Astronomy & Astrophysics

Astronomy & Astrophysics

  • Akira Kanda added an answer:
    Is dark energy merely an illusion?

    According to a report by Carlo Iorio and Timothy Clifton, dark energy may be an illusion. And LTb model or variations of it can be promising candidates to get rid of it. What do you think?

    Akira Kanda · University of Toronto

    Victor, 

    As I said, there is no such thing as energy as the very definition of kinetic energy contradicts the conservation of work. 

    Putting this aside, regardless of what energy is, what I see in these "dark" issues is attempt top cover up what their theory fails to agree with experiment. 

    Of course, as their theory is inconsistent logically, their theory can prove anything which contradicts any experimental result, a trivial of logic. So here they go again. Dark dark entities. 

  • Naïma Amrani asked a question:
    What is the three dimensional external graphical representation of black hole?

    .

  • Krzysztof Bohdan added an answer:
    Cosmic background microwave radiation - why can we still detect it?
    I have a difficulty in understanding how can we still detect CBMR. In literature I read that:"The photons that existed at the time of photon decoupling have been propagating ever since:"
    "The surface of last scattering refers to the set of points in space at the right distance from us so that we are now receiving photons originally emitted from those points at the time of photon decoupling." Now, if we observe a supernova explosion, the light will get past us and we can no longer detect these photons emitted during the explosion. I cannot understand how can we still detect the photons from the surface of the last scattering, why they just won't get past us and be gone?
    Krzysztof Bohdan · University of the West of England, Bristol

    Dear Prof Zekry, if the source of radiation is distant enough, this combined with the universe expansion will cause that the wave which travels through the so called vacuum, never to arrive at our location, ever. This is known as the cosmic event horizon, which is currently about 5 gigaparsecs. Universe expansion continues to accelerate, towards infinite speed. Also, as the wave travels through the space-time, maybe distorted it is not but the proper term to describe what is happening to it is that it becomes redshifted. As we continue to detect the CMBR from further away, the waves become longer and weaker, eventually reaching the cosmic event horizon and we will no longer see the surface of the last scattering. Of course this will be billions years into the future. I am sure these facts can be ignored in communication technology.

  • Charles Francis added an answer:
    Why are there different gases for different galaxies?
    In different galaxies there appear different gas structures. What is the mechanism adapting certain gases in certain galaxies while excluding the other gases?
    Charles Francis · Jesus College, Cambridge

    Of relevance may be that there is continuing star formation in spiral galaxies, but not in elliptical galaxies.

  • James Overduin added an answer:
    How do cosmologists determine the curvature of the universe based on astronomical data?

    Almost all cosmologists agree that the curvature of the universe is “flat” (k=0). Obviously this conclusion is based on solid astronomical observations - data. My question then is: Is there direct observational evidence that shows the universe is flat or is this conclusion solely a model-dependant parameter?

    James Overduin · Towson University

    Observational data (primarily the CMB) tell us that the curvature must be close to flat, but do not tell us that it is exactly flat. The belief that k=0 is a theoretical prejudice, usually based on fine-tuning arguments. For example, the "flatness problem" is the argument that any departure from flatness at the present time would imply that the total density of the universe at early times would have to be fantastically close to (but not equal to) the critical density. While widely believed, this argument has little logical force because the fine-tuning parameter here is simply an artifact of the definition of critical density (which evolves strongly with time). When curvature is expressed by more physical parameters, the fine tuning disappears and universes with nonzero curvature are found to be perfectly compatible with current and forseeable future CMB data. Some relevant papers here are by Adler & Overduin (gr-qc/0501061), Lake (astro-ph/0404319), and more recently by Carroll (1406.3057). In this context it is interesting to note that the Planck 2015 results (1502.01589) report a "2-sigma detection of positive curvature", which disappears when Planck data are combined with other datasets. A nice early discussion of these issues was by White and Scott (astro-ph/9508157).

