I had wondered whether the brightness fluctuations of 'KIC 8462852' could be caused by a planetary collision that happened some time ago and the resulting heat radiation would have disappeared.
But then that's probably not possible.

How would the Solar System and the Earth be as a whole if there were 2 stars that orbited every 29 hours or so around eachother, and the system being around the close binary? One being exactly like our sun, and the other being a K5V star. 

If there really is dark energy (the one that accelerates the expansion of the universe) shouldn't there be exotic matter too?
Energy is mass. And I assume that in order to have a mass you also need something like matter.
(The effect of both states (dark energy and exotic matter) is the same: antigravity.)
But apart from that, would the so-called exotic matter really be needed to keep wormholes open?
Wouldn't dark energy also suffice for this?

If there really is dark energy (the one that accelerates the expansion of the universe) shouldn't there be exotic matter too?

We don't really understand what dark energy is, but it's a name we give to whatever causes the accelerating expansion that we observe. So it's like a placeholder name, which apparently has stuck. We say "energy" because whatever it is, it is uniformly distributed and does not dilute with expansion, which makes it act like an energy density associated with space itself rather than a distribution of matter particles. And we say "dark" because we don't detect it directly with electromagnetic observations.

(The effect of both states (dark energy and exotic matter) is the same: antigravity.)

Now we get closer to the meat of your question: is dark energy like an antigravity, similar to exotic matter?

Weirdly, no. Dark energy accelerates the expansion of the universe, but not by being repulsive! Its mass or energy density is positive, so it's not like antigravity. But because it does not dilute with expansion, it generates a tension pulling inward, rather than a pressure pushing outward. That probably still sounds very weird or even backward. You might think that pressure should help increase the expansion rate. But there are no walls, no edge of the universe for pressure to push against, so it doesn't help expand the universe at all. Instead, the pressure modifies the geometry of spacetime according to Einstein's general relativistic field equations, which in turn change the expansion rate, and it works out in the opposite direction of our intuitions. In cosmology, pressure slows expansion down, and tension speeds it up!

Example: in the very early universe when things were extremely hot, all the particles were moving around so fast that the pressure they generated was the most significant form of mass-energy in the universe. We call this the "radiation dominated era". That pressure slowed the expansion rate down even more rapidly than a matter dominated universe would.


To really see why this is and why tension should speed up expansion while pressure would slow it down, we need to go through some mechanics and derive the equations that describe how the expansion rate changes -- those are the Friedmann equations. I've mentioned them before and still intend eventually to get around to writing a post for the cosmology thread to show where these equations come from and help demystify what they mean. They're quite possibly the most important equations in all of cosmology. But for now, I'll just say that it basically has to do with the laws of thermodynamics and the work done in expanding the contents of the universe. I promise that it all makes sense when we work through the details.

But apart from that, would the so-called exotic matter really be needed to keep wormholes open?
Wouldn't dark energy also suffice for this?

Now we're there. "Could dark energy hold wormholes open instead of us needing exotic matter?"

Maybe! The dark energy could support the wormhole throat against collapsing, and there are some papers exploring that idea, but as described above it would work in a different way than the "antigravity" of exotic matter.  With dark energy we utilize the tension or negative pressure it produces. The trick, though, is that dark energy appears to be uniformly distributed in the universe and a property of space itself, whereas we imagine exotic matter (if it exists) as something we could either generate or gather and then distribute it in any way we want. Dark energy might have the right properties, but it's apparently not something that we can use to our benefit. It just... makes the universe expand faster.

I hope that's at least somewhat helpful an explanation -- these are pretty difficult and mind-bending concepts, as is par for the course when cosmology is concerned. I'm sure much may still be unclear, and if so please feel free to ask more questions!

If there really is dark energy (the one that accelerates the expansion of the universe) shouldn't there be exotic matter too?

We don't really understand what dark energy is, but it's a name we give to whatever causes the accelerating expansion that we observe. So it's like a placeholder name, which apparently has stuck. We say "energy" because whatever it is, it is uniformly distributed and does not dilute with expansion, which makes it act like an energy density associated with space itself rather than a distribution of matter particles. And we say "dark" because we don't detect it directly with electromagnetic observations.

(The effect of both states (dark energy and exotic matter) is the same: antigravity.)

Now we get closer to the meat of your question: is dark energy like an antigravity, similar to exotic matter?

Weirdly, no. Dark energy accelerates the expansion of the universe, but not by being repulsive! Its mass or energy density is positive, so it's not like antigravity. But because it does not dilute with expansion, it generates a tension pulling inward, rather than a pressure pushing outward. That probably still sounds very weird or even backward. You might think that pressure should help increase the expansion rate. But there are no walls, no edge of the universe for pressure to push against, so it doesn't help expand the universe at all. Instead, the pressure modifies the geometry of spacetime according to Einstein's general relativistic field equations, which in turn change the expansion rate, and it works out in the opposite direction of our intuitions. In cosmology, pressure slows expansion down, and tension speeds it up!

Example: in the very early universe when things were extremely hot, all the particles were moving around so fast that the pressure they generated was the most significant form of mass-energy in the universe. We call this the "radiation dominated era". That pressure slowed the expansion rate down even more rapidly than a matter dominated universe would.


