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13 Oct 2017 00:42

Watsisname wrote:
Mr. Missed Her wrote:
Source of the post Since we're talking about dark energy, I've got to ask: What if we're just in a less dense region of space bigger than the observable universe? The more dense universe around our patch would be pulling everything away from our observable region, and it looks like the universe is expanding.

A-L-E-X wrote:
Source of the post Dark flow, which is what you were alluding to, is something that could be accounted for something outside our observable universe pulling the universe outward (perhaps another universe?

Counter-intuitively this would have zero effect on our volume of space.  This is because the gravitational field everywhere inside a hollow sphere, or spherical shell, is equal to zero. The only way to get a uniform accelerating expansion of the observable volume is for the volume to be uniformly filled with something that has this property of dark energy.

JackDole wrote:
Source of the post I was actually of the opinion that vacuum energy and dark energy are not quite the same.

Yeah, they're not quite the same, but share similar features as an energy density associated with the vacuum.  So there is this tantalizing idea that they should be related, but applying concepts of quantum field theory directly to try to predict dark energy's magnitude leads to the infamously wrong prediction by orders of magnitude.

That makes sense because space is nearly a vacuum.  Do we have any idea what dark flow is- is it just another property of dark energy?  Also, the tantalizing piece that links vacuum energy and dark energy together is Einstein's (in)famous cosmological constant.  It's unusual value has some thinking that it hints at an oscillating universe that runs down a bit more with each cycle (and thus the value of the CC gets slightly lower with each iteration.)

Is that new discovery the asteroid that missed earth by just 25,000 miles (one tenth the distance to the moon!)  That sure was a narrow escape and we just discovered it!
 
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13 Oct 2017 05:23

CPU: AMD FX-8350 8 core processor 4GHz / GPU: GeForce GT 730 @ 1920x1080, 60Hz with 1GB adapter RAM / RAM: Patriot Signature 4GB 1600MHz 240-Pin DDR3 (only 2GB work, don't buy it) / Motherboard: MSI 970 Gaming MS-7693
 
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13 Oct 2017 10:16

Wow, I take it this isn't in SE yet? I expect Jack to remedy that situation rather quickly!

Also might want to make some room for this

https://www.space.com/38431-new-evidenc ... tence.html
 
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14 Oct 2017 01:54

Gnargenox wrote:
Source of the post Six planet system at HD34445!

A-L-E-X wrote:
Source of the post I expect Jack to remedy that situation rather quickly!

They are only candidates. They are not confirmed yet. And I can not find any more data.
But here is a preliminary script.
scr00020.png

HD_34445.sc
(3.02 KiB) Downloaded 4 times
(Put this in 'addons\catalogs\planets')

The already existing 'Planet b' is probably the same as mine, but the orbit of the existing planet overlaps with other orbits because of the high eccentricity. That's why I disabled it and replaced it.
scr00021.png


A-L-E-X wrote:
Source of the post Also might want to make some room for this

Planet Nine is here.
 
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14 Oct 2017 08:16

Looks great Jack! I'll need to learn how to do Space Engine scripts soon. This will be a great chance to do that. Here's a screen shot from Celestia.
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14 Oct 2017 11:19

This was an interesting read on Einstein, John Nash and his last days, but in Norwegian.  Google translated, sort of conveying the original story.

EDIT: the paper mentioned.
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14 Oct 2017 17:36

Haumea has rings? Amazing. My day has just gotten better. Way better. 

I've made an addon for it.

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14 Oct 2017 19:12

midtskogen wrote:
This was an interesting read on Einstein, John Nash and his last days, but in Norwegian.  Google translated, sort of conveying the original story.

EDIT: the paper mentioned.

Einstein was trying to come up with a theory of everything from what I recall, but in his time only gravity and EM were known.  Interestingly he came to the same conclusion that those later did- you need to add more dimensions to make unification work.

BTW awesome work, Jack!
 
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15 Oct 2017 01:08

I am not in a position to judge whether Nash or others with similar ideas are on to something.  But I do find "dark energy" problematic from a methodology viewpoint. It has the traits of an ad hoc hypothesis which doesn't really add much.  It's mainly serves to save our current understanding of the universe from falsification. Unless we assume that this understanding is correct, how is "dark energy" falsifiable?  I'm leaning towards the position that we're missing something important.  Dark energy might merely be the shadow of a structure of the universe that we have not yet seen.
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15 Oct 2017 02:05

midtskogen wrote:
Source of the post  It's mainly serves to save our current understanding of the universe from falsification.

