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the fundamental forces <http://en.wikipedia.org/wiki/Fundamental_forces> other than 1 gravity <http://en.wikipedia.org/wiki/Gravity>. Since it is not very dense-roughly 101 29 grams per cubic centimeter-it is hard to imagine experiments to detect it in the 1 laboratory (but see the references for a claimed detection). Dark energy can only
have such a profound impact on the universe, making up 70% of all energy, be1 cause it uniformly fills otherwise empty space. The two leading models are quintes| sence and the cosmological constant. I 3.2.1 Cosmological constant 1 The simplest explanation for dark energy is that it is simply the "cost of having | space”: that is, that a volume of space has some intrinsic, fundamental energy. This I is the cosmological constant, sometimes presented as ë (Lambda). Since energy 1 and mass are related by E = mc2, Einstein's theory of general relativity <http:// 1 en.wikipedia.org/wiki/General_relativity> predicts that it will have a gravitational ef| fect. It is sometimes called a vacuum energy <http://en.wikipedia.org/wiki/ | Vacuum_energy> because it is the energy density of empty vacuum <http:// 1 en.wikipedia.org/wiki/Vacuum>. In fact, most theories of particle physics <http:// I en.wikipedia.org/wiki/Particle_physics> predict vacuum fluctuations <http://
en.wikipedia.org/wiki/Vacuum_fluctuations that would give the vacuum exactly this I sort of energy. The cosmological constant is estimated by cosmologists (Steinhardt,
Paul J. <http://adsabs.harvard.edu/cgi-bin/author_form? author = Steinhardt, + P & fullauthor = Steinhardt, % 20Paul%20J.&charset=ISO-8859-1&db_key=AST>;Turok, Neil <http://adsabs.harvard.edu/cgi-bin/author_form? author = Turok, +N & fullauthor = Turok,%20Neil&charset=ISO-8859-1&db_key=AST>, 2006) to be on the order of 10-29g/cm3, or about 10-120 in reduced Planck units <http://en.wikipedia.org/wiki/ Reduced_Planck_units>
The cosmological constant has negative pressure equal to its energy density and so causes the expansion of the universe to accelerate (see equation of state 1 (cosmology) <http://en.wikipedia.org/wiki/Equation_of_state_%28cosmology%29>). The reason why a cosmological constant has negative pressure can be seen from classical thermodynamics. The work done by a change in volume dV is equal to -p1 dV, where p is the pressure. But the amount of energy in a box of vacuum energy actually increases when the volume increases (dVis positive), because the energy is equal to nV, where ñ is the energy density of the cosmological constant. Therefore, p is negative and, in fact, p= -ñ.
A major outstanding problem <http://en.wikipedia.org/wiki/Unsolved_problems_in_physics> is that most quantum field theories <http:// en.wikipedia.org/wiki/Quantum_field_theory> predict a huge cosmological constant from the energy of the quantum vacuum <http://en.wikipedia.org/wiki/ Vacuum_fluctuation>, up to 120 orders of magnitude <http://en.wikipedia.org/wiki/ Orders_of_magnitude> too large. This would need to be cancelled almost, but not exactly, by an equally large term of the opposite sign. Some supersymmetric <http:/ len.wikipedia.org/wiki/Supersymmetry> theories require a cosmological constant that is exactly zero, which does not help. This is the cosmological constant problem,
