Re: the edge of the universe and after

There are a bunch of questions here – let’s take them one at a time! First, you ask whether the space inside of atoms is being stretched with the rest of the universe. If so, this could have pretty catastrophic consequences, as you point out. Fortunately, the answer is probably no. We can conceptually divide the universe’s expansion into two parts. First, there’s the “leftover” expansion from the Big Bang. This event imparted an expansion velocity to the universe, but the expansion is slowing down with time because of the gravitational attraction of all the matter in the universe. It’s basically like a car that begins at high speed but slows down because of friction with the street – in the universe’s case, the “friction” is gravity.

So this means that the expansion is much weaker than gravity. Atoms, as you probably know, are held together by electromagnetic forces. It turns out that the electromagnetic force is more than a billion billion times stronger than gravity. It may not seem that way in everyday life – after all, gravity does a pretty good job of holding us on the ground – but that’s only because the earth is so massive, while most things are (on large scales) electrically neutral. But because the electromagnetic force is so enormous inside of atoms (at least compared to gravity), it easily overcomes the expansion. In fact, even the gravity from a large blob of matter is enough to stop the expansion near that blob. The Milky Way is massive enough and concentrated enough that it does not expand: the gravity of all the stars (and dark matter) have halted the expansion near our galaxy. (You might find this answer helpful here.)

However, there’s another source to the expansion: the recently discovered dark energy. Dark energy is completely mysterious; we really have no idea what it is or how it behaves in detail. But the most important point is that it acts as a source of “negative pressure” that actually pushes outward on the universe, causing the expansion to accelerate. Most theoretical models for this dark energy predict that it is relatively weak, meaning that the acceleration is slow and doesn’t grow too rapidly with time. But there are some models, called phantom energy in which the dark energy gets stronger and stronger with time, causing the universe to expand more and more rapidly. In some of these models, the dark energy can even eventually dominate over the electromagnetic force and tear atoms apart. It’s not clear yet which direction the data points – but in any case, the “big rip” wouldn’t happen for many billions of years, so no reason to worry yet.

Next question – can light reach the edge of the universe? The answer is no. The most important reason is that we think that the universe itself is infinite, without any edges to hit! There’s no way to verify this, obviously, but we haven’t seen an edge and the current inventory of mass and energy in the universe is consistent with an infinite universe. You can learn more here (where you’ll also find many other answers to related questions about the infinite universe). (By the way, it’s also true that the universe itself can expand faster than the speed of light. This gets a bit tricky, because it depends on how you define things, but (relative to us) most of the universe appears to be expanding faster than the speed of light. This is okay because, locally, nothing actually moves faster than the speed of light. Again, you can find more details here.)

Finally: do photons exert gravity? Yes, they do. By Einstein’s famous E=mc^2, energy and mass are equivalent. Photons do carry energy (that’s why you get hot if you stand close to a lamp), so they do indeed create gravity. However, it turns out that they very rarely carry enough energy to exert a significant gravitational force. For example, there’s only about 1/10,000 as much total energy in photons as there is in matter in the universe right now, although for the first 100,000 years or so of the universe’s existence radiation did indeed dominate over matter.