Final answer:
The universe is homogeneous and isotropic on a large scale, leading to a uniform expansion rate everywhere. With mass density below critical density and the presence of dark energy, the universe is expected to expand forever. A steady-state cosmology was considered in the past but is not supported by current evidence of universal expansion.
Step-by-step explanation:
On the large scale, the universe at any given time is the same everywhere, meaning it is homogeneous and isotropic. Because of this, the expansion rate of the universe must be the same everywhere during any epoch of cosmic time. This constant expansion means that as space stretches, distances between objects like galaxies grow larger by the same factor everywhere. With the observation that the mass density of the universe is less than the critical density, and with the influence of dark energy accelerating the expansion, it is expected that the universe will continue to expand forever.
Additionally, the cosmological principle supports this view of a uniformly expanding universe, where the relationship between the velocities and distances of remote galaxies is proportional. The concept of a steady-state cosmology was once popular, suggesting that the universe was the same at all times and continually creating new matter. However, we now understand that the universe has been expanding since the beginning, an event that occurred about 13.8 billion years ago. Therefore, the expansion rate of the universe is not consistent with a steady state but indicative of an ongoing, dynamic process.