Why Consider Dark Matter Over New Space for Universe Expansion?

[mrxyz]

Hi

I rarely have a background in physics (yet!)

But based on my popular knowledge I understand that the Universe is 'expanding' in a loose sense of the word - which basically equates to the fact new space is being created all the time which in turn resembles a Universe where the distances between objects appear to grow and grow.

The 'Dark Matter' theory however hypothesizes some kind of matter which physicists don't understand and the fact due to some properties of this matter objects appear to be getting pushed away from each other.

My question is - why does there need to be this unverified substance 'dark matter'. Why is it never said that new space being created is the reason for the accelerating expansion of the Universe.

Why bring Dark Matter into the equation at all?

 

[Mordred]

dark matter has nothing to do with expansion. Dark energy does.

Dark matter is needed to explain the missing mass conundrum, why are galaxies spinning the way they do? Where is the missing mass that formed various gravitational lenses ?

 

[marcus]

You are confusing dark matter with something else. Dark Matter helps pull things together by its gravity. It has been enormously important in helping ordinary matter gather together into clouds and form clusters of galaxies, and even has aided in the formation of galaxies.

Maps have been made of the DM clouds showing varying density, they look like contour maps of hills and valleys.
One can "see" dark matter by its gravitational effect on light.

Very interesting stuff!

EDIT: Mordred is right, DM also plays a role in holding galaxies together, which because of the rate they are wheeling around, would not be stable without some DM. They would drift apart without the extra mass it contributes. DM has been deduced to play some half dozen essential roles (of which that is one)

 

[marcus]

...

My question is - why does there need to be this unverified substance 'dark matter'. Why is it never said that new space being created is the reason for the accelerating expansion of the Universe.

Why bring Dark Matter into the equation at all?

What you really meant to talk about was the popular buzzword Dark Energy. So let's correct the question so it makes sense:

My question is - why does there need to be this unverified substance 'dark energy'. ...

Why bring Dark Energy into the equation at all?

The answer is there is no need to assume an actual energy, because all the observed effects are adequately explained by a simple constant term in the classic Einstein equation (1915) which we still use to model the universe. Talking about "Dark Energy" is partly just a kind of "hype". It sounds jazzy and appeals to people more than talking about what Einstein called it: the Cosmological Constant.

Starting in 1998 people looked carefully for signs that the slight observed acceleration was NOT due simply to the constant in the Einstein equation. They looked for signs that it might be due to a form of energy, capable of change, growth, decay etc. It is good they did. Scientists must be careful and explore various possibilities. But after over 10 years of studying the data they have not found anything that needed a more complicated explanation. The simple constant Einstein called Lambda, written with a greek capital letter Λ or sometimes with the lower case lambda λ.

In 2010 the physicist Carlo Rovelli and one of his students wrote a debunking article called
"Why all these prejudices against a constant?" You might find parts of it readable, and even amusing. Just skip the overly technical parts and see what you can understand. To get a free download copy, google "prejudices against a constant", or click on this:
http://arxiv.org/abs/1002.3966

As Rovelli and his student Bianchi tell it, it is one of the funnier physics stories of the 2000-2010 decade.