[saspinski (OP)]

If an astronaut can see celestial bodies nearby, it is easy to know if he (she) is rotating or not, as shown in a scene of the film "Gravity". If there is no way to see outside, one way I can imagine is holding some object through a rope. If there is some rotation the rope will show some tension, while winding. That means: it would go away if released.

In this case, I suppose the gravitational field generated by the astronaut is negligible. If the case of a fantastic massive astronaut, it would be necessary to compensate for the gravitational field. If I could feel the object touch me after some time, without tension in the rope, I could conclude that the my gravitational field was strong enough to hold the object. But I would be not sure if was  rotating or not. On the other hand, if the rope kept tensioned, I could conclude that I was rotating, and the object would escape if I loose the grip.
 
If the star velocities at several distances from the center of a spiral galaxy are greater than expected for a steady state, based on its mass density, couldn't that galaxies be expanding? That means, they would come from huge rotating bodies, from which parts were being detached above the escape velocity. Wouldn't be a big coincidence if the spinning velocities were just the necessary to hold the system in equilibrium, not expanding or colapsing?

It is true that it is exactly the case of our solar system, but maybe we are extrapolating that stability to the spiral galaxies, and they are not as a rule, in a steady state.

[PmbPhy]

If the star velocities at several distances from the center of a spiral galaxy are greater than expected for a steady state, based on its mass density, couldn't that galaxies be expanding?

If your assumption is true then the conclusion is true. But the problem with that is that your assumption is not true, at least not with all the stars in a galaxy. A galaxy is a galaxy because the stars within it are orbiting at fixed distances.

With spiral galaxies I'd imagine all of those stars would either have left the galaxy, died or been combined with other stars.

That means, they would come from huge rotating bodies, ...

I don't understand. Who or what are the "they" that you're referring to and why do you conclude that?  What are these huge rotating bodies that you're referring to?

[evan_au]

If the star velocities at several distances from the center of a spiral galaxy are greater than expected for a steady state, based on its mass density, couldn't that galaxies be expanding?

Stars in a galaxy follow elliptical orbits around their central black hole, until they pass close enough to another star (or ex-star) to disturb them into a slightly different elliptical orbit around the central black hole. In an elliptical orbit, the star spends half of its time moving closer to the center, and half its time moving away (and hardly moving radially in-between). So at any instant in time, you would expect to see about half of the stars in a galaxy moving inwards, and half moving outwards. It is the average motion which is measured by galaxy rotation curves.

Depending on the eccentricity of the star's orbit, it may be almost circular (constant radius) or quite elliptical (such as stars cannibalised from a nearby dwarf galaxy). It is thought that stars which formed in a stable spiral galaxy would be in fairly circular orbits. On the other hand, elliptical galaxies formed from the merger of two or more spiral galaxies would have stars in highly elliptical orbits in all orientations, depending on which galaxy they originally came from. In either case, you expect to see about half moving inwards and half moving outwards, at any instant in time.

This post seems to be suggesting that all stars in a galaxy are all seen to be moving outward at the same time (ie the galaxy is expanding). This implies that at a later time, all stars in this galaxy would start to simultaneously move inwards, on the other leg of their elliptical orbits.

That all stars in a galaxy seem to be moving outwards at the same time seems highly unlikely - it would require something dramatic like the galaxy's central black hole suddenly losing a large part of its mass. This hypothesis can be disproven by looking at galaxy rotation curves, which show that as you look closer towards the central rotation axis of the galaxy, the radial speed drops to zero. ie the galaxy is not expanding.

This new theory seems to be suggesting that all stars in every galaxy, at many different distances, as seen in many different epochs, are all seen to be moving outward at the instant that we on Earth happen to be observing them. This seems highly unlikely. Even if all stars in a galaxy could simultaneously move outwards, another observer, at another point in space would see different galaxies at different phases of their expansion/contraction cycle.

If the star velocities at several distances from the center of a spiral galaxy are greater than expected for a steady state...

This seems to be referring to one of the lines of evidence for Dark Matter. So the post seems to be looking for an alternative to Dark Matter?

But there are numerous other lines of evidence which seems consistent with Dark Matter. It's not proof that Dark Matter exists, but it is suggestive...
https://en.wikipedia.org/wiki/Dark_matter#Galaxy_rotation_curves

I am afraid that this New Theory cannot explain away Dark Matter. We don't know exactly what Dark Matter is, but it is not expanding galaxies!

[puppypower]

A related question, connected to both galaxies and dark matter, is related to the four forces of nature. When a force lower potential, it gives off energy. As an example, as an electron lower EM force in an atom, it gives off a quanta of energy. If gravity is a force, what type of energy quanta is given off when gravity lowers potential?

In the case of the election, the photon that is given off can cause the EM force to increase elsewhere. It can excite an electron in another atom. In terms of gravity, if energy was given off due to the lowering of gravitational potential this should be reflected, in the opposite happening nearby or elsewhere. This is observed in all rotations, where matter gains kinetic energy and an opposing centrifugal force is created with a vector opposite the center of gravity. It has an antigravity affect in terms of math.

The next question is, is dark energy simply the energy output stemming from lowering gravitational force potential? It is not easy to find gravitons and gravity waves. These are theorized to exist, but are hidden in the lab, just like dark energy. We infer both from affect. I showed one affect.

If you use this theory; there is an energy output from the lowering of gravitational force potential, an accelerated expansion of the universe would simply be caused by more and more mass compacting as galaxies, stars and black holes. More and more will accelerate the output of energy for an accelerating antigravity affect; expansion. The question is, is there more  compacted mass today than yesterday? (present versa past)

The fact that galaxies are not expanding, even though new stars are forming, suggests the excess energy output from gravity is leaving the galaxy and becoming part of a larger expansion energy economy; dark energy.

