What Causes Red Shift in Cosmology: Expansion of Space or EM Red-Shift?

[jerromyjon]

Does the expansion of space cause cosmological red-shift or does EM red-shift cause space to expand?

 

[Nugatory]

The expansion causes the red shift.

 

[jerromyjon]

If expansion causes the wavelength of EMR to shift what would have happened in the early universe when inflation expanded space at such a phenomenal rate? Were there even photons at this point or just gravitational waves and density waves?

I know this is speculation and since I'm not nearly that good with the math I can't even use my intuitive power of estimation to get a grip on the concepts involved. The best description I've found of the events of the early universe, as the theory goes, doesn't go into much detail. Quoted from wikipedia: "Approximately 10−37 seconds into the expansion, a phase transition caused a cosmic inflation, during which the universe grew exponentially. After inflation stopped, the universe consisted of a quark–gluon plasma, as well as all other elementary particles."

It goes on a bit about annihilation and slight imbalances leading to the dominance of matter constituents and continued: "A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos)."

So the first official mention of photons isn't until after 1 second but annihilations create photons and heat which is mentioned after the quark-gluon plasma and other elementary particles. The first mention of the CMB isn't until 380,000 years later and that's when I get a good grasp on the evolution up to present. Polarization, doppler shift and gravitational shifting all make perfect sense to me... It is only cosmological shift I'm having trouble with.

 

[PeterDonis]

If expansion causes the wavelength of EMR to shift what would have happened in the early universe when inflation expanded space at such a phenomenal rate?

If photons were present, they would redshift at a phenomenal rate. However, as far as we can tell, they weren't present; see below.

Were there even photons at this point or just gravitational waves and density waves?

As I understand it, there weren't photons, and there also weren't gravity waves or density waves as we normally understand those things. The universe was filled with what's called an "inflaton field" (at least in the simplest model of inflation), which is something like a very large and positive cosmological constant (or vacuum energy, but in a "false vacuum" state where the vacuum energy in a given volume of space is much, much, much larger than it is now). The only things like "waves" that were present were quantum fluctuations in that inflaton field. Those fluctuations were indeed amplified (i.e., their wavelength increased) by a very large factor during the inflation period, because of the spatial expansion; when inflation ended, those fluctuations led to small variations in the density of the quark-gluon plasma, which in turn led to small variations in the CMBR when that was formed (and which the Planck satellite was able to observe).

 

[jerromyjon]

http://en.wikipedia.org/wiki/File:History_of_the_Universe.svg

I'm not sure if it is better to link the file or upload it but, anyway, I found this very detailed image which clears up most of my questions. According to this photons formed at 1 microsecond after the big hot mess started, but I haven't found anything about the forces diverging.

You may have left out a "much" or two about the vacuum energy, from what I understand it's much, much higher than what makes logical sense today!

Thanks for the help and I'm sorry if I am being a pain... I may not be able to prove much yet mathematically but I am working on it and trying to keep it all in perspective.

 

[timmdeeg]

The universe was filled with what's called an "inflaton field" (at least in the simplest model of inflation), which is something like a very large and positive cosmological constant

To my understanding the universe expands exponentially in case the cosmological constant is its only ingredient. So, having in mind that the 'condensation' of matter happened at the end of the inflationary era, the amount of the positive cosmological constant should be arbitrary then. Or do I miss something? I think only today - knowing more about the dynamics and the matter content etc. - we identify a certain amount of said constant.

 

[Orodruin]

Even if there were photons and/or gravitational wave and/or matter before the inflationary epoch, it would be quickly diluted (and red shifted in the case of radiation components) away by the expansion. What remains after inflation is essentially only the latent heat of the inflaton phase transition, which reheats the universe, and possibly quantum fluctuations from the inflationary epoch that are blown up out of proportion by inflation itself.

 

[PeterDonis]

the universe expands exponentially in case the cosmological constant is its only ingredient

Yes.

having in mind that the 'condensation' of matter happened at the end of the inflationary era, the amount of the positive cosmological constant should be arbitrary then.

Not really. During the inflationary era, before any matter/energy "condenses", the "amount of cosmological constant" is constant, and large. That equates to a large positive energy density everywhere in the universe. What happens when the inflationary era ends is that most of that large positive energy density converts from cosmological constant to ordinary matter and radiation (quark-gluon plasma, plus other stuff). The value of the positive cosmological constant after that happens is whatever is left over, i.e., whatever is not converted to ordinary matter and radiation. AFAIK we have no way of predicting theoretically how much that would be (how much gets converted vs. not); our only way of knowing is by measuring the cosmological constant today, and assuming that there have been no further "phase transitions" in it since inflation ended (which is a reasonable assumption), so its value now is the same as its value right after inflation ended.

 

[timmdeeg]

I agree. I didn't make clear that my (quite unimportant) point was related to an arbitrary universe, not to our universe in particular.

 

[Torbjorn_L]

The only things like "waves" that were present were quantum fluctuations in that inflaton field.

The expansion would supercool the universe, and else dilute any inflaton particles however created I take it.

If I remember Susskind's cosmology lectures correctly, when inflation stops part of the excitations will be inflaton particles before the potential energy is fully gone. But they are heavy and short-lived and contribute to the heating mechanism. The mass comes from the rapid drop in potential (high effective mass in steep potential wells), but the short lifetime I'm not sure on, not having studied particle physics. [Is it that the new field value does not support stable inflatons? Not a red shift question, I realize.]

 

[PeterDonis]

The expansion would supercool the universe, and else dilute any inflaton particles however created

But this does not affect the vacuum expectation value (VEV) of the inflaton field, which is what leads to the effective cosmological constant. The "inflaton particles" are quantum excitations of the field over and above the VEV; supercooling those excitations has no effect on the VEV itself, which is constant during the entire inflation era (i.e., until the phase transition happens that ends inflation).

 

[Orodruin]

Well, not constant, but slow rolling, or inflation would never end. Unless you go back to old inflation and as far as I remember that did not really work.