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FastFourierTransform
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25 Nov 2017 01:36

Yep, and not only that. All the observations can be explained by an interstellar asteroid with elongated shape, all. So even if dark matter could account for the same set of observations (as you say it probably does not) the idea is still unnecessary. It's occams razor.
In the end jumping from the asteoid idea to the dark matter chunk one you need a leap of faith since no observation pushes you farther and no jump is needed with those observations.
 
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midtskogen
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26 Nov 2017 06:14

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Watsisname
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26 Nov 2017 15:50

An interesting take; I don't think I've encountered models motivated by scale-invariance before.  

The basic framework is in the vein of modified gravity (MOND), where the model is built to explain the observed behavior of galactic rotations and virialized systems by assuming some new property for how gravity works at large scales.  This isn't terribly difficult to do from scratch.  For example a simple transition from gravity behaving as 1/r2 to 1/r at large distances will do the same thing.  The real test is not just how well this works for explaining rotation curves and velocity dispersion of clusters (the physics of which are very related), but whether it goes on to naturally predict additional cosmological observations usually attributed to dark matter and dark energy. 

Maeder argues that this model explains away dark energy by naturally predicting the universe's accelerated expansion history.  Unfortunately the earlier paper devoted to that aspect of it is not free to view, but from the abstract I find it a bit dubious.  His model is built under an assumption that the density parameters (matter density Ωm and his new parameters Ωk and Ωλ) sum to 1 and thus produces a flat universe.  This is a poor way to build a cosmological model.  One should not assume a flat universe to start with, because General Relativity potentially allows any value for the spatial curvature!

For comparison, Lambda-CDM cosmology allows the combination of parameters to sum to any fraction of the critical density, rather than being assumed to sum to 1.  That these parameters are independently found to sum to 1 and thus agree with the observation that the universe is flat is one of Lambda-CDM's predictive strengths.


Summary:  I don't think it's good that his model assumes Ωm + Ωk + Ωλ = 1.  It immediately misses out on a significant potential source of predictive power.  It's good that it agrees with observed rotation curves and velocity dispersions, but it does this by design. Further tests of his model are whether it correctly predicts the gravitational lensing, formation of structure, and the angular power spectrum of the CMB.  It should also be able to naturally explain the Bullet Cluster, which as far as I can tell it doesn't.


Edit:  Found a free-to-view e-print of his earlier paper on scale invariant cosmology here.
 
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midtskogen
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27 Nov 2017 01:02

On the other hand, wont simply assuming a flat universe significantly add to the falsiabilty of the hypothesis?  I'm not able to judge its merits, but Meader states that the list of problems is long, and he's basically calling for help to investigate whether his ideas can lead to a theory.
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Watsisname
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28 Nov 2017 05:14

Not significantly, because observations indicate the universe is very close to flat: the total density parameter is 1.002 +/- 0.005.  So the degree to which his model could be potentially falsified through further observations by this method is less than 1%.
 
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28 Nov 2017 21:15

Not everything that looks like liquid water on Mars is liquid water

Paywalled, but I think the idea is that new lines only form at pretty steep angles and stop quickly, which much better match the behaviour of sand grains of a typical sizes than liquid water.
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Spock1108
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02 Dec 2017 05:22

A really interesting discovery, that it would be nice to be able to see it in space engine ... maybe with a nice accretion disk around both black holes!

https://www.nasa.gov/mission_pages/chandra/news/giant-black-hole-pair-photobombs-andromeda-galaxy.html

Image
 
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Mosfet
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20 Dec 2017 15:10

Direct observation of granulation patterns on [math]1 Gruis thanks to ESO VLT
https://www.eso.org/public/unitedkingdom/news/eso1741/

Image
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FastFourierTransform
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21 Dec 2017 01:50

Mosfet wrote:
Source of the post Direct observation of granulation patterns on π1 Gruis thanks to ESO VLT

This is one of the most awesome images I've ever seen. Just wow. Human beings are incredible creatures.

