How fast galaxies are moving?

[Bill Phil]

Absolutely everything you're saying is self contradictory. You accept that there is no absolute coordinate system but then say to pick a point that's not moving, but not relative to anything. All you're doing is picking a coordinate system where the average speed of galaxies is 200km/s.

No, you're just bending what I have said.

I said that for any random point, where you are few megaparsecs (it was in an edit, earlier) away from a galactic cluster, the average speed of the galaxies moving away from you is 200 km/s.

The point, by virtue of being a point, is stationary. I didn't pick a coordinate system. This is just what happens.

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So it actually is a point in spacetime.

But with only a point you cannot measure speed. What you actually need is a reference frame (i.e. a point moving through spacetime) to define speed.

That's why it's not a point in space time. It's a point in space. And time is moving along. ( for the sake of just getting this over with, let's just say it's a point in space, and time is moving). This point is random, but then if that point is a few megaparsecs from the nearest galactic clusters, then their average velocity is 200km/s away from you.

[BlueCosmology]

No, you're just bending what I have said.

I said that for any random point, where you are few megaparsecs (it was in an edit, earlier) away from a galactic cluster, the average speed of the galaxies moving away from you is 200 km/s.

The point, by virtue of being a point, is stationary. I didn't pick a coordinate system. This is just what happens.

That makes no sense. Again, you accept that there is no such thing as an absolute coordinate system, then you pick a point that is stationary absolutely. You can not "pick a point" without picking a coordinate system, as you entirely agree by accepting that there is no absolute coordinate system. Nor can you pick one that is absolutely stationary, again as you accept.

[ZetaX]

The point, by virtue of being a point, is stationary. I didn't pick a coordinate system. This is just what happens.

No. Just no.

While a singular point may as an abstract concept exist, it is irrelevant here. What BlueCosmology and I tried to tell you is that you actually need a reference frame for all this, no exceptions.

You are probably just choosing the one of the galaxy you are in, but are for some weird reason denying that.

[Bill Phil]

That makes no sense. Again, you accept that there is no such thing as an absolute coordinate system, then you pick a point that is stationary absolutely. You can not "pick a point" without picking a coordinate system, as you entirely agree by accepting that there is no absolute coordinate system. Nor can you pick one that is absolutely stationary, again as you accept.

I clearly said earlier that it's relative to whichever point you choose as the origin.

[BlueCosmology]

I clearly said earlier that it's relative to whichever point you choose as the origin.

You also just said you didn't pick a coordinate system, hence you have not chosen an origin.

[Bill Phil]

No. Just no.

While a singular point may as an abstract concept exist, it is irrelevant here. What BlueCosmology and I tried to tell you is that you actually need a reference frame for all this, no exceptions.

You are probably just choosing the one of the galaxy you are in, but are for some weird reason denying that.

This point, is just a point. The reference frame doesn't matter. It's just a location. Like if you teleported a few meters in front of a bus you wouldn't care about reference frames, you would care about the speed of the bus.

I never said there wasn't a reference frame, but for this it simply doesn't matter.

[theend3r]

This point, is just a point. The reference frame doesn't matter. It's just a location. Like if you teleported a few meters in front of a bus you wouldn't care about reference frames, you would care about the speed of the bus.

I never said there wasn't a reference frame, but for this it simply doesn't matter.

No. In this case your position is near the bus and your speed it the speed of the ground below you. You need to specify both the location and the speed. Actually the location shouldn't matter, just the speed.

[Bill Phil]

You also just said you didn't pick a coordinate system, hence you have not chosen an origin.

I'm not specifying the point, it's random. The origin is chosen by you, which I also just said, and not me. And then you pick a point. And if that point is a few megaparsecs away from the nearest galactic clusters, the galaxies are moving at an average velocity of 200km/s away from you.

[BlueCosmology]

This point, is just a point. The reference frame doesn't matter. It's just a location. Like if you teleported a few meters in front of a bus you wouldn't care about reference frames, you would care about the speed of the bus.

