Why Does Dark Energy Make the Universe Accelerate Peter Coles has issued a challenge explain why dark energy makes the universe accelerate in terms that are understandable to non scientists. This is a pet peeve of mine any number of fellow cosmologists will recall me haranguing them about it over coffee at conferences but Im not sure Ive ever blogged about it directly, so here goes. In three parts the wrong way, the right way, and the math. Supernova Crack' title='Supernova Crack' />The Wrong Way. Ordinary matter acts to slow down the expansion of the universe. That makes intuitive sense, because the matter is exerting a gravitational force, acting to pull things together. So why does dark energy seem to push things apart The usual wrong way to explain this is to point out that dark energy has negative pressure. The kind of pressure we are most familiar with, in a balloon or an inflated tire, pushing out on the membrane enclosing it. Supernova Crackers' title='Supernova Crackers' />But negative pressure tension is more like a stretched string or rubber band, pulling in rather than pushing out. And dark energy has negative pressure, so that makes the universe accelerate. Peter Coles has issued a challenge explain why dark energy makes the universe accelerate in terms that are understandable to nonscientists. This is a pet peeve of. Why do most of the game of thrones torrents not have either hard baked foreign subsidiaries sub files with them The show has 2 fictional launage, so its not like. Echo The list of software which use Sentinel SuperPro dongle licensing. The ad for todays Nice Price or Crack Pipe Callaway Corvette claims the twinturbo convertible to be 51 out of just 184 produced. Lets see if this storied, and. Freeza, spelled as Frieza in the Funimation dub, is one of the main antagonists of Dragon Ball Z. The Devil Wears Prada is an American Christian metalcore band from Dayton, Ohio, formed in 2005. It consists of members Mike Hranica vocals, additional guitar. Star Wars A New Hope script at the Internet Movie Script Database. If the kindly cosmologist is both lazy and fortunate, that little bit of word salad will suffice. But it makes no sense at all, as Peter points out. Why do we go through all the conceptual effort of explaining that negative pressure corresponds to a pull, and then quickly mumble that this accounts for why galaxies are pushed apart So the slightly more careful cosmologist has to explain that the direct action of this negative pressure is completely impotent, because its equal in all directions and cancels out. Thats a bit of a lie as well, of course its really because you dont interact directly with the dark energy, so you dont feel pressure of any sort, but admitting that runs the risk of making it all seem even more confusing. What matters, according to this line of fast talk, is the gravitational effect of the negative pressure. Supernova Cracker' title='Supernova Cracker' />Supernova CrackleAnd in Einsteins general relativity, unlike Newtonian gravity, both the pressure and the energy contribute to the force of gravity. The negative pressure associated with dark energy is so large that it overcomes the positive attractive impulse of the energy itself, so the net effect is a push rather than a pull. This explanation isnt wrong it does track the actual equations. But its not the slightest bit of help in bringing people to any real understanding. It simply replaces one question why does dark energy cause acceleration with two facts that need to be taken on faith dark energy has negative pressure, and gravity is sourced by a sum of energy and pressure. The listener goes away with, at best, the impression that something profound has just happened rather than any actual understanding. The Right Way. The right way is to not mention pressure at all, positive or negative. For cosmological dynamics, the relevant fact about dark energy isnt its pressure, its that its persistent. It doesnt dilute away as the universe expands. And this is even a fact that can be explained, by saying that dark energy isnt a collection of particles growing less dense as space expands, but instead is according to our simplest and best models a feature of space itself. The amount of dark energy is constant throughout both space and time about one hundred millionth of an erg per cubic centimeter. It doesnt dilute away, even as space expands. Given that, all you need to accept is that Einsteins formulation of gravity says the curvature of spacetime is proportional to the amount of stuff within it. The technical version of curvature of spacetime is the Einstein tensor, and the technical version of stuff is the energy momentum tensor. In the case of an expanding universe, the manifestation of spacetime curvature is simply the fact that space is expanding. There can also be spatial curvature, but that seems negligible in the real world, so why complicate things. So the density of dark energy is constant, which means the curvature of spacetime is constant, which means that the universe expands at a fixed rate. The tricky part is explaining why expanding at a fixed rate means accelerating. But this is a subtlety worth clarifying, as it helps distinguish between the expansion of the universe and the speed of a physical object like a moving car, and perhaps will help someone down the road not get confused about the universe expanding faster than light. A confusion which many trained cosmologists who really should know better continue to fall into. The point is that the expansion rate of the universe is not a speed. Its a timescale the time it takes the universe to double in size or expand by one percent, or whatever, depending on your conventions. It couldnt possibly be a speed, because the apparent velocity of distant galaxies is not a constant number, its proportional to their distance. When we say the expansion rate of the universe is a constant, we mean it takes a fixed amount of time for the universe to double in size. So if we look at any one particular galaxy, in roughly ten billion years it will be twice as far away in twenty billion years twice that time it will be four times as far away in thirty billion years it will be eight times that far away, and so on. Its accelerating away from us, exponentially. Constant expansion rate implies accelerated motion away from us for individual objects. Theres absolutely no reason why a non scientist shouldnt be able to follow why dark energy makes the universe accelerate, given just a bit of willingness to think about it. Dark energy is persistent, which imparts a constant impulse to the expansion of the universe, which makes galaxies accelerate away. No negative pressures, no double talk. The Math. So why are people tempted to talk about negative pressure As Peter says, there is an equation for the second derivative roughly, the acceleration of the universe, which looks like this I use a for the scale factor rather than R, and sensibly set c1. Here, is the energy density and p is the pressure. To get acceleration, you want the second derivative to be positive, and theres a minus sign outside the right hand side, so we want 3p to be negative. The data say the dark energy density is positive, so a negative pressure is just the trick. But, while thats a perfectly good equation the second Friedmann equation its not the one anyone actually uses to solve for the evolution of the universe. Its much nicer to use the first Friedmann equation, which involves the first derivative of the scale factor rather than its second derivative spatial curvature set to zero for convenience Here H is the Hubble parameter, which is what we mean when we say the expansion rate. You notice a couple of nice things about this equation. First, the pressure doesnt appear. The expansion rate is simply driven by the energy density. Its completely consistent with the first equation, as they are related to each other by an equation that encodes energy momentum conservation, and the pressure does make an appearance there. Second, a constant energy density straightforwardly implies a constant expansion rate H. Inuyasha Game Psp. So no problem at all a persistent source of energy causes the universe to accelerate. Banning negative pressure from popular expositions of cosmology would be a great step forward.