Wikipedia:Reference desk/Archives/Science/2022 January 30

= January 30 =

Dark matter and quasars
One of the original reasons for the postulate that most of the matter in the universe is dark matter is that in the solar system, the inner planets like Mercury have a higher velocity than the outer planets like Neptune, however, the stars in galaxies do not exhibit the same phenomenon - the velocity of the outer stars is roughly the same as the inner stars. This article deals with the phenomena, https://en.wikipedia.org/wiki/Galaxy_rotation_curve, and it mentions a halo-effect, however, I don't think it specifically considers the effect of mass ejected by quasars. Quasars often eject matter perpendicularly to the galactic plane. If the matter from quasar jets were ejected in opposite directions perpendicularly to the galactic plane a significant portion of the radius of the galaxy, it could have a significantly greater inward pull on stars in the outer edges of the galaxy than it would on stars close to the centre of the galaxy because the forces on the inner-stars would mostly cancel-out (one component of the net force exerted on stars by the ejected matter would be the sine of the angle between the centre of mass of the ejected matter and the axis of rotation of the galaxy, which could easily more than offset the inverse square gravitational law).

The thing that I'm not sure about is whether or not over billions of years, the total amount of matter and energy ejected by quasars would be enough to have a dramatic effect on the relative speed of stars close to and far from the galactic centre.

The image in this article - https://cosmosmagazine.com/space/quasar-jets-are-thousands-of-light-years-long/ - based on an article published in the journal Nature, reveals the hidden truly massive extent of some quasar jet emissions, and I think it's worth re-examining the galactic orbital velocity & dark matter thesis in light of it.

MathewMunro (talk) 08:12, 29 January 2022 (UTC)
 * (Re-formatted to remove absurdly long heading. Bazza (talk) 09:59, 29 January 2022 (UTC))


 * The matter in the jets of an active galactic nucleus is ejected by the magnetic field in the inner part of the accretion disk. Whatever that matter is (it's just ordinary atoms), it must be able to interact with other matter electromagnetically. We also know that we can't see the matter that explains the fast rotation of galaxies (that's why it's called dark matter), so it can't interact electromagnetically, or only very weakly. If that dark matter were gas, we would have seen it with our radio telescopes. So that unknown matter cannot be the gas ejected by a quasar. Anyway, the gas wouldn't stick around for billions of years. If it pulls on the matter in the disk to make it rotate faster, the matter in the disk would pull back (Newton's third law), causing the matter to fall back into the disk. So whatever this dark matter is, it must be massive, gravitationally bound to the galaxy and able to fall through the disk without interacting in any way other than by gravity. Gas doesn't work. PiusImpavidus (talk) 10:15, 29 January 2022 (UTC)


 * Firstly, thanks for your reply. I'm not aware of what our radio telescopes can and can't see, but wouldn't the gas "go dark" after millions of years, after it radiates its heat? And in regard to it not sticking around for billions of years, true, but it also wouldn't just stop once it falls to the galactic plane - it would orbit in ellipses that gradually went from being very long in the perpendicular direction to being very much shorter in the perpendicular direction and longer in the galactic plane direction, I think. Also, given that it takes 230 million years for our solar system to orbit the galaxy, it could receive successive boosts (relative to the inner stars) from long-since faded quasar emissions. I just don't know if the amount of the effect would be enough to account for observations. MathewMunro (talk) 10:43, 29 January 2022 (UTC)


 * Space isn't in thermal equilibrium. The gas in the galactic halo is still bathing in the light of all those stars, which has a low intensity, but a fairly high energy. A very low density hydrogen and helium gas is a pretty poor radiator of infrared, but still pretty good at absorbing the ultraviolet light from those stars, so it doesn't get really cold.
 * Any cloud launched from the centre of the galaxy will be on an orbit still intersecting that centre, and if we were to put a lot of gas clouds on wide orbits around the galaxy, the shell theorem tells us they wouldn't affect gravity inside. So those gas clouds are on orbits (if indeed they are bound to the galaxy) intersecting the disk of the galaxy. But this is very low density gas. The density of the gas in the halo is certainly not much higher (in fact, much lower) than the density of the interstellar gas in the disk. That means that your gas cloud cannot pass through the disk; instead, it will collide. Halo stars can pass through, similar to how a bullet can travel for kilometres through air, but a cloud of smoke can't. PiusImpavidus (talk) 12:58, 29 January 2022 (UTC)


