How can space expand between individual hydrogen atoms? [Strange Universe Series, 2011]

Gravitational force becomes weaker over distance (inverse square). Also, gravity is the a very weak force. You can not even measure it in small scale (a.k.a sub-atomic scale). It does not help either that a Hydrogen atom does have a tiny, barely measurable gravitational pull, and with those points in mind, a metre becomes a really big distance, and the gravitational forces present in such a space will be (quite) smaller than the pressure exerted by zero point energy.
Also, the gravitational pull of a galaxy is not exactly tiny. After all, we are talking about billions of stars, dust clouds, nebulas, planets, plus a supermassive black hole that are thought to exist at the centre of each galaxy. While the gravitational pull does get weaker over distance, too, the remaining force, coupled with similar forces from the other galaxies, which then kind of add up, is enough to still be bigger than the ZPE’s pressure of the space between them.

@ragingloli It must be so based on observed phenomena. However, the density of hydrogen atoms in open space is a few per cubic centimeter (1/100 of a meter). They are not individually spaced more than a meter apart.

Gravity is so weak, that it is still a mystery why it is so weak. A tiny magnet can overcome the gravity of our entire planet when it picks up paperclips. The gravitational field of a hydrogen atom is almost nonexistant. The spacing between atoms is large enough that it hardly has any effect on other atoms. At the small range at which gravity becomes measurable, the repulsive effect of the electromagnetic field of the atom is more powerful than the attraction of gravity.

@PhiNotPi Interesting thought. Electrons are actually a long way from the proton at the center of a hydrogen atom. If, for instance, a proton were about the diameter of a pencil eraser, the entire atom with its electron cloud would be nearly the size of Gaint Stadium. In open space, could the electrons repel one another more than their gravity pulls them together?

@ETpro It would not be the electrons repelling each other, it would be the nucleus. The nucleus of an atom contains almost all of the mass, so it causes most of the gravity. From outside of the electron shell, the atom is electrically neutral. Outside of the atom, the positive charge of the nucleus and the negative charge of the electrons cancel out perfectly. However, inside of the electron shell, the negative charge cancels itself out (Gauss law- there is no charge inside a charged sphere). The positive charge of the nucleus is not canceled. Once two hydrogen nulcei are closer together than the electron shell, the two nuclei are repelled. This effect prevents things like cold fusion, because it takes energy to overcome this repulsion and make the nuclei fuse.