This comes from having only a low level of knowledge of physics and cosmology but here goes…
The sun emits a tremendous amount of energy. Only a tiny bit of it hits and is absorbed by things orbiting it (Me on the beach, for example) The rest just passes on into ‘space’.
Billions of other stars are doing the same thing.
But what happens to all that energy? It it just raising the ‘temperature’ of space?
Does it in any way to contribute to the expansion of the universe? (I assume its not an explanation for Dark Energy - somebody would have thought of that by now)
Imagine that in the center of the room you’re sitting in, an atom is emitting photons. Think about the size of that atom compared to the size of that room. Is that atom really affecting that volume to any measurable degree?
That’s like space. The sun emits a lot of energy. But it’s 25 trillion miles to the nearest star. That’s a volume of 6.5*10[sup]40[/sup] cubic miles. 65000000000000000000000000000000000000000 cubic miles. Up that another 18 zeroes to get to the volume of space to the Andromeda Galaxy. Which is part of the local cluster. That’s why Douglas Adams gets thought to be profound when he talks about how big space is.
Space is just too big to care about stars. We care because we’re next to one. They don’t matter in the big picture. Too small and puny.
Photons don’t contribute to expansion, either. Unless you’re next to an exploding star, in which case you’ll expand whole bunches.
Look up at night, at a random infinitesimal point on the sky. You’re vastly more likely to see nothing at that point than you are to see a star. That’s how empty space is, and it’s just as empty looking at it the other way around, starting at a star.
There are a lot of photons in any given region of space. The vast, vast majority of them are heading directly for Nowhere In Particular, but they’re still there. Very, very early in the Universe’s history, the energy in those photons was relevant for the Universe’s expansion (slowing it down, not speeding it up), but not any more. Now, matter is much more significant than light, and the dark energy (whatever it is) is a few times more significant than matter.
This is partly because all of that energy from starlight is, in some sense, being lost. Right now, photons emitted from stars have, more or less, the energy corresponding to the temperature of a star’s surface. But as they get billions of lightyears away, they’re then in regions of the Universe where everything is receding away from where they originated, and so they’re redshifted to lower energies. Meanwhile, some of the photons that left that region long ago were heading towards our region of space, and they’re redshifted, too. So any given region of space has about as many photons as it’s ever had, but they’re lower-energy than the photons it used to have.
And no, this doesn’t violate the Law of Conservation of Energy, because that law doesn’t actually mean what most people think it means.
To a first level approximation, space does not have a temperature. Or, it’s essentially 0 deg K. Remember that “temperature” is just the movement of atoms (or molecules), and space is mostly lacking of those things. If you picked a random part of space and stuck a thermometer there, it would read very close to 0 deg K. Unless, you picked a hot spot that just happened to have a bunch of matter banging around. But most of space is devoid of matter, and wherever we do encounter significant amounts of matter, we tend to think of those areas as “not space” (like the Earth).
At a more exact level, yeah, whenever a photon hits something and gets absorbed, it makes things bounce around a bit more, and raises the temperature. But if there’s nothing to hit in the area where you are, then there is no effect to measure.
Mass-energy is conserved. When a star emits energy it is losing mass.
The total amount of mass-energy averaged over a large* volume stays the same.
Think of a cube hundreds of millions of light years across. People tend to think in terms of tiny volumes like solar systems in this context. But star systems are relatively few and far between in space. They are basically oddities. Think in terms of the Big Picture.
Actually, no. What is true that the change in the amount of energy within a region, as measured in any given reference frame, is equal to the net amount of energy crossing the boundaries of that region. That’s what conservation of energy actually means.
This is not very accurate. Even a completely empty space has a temperature defined by the radiation environment. If you stick a thermometer there, the thermometer would not read 0 degrees K. It will reach an equilibrium where it radiates as much radiation as it receives. In interstellar space (i.e. inside our galaxy but not very close to any particular star), it will be somewhere around 10 to 20 degrees K. We know that because molecular clouds, which are just inert clouds of gas floating in interstellar space, reach equilibrium at these temperatures. Even in intergalactic space it won’t get colder than the cosmic microwave background, about 2.7 degrees K. Actually you still have starlight from nearby galaxies, so it’ll probably be a degree or two above that.
Also, even intergalactic space is not completely empty. There may be about 1 atom per cubic meter. In the vicinity of a star (e.g. in our solar system but away from any planet) there are millions of atoms per cubic meter.
Exactly. All the energy emitted by stars will make the universe brighter over time. Any given patch of sky is full of galaxies already. The reason the night sky is so dark is because the universe hasn’t existed for very long.
I’m not sure if this is true. Perhaps at present the number of photons emitted exceeds the number absorbed, but the expansion of space needs to be taken into account, both in terms of the number of photons reaching any given point in the universe, and their wavelength.
The “brightest” time in the history of the universe kind of depends on what you mean, I guess. There was a photon epoch, but photons did not traveled far in that period, they were scattered by free electrons, so I’m not sure that calling it “bright” is an apt adjective. The best candidate for the “brightest” time in the history of the universe is probably the end of the photon epoch, the time of recombination when neutral atoms first formed, and the universe became transparent, i.e. photons could now travel through the universe unimpeded. Those photons became the CMBR, and at that time they were much more energetic, the universe was uniformly as bright as the sun. The expansion of the universe has since shifted those photons to much lower microwave energies, far from visible light.
Olbers’ Paradox rested on a number of assumptions: that the universe had existed forever, that it was static, and that an infinite number of stars existed.
The logic behind this said that a star therefore existed on a line through any and every point in the 360° sphere that surrounds the earth. Light from that star would eventually hit the earth. It didn’t matter how far away it was. It would arrive sooner or later and because there were more stars at that distance, their total light would always be equal to the fewer but closer stars at any distance.
We know that none of these are true. Since the number of stars are finite, most points on the sphere surrounding earth have no stars on a direct line. In addition, they are moving away from the earth at increasing speeds so that most of the light will never reach us, and that’s true already even at this comparatively young age of our universe.
The assumptions in the OP seem to me to be a different set than Olbers’. I don’t see how those apply here.
You seem to have a rather high bar for what “similar” means! The OP’s “What happens to the energy emitted by stars?” is exactly what Olbers asked.
Obviously Olbers know nothing of Big Bang cosmology, and two centuries have passed. I wasn’t suggesting that it’s still an unresolved paradox, the resolution is described in the Wikipedia article that I linked to. But I think it’s certainly worth noting that the OP’s question is a good question, with an interesting history.
I do see them as different. Olbers, to my knowledge, did not ask what temperature space was. His concern was for received light. That is shown by some of the answers to his paradox, the ones that postulated dust clouds or other types of interference cutting the light off. (Those would themselves warm up and radiate photons of various wavelengths, just kicking the can down the road.) The OP asked about light during transmission, the light that does *not *reach the earth or anyplace else.
I may be wrong in my assumption. UncleFred never returned to his thread. Even so, I’m not doubting your physics, just seeing the question in a different light. So to speak.
The Olbers paradox is, why isn’t the why sky as bright as the surface of a star? It’s equivalent to saying why the radiation temperature of the night sky is not thousands of degrees.
While we’re ignoring our atmosphere, let’s go ahead and ignore the Milky Way as well … as scr4 points out … whichever direction we look, and if we look long enough, we will see another galaxy … so in a sense we do have “light” bombarding us from all directions all at once … the sky isn’t bright as day simply because the light isn’t strong enough … it’s there, just we have to count the individual photons …
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