    It's worth appreciating how impressive the CMB measurement of curvature really is. It is very much akin to the measurement that might be made by creatures in a 2D universe who measure the curvature of their world by adding up the angles in a triangle to see if they are equal to 180 degrees. In our case the "baseline of the triangle" is the patch of sky occupied by the largest fluctuations in the primordial plasma at the moment when the universe became transparent to light. This method gets its power from the fact that it leapfrogs "local" systems whose interpretation is messy and complex (supernovae, galaxies, quasars) and goes directly back to the radiation-dominated era when physics was simpler.

  • Mohamed Th. S. Heikal added an answer:
    Is it possible to find pure iron phase in meteorite without any inclusions?

    In the Chelyabinsk meteorite in particular.

    Mohamed Th. S. Heikal · Tanta University

    Iron meteorites were strongly used by ancient Egyptians as a tool in agriculture and other uses. I strongly  some colleagues about pure iron associated with Ni & Co as mentioned in Kamel impact crater crater of Egypt.

  • Arno Gorgels added an answer:
    As a researcher claimed that black holes do not exist , is any one interested in finding non singular general relativity?

    A researcher claimed that black holes do not exist the link is:

    http://phys.org/news/2014-09-black-holes.html

    https://www.researchgate.net/publication/265415056_Back-reaction_of_the_Hawking_radiation_flux_on_a_gravitationally_collapsing_star_II_Fireworks_instead_of_firewalls

    Is any one interested in solving the singularity problem in general relativity?

    What are the suggested ideas?

    Arno Gorgels · Principia Naturae

    Bon appetit, dear Daniele

  • Barry Turner added an answer:
    How significant is the discovery of Kepler-186f, an earth-sized habitable zone planet?
    Kepler-186f is the first earth-sized planet located in the habitable zone of another star that has been discovered. With this discovery, the search for life on other planets has entered into a new zone of discovery.
    Barry Turner · University of Lincoln

    Louis

    As you know I hope I am totally wrong in my pessimism about space travel or being able to communicate with other intelligent life forms.  I am not however optimistic that the quantum world offers us much assistance since we do not actually live in it.  You are quite right about general relativity and even Einstein accepted it was seriously flawed but we live in a universe of matter and energy for certain and it poses the problem.

    One of the biggest problems with travelling through space is the fact that it is not empty.  Since at present it is only feasible that humans would have to be in an energy propelled vessel to make a space voyage the presence of particles on matter in space would be a huge problem.

    Any space ship travelling through interstellar space at the speed of light, were that possible would be destroyed by the huge energy released by collision with even a tiny particle.  They would be unavoidable in a long space mission.

    Similarly navigation would be impossible due to the relative position of interstellar bodies being shifted.  

    Warp drive is the only possible method by which we could travel these distances but the physics of that require energy levels similar to those emitted from a quasar.  It is inconceivable that we could control let alone generate such forces.

    As I say, I hope I am wrong and physics is due for an overhaul!  

  • Mohammad Ayaz Ahmad added an answer:
    What is the structure of black holes?
    In some books popularizing science (e.g. “Astronomy for dummies” by S.P. Maran) it is written that black holes have the following structure: falling matter, event horizon, singularity. This structure does not coincide with the classification used in special literature where the accretion disk forming by falling matter is included. Is the black hole structure in the book above an adequate explanation for non-specialists?
    Mohammad Ayaz Ahmad · University of Tabuk

    Really, we do not know what the inside of a black hole. Describing the characteristics of the structure of a black hole still remains one of the challenges of modern relativistic astrophysics.

  • Dhanraj Sitaram Warjurkar added an answer:
    What are the popular plotting softwares that astromomers and astrophysicists use for publication? I am using XMGRACE and GNUPLOT? What about Matlab?

    Supermongo and IDL are also very popular among astrophysicists. Are Matlab, Mathematica, Xmgrace, Gnuplot also popular among some groups?

    Dhanraj Sitaram Warjurkar · Seoul National University

    MatLab is very good tool for plotting ...

