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camilus
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I read it is possible for the universe to be expanding faster than the speed of light.
What would be the implications of this?
What would be the implications of this?
The speed limit of the universe applies locally. The expansion of the universe is the expansion of the space - no object is moving faster than c locally, so it does not violate SR.camilus said:I read it is possible for the universe to be expanding faster than the speed of light.
What would be the implications of this?
DaveC426913 said:However, the expansion will prevent us from seeing anything that's moving away faster than c.
Search this forum for "superluminal expansion".
DaveC426913 said:The speed limit of the universe applies locally. The expansion of the universe is the expansion of the space - no object is moving faster than c locally, so it does not violate SR.
However, the expansion will prevent us from seeing anything that's moving away faster than c.
Search this forum for "superluminal expansion".
camilus said:Im going to do the search now, thanks.
DaveC426913 said:...
However, the expansion will prevent us from seeing anything that's moving away faster than c.
...
That one always throws me. I defer to greater wisdom.sylas said:Most of the galaxies we see in deep space are "moving" (or receding) faster than the speed of light. They have always been receding faster than the speed of light.
You need to be careful. There are sometimes people who confidently make statements that are not true. Sorry Dave, you just did this here yourself.
camilus said:Thats my initial thought as well. But if the rate of this expansion is in fact superluminal, what can we say about the size of the universe? Cant it be a lot bigger than we think. since we can't see that far?
camilus said:I read it is possible for the universe to be expanding faster than the speed of light.
What would be the implications of this?
Phrak said:Are the values of velocities for distant objects and radiation intrinsic properties, or are they an artifact of the map?
marcus said:Once you choose a map, then everything you derive from using that map must derive ipso facto from that map.
That's not the point. The question is what kind of coordinates and what kind of basic distance measure do cosmologists actually use. We have an educational responsibility to use terms and concepts compatible with normal cosmology. They use the Friedman metric and the Friedman equations are the basic model.
camilus said:I read it is possible for the universe to be expanding faster than the speed of light.
What would be the implications of this?
I didn't read between the lines. What did I not see.
I don't think this is actually true. First, I'd like to make a pedantic point:sylas said:Most of the galaxies we see in deep space are "moving" (or receding) faster than the speed of light. They have always been receding faster than the speed of light.
sylas said:Most of the galaxies we see in deep space are "moving" (or receding) faster than the speed of light.
...
Chalnoth said:I don't think this is actually true.
... it is just patently incorrect to state that we can see objects today there have always been receding at faster than the speed of light.
But what if the object today is expanding faster than light, and always will be? Well, in that case, we can never observe it.
Chronos said:Other interpretations are possible, marcus. I perceive no need to portray any particular model as the only one worth considering.
yogi said:I would agree - and while having learned much from reading marcus's informative and well written tutorials, the standard model of mainstream cosmology should be qualified..
Chalnoth said:I don't think this is actually true. ...
[...] First, I'd like to make a pedantic point:
In General Relativity, the operation of subtracting one velocity from another to get relative motion is only well-defined at a single point. So if I want to talk about recession velocity, it turns out that the question is very ambiguous, and whether or not such velocities exceed the speed of light just depend upon what ambiguous definition we choose. [...]
The actual speed of light limitation in GR is that no object can outrun a light beam. That is, no matter where the object is, and no matter what your perspective of looking at that object is, that object will always be moving more slowly than the photons in that object's vicinity.
Now, with that out of the way, we can define a very simple recession velocity:
v = Hd.
This is sort of what we naturally think of when we think recession, and I'll use this definition from here on out. With this definition, the distance at which things start to exceed the speed of light is about 14 billion light years away. As marcus points out, in terms of redshift this is about z = 1.5 or so. We currently can see galaxies out beyond redshift 7, and the CMB is at a whopping redshift of 1090.
The solution to this apparent problem is just that we have to take into account how the universe has expanded from the time the light was emitted to now. To take a look at the distance today of these objects, and notice that the Hubble law predicts a recession velocity greater than c, and then scratch our heads wondering why we can see these objects, is missing the point.
First, we can't say, without looking in more detail, whether or not these objects were receding faster than light. They might have been: as long as the universe reduced its expansion enough in the interim, we might see them today.
But what if the object today is expanding faster than light, and always will be? Well, in that case, we can never observe it. ...
Chalnoth said:Ah, right, sorry. I was failing to take into account an important fact: once the photon has left the object in question, it doesn't actually matter what that object does. It matters, instead, what the photon does. Even if the object retains a recession velocity greater than c, a photon which was emitted some time ago will be closer to us than the galaxy, and so won't have the same recession velocity to fight against, and so may well reach us.
However, far enough away, and it will always have more space to travel ahead of it than it crosses, and so it will never reach us.
Certainly :)sylas said:That is a really clear and concise statement. I'm going to steal it for future use, with your permission!
Quite right. I addressed this point in my previous post, but was just a bit short for the sake of brevity here. What I said is accurate if the correct explanation for the accelerated expansion is dark energy or something similar. Otherwise...it depends.sylas said:It depends on how the expansion rate develops. In a matter dominated universe, or an empty (constant expansion) universe, then no; photons from arbitrarily far away will reach us.
