Where Have All the Shadows Gone?

by Paul Gilster on August 31, 2006

Cosmological shadows? Theory predicts that objects between us and the source of the cosmic microwave background should cast them. Specifically, the hot gases found in clusters of galaxies should show a measurable shadow effect produced by that background radiation, and there are reports of such effects from various observers. The scattering of the cosmic microwave background by high-energy electrons is known as the Sunyaev-Zel’dovich effect.

However, a new study from the University of Alabama at Huntsville raises real problems. The first to study the phenomena with data from the Wilkinson Microwave Anisotropy Probe (WMAP), the team reports an oddly sporadic shadow effect for a background thought to be afterglow radiation from the Big Bang.

And that raises questions about the Big Bang model itself. Says physicist Richard Lieu, after an investigation involving 31 clusters of galaxies:

“These shadows are a well-known thing that has been predicted for years. This is the only direct method of determining the distance to the origin of the cosmic microwave background. Up to now, all the evidence that it originated from as far back in time as the Big Bang fireball has been circumstantial.”

And Lieu’s team is reporting that among the clusters studied, some showed the shadow effect and some did not. In fact, the actual shadow effect is about one-fourth of what was predicted. Which is decidedly odd, for the microwave background radiation’s effects ought to be obvious. “If you see a shadow…it means the radiation comes from behind the cluster,” Lieu adds. “If you don’t see a shadow, then you have something of a problem.”

All of which is certain to raise controversy and a spate of new studies using the publicly-available WMAP data. The Huntsville team examined hot ionized gases at the center of galaxy clusters; the free electrons there interact with the background radiation to create the shadow effect. Because the observed shadow effect more or less equals in strength the natural variations already observed in the microwave background, the question becomes acute. If the background radiation is closer than the galaxy clusters under study, what exactly is it and how did it arise? And what does this mean about the Big Bang?

The paper is Lieu, Mittaz and Shuang-Nan Zhang, “The Sunyaev-Zel’dovich effect in a sample of 31 clusters: A comparison between the X-ray predicted and WMAP observed decrement,” Astrophysical Journal, Sept. 1, 2006, Vol. 648, No. 1, p. 176. Now available here on arXiv.


andy August 31, 2006 at 18:12
Administrator August 31, 2006 at 18:23

Thanks, Andy. I’ll also move that link into the post.

Hans Bausewein September 2, 2006 at 13:52

What can gravitational lensing do here? If the diffuse background radiation bends around the cluster, can it fill the shadow?

Administrator September 2, 2006 at 15:08

Apparently not without producing observable effects that tell us something about the relative positioning of background radiation and cluster. It seems decidedly odd that the detected ‘shadows’ are roughly similar to the natural variations already detected in the CMB.

Eric James September 3, 2006 at 0:39

Aha! My Mach’s Principle hypothesis is beginning to make a little bit of sense, all of a sudden. That’s too weird. I just thought I was jus’ messin’ around!

JD September 3, 2006 at 3:57

Doesn’t this provide more support for the origional steady state adherents or am I looking at this wrong?

Eric James September 4, 2006 at 3:03

I’ve been reading what I can find on this. It seems this might indeed have serious ramifications in regards to The Big Bang hypothesis.

If correct, the observations imply that the microwave background radiation’s source might be a relatively local phenomenon, rather than the supposed afterglow of the big bang, emanating from the extremum of space/time. Perhaps it is a combination of things that have yet to be sorted out.

Obviously, more observations are necessary.

JD September 4, 2006 at 19:00

Okay, after reading up and refreshing my admittedly lagging memory these observations may tend to encourage the “quasi steady state theorists.” It will be interesting to see how the arguments go on this.

Administrator September 5, 2006 at 7:26

JD, yes, I do think this poses problems, though we may learn there are data gathering issues here that solve them. But we’ve used the microwave background for numerous important studies, and if it’s compromised in some way, a lot of things would have to be re-thought. It’s going to be fascinating to watch the reaction to this work on cosmic ‘shadows.’

pfdietz September 9, 2006 at 15:43

The idea that the microwave background is local has very serious problems. You need an optically thick emitter to get a black body spectrum, which doesn’t seem consistent with the evidence if it were local. Moreover, it would be difficult to achieve temperature uniformity across the sky.

Administrator September 9, 2006 at 16:01

Agreed. Temperature uniformity seems like the biggest issue for a ‘local’ CMB.

Eric James September 10, 2006 at 4:36

Why wouldn’t a local phenomenom from interstellar space be expected to be a uniform temperature? If all of flat (empty) space is consistent in regards to energy density, and supposing it’s the source of the CMB, wouldn’t you expect it to look fairly uniform?

Isn’t it the apparent uniformity (lack of shadows) that’s befuddling the astronomers in the article?

So therefore the uniformity is more of a problem for a distant CMB, more than for a local CMB, right?

Administrator September 11, 2006 at 13:26

Eric, as I read this work, some clusters show shadows while most do not, which seems to confound the picture utterly. Getting a uniform temperature out of this seems tricky if some of the radiation is coming from well beyond the clusters while most of it is more local. All of which translates into my saying that I can’t figure out what sort of mechanism might account for a local CMB, but if there’s some way to get uniforms temperatures out of it, just how that works is going to be interesting indeed!

pfdietz September 15, 2006 at 0:17

Why wouldn’t a local phenomenom from interstellar space be expected to be a uniform temperature?

Because, observably, it isn’t. We see gas and dust clouds at widely different temperatures, depending on what is heating them.

Everyone also missed another point I made, which is even more damning than the temperature argument. Blackbody radiation has a specific shape and intensity. The background radiation matches this shape & intensity with extreme accuracy, to more than five significant digits. This means the source must be a blackbody source — that is, a source that is also a perfect absorber of the wavelengths in question. If this source were near the Earth, we could not see beyond it at those wavelengths — it would be black! But we can see sources at the relevant millimeter waves out to cosmological distances.

Eric James September 15, 2006 at 0:42

Sorry, I have to ask the wiseacre question…

Can we see space itself? Could this be a blackbody radiation source? Perhaps from one or more of the proposed extra dimensions?

Russell Wallace September 20, 2006 at 23:49

I don’t get this. Suppose there’s a putative shadowing object between us and the CMB. The object will be at a temperature of at least 3K (if it wasn’t, the CMB would heat it up until it was). Therefore it will emit microwaves at least as brightly as the CMB. Therefore we will see no shadow – typically a bright spot, but definitely not a shadow. What am I missing?

pfdietz September 22, 2006 at 21:49

The intervening objects contain energetic electrons that upscatter microwave photons to higher energies. The objects are not themselves sources of thermal radiation; they are far too ‘thin’ for that.

Russell Wallace September 24, 2006 at 10:53

Oh, so it’s a brighter rather than darker patch? Okay that makes sense thanks.

kjk January 30, 2011 at 22:54

Objects exposed to sunlight absorb visible light and re-radiate it as infrared. The object does not get as hot as the surface as the sun because sunlight is “diluted” 40000 fold by the apparent small area of the sun compared to the entire sky. CMB comes from ALL directions and is extremely close to thermal. There is no “dilution”. Objects exposed to the CMB will heat up to 2.7 kelvin and emit the same amount of radiation that they absorb at every wavelength. There is no shadow since the shading object is also a light source. Sometimes gas can be cooled below 2.7 K by expansion (boomerang nebula) but these events are rare.

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