Over the weekend, Sean Carroll put up a blog post to address common misconceptions about cosmology. I understood most of his points, but was confused when I saw this one:
Dark energy is not a new force; it’s a new substance. The force causing the universe to accelerate is gravity.
Carroll was referring to the accelerating expansion of the universe. But gravity causing the acceleration, instead of dark energy? I asked in a comment, along with at least one other commenter, how this could be so. Carroll was kind enough to respond to us:
Gravity causes the universe to accelerate because gravity is not always attractive. Roughly speaking, the “source of gravity” is the energy density of a fluid plus three times the pressure of that fluid. Ordinary substances have positive energy and pressure, so gravity attracts. But vacuum energy has negative pressure, equal in size but opposite in sign to its energy. So the net effect is to push things apart.
I had always been under the impression that dark energy was simply the unknown force behind the accelerating expansion, a force I understood to be in opposition to gravity. However, it appears that dark energy actually affects gravity by causing it, on cosmological scales, to be repulsive, to repel distant parts of the universe apart from each other.
The force behind this appears to be negative pressure. Pressure, it turns out, is a source of gravity. Brian Greene in his book, The Fabric of the Cosmos, explains that pressure, in the sense of outward pushing, like what you might find with a coiled spring, is a form of energy, and energy generates gravity. Negative pressure, such as the tension in a rubber band that wants to contract when it’s stretched out, is energy going in a different direction. Negative pressure actually has a negative effect on the attractive force of gravity, causing it to be repulsive.
Albert Einstein understood this when he first formulated his Cosmological Constant to explain why gravity didn’t cause the universe to collapse. After Edwin Hubble’s discovery that the universe was in fact expanding, Einstein would regard the Cosmological Constant as his greatest blunder. It’s therefore ironic that several decades later it became useful again with the discovery that the expansion of the universe was actually accelerating.
So gravity can be repulsive, and dark energy, an energy apparently permeating all of space, due to its negative pressure, brings out this repulsive nature. Many of you who are more knowledgeable about physics no doubt already understood this, but it was a major revelation to me.
I briefly wondered if this might be a way to achieve the anti-gravity capabilities that often show up in science fiction. But after giving it some thought, no, it wouldn’t.
The problem is that most of what generates the gravity that attracts, say, a flying car, to the Earth is Earth’s overall mass. In order to overcome this with repulsive gravity, the car would have to generate so much negative pressure that it would cause the car to generate more repulsive gravity than the Earth’s attractive gravity. Such a force would violently repel everything around it, push the Earth out of its orbit, and probably cause a host of other catastrophes. Not exactly a practical solution.
Still, this is a fascinating effect and I learned something new!