The First Three Minutes - Steven Weinberg

twofish-quant

That's not the problem. What will happen is that even with decelerating universes you will have galaxies eventually go past the hubble horizon. v=Hr. You set v to c, and you'll get the radius at which galaxies are going away from you faster than light.

Now, I think he was referring to special relativistic corrections and talking about galaxies that we are observing now, but it's still potentially misleading.

phinds

Yes, clearly I didn't think it all the way through. Thanks for that correction. I was focusing on the age of the book more than the content being discussed ... big mistake.

RUTA

The v = c sphere can reside inside the particle horizon, so we can see galaxies with v > c. That is the case in the most popular model, i.e., Einstein-deSitter.

epenguin

However it was written unless there is a new edition, in 1977! When there was no data to indicate cosmic acceleration yet. And I think inflation was not yet in vogue.

Now if that disqualifies it as a strict information source I would suspect it should be longer lived than some others. Speaking from memory what was good about it was the solid and intuitive physics. An explanation of intuitively and physically why black body radiation is like it is, - instead of Rayleigh-Wien-Planck-this-formula-that-formula - helpful to any student. The explanation of how it comes about that the small nucleus (boron etc.) abundances are critically revealing of early-universe history even if his information must be out of date or limited. Again connecting with lasting parts of physics he emphasizes that after I think it is fifteen minutes from birth, known physical laws are adequate for all explanations. Maybe dark matter was not known then, maybe not much is known now - to know anything you have to use the known laws OK?

phinds

See post #17

epenguin

OK. Independent discovery.

jobigoud

Sorry if it looked like so, but I was definitely NOT referring to him not mentionning acceleration. (I have realized that acceleration of expansion has confused me before, and find it easier to first work out the concepts without it anyway.)

I was referring to the « velocities left over from a past explosion » which I think really invites the reader to think in terms of explosion in space, and « velocities gradually slowing down due to gravitation », probably my mistake, but again this invites me to think in term of an explosion, where you'd have some stuff exploding and then falling back to the ground due to gravity, except here it would be in all directions.

Even with linear expansion, (and even with decelerating expansion for that matter), isn't it better to describe expansion as some sort of cosmical stretching rather than a kind of inertial motion left over from an explosion ?
Maybe I was half asleep though

phinds

"Expansion" has several meanings/periods ... discussed here:

www.phinds.com/balloonanalogy