# I The Universe accelerating and light

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1. Sep 11, 2015

### Jaami M.

How are we, and the universe effected from its accelerating expansion? Also how will our perception of the world around us be effected, since the universe is expanding and light is constant? <-Will things appear to move slower? I know that we only experience just a small fraction since the universe expanding velocity is 68 km/sec or 42.253 mi/sec. But is there any type of effect?

2. Sep 11, 2015

### Simon Bridge

We are not... the world around us in unaffected, though what we see of large distances is affected.
"Universe expanding velocity" is a meaingless term.

3. Sep 11, 2015

### Drakkith

Staff Emeritus
The expansion velocity is actually expressed as km/s/megaparsec. In other words, it's 68 kilometers per second per megaparsec. This means that objects 2 megaparsecs apart recede from each other at 136 km/s, double the velocity at one megaparsec. So the recession velocity is actually different for all objects based on their distance apart.

4. Sep 11, 2015

### phinds

No, things seem to move slower when they are moving relative to you in the same inertial frame of reference. Receding galaxies are not IN the same inertial frame of reference so time dilation is not applicable.

5. Sep 11, 2015

### Drakkith

Staff Emeritus
What? What does "the same inertial frame" mean?

6. Sep 11, 2015

### phinds

I thought that was a well defined concept, no? If we are both on the same train and it is not accelerating then we are in the same inertial frame. If I'm on the platform and you're on the train and it's not accelerating (relative to me) then we are both in the same inertial frame. If I'm on the train OR the platform, a receding galaxy is not in an inertial frame relative to me.

7. Sep 11, 2015

### Drakkith

Staff Emeritus
I've never seen it used like that before. To me, the "same inertial frame" would be the two of us on the train or the two of us on the platform, not moving relative to each other. If you're on the train and I'm on the platform we will see things differently, so I wouldn't call it the same inertial frame. We're both in an inertial frame, but not the same one.

I'm also unsure about whether or not a receding galaxy is considered to be in an inertial frame. It's certainly not accelerating in its own frame, but I don't know how things like this are handled in GR.

8. Sep 11, 2015

### phinds

I think maybe it's the way we use the words. If I'm on the platform and you're on the train (moving but not accelerating) you are moving in my inertial frame so I say we're in the same inertial frame, but I see what you mean by the way you use the term.

9. Sep 12, 2015

### Orodruin

Staff Emeritus
The same inertial frame is not a very meaningful concept in GR because space-time is generally curved. The only way to make proper sense of it is if you have two observers at the same event. Those inertial frames are local and not global.

10. Sep 12, 2015

### phinds

Exactly. And if one of them is moving (but not accelerating) would you find it correct to say that they are "in the same inertial frame" as I do or do you hold with Drakkith's point of view that they are each in inertial frames but not the same one? Seems to me both points of view are right, actually.

11. Sep 13, 2015

### Simon Bridge

The "frame" of an object is usually (represented by or pictured as) the coordinate system attached to the object.
Two objects share the same reference frame if they are stationary with respect to each other.

However - it is common in physics to mix up technical and non-technical uses.
If two people are in the bath, moving about, then we say they share a bath ... we can put the grid on the bath and , similarly, in a non-technical sense, talk about the two people sharing the bath coordinate system ... but, in the technical use: that is the frame of the bath, not the people. If one were stationary in the bath, then that one would share the frame of the bath as well as being in the frame of the bath.

When a physicist writes about something moving in a frame, that's usually what is meant.
There are a lot of references about reference frames ... having trouble finding any that spell out when two objects share a frame.

12. Sep 13, 2015

### phinds

Thanks for that, Simon.