Redshift and Distance in the Expanding Distance! Isn?t This Interesting.
Imagine the ultimate version of a toy Universe: it's expanding, it's full of material, and through it all, there's one photon ?? or quantum of light ?? that we keep track of and forbid from interacting with any other particle. The photon, at any given time, will have all the properties you expect a quantum of electromagnetic radiation to have, including a direction of propagation, a polarization for its electric and magnetic fields, and a wavelength that dictates how much energy is inherent to that photon.
Well, as photons travel throughout the expanding Universe, they experience the effects of that expansion, which stretch it to longer wavelengths. Longer wavelengths imply a decreased energy, and a decrease in energy implies that either energy is not conserved, or that energy must go somewhere. Either way, it's a massive cosmic puzzle.
After all, if there's one thing we've learned about energy, it's that it can neither be created nor destroyed. When you burn wood to create fire, you might think that you're creating energy. But what's actually happening is far more subtle:
? Molecular bonds are being broken and reformed, from a less stable configuration (wood and oxygen) to a more stable one (ash and water vapor), releasing energy in the process.
? If you were to look at the amount of energy released and use Einein's famous conversion, E = mc2, you'd actually find there was a tiny, tiny mass difference between the mass of the product and reactant molecules.
? In truth, the total amount of energy in all its forms, including mass, is unchanged throughout every step in the reaction.
The mass difference is even more pronounced in something like a nuclear reaction, like something that takes place in the Sun. In fact, if you were to measure the mass of the Sun from its birth until now, you'd find that it has lost approximately the mass of Saturn over those 4.5 billion years of emitting energy.
In all the energy-conserving reactions that we know of, keeping track of where all the initial sources of energy and all the final sources of energy are is the hard part. To a physicist, this is just an accounting problem: one so rich that when some radioactive decays (beta decays) were seen to not conserve energy, we postulated a new particle to maintain energy conservation. Although it took 26 years from Pauli's proposal of the neutrino until it was detected, it remains a testament to the power of energy conservation.
But sometimes, things appear to lose energy, and nothing appears to gain energy (or mass) to compensate. That's the case with the expanding Universe. You see, one of the new things that came along with Einstein's theory of General Relativity was the notion that space itself is changeable, rather than a fixed coordinate "grid" that everything lives on. The Universe can and must curve dependent on the amount and configuration of matter and energy inside, and the fabric of the Universe is allowed to expand or contract as well.
The kicker, though, is that any photon ? or particle of light ?has its energy defined by its wavelength. And if the fabric of the Universe stretches (as it expands) or shrinks (as it contracts), the wavelength of that light, and hence its energy, changes as well.
This should bother you! After all, we think that energy should be conserved in any and all physical processes that take place in the Universe. Does General Relativity offer a possible violation of energy conservation?
The scary answer is yes, actually. It's eminently possible that, on a global scale in the Universe, energy isn't conserved. There are a lot of quantities that General Relativity does an excellent and precise job of defining, and energy is not one of them. If you have an expanding Universe, the Universe changes over time; if your Universe isn't invariant to time translations, then there's no rule that states energy must be conserved.
In other words, there is no mandate that energy must be conserved from Einstein's equations; energy is not even defined in an expandingDisclaimer- This information is entirely by a computer program and has not been created or edited by Just Learning.