The assumption that the universe is currently undergoing expansion is based on several lines of evidence and observations, not solely reliant on the redshift of galaxies. While it's true that the redshift of galaxies was one of the early pieces of evidence for cosmic expansion, subsequent discoveries and measurements have provided further support for the ongoing expansion.
Here are some key pieces of evidence that suggest the universe is expanding:
Hubble's Law: The relationship between the redshift of galaxies and their distance, as formulated by Edwin Hubble, has been confirmed by various observations. Hubble's Law demonstrates that the universe is expanding uniformly, with galaxies moving away from each other.
Cosmic Microwave Background Radiation (CMB): The CMB is the relic radiation from the early universe, emitted when the universe was just 380,000 years old. Precise measurements of the CMB, such as those conducted by the Planck satellite, provide detailed information about the properties of the universe. These measurements indicate that the universe has been expanding since the time of the Big Bang.
Large-scale Structure: The distribution of galaxies and galaxy clusters on large scales also supports the notion of cosmic expansion. The clustering patterns and the distribution of cosmic structures align with the predictions of an expanding universe.
Consistency with General Relativity: General relativity, Albert Einstein's theory of gravity, provides a mathematical framework to describe the expansion of the universe. The equations of general relativity predict an expanding universe, and the theory has been consistently successful in explaining a wide range of astronomical observations.
While these observations suggest that the universe is expanding, it's important to acknowledge that our understanding is based on our current data and models. It's always possible that new discoveries or evidence may challenge or refine our understanding of cosmic expansion.
Regarding the possibility of the universe reversing and collapsing, this is a valid question. The ultimate fate of the universe is still an active area of scientific investigation. Depending on the balance between the expansion rate and the gravitational forces in the universe, it could continue to expand indefinitely, reach a state of equilibrium, or eventually collapse in a process known as the Big Crunch.
However, current observations indicate that the expansion of the universe is accelerating, driven by a mysterious force called dark energy. This acceleration suggests that the expansion is unlikely to reverse and result in a collapse in the near future. But as our understanding of dark energy and the fundamental properties of the universe improves, our predictions about its long-term fate may evolve.
In summary, while we cannot predict the future with absolute certainty, the current evidence, including redshift measurements, CMB observations, large-scale structure, and general relativity, strongly supports the concept of an expanding universe.