaction strength, or are consistent with the extremely weak neutrino interaction found in ΛCDM, resulting in a bimodal posterior distribution for the neutrino self-interaction cross section. In this paper, we explore why current cosmological data select this particular large neutrino self-interaction strength, and by consequence, disfavor intermediate values of the self-interaction cross section. 

We show how it is the ℓ≳1000 CMB temperature anisotropies, most recently measured by the Planck satellite, that produce this bimodality. We also establish that smaller scale temperature data, and improved polarization data measuring the temperature-polarization cross-correlation, will best constrain the neutrino self-interaction strength. We forecast that the upcoming Simons Observatory should be capable of distinguishing between the models.

Minsu Park, et al., "ΛCDM or self-interacting neutrinos? - how CMB data can tell the two models apart" (April 4, 2019).

* Gravitational wave observations can determine if gravitational waves have polarizations present in certain kinds of modified gravity theories.

General metric theories of gravity in four-dimensional spacetimes can contain at most six polarization modes (two spin-0, two spin-1 and two spin-2) of gravitational waves (GWs). It has been recently shown that, with using four GW non-coaligned detectors, a direct test of the spin-1 modes can be done in principle separately from the spin-0 and spin-2 modes for a GW source in particular sky positions [Hagihara et al., Phys. Rev. D 98, 064035 (2018)]. They have found particular sky positions that satisfy a condition of killing completely the spin-0 modes in a so-called null stream which is a linear combination of the signal outputs to kill the spin-2 modes. The present paper expands the method to discuss a possibility that the spin-0 modes are not completely but effectively suppressed in the null streams to test the spin-1 modes separately from the other modes, especially with an expected network of Advanced LIGO, Advanced Virgo and KAGRA. We study also a possibility that the spin-1 modes are substantially suppressed in the null streams to test the spin-0 modes separately from the other modes, though the spin-1 modes for any sky position cannot be completely killed in the null streams. Moreover, we find that the coefficient of the spin-0 modes in the null stream is significantly small for the GW170817 event, so that an upper bound can be placed on the amplitude of the spin-1 modes as <6×10−23.

* More precise models make it easier distinguish ΛCDM from the alternatives without theoretical error adding to the uncertainties created by experimental errors.

The observational success and simplicity of the ΛCDM model, and the explicit analytic perturbations thereof, set the standard for any alternative cosmology. It therefore serves as a comparison ground and as a test case for methods which can be extended and applied to other cosmological models. In this paper we introduce dynamical systems and methods to describe linear scalar and tensor perturbations of the ΛCDM model, which serve as pedagogical examples that show the global illustrative powers of dynamical systems in the context of cosmological perturbations. This also allows us to compare perturbations with results in general relativity. Furthermore, we give a new approximation for the linear growth rate, f(z)=dlnδdlna=Ω611m−170(1−Ωm)52, where z is the cosmological redshift, Ωm=Ωm(z), while a is the background scale factor, and show that it is much more accurate than the previous ones in the literature.

* Experimental data strongly limits the strength of the interactions that dark matter can have with ordinary matter:

our experiment places an upper limit of 1.6×1013cm−3 on the density of DM at the Earth's surface. This in turn, significantly constrains the properties for any DM candidate that interacts with baryons.

Particle physics

* Belle finds that its data in certain muon v. electron semi-leptonic B meson decays is within 1.8 sigma of the Standard Model prediction of lepton flavor universality, undermining this new anomaly from other experiments.