Help with consequence of Hubble's law

[pajacek]

A bit boring introduce with my calculations. I hope I didn't make mistake. You can skip it.Lets take for example EGSY8p7. It is a galaxy which light reaching Earth shows its position about 13.2 billion years ago. We can convert its distance to parsecs: 13000000000ly= 4.047139E9 pc = 4.047139E3 Mpc.Lets use Hubble's law now. Planck Mission gives us that Hubble constant H0=67.80 (km/s). If I understand it well, galaxy is running away from us with 67.80 (km/s) for every one Mpc distance. So 4.047139E3 (Mpc) * 67.80 [(km/s)/Mpc]=274.4E3 (km/s)Speed of light is about 299.7E3 (km/s) so calculation above gives quite close similar order number.

And now is moment for my question. What would happened if distance between us and other galaxy would be more than about 4,42E3 (Mpc)? It gives velocity exceeding the speed of light. Does it mean that we could not see galaxies which are further away than 4,42E3 (Mpc) if we are moving away faster than light travels? Is it even possible to move away from each other faster than light?

P.S. I just noticec I was mistaken with speed of light. It should be 299.8E3 but I'm to lazy to recalculate this 4,42E3 (Mpc). Idea is unchanged.

[pajacek]

I have no idea how to edit my previous post. I didn't know code option would mess up my post. Can you repair it?

[Stu]

Others will answer your question far better, but my understanding is that objects can move away from each other faster than the speed of light if it is caused by the expansion of the universe, in fact that is what determines the limitation of how far back we can see. Beyond a certain point the universe is moving away too quickly so cannot be observed.

[ollypenrice]

Others will answer your question far better, but my understanding is that objects can move away from each other faster than the speed of light if it is caused by the expansion of the universe, in fact that is what determines the limitation of how far back we can see. Beyond a certain point the universe is moving away too quickly so cannot be observed.

My understanding is exactly this. 

Olly

[Alien 13]

The speed of light is much over rated.

Alan

[ronin]

Any object beyond the Hubble Radius means that it is "moving" away from us faster then c, so we never observe it.

That is where the term the "observable" universe comes from.

As has been pointed out the object is not necessarily moving but the "stuff" between us is expanding and so carrying the other object along with/on it.

The universe will therefore be larger then this observable boundry.

[Joe12345]

I know ABSOLUTELY NOTHING about this theory, but if the speed increase is caused by gravity reducing then surely there would be a point where it stops being affected by gravity?

I know that gravity travels at the speed of light, so if photons aren't reaching our galaxy, then gravitons wouldn't reach the other galaxy?

If that happens then there wouldn't be any boundaries to the speed of the other galaxy (assuming there aren't any other galaxies to affect it).

I wouldn't be surprised if I had got the wrong end of the stick completely!

[Stu]

I know ABSOLUTELY NOTHING about this theory, but if the speed increase is caused by gravity reducing then surely there would be a point where it stops being affected by gravity?

I know that gravity travels at the speed of light, so if photons aren't reaching our galaxy, then gravitons wouldn't reach the other galaxy?

If that happens then there wouldn't be any boundaries to the speed of the other galaxy (assuming there aren't any other galaxies to affect it).

I wouldn't be surprised if I had got the wrong end of the stick completely!

My albeit limited understanding is that the increased speed is not related to gravity but to the fact that there is more 'stuff' in between the distant objects which is expanding, therefore the further away the object is, the faster it appears to be traveling.

[acey]

Einstein showed that there is no absolute reference frame for space and time. Cosmologically, the nearest thing is to measure position, time and speed relative to the "Hubble flow". Imagine you're stationary on an expanding rubber sheet (e.g. surface of balloon). This is like being at rest relative to the Hubble flow. Other galaxies that are also at rest (with respect to the Hubble flow) will be seen to recede, because of the expansion of the "balloon". Very distant ones will recede faster than light, but this does not violate relativity, because information is not propagating faster than light.

http://astronomy.swin.edu.au/cosmos/h/hubble+flow

The region in which the recession velocity is no greater than the speed of light is called the Hubble sphere or Hubble volume.

https://en.wikipedia.org/wiki/Hubble_volume

It is not the same size as the observable universe.

https://en.wikipedia.org/wiki/Observable_universe

This paper is technical but explains it in more detail.

http://arxiv.org/abs/astro-ph/0310808v2

[pajacek]

So with every passing second there is less and less light coming to us from objects beyond the Milky Way... It's a bit sad

[acey]

So with every passing second there is less and less light coming to us from objects beyond the Milky Way... It's a bit sad

Not necessarily. M31 is approaching us.

[Stu]

Einstein showed that there is no absolute reference frame for space and time. Cosmologically, the nearest thing is to measure position, time and speed relative to the "Hubble flow". Imagine you're stationary on an expanding rubber sheet (e.g. surface of balloon). This is like being at rest relative to the Hubble flow. Other galaxies that are also at rest (with respect to the Hubble flow) will be seen to recede, because of the expansion of the "balloon". Very distant ones will recede faster than light, but this does not violate relativity, because information is not propagating faster than light.

http://astronomy.swin.edu.au/cosmos/h/hubble+flow

The region in which the recession velocity is no greater than the speed of light is called the Hubble sphere or Hubble volume.

https://en.wikipedia.org/wiki/Hubble_volume

It is not the same size as the observable universe.

https://en.wikipedia.org/wiki/Observable_universe

This paper is technical but explains it in more detail.

http://arxiv.org/abs/astro-ph/0310808v2

Thanks Acey

I am reluctant to click on the links as it will quickly lead to an unwelcome reminder of just how little I know, and how limited my intellectual abilities are to actually understand this stuff at anything other than a trivially superficial level. Ho hum!