Astronomers weigh galaxies, measure expansion

[MyOtherAccount]

I would like to ask a few questions as a result of reading this article.

• Wasn't there an experiment that was supposed to measure the rate of expansion?
  
• Weren't the experimenters expecting the results to help predict a slowing of the rate of expansion?
  
• Didn't it instead prove the universe to be ever increasing in the expansion rate?
  
• Wasn't the theory of dark matter with the characteristic of negative gravity developed to explain how such a result could occur?
  
• Weren't the two galaxies supposed to be the same mass and about the same size? And still would be if neither of them had dark matter--attributing the difference as being the extra dark matter in Andromeda?
  

Then how could Ma say,

It was surprising that we could see expansion happening from within our own local group of galaxies

that is consistent with expansion on a universal scale.

[my emphasis]

They just said that the two galaxies are different in about a 1:2 ratio. Seems to me that if dark matter has negative gravitation, and the dark matter density is 1:2, then one of those galaxies would have a rate of acceleration higher than the other, and accumulatively over eons of time much, much greater than the other.

Also, if various galaxies have various dark matter densities, why would Ma be emphasizing the claim that there is a Universal Expansion Rate? since he just proved, if indeed he did, that the rate varies from galaxy cluster to galaxy clusters.

One other thing... Doesn't their finding impact their experiment? If their models were developed assuming universal dark matter densities across the universe and their experiment has proven that not to be so, then little else can be said as to what the experiment has proven.

So, that makes for three discussion points, I think.

I know I'm confused! Could someone help me understand this possible blah, blah, blah jibberish?

--MOA

Edited September 13, 2014 by MyOtherAccount

[sepulchrave]

I think you are confusing dark matter with dark energy.

Dark matter behaves normally under gravity (so it would have ``positive'' gravity, to use your nomenclature). The existence of dark matter is postulated in part to explain the structure of galaxies - as these do not contain enough visible matter to explain their gravitational strength. Because dark matter has a normal gravitational interaction, it should cluster around large sources of visible matter.

Dark energy provides ``negative gravity'' to drive the expansion of the Universe. Dark energy should not cluster around matter, rather it should be more-or-less spread out evenly over the Universe.

[MyOtherAccount]

Thank you heaps. That was a great favor which should help with many other interpretations of things in the future.