1. Newbie
Join Date
May 2008
Posts
2

## Confused about galaxy distances and age of Universe

If the universe is somewhere between 14 and 18 billion years old, and the distance to the furtherst galaxy we have seen is about 13 billion light years (from an article I read), then the galaxies must be speeding away from each other at ridiculously high speeds.

In other words, if our galaxy and the furthest galaxy are 13 billion light years apart, and we both "started" in the same spot at the Big Bang some 15 billion years ago, then we would have to be moving apart from each other at over half the speed of light. Is that correct thinking?

Jon

2. dcl
Established Member
Join Date
Aug 2002
Posts
262
When discussing expansion of the Universe, it is unrealistic to talk about how fast objects are moving away from each other. They need not be moving at all if it's the space they're in that's expanding while each object is stationary at the point at which it happens to be located in the Universe. Consider two dots on the surface of an expanding balloon. The distance between them is increasing, but neither of them is moving.

By the way, the WMAP data have indicated that the age of the Universe is between 13.70 and 13.72 billion years

3. To be fair to upbeatjonm, the separation of co-moving coordinates is often described as an apparent speed of recession and it is valid to think of it in this way as long as you remember that nothing is actually moving faster than light, it just seems that way.

As you say, we can consider the galactic clusters to be at rest, relative to the Hubble flow (the expansion of the universe) and that the distances between the clusters increase over time.

A simple way to put it is that the galactic clusters do not move through space, but rather the space in between the clusters increases in size. This causes an object to have an apparent recession speed - if distances have doubled, for instance, then a galaxy initially 1 billion light years away is now 2 billion light years away, and a galaxy that was initially 2 billion light years away is now 4 billion light years away. The more distant galaxy has apparently receded at twice the speed of the closer one. If we look far enough we see galaxies that have apparently receded at the speed of light and we have also seen many galaxies that have apparent recession speeds that are faster than light.

The age of the universe is around 13.7 billion years, so that is all the time that light has had to travel. But we think that regions of space that were quite close to us when the CMBR was emitted (at that time our observable universe had a radius of only 40 million light years) are now something around 46 billion light years away, due to the expansion of the universe. This means that region of space has apparently receded from us at many times the speed of light.

4. Originally Posted by upbeatjonm
[...] then the galaxies must be speeding away from each other at ridiculously high speeds.
You may enjoy Sean Carroll's Cosmology Primer. The FAQ appears to address part of your question.

Are distant galaxies moving faster than the speed of light? Wouldn't that violate relativity?

A profound feature of relativity is that two objects passing by each other cannot have a relative velocity greater than the speed of light. An even more profound feature, one which has received much less publicity, is that the concept of "relative velocity" does not even make sense unless the objects are very close to each other. In Einstein's general theory of relativity (which describes gravity as the curvature of spacetime), there is no way to define the velocity between two widely-separated objects in any strictly correct sense. The "velocity" that cosmologists speak of between distant galaxies is really just a shorthand for the expansion of the universe; it's not that the galaxies are moving, it's that the space between them is expanding. [...]
Last edited by 01101001; 2008-May-11 at 09:38 PM.

5. Newbie
Join Date
May 2008
Posts
2
Originally Posted by 01101001
You may enjoy Sean Carroll's Cosmolgy Primer. The FAQ appears to address part of your question.
Thanks for the links. All the answers so far have helped explain. Still mind-boggling.

Jon

6. dcl
Established Member
Join Date
Aug 2002
Posts
262
speedfreak: WMAP has indicated that the Universe is 13.7 billion light years old. The CMBR was emitted 380,000 years later. For practical purposes, that's still very nearly 13.7 billion years ago. What is the basis for your statement that "regions of space that were quite close to us when the CMBR was emitted (at that time our observable universe had a radius of only 40 million light years) are now something around 46 billion light years away, due to the expansion of the universe. This means that region of space has apparently receded from us at many times the speed of light"?

By the way, "galactic clusters" is a misleading term. Star clusters inside the galaxy were commonly called "galactic clusters" to distinguish them from globular clusters. "Galaxy clusters" is a term that avoids ambiguity
Last edited by dcl; 2008-May-15 at 07:31 PM. Reason: to correct typos

7. That is a fair point about the clusters of galaxies, my bad.

As for the basis for my previous statement, that comes from the estimated proper distance to the surface of last scattering as it was when photons decoupled (when the CMBR was emitted). As you say, this happened 380,000 years after the big-bang, and at this time photons decoupled and moved through space independently for the first time. This is when the universe is considered to have become transparent, and filled with those photons. We receive those photons 13.7 billion years later and the photons we are receiving right now were emitted at an estimated original distance of only 40 million light years away. That was the size of our observable universe when the CMBR was emitted, although we assume the whole universe could have been any size larger than that and so we expect to, in the future, receive CMBR photons that were originally emitted at distances further than 40 million light years.

The edge of the observable universe, known as the particle horizon or the surface of last scattering, is estimated to have increased in distance by a factor of around 1100 since the CMBR was emitted. This means it was around 1100 times closer then, than it is now. The current mainstream view puts those figures as around 40 million light years radius when the CMBR was emitted and around 46 billion light years radius today.

My primary source is Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universe

And I also use the calculator from Ned Wright's Cosmology Tutorial page.

8. dcl
Established Member
Join Date
Aug 2002
Posts
262

9. Established Member
Join Date
Jun 2006
Posts
2,466

## Long Way

And just think, OP, 13 billion is where we see them, looking deep into the past. Could we see their instantaneous present position, they'd be about 45 billion light years away.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•