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Fraser
2006-May-10, 07:24 PM
SUMMARY: The Universe is filled with a diffuse glow of radiation coming from all the stars and galaxies. This cosmic fog is actually hard to detect because we have much brighter objects nearby that can wash it out; like how the city lights obscure the stars at night. One way to measure this radiation is by using the radiation from quasars, which are extremely bright and distant. The high-energy radiation from the quasars loses energy as it passes through this background radiation, and this can be measured.

View full article (http://www.universetoday.com/am/publish/extragalactic_hess.html)
What do you think about this story? post your comments below.

antoniseb
2006-May-10, 07:45 PM
This story doesn't really tell us the details of what they are looking at. What energy gamma rays are they looking at? HESS looks at the cascade from *very* high energy gamma rays as they hit Earth's upper atmosphere. The "reddening" that they are talking about has to do with the GZK interactions between high energy photons and the cosmic microwave photons.

If there is no GZK reddening, this is a major story about something in physics not working as expected.

Jerry
2006-May-11, 01:33 AM
This story doesn't really tell us the details of what they are looking at. What energy gamma rays are they looking at? HESS looks at the cascade from *very* high energy gamma rays as they hit Earth's upper atmosphere. The "reddening" that they are talking about has to do with the GZK interactions between high energy photons and the cosmic microwave photons.

If there is no GZK reddening, this is a major story about something in physics not working as expected.
Yes, it implies either much of the Cosmic Microwave Background is local (a possibility supported by the zodiac axial bias) or quasars are relatively local...or both.

antoniseb
2006-May-11, 12:53 PM
Yes, it implies either much of the Cosmic Microwave Background is local (a possibility supported by the zodiac axial bias) or quasars are relatively local...or both.
Or that GZK doesn't work the way we think it does for super-high-energy photons and microwave photons.

Duane
2006-May-11, 03:54 PM
cph-theory is unable to recieve private messagesm so I am forced to post this warning in public. ATM theories are to be discussed in the appropriate forum only. Please cph, review the rules (http://www.bautforum.com/showthread.php?t=32864).

punch_md
2006-May-11, 10:20 PM
This story doesn't really tell us the details of what they are looking at. What energy gamma rays are they looking at? HESS looks at the cascade from *very* high energy gamma rays as they hit Earth's upper atmosphere. The "reddening" that they are talking about has to do with the GZK interactions between high energy photons and the cosmic microwave photons.

If there is no GZK reddening, this is a major story about something in physics not working as expected.

First, you can find our full press release here
http://www.mpi-hd.mpg.de/hfm/HESS/public/PressRelease/EBLPress/PressRelease_E.html

The gamma rays we are looking at are a million million times more energetic than visible light (10^12 eV, or a TeV), which is "very high", but not "extremely high" :)

The GZK cut-off (named after Greisen-Zatsepin-Kuzmin) is a effect which applies:
- not to gammas, but to protons (as you say, due to their interaction with the cosmic microwave background, or CMB)
- at an energy over a million times beyond what we measure.

So, we're not measuring the GZK, but the effect of absorption of very high energy gamma rays hitting the near-infra-red (NIR) background light (well, not strictly absorbed but they make an electron-positron pair which then spiral off elsewhere).

The CMB is the "light" (shifted in microwaves) which was freed to travel in straight lines once the universe had expanded enough after the big bang to be cool enough so that electrons could pair up with protons, giving neutral hydrogen. Then, a few hundred million years later, galaxies and stars started to "condense" out of the hydrogen soup. The light from these is what we measure, and in particular we estimate that there isn't enough of this near-infra-red light for us to have to suppose that the stars and galaxies appeared sooner rather than later. Some researchers proposed very early star formation in order to explain other effects that had been measured; we pretty much rule this out.

We don't require any Lorentz-violation theories, or suchlike, and do require that quasars have cosmological redshift.

One other point on the original story:
The high-energy radiation from the quasars loses energy as it passes through this background radiation, and this can be measured. should be interpreted as "the highest-energy gamma-rays are preferentially absorbed", but the energy of the the gamma-rays that do get to us are not much changed (just slightly redshifted). The "reddening" referred to is like the reddening of a sunset, where the highest energy (blue) photons are lost going through a thick atmosphere, but the photons that do get to us have the same energy as when they left the sun.

antoniseb
2006-May-11, 10:34 PM
Hi punch_md, welcome to the BAUT forum.

Also, thanks for:
- providing the link to the paper
- clearing up my misconceptions.

punch_md
2006-May-11, 10:46 PM
Hi punch_md, welcome to the BAUT forum.

Also, thanks for:
- providing the link to the paper
- clearing up my misconceptions.

No problem :) Happy to help

ngeo
2006-May-11, 10:54 PM
punch_md wrote, "Some researchers proposed very early star formation in order to explain other effects that had been measured; we pretty much rule this out."

What are these other measured effects?

punch_md
2006-May-12, 12:01 AM
punch_md wrote, "Some researchers proposed very early star formation in order to explain other effects that had been measured; we pretty much rule this out."

What are these other measured effects?

First, the early abundance of heavy elements (which can't be produced in quantity in the big bang). Some emission lines in very distant quasars seem to indicate that there are more than expected.

Secondly, preliminary measurements from WMAP (the Wilkinson Microwave Anisotropy Probe), which measures the Cosmic Microwave Background, seemed to indicate that stars did form early. However, new analysis with more data has revised this result (about two months ago), so this second point is no longer a good justification for early stars.

The first point is still a puzzle, though, and the cosmologists will now have to think up another way to create heavy elements early on (other than early "population III" stars : http://en.wikipedia.org/wiki/Population_III_stars).

ngeo
2006-May-12, 01:00 PM
Thank you, punch_md. According to the wikipedia entry, these Population III stars are believed to have triggered reionization. If they are to be ruled out, what would be a possible explanation for reionization in the current Big Bang models? And what would happen to the time for creation of galaxies?

punch_md
2006-May-13, 10:20 AM
Thank you, punch_md. According to the wikipedia entry, these Population III stars are believed to have triggered reionization. If they are to be ruled out, what would be a possible explanation for reionization in the current Big Bang models? And what would happen to the time for creation of galaxies?
Good question... as I understand it, it just means that the reionization happened later, caused by the formation of "standard" galaxies and stars, at 400million years rather than 200million years. I've found the press release for the WMAP new data and re-analysis which supports this: http://physicsweb.org/articles/news/10/3/14/1 (from 17 march).

Jerry
2006-May-16, 05:55 PM
Welcome aboard, Punch_md, and thanks for the clarification...So where does the infrared background come from?

Yes, the WMAP II & III releases support a later reionization, but that just begs the question as to where all the heavy metal came from.

Also, the "Axis of Evil" emerged stronger than ever in the WMAP II & III. This alignment with the Zodiac can't be brushed under the rug, along with all the heavy metal. If someone discovers Dark Energy evolves into heavy metal and infrared radiation, I will just puke.