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Why is dark energy necessary?

by gregtomko
Tags: acceleration, dark energy, universe
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juanrga
#55
Nov5-11, 01:43 PM
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Quote Quote by gregtomko View Post
If the mass of the universe is constantly being converted to energy through nuclear fusion, and nothing can travel outside of space-time, then isn't the ratio of energy to mass increasing? If so, then wouldn't the only possible option be for an acceleration of the universe's expansion?
No. In fact the mass M of the Universe is constant.
gregtomko
#56
Nov5-11, 10:00 PM
P: 71
that is very interesting, where does the mass go if photons are massless?
gregtomko
#57
Nov5-11, 10:25 PM
P: 71
oh, OK, they don't have "rest" mass, but they aren't at rest. I wasn't aware they had mass when traveling.
juanrga
#58
Nov6-11, 09:47 AM
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Quote Quote by gregtomko View Post
oh, OK, they don't have "rest" mass, but they aren't at rest. I wasn't aware they had mass when traveling.
Rest mass m is a confusing name. It is not the mass of an object when it is only at rest. Rest mass is the mass of the object with independence of its motion and in modern literature it is best named invariant mass or just mass. When the object is moving its mass m is the same than if was at rest. m=0 for a photon always.
Drakkith
#59
Nov6-11, 04:09 PM
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Quote Quote by gregtomko View Post
oh, OK, they don't have "rest" mass, but they aren't at rest. I wasn't aware they had mass when traveling.
They do not have mass, ever. They have momentum and energy. Both mass and energy contribute to gravity. There is a confusing thing called "relativistic mass" that shouldn't have ever been called mass to begin with. When you think of mass only think of "rest mass" or "invariant mass". Both are the same thing. When a star emits light it does lose a small amount of mass thanks to the missing energy that the photon took. While in transit that photon is affecting the space around it through gravity. Once the photon is absorbed the energy it carried is turned back into mass, making whatever absorbed it slightly more massive.
gregtomko
#60
Nov6-11, 04:46 PM
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Quote Quote by Drakkith View Post
They do not have mass, ever. They have momentum and energy. Both mass and energy contribute to gravity. There is a confusing thing called "relativistic mass" that shouldn't have ever been called mass to begin with. When you think of mass only think of "rest mass" or "invariant mass". Both are the same thing. When a star emits light it does lose a small amount of mass thanks to the missing energy that the photon took. While in transit that photon is affecting the space around it through gravity. Once the photon is absorbed the energy it carried is turned back into mass, making whatever absorbed it slightly more massive.
That is exactly the way I thought it worked. How does that relate to the earlier post
Quote Quote by juanrga View Post
No. In fact the mass M of the Universe is constant.
If the mass is no longer in the star while the photons are in transit, how can the mass of the universe stay constant? Or maybe that was referring to the relativistic mass of the universe?
Drakkith
#61
Nov6-11, 05:12 PM
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Quote Quote by gregtomko View Post
That is exactly the way I thought it worked. How does that relate to the earlier post

If the mass is no longer in the star while the photons are in transit, how can the mass of the universe stay constant? Or maybe that was referring to the relativistic mass of the universe?
When talking about dark energy and expansion and all that, both energy and mass have the same effect. So turning the mass into energy does nothing to the universe as a whole. Everything is still conserved.

Edit: I think that radiation pressure DOES happen to everything. The Sun is pushing objects away from it all the time. However the force of this pressure is extremely small and it also falls off exponentially with range. So while it probably does contribute to a very very slight "expansion", it is many orders of magnitude too small to cause the effect we see on a universal scale. See the table here: http://en.wikipedia.org/wiki/Radiati...lanetary_space
Notice that at a distance of 1 AU the pressure is 100 times LESS than it is at 0.1 AU from the Sun. For 4 light years the amount of pressure is 63,990,987,667.36 times LESS than it is at 1 AU. So the nearest star experiences 64 billion times less radiation pressure from the Sun than the Earth does. (Pressure falls off at the square of the distance from the emitting object. 4 Lightyears = 252,964.4 AU. 252,964.4^2 = 63,990,987,667.36)