  • Xiangqian Wu added an answer:
    How can I calculate or where can I find the selenographic coordinate of the point where the Sun is at zenith?

    I need to know the time-varying location, in terms of selenographic latitude and longitude, of the point where the line connecting the centers of the Sun and Moon intersects with the lunar surface, to the accuracy of second and kilometer, from 2000 to 2020. Thanks!

    Xiangqian Wu · National Oceanic and Atmospheric Administration

    Thanks a lot!

  • Michael Peck added an answer:
    What is causing Dark Flow?

    Dark flow is an astrophysical term describing a possible non-random component of the peculiar velocity of galaxy clusters. The actual measured velocity is the sum of the velocity predicted by Hubble's Law plus a possible small and unexplained (or dark) velocity flowing in a common direction.

    Does Dark flow exits ? If yes what is the cause of it?

    1. There are many affects due to sample dependence, analytical methods and luminosity distance; I'm in no way saying you are unaware of them. However, I do think it is important to consider that the great attractor is estimated to be 80 Mpc away relative to the extent of the dark flow (at least 800 Mpc or almost 0.2z).

    2. I've only looked at articles by Kashlinsky and/or Atrio-Barandela, as there was controversy surrounding the Planck analysis by others.

    3. The question was asking about the source of the dark flow, which I suppose I can offer an answer to. Consider a sink-source universe with a cosmological scale gravitation potential, i.e. no energy is created or destroyed. The simplest solution would be a continuous, but collaminated 'big bang' arising from the center of the potential (CMB -> gravitational redshift). This than forms into the locally hot x-ray clusters, various galaxies and a Hubble flow, where the bulk flow begins to fall back into the potential (the "dark flow"). Gravitational lensing will then project local geodesics towards the center; i.e. we are observing objects accelerating back into a global gravitational potential through a cosmological-scale lens. This would further explain why volume element/angular scale observations support a static metric, increased entropy with redshift (cold baryonic matter, metallicity and mergers) and hemispherical anomalies versus homogenous universe.

  • Nestor Roman Voronka added an answer:
    What is the best model to predict Solar Cycle 25??

    Prediction of Solar cycle 24 based on various models that is a less active cycle is well known. What is the best model or way to predict Solar cycle 25?

    Nestor Roman Voronka · M42 Technologies

    I second the use of Dr. Ken Schatten's model - he is now a senior scientist at a.i. solutions.  You should also check out the work that Dr. W Dean Pesnell has been doing at NASA/GSFC for comparison.

    https://www.linkedin.com/pub/kenneth-schatten/33/aa5/978

    https://www.linkedin.com/pub/dean-pesnell/73/a23/405

  • Robert Loughnane added an answer:
    Is there a database of community-available antennas - beam size and efficiencies?

    Is it possible to source a URL or database of the available antennas in the field of millimeter and submillimeter astronomy?

    I need to be able to tabulate available frequency-dependent beamsizes and antenna efficiencies.

    Robert Loughnane · Universidad Nacional Autónoma de México

    True Johannes, but the beamsizes usually stated in the observing section of a specified article generally represent a frequency range. However, your point is noted. Thanks.

  • Parviz Parvin added an answer:
    An old question that is still fresh: Is gravity a Newtonian force or Einstein space-time curvature?
    No gravitational wave was measured yet, no graviton was detected accordingly. On the other hand no space- time curvature was observable. There is no successful experiment to validate the current theories. What is the nature of the mysterious gravity? What is the velocity of this effect ?
    Parviz Parvin · Amirkabir University of Technology

    Dear Tolga

    Happy new year.

  • Fatemeh Tabatabaei added an answer:
    What methods are the best to measure the metallicity in the ISM?

    There are several methods and observers which can be used to estimate the metallicity in the interstellar medium, most of which are based on the measurements in the HII regions. First, what methods give the most reliable estimate? Second, has there been any measurement in more diffuse ISM in the Milky Way?