I never said there wasn't a reference frame, but for this it simply doesn't matter.

Yes it entirely does. A point is a location in a reference frame. You don't seem to understand what you're saying, you're saying that there is no absolute coordinate system (i.e. there is no such thing as a location, there are just locations between different objects) but you're saying there are just absolute locations.

Ye syou would care about reference frames if you reported in front of a bus. If you teleport in front of it, and go at the speed of it, you will not get hit. If you teleport infront of it and it is travelling towards you at 50mph, you will.

[Bill Phil]

No. In this case your position is near the bus and your speed it the speed of the ground below you. You need to specify both the location and the speed.

No, your position is exact, but not known until you're there. The location is random, but the statement depends on factors.

Honestly you are all bringing up unnecessary arguments to this.

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Yes it entirely does. A point is a location in a reference frame. You don't seem to understand what you're saying, you're saying that there is no absolute coordinate system (i.e. there is no such thing as a location, there are just locations between different objects) but you're saying there are just absolute locations.

Ye syou would care about reference frames if you reported in front of a bus. If you teleport in front of it, and go at the speed of it, you will not get hit. If you teleport infront of it and it is travelling towards you at 50mph, you will.

No, there is a reference frame here, and it's the origin. You specify it. But once you get to the randomly chosen point, that becomes your new origin and thus your "reference frame". All objects, relative to you, are then shown on this plane.

[PakledHostage]

The point, by virtue of being a point.

[Bill Phil]

I think what the guys are getting at is that the selection of your point is arbitrary. There is no absolute reference frame. You could claim that your chosen reference frame is stationary, but someone else could choose another perfectly valid reference frame in which your "stationary" reference frame is moving. Why is your arbitrary coordinate frame any more valid than someone else's?

I'm saying the selection is arbitrary! It doesn't matter the origin, it can be anywhere, but the point is then randomly chosen from that origin.

If the reference frame is moving, how did it start moving. It's not moving by virtue of it being a point. Points don't change, but a vector could make a new point with different values.

[theend3r]

I'm saying the selection is arbitrary! It doesn't matter the origin, it can be anywhere, but the point is then randomly chosen from that origin.

If the reference frame is moving, how did it start moving. It's not moving by virtue of it being a point. Points don't change, but a vector could make a new point with different values.

Just like there are no straight lines in space, there are also no points in space(time).

[RainDreamer]

An interesting video relating to this topic:

[BlueCosmology]

I'm saying the selection is arbitrary! It doesn't matter the origin, it can be anywhere, but the point is then randomly chosen from that origin.

If the reference frame is moving, how did it start moving. It's not moving by virtue of it being a point. Points don't change, but a vector could make a new point with different values.

Again, as you accept there is no absolute coordinate system.

You pick your origin, I pick my origin moving relative to yours.

[-Velocity-]

Christ, four pages and no one mentioned you can measure your velocity with respect to the microwave cosmic background?! There is only one velocity at which the average CMB wavelength from all directions of the sky is the same (and thus, not affected by doppler shift). It's a coordinate system that can be seen anywhere in the visible universe. So it serves as a useful origin/reference point for "zero" velocity. Though, as has been pointed out ad nauseum in this thread, any reference point or reference velocity in space is arbitrary. Even the cosmic microwave background. But at least the CMB gives us a sense for what the average initial velocity of stuff in the universe should have been.

Here's a quote from this website:

Precise measurements made by the Cosmic Background Explorer (COBE) satellite launched in late 1989 determined the spectrum to be exactly characteristic of a blackbody at 2.735 K. The velocity of the satellite about the Earth, the Earth about the Sun, the Sun about the Galaxy, and the Galaxy through the universe actually makes the temperature seem slightly hotter (by about one part in 1,000) in the direction of motion rather than away from it. The magnitude of this effect--the so-called dipole anisotropy--allows astronomers to determine that the Local Group of galaxies is moving at a speed of about 600 km/sec in a direction that is 45 from the direction of the Virgo cluster of galaxies. Such motion is not measured relative to the galaxies themselves (the Virgo galaxies have an average velocity of recession of about 1,000 km/sec with respect to the Milky Way system) but relative to a local frame of reference in which the cosmic microwave background radiation would appear as a perfect Planck spectrum with a single radiation temperature.

