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Forum » SpaceEngine » Science and Astronomy Discussions » Science and Astronomy Questions
Science and Astronomy Questions
AlekDate: Thursday, 28.07.2016, 23:33 | Message # 631
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Quote FastFourierTransform ()
(even a brown dwarf would have made a huge influence on the solar system this close).


Not necessarily. I've done plenty of simulations of this in Universe Sandbox and a passing star would have to be nearly the same mass as the sun to disturb planetary orbits enough to make the whole system unstable. It changes the orbits slightly, yes, especially the object(s) closest to where the star passes, but usually unless the star gets close enough to the object to completely scorch it, the object's orbit won't get changed enough to unstabilize the whole system, or usually even neighboring planet's orbits.
Also, if a star goes through the solar system by a near-90-degree angle to the ecliptic, even if it passes right over Earth's orbit, it only seems to majorly effect the kuiper belt.
Though it might not seem like it, by no means am I saying it wouldn't effect the solar system enough to cause problems, and that particular equation probably isn't correct because of this, I'm just saying it's quite possible that a star could pass close without completely ruining everything.





Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.

Edited by Alek - Thursday, 28.07.2016, 23:36
 
WatsisnameDate: Friday, 29.07.2016, 02:22 | Message # 632
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Quote FastFourierTransform ()
So we can say that every 2 billion years a star crosses Neptune's orbit (D = 30 AU = 1,446 *10^(-4) Parsecs). But this figure seems quite strange to me. Can someone explain me what am I missing? This means that 2 or 3 stars have come closer to the Sun than Neptune in the past 5 billion years.


Did you forget to square D? I get N=9.5*10-8 per million years, meaning it's very improbable for any star to pass within Neptune's orbit during the lifetime of the solar system. (Expect about one per 10 trillion years).

Added: This is a neat calculation to play with. Another thing we can do is solve for D such that the expectation is one star passing within D over the age of the solar system, giving us:


which sounds plausible.





 
FastFourierTransformDate: Friday, 29.07.2016, 08:35 | Message # 633
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Quote Watsisname ()
I get N=9.5*10-8 per million years


You are totally right! I squared it but my calculator squared the entire equation for some reason.

Quote Watsisname ()
This is a neat calculation to play with

Yes, it is. That 1500AU mean that some star may have been closer to the Sun than planet 9! biggrin
 
WatsisnameDate: Friday, 29.07.2016, 09:41 | Message # 634
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Whew! I'm glad we can confirm that the solar system is safe, at least from very close stellar encounters. biggrin

Quote FastFourierTransform ()
Yes, it is. That 1500AU mean that some star may have been closer to the Sun than planet 9!


Now that is interesting. Alek, what do your US simulations suggest for how likely or severely this might have affected planet 9?





 
JackDoleDate: Friday, 29.07.2016, 15:13 | Message # 635
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And how much will increase the likelihood of a close stellar encounter if one day Milky Way and the Andromeda galaxy collide?




Don't forget to look here.

 
Wicker1MDate: Friday, 29.07.2016, 15:49 | Message # 636
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Could the Higgs field be the fifth fundamental force, or could the Higgs field be categorized as a fundamental force/interaction?

What forces/particles would account for dark matter and dark energy?
 
AlekDate: Friday, 29.07.2016, 17:14 | Message # 637
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Quote Watsisname ()
Now that is interesting. Alek, what do your US simulations suggest for how likely or severely this might have affected planet 9?


I tested it out and the passing star effected Planet 9's orbit, but only "slightly" (If it were closer to the sun and it was effected as much it might be considered a huge difference but because it's so far from the sun, the changes almost aren't noticeable at all besides the eccentricity change, especially if you don't pay too much attention to the orbital elements) It's inclination slightly changed toward being closer to where the star passed, the average Semimajor Axis became smaller by about 40-50 AU because of how long it took for the star to go past the rest of the solar system, and the eccentricity of it's orbit went from 0.59 to 0.75 and the parts indicating the "direction" of the orbit were changed slightly but I can't tell how much, really, because Jupiter makes the sun wobble a lot and that makes the orbital elements for Planet 9 vary a bit...

Quote Wicker1M ()
Could the Higgs field be the fifth fundamental force, or could the Higgs field be categorized as a fundamental force/interaction?

What forces/particles would account for dark matter and dark energy?


The Higgs field is what gives particles mass and in a way is considered a force, since it effects other particles, but it's different than the other four because its at every point in the universe, with no fade with distance, and no "origin point": it's everywhere. The other forces decrease with distance (the strong and weak nuclear forces decrease rapidly at any size greater than an atom, and gravity and electromagnetism are said to extend infinitely but they also get much weaker at high distances, and all of them are caused by a property of the matter particle they "originate from" like how particles exchange photons for the electromagnetic force) whereas the Higgs field is everywhere and has the same strength everywhere)
It's weird to think this way but gravity is caused by the Higgs field because that gives particles mass, which allows gravity to have an effect. If there were no Higgs field particles would have no mass and so would not gravitate toward each other in the slightest.