    Fatemeh Tabatabaei · Max Planck Institute for Astronomy

    The radial gradient is an observational fact (which even could have been complicated by migration of stars in some cases). In any case, the environmental effects and the star formation feedback should not be neglected.

  • Ksh. Newton Singh added an answer:
    What will be the speed of sound in quark star if we consider EOS proposed by MIT bag model for an anisotropic fluid?

    If we consider MIT Bag Model EOS, the square of speed of sound for quark star should be around 0.33. But whether this is true for an anisotropic quark star or not. So whether EOS from MIT Bag Model is for isotropic or anisotropic matter or for both?

    Ksh. Newton Singh · National Defence Academy, Khadakwas, Pune, India

    Thanks Prof. Uechi

  • Ksh. Newton Singh added an answer:
    Does a free fall collapsing dust radiate gravitational waves?

    A collapsing star when explode (supernova), due to the sudden ejection of massive mass around the central core, there is a disturbance in space-time leading to emission of gravitational waves. But what will happen if a collapsing goes on till black hole is formed, without any explosion? Will there be an emission of gravitational waves due to the continuous grow in curvature because of the growing mass?

    Ksh. Newton Singh · National Defence Academy, Khadakwas, Pune, India

    Thank You Dr. Yang

  • Marshall Eubanks added an answer:
    How do I determine the spin rate of solivagant exoplanets and substellar objects?

    Solivagant (nomadic) planets are roaming the interstellar space. Depending on the steepness of the mass ditribution law, There may be significantly more substellar objects in the vicinity of the Sun than there are normal stars. A few nearby extremely cool object of super-Jupiter mass have been discovered (e.g., one with WISE). Despite the absence of light, such systems of planetary mass may be teeming with life. According to M. Eubanks, more solivagant planets will be observed in the future with JWST, ALMA and SPICA. My calculations show that the tidal heating of Earth generated by the Moon may presently come up to ~5 TW. A heat source of this order can sustain a massive subsurface ocean on a lonely exoearth for gigayears. The question is, how to observationally verify that nearby solivagant planets rapidly rotate? The spin rate of some stars has been determined photometrically from the modulations caused by persistent features (dark or hot spots) on the photospheres. Would that be the best way to observe the spin of very cold planets? Are there other possibilities?

    Marshall Eubanks · Asteroid Initiatives LLC

    Most galaxies, projected on the sky, are elliptical, and so there are a number of papers dealing with lens ellipticity (oblateness) for strong lensing (see below). I  do not think there will be much difficulty in introducing this into the microlensing formalism. Whether or not the data will be good enough to actually determine nomad rotation oblateness is, of course, another question. 

    http://iopscience.iop.org/0004-637X/482/2/604/pdf/0004-637X_482_2_604.pdf

  • Alexander Yurkin added an answer:
    Is there a future for ground-based astrometry after GAIA?
    Considering the revolution that occurred in terms of optical astrometry due to astrometric satellites such as Hipparcos (even though there were parallel developments and major improvements on ground-based astrometric telescopes), an even larger jump is about to occur with the GAIA astrometric mission. Much work will need to be done to tie the radio reference frame (ICRF2) to the GAIA optical reference frame. There will be ground-based follow-up work following GAIA detections, proper motions and parallax work, as satellite missions are relatively short lived and expensive. But, what does the future hold for ground-based astrometry? Near Earth objects? Solar system measurements? Reference frame maintenance?
    Alexander Yurkin · Russian Academy of Sciences

    Dear Sirs,

     I the first time answered this question by means of a joke, but this time I will give two serious forecasts and in advance I ask astrophysicists not to appropriate two of my deep theoretical scientific results.

    1. It is necessary to leave telescopes and field-glasses on Earth since in space there was a lot of space debris which strongly scratches glasses of space telescopes, sharply lowering their resolution therefore in them it is visible nothing.

