Orbits

Author(s):  
David M. Wittman

Orbits are ubiquitous in the universe: moons orbit planets, planets orbit stars, stars orbit around the center of the Milky Way galaxy, and so on. Any theory of gravity will have to explain the properties of all these orbits. To pave the way for developing the metric theory of gravity (general relativity) this chapter examines the basics of orbits as observed and as explained by the Newtonian model of gravity. We can use our understanding of gravity to infer the masses and other properties of these cosmic systems. Te chapter concludes with four optional sections in this spirit, covering the slingshot maneuver; dark matter; binary star orbits and how they reveal the masses of stars; and extrasolar planets.

Symmetry ◽  
2021 ◽  
Vol 13 (5) ◽  
pp. 877
Author(s):  
Elena Arbuzova ◽  
Alexander Dolgov ◽  
Rajnish Singh

Evolution and heating of the universe in R2-modified gravity are considered. It is shown that the universe’s history can be separated into four different epochs: (1) inflation, (2) heating due to curvature oscillations (scalaron decay), (3) transition to matter dominated period, and (4) conventional cosmology governed by General Relativity. Cosmological density of dark matter (DM) particles for different decay channels of the scalaron is calculated. The bounds on the masses of DM particles are derived for the following dominant decay modes: to minimally coupled scalars, to massive fermions, and to gauge bosons.


Author(s):  
Karel Schrijver

How many planetary systems formed before our’s did, and how many will form after? How old is the average exoplanet in the Galaxy? When did the earliest planets start forming? How different are the ages of terrestrial and giant planets? And, ultimately, what will the fate be of our Solar System, of the Milky Way Galaxy, and of the Universe around us? We cannot know the fate of individual exoplanets with great certainty, but based on population statistics this chapter sketches the past, present, and future of exoworlds and of our Earth in general terms.


2020 ◽  
Vol 29 (14) ◽  
pp. 2043028
Author(s):  
M. Ángeles Pérez-García ◽  
Joseph Silk

Neutron Stars (NSs) are compact stellar objects that are stable solutions in General Relativity. Their internal structure is usually described using an equation of state that involves the presence of ordinary matter and its interactions. However there is now a large consensus that an elusive sector of matter in the universe, described as dark matter, remains as yet undiscovered. In such a case, NSs should contain both, baryonic and dark matter. We argue that depending on the nature of the dark matter and in certain circumstances, the two matter components would form a mixture inside NSs that could trigger further changes, some of them observable. The very existence of NSs constrains the nature and interactions of dark matter in the universe.


2018 ◽  
Vol 27 (14) ◽  
pp. 1830009
Author(s):  
Virginia Trimble

A large majority of the physics and astronomy communities are now sure that gravitational waves exist, can be looked for, and can be studied via their effects on laboratory apparatus as well as on astronomical objects. So far, everything found out has agreed with the predictions of general relativity, but hopes are high for new information about the universe and its contents and perhaps for hints of a better theory of gravity than general relativity (which even Einstein expected to come eventually). This is one version of the story, from 1905 to the present, told from an unusual point of view, because the author was, for 28.5 years, married to Joseph Weber, who built the first detectors starting in the early 1960s and operated one or more until his death on 30 September 2000.


2021 ◽  
Vol 52 (1) ◽  
Author(s):  
Alexander P. Sobolev

AbstractThe gravitational equations were derived in general relativity (GR) using the assumption of their covariance relative to arbitrary transformations of coordinates. It has been repeatedly expressed an opinion over the past century that such equality of all coordinate systems may not correspond to reality. Nevertheless, no actual verification of the necessity of this assumption has been made to date. The paper proposes a theory of gravity with a constraint, the degenerate variants of which are general relativity (GR) and the unimodular theory of gravity. This constraint is interpreted from a physical point of view as a sufficient condition for the adiabaticity of the process of the evolution of the space–time metric. The original equations of the theory of gravity with the constraint are formulated. On this basis, a unified model of the evolution of the modern, early, and very early Universe is constructed that is consistent with the observational astronomical data but does not require the hypotheses of the existence of dark energy, dark matter or inflatons. It is claimed that: physical time is anisotropic, the gravitational field is the main source of energy of the Universe, the maximum global energy density in the Universe was 64 orders of magnitude smaller the Planckian one, and the entropy density is 18 orders of magnitude higher the value predicted by GR. The value of the relative density of neutrinos at the present time and the maximum temperature of matter in the early Universe are calculated. The wave equation of the gravitational field is formulated, its solution is found, and the nonstationary wave function of the very early Universe is constructed. It is shown that the birth of the Universe was random.


