# Distance and space expansion

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Let say I am observing a galaxy situated 1 billion light-years from me.

At that distance the expansion of the universe is apparent.

My question: If was able to live long enought and mesure the distance to that same galaxy in 100 million years, will I mesure the same distance 1 billion light-years ?

My questioning come from what I readed in a thread that was going something like that : space expansion is not the addition of space unit but the space unit expanding. In an expanding universe, 1 meter (or a billion light-years) still is 1 meter (or a billion light-years) after the universe expand for a time. The meter is just longueur...

I am asking the expert there interpretation of the expansion of the universe on distance.

Thanks

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You would measure it as a greater distance as far as I know.

The universe is quite spread out now, so its obvious that it can appear to increase. Otherwise if we looked up the sky it would still look like what it looked like right after the big bang. But things move apart with time, making things a great deal furthar appart now.

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Then am I right by saying that space doesn't expand but space is being created. One unit of lenght does not stretch, but new units of space are added ?

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yeah, otherwise atoms and whatnot would get bigger too...

plus it wouldn't seem any different

things are just spreading out, as far as I know

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Then am I right by saying that space doesn't expand but space is being created. One unit of lenght does not stretch' date=' but new units of space are added ?[/quote']

This works as an analogy, but be careful with "space is being created". This gives the impression that space is actually a thing, when its actually the lack of things. And in some ways it does seem to stretch: light passing through space for very long periods of time reaches earth as being redshifted (its wavelenght has been made longer).

Try reading: http://en.wikipedia.org/wiki/Redshift#Expansion_of_space and follow any interesting links about the expansion of space, hubbles constant, and stuff like that. I can't give you a definitive answer but that site might help.

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The thing that has always bothered me about the light from distance galaxies is that it takes a finite time to reach us. As such, the light we see is from the distant past. The light from something 10Billions lights years away actually left the object 10 billion years ago. We do not know what it is doing today, only what it did 10 billion years ago. If the universe is say 10-15 billions years old and the most distance objects are that old and are accelerating away, doesn't that really imply what these objects were doing near the beginning of the universe. What they are doing today, we will not see for another 10-15billions years.

For example, if we sent a robot to Mars and say it about to fall into a hole and we sent it commands to turn, it would have already fell in the hole before the visual signal ever reached us, and our commands to turn would reach it after its had been lying in the hole for weeks.

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Hi,

I think, it would not measure the same. No, its not because I go on with what the people think, but its because I feel that there willl be a change in the orbit, and the star most probably would not/could not exist.

But if you rule out the above two facts, even then it would be the same because, the rate of expansion on different sides of the Universe would be different.

gagsrcool

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Thanks Tycho

I followed the link and found that they also talk about "space stretching" :

While this redshift of distant galaxies closely resembles what would be seen if distant galaxies simply had recessional velocities, in general relativity stretching of spacetime is different from the physical movement of the source. These galaxies are not believed to be receding; instead, the intervening space is believed to be stretching, which is subtly different. Nevertheless, astronomers (especially professional ones) sometimes refer to "recession velocity" in the context of the redshifting of distant galaxies from the expansion of the Universe, even though it is only an apparent recession. More mathematically, the viewpoint that "distant galaxies are receding" and the viewpoint that "the space between galaxies is expanding" are related by changing coordinate systems. Expressing this precisely requires working with the mathematics of the Robertson-Walker metric. [2]

I followed the link to the Robertson-Walker metric and found that equation:

$ds^2 = c^2 dt^2-a(t)^2[dr^2+\bar{r}^2 d\Omega^2]$

Can you explain it a little bit, in layman term ?

Thanks again

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For example, if we sent a robot to Mars and say it about to fall into a hole and we sent it commands to turn, it would have already fell in the hole before the visual signal ever reached us, and our commands to turn would reach it after its had been lying in the hole for weeks.

Weeks? How about less than an hour. The distance varies because of the relative orbits, but at ~300 million km it's 2000 seconds round-trip at c.

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I followed the link to the Robertson-Walker metric and found that equation:

$ds^2 = c^2 dt^2-a(t)^2[dr^2+\bar{r}^2 d\Omega^2]$

The scale factor [imath]a(t)[/imath] captures the scale of the space component (the [imath]dr^2 + \bar{r}^2d\Omega^2[/imath] stuff). The scale factor is given as 1 in the present day' date=' and if the universe is spatially flat, then the FLRW metric reduces to the polar coordinate version of the Minkowski metric (which is [imath']ds^2 = c^2 dt^2 - (dx^2 + dy^2 + dz^2)[/imath] in Cartesian coordinates).

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Thanks Tycho

I followed the link and found that they also talk about "space stretching" :

I followed the link to the Robertson-Walker metric and found that equation:

$ds^2 = c^2 dt^2-a(t)^2[dr^2+\bar{r}^2 d\Omega^2]$

Can you explain it a little bit' date=' in layman term ?

