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Cheap Astronomy
2009-Apr-26, 09:40 AM
Hi,

I'm down to do a talk on neutrinos in the early universe and suspect my preliminary research has failed to grasp the key issues.

I get that some time during the first second or the second second - and well after cosmic expansion - you have a decoupling of neutrinos which shoot out into the presumably fairly empty space-time of the newly inflated universe - giving the hypothesised cosmic neutrino background - since they move at close to the speed of light, don't interact with anything much and photons are still caught up interacting with matter and don't shoot out into the void for another 400,000 years.

So, all very interesting, but so what? I get the impression that the early emergence of neutrinos might have had deep and meaningful effect on subsequent events, maybe even the shape and nature of the universe that now is. But what exactly are the key linkages that apparently make this such a tantalising puzzle for astronomy?

So, my question is - what's so puzzlingly strange about neutrinos in the early universe??

Cougar
2009-Apr-26, 01:31 PM
So, my question is - what's so puzzlingly strange about neutrinos in the early universe??

I don't think it's that they're so strange, but it's what the neutrino background radiation might tell us. Currently, our deepest observations are restricted to the photons of the CMB, as you say, ~400,000 years after the point of real interest. Look what careful measurements of the CMB have been able to tell us. If we could detect the neutrino background, we could have direct observations of the universe much earlier than 400,000 years and be able to infer much about what had to be going on at that very early time.

Ken G
2009-Apr-26, 09:19 PM
Yes I agree, it's not that the neutrinos are doing anything important, it is that they carry information we want to extract. But I'm not aware of any serious suggestion that we will ever be able to directly detect the cosmic neutrino background and extract that information. As such, it's a lot like dark matter, except that neutrinos themselves have indeed been detected in other contexts (lab experiments, the Sun, supernova, cosmic rays-- just not the cosmic background). The bottom line is, you do indeed have a problem on your hands: you are giving a talk about a population that itself does not interact with anything in any important ways, and is not directly detectable in any way! Good luck.

(Seriously, you might take a historical perspective of how amazing it is that the discovery of the neutrino happened after its existence had been postulated based on failures to conserve energy in particle experiments. So we have a case where theory preceded observation. Then you can point out that Einstein's relativity also would have predicted a dynamical, not static, universe, even though he himself resisted the idea so lost that opportunity. So when united, we have two situations where purely theoretical predictions were trying to tell us something about actual reality, before it was observed, and the result is the expectation of a cosmic neutrino background-- even before we can observe it. Sadly, we won't be observing it any time soon!

Or, alternatively, you can look at the importance of the neutrino as a channel for carrying away energy. When neutrinos decoupled from matter, you had a sudden loss of energy that was shunted off into a reservoir with which normal matter had no further contact. That must have left an imprint on the brightness of the radiation field at the time, and on the subsequent freezing out of other types of matter, so could be seen as a way to indirectly infer the existence of the neutrino population. So the neutrinos did do something, it's just that they did it when they were created, it's not something they actually themselves did having been created. So far as I know anyway, perhaps your research will point you in a very different direction! )

blueshift
2009-Apr-27, 03:00 AM
All good comments above. Here's my two cents.

To add, primordial neutrinos would have had the greatest gravity wells to pull away from in the early universe and, hence, should be the most redshifted. A lot also depends upon when gravity froze out and when the first neutrinos were realized.The trouble with all of that is detection. It seems difficult enough right now to detect but a few each year. Newer technology will develope better detectors.

Ken G
2009-Apr-27, 05:22 AM
A few a year, that's not hopelessly bad if it is above instrumental background (and the solar flux) so one can really associate it with the cosmic background. Are there ways to control against the solar neutrino background?

Kwalish Kid
2009-Apr-27, 03:15 PM
I get that some time during the first second or the second second - and well after cosmic expansion - you have a decoupling of neutrinos which shoot out into the presumably fairly empty space-time of the newly inflated universe - giving the hypothesised cosmic neutrino background - since they move at close to the speed of light, don't interact with anything much and photons are still caught up interacting with matter and don't shoot out into the void for another 400,000 years.
I don't really like the "shoot out into the void" language. The neutrinos, like the CMB, don't escpae from an area of density. Rather, at some point, the universe loses enough density that it becomes a void, as far as the particle is concerned. The neutrinos and the CMB, when they are released, still fill the same area that they always did (though technically, we should think of it as a co-moving volume).

Ken G
2009-Apr-27, 04:47 PM
The neutrinos, like the CMB, don't escpae from an area of density. Rather, at some point, the universe loses enough density that it becomes a void, as far as the particle is concerned.That's true, the neutrinos do not spatially travel between regions of different density, they merely find themselves in a region of changing density, which eventually becomes sparse enough that the neutrinos don't interact any more (and "stream out into the void").

The neutrinos and the CMB, when they are released, still fill the same area that they always did (though technically, we should think of it as a co-moving volume).That isn't true-- when the density is high, they are constrained to a very small comoving volume, and when the density is low, they really do stream out into far-flung regions that are quite distant from where they were born. Their motion starts out like a drunken person staggering around randomly and not going very far (except insofar as that region is expanding), but ends up like they "sobered up" and are moving in a straight line, covering vast distances even against the prevailing expansion. So there is some validity to the "out into the void" picture.

trinitree88
2009-Apr-27, 10:50 PM
That's true, the neutrinos do not spatially travel between regions of different density, they merely find themselves in a region of changing density, which eventually becomes sparse enough that the neutrinos don't interact any more (and "stream out into the void").
That isn't true-- when the density is high, they are constrained to a very small comoving volume, and when the density is low, they really do stream out into far-flung regions that are quite distant from where they were born. Their motion starts out like a drunken person staggering around randomly and not going very far (except insofar as that region is expanding), but ends up like they "sobered up" and are moving in a straight line, covering vast distances even against the prevailing expansion. So there is some validity to the "out into the void" picture.

Ken G. True enough. There's a corollary here, too. One frequently hears the age of hydrogen recombination as ~380-400,000 years post BigBang. That would give the neutrinos a big head start in their rabbit and hare race. If the neutrinos have rest mass, the photons should overcome that early lead, eventually. But, in a recent article in Discover, Sunyaev postulate an interesting idea. There was some helium in the early universe, too. With twice the coulombic charge on it's nuclei, He-4, and He-3...helium captures it's first electron, turning a He-4++ ion into a He-4+ ion, at about 18,000 years post-BB, and captures it's second electron, becoming neutral He-4 at ~ 100,000 years. (Similar temporal data for He-3).
These two wavefronts should overtake the neutrinos before the hydrogen, if they're subluminal, and the singly ionized He-3 is polarizable, showing a distinct dipole in the symmetry of it's electromagnetic and weak interactions (Bates Accelerator, He-3 partially polarized gaseous target study, 1991).
Sunyaev has proposed instrumentation examining the same spatial context of the CMB, while varying the frequency, as opposed to the WMAP/Planck satellites which examine the same sweep of frequency over a varying spatial context, which showed up the angular anisotropies as expected. He likens it to searching for a small blip on a very smooth surface by running your fingers over it. The instrumental question is whether or not a signal is lost in ~ a billion photons of noise. Interesting, and might give data on two more temporal epochs. pete

Cheap Astronomy
2009-Apr-28, 11:09 AM
Hi

Thanks everyone. I am thrilled to hear that something I cobbled together from some Wikipedia searches "may have some validity". Got to love this forum.