To expand on the subject title, does the conversion of matter to energy by nuclear fusion inside stars mean that the amount of matter in the universe has been steadily declining since the first star flashed into life?

If the answer is "no", then are there processes reversing this transformation, i.e. converting energy to matter? What the heck are they?

If the answer is "yes", then does the depletion of matter have any significant effects on the cosmos? Like a lessening of overall gravitational force? (If this turns out to be the explanation for the universe's accelerated expansion, you are all hereby invited to my Nobel Prize party.)

The last question begs another: is gravitation manifested only in matter? Or does the maxim that matter and energy are really the "same thing" mean that energy also has gravitational properties?

A quick response: There are processes where matter absorbs energy (protons for example hitting a rock). My assumption is that at large scales, big energetic events also create big masses. How the tendency over the life of the universe is going, I can't say.

To your last question, in gravitational terms, matter matters. Matter is solid, clumps together in big things, and is pretty stable. Energy does not tend to stick together long enough for a significant mass to accumulate long enough for a gravitational force to concentrate.

um... if you meant that if energy is affected by gravity, then yes. Gravity causes light to bend, which can cause lensing effects that astronomers can measure. this is also why black holes are called black holes, because the gravity is so strong that not even light can escape.

If you meant that if energy has gravity, I have no clue. Interesting question.

Matter doesn't take in energy and create more matter. Matter just transforms energy. n this case, the photon absorbed by the matter could be turned into kinetic energy in the form of heat, or it could turn it into potential energy when an electron jumps an energy level... this however is unstable and when the electron "jumps" back down, a photon is emitted.

several hundred? thousand? tons of matter is converted to light by the Sun every second.

yes fusion converts matter to energy - and matter is lost (not alot but some). Maybe a percent or two by the end of a stars life? not sure. same with thermonuclear bombs.

we also have Fussion (both bombs and radioactive elements) - it too converts matter to energy (less mass is converted compared to fussion).

the natural radioactive decay of Uranium (mostly) is half over here on Earth - and some of that mass was lost to energy when Uranium transmutated through its various elements and finally ended up with Lead (which has less mass per Mole). So the Earth is less massive today (though infinatesimal I'd wager - know know how much really).

and finally we have the question of whether the Proton itself is unstable and simply "explodes" into energy on a whim every once in a VERY LONG WHILE. If so then eventually all Protons will pop into energy and alot of mass will be turning into energy in the infinate future.

of course does this apply when the proton is crushed into an electron to make a neutron (is it them stable?) and what about the matter in a Black Hole - is it stable? who knows.

surely there is matter and huge amounts in those Black Holes - just what type?

excellent questions BTW.

Fission - sorry.

Well, there IS stuff coming into the earth. Dust, meteors, etc.

I thought a photon WAS energy?

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"As long as the prerequisite for that shining paradise is ignorance, bigotry and hate, I say the hell with it." Henry Drummond, inherit the wind.

Yes, not only is "energy" affected by gravitation, as DDPP says, it also has a gravitational effect via m=e/c². Of course, when you divide by c², the effect becomes quite small. As I understand it, according to Einstein's field equations, even gravitational energy adds an additional gravitational effect.

That I don't know. If the amount of matter in the universe has been steadily declining, it is apparently not declining very fast!

Yes, there are such processes, but it takes a heck of a lot of energy. For a few more details, see here, here, or here.

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Everyone is entitled to his own opinion, but not his own facts.

Yes, like we'll ever run out.

He said "proton", not "photon".

oh. whoopppss! stupid me.

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"As long as the prerequisite for that shining paradise is ignorance, bigotry and hate, I say the hell with it." Henry Drummond, inherit the wind.

Do we actually know that?
I mean, most things we see in our universe, have actually happened a long long time ago, right?

May be this question is dumb but I often think about this:
could it (theoretically) be that everything we see - let's say outside our galaxy - is not really around anymore?
Is there any evidence for all that stuff still being around today?

Obviously, since light takes time to travel, we are looking back in time. However, we are not the center of the universe. Unless our average solar system or our average galaxy was not really average and was in fact EXTREMELY special, we really are just another speck in the universe.

Besides, astronomers can see the "life cycle" of stars and galaxies and whatnot. They know roughly how long each stage lasts (by using physics), and there is no way that most of that stuff is not really around anymore. Some stars VERY far away or some that we see as JUST ABOUT to explode won't be there (in the form we see them as), but the rest of them are still there.

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"As long as the prerequisite for that shining paradise is ignorance, bigotry and hate, I say the hell with it." Henry Drummond, inherit the wind.

Also there is beleived to be galaxies that are greater than 15 billion light years distant.

We will never EVER see them because the speed of Expansion of the space between us and them is GREATER than the speed of light.

From what I've heard we see 20-percent of the actual radius of the universe.

how/why they beleive this is beyond me.

Well, When Alan Guth was formulating the theory of inflation, his original calculations came up with the size of the "actual" universe being something like 10²³ times the size of the visible universe. The original form of the theory has been modified a bit, and there are competing versions, but the general inflationary idea has become fairly well accepted, I think, within the astronomical community. That doesn't mean it's set in stone, but there is a modest amount of observational and theoretical support for it.

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Everyone is entitled to his own opinion, but not his own facts.