Try UTube "Astronomy Notes" and use the rotation curves from there. The results, if you do as I suggest, are startling.
You may argue that the time dilation varies only imperceptibly at these distances from the galactic central mass, but don't forget the vast distances involved. The slope of the curve is tiny, but on galactic scales it becomes significant in relation to our observations of rotation speeds. This is a better fit to science than the invention of CDM which has impossible properties.
Continued;- The only radius at which there is no temporal distortion of velocities is the Solar system radius. Newton's curve MUST then pass through the observed velocity at Earth radius since the observed speed here is real or "proper" and must equal Newtonian or Kepler predictions. This means the equivalent central mass is much larger than currently believed since we have raised the velocity curve. These "proper" Keplarian speeds appear slowed inboard, and faster outboard when viewd from Earth
t = ROOT(1 - 2GM/rcSQUARED) (Sorry about the math type) This is the curve of time dilation against radius "r". Imagine this curve from the galactic centre out to the rim. (Yes I know this curve will be disrupted due to the uneven distribution of stars in the spiral arms, but let's look at the principle before the detail). If time is faster further out than at the solar system, velocities will look faster as viewed from Earth. If time is slower further in, velocities will look slower from Earth?
is it possible that the stars/matter in a galaxy are not actually moving but that the space in which they are is rotating due to maybe a black hole. wouldnt that give a good reason why the stars further out move as quick as the stars near to the centre ?
No, adding more mass to the center does not reproduce the observed dependence of orbital velocity on radius. If you are talking about frame dragging, that happens only very near the black hole.
@drdwittman Hi, just had a quick question. Lets assume we are looking at a galaxy which has about 90% dm. What would be the distribution of dark matter throughout the galaxy. Can we assume the dm mass is evenly distributed?
@givemeblowjob69 No, we don't need to assume anything. We use the rotation curve to tell us the distribution of mass (all types of mass). We can see the distribution of mass in stars, gas, dust, etc, so the remainder must be dark matter. We can also use the vertical (in/out of plane) motions of stars in the disk to tell us the mass in the disk. Result: most of the mass is not concentrated in the disk like the stars, gas, and dust are. The DM must therefore be a roughly spherical halo.
@drdwittman Ok thanks. one more question. What is the dm mass estimated in the spherical halo compared to the centre of the galaxy or the galactic bulge (If its a spiral galaxy)
@givemeblowjob69 The DM mass is a lot more. The DM halo extends out much further than the bulge (further even than the stars in the disk, judging by the rotation curves of gas out there) and so has a huge volume in which to accumulate mass, even if its density is not incredibly high.
@drdwittman Thanks for the response. So the overall dm mass is greater than the galactic centre mass. But the dm mass is more widespread so less dense. Otherwise the stars would orbit the dark matter region, is that correct?
@givemeblowjob69 The DM is centered on the same place that the stars are centered on. If they were not, they would fall into each other until the were concentric...and we would notice the disturbance on the stars' orbits.
Well that depends on the exact modification to the laws of gravity, I was not really referring to MOND. To me the energy mass equivalence is a reminder of how close to the top we live. Mass energy is really potential energy, the energy that would be available if the mass was allowed to fall to it's SR radius. So in the reverse case you could create any amount of real mass by lifting some small amount of mass out of a gravitational field, and the mass increase will turn out to be linear.
This used to be a viable interpretation of the evidence: the law of gravity, rather than the distribution of mass, is what needs to be modified. However, when a modification was invented to explain galaxy rotation curves, it failed to work in other contexts where we invoke dark matter, such as cores of galaxy clusters. And now the Bullet Cluster provides direct nongravitational evidence.
Invisible mass may be the simplest solution, but consideration should be given to alternative explanations. Newtons force law may not hold for systems as big as a Galaxy. I suspect that a potential energy mass gain factor may need to be applied. Incidentally if you measure the radius in KPC then the factor 1, 2, 3, 4, ... etc. applied to gm/r^2 will do the trick. This tells us that the mass increases linearly from the radius 2GM/c^2.
