Noob question about universe expansion

[smeagol69]

http://www.space.com/scienceastronomy/mystery_monday_040524.html

 

I was reading an article about the size of the universe & got very confused. It explained it thus:

 

Need a visual? Imagine the universe just a million years after it was born, Cornish suggests. A batch of light travels for a year, covering one light-year. "At that time, the universe was about 1,000 times smaller than it is today," he said. "Thus, that one light-year has now stretched to become 1,000 light-years."

 

My problem is this: A star produces X number of photons along a hypothetical straight line in one "year". The first photon is a light-year away from the sun, the last one is just coming out of the sun. According to the quote above, the line has now stretched 1,000 times it's original length. Since there are still the same number of photons along that line, wouldn't the light be 1,000 times "weaker" to our eyes?

 

What piece is my layman's brain missing?

[[Tycho?]]

You sorta have a point. Look up redshift effect on wikipedia, one of its causes is the expansion of the universe.

[Sisyphus]

It would be weaker, yes.

[CanadaAotS]

wait wait, what do they mean by "size of the universe"? I would think the size of the universe would be 15 billion ly's...

[Martin]

http://www.space.com/scienceastronomy/mystery_monday_040524.html

 

... Since there are still the same number of photons along that line' date=' wouldn't the light be 1,000 times "weaker" to our eyes?

 

What piece is my layman's brain missing?[/quote']

 

it (brain) seems to be all there

 

your question has been well replied by [Tycho] and Sisyphus

it was a good question. hope you keep on asking ones like that!

 

do you want extra stuff thrown in for free, that you didnt ask for?

 

like what has happened to the wavelength of the light?

 

(it is 1000 times longer, so if it was originally visible wavelength say half a micron, its new wavelength is half a millimeter-----like microwaves

that is the origin of the famous "cosmic microwave background" which used to be a mix like sunlight containing visible and got stretched 1000-fold so it is now all invisible infrared and microwave)

 

you could argue that the light is more than 1000 times weaker because there are 1000 times fewer photons arriving from the source, per second, and also each photon is 1000 times less energetic because it is longerwavelength lower frequency

 

but instead of us throwing in extra stuff, you probably learn more by first thinking of a new question

 

thanks [tycho] and sisyphus

 

BTW the size of the observable universe is far greater than the age of the universe multiplied by the speed of light----roughly 3 times larger. check out Ned Wright cosmology FAQ for instance. just google Ned Wright, who teaches cosmology at UCLA and has a good website about it

[sunspot]

Doesn't this also mean that a large fraction of the light energy within the expanding universe will outrun the matter? In other words, if we very start small, the first blast of energy will outrun any matter into space, since it travels at C and matter is constrained to less than C. As the universe expands, radiate energy will expand out from all the expanding particles points, recycling more and more energy, everywhere, except the outer perimeter. This matter will have an open side. How much energy was lost in the process of the universe reaching its current size?

[[Tycho?]]

"...recycling more and more energy, everywhere, except the outer perimeter. This matter will have an open side."

 

This part doesn't seem to make sense. How is radiating energy the same as recycling it? Matter will have an open side? What does that even mean?

[Martin]

sunspot, I agree with Tycho that your post does not make sense

 

you seem to have a non-standard idea of big bang cosmology in mind

 

in the standard picture that mainstream cosmologists use, the big bang was not an explosion in the midst of empty space

 

so there is no empty space for the light to "run off into" and be lost in

 

in mainstream cosmology the expansion is not an expansion OF something (light and matter) out into space

 

the expansion is an expansion of SPACE ITSELF

and the space has stuff randomly distributed more or less evenly scattered

so that except for the random gaps in between----just looking at the overall picture:

 

there is no place for light to go that does not already have light

and there is no place for matter to go that does not already have matter

 

 

light and matter are approximately uniformly distributed throughout and the "where" is what is expanding

 

General Relativity of 1915 was the first theory with a dynamic geometry so that space itself can expand

 

a lot of physics is still based on a rigid concept of space-----the space of 1905 Special Relativty, for example, cannot expand---it is a fairly rigid framework although different observers can have different perspectives on it. But this is wrong, nature isnt like that, so the parts of physics based on the absolute rigid space framework are having to be put on new foundations.

 

If all we had was the static space of Special Rel, and all the light and star crud was a little ball in the middle of it and went boom, then your question would make sense. But contemporary cosmology is not based on Einstein Special Rel, it is based on the 1915 theory-----different theory, different idea of space

 

Causes lots of confusion:-)

 

there is a good Scientific American article about this if you want to get the ideas straight.

 

It is called "Common Misconceptions of the Big Bang" .

 

Links to the article (which is free online) are here

http://www.scienceforums.net/forums/showpost.php?p=142965&postcount=65

 

http://www.scienceforums.net/forums/showthread.php?p=142965#post142965

[s pepperchin]

one thing that you need to remember aboaut light is that it doesn't travel out in a straight line. electromagnetic radiation is given off by an object at a certain rate but it also has an energy density. the energy density is usually looked at in terms of energy per square meter. The reason for this is that light travels from the sphere in a direction that is perpendicular to the surface.

 

At the beginning when the first photon leaves there is a photon leaving in every direction we will say that the energy density is X J/m^2 and the equation for the surface area of a sphere is 4 Pi r^2, this means that the total energy emitted from a star of radius R is X*4 Pi R^2. This must be taken into consideration along with the expansion of the universe.