Apparently all light created since the Big Bang is still moving through the Universe. Will there ever be a point where the light created 14 billion years will totally disperse and the entire universe will be dark?
You’re probably talking about the cosmic background radiation. Strictly speaking, it’s not from the Big Bang, but from a time about 370,000 years after the Big Bang when the gas making up the early universe became thin and cool enough to be transparent.
As the universe expands, the cosmic background radiation stretches along with it. Its wavelength gradually drops, like the whistle of a receding train. The wavelength of the CBR is now about 2 mm, in the microwave range. It will never disperse, but its wavelength will get longer and longer, making it harder to detect.
Not correct. Much of the light is absorbed.
For example, touch the top of a black car sitting outside on a sunny day – the heat you feel is from sunlight created about 8 minutes ago, traveling through space, and being absorbed by the black paint on the car.
True, but it took thousands of years (between 20,000 and 1,000,000 depending on the source) for that light to travel from the sun’s core to its surface.
I was going to bring that up too, KarlGauss, but I guess that depends on how you think of “that light.” The energy started life as a photon created by fusion, right? But then that photon was almost immediately absorbed by the dense matter around it, which then re-emitted another photon because it was already really hot, and so forth until it finally reaches the surface of the sun and comes toward us. But since the photons gets absorbed and re-emitted at each step, you could say that the photons we see really are only 8 minutes old, even though the photon that started each of those visible photons’ life was originally emitted eons ago… right?
[further nitpick]The temp of the black car is caused by a lot more than just the photons which left the Sun 8 minutes ago and are striking the car just at the moment. The temp is the result of the cumulative effect of the incoming photons over the last few minutes or hours since the car was moved outdoors and the sun came up.
[/further nitpick]
But the larger point remains as previously described: Photons have been zinging around since near the beginning of time. They can be absorbed and later re-emitted by matter. When that happens, the re-emission is usually at a different (lower) energy level = frequency. On a large enough scale, the total absorbtion is more or less constant over time. So in a sense, the same photons have been here since the beginning.
As the Universe expands and entropy does its thing along the arrow of time, the average photonic energy goes down and the frequency goes down. This applies to all frequencies of EMR, not just the narrow frequency range our eyes are sensitive to which we naively call “light”.
So what was “light” is now “heat”.
Indeed, I think you are spot on.
Perhaps it’s better to say that the photon we see or feel here on Earth, although leaving the Sun just 8 minutes ago, is the result of an event that took place tens of thousands, or even millions, of years ago.
The light from the CMB will continue to bathe the universe forever, but the expansion of the universe will stretch its wavelength arbitrarily high, making it practically undetectable. Light can’t be destroyed, it can only lose some of its energy to exciting the motion of a particle, e.g. heating up a substance. Once the substance is heated up, it will continue to emit photons and lose its energy unless more photons from an outside source energize it again. Once all the stars burn out, there will be no more source of high energy photons and everything will cool down to near absolute zero. There will be a few fluctuations here and there when black holes evaporate and possibly if protons decay, but even the high energy photons resulting from these events will be stretched out into oblivion by the expanding universe, so ultimately all light will still be around, but be so low energy that it might as well not exist.