Astronomy & Astrophysics

Astronomy & Astrophysics

  • G. Bothun added an answer:
    Where is the center of the universe?
    I was just thinking about inflation of the universe and this question came into my mind. Since it inflates because of an explosion, there should be a point of no velocity. Am I wrong? Feel free to answer and criticize.

    P.S. My knowledge about planets and the big bang is limited to some documentaries and personal readings. So the question might be funny.
    G. Bothun · University of Oregon
    the microwave background is our cosmological inertial frame - all motions are measured relative to that.

    The universe is a surface and a surface has no center. All parts of the Universe are moving away from each other because the surface is expanding.
  • Oliver Manuel added an answer:
    Could a supermassive black hole singularity experience amplification of quantum fluctuations similar to AQFs that Linde claims cause a big bang?
    Could a singularity in a supermassive black hole experience amplification of quantum fluctuations similar to quantum fluctuations that form ever larger cycles, causing a big bang as described by Linde?

    If it is reasonable to assume that an amplification of quantum fluctuations can occur spontaneously at the quantum level and continue to fluctuate in ever larger cycles until it produces a universe--forming big bang (Andrei Linde, inflationary multiverse and eternal chaotic inflation), then would it also be reasonable to assume that a singularity in a supermassive black hole could experience a similar amplification of quantum fluctuations?

    ( – “In a new study by physicists Vanzella and Lima, it is proposed that gravity could trigger a runaway effect in quantum fluctuations, causing them to grow so large that the quantum field’s vacuum energy density could dominate its classical energy density. This vacuum-dominance effect, which emerges under some specific but reasonable conditions, contrasts with the widely held belief that the influence of gravity on quantum phenomena should be small and subdominant.”
    “If the vacuum-dominance effect exists and is strong enough to have such consequences, scientists will still have to discover a new kind of quantum field that would react to gravity in this way, since none of the quantum fields based on known forces could induce these effects. Still, the physicists note that the possibility of vacuum dominance itself is surprising to discover within “a simple and classically well-behaved situation.” Read more at: Daniel Vanzella and William Lima. “Gravity-Induced Vacuum Dominance.” Phys. Rev. Lett. 104, 161102 (2010).

    Such an amplification of a quantum fluctuation in a supermassive black hole singularity could occur either spontaneously or possibly caused by some unusual event. The following are proposed as possible causes of cyclic amplification of quantum fluctuations: 1. Spontaneous event similar to the subatomic quantum fluctuation proposed by Linde; 2. The merging of two very large supermassive black holes; 3. The mass of the black hole exceeding the mass of the rest of the galaxy, thus causing an extreme warp of space; or 4. An usual interaction with dark matter.
    Oliver Manuel · University of Missouri

    Today JoNova and David Evans are in the process of discovering Earth’s climate is driven by the Sun’s deep-seated magnetic fields (and the X Force) from the Sun’s compact innards (Fe-mantle and/or pulsar core).

