# Faster than light expansion speeds are standard

Gold Member
Dearly Missed

## Main Question or Discussion Point

We need a stickythread that makes this point. Prominently so it catches newcomers' attention. Otherwise questions about it keep coming up.

If anyone wants to try saying this in an especially clear way, please go ahead. Everybody I can think of has made this point----the mods, the professional astronomers that post here etc.

We should have a Cosmology FAQ. the kindergarten basics. Any one of half a dozen people here could either write the whole thing or contribute paragraphs. Russ, Wallace, SpaceTiger, Kurdt, Janus...

Now I can think of half-a-dozen more! How would YOU explain to a noob that it is standard in cosmology for distances between stationary points to expand FTL?

Relativity prohibits v>c for inertial reference frames. Inherent in this requirement is that a relative velocity exists between two points for relativistic effects and boundaries to come into effect. The thing people have trouble getting their head around is that expansion of space does not induce a relative velocity. A way of thinking of this is a sheet of cloth with a 1D axis on it. If you make a tear and sew in new cloth along a line, the points a and b on the axis have moved apart a bit but their distance is still a-b and are anchored in space. Simply looking at the mathematics of special relativity this is allowed because again we are only concerned with relative movement IN SPACE if space itself expands theres nothing to say that two points can be 'carried away' from each other faster than the speed of light. We don't witness this expansion on a local scale because of gravitational effects. The mass of galaxies and clusters of galaxies like our own local group reigns in the effects of this but the drift of galactic clusters shows clear indication of this happening all the time.

Thats as best as i can put it for now :P

Gold Member
Dearly Missed
... Simply looking at the mathematics of special relativity this is allowed because again we are only concerned with relative movement IN SPACE if space itself expands theres nothing to say that two points can be 'carried away' from each other faster than the speed of light. We don't witness this expansion on a local scale because of gravitational effects. ...
That works for me. Does anybody have problems with that way of putting it, or anything to add?

Wallace
Personally I don't like the 'space can expand faster than light' thing since it gives the idea that 'space' is something that can do things and helps reinforce the (incorrect) notion that the expansion of space causes galaxies to move apart.

In any case I've been waiting till the wiki style 'content library' appears then I think we should mine old threads for the basis of the content to go in this, with some polishing and new additions.

Chronos
Gold Member
John Baez has already done this on his website with great clarity and expertise. A link there would be sufficient, IMO.

I find the balloon analogy to be helpful. Exaggerating the expansion speed for simplicity -- if the balloon (universe) is expanding at a rate such that distance between objects increases by say, one percent per year, then after 100 years everything will be twice as far away as it was. So an object that was 200 light years away on this giant balloon 100 years ago is now 400 light years away. It has apparently moved 200 light years in 100 years, a recession speed of twice the speed of light. With respect to local space it remained stationary, there is just more space due to the expansion of the universe. If the universe is large enough and expanding even at a slow rate, superluminal recession speeds at great distances are unavoidable.

russ_watters
Mentor
We need a stickythread that makes this point. Prominently so it catches newcomers' attention. Otherwise questions about it keep coming up.

If anyone wants to try saying this in an especially clear way, please go ahead. Everybody I can think of has made this point----the mods, the professional astronomers that post here etc.

We should have a Cosmology FAQ. the kindergarten basics. Any one of half a dozen people here could either write the whole thing or contribute paragraphs. Russ, Wallace, SpaceTiger, Kurdt, Janus...

Now I can think of half-a-dozen more! How would YOU explain to a noob that it is standard in cosmology for distances between stationary points to expand FTL?
I agree that a faq would be a good idea - there are lots of questions people have that are repeated over and over again.

As you probably know, I am an engineer and an amateur astronomer, not a professional physicist/academic. While I do agree that such a faq should probably be very basic (and such explanations are what I try to contribute when I see people getting hung up on or confused by the minutae), I know a lot of the higher level guys worry about oversimplification of the issues. It's a tough balance. I'll bring the issue up with the other mods.