Edit 2: For a star at the other end of our galaxy, 100,000 ly away, assuming the light could even reach it without being absorbed first, which it cant, the pressure would be 39,994,367,292,100,000,000 times less. (That's almost 40 quintillion times less. About 40 billion billion times less)
I really hope all my math is correct lol.
gregtomko
#62
Nov6-11, 06:52 PM
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What I am questioning is if the total quantity of energy released through nuclear fusion throughout the history of the universe is enough to equal the acceleration we observe. Not if we see certain effects on certain systems. As a whole, the proportion between how much energy is necessary to accelerate the universe as we see, and the amount of energy released in stars throughout time.... is that a known ratio?
phinds
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Nov6-11, 06:55 PM
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Quote Quote by gregtomko View Post
What I am questioning is if the total quantity of energy released through nuclear fusion throughout the history of the universe is enough to equal the acceleration we observe. Not if we see certain effects on certain systems. As a whole, the proportion between how much energy is necessary to accelerate the universe as we see, and the amount of energy released in stars throughout time, is that a known ratio?
I have no idea what the quantitative answer to your question is, but I doubt it matters because you have to have a plausible mechanism for transferring the energy released in the middle of stars to points MANY light years away, and there isn't any.
gregtomko
#64
Nov6-11, 07:32 PM
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Quote Quote by phinds View Post
you have to have a plausible mechanism for transferring the energy released in the middle of stars to points MANY light years away, and there isn't any.
I only ask the question because I am not an astrophysicist. I am not asking about the mechanism, just about the relationship in energy.
phinds
#65
Nov6-11, 07:49 PM
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Quote Quote by gregtomko View Post
I only ask the question because I am not an astrophysicist. I am not asking about the mechanism, just about the relationship in energy.
Well, to be fair, you were NOT asking "JUST" about the relationship, you said

What I am questioning is if the total quantity of energy released through nuclear fusion throughout the history of the universe is enough to equal the acceleration we observe
so your full question does bring up the need for a mechanism since otherwise equating the two is not in any way meaningful.

Now, don't get me wrong, I DO get (now that you've mentioned it twice ... I'm a little slow sometimes) that you are NOT asking about a mechanism, but do you see my point that your question as asked really does bring in a mechanism, else is somewhat meaningless?
gregtomko
#66
Nov6-11, 08:04 PM
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It seems to me that the question of meaningless or meaningful, rests on the relationship of the quantities of energy involved.
gregtomko
#67
Nov6-11, 08:13 PM
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I am just curious, is this an understood proportion?
phinds
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Nov6-11, 08:13 PM
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Quote Quote by gregtomko View Post
It seems to me that the question of meaningless or meaningful, rests on the relationship of the quantities of energy involved.
But that's my point ... there IS NO realationship. I mean, comparing numerical quanties doesn't have any meaning unless the quantities describe things that are somehow related.

You might as well add up all the fuel costs for all ocean going vessels this year and compare it to all the fuel costs for automobiles for this year. What does that comparison tell you? Nothing meaningful about the realtionship between ships and cars.
phinds
#69
Nov6-11, 08:14 PM
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Quote Quote by gregtomko View Post
I am just curious if this is an understood proportion?
That's a reasonable question and I don't know the answer, but once you HAVE the answer, what do you DO with it? I just can't get how it MEANS anything.

EDIT: I think I'm coming across as being hard to get along with in this. I don't mean to be. What I SHOULD be doing is ASKING, what do you think it will TELL you if/when you get a quantitative answer?
gregtomko
#70
Nov6-11, 08:18 PM
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I am not sure where I am going with it either, it just seems like there might be some significance if there was a similarity. It would at least be a way to disprove an intuitive, yet overly simplistic connection, if there were no similarity.
phinds
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Nov6-11, 08:21 PM
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Quote Quote by gregtomko View Post
I am not sure where I am going with it either, it just seems like there might be some significance if there was a similarity. It would at least be a way to disprove an intuitive, yet overly simplistic connection.
I see what you are saying, but even if the quantities were exactly the same, it would have no significance because of the lack of a mechanism relating the two. They just really don't have anything to do with each other.
Drakkith
#72
Nov6-11, 08:22 PM
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Quote Quote by gregtomko View Post
What I am questioning is if the total quantity of energy released through nuclear fusion throughout the history of the universe is enough to equal the acceleration we observe. Not if we see certain effects on certain systems. As a whole, the proportion between how much energy is necessary to accelerate the universe as we see, and the amount of energy released in stars throughout time.... is that a known ratio?
It is not. It isn't even close.


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