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Bogie
2007-Feb-06, 06:41 PM
http://www.mpi-hd.mpg.de/hfm/HESS/HESS.html

This quote comes from the H.E.S.S. page, "April 2006: Measurements of two distant quasars by H.E.S.S. reveal that intergalactic space is more transparent to gamma-rays than previously thought."

What is the definition of "transparent"? How does "transparent" make a difference in the context that it is used in that quote?

In other words, what is there about intergalactic space that would make it less transparent to less energetic radiation if anything, i.e. what is the characteristic of gamma-rays that makes space more transparent to them than to less energetic particles?

Ken G
2007-Feb-06, 07:13 PM
Transparent just means that light can pass through without interacting with anything. The "thing" the gamma rays would interact with, in this case, is something quite unusual-- other light! But if a high enough energy gamma ray encounters another photon (here a background photon from starlight), it can create an electron-positron pair. That would destroy both the gamma photon and the background photon, and the signal of this removal of gamma photons could then be used to infer the presence of the starlight photon. (It's not so easy to directly observe starlight from ultra-distant galaxies, so this is an indirect way to do it, because it is easy to observe bright gamma-ray sources.) So "starlight" is what there is in intergalactic space that makes it less transparent to gamma rays than to lower energy photons (your question had this backward, the gamma rays are expected to be more likely to interact, because they have enough energy to make particles, it's just not quite as likely as thought, apparently).

Argos
2007-Feb-06, 07:29 PM
From the article you referenced


A beam of gamma-rays from a distant galaxy is thus attenuated in its way to the Earth, owing to these collisions with the diffuse-light photons. The effect is stronger for more energetic gamma-rays, so the original gamma-ray spectrum gets "reddened", somewhat like the Sun looking redder at sunset because the blue light is more heavily scattered by the atmosphere than the red light. Since the "reddening" depends on the thickness of the absorber (the density of the background light photons, in this case), a measure of this thickness becomes possible.

dhd40
2007-Feb-06, 07:43 PM
http://www.mpi-hd.mpg.de/hfm/HESS/HESS.html

This quote comes from the H.E.S.S. page, "April 2006: Measurements of two distant quasars by H.E.S.S. reveal that intergalactic space is more transparent to gamma-rays than previously thought."

What is the definition of "transparent"? How does "transparent" make a difference in the context that it is used in that quote?

In other words, what is there about intergalactic space that would make it less transparent to less energetic radiation if anything, i.e. what is the characteristic of gamma-rays that makes space more transparent to them than to less energetic particles?

Your link doesn´t say that intergalactic space is less transparent to less energetic radiation, it only says that intergalactic space is more transparent to gamma-rays than previously thought
But what I don´t understand is the mechanism they propose. I´ve never heard of pair production by the interaction of two photons, only by photons interacting with electric fields (atomic nucleus, electron). But I may be wrong.
Would like to hear more about this from the experts.

Oh, I just see that Ken G already answered your and my questions. My typing speed is far too slow

Bogie
2007-Feb-06, 07:53 PM
Your link doesnīt say that intergalactic space is less transparent to less energetic radiation, it only says that intergalactic space is more transparent to gamma-rays than previously thought
But what I donīt understand is the mechanism they propose. Iīve never heard of pair production by the interaction of two photons, only by photons interacting with electric fields (atomic nucleus, electron). But I may be wrong.
Would like to hear more about this from the experts.

Oh, I just see that Ken G already answered your and my questions. My typing speed is far too slowMine too :).

Thanks. OK, so gamma-rays that come through intergalactic space (space between galaxies) is expected to encounter photons on the way. These "background" photons cause intergalactic space to be less transparent and the degree of transparency depends on the number of photons that are likely to be encountered.

So when space turns out to be more transparent than expected it means that there are fewer background photons in intergalactic space than expected.

Is there an implication from this observation about intergalactic space other than it contains fewer background photons than thought? Does it mean that the universe is somehow different than previously expected and does that tell us anything new about the early universe?

Nereid
2007-Feb-06, 08:48 PM
Mine too :).

Thanks. OK, so gamma-rays that come through intergalactic space (space between galaxies) is expected to encounter photons on the way. These "background" photons cause intergalactic space to be less transparent and the degree of transparency depends on the number of photons that are likely to be encountered.

So when space turns out to be more transparent than expected it means that there are fewer background photons in intergalactic space than expected.

Is there an implication from this observation about intergalactic space other than it contains fewer background photons than thought? Does it mean that the universe is somehow different than previously expected and does that tell us anything new about the early universe?There are several things it may mean ... the relevant scattering cross-sections may be different than expected, the background radiation field may be different (in one of many possible different ways) than expected, the source may be more luminous than expected, the experimental design (a.k.a. the whole theory chain leading to the conclusions reached by the H.E.S.S. team, wrt high energy gamma detections) may be (slightly) wrong, ...