I tend to agree with this postion.
And it could be easily said also for dark matter; It's starting to look mostly like the luminiferous ether debate. Now we have again a very massive substance that magically can't interact with anything (if not gravitationally), it's transparent, doesn't emmit light, it doesn't generate friction, it's ... etherial. Maybe there's something about that idea that atract us.
Like in the case of dark energy we are trying to assume there is mass somewhere (just because we have observed certain gravitational effects: in galactic rotation, in the bending of light rays etc...) because in our current knowledge of physics it's just matter that can bend space-time. As I said, maybe also dark matter as matter is an idea that tries to save our current understanding against the posibility that our description of gravity is not yet complete.
 
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15 Oct 2017 07:01

Such critique of dark matter and dark energy as being ad hoc or unfalsifiable is quite common.  It is also based on significant misconceptions.


Dark energy is a name we give to the observed accelerating expansion of the universe (measured from supernovae data).  When the cause of this acceleration is modeled as a cosmological constant (a term which appears in the field equations of general relativity), that model makes a set of predictions that go beyond the supernovae data it was originally formulated to explain.


1)  Dark energy as a cosmological constant cannot produce any arbitrary curve for expansion over time.  Put another way, any arbitrary expansion history (even an accelerating one) generally cannot be fit by a combination of matter and dark energy.  The reason is because the relative density of matter and dark energy must change in a specific way as the universe expands.  Matter density drops with the scale factor cubed.  Dark energy as a cosmological constant... stays constant.  

So for any combination of cosmological parameters (matter density, dark energy density, Hubble Constant, etc), there is a specific solution for the expansion history as given by the Friedmann equations, and this in turn has observable consequences.  One of those consequences is the angular size of fluctuations in the Cosmic Microwave Background, which is measured with WMAP and Planck data.  When the frequency of these fluctuations is plotted against their angular size to produce an "angular power spectrum", the shape of that spectrum depends on the cosmological parameters:


Image

We see that a flat universe (Ωtot = 1) with dark energy ~70% of the critical density (ΩΛ ~0.7), correctly fits the observed angular power spectrum.  This is remarkable because the initial motivation for proposing dark energy was to explain accelerating expansion, not a CMB angular power spectrum.

2)  The amount of dark energy necessary to fit the expansion history should be consistent with knowledge of the universe's spatial curvature.  Because the curvature is measured to be nearly flat, the dark energy density and matter density should sum to nearly the critical density.  In fact, they do!  The constraints of three different datasets yield a best value of dark energy as ~0.7 of the critical density, and matter as ~0.3 of the critical density, which sums to ~1 to yield a flat universe:

Image


3)  A further test of dark energy as a cosmological constant is the constancy of it.  If it instead varies over time, then this would be modeled by having a different "equation of state".  In cosmology we write that with a letter w, where a Cosmological Constant must have w = -1.  If we measure any departure of w from -1, then that indicates that dark energy is something dynamic, not constant.  This is what the data show:


Image


So not only is dark energy necessary, but the cosmological constant also continues to be the best model for it.


FastFourierTransform wrote:
Source of the post Like in the case of dark energy we are trying to assume there is mass somewhere (just because we have observed certain gravitational effects: in galactic rotation, in the bending of light rays etc...) because in our current knowledge of physics it's just matter that can bend space-time.

Energy and momentum also bend space-time!  This is described in general relativity by the stress-energy-momenta tensor.  So dark energy bends space-time, too.  Radiation in the form of light does as well (since photons have momentum), and this was an important effect in the early universe.

For dark matter, galactic rotation curves and gravitational lensing of light passing through clusters are two of the original motivating arguments for its existence.  Now we know that dark matter also explains cosmic web formation, the separation of luminous and nonluminous matter in the Bullet Cluster, and the observed density being close to the critical density. (The same 25% dark matter needed to explain the other observations is also the same 25% needed to reach flatness when summed with baryon density and dark energy density).  So the evidence for dark matter is considerable.


And it could be easily said also for dark matter; It's starting to look mostly like the luminiferous ether debate.


The luminiferous aether made predictions too.  If the aether exists, then the speed of light should be variable since the velocity of Earth would change with respect to the aether.  Or, if the Earth drags the aether with it as some proposed, then we would not observe stellar aberration.  Since aberration of light is a real thing, and interferometry experiments reveal no variation in the speed of light, we can unambiguously reject the luminiferous aether in favor of relativity.  