    That was also the conclusion of a paper by Professors Barry Ninham, Stig Friberg and I [“Super-fluidity in the solar interior: Implications for solar eruptions and climate”, Journal of Fusion Energy 21, 193-198 (2002)].
  • Victor Christianto added an answer:
    Can entropic force explain dark energy properly?
    Since Verlinde's proposal that gravitation is related to entropy, there have been many papers discussing or extending his hypothesis. In a recent paper, Basilakos and Sola reconsidered entropic-force dark energy ( They wrote: "We reconsider the entropic-force model in which both kind of Hubble terms appear in the effective dark energy (DE) density affecting the evolution of the main cosmological functions, namely the scale factor, deceleration parameter, matter density and growth of linear matter perturbations. However, we find that the entropic-force model is not viable at the background and perturbation levels due to the fact that the entropic formulation does not add a constant term in the Friedmann equations."
    So do you think that entropic force can explain dark energy?
    Victor Christianto · University of New Mexico
    Dear Antonio, thank you for your answer. For your information, since 2010 there were many papers discussing entropic dark energy, including by Smoot et al. The file that i attached is one of the most recent papers. Best wishes
  • Remudin Reshid Mekuria added an answer:
    How can I perform weighted averages over mass or volume of the cell for cell count to get dark matter density?
    Having count in cell for different bins provides different densities in a cell. Therefore I think I need to perform weighted averages over mass or volume of the cell. I am looking for an explanation how to do the averaging in either over cell-mass or cell volume?
    Remudin Reshid Mekuria · University of the Witwatersrand
    Thank you very much!!
  • Vladan Celebonovic added an answer:
    Is there a possibility of collision of a heavenly body that we can't predict?
    Can this happen that our galaxy falls in the trajectory of a star/galaxy that have not been located so far, and that star/galaxy appears(have still not been spotted) & collide with the Milky Way?
    Vladan Celebonovic · University of Belgrade
    Rock of various sizes which exist in our planetary system are mostly made up of materials with low albedos, implying that they are dark. This makes their discovery even more difficult, and making the decision whether or not one of these rocks is on collision course with the Earth even more difficult.
  • Demetris Christopoulos added an answer:
    How many dimensions are there in the universe?
    In particular, why do we only experience three spatial dimensions in our universe, when superstring theory, for instance, claims that there are ten dimensions — nine spatial dimensions and a tenth dimension of time?
    Demetris Christopoulos · National and Kapodistrian University of Athens
    @Marshall, I don't refer to this conversation only, but to many past ones, where you couldn't even express your opinion and then... suddenly a downvote was always present at every post of you!
    Probably the publicity to this process has led to an improvement... we will see.
    As for the core of the problem, I agree with Bernard, since, as the 'like' process is so popular, then we don't need a 'negative vote'.
  • James Dwyer added an answer:
    Can someone suggest galaxies, for which both the total baryonic mass and the mass of its central supermassive black hole have been calculated?
    Of the 72 central supermassive black holes on the McConnell and Ma table in their paper “Revisiting the Scaling Relations of Black Hole Masses and Host Galaxy Properties”, I have only found six galaxies in which the baryonic mass has been estimated for the entire galaxy. McConnell and Ma have listed the mass of 41 central bulges of these galaxies; however, in this project the central bulge is not a good proxy for total mass. In addition to publications, any leads to people who or institutions that may have done these calculations would be helpful.
    I also suggest a recent paper -
  • Victor Christianto asked a question:
    Is the Sun primarily comprised of condensed matter?
    According to conventional paradigm, the sun is assumed to consist of hot gaseous plasma. But that assumption is not supported by direct astronomical evidence.

    Another idea is suggested by Dr. Oliver Manuel, who proposed that the Sun's core consist mainly of iron and the source of energy in the sun is not fusion but neutron repulsion.

    But Prof. Pierre-Marie Robitaille has different idea. He suggests that the Sun is primarily comprised of condensed matter. In a recent report, he argues that there are 40 evidences supporting his hypothesis. See

    If Robitaille's hypothesis holds true, then it could imply that the process inside the Sun may be modeled as Condensed Matter Nuclear Science.

    So do you think that the Sun is comprised as condensed matter? Your comments are welcome
  • Michael Peck added an answer:
    What verification we need on the latest BICEP2's universe inflation?
    There are a few points being raised lately on the first direct evidence on the universe inflation and the detection of gravitational waves:

    1. The cosmic microwave polarization patterns referred to carry a lot of information. They do not solely refer to the gravitons (the gravitational waves - the ripples in space-time amalgam)

    2. The indication to gravitons were carefully disentangled from many other irrelevant things.

    3. Which could refer to objects - like dust or any thing of density that would simply cause the same (more or less) type of distortions in the cosmic microwave background that indicated the existence of gravitons.

    4. For one thing, most importantly, no previous indication from a different method other than the polarization in cosmic microwave background has ever been made. An independent method is needed to confirm this uniquely new result against different set of results that would lead to same findings.