When estimating the velocities of stars in galaxy have they (besides Doppler effect) also taken into account the gravitational reshift caused by the mass of galaxy to the light escaping from stars? The total redshift should be propotional to 1/sqrt(r) - 1/r, which decays much slowly than just 1/sqrt(r). Or is the effect of gravitaional redshift so tiny that it cannot be observed at all while measuring the spectra of similar stars at different distancies from galactic center?
Let's look at why things would or would not orbit in a plane. If you start out with a big cloud of gas that has enough mass to start collapsing gravitationally, the disk configuration is the smallest it can collapse to while conserving angular momentum. But to get there, the particles have to have lots of interactions that let them settle into those flat disk orbits. Unlike gas, dark matter does not interact much and therefore can't do that.
so the distant stars (from the center of the gallaxy) has the speed the same like a closer star?
I know that the evidence for a superblack hole is that stars have a high speed at the center of the gallaxy and lowest at the edge! (i hope you will understand me sorry for such a bad english ;) )
Good question. We have to be more specific about what "close" and "distant" mean. Evidence for a supermassive black hole (SMBH) at the center of our galaxy comes from the high velocities of stars VERY close (<0.01 pc) to the center. Evidence for a diffuse halo of dark matter comes from the flat rotation curve seen in the video, from about 1000 pc to as far out as we can see. So we have both, but it's not easy to see both at the same time on a single plot of rotation speed vs. radius.
Try UTube "Astronomy Notes" and use the rotation curves from there. The results, if you do as I suggest, are startling.
You may argue that the time dilation varies only imperceptibly at these distances from the galactic central mass, but don't forget the vast distances involved. The slope of the curve is tiny, but on galactic scales it becomes significant in relation to our observations of rotation speeds. This is a better fit to science than the invention of CDM which has impossible properties.
MrKENHUGHES 3 months ago
Continued;- The only radius at which there is no temporal distortion of velocities is the Solar system radius. Newton's curve MUST then pass through the observed velocity at Earth radius since the observed speed here is real or "proper" and must equal Newtonian or Kepler predictions. This means the equivalent central mass is much larger than currently believed since we have raised the velocity curve. These "proper" Keplarian speeds appear slowed inboard, and faster outboard when viewd from Earth
MrKENHUGHES 3 months ago
t = ROOT(1 - 2GM/rcSQUARED) (Sorry about the math type) This is the curve of time dilation against radius "r". Imagine this curve from the galactic centre out to the rim. (Yes I know this curve will be disrupted due to the uneven distribution of stars in the spiral arms, but let's look at the principle before the detail). If time is faster further out than at the solar system, velocities will look faster as viewed from Earth. If time is slower further in, velocities will look slower from Earth?
MrKENHUGHES 3 months ago
is it possible that the stars/matter in a galaxy are not actually moving but that the space in which they are is rotating due to maybe a black hole. wouldnt that give a good reason why the stars further out move as quick as the stars near to the centre ?
MrAndymasters 2 years ago
No, adding more mass to the center does not reproduce the observed dependence of orbital velocity on radius. If you are talking about frame dragging, that happens only very near the black hole.
drdwittman 2 years ago
@drdwittman Hi, just had a quick question. Lets assume we are looking at a galaxy which has about 90% dm. What would be the distribution of dark matter throughout the galaxy. Can we assume the dm mass is evenly distributed?
givemeblowjob69 4 months ago
@givemeblowjob69 No, we don't need to assume anything. We use the rotation curve to tell us the distribution of mass (all types of mass). We can see the distribution of mass in stars, gas, dust, etc, so the remainder must be dark matter. We can also use the vertical (in/out of plane) motions of stars in the disk to tell us the mass in the disk. Result: most of the mass is not concentrated in the disk like the stars, gas, and dust are. The DM must therefore be a roughly spherical halo.
