Cosmological perturbation theory is a key tool to study the universe.The linear or first order theory is well understood,however,developing and applying the theory beyond linear order is at the cutting edge of current...Cosmological perturbation theory is a key tool to study the universe.The linear or first order theory is well understood,however,developing and applying the theory beyond linear order is at the cutting edge of current research in theoretical cosmology.In this article,I will describe some signatures of non-linear perturbation theory that do not exist at linear order,focusing on vorticity generation at second order.In doing so,we discuss why this,among other features such as induced gravitational waves and non-Gaussianities,shows that cosmological perturbation theory is crucial for testing models of the universe.展开更多
We investigate the influence of population density on radio-frequency inter- ference （RFI） affecting radio astronomy. We use a new method to quantify the thresh- old of population density in order to determine the m...We investigate the influence of population density on radio-frequency inter- ference （RFI） affecting radio astronomy. We use a new method to quantify the thresh- old of population density in order to determine the most suitable lower limit for site selection of a radio quiet zone （RQZ）. We found that there is a certain trend in the population density-RFI graph that increases rapidly at lower values and slows down to almost fiat at higher values. We use this trend to identify the thresholds for pop- ulation density that produce RFI. Using this method we found that, for frequencies up to 2.8 GHz, low, medium and high population densities affecting radio astronomy are below 150 ppl km-2, between i50 ppl km-2 and 5125 ppl km-~, and above 5125 ppl km-2 respectively. We also investigate the effect of population density on the environment of RFI in three astronomical windows, namely the deuterium, hydro- gen and hydroxyl lines. We find that a polynomial fitting to the population density produces a similar trend, giving similar thresholds for the effect of population density. We then compare our interference values to the standard threshold levels used by the International Telecommunication Union within these astronomical windows.展开更多
Jupiter is one of the major targets for planetary exploration,and dust in the Jovian system is of great interest to researchers in the field of planetary science.In this paper,we review the five dust populations outsi...Jupiter is one of the major targets for planetary exploration,and dust in the Jovian system is of great interest to researchers in the field of planetary science.In this paper,we review the five dust populations outside the ring system:grains in the region of the Galilean moons,potential dust from plumes on Europa,Jovian stream particles,particles in the outer region of the Jovian system ejected from the irregular satellites,and dust in the region of the Trojan asteroids.The physical environment for the dust dynamics is described,including the gravity,the magnetic field and the plasma environment.For each population,the dust sources are described,and the relevant perturbation forces are discussed.Observations and results from modeling are reviewed,and the distributions of the individual dust populations are shown.The understanding of the Jovian dust environment allows to assess the dust hazard to spacecraft,and to characterize the material exchange between the Jovian moons,their surface properties and distribution of non-icy constituents.展开更多
We are interested in stable periodic orbits for spacecraft in the gravitational eld of minor celestial bodies.The stable periodic orbits around minor celestial bodies are useful not only for the mission design of the ...We are interested in stable periodic orbits for spacecraft in the gravitational eld of minor celestial bodies.The stable periodic orbits around minor celestial bodies are useful not only for the mission design of the deep space exploration,but also for studying the long-time stability of small satellites in the large-size-ratio binary asteroids.The irregular shapes and gravitational elds of the minor celestial bodies are modeled by the polyhedral model.Using the topological classi cations of periodic orbits and the grid search method,the stable periodic orbits can be calculated and the topological cases can be determined.Furthermore,we nd ve di erent types of stable periodic orbits around minor celestial bodies:(1)stable periodic orbits generated from the stable equilibrium points outside the minor celestial body;(2)stable periodic orbits continued from the unstable periodic orbits around the unstable equilibrium points;(3)retrograde and nearly circular periodic orbits with zero-inclination around minor celestial bodies;(4)resonance periodic orbits;(5)near-surface inclined periodic orbits.We take asteroids 243 Ida,433 Eros,6489 Golevka,101955 Bennu,and the comet 1P/Halley for examples.展开更多
文摘Cosmological perturbation theory is a key tool to study the universe.The linear or first order theory is well understood,however,developing and applying the theory beyond linear order is at the cutting edge of current research in theoretical cosmology.In this article,I will describe some signatures of non-linear perturbation theory that do not exist at linear order,focusing on vorticity generation at second order.In doing so,we discuss why this,among other features such as induced gravitational waves and non-Gaussianities,shows that cosmological perturbation theory is crucial for testing models of the universe.