    5. The detection was made 3 years ago, after which tedious calculations and investigations were made. Can such a polarization pattern be detected again? Why or why not?

    6. We need to know how would, at least theoretically, the cosmic microwave polarization pattern purely (without the interference from any thing, like dust) look under the effect of gravitons? Can we do gravitons in a lab?

    What methods should be followed to verify the findings of this discovery?
    "Now, serious flaws in the analysis have been revealed that transform the sure detection into no detection."
  • G. Bothun added an answer:
    What is the Inflation model of the Big Bang?
    It refers to the existence of the big bang, and it predicts the predominal energy that is universally spread out.
    G. Bothun · University of Oregon

    remember that the "big bang" really means the instant at which space-time, as a geometry, came into existence.
  • Ramanand Jha added an answer:
    Is it correct that Kepler gave four laws but one of them was wrong?
    The three laws of Kepler are well-known. These are (i) Law of Orbits/Ellipses ( in 1605) (ii) Law of Areas (in 1602) (iii) Law of Periods/Harmonic Law (in 1618). What was the 4th law (if possible please give the reference)?
    Ramanand Jha · Skyline Institute of Engineering & Technology, Greater Noida (UP) INDIA
    Dear Eric Lord, Thanks for your answer and the reference !!!
  • Yuri Shchekinov added an answer:
    In a two temperature plasma, we have the ratio of electron and ion temperature given by T_e = (m_e/m_i)^0.5 T_i. How do we arrive at this relation?
    The above relation has been used in the synchrotron cooling expression in the paper "Behaviour of dissipative accretion flows around black holes" 2007MNRAS.376.1659D.

    We know that the electron thermal velocity is v_e = (k T_e/m_e)^0.5 and ion thermal velocity is v_i = (k T_i/m_i)^0.5. If we say that ions and electrons have same kinetic energy then...m_e (v_e^2) = m_i (v_i^2)...and we arrive at the ralation...

    T_e = (m_e/m_i)T_i

    But under what assumptions can we arrive at the relation T_e = (m_e/m_i)^0.5 T_i ??
    Yuri Shchekinov · Southern Federal University
    have to think -- I am not quite familiar with what's going on with T_e/T_i when you do approach black hole.. depends on heating sources of the electron, I guess.
  • Luc Arnold added an answer:
    Is it possible to see mercury transit the same way that I saw venus transit i.e without a telescope?
    I saw venus transit in the morning 6-june-2012 at sunrise just by putting a sun glass and looking at the sun as it was rising. I calculate that mercury during transit would have
    about one fifth the angular size of venus. My question : Is it possible to see mercury
    transit the same way that I saw venus transit in 2012?
    Luc Arnold · French National Centre for Scientific Research
    Yacoub, yes with binocular with 7x or more power, you should see Mercury.
  • Parviz Parvin added an answer:
    The sun is a critical fusion reactor and needs negative feedback control. How does the sun stabilize itself?
    Negative feedback is essential for the steady state operation of any dynamic system. The sun generates heat based on nuclear fusion. Therefore the sun is a nuclear reactor operating in a critical condition. How does the sun remain stable? What mechanisms are the control feedback of sun?
    Parviz Parvin · Amirkabir University of Technology

    Dear John

    In fact , the hot plasma form of sun surface does not allow to measure the events in the  core. however if there is good estimation of core pressure and temperature, then the according to thermodynamic state of hydrogen , its phase in core may be estimated.