The aether was a hypothesis with a good theoretical motivation, which happened to be falsified by observations.  Dark matter and dark energy were motivated by observations, and through continued predictive successes have graduated into the concordance model of cosmology.
 
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15 Oct 2017 11:38

Maybe criticism of such criticism is based on misconceptions? :)

Does anyone really categorically deny that dark energy can be described as an ad hoc hypothesis?  Even if we agree that it's ad hoc because there is still much we don't understand, that doesn't mean that it's wrong.  The question is rather whether we should shut the other doors or not.

Consider the discovery of Neptune.  It was discovered that there were irregularities in Uranus' orbit.  So were Newton's laws wrong?  Naturally, one is reluctant to accept that the current knowledge of the universe is flawed.  So it was proposed that there was an undiscovered planet beyond Neptune.  Calculations were made that could explain Uranus' orbit well assuming a new planet.  And indeed a new planet was just a Moon's diameter or so away from the predicted position.  But let's imagine that no matter how hard people were looking for the planet, none was found.  Would scientists declare Newton wrong?  Probably not.  Perhaps it would have been postulated that there was a planet, but entirely made up of an unknown transparent type of matter.  And people would insist that invisible matter is a fact using the irregular orbit of Uranus as strong proof.  But let's say in this hypothetical scenario that much later, when relativity had been discovered, that instead of a transparent planet there was a tiny black hole, which previously had been difficult to imagine and impossible to observe.

Then consider Vulcan or vulcanoids, the bodies that could explain the irregularities of the orbit of Mercury.  Like the irregularities of Uranus.  But eventually it was of course discovered that the laws of Newton were indeed wrong or at least incomplete.

I've described three possibilities of what started out as an ad hoc hypothesis:
1) The understanding of the universe was correct, and the discovery of transparent matter was universally accepted based on the observational evidence.
2) The understanding of the universe was (adequately) correct, but the original explanation was incorrect and replaced after further advances in technology and understanding
3) The understanding of the universe was incomplete, and the proposed explanation was merely an illusion.

The jury may still be out on which of the above possibilities is most like the case of dark energy.  And perhaps we may never find out for sure.  What if dark energy is just that shadow of an undiscovered structure of the universe and the laws of physics themselves forbid us to turn our heads and look directly?
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15 Oct 2017 14:36

midtskogen wrote:
Source of the post Does anyone really categorically deny that dark energy can be described as an ad hoc hypothesis?  Even if we agree that it's ad hoc because there is still much we don't understand, that doesn't mean that it's wrong.  The question is rather whether we should shut the other doors or not.

You claimed it is ad hoc (which it is not), and you asked if it is unfalsifiable, or if it serves as avoiding the falsification of the application of the field equations to cosmology (which it also is not).  Could a different model turn out to work better for explaining the accelerating expansion? Sure! Different models and different variations of the parameters of this model are tested all the time by many different researchers.  I happen to be one of them because my measurements of mass profiles vs. velocity dispersion in galactic clusters also act as observational constraints on dark matter, and by extension the Lambda-CDM model parameters.  My work is not specifically meant to study dark matter or dark energy, and does not give the strongest of constraints, but it does serve to help the process, and it also means that I am fairly well qualified to speak about it.

A hypothesis is ad hoc if it is motivated purely by fitting to data, with no further justification for those fits through producing new predictions.

Putting epicycles to orbits of objects around the Earth is an ad hoc adjustment of the geocentric model.  Einstein originally adding a cosmological constant to the field equations was an ad hoc forcing of a static solution.

The status of dark energy is now not an ad hoc hypothesis, nor is it unfalsifiable.  It is a component of a model of cosmology that makes predictions beyond what it was originally formulated to explain, and as I have shown, those predictions have been and continue to be tested.

If, at the time accelerating expansion was measured from supernovae data, there was not already a model for the behavior of the universe in general relativity with a cosmological constant, then dark energy would have been completely ad hoc.  An explanation made purely after the fact to explain the data.  But because the mathematics was already there, and behaviors of universes with different cosmological parameters including a cosmological constant worked out in the 60s and 70s, the observation of accelerating expansion acted as a test of the cosmological constant.