To really see why this is and why tension should speed up expansion while pressure would slow it down, we need to go through some mechanics and derive the equations that describe how the expansion rate changes -- those are the Friedmann equations. I've mentioned them before and still intend eventually to get around to writing a post for the cosmology thread to show where these equations come from and help demystify what they mean. They're quite possibly the most important equations in all of cosmology. But for now, I'll just say that it basically has to do with the laws of thermodynamics and the work done in expanding the contents of the universe. I promise that it all makes sense when we work through the details.

But apart from that, would the so-called exotic matter really be needed to keep wormholes open?
Wouldn't dark energy also suffice for this?

Now we're there. "Could dark energy hold wormholes open instead of us needing exotic matter?"

Maybe! The dark energy could support the wormhole throat against collapsing, and there are some papers exploring that idea, but as described above it would work in a different way than the "antigravity" of exotic matter.  With dark energy we utilize the tension or negative pressure it produces. The trick, though, is that dark energy appears to be uniformly distributed in the universe and a property of space itself, whereas we imagine exotic matter (if it exists) as something we could either generate or gather and manipulate how it is distributed. Dark energy might have the right properties, but it's apparently not something that we can use to our benefit. It just... makes the universe expand faster.

I hope that's at least somewhat helpful an explanation -- these are pretty difficult and mind-bending concepts, as is par for the course when cosmology is concerned. I'm sure much may still be unclear, and if so please feel free to ask more questions!

Great explanation, Wat, and we can make things even more interesting when we add "Dark Flow" to the equation.  I've always wondered if there could be the gravitic influence of other universes in here somewhere....especially if we dont find other candidates that explain dark matter, dark energy and dark flow.  I know NASA has been looking into this also.  I remember we once discussed if the power of dark energy could somehow be harnessed to create a warp drive, after all, you'd actually be working with the expansion of the universe, which does not have the limitations of the universe itself- like "riding a wave."  

This mentions some emerging tech that could be used for interstellar travel:

https://www.livescience.com/55981-futur ... ravel.html

The top few utilize tech we all know, there are some farther down the list that sound interesting to me like black hole drives that could get us to 0.2c within a few days of travel, then there are the hypothetical methods that utilize an "end around" Relativity to attain hyperluminal velocity.  They dont seem plausible because they violate causality (which, as you know I absolutely HATE, I would like nothing more than to topple it, since there is no scientific law per se that demands its existence.  I would like nothing more than for causality to be observer's bias, where it can be violated but no observer can detect the violation.  Something similar has already been done to *partially* violate the Uncertainty Principle.)  And as you know, Relativity doesn't necessarily say that superluminal travel is impossible, it just places restrictions on it and the existence of exotic matter becomes necessary....but just like with "below absolute zero temperatures" the barrier is more of a wall than an actual barrier.  (And theoretically at least, walls can be jumped over.....)

Actually, the pressure vs tension property makes sense, as I'd expect pressure to slow the universe down (think of denser heavier air with a higher air pressure moving more slowly because of it being heavier) vs increasing the tension on a rope by pulling it from both directions to stretch it out.  Using that analogy it can be seen that higher pressure would slow the universe down while more tension is the result of faster expansion.  In my analogy, the rope model is closer to how I envision this universe to be.

You might find this interesting:

https://www.space.com/making-stable-wormholes.html

I like how they leave an opening for traversable wormholes because relativity breaks down at the quantum level, pending the working out of a theory of quantum gravity, but the opening as of now for non-exotic matter traversable wormholes, only seems to be 30% wider than the planck length.

but the opening as of now for non-exotic matter traversable wormholes, only seems to be 30% wider than the planck length.

This might still be useful, if we can work on it. 30% larger than the plank length is really small and probably isn't useful but if we can find a way to use preexisting wormholes like this and pry them open a bit wider with cleverly used quantum effects we maybe could send light through, maybe even one photon, but that could be enough to do something interesting with. 

but the opening as of now for non-exotic matter traversable wormholes, only seems to be 30% wider than the planck length.

This might still be useful, if we can work on it. 30% larger than the plank length is really small and probably isn't useful but if we can find a way to use preexisting wormholes like this and pry them open a bit wider with cleverly used quantum effects we maybe could send light through, maybe even one photon, but that could be enough to do something interesting with. 

I like that they mentioned this is all pending a workable theory of quantum gravity, because at that level where relativity breaks down, even a small "opening" may have huge implications if quantum gravity can take over and make possible an entirely new kind of physics that we cant even comprehend right now.

Why do some meteors look green?

Green is often associated with evaporated nickel.  But it's complex.  Most of the light you see from meteors is not the meteoroid itself glowing, but the surrounding air glowing from the immense pressure caused by the meteoroid.  So speed is a major factor for what colour you see, possibly also altitude.  And certainly altitude above the horizon.  Near the horizon meteors become red, just like the setting sun.
Perseids, however, look blue to me,  but I've never seen Perseids in proper darkness.  It's still a bit early for night sky observation over here.

