# Astronomy & Astrophysics

• Allard Jan Van Marle added an answer:
What are the main astrophysical radio wave sources?
I have a project and I need to know the sources of electromagnetic radiation along with the processes by which they are emitted in space. Any help would be appreciated.
Allard Jan Van Marle · University of Leuven

Emission from molecular electronic transitions (rotational and vibrational)

Scaling solution to BBGKY-hierarchy in Astrophysics
I am interested in finding closure relations for the BBGKY-equations and came across an old article On the integration of the BBGKY equations for the development of strongly nonlinear clustering in an expanding universe'' by M DAVIS, PJE PEEBLES, Astrophysical Journal Supplement Series, 1977. I am used to closures where the three-particle correlations are either neglected or are approximated by Kirkwood's superposition principle, which appears ad hoc to me and is inaccurate for certain applications.
In the article by Davis and Peebles, something I haven't seen before was used, a scaling solution where n-particle correlation functions are approximated by a simple power law. It looks like this only works if the particles are far apart from each other and if there is no characteristic length scale in the particle interactions, which is the case for gravity. This made me curious: How useful did this approach become in Astrophysics simulations, is it still used or was this a dead end? If not, has there been any recent improvements on this approach?
Has such a scaling approach to the BBGKY-hierarchy been used in other areas of physics, like plasma physics where there is also a power law (Coulomb) interaction?
Thomas Ihle · North Dakota State University

Thank you for your answer. If your truth would be embarrassing for me, it would also embarrass such icons of soviet physics as Bogolubov and Landau, and I'd be totally fine with being part of that axis-of-shame :-) I agree that one has to be careful with ensemble averages when talking about inhomogeneous systems, for example, when describing soliton-like waves in active matter (see my Phys. Rev. E from 2013, and the Discussion on Ohta paper in 2014)). However, I disagree that hierarchy-equations like the BBGKY-hierarchy are useless in general. In your own papers, you also use such hierarchies of equations for multi-point correlation functions. In my opinion, for the ensemble average, one just has to imagine that only  the proper microscopic members that describe a particular inhomogeneous state are part of the ensemble. For example, for a single soliton, only those microscopic states should be  included that have the same density and momentum profile (coarse-grained over small cells in phase space) at the same time. Another argument to not dismiss BBGKY-like hierarchies is to consider the extreme case of a N-particle ensemble distribution that consists of products of delta-functions at time t. That means, all ensemble members have particles at the same positions with the same momenta and the BBGKY-equations would be equivalent to the evolution of a particular real system with those initial conditions. A more pragmatic argument: I recently used the first two BBGKY equations (adopted to my system) for self-propelled particles in a weakly-correlated limit where three-particle correlations are negligible compared to two-particle ones and I found perfect quantitative agreement with particle-based direct simulations. I didn't have to explicitly select the proper members of the ensemble, I did neither use thermodynamics nor Gibbs distributions, I just solved the time-dependent hierarchy equations until  a stationary state was reached. Thus, if you have an alternative theory, I suggest testing it quantitatively with Molecular Dynamics simulations and see how it does. In contrast to experiments, simulations can be adjusted to mirror all the approximations made in a theory and thus allow comparing apples with apples''. In my opinion, the more serious problem with kinetic theory is the closure of the hierarchy. You mentioned in your papers that you somehow close your equations at the two-particle level but I couldn't find any details. For self-propelled particles, there is parameter ranges where the three-particle correlations are stronger than the two-particle ones and the four-particle-ones are even stronger than three-particle ones and so on. Thus, in this strongly-correlated system, some type of generating function should be found that contains correlations to all orders, at least approximately, or a systematic closure for all multi-particle correlations is needed. In your papers, you mention discrepancies between experiments and theory. Could that also be due to neglecting these higher multi-particle correlations or using an inappropriate closure ?

Time freezes at the event of horizon therfore for observer the mass falling into it looks like it doesn't fall, so how could you get this pre and post flare?
At the center of our Milky Way Galaxy, a mere 27,000 light-years away, lies a black hole with 4 million times the mass of the Sun. Fondly known as Sagittarius A* (pronounced A-star), the Milky Way's black hole is fortunately mild-mannered compared to the central black holes in distant active galaxies, much more calmly consuming material around it. From time to time it does flare-up, though.

The flare sequence is illustrated in the panels at the far right. X-rays are generated in material heated to over 100 million degrees Celsius, accelerated to nearly the speed of light as it falls into the Miky Way's central black hole. The main inset X-ray image spans about 100 light-years. In it, the bright white region represents the hottest material closest to the black hole, while the pinkish cloud likely belongs to a nearby supernova remnant.