This idea came to me due to gravity and rotations. If we started with a cloud of dust in space and it starts to compact via gravity, the theory say energy will be given off, albeit, not easy to see. The impact is almost always a rotation which is a form of acceleration that counters gravity.

If this rotation was to go too fast, it would overcome gravity, which rarely happens. The rotation is not leading the energy balance. This means the engine has to be gravity. There is excess energy for bulk universe antigravity affects like expansion. This just so happens to be relative to the main centers of mass compression; galaxies. There is your dark energy. The proof of this theory would be doing a universal star or mass density count, as function of time, too see if this parallel the expansion. It comes down to an energy balance.

[saspinski (OP)]

As far as I know the velocities are measured through doppler effect. From this method we know the velocities, that is, the first derivative of the position, but not the second derivative. As the time scales involved are much larger than our life span, we donīt measure rotations as we do in our solar system. For example, if our sun is a good example of star velocity, it takes 250 millions of years to orbit the milky way. Its distance from the center is 26000 light years. That means: our assumption that it is rotating and not going in a straight line for example, is a extrapolation of an angle of 2,5x10^-6 rad (supposing 100 years of observations) to 2π rad.

So my doubt is: do we have any evidence from observations that the stars are really orbiting the center of their galaxies?

[Janus]

Wouldn't be a big coincidence if the spinning velocities were just the necessary to hold the system in equilibrium, not expanding or colapsing?

It would be even a greater coincidence that we would be seeing all these expanding galaxies at the same point of their expansion.   As we look further and further out  into the universe we are seeing it as it was during earlier and earlier ages. If the galaxies were expanding over time, we should be seeing more and more "compact" galaxies as we look deeper into space.   Also, given the magnitude of these measured star velocities,  If they weren't stable, they would dissipate in a few hundred million to a billion years.  Since stars need relatively dense regions in which to form, Stars would only form while the galaxy was in its compact form.  But out own Sun is some 4+ billion years old already.  Ergo, if our galaxy were expanding as you suggest, our galaxy would dissipated by now and we would not see ourselves as being in the midst of a relatively compact structure like we presently do.  Put another way, the lifetime of a galaxy as a fairly compact object would be much, much, shorter than the lifetime of the stars it is made of.

[Janus]

As far as I know the velocities are measured through doppler effect. From this method we know the velocities, that is, the first derivative of the position, but not the second derivative. As the time scales involved are much larger than our life span, we donīt measure rotations as we do in our solar system. For example, if our sun is a good example of star velocity, it takes 250 millions of years to orbit the milky way. Its distance from the center is 26000 light years. That means: our assumption that it is rotating and not going in a straight line for example, is a extrapolation of an angle of 2,5x10^-6 rad (supposing 100 years of observations) to 2π rad.

So my doubt is: do we have any evidence from observations that the stars are really orbiting the center of their galaxies?

Consider the following image:   The green circle represents an object traveling in a circular orbit at the correct speed for orbiting a given mass.    The red curve is the trajectory followed by an object that at the moment it is tangent to the circular orbit has a greater speed than that needed to maintain that circular orbit around that same mass.  This trajectory follows a hyperbola.  It sweeps in from a further distance, briefly grazes the circular orbit and then move back out into space.   It our galaxy were to be a confluence of such objects, it would be the wildest of coincidences that they would all be crossing the same small volume of space at the same time while we are around to see it.

So basically it comes down to this: All our observations lead to the conclusion that galaxies must be long-lived structures.   However, given the amount of visible mass of these structures, the measured velocities of the stars making them up should make them short-lived ones.   This leads to the suggestion that there is more mass holding these structures together than we see. Not only that, but the particular pattern of stellar velocities indicates that the distribution of this mass differs from the distribution of the visible mass of the galaxies involved.

trajectories.gif (5.11 kB . 720x480 - viewed 2634 times)

[jeffreyH]

The extra 'mass' could be due to all the photons emitted during acceleration of the combined matter of the galaxy. This could produce the halo required.

[Janus]

The extra 'mass' could be due to all the photons emitted during acceleration of the combined matter of the galaxy. This could produce the halo required.

Let's see if that is even a reasonable suggestion:
Taking the rotation curve of galaxy M33, the actual measured velocity of stars 15,000 light years our from the center is ~100 km/sec. The predicted velocity based on the visible matter is ~60 km/sec.  The extra mass required to make up the difference is ~1.8 times that of the visible mass.  The amount of energy that amounts to if converted to photons is enormous! Not only would these galaxies have to be radiating at rates magnitudes greater than what we measure,  That much energy being extracted from accelerating matter would damp out the motion of that matter in short order. (There is a reason bremsstrahlung means "braking radiation".)   The galaxies would quite quickly collapse into SMBs as the energy for this emission would have to come at the expanse of  the rotational energy of the Galaxy. In addition, The total mass equivalence for the rotational energy of a galaxy falls well short of that needed to explain the extra mass.

So, no, this is not a reasonable explanation for the missing mass.

[saspinski (OP)]

 

But out own Sun is some 4+ billion years old already

It is really a good point. If stars like the sun were "migrants" from other galaxies, being only temporary in the milky way describing hyperbolas, they should be older than they are. Taking 50 times the diameter of a galaxy as a standard distance between them, it would take 7 billions of years for it come here, keeping its present velocity. On the other hand, if they were born here at the milky way, but not in orbit, in less than one billion of years it would be out.