Ah that makes sense....I noticed something strange with this shower that I hadn't noticed in others.  Most of the time, I watch meteor showers at a sea level location on Long Island.  But this time, I decided to do it at a less light polluted place, in the Poconos Mtns, about 2,000 ft above sea level.  I didn't see a lot of meteors, but the ones I did see were very bright, as bright as Venus or even a little brighter.  I saw about 10 of these per hour the night before the peak and the night of the peak.  Here is what was really unusual- they seemed really low, maybe only a couple of hundred feet above the trees (?)  That is just an estimate.  But two of them I know for sure really seemed like they were only a little above the roof of my house.....and one of them changed color from golden to green to orange to red and I heard a sound at the end of the flash- it sounded like- as best I can describe it- like the sound of a pebble hitting a hard surface?  I have never seen or heard of anything like this before!  A bolide?
Meanwhile, in the background sky, just at the limit of visibility there was a near constant flashing and a sensation of the sky moving (?)- I suspect this was the vast majority of meteors, just at the limit of naked eye visibility?  Whatever it was it was below the brightness of the Milky Way, which I can clearly see from this location (limiting magnitude about 6.5)

Here is what was really unusual- they seemed really low, maybe only a couple of hundred feet above the trees

Your eyes deceive you. You were seeing them at more like a couple hundred thousand feet.

People often misjudge how far away meteors are, thinking they are "just over those trees", or must have landed "just over that hill". But no. Meteors burn up several times higher than airplanes fly. Especially Perseids, which are typically small and come in so fast that they burn up very quickly in the upper atmosphere.

Larger meteors that reach lower altitudes or drop fragments on the ground (also usually of asteroid origin unrelated to a meteor shower) are also not visible so close to the surface. They slow down significantly and enter "dark flight", no longer draped by glowing plasma.  And if you *do* see a meteor reach just 100s of meters altitude and is still burning bright, then you're probably about to be killed by it and the impact will make global news.

Watsisname, can you tell me how to convert the values given in this table into values for 'AscNodePreces' (or 'ArgOfPeriPreces')?  (Values for these parameters must be given in years.)

People often misjudge how far away meteors are, thinking they are "just over those trees", or must have landed "just over that hill". But no. Meteors burn up several times higher than airplanes fly. Especially Perseids, which are typically small and come in so fast that they burn up very quickly in the upper atmosphere.

Very true.  I receive a lot of reports about meteors, and very frequently people insist that they saw it land just behind that small hill a few hundred meters away, or even just behind the neighbour's house.  In those cases I can safely know that in reality they fell several 100 km away, since they saw it very low in the sky.  If it really falls nearby, it's always visible very high in the sky.  If it's heading for you, it will appear fairly high in the sky and travel slowly and straight up.  In fact, I recorded just such a meteor a couple of weeks ago.  This is what it looks like:

Meteor heading straight towards you

Since it was also recorded from other directions, the bright path is known, and even the dark path could be modelled based on this and wind data.  This was a fairly small object with an entry mass of a few kg most likely and an entry speed of about 15 km/s (dropping to just a couple at the end of the bright path at nearly 30 km altitude), so not many fragments have survived, most or all probably smaller than 100g, but the model shows them landing 3 to 6 km from here.  The debris field pretty extends pretty much straight over my house.

JackDole, it looks like in that table, δϕ column is the apsidal precession rate in arcminutes per orbit, while Ω'_LT is the precession rate of the nodes (arcseconds per year), due to the effect of black hole spin. Now I'm not completely sure what the value being given in years means in SE, exactly, but my best guess based on how other parameters involving rotations were defined is that it means the number of years for one radian of precession. So I'll show how the conversions work using that assumption, but if it's something else then it should be easy to modify.

For ArgOfPeriPreces, divide the the value in the δϕ column by 60 (to convert from arcminutes to degrees), then multiply by pi and divide by 180 (to convert from degrees to radians), and finally divide by the orbital period in years. (Edit: and then invert). That should give you the number of years for 1 radian of apsidal precession.

For AscNodePreces, divide the value in the Ω'_LT column by 3600 (converting from arcseconds to degrees), then multiply by pi and divide by 180 (converting degrees to radians), and then take the reciprocal to get the number of years for 1 radian of nodal precession. As an example, for the first entry 'S62', you should get 40444 years.

I hope that's enough to go on, and if it looks like the results you get from this are wildly off from what they should be, then I am probably mistaken about what SE's value of years means and we'll adjust as necessary.

Thank you, I'll try it out. In any case, the value for S62 seems more likely to me than the value I calculated myself for 'AscNodePreces'.

For ArgOfPeriPreces, divide the the value in the δϕ column by 60 (to convert from arcminutes to degrees), then multiply by pi and divide by 180 (to convert from degrees to radians), and finally divide by the orbital period in years.  That should give you the number of years for 1 radian of apsidal precession.

I suspect that with 'ArgOfPeriPreces' I also have to take the reciprocal value.
Then it seems to work. Otherwise not.
https://imgur.com/FMrZPFK.mp4

I suspect that with 'ArgOfPeriPreces' I also have to take the reciprocal value.

Oops, yes, that's correct.