Answer to Question #78957 in Astronomy | Astrophysics for R H Busch

Question

Universe expanding at accelerating rate seemingly overcoming force of gravity. Why don't massive objects, planets and stars, likewise expand accordingly?

Answer 1

This question can not have a definite answer at the moment because it deals with fundamental cosmological concepts and hypotheses that still need to be checked experimentally. One of them states that there exists unknown form of energy – the Dark Energy (DE) [1] – that is responsible for the expansion of the Universe with positive accelerating rate (and this is the factor that overcomes the gravitational attraction of matter). It contributes around 70 percent to the total energy in the observable universe. The open question (alongwith the existence of DE at all) is the relation of its intensity to other types of interaction (like gravitational, electromagnetic, strong and weak) in time and space. One of the hypotheses (see, for example, the paragraph "Implications for the fate of the universe" in [1] and references therein) states that for low densities of matter (when you measure it for the big scale objects like sets of galaxies, etc., taking into account the huge distances between them as well) the DE dominates over the other types of matter, so that the forces of its repulsion are much stronger than that of the gravitational attraction. On the other hand, the objects like planets and stars are extremely dense, so that one can neglect the influence of DE on them. However, according to the so-called phantom energy concept of DE [2], the forces of DE continue growing and one day it will dominate over all other forces (including gravitational and electromagnetic) on all scales. As a result, all bound structures may be torn into pieces, so that the problem in the question may come true (see the theory of Big Rip [3] for details).

[1] (Electronic resource) https://en.wikipedia.org/wiki/Dark_energy
[2] (Electronic resource) https://en.wikipedia.org/wiki/Phantom_energy
[3] (Electronic resource) https://en.wikipedia.org/wiki/Big_Rip