Gravity and Dark Matter

talksabcd

Hi,

Many astronomers believe that there is no center to the universe.
So where ever you go, you would approximately see same amount of matter (stars galaxies etc) around you. So we can assume that there wont be net
gravitational effect on any body at all the places in the universe (neglecting local gravitation like the one between Earth and Sun).

1) So why always dark matter is said to be of overcoming gravity when there is no net gravity on any object ?

2) If the universe came from Big Bang then shouldn't it have a boundary ?
Boundary came to my mind because a Bang will always push the matter outwards and my visualization clearly shows a boundary to the expansion of matter from a single point ?

3) Many believe that we are in 3D space on a 4D platform. How can we visualize this ?. I can easily vizualize time as fourth dimension because
gravity wraps space but I cant visualize a directional fourth dimension like
X, Y and Z. Please help me to Vizualize the shape of the universe.

Thanks,
talksabcd

mathman

The stars in each galaxy are revolving around the center of the galaxy. Their speed of revolution is too high to be held in place by the baryonic matter of the galaxy. Dark matter is needed to give enough mass for the stars to stay in orbit and not fly off. Astronomers have also noted the same problem in describing clusters of galaxies.

talksabcd

Now I have realized that I have put 'Dark Matter' instead of 'Dark Energy'.
Its a typo.. It should be Dark energy.

talksabcd

Can someone answer the above questions ?. Please replace Dark Matter with Dark Energy in the above questions listed in the 1st
post of this trail.

Thank You
talksabcd (talks basics)

mathman

Feb. 2007 issue of Scientific American has an article which discusses dark energy in great detail. It might help you get the answers you are looking for.

natski

In regard to question 1), dark energy does not even have to work on the cosmological scale but on smaller scales too. The Big Crunch is an idea that dark energy will continue to increase overtime until eventually atoms will be ripped apart by it's force. This is however just one of three commonly quoted endings for the Universe.

The shape of space is difficult to describe and is best done so with GR. The universe can be either flat, open or closed in shape. Data suggests we are on a universe that is just closed. It's not really meaningful to talk about geometric shapes, but I believe a useful analogy would be that flat corresponds to a flat sheet, open corresponds to a saddle like shape, and closed corresponds to a spherical like shape. Hope that helps.