2019 ◽  
pp. 64-72
Author(s):  
Nicholas Mee

Most of the matter in the universe exists in an unknown form called dark matter. All estimates of the mass of galaxies and galaxy clusters suggest they contain far more matter than is visible to us in the form of stars. Conventional explanations, such as the existence of large quantities of burnt-out stars known as MACHOs or dark gas clouds, have been ruled out. The most popular explanation is that dark matter consists of vast quantities of hypothetical stable particles known as WIMPs that were produced in vast quantities in the very early universe. Many laboratories around the world are searching for signs of these particles. These include the Italian Gran Sasso laboratory running the XENON100 experiment. Some theorists have suggested the evidence for dark matter would disappear if we had a better theory of gravity. Analysis of the Bullet Cluster indicates such proposals will not work.


2019 ◽  
Vol 79 (10) ◽  
Author(s):  
Surajit Kalita ◽  
Banibrata Mukhopadhyay

Abstract A number of recent observations have suggested that the Einstein’s theory of general relativity may not be the ultimate theory of gravity. The f(R) gravity model with R being the scalar curvature turns out to be one of the best bet to surpass the general relativity which explains a number of phenomena where Einstein’s theory of gravity fails. In the f(R) gravity, behaviour of the spacetime is modified as compared to that of given by the Einstein’s theory of general relativity. This theory has already been explored for understanding various compact objects such as neutron stars, white dwarfs etc. and also describing evolution of the universe. Although researchers have already found the vacuum spacetime solutions for the f(R) gravity, yet there is a caveat that the metric does have some diverging terms and hence these solutions are not asymptotically flat. We show that it is possible to have asymptotically flat spherically symmetric vacuum solution for the f(R) gravity, which is different from the Schwarzschild solution. We use this solution for explaining various bound orbits around the black hole and eventually, as an immediate application, in the spherical accretion flow around it.


2018 ◽  
Vol 33 (29) ◽  
pp. 1850181 ◽  
Author(s):  
Saleh Hamdan ◽  
James Unwin

We highlight the general scenario of dark matter freeze-out while the energy density of the universe is dominated by a decoupled non-relativistic species. Decoupling during matter domination changes the freeze-out dynamics, since the Hubble rate is parametrically different for matter and radiation domination. Furthermore, for successful Big Bang Nucleosynthesis the state dominating the early universe energy density must decay, this dilutes (or repopulates) the dark matter. As a result, the masses and couplings required to reproduce the observed dark matter relic density can differ significantly from radiation-dominated freeze-out.


2015 ◽  
Vol 30 (11) ◽  
pp. 1550056 ◽  
Author(s):  
Ramil Izmailov ◽  
Alexander A. Potapov ◽  
Alexander I. Filippov ◽  
Mithun Ghosh ◽  
Kamal K. Nandi

We investigate the stability of circular material orbits in the analytic galactic metric recently derived by Harko et al., Mod. Phys. Lett. A29, 1450049 (2014). It turns out that stability depends more strongly on the dark matter central density ρ0 than on other parameters of the solution. This property then yields an upper limit on ρ0 for each individual galaxy, which we call here [Formula: see text], such that stable circular orbits are possible only when the constraint [Formula: see text] is satisfied. This is our new result. To approximately quantify the upper limit, we consider as a familiar example our Milky Way galaxy that has a projected dark matter radius R DM ~180 kpc and find that [Formula: see text]. This limit turns out to be about four orders of magnitude larger than the latest data on central density ρ0 arising from the fit to the Navarro–Frenk–White (NFW) and Burkert density profiles. Such consistency indicates that the Eddington-inspired Born–Infeld (EiBI) solution could qualify as yet another viable alternative model for dark matter.


Author(s):  
Viktor T. Toth

Beyond the Newtonian approximation, gravitational fields in general relativity can be described using a formalism known as gravitoelectromagnetism. In this formalism, a vector potential, the gravitomagnetic potential, arises as a result of moving masses, in strong analogy with the magnetic force due to moving charges in Maxwell’s theory. Gravitomagnetism can affect orbits in the gravitational field of a massive, rotating body. This raises the possibility that gravitomagnetism may serve as the dominant physics behind the anomalous rotation curves of spiral galaxies, eliminating the need for dark matter. In this essay, we methodically work out the magnitude of the gravitomagnetic equivalent of the Lorentz force and apply the result to the Milky Way. We find that the resulting contribution is too small to produce an observable effect on these orbits. We also investigate the impact of cosmological boundary conditions on the result and find that these, too, are negligible.


Sign in / Sign up

Export Citation Format

Share Document