Is Dark Energy a particle due to the mass-energy equivalence ($E=mc^2$)? I mean surely something cannot have energy without mass. Hence my question, I've been trying to figure this question out, but have come to the conclusion that Dark Energy is a particle. Perhaps it acts like hydrogen? Being abundant everywhere throughout the universe.
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2$\begingroup$ Consider that if something that was dark energy was actually a particle...it would instead be dark matter. EM fields are not built from matter yet they can have an energy density in the fields themselves which isn't made of matter particles. $\endgroup$– TriatticusCommented Apr 15, 2018 at 18:07
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First of all:
I mean surely something cannot have energy without mass.
It can. Photons do not have mass but have energy. The misleading thing that made you think like that is the relation $E=mc^2$. This is true only if the object is free and at rest. The true relation between the mass and the energy depends also on the momentum, i.e., $$ E^2 = p^2 c^2 + m^2 c^4 $$ Therefore, a particle can have no mass but energy or so forth.
I've been trying to figure this question out, but have come to the conclusion that Dark Energy is a particle.
It is not. Because what dark energy is referring to is that the total energy of the universe has a constant amount of extra energy. This kind of energy actually does not affect anything in the standard model of particle physics, because what does really matter is the energy differences.
However, the gravity changes the game. A constant energy to the total energy of the universe would change the gravitational effects and the geometry of the space-time. So, it is entirely different than any particle.
For example, the vacuum energy can be related to the dark energy, or an overall interaction energy of some fields, etc. Whatever it is, it can be detected gravitationally because it changes how the universe expands and how the overall space is curved.
There is also dark matter which is completely a different concept. It is simply some other category of particles, indeed, that we could not understand but exist in such amounts that the galaxies are affected more than the ordinary matter does.
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$\begingroup$ Perhaps "It is not" is a bit too emphatic? en.m.wikipedia.org/wiki/Chameleon_particle $\endgroup$– KaiCommented Apr 15, 2018 at 23:06
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$\begingroup$ Dark Energy as a concept is definitely not a particle. There are some hypothetical particle fields that predicts the dark energy as an effective consequence. $\endgroup$ Commented Apr 16, 2018 at 19:31
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$\begingroup$ Thank you for this, it makes much more sense. And thank you for clarifying photons, I don't know why I didn't think of that!! $\endgroup$ Commented Apr 17, 2018 at 16:28