This is very much cutting-edge research, and way, way beyond anything that can be directly checked by experiments in earthly labs (the relevant parameter space is just too far from anything we can create here).

Bogie
2007-Feb-06, 09:01 PM
There are several things it may mean ... the relevant scattering cross-sections may be different than expected, the background radiation field may be different (in one of many possible different ways) than expected, the source may be more luminous than expected, the experimental design (a.k.a. the whole theory chain leading to the conclusions reached by the H.E.S.S. team, wrt high energy gamma detections) may be (slightly) wrong, ...

This is very much cutting-edge research, and way, way beyond anything that can be directly checked by experiments in earthly labs (the relevant parameter space is just too far from anything we can create here).Thanks. So the H.E.S.S. array is telling us things we had no way to know before H.E.S.S.? And being cutting edge, If we keep abreast of H.E.S.S. findings we might see science unfolding from its observations as time goes on, right? I'll check it out from time to time. Thanks for providing the link (elsewhere).

Ken G
2007-Feb-06, 09:11 PM
I think the big question they were after was, after we account for all the "mundane" types of starlight we expect to be lingering around in intergalactic space, will there also be additional "more interesting" sources of light, that is still too dim to see directly? It sounds like one important speculation was that "first stars", big superbright monsters that were not necessarily associated with the first galaxies, might have contributed a lot of excess background light, and this observation would seem to be discouraging in regard to that speculation.

Nereid
2007-Feb-06, 09:21 PM
Thanks. So the H.E.S.S. array is telling us things we had no way to know before H.E.S.S.? And being cutting edge, If we keep abreast of H.E.S.S. findings we might see science unfolding from its observations as time goes on, right? I'll check it out from time to time. Thanks for providing the link (elsewhere).There are several other CATs, including CANGAROO (http://icrhp9.icrr.u-tokyo.ac.jp/).

So far, it seems that they all give consistent results, albeit within quite large error margins.

There are several ways to get a handle on the relict light from Pop III stars, including analysis of deep Spitzer observations. Future observations, using the JWST, the SKA, and other planned 'telescopes', will also hone in on that particular background, as will (hopefully) more detailed studies of the mean free path of very high energy cosmic rays and the GZK effect.

As happens when you do this kind of leading edge research, you may discover something completely unexpected ... something which has nothing to do with the original question you set out to try to answer ...

Bogie
2007-Feb-06, 09:24 PM
I think the big question they were after was, after we account for all the "mundane" types of starlight we expect to be lingering around in intergalactic space, will there also be additional "more interesting" sources of light, that is still too dim to see directly? It sounds like one important speculation was that "first stars", big superbright monsters that were not necessarily associated with the first galaxies, might have contributed a lot of excess background light, and this observation would seem to be discouraging in regard to that speculation.Big "first" stars, superbright monsters would burn very fast and burn out quickly. Maybe there was a period of thermalization after the burn out that converted that "light" to the CMB before newer stars that formed after thermalization took on galactic shape and the associated anisotropy.

Why wouldn't the greater transparency be encouraging in regard to that speculation?

Nereid
2007-Feb-06, 09:31 PM
Big "first" stars, superbright monsters would burn very fast and burn out quickly. Maybe there was a period of thermalization after the burn out that converted that "light" to the CMB before newer stars that formed after thermalization took on galactic shape and the associated anisotropy.Not in any model that I am aware of - the footprints in the CMB are all wrong, for it (the CMB) to be due to such a source ...
Why wouldn't the greater transparency be encouraging in regard to that speculation?Because all the other footprints are missing ...

Bogie
2007-Feb-06, 09:35 PM
Not in any model that I am aware - the footprints in the CMB are all wrong, for it (the CMB) to be due to such a source ...Because all the other footprints are missing ...Do you mean the slight anisotropy in the CMB (1 part in 100,000)? Why couldn't that be caused by slight differences in the burn out rate of the first stars, leaving a pattern of anisotropy in the CMB itself?

Nereid
2007-Feb-06, 09:42 PM
Do you mean the slight anisotropy in the CMB (1 part in 100,000)? Why couldn't that be caused by slight differences in the burn out rate of the first stars, leaving a pattern of anisotropy in the CMB itself?That's one, the polarisation is another, and the astonishingly BB nature of the CMB SED is a third ...

Note that it's not just a "slight anisotropy in the CMB (1 part in 100,000)", the angular power spectrum has now been well measured, and constrained. Maybe someone could develop a model, based on Pop III stars, which produced the observed CMB, but I'm certainly not holding my breath ...

Oh, and the biggest hurdle for such a model would be, I think, the CMB temperature ... how could such a model produce something as low as 2.73 K?

Bogie
2007-Feb-06, 10:05 PM
That's one, the polarisation is another, and the astonishingly BB nature of the CMB SED is a third ...

Note that it's not just a "slight anisotropy in the CMB (1 part in 100,000)", the angular power spectrum has now been well measured, and constrained. Maybe someone could develop a model, based on Pop III stars, which produced the observed CMB, but I'm certainly not holding my breath ...

