Planet X student debunking

Modelling the Dynamics of a Hypothetical Planet X by way of Gravitational N-body Simulator

ABSTRACT: This paper describes a novel activity to model the dynamics of a Jupiter-mass, trans-Neptunian planet of a highly eccentric orbit. Despite a history rooted in modern astronomy, “Planet X”, a hypothesised hidden planet lurking in our outer Solar System, has often been touted by conspiracy theorists as the cause of past mass extinction events on Earth, as well as other modern-day doomsday scenarios. Frequently dismissed as pseudoscience by astronomers, these stories continue to draw the attention of the public by provoking mass media coverage. Targeted at junior undergraduate levels, this activity allows students to debunk some of the myths surrounding Planet X by using simulation software to demonstrate that such a large-mass planet with extreme eccentricity would be unable to enter our Solar System unnoticed, let alone maintain a stable orbit.


Fast metallisk hydrogen?

Using two diamonds, scientists squeezed hydrogen to pressures above those in Earth’s core.

Diamond vise turns hydrogen into a metal, potentially ending 80-year quest

By Robert F. Service

The feat, reported online this week in Science, is more than an oddity. Solid metallic hydrogen is thought to be a superconductor, able to conduct electricity without resistance. It may even be metastable, meaning that like diamond, also formed at high pressures, the metallic hydrogen would maintain its state—and even its superconductivity—once brought back to room temperatures and pressures.

Still, claims of solid metallic hydrogen have come and gone before, and some experts want more proof. “From our point of view it’s not convincing,” says Mikhail Eremets, who is pursuing solid metallic hydrogen at the Max Planck Institute for Chemistry in Mainz, Germany. Others in the contentious field are downright hostile to the result. “The word garbage cannot really describe it,” says Eugene Gregoryanz, a high-pressure physicist at the University of Edinburgh, who objects to several of the experiment’s procedures.

Observation of the Wigner-Huntington transition to metallic hydrogen

ABSTRACT: Producing metallic hydrogen has been a great challenge to condensed matter physics. Metallic hydrogen may be a room temperature superconductor and metastable when the pressure is released and could have an important impact on energy and rocketry. We have studied solid molecular hydrogen under pressure at low temperatures. At a pressure of 495 GPa hydrogen becomes metallic with reflectivity as high as 0.91. We fit the reflectance using a Drude free electron model to determine the plasma frequency of 32.5 ± 2.1 eV at T = 5.5 K, with a corresponding electron carrier density of 7.7 ± 1.1 × 1023 particles/cm3, consistent with theoretical estimates of the atomic density. The properties are those of an atomic metal. We have produced the Wigner-Huntington dissociative transition to atomic metallic hydrogen in the laboratory.


Einstein’s 1917 Static Model

Einsteins kosmologiske model fra 1917 er baseret på 3 principper:

  1. Ækvivalensprincippet (tung masse er lig inertimasse).
  2. Generel relativitet (anvendelse af generelle koordinater).
  3. Machs princip (acceleration bestemmes alene af Universets masser).

Ækvivalensprincippet betyder (a) alle massive legemer med samme begyndelsbetingelser (sted og hastighed) følger den samme bane og (b) tyngdekraften ophæves lokalt i frit fald. Dette betyder, at der ikke lokalt er nogen forskel mellem en tyngdekraft og et et accelereret referencesystem. Det er derfor indlysende, at gravitationen kun kan beskrives i generelle accelererede koordinater (generel relativitet). 1. og 2. medfører, at gravitationen er en metrisk teori for en generel krum rumtid defineret ved et metrisk tensorfelt gμν. Et legemes bane en geodætisk kurve i rumtiden, som gestemmes ud fra det metriske tensorfelt. Et legemes acceleration er altså bestemt af gμν. Det metriske felt findes ved løsning af Einsteins feltligninger, som kæder rumtidens krumning sammen med massernes fordeling og bevægelser.

Men Einstein har et mere kontroversielt krav: Et legemes acceleration skal alene bestemmes ud fra massernes fordeling i Universet (Machs princip). Dette krav er ensbetydende med, at det metriske felt alene er bestemt af fordelingen af universets masser. Einstein antager desuden at Universet er statisk. Langt de fleste astronomer satte lighedstegn mellem Mælkevejen og Universet. Hvad med ideen om, at tågerne var andre Mælkeveje? Tågerne er stærkt koncentrerede mod Mælkevejens poler. Dette blev taget som bevis for, at de var en del af Mælkevejen.

Einstein mente først, at feltligningerne sikrede, at Machs princip var opfyldt; men astronomen de Sitter argumenterede for, at feltligningernes løsning krævede angivelse af det metriske felt på randen af rummet, dvs i uendelig afstand. Det er sådan, at man løser bevægelser i Solsystemet. Man lader det metriske felt gå mod feltet for den specielle relativitetsteori (altså et tomt rum). Dette betyder imidlertid, at accelerationer i store afstande måles i forhold til et tomt rum (Machs princip er ikke opfyldt).

