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According to a little I have understood whatever we see in the universe is its postive energy and there is so an equal negative energy.

  1. Is this Negative energy the Dark energy? which is responsible for the expansion of the universe? which we have experienced by blue shift and red shift or particularly galaxies moving away from us.

  2. why we cant see this energy?

  3. Is it hidden?

Please answer this regarding me as a starter physics Student.

Qmechanic
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Tushar Bhalla
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2 Answers2

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Dark energy is not negative energy. It causes a repulsion because of its unusual equation of state, which causes it to behave as if it has a negative pressure. There is some discussion of this in the answers to Have negative pressures any physical meaning? and possibly also 'Negative pressure' counteracting gravity?.

When general relativists talk about the energy of the universe they normally mean an object called the stress-energy tensor. The mathematical description of this looks a bit scary, but basically it measures the amount of stuff in the universe, where stuff includes matter, dark matter, photons, dark energy and anything similar that we haven't discovered yet. As far as we now all this stuff, has a positive energy density.

However when discussing the energy of the universe some commentators like to include the gravitational potential energy, and depending on how you define this it can be negative. There is an argument that the total energy of the universe is zero, and this is based on the idea that the positive energy of matter, photons and dark energy is balanced by an equal negative potential energy so the two add up to zero.

If the last paragraph reads as if I'm being deliberately neutral, that's because I am. It's not obvious that the energy of the universe or the gravitational potential energy of the universe is a well defined and useful concept. Physicists have differing views about this.

Community
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John Rennie
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  • Ya I have understood a little from your precised answer but as we refer the gravitational potential energy of our old theories energy possessed due to earth's gravitational field is – Tushar Bhalla Feb 26 '15 at 05:48
  • The gravitational potential energy then the energy you are referring in ur answer is the energy possessed due to bending of space time by sun – Tushar Bhalla Feb 26 '15 at 05:49
  • Or any other energy – Tushar Bhalla Feb 26 '15 at 05:52
  • PLEASE #johnrennie clarify my doubt {{ urgent }} – Tushar Bhalla Mar 04 '15 at 14:32
  • @TusharBhalla: I don't understand what you're asking – John Rennie Mar 04 '15 at 15:36
  • I AM asking what is the cause of the potential energy u r referring in your answer – Tushar Bhalla Mar 05 '15 at 06:30
  • @TusharBhalla: in the absence of dark energy the origin of the PE is obvious because it's just due to the mutual attraction of all the matter. In the presence of dark energy I can't think of an intuitive explanation. If you follow the links from the article by Phil Gibbs that I linked in my answer then that will give you all the maths. But I can't think of a way to express this in everyday terms. – John Rennie Mar 05 '15 at 07:20
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Negative energy can refer to several different phenomena:

By definition, the potential energy between attracting bodies is negative, so that together with their kinetic energy, the total energy is zero. This is nothing magic, just a convention.

In quantum theory, there are several forms of energy which are negative, e.g. the Casimir effect and virtual negative-energy particles created along with normal particles for a short perdiod of time (as in Hawking radiation).

"Energy" cannot by itself be seen, but it can affect bodies and particles in a way that can be seen, as when the negative potential energy of an apple is converted to positive kinetic energy as it falls to the ground.

None of these energies have anything to do with dark energy, which is (perhaps, we don't know, really) more likely a form of energy behaving like a negative pressure, causing a repulsive force on space.

pela
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