why is dark energy necessary?

DaveC426913

I guess my difficulty was that relies on some assumptions about the shape of our universe, which we know little about. We'd have to accept them as true first before your argument is granted.

Doesn't feel right trying to debunk something that's concrete and right in front of us, by invoking something so speculative and distant.

yoron

Ahh, but then we have all those virtual particles too :)

I think they lay in wait, and as they realize that they are in a SpaceTime with planets they pop up to 'push', our typical party pranksters. To get around the question of why they shouldn't push 'evenly' I will now suggest that they 'push' on 'space'. To get around those now arguing that 'space' therefore is a aether, ahh, I will now suggest that they 'push' on 'gravity'..

So there, a perfectly reasonable explanation, covering ahh, a lot.

No I'm not serious, but I've been waiting on someone using 'virtual particles' for the longest time :)

gregtomko

Could someone explain the rational behind, "same pressure from all directions ==> no movement"? It seems counter intuitive to me. Specifically why wouldn't equal pressure in all directions translate into equal expansion in all directions. Doesn't the idea that the universe may someday have a "big crunch" rely on gravity, a force pulling all objects together? How is that contraction possible, but the opposite expansion not?

juanrga

No. In fact the mass M of the Universe is constant.

gregtomko

that is very interesting, where does the mass go if photons are massless?

gregtomko

oh, OK, they don't have "rest" mass, but they aren't at rest. I wasn't aware they had mass when traveling.

juanrga

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

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

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?

Drakkith

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

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

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

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

Well, to be fair, you were NOT asking "JUST" about the relationship, you said

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

It seems to me that the question of meaningless or meaningful, rests on the relationship of the quantities of energy involved.

gregtomko

I am just curious, is this an understood proportion?

phinds

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.