Einstein indså nu, at man kan omgå problemet, hvis man antager, at rummet ikke har nogen rand. Einstein antager, at rummet er en 3-dimensional kugleflade med en bestemt krumningsradius. Han antager desuden, at Universet har en bestemt middelmassetæthed (den simpleste mulighed).  Der var kun et problem: Modellen er ikke en løsning til feltligningerne!

Einstein havde allerede ved introduktionen af feltligningerne benærket, at man kan adere leddet λgμν uden, at det ændrer energi- og impulsbevarelse. Introduktionen af den kosmologiske konstant λ tillader den ønskede statiske løsning uden rand. Einstein har således den ønskede statiske model, som opfylder Machs princip. Einsteins største bommert var, at han ikke undersøgte modellens stabilitet. Det var sandsynligvis Eddington, som fortalte ham om modellens ustabilitet i juni 1930. I mellemtiden var Universets ekspansion blevet opdaget, så Einstein opgav helt den kosmologiske konstant, ligesom han et par år senere sammen med de Sitter opgav det krumme endelige rum til fordel for et fladt uendeligt rum.

Einstein’s 1917 Static Model of the Universe: A Centennial Review

ABSTRACT: We present a historical review of Einstein’s 1917 paper ‘Cosmological Considerations in the General Theory of Relativity’ to mark the centenary of a key work that set the foundations of modern cosmology. We find that the paper followed as a natural next step after Einstein’s development of the general theory of relativity and that the work offers many insights into his thoughts on relativity, astronomy and cosmology. Our review includes a description of the observational and theoretical background to the paper; a paragraph-by-paragraph guided tour of the work; a discussion of Einstein’s views of issues such as the relativity of inertia, the curvature of space and the cosmological constant. Particular attention is paid to little-known aspects of the paper such as Einstein’s failure to test his model against observation, his failure to consider the stability of the model and a mathematical oversight in his interpretation of the role of the cosmological constant. We discuss the insights provided by Einstein’s reaction to alternate models of the universe proposed by Willem de Sitter, Alexander Friedman and Georges Lemaitre. Finally, we consider the relevance of Einstein’s static model of the universe for today’s ’emergent’ cosmologies.


Orbital motion of the Earth over 40 Myr

NOTE: This example has been chosen as this solution is widely used for the reconstruction of the climates of the past.

Frequency analysis and the representation of slowly diffusing planetary solutions

ABSTRACT: Over short time intervals planetary ephemerides have been traditionally represented in analytical form as finite sums of periodic terms or sums of Poisson terms that are periodic terms with polynomial amplitudes. Nevertheless, this representation is not well adapted for the evolution of the planetary orbits in the solar system over million of years as they present drifts in their main frequencies, due to the chaotic nature of their dynamics. The aim of the present paper is to develop a numerical algorithm for slowly diffusing solutions of a perturbed integrable Hamiltonian system that will apply to the representation of the chaotic planetary motions with varying frequencies. By simple analytical considerations, we first argue that it is possible to recover exactly a single varying frequency. Then, a function basis involving time-dependent fundamental frequencies is formulated in a semi-analytical way. Finally, starting from a numerical solution, a recursive algorithm is used to numerically decompose the solution on the significant elements of the function basis. Simple examples show that this algorithm can be used to give compact representations of different types of slowly diffusing solutions. As a test example, we show how this algorithm can be successfully applied to obtain a very compact approximation of the La2004 solution of the orbital motion of the Earth over 40 Myr ([-35Myr,5Myr]). This example has been chosen as this solution is widely used for the reconstruction of the climates of the past.


Distant Kuiper Belt in a Nice Model

The Structure of the Distant Kuiper Belt in a Nice Model Scenario

ABSTRACT: This work explores the orbital distribution of minor bodies in the outer Solar System emplaced as a result of a Nice model migration from the simulations of Brasser & Morbidelli (2013). This planetary migration scatters a planetesimal disk from between 29-34 AU and emplaces a population of objects into the Kuiper belt region. From the 2:1 Neptune resonance and outward, the test particles analyzed populate the outer resonances with orbital distributions consistent with trans-Neptunian object (TNO) detections in semi-major axis, inclination, and eccentricity, while capture into the closest resonances is too efficient. The relative populations of the simulated scattering objects and resonant objects in the 3:1 and 4:1 resonances are also consistent with observed populations based on debiased TNO surveys, but the 5:1 resonance is severely underpopulated compared to population estimates from survey results. Scattering emplacement results in the expected orbital distribution for the majority of the TNO populations, however the origin of the large observed population in the 5:1 resonance remains unexplained.


Test af gravitationsbølgers hastighed

Testing the speed of gravitational waves over cosmological distances with strong gravitational lensing

ABSTRACT: Probing the relative speeds of gravitational waves and light acts as an important test of General Relativity and alternative theories of gravity. Measuring the arrival time of gravitational waves and electromagnetic counterparts can be used to measure the relative speeds, but only if the intrinsic time-lag between emission of the photons and gravitational waves is well understood. Here we suggest a method that does not make such an assumption, using future strongly lensed GW events and EM counterparts; Biesiada et al forecast that 50-100 strongly lensed GW events will be observed each year with the Einstein Telescope. A single strongly lensed GW event would produce robust constraints on the ratio of speeds of GWs and light at the 10-7 level, if a high energy EM counterpart is observed within the field-of-view of an observing gamma ray burst monitor.


