The Shape of Spacetime.

phinds

Quote by denism

I asked this question because I read an other interpretation (with which I have some concerns) that the unobservable universe located behind the horizon, corresponds to galaxies for which the recession velocities exceed light speed. Is it erroneous according to you?

Not only is it NOT erroneous, it is an understatement in that galaxies IN the observable universe are "now" ("now" gets a bit tricky) are already receding from us FTL. In fact, those at the edge of the OU are receding at about 3c

denism

Quote by phinds

Not only is it NOT erroneous, it is an understatement in that galaxies IN the observable universe are "now" ("now" gets a bit tricky) are already receding from us FTL. In fact, those at the edge of the OU are receding at about 3c

I definitely cannot understand this point. My intuitive expectation was that the expansion rate cannot exceed c because it it was the case, everything would disconnect at once, even between the sun and earth and between your eyes and your screen..

Did you read the problem of the ants on a rubber rope <http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope>

phinds

Quote by denism

I definitely cannot understand this point. My intuitive expectation was that the expansion rate cannot exceed c because it it was the case, everything would disconnect at once, even between the sun and earth and between your eyes and your screen..

Did you read the problem of the ants on a rubber rope <http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope>

The first thing you need to do when studying either cosmology or quantum mechanics it TOTALLY get rid of the concept that your intuition is worth squat. It is not. It's something we all have to get used to.

What you are not understanding is that nothing is traveling FTL in the same reference frame. The universe is expanding. The recession rate has nothing to do with c.

Drakkith

Quote by denism

I definitely cannot understand this point. My intuitive expectation was that the expansion rate cannot exceed c because it it was the case, everything would disconnect at once, even between the sun and earth and between your eyes and your screen..

Did you read the problem of the ants on a rubber rope <http://en.wikipedia.org/wiki/Ant_on_a_rubber_rope>

The expansion is a rate, meaning that the recession velocity increases over distance. The further away something is the faster it recedes. So a galaxy can receded from us at 0.01c that is relatively nearby while another that is very very far away can recede at 2c or 3c or whatever. Note that this also includes light emitted from those galaxies. A galaxy receding from us at 2c that emits light away from us would NOT catch up to the light. In the galaxies frame of reference it is stationary and the light moves at 1c away from it.

denism

I know that v=H*D. If the Hubble constant was constant, this would lead to an exponential recession.
I understood how H is determined (through the variations of redshifts as fonction of distance). I also see how distances can be determined.
But I dont know how recession velocities are measured? do you knwo?

Chronos

There are no galaxies residing beyond the surface of last scattering whose photons have not yet reached us. Nothing but hot plasma exists beyond z~1100 [source of the cmb].

Whovian

Recession velocities are measured via redshift. Research the Doppler Effect to understand why Redshift comes about. We can determine which spectral emission lines we can expect to see in stars of certain materials (I can't remember how,) and see how redshifted the stars are. Plug into our easily derivable formula for the relativistic doppler effect, and we're done.

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denism	I know that v=H*D. If the Hubble constant was constant, this would lead to an exponential recession. I understood how H is determined (through the variations of redshifts as fonction of distance). I also see how distances can be determined. But I dont know how recession velocities are measured? do you knwo?

Chronos Recognitions: Gold Member Science Advisor	There are no galaxies residing beyond the surface of last scattering whose photons have not yet reached us. Nothing but hot plasma exists beyond z~1100 [source of the cmb].

Whovian	Recession velocities are measured via redshift. Research the Doppler Effect to understand why Redshift comes about. We can determine which spectral emission lines we can expect to see in stars of certain materials (I can't remember how,) and see how redshifted the stars are. Plug into our easily derivable formula for the relativistic doppler effect, and we're done.