A recent news story about a new measurement of the “diameter” of the universe (156 billion light years!!) led to a discussion about spatial expansion. If the universe has expanded to 156 billion light years in ~13+ billion years, it must be expanding at an observable rate (since current evidence suggests that the expansion is still continuing). Indeed, the Doppler effect/red-shift of light from sufficiently distant objects does in fact reflect (no pun intended) this. In searching the board, I came across this post that suggests that the rate is thought to be 6.6 centimetres per metre per billion years. Fascinating bit of information, and I understand the impossibility of performing experiments, but how was this value deduced?
However, the real questions that prompted me to start this thread are around red-shifted light: Where does the energy lost by photons go? Thermodynamics says it has to go somewhere. In our intellectual flailing around, we wondered if the lost energy is somehow connected to the expansion of the universe. In other words, does the energy lost to the expansion of the univsere actually fuel the process of expansion? That’s something none of us have heard of, so it’s likely not the case, but we’re at a loss to explain how the expansion of the universe complies with thermodynamics.
You need to think in terms of reference frames. From the reference frame of the atom that emitted the photon, the photon never redshifts, no energy change. From the view of an observer a long way off, the photon was born redshifted. It never lost energy since it had the same energy all along. For the remote viewer’s point of view, the reason the photon is redshifted is because of Lorentz contraction/increased mass of the emitting atom type stuff. So the atom generated a photon with the “right amount” of energy. Nothing is lost.
Point of view doesn’t alter laws of conservation of energy. This is one reason why relativity had to come out the way it did.
As for your first question, the expansion rate of the Universe is measured by looking at distant galaxies (distant enough that they’re not significantly affected by the gravity of our clump of galaxies), and measure their recessional velocity and their distance from us. The recessional velocity is easy to measure, since you can get that directly from the Doppler shift. Distance is more difficult to measure, but it’s generally done by using some sort of “standard candle” method. You find something in the other galaxy for which you know its true brightness, and see how bright it actually looks. Distance will make it look dimmer, and how much dimmer will tell you how far away it is. This method is often used with a particular type of supernova (type 1a), since we understand that type of supernova well enough to know their brightness, and they’re bright enough that we can see them even in the most distant galaxies.
The decreasing density of the universe as it has expanded caused it to slow down due to the decreasing effects of gravity. However, about 6 years ago it was discovered that the universe is accelerating its rate of expansion. Researchers are considering two divergent scenarios: dark energy and modification of the laws of gravity. Dark energy is preferred now. This, too, has two different theories: negative pressure of the interstellar vacuum and the 11-dimensional universe.
The vacuum seethes with virtual particles. The general theory of relativity holds that pressure also exerts a gravitation force, and it can be either positive or negative. However, scientists say that the density of this vacuum energy is much bigger than that of dark energy.
The other theory says that gravity parts company with GR because some of it leaks away into extra, hidden dimensions. The universe as we know it has 4 dimensions, called a “brane” (short for “membrane”), but the theory holds that this brane is embedded in a higher-dimensional world. Because gravity is an intrinsic property of all of space-time, it may be the only component of the cosmos that isn’t trapped in this brane. When gravitons, the particles that mediate gravitation attraction, escape the local brane, the gravitational force that remains within the brane diminishes, which shows up as an increase in the rate of cosmic expansion.
