Is the Universe Expanding Faster Than the Speed of Light?

[Chaos' lil bro Order]

Seeing objects receeding faster than the speed of light is not a problem - which includes all objects with a redshift [z] more than about 1.6. We routinely observe objects in visible wavelengths up to z~10. We also routinely detect photons in the microwave band that were emitted from the surface of last scattering [i.e., the CMB], which is receeding at z~1100. This is perfectly fine under the rules of GR. Take a look at:
http://www.astro.ucla.edu/~wright/cosmolog.htm

We can see as much of the universe now as we could ever have seen since one Planck tick of time after the big bang - always have, and always will. Distant objects do not disappear from view, they merely redshift and time dilate into frozen obscurity.

I am curious what objects you are referring to that we routinely observe in visible wavelengths of up to z~10. As far as I know, the highest redshift observed of a galaxy is a bit under z=7. Did Ned Wright say this or where did you get this information?

 

[Chronos]

Correct. Around 6.8 is still the reigning champion. I merely wished to emphasize the point we can [and forever will] continue to see objects receeding at velocities >c.

 

[hellfire]

What about a star at a distance of 8 billion light years, that was formed 4 billion years ago? Is that dark matter?

It is not, because stars usually form in giant molecular clouds that are part of the interstellar medium within galaxies. The density of the interstellar medium is known and is assumed to comprise about 5% of the mass of the visible stars in a galaxy. Simulations and some observations suggest that most of the baryonic dark matter is located in the intergalactic medium. Moreover, most of the dark matter that contributes to the rotation curves of galaxies is assumed to be non-baryonic.

We can't see it, because the light hasn't gotten the time yet to reach us. Is this star part of the "visible" universe?

Is the visible universe merely limited by distance or is it literally that what we can actually see?

The term visible universe or observable universe is independent of our current tecnology and relates to the current distance from which the most distant photons we should be able to detect today were sent.

 

[marcus]

PhysicsLearner, I didn't mean to suggest that dark matter is matter that is beyond the event horizon (whatever that is estimated to be, something like 60 billion LY IIRC)

dark matter is a "different matter"-----our own galaxy preumably has lots of it, if that turns out the be the correct explanaition of how galaxies manage to whirl so fast and yet hang together

now hellfire (who is the guru) has checked in so you should address your questions to him and the others. I'll migrate to another forum where I can be of more use

 

[Physics-Learner]

hi marcus,

i wasnt sure if you were suggesting that or not. but i guess our understanding is the same, in that if it exists, then it is everywhere. this is what i had read and seen it to be.

 

[Peter (IMC)]

Thanks Hellfire, Your explanations are very much appreciated!


Suppose you´re a particle traveling at light speed... What does space-time look like to you? Is it simply not observable?

Then suppose you´re slowing down. How do you observe space-time during the slowing down process? Does it seem to suddenly come into existence and start expanding?

If you're just you and you´re slowing down, does the universe seem to be expanding?

Suppose you´re still just you and stop slowing down, does the universe seem to stop expanding?

Suppose you´re speeding up, does the universe seem to be contracting?

If I understood it correctly, slowing down is only possible if you can do so relative to something else. so perhaps the above makes absolutely no sense at all. But I haven't figured out yet why it doesn't make sense. If anybody can help?

 

[hellfire]

Cosmological expansion is a general relativistic effect. If I understand correctly you are thinking about Lorentz contraction or something similar. Note that unlike cosmological expansion this effect is not isotropic and is only in the direction of motion. But, in general, to explain features of the universe at large scales one must use of general relativity instead of special relativity.

 

[Peter (IMC)]

Cosmological expansion is a general relativistic effect. If I understand correctly you are thinking about Lorentz contraction or something similar. Note that unlike cosmological expansion this effect is not isotropic and is only in the direction of motion. But, in general, to explain features of the universe at large scales one must use of general relativity instead of special relativity.

What if your direction is constantly changing?

 

[Physics-Learner]

there is no way to change direction without accelleration.

 

[hellfire]

What if your direction is constantly changing?

Special relativity is not appropriate to explain expansion. What has to be explained is cosmological redshift, as well as cosmological time dilation, Tolman's surface brightness variation, etc. which are phenomena that require of general relativity to be explained.

 

[Peter (IMC)]

Thanks hellfire, I know where to do some reading now,. :)

I´m just confused I guess by that an electron is going around the core of an atom at light speed. Which seems to be an impossibility because being in orbit means changing direction all the time.

I know there´s that probability cloud of where the electron could be, but that too seems to make sense to me. If it is going at light speed and time stands still for it (compared to us) then time around it is happing all at once from the electron point of view... so from our point of view he´s in all places it's ever been at the same time. In fact, it feels to me like he could be anywhere in the universe as it's all the same place to him, a dot.

But I guess here too I need to do some more reading on when to use general relativity and when to use special relativity.

general and special relativity, that already sounds like a paradox as well... ( :) Just kidding )

 

[Chronos]

Lets not mix GR with QT, they don't play well together. Most physicists would object to the characterization of an electron as 'orbiting' an atomic nucleus. This is a pretty basic discussion, but captures the issues:
http://www.colorado.edu/physics/2000/quantumzone/frequency2.html