Upgrade your account to a Community Supporter account and remove most of the site ads.

Reply to thread

Message: <blockquote data-quote="fuindordm" data-source="post: 2407028" data-attributes="member: 5435">I thnk that's what I said, in another way...I'm more of a simulationist too, and unfortunately my QM isn't strong enough to get deep understanding of the possible nature of the dark energy.  From the perspective of cosmologists, the evolution of the universe can be modeled as the adiabatic expansion of a perfect fluid with more than one component.  The radiation component has one equation of state, the matter and dark matter components have another equation of state (zero pressure, since it's so sparse), and the dark energy has a third exerting negative pressure proportional to the energy density.  The equations that model the distribution of supernovae redshift come from following the evolution of these three components--these are the curves drawn through the data.The dark energy component has some wiggle room, since we don't know the constant of proportionality between energy density and pressure.  But if the pressure weren't negative then the curves would swing the opposite way and lie well outside the data, large error bars or no. (And keep in mind, even if the error bars are large when there's a lot of data you can still get pretty good statistical constraints.)As for whether the energy density of the dark energy is *really* negative, from my point of view the difference between saying that it creates negative pressure or that it has negative energy density is purely semantic.  A substance with an equation of state like Pressure = -C*density will have the effect in general relativity of promoting an expansion of space-time, wheras normal matter will have the effect of contracting space-time.  So what is needed for a warp bubble is something like the dark energy, but in a controlled form.  The surveys going on right now are trying to constrain the value of C.A cosmological constant would still fit the data.  The difference between a cosmological constant and dark energy is that the cosmological constant behaves like a 100% uniform field of dark energy with equation of state P = -density (C=1) --for example, the quantum mechanical zero-point field energy.  I do know enough about quantum field theory, however, to say that all calculations of this field energy so far have either given a value of zero or a number dozens of orders of magnitude higher than the observed universe allows.  So if it is a cosmological constant, its physical origin begs explanation.To sum up, I would give a cosmological constant even odds against a new, exotic field.  Both explanations have problems that can't be resolved right now, but both have the correct equation of state and fit the data.  Both also have "negative energy density" in the sense that they provoke an expansion of space rather than the usual contraction, but you're right that if it turns out to be simply a cosmological constant there's probably little we can do with that.On a final note, however, the cosmological constant isn't really constant; its value evolves along with the universe.  In the far distant future, when the universe has expanded so much as to be effectively empty, its value will approach 1.Ben</blockquote>

Verification