Credit: NuSTAR project,APOD(NASA)
F. Leyvraz · Universidad Nacional Autónoma de México

The motion of a particle near a black hole is given by the geodesics corresponding to the metric describing the black hole. The motion can be described in various coordinate systems, but it is, of course, always the same motion. This motion has no particular singularity at the horizon, if we are talking of an ordinary Schwarzschild black hole. However distant observers will in fact see strange things as the paticle nears the horizon, but these are simply due to the fact that the coordinate system of the distant observer cannot be used to describe the neighbourhood of the horizon.

Saying that time freezes'' is doubtless easier than attempting to describe in detail what a particle does when it comes near a black hole. But it is not nearly as clear, nor nearly as precise. What is the time we mean? In what sense does it freeze''? I do not deny that such expressions may be used to describe what happens, but it is very easy to be led to misconceptions when abstract concepts are used without reference to their concrete use and their precise definitions....

• Mahak Singh Chauhan added an answer:
How do I check the performance of the Genetic Algorithm in MATLAB ?
As every time GA gives different results but how do I know which one is best ?

Maybe it is possible to check the fitness error but I do not know how to check it

Thanks a lot.
Mahak Singh Chauhan · University of Naples Federico II
What is the possibility of a Carrignton Type event in the recent future ?
I got pulled to the Solar Physics field by interest and now exploring it in its full depth. One major area I have found highly intersting is the Effect of Solar Flares on the technology we possess. In accordance to several anomalies put forward by different research groups, I am curious to know the possibility of a Solar Super storm in the near future. Calling in seasoned researchers to spark a discussion and also come up with ideas to counter attack such an event.
Abhijeet Khandagale · Albedo Informatics (A.I.)
Thanks to all, this discussion has truly beefed up my current understanding. For a young researcher like me, its a gold mine :)
@yuri great work. if that's the frequency, i hope we should be ready in the next 450yrs for such an event. maybe on Mars?
@gary thanks for the link. I had read that. Interesting to note.
What is the effect of microlensing and weak lensing on the stability of the International Celestial Reference Frame?
Spacetime curvature creates a space time "seeing" effect, which is analogous to astronomical seeing. This is due to microlensing and weak lensing, which becomes problematic from an ICRF stability viewpoint as astrometric VLBI and optical measurements (e.g. GAIA) moves into the tens of micro-arcsecond accuracy region. This space time seeing effect will create a noise floor, limiting ICRF stability due to apparent source position distortions and amplitude variations. This will of course have an impact on the high accuracy requirements of the Global Geodetic Observing System (GGOS), which has the objectives of 0.1 mm stability and accuracy of 1 mm for the ITRF. The distribution of dark matter complicates the problem.
Daniele Sasso · Progetto Indipendente ARS
Dear Matts, thank you for your useful clarifications that confirm what I think and wrote in my previous comments. You are right, cosmic muons quoted by me in the previous comment are secondary and derive from other charged primary cosmic particles. The astronomy in the twentieth century considered justly first of all neutral cosmic radiations (photons, neutrinos, etc..). Now we know the astronomy for the future will consider always more charged cosmic rays (leptonic or baryonic) that allow to have always more precise knowledges on the structure of the Universe. Thechnologies and methodologies
for this type of astronomy are today more complex but it represents certainly the way for the future astronomy.
• Marcelo Iván Rojas added an answer:
Why we are interested in Astronomy and Astrophysics? Why we are interested in studying the Cosmos and solving the mysteries of our Universe?
Well... for me, I will tell why i am interested in studying the Cosmos...
"Knowing and understanding the stage on which our life is being played is crucial for any existence to have real meaning."
Astronomy has always been my passion since my childhood. I started looking at the pictures in astronomy books before I could even read. The pictures were beautiful,fascinating and intriguing. As I grew up I wanted to know all about what those pictures meant.I am still learning. Perhaps i like astronomy because we are personally a way of the Universe experiencing itself,or that we are all made of star-stuff,as Carl Sagan so elegantly put it. Astronomy is a science that seeks to explain everything that we observe in the Universe,from the comets and planets in our own solar system to distant galaxies to the echoes of the Big Bang. By studying the cosmos beyond our own planet,we can understand where we came from,where we are going,why is the universe organized as it is?Are we alone? And how physics works under conditions which are impossible to recreate on Earth. In astronomy,the Universe is our laboratory! Astronomy provides an example of an alternative approach to the scientific method- observation,simulation,and theory. It is an enjoyable,inexpensive hobby for millions of people- the naturalists of the night. But above all astronomy has shown us how insignificant the human physical existence is,and how great the human mind with an intelligence that is now embracing a dazzling range of phenomena,from the astronomically large to the infinitely small. I have very little doubt that what astronomy will still give us in the future will dwarf all of its past contributions by comparison. I am motivated by curiosity and a deep desire to understand some of the grandest and most beautiful phenomena in the universe,as well as a desire to share these wonders with others. To me, research is often a bridge to an ambitious goal - a bridge that needs to be crossed in steps..