Why?  Because the accelerating expansion could easily have worked out to follow a history which is inconsistent with general relativity with a cosmological constant.  But it does work and thus is a predictive success.  A very unexpected one that made a lot of researchers scratch their heads! :)  This is Einstein being such a genius that even his biggest mistakes turn out correct.

Now we have further tests of the cosmological constant.  It affects the spatial curvature of the universe, the evolution of structure (run models of a Lambda-CDM universe to compare the how the evolution of the cosmic web in the model with what we observe in nature), and the fluctuations in the CMB.  We test all of those, and we find that it still works.

So this is not ad hoc model tweaking.  It is as good of an example of the scientific method at work as any.


midtskogen wrote:
Source of the post What if dark energy is just that shadow of an undiscovered structure of the universe and the laws of physics themselves forbid us to turn our heads and look directly?

Plato's cave again. :)  The lesson of Plato's Cave is that science cannot, and does not, determine what the true nature of reality is.  What is a particle really?  What is a magnetic field really?  I have no idea.  Science does not answer. 

An observer watching shadows shimmering on the walls of the cave has no way to know what agent is casting them.  But he can study them and attempt to build a model for how they behave.  A good model will not only predict the original observations, but also predict new ones.  This is the caveman scientist's power: he knows not what the agent is, but he can obtain a predictive power of how the agent operates.

I have no idea what the physical cause of accelerating expansion really is.  I only know that we observe that it happens, that the manner in which it happens is consistent with the field equations of general relativity with a nonzero cosmological constant, and that this model successfully predicts things besides just accelerating expansion.  
 
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15 Oct 2017 22:21

Watsisname wrote:
Source of the post A hypothesis is ad hoc if it is motivated purely by fitting to data, with no further justification for those fits through producing new predictions.

Putting epicycles to orbits of objects around the Earth is an ad hoc adjustment of the geocentric model.

But the epicycles did more than purely fitting to data.  It also preserved a persistent (and irrelevant) insight of the universe since antiquity: the circle and its divinity.

Such hypotheses appear much more ad hoc after they've been replaced than before while people are still overwhelmed by their agreement with observations, explicative force and agreement with existing ideas.  It's clear now that epicycles were ad hoc and couldn't actually be falsified (but I regard that as distinct properties of hypotheses), as one could add epicycles forever and always match the observations (proven mathematically much later, though).
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15 Oct 2017 23:07

Indeed.  The motivation for proposing epicycles was to explain variable and retrograde apparent motion of the planets in a way that preserves the notion that things move around the Earth in heavenly circles.  What predictive power did epicycles have which help justify the proposition?  

I don't know of any.  In principle, any new observations can be handwaved away by adding more complex epicycles.  There's no independent justification for them.


The motivation for proposing a nonzero cosmological constant to explain the cause of accelerated expansion is that it agrees with the data.  What predictive power does this have which helps justify the proposition? 

1)  Angular power spectrum of the CMB.
2)  The formation of the cosmic web vs. how it works in a universe without a cosmological constant.
3)  The value of cosmological constant needed to explain expansion history is consistent with the value needed to yield a critical density flat universe.

This is why these two things are of a totally different character.  Lambda-CDM follows through the scientific method and is a testable, falsifiable model.  Epicycles was stuck at validation/falsification, and failed to be a model.


There is a further difference which is the strength of the starting assumptions.  For epicycles, what independent evidence was there for the notion that heavenly motions must be circles?  None.  Only an affinity for circles.

For the cosmological constant, what evidence is there for the notion that general relativity is useful to study the universe?  For one thing, general relativity is exceedingly well verified by experiments.  Second, the large scale universe is observed to be homogenous and isotropic, so we can apply general relativity to yield the Friedmann equations.

It might be that general relativity does not correctly describe the universe's expansion, and thus is why we have this weirdness with dark energy and dark matter.  It could be our understanding of how gravity applies on that scale is wrong.  Many people are working that angle as well.  But if that is the case, then this consistency across different lines of observational tests of the LCDM model is very surprising.

Finally, ask yourself what you would think if it turns out that cosmologists abandon LCDM for some reason and turn to modified gravitation.  What would distinguish modified gravity from an ad hoc hypothesis?  Same answer!  It would be ad hoc as far as it is an adjustment of our understanding of gravity to fit observations.  What would turn it into a bona fide model is if it makes new falsifiable predictions for cosmology, and better yet, since it modifies GR, experimental tests for those same modifications that can be done in the lab.

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