- Why does dark energy cause the universe’s expansion at the intergalactic level but not at the interstellar level? One problem is that many popular accounts make it appear that “space” is some substance expanding and dragging stuff along. THIS IS WRONG. Let me tell you something interesting. The equations that describe an expanding universe, the so-called Friedmann equations, can be derived from purely Newtonian physics. No relativity theory is required! So, no, space is not a substance. It is not doing any expanding. It is not dragging things along. Instead, when matter appeared at the Big Bang, it had initial momentum. The best way to think about it is that matter came into being in the state of flying apart.
- If all that matter had been what cosmologists call “dust,” matter with no pressure or internal friction, and if it had been entirely homogeneous, it would continue to fly apart at the same rate everywhere. But matter was not completely homogeneous. There were small initial fluctuations. And where matter was a little denser, it had a little more self-gravity. Which means that over time, it was flying apart a little more slowly at such locations.
- In some places, the overdensity of matter was large enough to stop the flying apart completely. Pressure and internal friction also helped, making it possible for matter’s kinetic energy to dissipate as heat, further slowing matter down. If the flying apart was stopped entirely, gravity took over and caused matter to clump. These clumps eventually became clusters of galaxies, and within those clusters, individual galaxies, star clusters in those galaxies, and solar systems in those star clusters. So, the point is that matter inside a galaxy has stopped flying apart. Its self-gravity prevailed over the initial “kick” that this matter received from the Big Bang.
- And just to be clear, when I talk about this initial “kick,” I don’t mean matter flying into pre-existing space. All of space has been filled with matter since the beginning. It’s just that, on average, matter is a lot less dense today than it was just after the Big Bang. If the universe were finite, it must be more significant today than it was back then. But to our knowledge, the universe has always been spatially infinite. And any mathematician can tell you that there are (very counterintuitively) precisely as many points on the real line between 0 and 1 as between o and 10 or 0 and 1,000,000. Dark energy contributes essentially repulsive gravity. That is, under its own gravity, dark energy expands instead of contracting. So, in any region of space dominated by dark energy, dark energy speeds up the expansion.
- Dark energy must first dominate. And the energy density of dark energy is very low. So it is only in space between galaxy clusters, and only in the past few billion years, that dark energy came to dominate over other forms of matter. In regions where the expansion stopped billions of years ago, such as inside a cluster of galaxies or in a galaxy, the density of ordinary (and dark) matter never dropped below the threshold where dark energy becomes dominant. So, the repulsive gravitational response of dark energy never got a chance to prevail over the attractive self-gravity of other forms of matter.
- Stars are hot because nuclear fusion in their cores converts stored energy into heat. It is perfectly valid that nuclear fusion occurs in the centers of stars, producing energy as long as the star fuses elements lighter than iron. But, as plasma physicists have been struggling with for years, nuclear fusion is challenging to get going!
- To get two elements to fuse, you need to force the respective nuclei together until they are “touching” (in a quantum mechanical sense—you need to get them close enough to a quantum tunnel), but the problem is that nuclei repel each other—they both have positive charges.
- That means that to get fusion started, you need to have an incredibly high ignition temperature. A high temperature means the particles smash into each other at much higher speeds, which increases the likelihood of “touching” (or tunneling) and hence makes fusion possible. This ignition temperature is a few million kelvins for the ongoing fusion projects.
- Once you start the process, you can (hopefully, for projects like JET and ITER) get it self-sustaining and outputting a net amount of energy, but there’s no getting around that initial ignition phase. So—for the nuclear fusion to get started in the sun, it already needed to be hot! On the order of millions of degrees!
- Using this information, we can see why the nuclear fusion argument doesn’t answer this question: Your sun was once just a giant cloud of gas floating in space. Not unlike this lovingly rendered mockup, this cloud is far from anything else—no other stars nearby.
Eg=−GMmr
- Into kinetic energy—the particles get faster and faster as they fall towards the center. Pretty obvious, right? You drop something, and it gets faster and faster as it falls downwards. We’ve got a bunch of tiny particles moving super fast. This cloud of gas is just going to keep on contracting and expanding. The energy keeps on changing between kinetic and gravitational. That means it can’t collapse into a dense object because there’s nothing to remove energy from this kinetic/gravitational switching process. Particles can’t pass through each other like I said they could in the previous image! The cloud can’t just turn itself inside out! The accurate picture is that the particles speed up as the cloud contracts. But as they pick up speed, they also bump into each other.
- This “bumping” converts energy from the contraction process into randomly oriented motion—the cloud is still moving inwards overall, but each particle can move in any direction at any given moment.
- The more the cloud contracts, the more energy is put into this “random motion.” The expansion of the cloud also slows down because, at any given moment, some particles might be moving outwards, and as they bump into other particles, they exert a force in this direction.
