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ejjr661
2012-Feb-15, 11:19 AM
Hi, I'm new to the site and love it.

If I understand, the sources of the light captured from the farthest galaxies in the Hubble Deep Field are +/-13 billion light years away. So the light photons captured in the photo left their respective sources 13 billion years ago. Furthermore, it is believed the age of the universe is +/-13 billion years. So, those same sources of light have now traveled for an additional 13 billion years, putting them well past a distance of 13 billion light years. Doesn’t this mean that those sources were at the location we now measure at nearly the beginning of the universe?

If those same sources are now beyond 13 billion light years, doesn’t that mean they traveled in excess of the speed of light in order to have traveled so far?
:confused:

antoniseb
2012-Feb-15, 01:58 PM
For the sake of explaining, let's imagine that the universe started expanding 13.7 billion years ago, and that when it was 0.7 billion years old a sufficient number of bright stars had formed and the rest of the universe was transparent enough for light to pass through it. At that time, the atoms we are made of mostly hadn't been created yet, but the Hydrogen and Helium that formed them were here (I'm using the term 'here' to mean a place where space isn't expanding away from us fast enough to make a difference, so here means in our galaxy group).

As time moved forward, the most distant photons from galaxies coming to 'here' were coming from further and further away, but it was always photons from that same period... just coming from further and further away. Today that era is thirteen billion years ago, and those galaxies are moving away from us at somewhere between five and fifteen times the speed of light (I think there are reports of observations near ten times, but the science is based on faint data, plus there is also some doubt about when exactly the universe became transparent).

Now, your question has to do with the speed of these galaxies, and no, the light from those galaxies that we see now, has only traveled 13 billion light years, and where the galaxies are now doesn't affect that... HOWEVER, the galaxies you mention are probably now (I use the word 'now' loosely here) more than 45 billion light years away from us, so their speed (taking cosmic expansion into account) IS faster than light. This is not a problem. Their speed relative to us is large and increasing because of Cosmic Expansion. You can think of it as tiny amounts of space being inserted between the space that is already here. The speed of light is something that can't be exceeded as two objects pass each other, but these galaxies are far from us, and so that rule doesn't apply.

Kuroneko
2012-Feb-15, 02:06 PM
You're conflating different distance measures. If we see light from a galaxy that was emitted T time ago, we can say that's Tc light travel distance away. So those galaxies at upwards 13 billion ly in light-travel distance are currently 40-something billion ly away in proper distance, because space expanded quite a bit in the time it took the light to reach us.

Some of the galaxies we are looking at are moving away faster than the speed of light in the sense that the proper distance between us is increasing at such a rate at the present moment (in the comoving frame). When the light was emitted, they were not.

WayneFrancis
2012-Feb-15, 03:28 PM
Hi, I'm new to the site and love it.

If I understand, the sources of the light captured from the farthest galaxies in the Hubble Deep Field are +/-13 billion light years away. So the light photons captured in the photo left their respective sources 13 billion years ago. Furthermore, it is believed the age of the universe is +/-13 billion years. So, those same sources of light have now traveled for an additional 13 billion years, putting them well past a distance of 13 billion light years. Doesn’t this mean that those sources were at the location we now measure at nearly the beginning of the universe?


Yes the light was emitted very early on in the universe. The Cosmic Microwave Background Radiation CMBR is the oldest light we see being emitted about 300,000 years after the big bang.




If those same sources are now beyond 13 billion light years, doesn’t that mean they traveled in excess of the speed of light in order to have traveled so far?
:confused:

The light was actually emitted closer to us then 13 billion light years away. It is just that in the last 13 billion years space has expanded so the total distance travelled by the photons is about 13 billion light years. The objects don't move much in space. It is space expanding and carrying them along for the ride. There is no limit in SR to how fast 2 objects can recede from each other. Just that no information can be transmitted faster then the speed of light and that included objects passing each other. Since no objects in the universe do this because of cosmic expansion there is no violation of special relativity.

RandyD123
2012-Feb-15, 04:46 PM
If the red shifted light we see now is over 13 billion years old, is it possible that the universe has stopped expanding and we won't see that evidence for another 13 billion years?

antoniseb
2012-Feb-15, 05:11 PM
If the red shifted light we see now is over 13 billion years old, is it possible that the universe has stopped expanding and we won't see that evidence for another 13 billion years?
Possible in the sense that there is no way we could have observed it? yes. Such behavior, however, would be inconsistent with our models of the universe in so many ways that we don't consider the possibility.

Extrasolar
2012-Feb-15, 07:41 PM
Possible in the sense that there is no way we could have observed it? yes. Such behavior, however, would be inconsistent with our models of the universe in so many ways that we don't consider the possibility.

You mean those models that are 90%-95% directly unobserved stuff?

Luckmeister
2012-Feb-15, 07:59 PM
The light was actually emitted closer to us then 13 billion light years away. It is just that in the last 13 billion years space has expanded so the total distance travelled by the photons is about 13 billion light years. The objects don't move much in space. It is space expanding and carrying them along for the ride. There is no limit in SR to how fast 2 objects can recede from each other. Just that no information can be transmitted faster then the speed of light and that included objects passing each other. Since no objects in the universe do this because of cosmic expansion there is no violation of special relativity.

Well put. I avoid using the misleading term "moving away from us" when discussing distant galactic recession relative to our reference frame.

antoniseb
2012-Feb-15, 08:30 PM
You mean those models that are 90%-95% directly unobserved stuff?
Umm, no. Dark Energy and Dark Matter are part of the universe models, but for RandyD123's question about the distant universe potentially halting its expansion to be meaningful a lot of more basic physics would have to be wrong as well.

Extrasolar
2012-Feb-15, 09:25 PM
Umm, no. Dark Energy and Dark Matter are part of the universe models, but for RandyD123's question about the distant universe potentially halting its expansion to be meaningful a lot of more basic physics would have to be wrong as well.

I wasn't implying otherwise. I was merely pointing out that any model of the universe, however flawless, is based on the understanding of very little directly observed material.

antoniseb
2012-Feb-15, 09:29 PM
... any model of the universe, however flawless, is based on the understanding of very little directly observed material.
Let's distinguish material and phenomena. Your 'material' argument is misleading to the topic at hand.

ejjr661
2012-Feb-16, 12:46 PM
Thank you for those explanations. I always envisioned the expansion taking place at the outside "edges" rather than throughout entire universe. Galaxies aren't simply moving away, rather all of space is being stretched. In defense of my ignorance, I am a carpenter by profession, just fascinated with these concepts.
Thanks again :D

Luckmeister
2012-Feb-16, 10:41 PM
In defense of my ignorance, I am a carpenter by profession, just fascinated with these concepts.
Thanks again :D

Hi ejjr661. I'm not surprised that you, being a carpenter, are fascinated with cosmological concepts. After all, you work with physics concepts every day; dimensions, mathematics, energy forces (gravity, density, etc.) chemistry (natural and synthetic building materials and bonding agents), heat propogation (insulation, etc.) and more. So you're not approaching this subject from a point of total ignorance.

Welcome to BAUT. :)