The cosmic voids of the universe are empty of matter. But we all know there’s more to the universe than just matter. Nothing in this universe is completely empty, and that’s because there’s always your constant companions. Me? No, not me, I only visit once a month.
For one, there are the neutrinos and the cosmic rays. These are high-energy particles zipping around the cosmos. They’re blasted out by all sorts of fun and energetic processes, like stars blowing up, stars getting torn to shreds, stars smashing into each other, and other various and sundry star-destroying processes. Oh, and the giant black holes, neutron stars, white dwarves, giant molecular clouds, fast radio bursts, accretion disks…man there’s a how lot of astrophysics out there, isn’t it?
Anyway, all these processes ten to produce showers of particles that go flying out at nearly the speed of light. These include protons and other misc atomic nuclei, which we call the cosmic rays (even though they’re not rays at all, but by the time we figured that out the name had stuck) and the neutrinos, which are the super-light, ghostly particles that hardly ever interact with matter.
On a typical day if you’re standing outside, about one cosmic ray strikes you every single second, and that’s AFTER many more have been blocked or absorbed by the Earth’s magnetic field and atmosphere. As for neutrinos, on that same typical day about 100 trillion neutrinos pass through you every single second, but they’re so slippery that you should expect only one of those to ever actually hit you over the course of your entire lifetime.
And if cosmic rays and neutrinos can do it here, they can do it anywhere. We know for a fact that cosmic rays and neutrinos can cross intergalactic distances, because we’ve traced back their origination points and, lo and behold, they often come from other galaxies. This means that neutrinos and cosmic rays and constantly crisscrossing the voids, spending tens of millions of years in their traversals of the great cosmic deserts before reaching another galaxy.
They don’t exactly add up to much, however. Yeah if you were sitting inside a void and had a sensitive enough detector, you would get ping ping pings as these particles zipped through. And that’s about it.
In the deepest centers of the voids besides the cosmic rays and the neutrinos and the dark matter that occasionally pay you a visit, there is still the ever-present bath of radiation. Some of this radiation is from starlight, although as you can imagine it's not exactly going to add up to a lot. The nearest galaxies are so far away that they're only visible with the most powerful telescopes. So yeah while they are technically sending photons in your direction it's not exactly a significant number .
But there's more to light in the universe than just Stars and galaxies. In fact even in our corner of the universe, which is a relatively dense neighborhood, starlight accounts for only an insignificant fraction of all the radiation in the cosmos. By far, the vast vast vast majority of light in the universe is due to the cosmic microwave background .
The cosmic microwave background was created in an event that occurred when the universe was only 380,000 years old. At that time the universe had expanded and cooled to the point that it turned from a hot dense plasma into a slightly less hot slightly less dense neutral gas. This freed the radiation. In physics we like to call this kind of situation free streaming. The light was finally able to just traverse the universe without really encountering any significant collisions or resistance or absorption.
The whole universe Participated in the generation of the cosmic microwave background. Every part of the universe released that light and that light has been traveling through the universe ever since so no matter where you go you are always surrounded by that radiation. When it was released that radiation had a temperature of around 10,000 kelvin, the same as the surface of a typical star. It was literally white hot. But it's been a while and the universe is a lot colder and a lot older and a lot more stretched out nowadays, and so that intensity of radiation has really fallen off and it is now firmly in the microwave band. Hence the name cosmic microwave background .
But still even in that fossil state that has been preserved for over 13 billion years the cosmic microwave background is responsible for something like 99.999 and you could probably add a few more nines just to get the point across percent of all the radiation in the universe .
So even if you're in a deep deep void you're still surrounded by the cosmic microwave background because the cosmic microwave background literally fills up the universe. However even though it's present it's not exactly going to do a lot for you. Yes if you have a sensitive microwave detector you're going to pick it up, you're going to see that those photons are washing over you. But the energy carried by the CMB is NOT a lot .
Even though the CMB is by far the brightest thing in the universe with by far the most amount of photons, there's a whole lot of universe out there for the CMB to be spread through. The temperature of the cosmic microwave background is a little less than three Kelvin. That's just three° above pure absolute zero. So if you were stuck in a void and had no other source of warmth, no other source of light, no other source of heat, you could at least rest assured that your frozen corpse would be prevented from reaching a temperature of absolute zero because of the presence of the cosmic microwave background. so you have that going for you which is nice I suppose but also you're not going to really notice it on a day-to-day basis in the deep corners of the voids.
But it's technically there. And like I said this is science ,and in science precision matters, so we have to add it to the list.