Oh, and the biggest hurdle for such a model would be, I think, the CMB temperature ... how could such a model produce something as low as 2.73 K?The angular power spectrum confirms the isotropy of the CMB in that the angular power spectrum shows no particular direction to the power spectrum.

Polarization, from the links I read when you mentioned it on my ISU thread did not seem to eliminate the possibility of a period of thermalization occuring after the burn out of a first round of hydrogen stars as I proposed in my OP in the ISU thread.

Doesn't the spectral energy distribution show a black body spectrum that could come from the thermalization period I proposed in the ISU OP?

Nereid
2007-Feb-06, 10:10 PM
The angular power spectrum confirms the isotropy of the CMB in that the angular power spectrum shows no particular direction to the power spectrum.

Polarization, from the links I read when you mentioned it on my ISU thread did not seem to eliminate the possibility of a period of thermalization occuring after the burn out of a first round of hydrogen stars as I proposed in my OP in the ISU thread.

Doesn't the spectral energy distribution show a black body spectrum that could come from the thermalization period I proposed in the ISU OP?You should ask such questions in the ATM thread devoted to such ATM ideas (or re-state your question so that it is completely free of any ATM idea).

Bogie
2007-Feb-06, 10:16 PM
...
Oh, and the biggest hurdle for such a model would be, I think, the CMB temperature ... how could such a model produce something as low as 2.73 K?If it was possible for these stars to heat up the CMB to around 3000 degrees, and then 13 billion years past, couldn't the temperature fall to 2.73 K in an expanding universe?

Bogie
2007-Feb-06, 10:21 PM
You should ask such questions in the ATM thread devoted to such ATM ideas (or re-state your question so that it is completely free of any ATM idea).OK, thanks for pointing that out.

Let me change that from, "Doesn't the spectral energy distribution show a black body spectrum that could come from the thermalization period I proposed in the ISU OP?" to, "I think the spectral energy distribution shows a black body spectrum that could come from the thermalization period after the burn out of the first round of hydrogen stars and before the formation of stars that began to show galactic structure.

Nereid
2007-Feb-07, 02:34 PM
If it was possible for these stars to heat up the CMB to around 3000 degrees, and then 13 billion years past, couldn't the temperature fall to 2.73 K in an expanding universe?
"I think the spectral energy distribution shows a black body spectrum that could come from the thermalization period after the burn out of the first round of hydrogen stars and before the formation of stars that began to show galactic structure.I'm not clear on what you're asking ... the stars could create a CMB, by their emission of EM radiation, but it wouldn't be ~3000 K (the Pop III stars would almost certainly have been far hotter than that!), and if these stars' EM radiation heated up something else, which subsequently re-radiated at ~3000 K, then what was that 'something else'?

Either way, you seem to be asking about some EM radiation which somehow became thermalised, streamed free, then as the universe expanded, 'cooled' to 2.73 K.

Without any details, it would be quite difficult to rule out this kind of idea ... however, you could put some pretty strong constraints on it, I expect, constraints which may serve to rule out whole classes of these models (should you get around to building them).

Squashed
2007-Feb-07, 03:01 PM
Transparent just means that light can pass through without interacting with anything. The "thing" the gamma rays would interact with, in this case, is something quite unusual-- other light! But if a high enough energy gamma ray encounters another photon (here a background photon from starlight), it can create an electron-positron pair. That would destroy both the gamma photon and the background photon, and the signal of this removal of gamma photons could then be used to infer the presence of the starlight photon. (It's not so easy to directly observe starlight from ultra-distant galaxies, so this is an indirect way to do it, because it is easy to observe bright gamma-ray sources.) So "starlight" is what there is in intergalactic space that makes it less transparent to gamma rays than to lower energy photons (your question had this backward, the gamma rays are expected to be more likely to interact, because they have enough energy to make particles, it's just not quite as likely as thought, apparently).

If gamma rays collide with starlight to produce particle pairs then wouldn't the particle pairs annihilate each other to produce the same two photons - or are the resulting photons different than the original photons?


Energy conservation:

If gamma = 4 and starlight = 1 and the two collide, produce particle pairs and then annihilate then can the resulting photons be new_gamma = 3 plus new_starlight = 2?

Nereid
2007-Feb-07, 03:18 PM
If gamma rays collide with starlight to produce particle pairs then wouldn't the particle pairs annihilate each other to produce the same two photons - or are the resulting photons different than the original photons?


Energy conservation:

If gamma = 4 and starlight = 1 and the two collide, produce particle pairs and then annihilate then can the resulting photons be new_gamma = 3 plus new_starlight = 2?In a word, no.

The newly created anti-matter/matter particle pair go flying off in different directions (conservation rules). The anti-matter particles might, eventually, somewhere else, collide with matter particles and annihilate, each annihilation creating a pair of photons (conservation rules again).

There's a longer answer, in which one more layer of detail is presented ...