# Hubble Constant, particle horizon and density (rho) of the observable universe

Would the Hubble Constant be affected as galaxies in our observable universe pass outside our particle horizon, by reducing the density (rho) of the observable universe? I am assuming that once outside the particle horizon gravitational information is lost.

Galaxies do not "pass outside the particle horizon". In fact the opposite is the case, as the light (and gravitational attraction) from more and more distant galaxies eventually has had the time to reach us. Once a galaxy is inside the observable Universe, it stays inside.

But the mean density of the Universe does decrease, because the volume of the Universe increases. This, in turn, affects the expansion rate since the mutual attraction of matter decreases. Note though that the effect is dominated by the accelerating expansion due to dark energy.

These interrelations (expansion rate and densities of the constituents of the Universe) are calculated by intergrating the Friedmann equation.

• Good point, I think I should have referred to galaxies moving beyond our event horizon. And I do understand that the mean density would effectively go down by virtue of the expansion of the volume of our universe (which I think is the Hubble Volume) but my question is whether the gravitational "information" of the galaxies that "disappear" is also lost, thereby contributing to the loss of mean density. May 1, 2017 at 1:03
• @J.Oppenheim: That galaxies move beyond the event horizon means that no "new" signal (light or gravity or whatever) that is emitted will be able to reach us in a finite time. But we still receive the "old" signals. Light is being constantly diminished, so that they gradually fade away. They same can be said about the gravitational information, although I'd prefer to say that their gravitational attraction gradually decreases as they move farther away.
– pela
May 1, 2017 at 6:22
• Thanks for that answer. I presume you would be indicating that the contribution of those galaxies to the density of the universe would also fade away and I am wondering if that might not contribute to accelerating the Hubble constant (dark energy notwithstanding). May 1, 2017 at 23:37
• @J.Oppenheim: No, that's not the case. On such large scales, matter is extremely homogeneously distributed. If you remove a chunk of a homogeneous fluid, the density of the remaining fluid stays constant. Like if you close the door to your bedroom, the density of the air in your living room doesn't change.
– pela
May 2, 2017 at 6:15
• Hmm… was that a bad analogy? No, I think it was decent :)
– pela
May 2, 2017 at 6:15