(Funny how SE was on point with its renderings of supergiants once again)
 
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Mosfet
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21 Dec 2017 05:15

FastFourierTransform wrote:
Source of the post (Funny how SE was on point with its renderings of supergiants once again)

I must say I was impressed too. Just for fun, let's look at this comparison:
Attachments
scr00015.jpg
Pi 1 Gruis as rendered in SE 0980
scr00015b.jpg
Same image, gaussian blur filter applied
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22 Dec 2017 13:23

Japan's space agency released a lot of images from their Akatsuki mission orbiting Venus.

Damia Bouic has used them to create false color IR+UV images of Venus. Given how rarely this planet is well-imaged, these are probably among the best we have so far.
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Websites with Original Calibrated images:
UV1IR1IR2LIR
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17 Jan 2018 16:13

 ESO used Space engine in its official annoucement 
https://www.eso.org/public/usa/news/eso1802/?lang
[img]https://cdn.eso.org/images/screen/eso1802a.jpg[/img]
 
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19 Jan 2018 13:47

The first time a non-feeding stellar-mass black hole has been found! ^

-----------------------------------

There's a new study out on moon formation around gas giants. They find that moon formation happens quickly in a circumplanetary disk, so several generations of moons are formed and fall into the planet before the final generation is kept simply because there's no more circumplanetary material to facilitate inward migration. The mass of the moon system around Jupiter appears to be less massive than the "typical" gas giant moon system, and planetary mass moons seem likely.

More massive moons around gas giants would be great for exobiology!

Satellites Form Fast & Late: a Population Synthesis for the Galilean Moons
Satellites masses distribution.jpg
Satellites masses distribution.jpg (24.97 KiB) Viewed 447 times

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Kepler/K2 has discovered two small transiting planets around the bright (V=9) rapidly rotating F-type star HD 106315. Radial velocity follow-up also finds a third, long-period, more massive planet but it's properties aren't yet well constrained.

Ever since the discovery of hot Jupiters in misaligned orbits (e.g., orbiting their star in polar, retrograde, or anywhere in between kind of orbit), an outstanding theoretical question has been the origin of these systems. The proposed explanations broadly fall into two categories: planet formation is a neat process and stellar equators aren't necessarily always aligned with their protoplanetary disks (having a rotation state similar to Uranus), or stars are aligned with the protoplanetary disks and chaotic planet formation involving scattering of planets into crazy orbits produces the observed population of misaligned planets.

Having a bright multi-planet transiting system with a rapidly rotating host star makes HD 106315 a perfect laboratory for exploring this problem. A co-planar multi-planet system is a hallmark of neat, orderly planet formation without violent scattering. Few such systems have been found around stars bright enough to pursue these sorts of investigations, making HD 106315 rather valuable. If the planets orbiting HD 106315 are misaligned with the stellar equator, it would suggest some stars can start out with equators misaligned with the protoplanetary disk, but ultimately the question will need to be investigated using a large sample of multi-planet systems of the kind the TESS mission (scheduled for launch in March 2018) is likely to find. The rapid stellar rotation makes the projected alignment between the stellar spin axis and planet orbit axis measurable via the Rossiter-McLaughlin effect.

A Multi-Planet System Transiting the V = 9 Rapidly Rotating F-Star HD 106315

Two Small Transiting Planets and a Third Body Orbiting HD 106315
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evildrganymede
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22 Jan 2018 11:48

https://cosmosmagazine.com/space/how-bi ... black-hole

The maximum mass of a non-rotating neutron star seems to be about 2.16 solar masses? (rotating ones can be about 20% higher, if I read that right). This may be a good limiter for neutron stars in SE (I haven't checked what it thinks the max mass of NS are there).

EDIT: I did find a few overweight neutron stars in SE - some around 2.6-2.9 solar masses. With the 20% extra allowance for rotating ones, we should expect the max mass in SE to be about 2.6 solar masses. More massive NS should be converted to black holes.
 
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midtskogen
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26 Jan 2018 07:50

NIL DIFFICILE VOLENTI

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