Is the universe still expanding?

[hellfire]

I just cannot imagine how physics can make a strict sense of any kind of infinite value. In this case an infinite amount of time from an infinite past. This would need of an infinite number of events to reach present. Does this make sense? Is present actually possible in such a model? It remembers me to the paradoxes related to the spatial infinite. For example, can a force (e.g. gravity) be acting on a body located at an infinite distance? This body will never change its position as it will always stay at infinity (oo - N = oo). It seams that the concept of position makes no sense at infinity. I have a bad feeling with this, and I think something similar goes on with an infinite past. I would be more happy with the existence of a state which is uncaused and from which the causal chain and time arise (may be as some fluctuation or break of symmetry).

 

[Simetra7]

Remember that this "motion" that they appear to have from our point of view is only a consequence of where we're viewing them from. If we were on another galaxy, they'd appear to be moving in a different direction.

Is there a distinct point (distance from Earth in every direction), where this apparent motion starts to become observable, or is the observed expansion rate very different at a particular distance in one direction than it is at the same distance in another direction?

 

[SpaceTiger]

Is there a distinct point (distance from Earth in every direction), where this apparent motion starts to become observable, or is the observed expansion rate very different at a particular distance in one direction than it is at the same distance in another direction?

It depends on the magnitude of the local motions, but it will be roughly the same no matter what direction you look. Basically, you'll see the effects of expansion when,

H_0d \sim v_{gal}

The magnitude of the galaxy's peculiar velocity (v_gal) will depend on where it's located, but on average, does not depend on direction. As for H₀, it certainly doesn't depend on direction.

 

[Simetra7]

Does the expansion of space effectively give us an almost static view of distant objects in space? For example, the furthest observable galaxy in the Hubble deep field has been estimated to be approximately 13 billion light years away, so the light that is observed from that galaxy is from 13 billion years ago, when the universe was only an estimated 1 billion years old. At that time, the position that Earth now occupies in space would have been much closer to that particular galaxy, so an observer, (had there been anyone around at that time to observe), would have seen that galaxy at almost the same period in it's evolution as we are seeing it now. So in 14 billion years, only 1 billion years worth of the evolution of that galaxy has been observable, but it has been stretched out over the whole of those 14 billion years. Does this make any sense?

 

[Garth]

It depends on the magnitude of the local motions, but it will be roughly the same no matter what direction you look. Basically, you'll see the effects of expansion when,

H_0d \sim v_{gal}

The magnitude of the galaxy's peculiar velocity (v_gal) will depend on where it's located, but on average, does not depend on direction. As for H₀, it certainly doesn't depend on direction.

When the Earth's velocity around the Sun has been taken into account the Solar System is traveling at 390 +/- 60 km/sec relative to the surface of last emission of the CMB. However when the Sun's motion around the Galaxy is also taken into account this translates into the fact that the Galaxy is traveling relative to the surface of last emission of the CMB, which probably defines the C.M. reference frame of the universe, at 603 km/sec or about 0.2%c! (Nature, Vol 270, 3 Nov 1977, pg 9) Therefore, if our galaxy's velocity is 'typical', then
H_0d \sim 0.002c or d ~ 10 Mpsc

So in 14 billion years, only 1 billion years worth of the evolution of that galaxy has been observable, but it has been stretched out over the whole of those 14 billion years. Does this make any sense?

Yes, you are describing cosmological time dilation, otherwise detected as cosmological red shift.

Garth

 

[Chronos]

I usually avoid these conversations. You are not seeing the 'big picture' here, Simretra7. You are making unfounded assumptions about the initial state of the universe and extrapolating them way beyond what is reasonable. It makes no sense.

 

[Simetra7]

I usually avoid these conversations. You are not seeing the 'big picture' here, Simretra7. You are making unfounded assumptions about the initial state of the universe and extrapolating them way beyond what is reasonable. It makes no sense.

My intention was not to make any sort of an assumption but just to try to make sense out of what is a fascinating but extremely complicated subject.
Are you saying that our view of far off objects is not a "slow motion" view, or is it just my reasoning that is unreasonable.

 

[hellfire]

Does the expansion of space effectively give us an almost static view of distant objects in space? For example, the furthest observable galaxy in the Hubble deep field has been estimated to be approximately 13 billion light years away, so the light that is observed from that galaxy is from 13 billion years ago, when the universe was only an estimated 1 billion years old. At that time, the position that Earth now occupies in space would have been much closer to that particular galaxy, so an observer, (had there been anyone around at that time to observe), would have seen that galaxy at almost the same period in it's evolution as we are seeing it now. So in 14 billion years, only 1 billion years worth of the evolution of that galaxy has been observable, but it has been stretched out over the whole of those 14 billion years. Does this make any sense?

Yes, this makes sense. Further on, objects located very near to our spatial position at an instant of time after t = 0 would have been observable to an observer located at our spatial position, but after 13.7 Gyr we would see them now exactly in the same instant of their evolution as this observer did.

This is due to the fact that in a dynamic space with an initial singularity the past light cone is not a cone but a "teardrop". In a static space, the size of the particle horizon now is the same as the size of the past light cone at t = 0. For a dynamic space this is not true, since the size of the light cone is zero at t = 0.

The term particle horizon refers to the current location of the objects which sent us a light ray at t = 0 that we are observing now. The term past light cone refers to the objects located in past which have a causal influence on us.

May be figure 1 in http://arxiv.org/astro-ph/0310808 will help.

 

[Simetra7]

May be figure 1 in http://arxiv.org/astro-ph/0310808 will help.

Could you check this link. I think it may be the wrong one.

 

[hellfire]

The link is correct. Go to Full-Text: PDF, and open the document ("Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe"). You may take a look to Figure 1 and try to understand what happens with the light cone and the particle horizon. If you have questions, please ask.