    2. Industrial activity of the person causes greenhouse effect and destruction of the Terrestrial atmosphere. Soon on our planet Earth the atmosphere will become same transparent as on the planet Mars both all stars and black holes can be observed without hindrances through magnifying glass of an average aperture.

    Sincerely, Alexander

  • Rudolph E. Schild added an answer:
    What radiation processes are used to fit the observed spectra from the accretion disk around a supermassive black hole?

    Which forms are radiations are dominant in the case of AGNs or in general a super massive black holes?

    Rudolph E. Schild · Smithsonian Institution

    The Shakura and Sunyaev (1973) model is accepted by many, but cannot be the entire story. Recall that the inner part of the accretion disc is inferred from microlensing observations to be a powerful X-ray source at 0.1 to 10 Kev, so something besides the SS model is present. Moreover, the X-ray observations show that most of the X-ray emission comes from a much larger source, which must therefore be farther away from the central black hole.

    Similarly optical observations of microlensing of the radiation show that the SS (1973) emission also has a reverberation in the quasar outer structure at 437 light days. See J. Lovegrove et al 2011.  It has been measured that 3/4 of the light at optical frequencies from this large outer reverberating source in all quasars. Recall that quasars have brightness fluctuation from microlensing that are just a few percent. The quasar microlensing signal from a star is a 30% brightness increase with a 30-year time scale. If the microlensing were of a point source as in MACHO searches, the amplitude would be a factor 10-30. From this we conclude that only a small fraction of the quasar microlensing is due to a compact central source, like the accretion disc inner edge at 6 R_S

    Microlensing from a single planet mass body has an amplitude of only 10%. This is understood to mean that the SS radiation mentioned by Nozhan arising from the inner edge of the accretion disc can only be a small fraction of the total quasar emission, and most of the emission you would see with your eye comes from the larger outer region.

    A good description of quasar structure observed in optical. radio, and X-ray regions can be found in Schild, Leiter, and Robertson, AJ, 2006. It gives the size scales of the luminous structures and their locations, in cm. 

  • Marcelo Negri Soares added an answer:
    What is antigravity?
    I want to know the basics.
    Marcelo Negri Soares · Universidade Nove de Julho

    Anti-gravity is an idea of creating a place or object that is free from the force of gravity. It does not refer to the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism or aerodynamic lift. Anti-gravity is a recurring concept in science fiction, particularly in the context of spacecraft propulsion. An early example is the gravity blocking substance "Cavorite" in H. G. Wells' The First Men in the Moon.

    In Newton's law of universal gravitation, gravity was an external force transmitted by unknown means. In the 20th century, Newton's model was replaced by general relativity where gravity is not a force but the result of the geometry of spacetime. Under general relativity, anti-gravity is impossible except under contrived circumstances. Quantum physicists have postulated the existence of gravitons, a set of massless elementary particles that transmit the force, and the possibility of creating or destroying these is unclear.

    "Anti-gravity" is often used colloquially to refer to devices that look as if they reverse gravity even though they operate through other means, such as lifters, which fly in the air by using electromagnetic fields.

  • Alexander Chepick added an answer:
    How can we explain Tifft's quantization of galaxy redshift?
    The reports by Tifft on quantization of galactic redshift are well-known to astronomers. Read for example http://www.vixra.org/abs/1309.0011. See also a recent review on redshift theories by Marmet at http://www.marmet.org/cosmology/redshift/mechanisms.pdf

    I don't have articles on Tifft's quantization, but I will send you two of my articles on static models.
    I think that the space of the Universe is not expanding. Test Tifft's quantization will show it, because in the framework of GR the distance between galaxies previously had to be less than it is now. Therefore, in GR dependence should be approximately inversely proportional to z.

