# The universe should be billions of times brighter than the Milky Way?

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1. Dec 13, 2006

### kmarinas86

The universe should be billions of times brighter than the Milky Way????

I miscalculated =P

The sun is 1 AU away.
1. The center of the galaxy is 2 billion AU away.
2. A sun at 2 billion AU away is 4 billion billion times dimmer than our sun.
3. There are about 200 billion stars within the radius of the sun's orbit.
4. It would take less than 20 million times as many stars to be as bright as our sun.
1. The Virgo SuperCluster is 6 trillion AU in radius.
2. A sun at 6 trillion AU away is 36 trillion trillion times dimmer than our sun.
3. There are about 10 trillion stars at the Virgo SuperCluster (10% of visible mass and 1% of virial mass).
4. It would take less than 3.6 trillion times as many stars to be as bright as our sun.
1. The visible "Big Bang" universe is 900 trillion AU in radius.
2. A sun at 900 trillion AU away is 810 thousand trillion trillion times dimmer than our sun.
3. There are about 70 thousand billion billion stars[1] in the visible "Big Bang" universe
4. It would take less than 12 million times as many stars to be as bright as our sun.

Overall:
Our overall galaxy is less than 20 million times dimmer than our sun.
Our overall super cluster is less than 3.6 trillion times dimmer than our sun.
Our overall universe is less than 12 million times dimmer than our sun.

It was my calcs.. nvm!

Last edited: Dec 13, 2006
2. Dec 14, 2006

### Jheriko

Don't forget about all of the dust blocking our view, and the relativistic effects too... there is a paradox named after this, but can't think of the name at the moment.

3. Dec 14, 2006

### DaveC426913

This is how we know that
- the universe is not infinite and unchanging
- it did not exist forever
- it is not infinite in size

4. Jan 3, 2007

### heusdens

Olbers' Paradox ony arises when the following conditions are met:
* The universe is homogenous with lumninous matter evenly spread (on average) through the universe
* The universe existed in that state either for infinite time and is infinite in size

So, in theory (and purely on the basis of Olbers' Paradox) the universe can be infinite in size and infinite in time, when it did not always contain luminous matter, or it could be infinite in size but not in time and at any moment evenly spread with luminous matter, or it could be finite in size but infinite in time and evenly spread with lumnious matter.

5. Jan 4, 2007

### hellfire

Additionally, the paradox bases on the assumption that space is static and the paradox is also resolved dropping this assumption. In an infinite, eternal universe with homogeneous luminous matter in which space expands there is no Olbers paradox. For example, a de-Sitter universe, as postulated in the Steady-State Cosmology.

As an interesting aside, the paradox is also resolved for an infinite, eternal and static universe with a fractal distribution of light sources with fractal dimension less than two. This is not difficult to prove. Peebles analizes in detail this scenario in his book "Principles of Physical Cosmology" when discussing the cosmology that was proposed by Benoit Mandelbrot.

Last edited: Jan 5, 2007