  • Alexander E Kaplan added an answer:
    Why you think that you should not give importance to the study of untested theories?
    Why you think you should not give importance to the study of the possibility of the existence of dark matter / energy , multidimensional theories , worm tubes , and other theories that have not been tested yet? (They are made ​​on paper ! )
    Recall that the theory of relativity, theory of evolution of Darwin and others were not accepted by the scientific community in its principles. There were writen on paper before being confirmed after many years. An example of this is: Albert Einstein in his theory of general relativity predicts that the path of light bends in a gravitational field. This was no positive proof until many years after his death. This was confirmed during observasion of light in an eclipse.
    Alexander E Kaplan · Johns Hopkins University
    The question by Luis is formulated in a very awkward way -- is the author implying that untested theories should not be pursued or the other way around? I guess what he wanted to say is that they should be pursued, with which most of us would agree, but then it is truism, a bit too trivial to have a serious discussion about it.
  • Yurij Kulinich added an answer:
    How to calculate power spectrum from density perturbation?
    What is the method for finding the power spectrum from the distribution of density? Considering I have the distribution of density over a certain volume.
    Yurij Kulinich · Ivan Franko National University of Lviv
  • Chris Gueymard 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.
    Chris Gueymard · Solar Consulting Services
    The formula of Dr. Dong cannot be precise because it ignores the effect of leap years and thus the exact (fractional) number of days in a year. It's only an approximation compared to the more complex methods discussed earlier.
  • James Dwyer added an answer:
    Is it possible to have some quasi-periodic solutions for the accretion disk equations?
    Can the accretion disk disappear and form again for the same compact star? And If yes what is the time scale for this process?In which type of disk this quasiperiodicity can appear?
    It seems the Marscher work was published in 2002 - I'm not sure that the regularity of AGN cycle has been borne out... Here's some more recent research from 2012: It also provides other, more recent references. As I understand, the case of galaxy 3C120 accretion is peculiar...
  • Dominic Napolitano added an answer:
    Can we build nuclear power plants on the Moon and transport energy to the Earth? Wouldn't that be very safe for all of us?
    Nuclear power plants are some of the most sophisticated and complex energy systems ever designed. Any complex system, no matter how well it is designed and engineered, cannot be deemed failure-proof. Veteran anti-nuclear activist and author Stephanie Cooke has argued:
    The reactors themselves were enormously complex machines with an incalculable number of things that could go wrong. When that happened at Three Mile Island in 1979, another fault line in the nuclear world was exposed. One malfunction led to another, and then to a series of others, until the core of the reactor itself began to melt, and even the world's most highly trained nuclear engineers did not know how to respond. The accident revealed serious deficiencies in a system that was meant to protect public health and safety.
    The 1979 Three Mile Island accident inspired Perrow's book Normal Accidents, where a nuclear accident occurs, resulting from an unanticipated interaction of multiple failures in a complex system. TMI was an example of a normal accident because it was "unexpected, incomprehensible, uncontrollable and unavoidable".
    Perrow concluded that the failure at Three Mile Island was a consequence of the system's immense complexity. Such modern high-risk systems, he realized, were prone to failures however well they were managed. It was inevitable that they would eventually suffer what he termed a 'normal accident'. Therefore, he suggested, we might do better to contemplate a radical redesign, or if that was not possible, to abandon such technology entirely..
    A fundamental issue contributing to a nuclear power system's complexity is its extremely long lifetime. The timeframe from the start of construction of a commercial nuclear power station through the safe disposal of its last radioactive waste, may be 100 to 150 years.

    The abandoned city of Prypiat, Ukraine, following the Chernobyl disaster. The Chernobyl nuclear power plant is in the background.
    The nuclear power debate is about the controversy which has surrounded the deployment and use of nuclear fission reactors to generate electricity from nuclear fuel for civilian purposes. The debate about nuclear power peaked during the 1970s and 1980s, when it "reached an intensity unprecedented in the history of technology controversies", in some countries.
    Proponents argue that nuclear power is a sustainable energy source which reduces carbon emissions and can increase energy security if its use supplants a dependence on imported fuels. Proponents advance the notion that nuclear power produces virtually no air pollution, in contrast to the chief viable alternative of fossil fuel. Proponents also believe that nuclear power is the only viable course to achieve energy independence for most Western countries. They emphasize that the risks of storing waste are small and can be further reduced by using the latest technology in newer reactors, and the operational safety record in the Western world is excellent when compared to the other major kinds of power plants.
    Opponents say that nuclear power poses many threats to people and the environment. These threats include health risks and environmental damage from uranium mining, processing and transport, the risk of nuclear weapons proliferation or sabotage, and the unsolved problem of radioactive nuclear waste. They also contend that reactors themselves are enormously complex machines where many things can and do go wrong, and there have been many serious nuclear accidents.Critics do not believe that these risks can be reduced through new technology.They argue that when all the energy-intensive stages of the nuclear fuel chain are considered, from uranium mining to nuclear decommissioning, nuclear power is not a low-carbon electricity source.
    Dominic Napolitano · Enercon
    It is not necessary or desirable to build nuclear plant on the moon. Nuclear power is, based on actual statistics, one of the safest ways to produce energy and also has one of the least environmental impacts. The reasons are simple:

    1. There is no air pollution and no carbon dioxide. Air is not used in the process
    2. The amount of fuel and waste produce is small. The fuel in a 1000 MWe reactor core can fit in a two garage.
    3. Since the amount of fuel required is smaller than coal, gas, wood or any other burning fuel the mining impact of nuclear is the least of all the major energy opitons.
    4. The amount of land usage for a reactor is the least compared to Wind or Solar or bio mass
    5. Nuclear energy production is 24/7 except for a refueling outage which takes 3 weeks every two years. Solar and Wind have capacity factor 25 and 35% are are very intermittent.
    6. Concerning accidents, even with TMI, Chernobol and Fukushima, the loss of life (none in the case of TMI and none in the case of Fukushima), is still very low and cancer rates are not significantly above the norm.

    FinallyAll energy options should be assessed against one another for there pro and cons. There is a serious problem with climate change and nuclear power must be part of the energy mix.
  • James Dwyer added an answer:
    Can the Lambda-CDM cosmological model survive the discrepancy between galaxy cluster observations and CMB projections?
    See It states:
    "… The background radiation shows the small density variations in the early universe that would eventually cause matter to clump in some places and form voids in others. We can see the end product of this clumping in the recent universe by observing the spread of galaxy clusters across space.
    "The best measurements of the cosmic background radiation came from the European Space Agency’s orbiting Planck telescope in March 2013. Galaxy-cluster measurements, on the other hand, come from various methods that include mapping the spread of mass across the universe by looking for the gravitational lensing, or warping of light, it causes. The two measurements, however, are inconsistent with one another. "We compare the universe at an early time to a later time, and we have a model that extrapolates between the two," says Richard Battye of the University of Manchester, UK, co-author of the new study1 published on 7 February in Physical Review Letters (PRL). "If you stick to the model that fits the CMB data, then number of clusters you find is a factor of two lower than you expect.""

    Recent reports of X-ray signals that may signify the decay of sterile neutrinos have raised hopes for a cosmological solution, as some mix of Cold Dark Matter (CDM), Warm Dark Matter (WDM) and/or Hot Dark Matter (HDM) could fit CMB projections of galaxy cluster formations to observations since WDM and HDM would prevent structure formation at increasingly larger scales. See and
    Also see and

    However, if the composition of universal mass-energy included HDM or WDM and less total CDM than now thought, how would that affect the enormous gravitational effects routinely attributed to CDM in the observed universe?

    Adding HDM and/or WDM may help with LCDM problems such as 'the small scale structure problem', 'the missing satellite problem', the 'cuspy halo problem' and others, but it must do so while maintaining alignment with other observations. What other L-CDM results would be affected by the inclusion of HDM and/or WDM?
    Dear Biswajoy,
    There have been a few L-CDM anomalies identified even in the Planck CMB data - see
    "One of the most surprising findings is that the fluctuations in the CMB temperatures at large angular scales do not match those predicted by the standard model – their signals are not as strong as expected from the smaller scale structure revealed by Planck.
    "Another is an asymmetry in the average temperatures on opposite hemispheres of the sky. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look.
    "Furthermore, a cold spot extends over a patch of sky that is much larger than expected."