Breakthrough Search for Intelligent Life

The Breakthrough Listen Search for Intelligent Life: Target Selection of Nearby Stars and Galaxies

ABSTRACT: We present the target selection for the Breakthrough Listen search for extraterrestrial intelligence during the first year of observations at the Green Bank Telescope, Parkes Telescope and Automated Planet Finder. On the way to observing 1,000,000 nearby stars in search of technological signals, we present three main sets of objects we plan to observe in addition to a smaller sample of exotica. We choose the 60 nearest stars, all within 5.1 pc from the sun. Such nearby stars offer the potential to observe faint radio signals from transmitters having a power similar to those on Earth. We add a list of 1649 stars drawn from the Hipparcos catalog that span the Hertzprung-Russell diagram, including all spectral types along the main sequence, subgiants, and giant stars. This sample offers diversity and inclusion of all stellar types, but with thoughtful limits and due attention to main sequence stars. Our targets also include 123 nearby galaxies composed of a “morphological-type-complete” sample of the nearest spirals, ellipticals, dwarf spherioidals, and irregulars. While their great distances hamper the detection of technological electromagnetic radiation, galaxies offer the opportunity to observe billions of stars simultaneously and to sample the bright end of the technological luminosity function. We will also use the Green Bank and Parkes telescopes to survey the plane and central bulge of the Milky Way. Finally, the complete target list includes several classes of exotica, including white dwarfs, brown dwarfs, black holes, neutron stars, and asteroids in our Solar System.


Stjernedannelse nær Sgr A*

ALMA and VLA Observations: Evidence for Ongoing Low-mass Star Formation near Sgr A*

ABSTRACT: Using the VLA, we recently detected a large number of protoplanetary disk (proplyd) candidates lying within a couple of light years of the massive black hole Sgr A*. The bow-shock appearance of proplyd candidates point toward the young massive stars located near Sgr A*. Similar to Orion proplyds, the strong UV radiation from the cluster of massive stars at the Galactic center is expected to photoevaporate and photoionize the circumstellar disks around young, low mass stars, thus allowing detection of the ionized outflows from the photoionized layer surrounding cool and dense gaseous disks. To confirm this picture, ALMA observations detect millimeter emission at 226 GHz from five proplyd candidates that had been detected at 44 and 34 GHz with the VLA. We present the derived disk masses for four sources as a function of the assumed dust temperature. The mass of protoplanetary disks from cool dust emission ranges between 0.03 — 0.05 solar mass. These estimates are consistent with the disk masses found in star forming sites in the Galaxy. These measurements show the presence of on-going star formation with the implication that gas clouds can survive near Sgr A* and the relative importance of high vs low-mass star formation in the strong tidal and radiation fields of the Galactic center.


White-light Flare on 10 Sep 1886

Evidence for a White-light Flare on 10 September 1886

ABSTRACT: We present evidence for the occurrence of a white-light flare on 10 September 1886. It represents the third of such rare events reported in the history of astronomy. The flare was mentioned by Valderrama (1886, L’Astronomie 5, 388). In this article we have used the original logbook of the observer, J. Valderrama y Aguilar, an amateur astronomer that lived in Madrid and Santa Cruz de Tenerife at that time.


Ikke-baryonisk mørkt stof

Jim Peebles slutter en kort gennemgang af, hvordan ikke-baryonisk stof kom ind i astronomien, med disse ord:

Roberts, referring to developments up to 1980, asked: “What took us so long to accept dark matter? how does it differ from the instant acceptance of the extragalactic nature of  the nebula after Hubble’s announcement of Cepheids in M31?” The dark matter story raises similar questions: why was the community so reluctant to accept Λ, yet so willing to accept nonbaryonic dark matter? Perhaps those looking back on what is happening now wil ask similar questions.

Et godt spørgsmål. Den kosmologiske konstant Λ fremkommer helt naturligt, hvis man udvider det specielle relativitetsprincip til at omfatte en rumtid med en rumtids-krumning forskellig fra nul. Holdningen til denne udvidelse afhænger helt af, om man er fysiker eller astronom. Det fysiske samfund kræver et argument for den kosmologiske konstants størrelse, og et sådant findes ikke. Astronomer ved udmærket, at et sådant argument heller ikke findes for størrelsen af gravitations-konstanten G. Astronomer kan ikke se den store forskel mellem G og Λ.

How the Nonbaryonic Dark Matter Theory Grew

ABSTRACT: The evidence is that the mass of the universe is dominated by an exotic nonbaryonic form of matter largely draped around the galaxies. It approximates an initially low pressure gas of particles that interact only with gravity, but we know little more than that. Searches for detection thus must follow many difficult paths to a great discovery, what the universe is made of. The nonbaryonic picture grew out of a convergence of evidence and ideas in the early 1980s. Developments two decades later considerably improved the evidence, and advances since then have made the case for nonbaryonic dark matter compelling.