Astrophysics strike me as one of the most exciting areas of research today. Thanks to new observations,better methods and more powerful simulations that we can now hope to answer,with reasonable confidence,some of the most profound questions ever raised.Astronomy has emerged forever from the old books of mythology to assert itself as an exact science,an unclouded crystal ball of the universe. I would like to be a part of it. Thank You.
Marcelo Iván Rojas · Salieri Software
For me, it is the same curiosity that human kind have since its early existence: explore the environment where we are. Nowadays, we have resources to explore even far away from our home. But, in universal terms, the universe is also our home.
Is there any experimental evidence for how much time a photon takes to flow from the core to the surface of the sun?
I have seen different estimations of photon diffusion time in different papers (30000 to a million years). There are many different mean free paths used. I want to know if these results are just speculations or if there is a way to verify these results?
I also want to know what the current standard time of diffusion is. It would be helpful if you could give me the links to some important research papers on this subject.
G. Bothun · University of Oregon
again

the original and most correct source for this kind of calculation is

http://adsabs.harvard.edu/full/1992ApJ...401..759M (a 2 page paper)
How can one explain existence of acoustic peaks in cosmic microwave background anisotropies?
When delta T/T is plotted against angular separations between points we see several peaks in the plot. The first one is most prominent one is between l value 200-300, or in terms of angular scales at about 1 degrees. How to explain the existence of these
peaks. We know that CMBR anisotropies provide a snapshot of the surface of last scattering. How does acoustic oscillations deform that surface? What are acoustic oscillations in primordial universe?
BTW - I found a very approachable article reviewing cosmological acoustic oscillations at http://dx.doi.org/10.1063/1.2911177 - and http://aether.lbl.gov/bccp/PDFs/BAO.pdf - to be very helpful...
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?

(PhysOrg.com) – “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: http://phys.org/news193330592.html#jCp 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
THE SUN's PULSAR CORE

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).

http://joannenova.com.au/2014/06/big-news-part-iv-a-huge-leap-understanding-the-mysterious-11-year-solar-delay/

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)].

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 (http://arxiv.org/pdf/1402.6594v3.pdf). 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!!
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'.
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 - http://arxiv.org/abs/1212.5317.
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."

http://www.nature.com/news/big-bang-blunder-bursts-the-multiverse-bubble-1.15346
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
All,

remember that the "big bang" really means the instant at which space-time, as a geometry, came into existence.
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.
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.
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
https://www.researchgate.net/publication/221936046_Non-linear_power_spectra_of_dark_and_luminous_matter_in_halo_model_of_structure_formation?ev=prf_pub
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: http://arxiv.org/abs/1205.3175. It also provides other, more recent references. As I understand, the case of galaxy 3C120 accretion is peculiar...
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.
Controversy

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.
Can the Lambda-CDM cosmological model survive the discrepancy between galaxy cluster observations and CMB projections?
See http://www.nature.com/news/cosmic-mismatch-hints-at-the-existence-of-a-sterile-neutrino-1.14752. 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 http://arxiv.org/abs/1308.3255 http://arxiv.org/abs/1402.2301 http://arxiv.org/abs/1402.4119 http://dx.doi.org/10.1103/PhysRevLett.112.051303 and http://dx.doi.org/10.1103/PhysRevLett.112.051302.
Also see http://phys.org/news/2014-02-massive-neutrinos-cosmological-conundrum.html https://www.technology.org/2014/02/25/neutrino-replaces-higgs-boson-sought-particle/ http://news.sciencemag.org/physics/2014/02/x-rays-other-galaxies-could-emanate-particles-dark-matter and http://www.nature.com/news/physics-broaden-the-search-for-dark-matter-1.14795.

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 http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_an_almost_perfect_Universe
"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 http://physics.aps.org/articles/v1/31.

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 http://arxiv.org/abs/1401.7063.

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 http://arxiv.org/abs/1109.2314.
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 (http://www.newscientist.com/article/dn24464-milky-way-galaxy-is-fluttering-like-a-flag.html#.U1m9laIa7pc), 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.
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.
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.
• Chris Vincent Boretzky added an answer:
How much energy hits Mars' surface per second?
55 x 10^6 mi^2 (surface area)/4 (cross section of Mars) x (5,280 ft)^2 x (12)^2 x (2.54 cm)^2 x 117 W/m^-2 (insolation)/(100 cm)^2 = 4.16 x 10^15 Watts.