    Moreover, to this pedestrian, it seems that L-CDM power spectrum analyses may be more an exercise in parametric curve-fitting than stringent testing of model results. For example, see

    There are also some very fundamental observations of low mass, large separation stellar binary systems in our galactic neighborhood that do not comply with Keplerian expectations for inverse-square diminishment of rotational velocity as a function of separation distance. While they do fit with the general expectations of modified gravity theories, there seems to be no configuration of dark matter that could produce the observed rotational characteristics. See

    Dark matter's only purpose, even in the primordial universe, is to increase the effects attributed to mass-energy to fit observations. Alternatively, the analytical determination of effects produced by mass-energy, especially under widely varying conditions, could be different than currently established evaluation methods presume. If the effects of mass-energy in the early universe were different than determined by the L-CDM model, results might still fit with observations despite the absence of any dark matter.

    Again, our interpretation of early, large scale conditions may be skewed by averaging methods and other scale dependent factors - see
  • Michael Y.T. Hwang asked a question:
    What is the standard for a new physics discovery?
    I discovered a EIDIP function that can be used to model the upper bound of nuclear binding energy, the relationship between the Higgs boson mass and nuclear binding energy, the relationship between quantized mass number and speed of light, the repulsive/attractive characteristic of the inter-nucleon nuclear force and inter-molecule covalent bonding force, the short range asymptotic freedom and long range color confinement behavior of the strong force at inter-atomic range, the Pioneer 10/11 spacecraft sunward acceleration anomaly at interplanetary distance, and the galaxy rotational velocity anomaly curve at interstellar distance. In order to prove it, I have developed 9 EIDIP function application models; Comparison between empirical data sets and model predictions are summarized in this slidedoc (in MS PowerPoint).
  • James Dwyer added an answer:
    Big bang is sometimes explained as an hydrogen generation explosion. Is that correct and why hydrogen?
    That should give hints about the preceeding origin, no?
    Isn't it thought that the very early universe was so hot (energetic) that perhaps only transient virtual particles could (fleetingly) exist? As the universe expanded and cooled, progressively more stable particles condensed until, eventually, hydrogen nuclei became stably bound...
    See and
  • Victor Ostrovskii added an answer:
    Do dark matter and dark energy constitute valid evidence of large spatial dimensions higher than 3?
    There are speculations that the gravitational effects of matter in 4 or greater LARGE spatial dimensions might account for the substantial discrepancy that exists between measured gravitational effects on normal baryonic matter and the amount of that matter that exists according to measurements. Could we be measuring the gravitational effects of "normal" matter in higher dimensions? And can large higher dimensions also explain the huge amount of dark energy that seems to be around?
    Victor Ostrovskii · Karpov Institute of Physical Chemistry
    Dear Dr. Juan Casado,

    Please, I send you the addresses of the two-page abstract and of the more comprehensive presentation.

  • Michael Peck added an answer:
    From dwarfs to giants: how were ellipticals formed ?
    Updating a basic question: what are the possible ways to make elliptical galaxies with various masses? Is merging the only way for the massive ones?
    I have not done any simulations in regards to monolithic collapse producing unstable rotation curves. However, it is relatively easy to conclude that if we initially have a spheroidal galaxy with mostly radial motion transitioning to a more tightly concentrated disk, then the exchange of gravitational potential to momentum along the z-axis would need to be conserved in some manner (x-y plane is aligned with the disk). There was a recent article about wavy motion in the Milky Way disk and I believe that this is a residual of collapse along the z-axis (, along with the motion of UV excess stars.

