Why did Isaac Newton conclude that the universe was static? Was he correct?

The concept of 'gravitational collapse' is a phenomenon that occurs when an astronomical body, such as a star or gas cloud, falls in on itself due to the force of gravity overpowering any other internal pressures that might be resisting the collapse. This is an important process in the universe as it's responsible for the formation of various structures, including stars, black holes, and galaxies.

Isaac Newton, renowned for formulating the law of universal gravitation, theorized that a static universe would eventually succumb to this gravitational collapse, as all matter would be drawn together purely by gravity. He envisioned the universe like a crowded dance floor, where dancers (stars) would eventually huddle up (collapse) without some sort of outward motion or repulsion keeping them apart. Newton proposed that for the universe to remain static, there had to be a precise balance between the pull of gravity and the motion of the stars.

However, this idea of a precise balance is more a thought experiment than a reality. In the vast cosmos, such a delicate balance is not observed. Modern astrophysics tells us that gravitational collapse is common and is the driving force behind the birth of new stars, when regions within giant molecular clouds succumb to gravitational pull, resulting in protostars that eventually heat up and sustain nuclear fusion.

Albert Einstein's theory of relativity revolutionized the way we understand gravity and the fabric of the cosmos. In essence, it transcends the idea of gravity as a force and instead describes it as the curvature of spacetime. This theory comprises two parts: the special theory of relavity, which provided a new framework for all of physics by proposing new concepts of space and time, and the general theory of relativity, which introduced the modern understanding of gravity.

Einstein's general relativity posits that massive objects cause a distortion in the spacetime continuum, much like a heavy ball placed on a trampoline distorts the surface. Smaller objects roll towards the heavier object not because they are pulled by a mysterious force but because the fabric of space itself is curved by the mass. This concept helped to explain why Newton's notion of a static universe was flawed. As it turns out, rather than being unchanging, the universe's spacetime fabric is dynamic and constantly evolving, leading to phenomena such as the expansion of the universe and black holes, which were not predicted by Newtonian physics.

Moreover, general relativity predicts that the universe could not be static: either it is expanding or contracting, but it cannot just hang in balance everlasting, as Newton had thought.

The discovery of an 'expanding universe' is one of the most remarkable achievements of 20th-century astronomy. Employing the Doppler Effect, astronomers like Edwin Hubble observed that galaxies appear to be moving away from us, with their light being redshifted—a shift towards the longer wavelengths—which indicates that they are moving farther away.

These observations directly conflict with Newton's image of a static universe. If all galaxies are moving away from each other, then the universe must have been denser and smaller in the past. This revelation is one of the foundational pillars of the Big Bang theory, the leading explanation of the universe's origins and development.

The observation of an expanding universe implies that not only is there no gravitational collapse pulling everything together, but also that the cosmos is getting more spacious over time. With this perspective, the focus shifted from an eternal, unchanging realm to a dynamic, evolving entity that began at a certain point in time and has been expanding and evolving over billions of years. The understanding of an expanding universe lays the groundwork for cosmology, the study of the universe's birth, structure, and eventual fate.