Does this seem accurate?
Chris Vincent Boretzky · Robert Morris University
Yes i rounded, but you were right when i took the cross section twice. Thank you
Is the cosmic axis due to a large-scale magnetic field?
http://www.sciencedirect.com/science/article/pii/S0370269311003947
http://arxiv.org/abs/astro-ph/0703694
http://www.newscientist.com/article/dn23301-planck-shows-almost-perfect-cosmos--plus-axis-of-evil.html#.UunXEtGYZok
A preference for spiral galaxies in one sector of the sky to be left-handed or right-handed spirals has indicated a parity violating asymmetry in the overall universe and a preferred axis.
Could the large-scale magnetic field be related to the predominant left-handed neutrinos in our cosmic sphere as well?
Guoliang Liu · Independent Researcher
In 1915 Emmy Noether demonstrated that the conservation laws are consequences of properties ("symmetries") of the space and time.
Briefly :
- Homogeneity of space -> Conservation of momentum
- Isotropy of space -> Conservation of angular momentum
- Uniformity of time - > Energy Conservation .
These 3 symmetries are always present in Galileo-Newton space as in Minkowski space.

The General Relativity theory requires the loss of these symmetries to explain gravitation. Only the space isotropy may be possible with spherical symmetry condition (Schwarzschild), but this is a particular case.

Curved space-time in GR is incompatible with - Homogeneity of space - Isotropy of space - Uniformity of time, and will violate the conservation laws associated with those symmetries.
Who can define the G without using Newton's gravitational law?
Any theory giving up the absolute space has to give its own definition first, because the G is defined by Newton's absolute space.
Without a universal flat space-time reference frame (Newton's absolute space), talking about conservation of energy and momentum is nothing more than daydreamer talking nonsense.

http://arxiv.org/pdf/gr-qc/0210005.pdf

Only experimental results can give final judgment to any physics theory, no matter how beautiful it was. Even Einstein said that no matter how many experiments can not prove his theory right, but just need one experiment to prove it wrong.
http://alpha.web.cern.ch/
If the ALPHA experiment in CERN can confirm the gravitational repulsive force between matter and anti-matter, then it will set the last nail on the coffin of the old dogmas, and set the cornerstone for the new theory of gravitation.

CPT and Lorentz Violation along with the emergence of the gravitational repulsive force towards anti-matter seems to be the only way to avoid the singularity in both special and general relativity.
http://cds.cern.ch/record/401916
http://arxiv.org/pdf/gr-qc/0505070.pdf

Logunov - Author Index All archives
http://eprintweb.org/S/authors/All/lo/Logunov
Is Lorentz-invariant gravitation theory a valid alternative to general relativity?
Krogdahl in his critique to general relativity suggests that we should better consider Lorentz-invariant cosmology (see http://arxiv.org/pdf/0711.1145.pdf).

I tried to search on this issue and only find few articles discussing Lorentz invariant gravitation theory, one of them from wikiversity, see http://en.wikiversity.org/wiki/Lorentz-invariant_theory_of_gravitation. Then i can only locate few papers discussing Maxwell-like Lorentz-invariant gravitation theory, one of them is perhaps worth mentioning here that is by Jeffrey Kaplan, David Nichols and Kip Thorne from Caltech. They summarize DSX paper, their paper can be found at http://arxiv.org/pdf/0808.2510.pdf.

So, do you think that Lorentz-invariant gravitation theory is a valid alternative to General relativity theory? Your comments are welcome.
Manuel Morales · Institute of Prephysical Research
"Is Lorentz-invariant gravitation theory a valid alternative to general relativity?"

To speculate valid alternatives there needs to be a way validate them. So 'how' would we validate such a theory?

Prediction is important if you start with "existence" of phenomenon (numerically or physically) in order to try to understand its cause. However, being able to successfully predict gives us a false sense of correctness. There is a HUGE difference between guessing correctly and "being" correct. If science is about the search for objective truths, then "guessing correctly" will never get you there for guessing will always be subjective.

Without Knowledge Of First Cause Can Science Correctly Predict Effects?:
https://www.researchgate.net/post/Without_Knowledge_Of_First_Cause_Can_Science_Correctly_Predict_Effects

I find it curious how we all talk about the importance of "prediction" in science while failing to take stock of the "superstitious" quality of the construct of such a method. So who is fooling who?

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