 * The thing is though, AGN jets & accumulated plumes aren't exactly "shell" shaped, and by the look of it, I'd say that the distribution of matter likely provides greater acceleration to the outer stars (see my crude mathematical logic for that in reply to Lambiam below). For an example of AGN plumes (visible in x-rays & gamma-rays), see https://cdn.shortpixel.ai/spai/q_lossy+ret_img/https://cosmosmagazine.com/wp-content/uploads/2020/06/200618-Centaurus-1.jpg (the image from the article linked above). MathewMunro (talk) 03:05, 30 January 2022 (UTC)


 * The vast majority of galaxies does not have an active galactic nucleus. Yet, for example, the Triangulum Galaxy, aka Messier 33, has a galaxy rotation curve that cannot be explained by its visible matter. The observed curve even shows an increase in speed as the distance from the centre increases. Matter ejected from the centre perpendicularly to the galactic plane should remain near the axis of rotation. Even if all gravitational matter is concentrated in a very long cylinder coinciding with the axis, the speed should still decrease with the distance. --Lambiam 13:09, 29 January 2022 (UTC)


 * 'Even if all gravitational matter is concentrated in a very long cylinder coinciding with the axis, the speed should still decrease with the distance.' - I'm not convinced of that. Rigorously disproving it is well beyond my capability, however, I note that the matter in the central column that's perpendicular to the galactic disk may spend more time far away from the galactic core, as it's initially travelling extremely fast, and presumably when it eventually falls back in and crosses the galactic plane, it would likely be travelling at a tremendous speed once more, whereas when it's far away, it's going slower, and if it just so happens that the average distance from the galactic centre is something like two times the galactic radius, then obviously stars very close to the galactic nucleus would experience lower net inward pull than stars further out. Even comparing a star on the edge of the galaxy to one halfway between the galactic centre and the edge, you can see that the one on the edge would get greater acceleration under those assumptions - setting the galactic radius to 1: sin(tan^-1(1/2))/(1^2+2^2) > sin(tan^-1(0.5/2))/(0.5^2+2^2). Granted that's a very sketchy argument with some questionable assumptions, but I think it demonstrates that it's at least possible for matter ejected in quasar jets to accelerate outer stars more than inner ones.


 * As for M33, a possible explanation is in the Wiki article: 'M33 is linked to M31 by several streams of neutral hydrogen and stars' - that may have an effect like an AGN plume of providing greater orbital acceleration to outer than inner stars? MathewMunro (talk) 19:14, 29 January 2022 (UTC)


 * These "streams" are for the larger part tenuous wisps of gas. Since they stretch out over the 750,000 light years the two galaxies are apart, they mostly exert hardly any gravitational influence. For a galaxy with an active nucleus that ejects a considerable jet of matter, it would give a much more spectacular sight than we see now if that jet did not reach escape velocity but fell back to the centre with the supermassive black hole powering the active nucleus. A centripetal force increasing with distance can only be expected if most of the inner matter is far distant from the galactic plane. --Lambiam 11:42, 30 January 2022 (UTC)


 * There just isn't enough mass in those jets. I've never really been involved with AGNs, so I don't have the numbers in my head, but the supermassive black hole powering an AGN has a mass that's only a small fraction of that of the host galaxy. Even though they are quite messy eaters, they can't put many times their own mass into a jet within just a few gigayears. Even worse as the jets don't stay in place. BTW, the jets are perpendicular to the accretion disk of the central black hole, but there's no tight correlation with the orientation of the disk of the galaxy. And we know from cosmology that only a small part of the mass that we don't see must be in the form of ordinary matter. PiusImpavidus (talk) 12:58, 30 January 2022 (UTC)


