Will CMB ever stop being detected on Earth

[Sleve123]

Not that it has become so small it is undetectable; but the point from which it was emitted is such a distance away that the expansion of space between Earth and that point is greater than the speed of light (like swimming upstream in a river going faster than you can swim).

 

[bapowell]

The CMB was not emitted from a single point. It was emitted from every point in the universe at (roughly) the same time. The CMB photons that we detect tomorrow will simply have originated from a point a little bit further away than the point emitting the CMB photons that we detect today.

 

[Sleve123]

When I mean point then, I mean a sphere, ie the radiation coming from all directions.

I'm getting this idea from the fact that some parts of the Universe will never be observable to us on Earth because the vast distances inbetween meaning that the space is expanding faster than the speed of light.

So as time passes we are getting CBM that was a little bit further away from us because it had a 'longer time of flight', then there must be a point where the distances between the points originally was so great that the expansion of space was greater than the speed of light. So the CBM is still there, it just won't reach us.

Please try to clear any flaws I have in my thinking, thanks.

 

[bapowell]

I see your question now. Sorry for the misunderstanding. The point beyond which distant objects recede with a speed surpassing that of light is given by the Hubble radius. As long as the Hubble radius is increasing in time at a rate greater than the expansion of space, then there is no point in the universe that will not eventually become visible. In other words, while indeed there are places in the universe that are now receding from us at superluminal rates, they won't always be. Specifically, if the comoving Hubble distance is increasing, then eventually all points in the universe will be within this Hubble distance.

The above scenario corresponds to universes undergoing decelerated expansion. If the rate of expansion of the universes is accelerating (as it appears to be), then the universe possesses an event horizon beyond which things are forever unobservable. CMB photons emitted from regions of the universe outside our event horizon will never be visible to us. Our event horizon lies a bit of ways outside of our Hubble sphere.

 

[Tanelorn]

I am trying to understand the difference between the doppler effect and the expansion of space when observing the CMB frequency.

The theory says that when the universe was 379,000 years old the temperature of the comoving sphere was approx 3000K. Now the temperature is 2.7K due to the expansion of space between us and the current comoving sphere.

Could not the presently observed CMB frequency be at least partly as a result of the matter moving away from us at high relative, (doppler) velocities at the time of emission, rather than the expansion of space between us and the comoving sphere over the past 13.7B years?

How did we decide the present CMB frequency was due to the expansion of space and not related to doppler?

 

[Sleve123]

Thanks that helps a lot.

 

[bcrowell]

Could not the presently observed CMB frequency be at least partly as a result of the matter moving away from us at high relative, (doppler) velocities at the time of emission, rather than the expansion of space between us and the comoving sphere over the past 13.7B years?

How did we decide the present CMB frequency was due to the expansion of space and not related to doppler?

FAQ: What does general relativity say about the relative velocities of objects that are far away from one another?

Nothing. General relativity doesn't provide a uniquely defined way of measuring the velocity of objects that are far away from one another. For example, there is no well defined value for the velocity of one galaxy relative to another at cosmological distances. You can say it's some big number, but it's equally valid to say that they're both at rest, and the space between them is expanding. Neither verbal description is preferred over the other in GR. Only local velocities are uniquely defined in GR, not global ones.

Confusion on this point is at the root of many other problems in understanding GR:

Question: How can distant galaxies be moving away from us at more than the speed of light?

Answer: They don't have any well-defined velocity relative to us. The relativistic speed limit of c is a local one, not a global one, precisely because velocity isn't globally well defined.

Question: Does the edge of the observable universe occur at the place where the Hubble velocity relative to us equals c, so that the redshift approaches infinity?

Answer: No, because that velocity isn't uniquely defined. For one fairly popular definition of the velocity (based on distances measured by rulers at rest with respect to the Hubble flow), we can actually observe galaxies that are moving away from us at >c, and that always have been moving away from us at >c.[Davis 2004]

Question: A distant galaxy is moving away from us at 99% of the speed of light. That means it has a huge amount of kinetic energy, which is equivalent to a huge amount of mass. Does that mean that its gravitational attraction to our own galaxy is greatly enhanced?

Answer: No, because we could equally well describe it as being at rest relative to us. In addition, general relativity doesn't describe gravity as a force, it describes it as curvature of spacetime.

Question: How do I apply a Lorentz transformation in general relativity?

Answer: General relativity doesn't have global Lorentz transformations, and one way to see that it can't have them is that such a transformation would involve the relative velocities of distant objects. Such velocities are not uniquely defined.

Question: How much of a cosmological redshift is kinematic, and how much is gravitational?

Answer: The amount of kinematic redshift depends on the distant galaxy's velocity relative to us. That velocity isn't uniquely well defined, so you can say that the redshift is 100% kinematic, 100% gravitational, or anything in between.

Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97, msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf

 

[Tanelorn]

Hi Ben, Davis and Lineweavers explanations sounds like a cop out to me, almost like saying it is what it is because we say so.

In the real world (universe) the doppler effect is real. Therefore in order to estimate the expansion of space and dark energy acceleration of the expansion of space, someone somewhere would have had to estimate the doppler contribution to the change in the CMB frequency? Surely?

Space could be not expanding at all and yet the frequency of light could be just as red shifted just due to relative velocity of matter. Yet cosmologists say that it is space that is expanding, therefore what is their reasoning for believing that it is the space that is expanding?