  • Rahul K Sharma added an answer:
    Does CME affect the pulse of the heart?
    CME can leads to blackout on Earth due to disturbance in Earth's magnetic field.
    Rahul K Sharma · National Association of Nanotechnology and Ultra-science(NANU)

    Yuri, thank you very much.its ok.my english is not very nice.Sure i will read the book u have mentioned. i am following both link in researchgate.thanks for those links.They are Russians.I am in Russia now.It is really helpful.Though I am far from Moscow, it will be easier for me in future. after reading book i will connect with them. If any thing i will not understand about CME or anything related to concerned topic, i will bother you for help.thanks a lot for kind replies..

  • Raul Simon added an answer:
    How energy is transfered from photosphere to corona on the bais of elemental abundance variation from photosphere to corona ?

    As some of the research papers suggest that low FIP (below 10 KeV) elements get enhance by factor of 3-4 in Corona from Photosphere, while high FIP (above 10 KeV) elements don't show this characteristics.

    Does this effect conclude anything about energy transfer from Photospere to Corona ?

    Raul Simon · L A M B

    Thanks, Phil.

  • John Houghton added an answer:
    Proton- proton reaction in star center can anyone help?
    The proton-proton reaction in star center goes on for billions of years. But when the reaction starts on the surface as in the case of the nova, it only lasts for a few weeks. Can somebody explain this difference?
  • William Dean Pesnell added an answer:
    How can we analytically calculate the Hurst exponent for a periodic function?

    Let's focus on a sin(x) function. I tried using the DMA (changing sums into integrals) and the series width w(l) from Katsev & L'Hereux, Computers & Geosciences 29 (2003) 1085–1089. In the first case I got stuck with some crazy functions, and in the second (expanding logs in time series to first order as l<<T) I got... H= -1/2. I want to precisely understand why H=1 for strict periodicity.

    An R/S analysis compares the properties of the time series over many different time scales. The slope of the curve then approximates the Hurst exponent. If you analyze a series with noise and a sine curve you see a discontinuity in the slope at the period of the sine curve (the Suyal et al. 2009 paper has some examples). In the absence of variations at longer timescales, such as trends, there is no information above that period to derive a slope. For the sunspot number we see a discontinuity at 11 years, just as expected, but there are longer-term signals to continue the R/S analysis at longer periods. That gives an estimate of H for the entire time series. A linear trend indeed has H=1 because the variation over a time bin is comparable to or larger than the noise within that time bin.

  • Z. Osmanov added an answer:
    Does anyone know what is the minimum level for detectability of PeV photons by modern telescopes?

    One of the important questions concerning this topic is to know sensitivity of instruments - the minimum detectable flux.

    Z. Osmanov · Free University of Tbilisi

    Dear Dr Samvel Ter-Antonyan, thank you for the useful references

    ZO

  • Gerro Prinsloo added an answer:
    How can we compute solar position at a given place on a given day and time?
    I have GPS obtained UTC time (hours, minutes, seconds), longitude(deg E), latitude (deg N) and date. I have thoroughly search on internet for step-by-step procedure to obtain solar position variables - solar zenith angle, solar azimuth angle, Sun-Earth distance. But every method is different. Some followed geometrical method while most others have some complicated formulae with varying coefficients. I never found a generalized way to obtain solar position variables. Is there any reference which provides step-by-step procedure to obtain them in the most accurate way? Can anyone provide the step-by-step procedure with equations, corresponding explanation for coefficients, accuracy of output and literature references for each equation? Please don't provide me readily available codes / functions or links on internet search.
    Gerro Prinsloo · Stellenbosch University

    Dear Lakshmi

    you already have many responses, but if you require open source algorithms for sun tracking with matlab simulink or PLC or microprocessors then you can also check Chapter 3 of our free eBook for links to the code: 

     https://www.researchgate.net/publication/263085113_Solar_Tracking_High_precision_solar_position_algorithms_programs_software_and_source-code_for_computing_the_solar_vector_solar_coordinates__sun_angles_in_Microprocessor_PLC_Arduino_PIC_and_PC-based_sun_tracking_devices_or_dynamic_sun_following_hardware

    regards

    Gerro

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