Dark Matter and Dark Energy have a lot of hypotheses as to what they are but none have truly been proven yet, they may be new types of particles that are just "scared" of the the electromagnetic force aka don't interact with it whatsoever, Dark Matter may just signify that there's more of something in galaxies than we expected, somehow, and Dark Energy may just be some odd form of antimatter in intergalactic space that has negative-gravitational effects, and so pushes galaxies apart...we just don't know.





Living among the stars, I find my way. I grow in strength through knowledge of the space I occupy, until I become the ruler of my own interstellar empire of sorts. Though The world was made for the day, I was made for the night, and thus, the universe itself is within my destiny.
 
WatsisnameDate: Friday, 29.07.2016, 22:01 | Message # 638
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Quote JackDole ()
And how much will increase the likelihood of a close stellar encounter if one day Milky Way and the Andromeda galaxy collide?


Depends on what happens to the Sun's orbit through the merger. If it becomes a highly eccentric orbit that plunges deeply into the galactic center, then the probability increases (but is still so small that the chance of a collision between stars is still pretty much nil). If the Sun ends up being flung off in one of the tidal tails, then the probability decreases.

Quote Wicker1M ()
Could the Higgs field be the fifth fundamental force, or could the Higgs field be categorized as a fundamental force/interaction?


It could be called that, but usually isn't because of a key difference from the other fundamental interactions. This gets somewhat technical, but it has to do with gauge theory. The beautiful thing about gauge interactions is that they arise naturally through any theory which requires their Lagrangian to be invariant under certain transformations (which has to do with their symmetry). That is, under certain reasonable assumptions about symmetries, you can predict that the other interactions must exist. But this is not true for Higgs.

Quote Wicker1M ()
What forces/particles would account for dark matter and dark energy?


Dark matter must be particles with the following properties:
-Massive, obviously. It must provide enough gravitation to explain the universe's "missing mass".
-No charge, or else it would interact with electromagnetism and we would see them.
-Weakly interacting, or else we would have already easily seen them in particle detectors.
-Non-relativistic (move at speeds much less than the speed of light), or else it would not form structures on galactic/cluster scales.

This is like combining certain properties of two particles that we already know about: the neutron and the neutrino. Dark matter particles have the massiveness of the neutron, its lack of charge, and non-relativistic nature, but also have the weakly interacting nature of the neutrino. Neutrons also are unstable outside of atomic nuclei, whereas dark matter must be stable over the lifetime of the universe.

Dark energy isn't so much a particle as a property of space itself. Specifically, under the concordance model (current, most simple accepted model which ascribes "dark energy" to a Cosmological Constant), it's an energy density associated with the vacuum. This is because the energy density is constant even as the universe expands.

Alek, thanks for checking that out! It seems that an expected close encounter would have affected its orbit in a measurable way (if we could measure it), but not so much as to greatly affect the solar system's dynamics. That's pretty neat!

Good discussion as well of Higgs and dark matter/energy, but there needs some corrections. Higgs mechanism is not responsible for giving all particles all their mass. This is a common misunderstanding due to over-simplified press reports. :/

Higgs mechanism only gives mass to the fundamental particles. Composite particles like protons have masses which are much greater than by Higgs alone, because there is mass associated with the way in which the constituent particles are arranged. Most of the mass of our bodies is due to the confined quarks inside of the protons and neutrons. Higgs mechanism is only responsible for about 20 grams out of our whole bodies.

Quote Alek ()
If there were no Higgs field particles would have no mass and so would not gravitate toward each other in the slightest.


This is not true even without considering the above. For example, photons are massless, but they still gravitate. This is because it is not just mass, but also energy and momentum, that gravitate. This comes from the stress-energy-momenta tensor in general relativity.

Quote Alek ()
Dark Energy may just be some odd form of antimatter in intergalactic space that has negative-gravitational effects


This wouldn't work. If dark energy is simply particles, then its energy density is not constant as the universe expands (it will have the wrong equation of state). And remember dark energy comprises about 70% of the mass-energy of the universe. If it is composed of anti-matter, then it would have annihilated huge amounts of regular matter and the universe would look very different.

We have really good models of dark matter and dark energy. We don't yet know exactly what they are physically, but we know what properties they must have and how they behave. smile





 
midtskogenDate: Saturday, 30.07.2016, 11:44 | Message # 639
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Quote Watsisname ()
We have really good models of dark matter and dark energy. We don't yet know exactly what they are physically, but we know what properties they must have and how they behave.

That bothers me. Doesn't dark matter/dark energy have the qualities of an ad hoc hypothesis?





NIL DIFFICILE VOLENTI
 
WatsisnameDate: Saturday, 30.07.2016, 13:14 | Message # 640
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No, this is a common but sloppy understanding of the history of modern cosmology. Because the history of these developments, their motivation, and how we test them, is complex. It's hard to do it justice in the format of a forum post. If you'd like to learn it in more detail, I can recommend books at various levels of technicality -- from popular to university textbook. But what I can say here is this:

Dark energy is the popularized name we give for the cause of the accelerating expansion. It is a real, measurable thing.