Yes, actually, this one I'm certain about. I'm not used to thinking in terms of recession velocity, hence my previous mistake. But this is a standard result for de Sitter cosmology (which our universe will asymptotically approach if dark energy = cosmological constant): light can only travel a finite distance in comoving coordinates in de Sitter space.sylas said:But in the current consensus model, with dark energy and subcritical matter, I think you may be correct... but I am not sure. I have not tried to prove it one way or the other. Marcus might know...
Chalnoth said:Of course, we're not in a perfectly de Sitter universe, so this isn't actually true: H is decreasing with time, which corresponds to a somewhat larger distance at which things will eventually communicate. Using [tex]\Omega_m = 0.27[/tex] and [tex]\Omega_\Lambda = 0.73[/tex], I get [tex]r = 1.12 \frac{c}{H_0}[/tex]. So objects currently receding up to about 12% higher than the speed of light are emitting photons that we will detect at some point. But beyond that, we can never see those photons (and bear in mind that this is assuming the cosmology is accurate, which is by no means certain).
Well, no, actually it doesn't mean this. Because the Hubble parameter has been decreasing with time, galaxies that have always been receding faster than c, and are currently doing so, are actually visible. There is a limit to how far we can see, but it's not so simple or obvious. Instead, the limit to our vision is the CMB (before which the universe was opaque), which denotes a surface that currently lies at roughly 45 billion light years away or so.Naty1 said:They fact that distant galaxies are expanding at a rate greater than "c" does not mean we can never observer them, only that we can't observe some right now.
Chalnoth said:Using [tex]\Omega_m = 0.27[/tex] and [tex]\Omega_\Lambda = 0.73[/tex], I get [tex]r = 1.12 \frac{c}{H_0}[/tex]. So objects currently receding up to about 12% higher than the speed of light are emitting photons that we will detect at some point. ...
sylas said:We are again all on the same page... I've not actually gone through the with calculation myself of 1.12 you give for the horizon of visibility in this model.
marcus said:I guess the asymptotic value for H(t) will be about sqrt(.75) times 74. Let us see what that is. It comes out to 64. So H(t) will decline from present 74 and approach 64 from above. This is just what the standard model predicts. Of course we do not know the future.
But going by the standard model, we see that the Hubble distance still has another 15 percent to expand. From the present 13.2 out to 15.3 billion LY.
That means that any photon which can manage to stay within 15.3 billion LY of us, and is heading our way, will eventually make it to us.
marcus said:All this discussion assumes we are talking proper distance. Here's where I calculated that 15.3. It's trivial. In the LCDM the energy density eventually goes to 0.75 of what it is today and putting that into the Friedman equations tells what the Hubble rate will be in the limit.
camilus said:I read it is possible for the universe to be expanding faster than the speed of light.
What would be the implications of this?
Naty1 said:Some implications (back to the original question):
I'll bet there are many implications that have not yet even been thought through nor even conceived! (let's hope so.)
We are likely able to currently observe only an infinitesimal portion of our own universe...
4D-UGRA said:please correct me if I am wrong everyone, but something like 46.5 billion light-years in all directions. This means that we can not and will not see any light from any further away then that.
4D-UGRA said:All of this in-fighting...
4D-UGRA said:Do a search on "particle horizon"
4D-UGRA said:It does appear that space can and does exceed the speed of light
4D-UGRA said:Camilus.
I have wondered if a serial set of hubble-like telescopes could be positioned throughout space in a line, and if they could broadcast images from beyond this X=46.5 billion light-year visible limit, since at some point the telescopes would pass into their own visible limit and may not be able to see Earth anymore, but could see one to the satilites/telescopes and relay data to that device. Allowing us to break this barrier. "
When you consider that the farther away telescope would send its signal back at the speed of light, and will at best add a small delay in sending said signal, there's no way the signal from the telescope can get back before the photons from the object the telescope is observing get back. So yeah, clearly won't work.yogi said:Unfortunately, that will, in general, not work - take a look if you have access to the Harrison Book "Cosmology" at page 441 The horizon limitation cannot be extended by the farther away telescope sending signals to the nearer telescope
camilus said:I read it is possible for the universe to be expanding faster than the speed of light.
What would be the implications of this?
The magnitude of the cosmological constant so small that I'm not sure that we'll ever be capable of measuring it.yogi said:In the context of the original question posed - the answers have been aimed mostly to academic topics such as how much we can see and how big is the universe. But there are some other issues embraced within the inquiry - for example - if we live in an accelerating universe - will there be any measurable affect upon the gravitational constant - or inertia ...Einstein reasoned that the same Newtonian forces would arise if the universe itself were accelerated rather than the local mass - if that is correct, changes in the expansion rate and changes in the distance at which global acceleration is communicated to local matter may have an influence upon local measurements, at least if one is of a bias that regards the universe as holistic.
objects currently receding up to about 12% higher than the speed of light are emitting photons that we will detect at some point. ...