 * I accept that AGN plumes probably don't single-handedly solve the riddle of why the outer stars don't orbit much slower than the inner stars, however, at the least, quasar plumes would decrease the amount of dark matter that we have to come up with to account for the orbital speed of outer-galactic stars. I get the impression that previous calculations have presumed a spherical shape for the matter outside the galactic plane, but it's likely concentrated in a central column which would have a greater relative impact on the velocity of outer vs inner stars.


 * Also, regarding M33's galaxy rotation curve, in addition to the aforementioned matter between M33 & M31, there's also M31 itself, which would provide orbital acceleration the outer stars of M31 more than its inner stars, just like quasar plumes would, albeit not symmetrically - a component of the force from M31 on the outer-galactic stars of M33 would pull them inward toward the centre of M33 just like water-skiers who were attached to a single point on a boat, except that because they're orbiting, the effect would be to increase their orbital velocity. I expect that if the pull of other galaxies is taken into account, the mystery of the relatively high velocity of outer-galactic stars would be a little closer to being solved. I also wonder if the expansion of the universe itself might create a similar effect. This article shows just how close M31 & M33 are, how much material lies between them, how large their bulges or plumes are, and the orientation of each galaxy in three different pairs of axis: https://academic.oup.com/mnras/article/493/4/5636/5721546 MathewMunro (talk) 13:56, 30 January 2022 (UTC)


 * The rotation curves of galaxies are close to flat, that is, the orbital velocity doesn't change with distance from the centre. The centripetal acceleration must therefore be proportional to the inverse of the distance to the centre, using a Newtonian approximation. There are multiple mass distributions that can lead to this effect. First, all mass is concentrated in a long rod along the axis of rotation and all mass is spread evenly along this rod. Second, the mass is in a spherically symmetric distribution with a density falling back as 1/r2. Third, a combination of these two. So, indeed, your rod could explain this, but AGNs don't produce such rods.
 * And now that I think about it again, those jets are pretty hot. To get in a pressure equilibrium with the surrounding gas, they must have a lower density, so we may be dealing with a rod of effectively negative mass. But as I stated above, this mass is pretty low and the rod isn't properly aligned in the first place. PiusImpavidus (talk) 11:24, 31 January 2022 (UTC)


 * You're correct that the flat rotation curve doesn't fully determine the spatial distribution of mass. Here's an article that uses tidal streams of stars around the Milky Way to further constrain the shape: . This supports an almost spherical halo shape. In a model where most of the mass is distributed linearly along the galaxy's axis, you should see quite a different orbit for an inclined tidal stream. (In particular, I'd expect halo stars in this case to follow a helical "corkscrew" orbit unlike the orbits we observe, which are highly elliptical and pass close to the galactic center; and I do not expect your axial mass distribution would support disk formation.) --Amble (talk) 21:40, 31 January 2022 (UTC)


 * I just thought of yet another potential cause of the higher relative velocities of outer vs inner stars in galaxies but not planets in solar systems: spiral arms. And even when stars aren't clumped in spiral arms, I think they'd get a boost from passing inner stars perhaps hundreds of times in their orbit of the galaxy, whereas outer planets might get a boost from inner planets once or twice per orbit. In a solar system, nearly all the mass is concentrated in the centre, but it's not like that with galaxies.
 * Having thought about it, I'm now less convinced than ever of the existence of WIMPs or some other form of exotic matter. There are numerous other potential causes of fast-moving outer galactic stars:
 * 1. They get a tow from passing inner-stars;
 * 2. There's a significant amount of matter outside the galactic centre, especially outside the galactic disk, including as a result of past galactic collisions, and possibly most effective of all per unit of mass - axial concentrations of matter a significant distance from the galactic centre as if often produced by quasar jets;
 * 3. Every nearby galaxy that's not in the same plane as the Milky Way's disk is pulling the outer stars of our galaxy toward the centre of our galaxy, increasing their velocity;
 * 4. If you Google 'expansion of the universe galactic rotation curve', you will see that a tone of papers have been written speculating that the expansion of the universe may be a factor in galactic rotation curves. And it makes sense to me - the expansion of the universe is equivalent to a gravitational attractive force in all directions. The force coming from the directions in the plane of the galactic disk would have no effect on the relative velocity of inner & outer galactic stars, but in every other direction, it would accelerate the outer stars' galactic orbital speed more than the inner stars, because at every point that it's pulling from, it is pulling on opposite sides of the galaxy from a different angle - pulling the opposite sides inwards.
 * So what's left in support of exotic Dark Matter? Convoluted arguments about the blotchiness/power-spectrum of the CMB? MathewMunro (talk) 17:33, 1 February 2022 (UTC)


