Certain classes of modified gravity theories are also ruled out by new observations:Can a negative-mass cosmology explain dark matter and dark energy?
(Submitted on 21 Feb 2019)A recent work by Farnes (2018) proposed an alternative cosmological model in which both dark matter and dark energy are replaced with a single fluid of negative mass. This paper presents a critical review of that model. A number of problems and discrepancies with observations are identified. For instance, the predicted shape and density of galactic dark matter halos are incorrect. Also, halos would need to be less massive than the baryonic component or they would become gravitationally unstable. Perhaps the most challenging problem in this theory is the presence of a large-scale version of the `runaway' effect, which would result in all galaxies moving in random directions at nearly the speed of light. Other more general issues regarding negative mass in general relativity are discussed, such as the possibility of time-travel paradoxes.
Also, LIGO rules out certain kinds of compact dark matter objects in the solar system:Constraints of general screened modified gravities from comprehensive analysis of binary pulsars
(Submitted on 22 Feb 2019)Testing gravity by binary pulsars nowadays becomes a key issue. Screened modified gravity is a kind of scalar-tensor theory with screening mechanism in order to satisfy the tight Solar System tests. In this paper, we investigate how the screening mechanism affects the orbital dynamics of binary pulsars, and calculate in detail the five post-Keplerian (PK) parameters in this theory. These parameters differ from those of general relativity (GR), and the differences are quantified by the scalar charges, which lead to the dipole radiation in this theory. We combine the observables of PK parameters for the ten binary pulsars, respectively, to place the constraints on the scalar charges and possible deviations from GR. The dipole radiation in the neutron star (NS) - white dwarf (WD) binaries leads to more stringent constraints on deviations from GR. The most constraining systems for the scalar charges of NS and WD are PSR~B1913+ 16 and PSR~J1738+ 0333, respectively. The results of all tests exclude significant strong-field deviations and show good agreement with GR.
Gravitational waves from compact dark matter objects in the solar system
(Submitted on 21 Feb 2019)
Dark matter could be composed of compact dark objects (CDOs). We find that a close binary of CDOs orbiting {\it inside} solar system bodies can be a loud source of gravitational waves (GWs) for the LIGO and VIRGO detectors. An initial search of data from the first Advanced LIGO observing run (O1), sensitive toOn the other hand, this new gravity based paper looks interesting:h0≈10−24 , rules out close binaries orbiting near the center of the Sun with GW frequencies (twice the orbital frequency) between 50 and 550 Hz and CDO masses above approximately 10^{-9} M_sun.
Dark matter effect attributed to the inherent structure of cosmic space
(Submitted on 21 Feb 2019)We propose that anomalous gravitational effects currently attributed to dark matter can alternatively be explained as a manifestation of the inherent structure of space at galactic length scales. Specifically, we show that the inherent curvature of space amplifies the gravity of ordinary matter such that the effect resembles the presence of the hypothetical hidden mass. Our study is conducted in the context of weak gravity, nearly static conditions, and spherically symmetric configuration, and leverages the Cosmic Fabric model of space developed by Tenev and Horstemeyer [T. G. Tenev and M. F. Horstemeyer, Int. J. Mod. Phys. D 27 (2018) 1850083; T. G. Tenev and M. F. Horstemeyer, Rep. Adv. Phys. Sci. 2 (2018) 1850011]
This is summed up in the preprint's conclusion:
We showed that the inherent curvature of physical space (that is curvature uncaused by matter) amplifies the gravitational effects of ordinary matter and produces the kind of gravitational anomalies that are currently attributed to the presence of dark matter (DM). We proposed the Inherent Structure Hypothesis (ISH) stating that the so called DM effect is the manifestation of the inherent structure of space at galactic length-scales, and not the result of invisible mass.
We demonstrated that any DM effect, which can be explained by the Modified Newtonian Dynamics (MOND) theory or by the presence of a DM halo, can be equally well explained by the ISH. At the same time, we showed, ISH allows for DM effects that cannot be explained by MOND or by DM halos. Therefore, we concluded that the Inherent Structure and DM explanations are observationally equivalent with each other to within some distance from the center of a gravitating system. However, beyond such distance, the ISH predicts that the gravitational impact of the hypothetical dark matter begins to be reversed and is nearly completely eliminated at sufficiently far distances. This is a verifiable prediction that would distinguish our model from other explanations of the DM effect.
In the comparison between the ISH and MOND we noted an interesting relationship between the size of a gravitational system and its Schwartzchild radius through the MOND parameter a0. Such relationship hinted at the structural underpinnings of the DM effect.
The Inherent Structure Hypothesis stems from the principle that structure is a fundamental aspect of matter, space, and nature in general, and as such can be incorporated into cosmological models that subscribe to the same principle.
1 comment:
The author of the negative mass article responds on Twitter. https://twitter.com/Astro_Jamie/status/1100050152734248962
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