    The theory that seems to match observations best is an origin from dense, x-ray emitting gas. Therefore, initial star formation begins due to a prior higher density of the ISM and then is halted in early-type galaxies due to expansion of the ISM (decreased density). The next phase would be cooling of the x-ray emitting gas, which relative to the Jean's criterion would allow a new generation of stars to form (high redshift ETG in the paradigm that I am arguing for). This is the only way I can think of that would reconcile the bottom heavy IMF in massive ETGs with its correlation to metallicity, stellar dispersion and x-ray temperature. For example, a very dense hot initial state would induce significant fragmentation while producing fewer massive stars. In regards to elliptical galaxies in cluster and field environments, the field varieties have more neutral hydrogen and increased ring/disk formation. One can therefore infer that cluster environments provide a look-back as to the previous evolution and state of field galaxies. Also remember that the gas in ETG's is on the order of that contained in their stellar populations, so a lot of future star formation is still possible.
  • Rolando Gaitan Deveras added an answer:
    How should I choose a particular gauge in first order perturbation in general relativity?
    In the case of general relativity the change in coordinate can give rise to fictitious perturbations, for which we can use gauge invariant variables or we can also choose a specific gauge. My question is how to decide which gauge is suitable for a particular problem?
    Rolando Gaitan Deveras · Universidad de Carabobo, UC
    Following Dr. Eubanks, if one assume gravity as a gauge theory, there is a nice analogy with Yang-Mills theory about possible gauge fixations. For example, let $h_{\mu\nu}$ be the graviton field in a perturbative regime, then the "Coulomb" gauge must be $\partial_i h_{i\nu}=0$. Observe that this fixation is no more a general relativistic covariant constraint. So, in gravity a gauge fixation and covariance constraint go hand in hand.
  • V. T. Toth added an answer:
    Big bang theory indicates that universal expansion will continue but where it is expanding and is it going to stop after several million years.
    Once the universal expansion stops will our universe start converge or collapse and will form another big bang and the new universe will start its creation and this process will go on and the creation and recreation will continue and the universe will remain infinite with expansion and contraction.
    V. T. Toth · N/A
    Whether or not the expansion comes to a halt or continues forever depends on whether or not the gravity of all matter and energy in the universe is sufficient to overcome the expansion itself. The recent discovery of accelerating expansion suggests that it is not; that in fact (due in part to the presence of "dark energy", about which we know next to nothing) not only will the expansion continue forever, but it will accelerate.
  • G. Bothun added an answer:
    When the Big-Bang exploded in a millisecond, what created the space in which primordial matter traveled?
    Did a certain type of space exist before the explosion took place? Even when the explosion itself created the space by its own strength, there had to be something that existed before the Big-Bang, or was there?
    G. Bothun · University of Oregon
    the "Big Bang" event does not need to be a singularity. Generally speaking, the
    big bang should be equated with the creation of space time and the point when
    the Universe starts to evolve. What exists before known physics its not a relevant question.
  • Biplob Sarkar added an answer:
    What is the Rayleigh Taylor Stability criterion in the case of a rotating viscous fluid?
    Why does the outward increase of angular momentum in an accretion disk make the disk Rayleigh stable? Since the density profile in an accretion disk increases inward in the case of constant accretion rate, there should not be an instability in between consecutive rings. Is that the reason why they are Rayleigh stable?
    Biplob Sarkar · Indian Institute of Technology Guwahati
    The Rayleigh criterion, relies on fluid elements retaining their angular momenta.
    Following Rayleigh, suppose we interchange the fluid in two rings, of equal masses, at radii r1 and r2 . So the fluid initially at r1 is moved to r2 , conserving its angular momentum L1 . Similarly the fluid initially at r2 is moved to r1 , conserving its angular momentum L2 . Then,

    change in KE ∼ (L2^2 − L1^2 )(r1^-2 - r2^-2)

    Now a system always wants to go to the lowest energy state. If angular momentum is a increasing function of r, then the swapping of fluid parcels is going to cost the system to increase its energy, so the system will not try to do it and the system is stable to this perturbation.
    However in case when angular momentum is a decreasing function of r, then the system can release its energy by swapping of fluid parcels and the system will try to do so, so the system is unstable to this perturbation.

    So we can say that when the angular momentum is a increasing function of r in an accretion disk, the disk is Rayleigh stable.

About Astronomy & Astrophysics

Everything about Astronomy & Astrophysics.

Topic Followers (11041) See all