The assumption that the universe is constantly expanding is based on observations and evidence gathered by astronomers and physicists over many decades. One of the key pieces of evidence supporting the idea of an expanding universe comes from the observations of distant galaxies. In the early 20th century, astronomers noticed that galaxies outside our own Milky Way were moving away from us, and the farther away they were, the faster they appeared to be receding. This observation is known as Hubble's law, named after the astronomer Edwin Hubble.
The concept of an expanding universe was further supported by the discovery of cosmic microwave background radiation. In the 1960s, scientists detected a faint background radiation throughout the universe, which is considered residual heat from the early stages of the universe known as the Big Bang. The existence of this radiation and its distribution across the cosmos provided strong evidence for the expansion of the universe.
Based on these observations, scientists developed the Big Bang theory, which suggests that the universe originated from a hot and dense state around 13.8 billion years ago and has been expanding ever since. According to this theory, the expansion of the universe is driven by a property known as dark energy, which acts as a kind of repulsive force, pushing galaxies apart.
Regarding the question of whether the expansion has an end or if it continues forever, scientists are actively researching and exploring this topic. It is currently an area of ongoing investigation in cosmology. One possibility is that the expansion of the universe could continue indefinitely, leading to what is known as a "big freeze" or "heat death" scenario, where galaxies drift farther apart and energy dissipates, resulting in a universe that becomes increasingly cold and sparse.
On the other hand, there are alternative theories and hypotheses that suggest different scenarios for the future of the universe, such as a cyclic model where the universe undergoes cycles of expansion and contraction or a scenario where the expansion accelerates to the point of tearing apart galaxies and even fundamental particles. These ideas are still the subject of scientific study and debate, and our understanding of the ultimate fate of the universe continues to evolve as more evidence is gathered and analyzed.