We don't know what the physical mechanism behind it is. It is very weird. But we do know that the simplest model that works is the cosmological constant, which comes straight out of mathematics of general relativity. You could say general relativity predicts dark energy, but not its strength (it could be zero, and mid 20th century it was generally assumed to be zero or negligibly small because we did not yet have the high redshift supernova data). You could say the supernova data support the existence of a non-zero cosmological constant. But that's somewhat weak. What's even better is it makes a variety of testable predictions beyond accelerating expansion. For example, it also affects the spatial curvature. It affects the angular size of fluctuations in the CMB. We can measure those. It works. smile

Dark matter could be said to have started out ad hoc, in the sense that it was one explanation for the large scale kinematics that contradict our understanding of gravitation with our knowledge of how much stuff there is. Dark matter says the solution to the contradiction is that there really is more stuff there. Another explanation was that our understanding of gravitation needed to be modified. Maybe the gravitational force law changes and drops off more slowly at large distances. But that hypothesis has not been found to work across all available evidence. The idea that dark matter is a real, additional form of weakly interacting matter in the universe does work. It works with the gravitational lensing data. It works with the formation of the cosmic web. It works with the CMB angular power spectrum. We even see places where dark matter is separated from the bulk of the visible gravitating matter through galactic collisions -- just as we predict ought to happen.





 
WatsisnameDate: Saturday, 30.07.2016, 13:35 | Message # 641
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Here is a model of the CMB angular power spectrum based on dark energy and dark matter (the Λ and CDM in ΛCDM model -- "cosmological constant + Cold Dark Matter"), compared with observational data. The agreement is remarkable. This is not an ad-hoc explanation for the universe. It's a model with incredible predictive success.






 
midtskogenDate: Saturday, 30.07.2016, 16:46 | Message # 642
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Quote Watsisname ()
We don't know what the physical mechanism behind it is. It is very weird. But we do know that the simplest model that works is the cosmological constant,

To be sure you get me right, by "ad hoc" I don't mean it's not to be trusted. It's more about the "cleanliness" of the associated theories. The Wikipedia article actually touches on what you mention:

Quote Wikipedia
An ad hoc hypothesis is not necessarily incorrect; in some cases, a minor change to a theory was all that was necessary. For example, Albert Einstein's addition of the cosmological constant to general relativity in order to allow a static universe was ad hoc. Although he later referred to it as his "greatest blunder", it may correspond to theories of dark energy

That something can be "ad hoc" doesn't mean that it's not reconcilable with mature theories. It's just that the "ad hoc" face of it makes it more difficult to free oneself from the thought that we could be missing something in the theories which would simplify them. Are we seeing dark energy and dark matter because our perspective of the universe isn't optimal?





NIL DIFFICILE VOLENTI
 
HuesudoDate: Saturday, 30.07.2016, 18:31 | Message # 643
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Hello everyone! I have a small question about the info SE gives about black holes.
Tidas force shows a number followed by g/m. What do those stand for? Gravity? Grams? Mass? Meters?
r/rg is the radius from the singularity, i guess? And z is the experienced blueshift
 
WatsisnameDate: Saturday, 30.07.2016, 23:45 | Message # 644
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Quote Huesudo ()
Tidas force shows a number followed by g/m. What do those stand for?


This is units of Earth surface gravities (9.8m/s2) per meter.

r/rg is your distance from the singularity in units of the event horizon radius, or "gravitational radius", 2GM/c2. For a solar mass black hole, rg is just under 3 kilometers.

z is the gravitational redshift relative to an observer infinitely far away. This goes infinite at the event horizon.

midtskogen, I know. smile What I'm showing is that these have long since evolved from ad hoc hypotheses, to become bona fide theories. They have successful predictive power beyond the observations they were originally formulated to explain.

Einstein's original use of a cosmological constant to force a static solution out of the field equations is a good example of an ad hoc hypothesis. It's an adjustment of the model to fit with expectations, without further observational support. Dark matter could originally be said to be in the same vein -- an ad hoc explanation used to rectify the contradiction between observations and understanding of universe's contents. But it and dark energy have progressed well beyond that status, especially in the last few decades.

Quote midtskogen ()
Are we seeing dark energy and dark matter because our perspective of the universe isn't optimal?


I'm not sure how that could be. The universe is homogeneous and isotropic (looks the same on large scales regardless of where you are or what direction you look). With various surveys we have pretty good knowledge of the large scale structure and its evolution, and these are not biased by our point of view. (Our point of view actually does matter in the sense that our motion relative to the CMB causes doppler shifts, but we correct for this.)





 
spacerDate: Sunday, 31.07.2016, 07:48 | Message # 645
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Watsisname, I always wondered, does dark energy and dark matter. are so hard to detect maybe because they dont react to any force exept gravity? and gravity is the weakest force, and we havent even detected graviton yet.




"we began as wanderers, and we are wanderers still"
-carl sagan

-space engine photographer
 
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