The Galaxy rotates as if it contains more mass than can be seen.
Due to E=mc^2, matter and energy are interchangeable.
Heat is a form of energy.
Energy can distort spacetime like mass.
Can a galaxy full of heat from stars and black holes and whatnot account for “missing mass”?
Furthermore, could the cold, massless voids between galactic superclusters be Dark Energy?
I do not think so. As space expands the energy density in the universe from heat would decrease (more space, same amount of heat). Dark energy’s pressure (for lack of a better word) seems to remain constant as space expands.
Nope. The indications are that the great majority of matter in the universe is dark matter; if there was that much heat we’d know because the galaxy in question would be literally exploding like a supernova.
A standard example of a mistake in syllogistic reasoning is:
All pennies are made of copper. This thing is made of copper. Therefore it is a penny.
I mean, heat does have mass. But unless you’re at a truly ludicrous temperature, it’s a very, very small amount of mass.
Yes. There’s even the concept of a kugelblitz, a black hole created entirely from energy.
A kugelblitz (German: [ˈkuːɡl̩ˌblɪt͡s] ⓘ) is a theoretical astrophysical object predicted by general relativity. It is a concentration of heat, light, or radiation so intense that its energy forms an event horizon and becomes self-trapped. In other words, if enough radiation is aimed into a region of space, the concentration of energy can warp spacetime so much that it creates a black hole.
But that doesn’t mean you can significantly curve space without so much energy that it would be very noticeable. And, well, it’s called “dark matter” in the first place because we can’t see it.
Wouldn’t that much energy in one spot just get converted into matter anyways, like when energy was converted into protons and electrons during the Big Bang?
I don’t…think so. It’s a lot of energy, but a galaxy is really huge so it would be spread out quite a lot. But that’s a question well beyond my knowledge of physics to be sure of.
Hah, excellent example, but it won’t age well!
Arguably it already hasn’t, since pennies haven’t actually been even mostly copper since 1982, and haven’t been 100% copper since 1857. Yes, I’m being pedantic, but with a smile on my face–so typical for something “everybody knows” to become no longer true, such that at some point it will just seem wrong! And you kids get off of my lawn.
As I understand it, dark matter does not interact with ordinary matter; it only makes its presence known through gravity. “Heat” is photons and photons interact via electromagnetic forces. Current theory might be entirely wrong, but that’s very low probability.
Also, there are no “massless” voids, merely voids with a low number of galaxies, plus whatever dust and gas and rogue stars that are too faint to easily see with our current technology.
It really can’t be, though. Heat is the kinetic energy of a bunch of particles going in a bunch of different directions. Which means that you also have the mass of those particles. And unless the particles are moving at relativistic speeds, the heat will be much less than the rest mass.
Temperature is a function of how fast those particles are moving. If they were moving at relativistic speeds, the matter would be really, really hot.
The problem is that if they were moving at relativistic speeds then the galaxy wouldn’t last long, because they would have enough energy to fly away from each other and dissipate. Since [checks notes] the galaxy hasn’t yet flown apart, we can conclude that its constituents aren’t traveling at the speed of light.
Physicists do think about unknown relativistic particles as a possible constituent of dark matter; it’s known as hot dark matter, to distinguish it from particles that are moving at much less than the speed of light (which are cold dark matter.) But having a certain amount of cold dark matter is required to get galaxies to form in the early Universe; hot dark matter is (roughly speaking) moving too fast to clump together and attract regular matter to form galaxies. So you still need som unknown slow-moving particles to explain things.
As an aside, I’m quite fond of scientific observations based on facts like “The galaxy hasn’t flown apart yet”, or “Mars is still in its orbit”, or the like. You can reach solid conclusions that way surprisingly often.