drdwittman 4 months ago
@drdwittman Ok thanks. one more question. What is the dm mass estimated in the spherical halo compared to the centre of the galaxy or the galactic bulge (If its a spiral galaxy)
givemeblowjob69 4 months ago
@givemeblowjob69 The DM mass is a lot more. The DM halo extends out much further than the bulge (further even than the stars in the disk, judging by the rotation curves of gas out there) and so has a huge volume in which to accumulate mass, even if its density is not incredibly high.
drdwittman 4 months ago
@drdwittman Thanks for the response. So the overall dm mass is greater than the galactic centre mass. But the dm mass is more widespread so less dense. Otherwise the stars would orbit the dark matter region, is that correct?
givemeblowjob69 4 months ago
@givemeblowjob69 The DM is centered on the same place that the stars are centered on. If they were not, they would fall into each other until the were concentric...and we would notice the disturbance on the stars' orbits.
drdwittman 4 months ago
Well that depends on the exact modification to the laws of gravity, I was not really referring to MOND. To me the energy mass equivalence is a reminder of how close to the top we live. Mass energy is really potential energy, the energy that would be available if the mass was allowed to fall to it's SR radius. So in the reverse case you could create any amount of real mass by lifting some small amount of mass out of a gravitational field, and the mass increase will turn out to be linear.
beeresearch 2 years ago
Well, no one to date has made a modification of gravity that convincingly explains all the evidence for dark matter.
drdwittman 2 years ago
This used to be a viable interpretation of the evidence: the law of gravity, rather than the distribution of mass, is what needs to be modified. However, when a modification was invented to explain galaxy rotation curves, it failed to work in other contexts where we invoke dark matter, such as cores of galaxy clusters. And now the Bullet Cluster provides direct nongravitational evidence.
drdwittman 2 years ago
Good video..
Invisible mass may be the simplest solution, but consideration should be given to alternative explanations. Newtons force law may not hold for systems as big as a Galaxy. I suspect that a potential energy mass gain factor may need to be applied. Incidentally if you measure the radius in KPC then the factor 1, 2, 3, 4, ... etc. applied to gm/r^2 will do the trick. This tells us that the mass increases linearly from the radius 2GM/c^2.
beeresearch 2 years ago
Great video. Very concise & informal.
zzt32 2 years ago
When estimating the velocities of stars in galaxy have they (besides Doppler effect) also taken into account the gravitational reshift caused by the mass of galaxy to the light escaping from stars? The total redshift should be propotional to 1/sqrt(r) - 1/r, which decays much slowly than just 1/sqrt(r). Or is the effect of gravitaional redshift so tiny that it cannot be observed at all while measuring the spectra of similar stars at different distancies from galactic center?
takuoinas 3 years ago
Good question. It's too tiny.
drdwittman 3 years ago
Paraphrasing a question from a student: why does luminous matter orbit in a disk, but dark matter orbits in more of a spherical halo?
drdwittman 3 years ago
Let's look at why things would or would not orbit in a plane. If you start out with a big cloud of gas that has enough mass to start collapsing gravitationally, the disk configuration is the smallest it can collapse to while conserving angular momentum. But to get there, the particles have to have lots of interactions that let them settle into those flat disk orbits. Unlike gas, dark matter does not interact much and therefore can't do that.
drdwittman 3 years ago
so the distant stars (from the center of the gallaxy) has the speed the same like a closer star?
I know that the evidence for a superblack hole is that stars have a high speed at the center of the gallaxy and lowest at the edge! (i hope you will understand me sorry for such a bad english ;) )
Arghira 3 years ago
Good question. We have to be more specific about what "close" and "distant" mean. Evidence for a supermassive black hole (SMBH) at the center of our galaxy comes from the high velocities of stars VERY close (<0.01 pc) to the center. Evidence for a diffuse halo of dark matter comes from the flat rotation curve seen in the video, from about 1000 pc to as far out as we can see. So we have both, but it's not easy to see both at the same time on a single plot of rotation speed vs. radius.
drdwittman 3 years ago