Thanks again[/quote']

That's the Robertson-Walker metric for a globally flat space-time. The general metric is

$ds^2=c^2dt^2-R(t)^2\left[\frac{dr^2}{1-kr^2}+r^2d\Omega^2\right]$

The k can be +1, 0 or -1, as the radial coordinate r can always be rescaled so that k takes on one of these values. k is the value of Ricci scalar, which is a constant becasue space-time is modelled as being homogeneous and isotropic. k=1 is positive curvature, and the resulting model is a closed spherical topology. k=0 we have zero curvature, a globally flat model. k=-1 we have negative curvature, and the resulting model is open and hyperbolic.

The equations governing the actual expansion can be derived from energy-momentum conservation.

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That's the Robertson-Walker metric for a globally flat space-time.

It's looks bad with inline TeX, but that's the general FLRW metric. The [imath]\bar{r}[/imath] is defined as:

$\bar{r} =\begin{cases} R \sinh(r/R), &\mbox{globally hyperbolic} \\ r, &\mbox{globally flat} \\R \sin(r/R), &\mbox{globally spherical} \end{cases}$

...where [imath]R[/imath] is the radius of curvature.

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Sorry about my laziness with respect to calculating the distance to Mars divided by the speed of light. But the point still remains that the light from the edge of the universe is not real-time data. It is what happened billions of years ago, because of the time it takes for light traveling at C to reach us from those long distances. As we get closer, the red shift is less and the data is more recent (relatively speaking). If I am not mistaken the closet galaxies (almost realtime data) are slightly blue shifted.

If would almost appear that the universe is a scrapbook of the life of the universe with the farthest objects the universe's baby pictures, when it was most rapidly expanding. As we get closer and closer we go through its toddler years, then its childhood, then its adolescence, finally the closest data are its adulthood pics. Although this is consistent with the data time delay, it logically creates a very different picture of the original universe.

The logically consistent scenario would imply an expansion that does not condense out into matter/energy, until the universe was already highly expanded. While the condensation would have happened as trillions of galaxy level mini big bangs already distributed thoughout the universe. This scenario is consistent with the time delayed data and the most energetic state of things happening roughly 10-15 billions years ago. The original energy pressure waves caused the highly distributed universe to expand further. This scenario readily accounts for the uniform microwave background, the holographic expansion of the universe, galaxies forming within the first 100M years, and the data time delay.

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The logically consistent scenario would imply an expansion that does not condense out into matter/energy, until the universe was already highly expanded. While the condensation would have happened as trillions of galaxy level mini big bangs already distributed thoughout the universe. This scenario is consistent with the time delayed data and the most energetic state of things happening roughly 10-15 billions years ago. The original energy pressure waves caused the highly distributed universe to expand further. This scenario readily accounts for the uniform microwave background, the holographic expansion of the universe, galaxies forming within the first 100M years, and the data time delay.

How can you say its consistent with anything when it doesn't make a lick of sense, and you dont back it up with any info? All your posts seem to be like this, talking about logical consistancy while rambling on with sentances that dont even make sense from a physics point of view. Case in point:

While the condensation would have happened as trillions of galaxy level mini big bangs already distributed thoughout the universe.

Are we supposed to know what you mean by galaxy level mini big bangs? What caused these? What evidence supports their existence? How is this logically consistant with anything? What in the world are you talking about?

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Thanks patcalhoun and h=-16nT

If I understand right then a metric is an equation to calculate the distance between 2 locations or more precisely, since it is spacetime, between to events.

I see that time is not subject to the scale factor, so time doesn't expand like space. Why since we are suppose to calculate spacetime and time is a dimesion like the others of space ?

I have a question that is bugging me since I started studying Relativity, why use ct for the time dimension instead of just t ?

Thanks

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Thanks patcalhoun and h=-16nT

If I understand right then a metric is an equation to calculate the distance between 2 locations or more precisely' date=' since it is spacetime, between to events.

I see that time is not subject to the scale factor, so time doesn't expand like space. Why since we are suppose to calculate spacetime and time is a dimesion like the others of space ?

I have a question that is bugging me since I started studying Relativity, why use ct for the time dimension instead of just t ?

Thanks[/quote']

Because ct has units of length in SI, which makes the metric dimensionally consistent. The convention is to adopt units in which c=G=1 in S/GR, to make things simpler.

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It's looks bad with inline TeX' date=' but that's the general FLRW metric. The [imath']\bar{r}[/imath] is defined as:

$\bar{r} =\begin{cases} R \sinh(r/R), &\mbox{globally hyperbolic} \\ r, &\mbox{globally flat} \\R \sin(r/R), &\mbox{globally spherical} \end{cases}$

...where [imath]R[/imath] is the radius of curvature.

I'm not familiar with the bar notation, so I either didn't see it or neglected it.

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I'm not familiar with the bar notation, so I either didn't see it or neglected it.

Chances are you didn't see it. I'm still two minds about inline TeX, but hey...its fun.

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