 * I'm closing this thread down. Per the instructions on this page "We don't answer requests for opinions, predictions or debate." It is clear from their repeated posts on this thread that MathewMunro is primarily interested with debating established physics.  They have been told repeatedly why their hypotheses are just wrong, but are uninterested in learning what the established physics is.  You don't know more than the physicists do.  You don't change established science by engaging in a debate with randos on a message board.  Just no.  -- Jayron 32 18:01, 1 February 2022 (UTC)

How did NASA know that JWST reach L2 point before deploying its high gain antenna?
JWST reached L2 points then after few hours, it deployed its high gain antenna.

But NASA released statement about reaching L2 point even before deployment of high gain antenna. How did NASA without using its antenna? Rizosome (talk) 01:38, 30 January 2022 (UTC)


 * Physics. Sagittarian Milky Way (talk) 02:19, 30 January 2022 (UTC)
 * Very useful answer, thanks for that . Jthistle38 (talk) 10:54, 30 January 2022 (UTC)


 * If they knew its initial velocity, they could calculate its trajectory & time of arrival, at least approximately. If you want the formulas they used to make that calculation, you will have to wait for a physicist to reply :) MathewMunro (talk) 03:20, 30 January 2022 (UTC)


 * The high gain antenna is used for scientific data. The spacecraft also has a medium gain antenna for telemetry, commands, and ranging.  It has been in contact since even before it separated from the Ariane upper stage. 85.76.76.8 (talk) 05:26, 30 January 2022 (UTC)


 * The statement that JWST "deployed its high gain antenna", instead of "deployed its antenna", implies that there's at least one other antenna. And indeed, it's common for space probes to have at least two antennas. Why? Well, a high gain antenna sends a radio signal in a narrow beam, a low gain antenna in a wide beam. The narrower beam means that a larger fraction of the beam hits the receiving antenna. Both antennas transmit at a similar power, limited by the solar panel of the space probe, so when the probe uses its high gain antenna, the receiver receives more power. More power means that it has to listen for a shorter period of time before the signal gets clear above the noise, so it needs less time before it can see if the signal represents a zero or a one. In short, a high gain antenna provides a higher bitrate, which is required for science operations, but not really for just steering the probe to the L2 point.
 * The high gain antenna also has disadvantages and for that reason a low gain antenna is also provided. Because the high gain antenna has such a narrow beam, it must be pointed accurately at the receiver, which may be hard when the space probe is manoeuvring or its control systems are malfunctioning. The high gain antenna is also large, so it has to be folded somehow to make it fit in the launch vehicle, so it cannot be used right after launch. The Galileo probe famously failed to deploy its high gain antenna, but thanks to its low gain antenna (and a lot of time and upgrades on the receiving end) still managed to send a decent amount of science data to Earth. PiusImpavidus (talk) 13:27, 30 January 2022 (UTC)

I got the answer from this line: The spacecraft also has a medium gain antenna for telemetry, commands, and ranging. It has been in contact since even before it separated from the Ariane upper stage. Rizosome (talk) 01:19, 31 January 2022 (UTC)