基金made possible by the usage of the University of Malaya’s grants UMRG(RG118-10AFR),IPPP(PV025-2011A)and HIR(H-21001-F000028)Universiti Sultan Zainal Abidin(UniSZA)and the Ministry of Higher Education(MOHE)for their sponsorship
文摘We investigate the influence of population density on radio-frequency inter- ference （RFI） affecting radio astronomy. We use a new method to quantify the thresh- old of population density in order to determine the most suitable lower limit for site selection of a radio quiet zone （RQZ）. We found that there is a certain trend in the population density-RFI graph that increases rapidly at lower values and slows down to almost fiat at higher values. We use this trend to identify the thresholds for pop- ulation density that produce RFI. Using this method we found that, for frequencies up to 2.8 GHz, low, medium and high population densities affecting radio astronomy are below 150 ppl km-2, between i50 ppl km-2 and 5125 ppl km-~, and above 5125 ppl km-2 respectively. We also investigate the effect of population density on the environment of RFI in three astronomical windows, namely the deuterium, hydro- gen and hydroxyl lines. We find that a polynomial fitting to the population density produces a similar trend, giving similar thresholds for the effect of population density. We then compare our interference values to the standard threshold levels used by the International Telecommunication Union within these astronomical windows.
基金the European Space Agency under the project“Jovian Micrometeoroid Environment Model”(JMEM)(Contract No.4000107249/12/NL/AF)at the University of Oulu,and by the Academy of Finland under the project“Earth and Near-Space System and Environmental Change”.
文摘Jupiter is one of the major targets for planetary exploration,and dust in the Jovian system is of great interest to researchers in the field of planetary science.In this paper,we review the five dust populations outside the ring system:grains in the region of the Galilean moons,potential dust from plumes on Europa,Jovian stream particles,particles in the outer region of the Jovian system ejected from the irregular satellites,and dust in the region of the Trojan asteroids.The physical environment for the dust dynamics is described,including the gravity,the magnetic field and the plasma environment.For each population,the dust sources are described,and the relevant perturbation forces are discussed.Observations and results from modeling are reviewed,and the distributions of the individual dust populations are shown.The understanding of the Jovian dust environment allows to assess the dust hazard to spacecraft,and to characterize the material exchange between the Jovian moons,their surface properties and distribution of non-icy constituents.
基金the State Key Laboratory of Astronautic Dynamics Foundation(No.2016ADL0202).
文摘We are interested in stable periodic orbits for spacecraft in the gravitational eld of minor celestial bodies.The stable periodic orbits around minor celestial bodies are useful not only for the mission design of the deep space exploration,but also for studying the long-time stability of small satellites in the large-size-ratio binary asteroids.The irregular shapes and gravitational elds of the minor celestial bodies are modeled by the polyhedral model.Using the topological classi cations of periodic orbits and the grid search method,the stable periodic orbits can be calculated and the topological cases can be determined.Furthermore,we nd ve di erent types of stable periodic orbits around minor celestial bodies:(1)stable periodic orbits generated from the stable equilibrium points outside the minor celestial body;(2)stable periodic orbits continued from the unstable periodic orbits around the unstable equilibrium points;(3)retrograde and nearly circular periodic orbits with zero-inclination around minor celestial bodies;(4)resonance periodic orbits;(5)near-surface inclined periodic orbits.We take asteroids 243 Ida,433 Eros,6489 Golevka,101955 Bennu,and the comet 1P/Halley for examples.
