# Gravity and Dark Matter

• talksabcd
In summary, the article discusses how dark energy exists and how it works to keep the universe in a closed shape. It also provides an analogy for how geometric shapes play into the theory.

#### 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 won't be net
gravitational effect on anybody 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 can't visualize a directional fourth dimension like

Thanks,
talksabcd

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

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

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)

Last edited:
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.

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.

## 1. What is gravity?

Gravity is a fundamental force of nature that causes any two objects with mass to be pulled towards each other. This force is responsible for keeping planets in orbit around the sun, and for keeping us grounded on Earth.

## 2. How is gravity related to mass?

Gravity is directly proportional to the mass of an object. This means that the greater the mass of an object, the stronger its gravitational pull will be.

## 3. What is dark matter?

Dark matter is a type of matter that is believed to make up about 85% of the total matter in the universe. It does not emit or interact with light, making it invisible to telescopes and difficult to detect.

## 4. How does dark matter affect gravity?

Dark matter is thought to have a significant influence on gravity, as it is believed to be responsible for the extra gravitational pull that holds galaxies together. Without the presence of dark matter, galaxies would not have enough mass to maintain their shape and would fly apart.

## 5. Why is studying dark matter important?

Studying dark matter is important because it can help us better understand the structure and evolution of the universe. It also plays a crucial role in the formation of galaxies and the distribution of matter in the universe. By studying dark matter, we can gain insight into the fundamental laws of physics and potentially discover new particles and forces in the universe.