Cluster motion

[Bzoodie]

From a latest news of Science Daily (http://www.sciencedaily.com/releases/20 ... 104410.htm) There is a sentence as following:

Hot X-ray-emitting gas in a galaxy cluster scatters photons from the cosmic microwave background. Clusters don't precisely follow the expansion of space, so the wavelengths of scattered photons change in a way that reflects each cluster's individual motion.

So, I don't understand why cluster don't precisely follow the expansion of space.
Can any one brother tell me the reason.
Thanks

[Wah!!]

原文話:

"The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase," says lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "We never expected to find anything like this."

Kashlinsky calls this collective motion a "dark flow" in the vein of more familiar cosmological mysteries: dark energy and dark matter. "The distribution of matter in the observed universe cannot account for this motion," he says.

都係等 WenXP 版主出來解畫啦~

[quantumkit]

Clusters of galaxies means a few galaxies that are gravitiationally affecting each others. So the motion of each galaxy also depends on other members' positions in the cluster. Therefore the light scattered from these galaxies are blue / red -shifted because of their motion.

If the galaxy is very far from others, its "motion" will be dominated by the universe expansion.

Hope this helps =)

[Bzoodie]

According to your explanation, I guess that the red-shift of each galaxy in the cluster is smaller than expected but there should not possible to have blue-shift (unless two galaxies are too close, and the speed of galaxy move to us great than its speed move away due to universe expansion). If it is true, how can we estimate or measure their distance from us?

[Wah!!]

I think scientists also use other ways to measure the distance of the galaxies such as measuring the brightness of Cepheid variable.

[sn-10]

Why shouldn't there be blue shift?

The shifting along the spectrum is just a relative concept, isn't it?

[Wah!!]

Scientists cannot determine the distances of blue shift objects by Hubble's law.

[quantumkit]

According to your explanation, I guess that the red-shift of each galaxy in the cluster is smaller than expected but there should not possible to have blue-shift (unless two galaxies are too close, and the speed of galaxy move to us great than its speed move away due to universe expansion). If it is true, how can we estimate or measure their distance from us?

I guess it depends what cluster you are measuring. If you have a cluster of galaxies that you can measure individual member's velocity, the average velocity should be mainly due to the Hubble flow (expansion of the universe). It also depends how far they are, e.g. they are not in our local group of galaxies.

So in the passage (I haven't read through it very carefully yet..), I guess what they have found is the redshift of some galaxy has an unexpectedly low redshift, that is the galaxy moves inside the cluster faster than normal.

The speed of the galaxy in a cluster depends on its total mass. So to get a conclusion of the galaxy being influenced by any other stuff, they have to measure the total mass of the host cluster very well, which I think the major error is here...

[WenXP]

原文話:

"The clusters show a small but measurable velocity that is independent of the universe's expansion and does not change as distances increase," says lead researcher Alexander Kashlinsky at NASA's Goddard Space Flight Center in Greenbelt, Md. "We never expected to find anything like this."

Kashlinsky calls this collective motion a "dark flow" in the vein of more familiar cosmological mysteries: dark energy and dark matter. "The distribution of matter in the observed universe cannot account for this motion," he says.

都係等 WenXP 版主出來解畫啦~

這段是說那個星系團以非哈伯式後退的方式移動, 因此科學家推繼該星系團可能是受大片的黑暗物質或黑暗能量所影響.

反倒是以下這一段我不明白:

Big-bang models that include a feature called inflation offer a possible explanation for the flow. Inflation is a brief hyper-expansion early in the universe's history. If inflation did occur, then the universe we can see is only a small portion of the whole cosmos.

WMAP data released in 2006 support the idea that our universe experienced inflation. Kashlinsky and his team suggest that their clusters are responding to the gravitational attraction of matter that was pushed far beyond the observable universe by inflation. "This measurement may give us a way to explore the state of the cosmos before inflation occurred," he says.

好像是說該片黑暗物質是在可見宇宙之外? 那麼該星系團應該會以更高速向外(即向後)移吧... 但實情是橫向移動.

[WFPC2]

好像是說該片黑暗物質是在可見宇宙之外? 那麼該星系團應該會以更高速向外(即向後)移吧... 但實情是橫向移動.

let me try....
他說WMAP的數據支持宇宙在極早期發生過暴漲的說法,如果暴漲是真的,那麼可觀測的宇宙都祗佔整個宇宙的極小部份,在可見區域之外一樣佈滿星系,暗物質和暗能量,但它們以更快的速度遠離我們而去.....

[Bzoodie]

I am wondering that if the inflation did occur and the galaxies in those region was unobservable. Then, we need to re-figure out the age of universe and stars, elements formation sequence, etc.
But how can they prove that?

[Wah!!]

Why do we need to re-figure out the age of universe if the galaxies can not be observed?

[Bzoodie]

Wah,

I think that the estimation of the age of universe is according to present observation. If there are some out of our sight, do you know there are another new phenomena observed? So, we may re-figure out the age of universe.
It is just my assumption.

[WFPC2]

Wah,

I think that the estimation of the age of universe is according to present observation. If there are some out of our sight, do you know there are another new phenomena observed? So, we may re-figure out the age of universe.
It is just my assumption.

For Standard Big Bang Model ,the age of the universe is depands on the value of Hubble Constant.

[WenXP]

What do you mean by "standard"?
For the very first version of big bang model, yes, the age of universe depends on Hubble constant only.
For later versions, some more parameters were added, e.g. curvature of space, and inflation was added.
For latest versions, dark energy is taken into concern.