So from a distance, we can figure out how fast some galaxy is moving compared its CMB zero velocity (which would be different than ours, due to the expansion of the universe).

I think I should probably explain even further. Before the universe began to condense, it was very uniform and didn't have much motion- except for the expansion of the universe. Before things could start moving fast, large structures had to condense and form first, concentrating angular momentum and gravity, causing much higher rates of relative motion. The CMB was emitted before this era, when the universe was only a few hundred thousand years old, so relative velocities were very low. Thus, like I said, it represents an accurate measure of what the average initial velocity of stuff in the universe was.

I think it would be tough to determine the relative velocity of a galaxy though with respect to the CMB though, if it's too far away to use Cephid variables and we don't observe any Type Ia supernovae in it or nearby galaxies it is gravitationally bound to. If we can't get an absolute measure of distance via some known stellar source, then the relative velocity with respect to the CMB should disappear into our estimate of distance based solely off of redshift.

Oh and of course, galaxies are too far away to detect their proper motion, so we can only get the radial term.

Edited May 3, 2015 by |Velocity|

[PakledHostage]

If the reference frame is moving, how did it start moving. It's not moving by virtue of it being a point. Points don't change, but a vector could make a new point with different values.

But that is where the confusion lies... Moving relative to what? A point may be a point in your chosen reference frame but your "point" is moving in my reference frame. I maintain that my reference frame is stationary and that any point in it is stationary. It is your chosen reference frame (and your point in it) that is moving.

And for the record, I am not saying you don't know what you are talking about. I am saying you aren't doing a good job of explaining your, er, point. There seem to be inconsistencies in your argument that need to be clarified. |velocity| gave a good answer above. Maybe you could explain your own answer in another context.

[K^2]

Christ, four pages and no one mentioned you can measure your velocity with respect to the microwave cosmic background?!

Do you happen to know how precisely our velocity relative to CMB has been measured? It'd be a very curious number to know.

[Camacha]

But that is where the confusion lies... Moving relative to what? A point may be a point in your chosen reference frame but your "point" is moving in my reference frame. I maintain that my reference frame is stationary and that any point in it is stationary. It is your chosen reference frame (and your point in it) that is moving.

This is the whole issue. Without a neutral omnipresent dimensional grid, any chosen point is arbitrary and could be moving in any direction. As far as I know, there is no universally valid value to take as a baseline.

[Xannari Ferrows]

Good question, but very hard to answer. As it move farther away relative to us, it accelerates relative to us as well, so there's no fixed speed for any number of galaxies.

[YNM]

Do you happen to know how precisely our velocity relative to CMB has been measured? It'd be a very curious number to know.

There is an expected apex / antapex of Local Group movement, even with a reported velocity (although very huge errors). There's another one from COBE data here. Indeed, explanation isn't just due to peculiar velocity : some says that it could be due to cosmological things.

Mind you that I just link - all are from arxiv, and only two that I fairly understand. I don't understand the last one.

[LLlAMnYP]

Different proposition. Consider the observable universe as a gas with galaxies (or maybe clusters would be better?) as particles. What is its temperature (or average variation of velocity squared)?

[-Velocity-]

Different proposition. Consider the observable universe as a gas with galaxies (or maybe clusters would be better?) as particles. What is its temperature (or average variation of velocity squared)?

I'm reasonably certain that temperature is a measure of randomized particle kinetic energy, not randomized particle velocity.

[LLlAMnYP]

Sure. It's just a matter of picking out comparable (by mass) particles. Then the width of the distribution of their velocities (which is independent of the reference frame in a non-relativistic case) will be roughly proportional to their average (internal) kinetic energy.