Static versus Dynamic universe

How can we determine the state of the universe?
Is the universe a static manifestation of the cosmos, or a dynamic, expanding phenomenon?
What did and do renowned scientists say about the state of the universe?
Let's start with Newton's law of gravity.

A. Newton's Gravity Theory

Isaac Newton (1643-1727) had already given the world a theory of gravity in the late 17th century. Newton's theory still works well: It can describe the movements of planets, as well as the movement of the moon, and so on. And, not to forget, the textbook example told about Newton: Newton got the ideas about his theory of gravity when he saw an apple fall from an apple tree and perhaps land on someone's head.
The working mechanisms of gravity on a falling object is one of many topics, all of which can be described and calculated according to the laws that Newton had already worked out. Newton's laws are still useful for 'normal' phenomena that we see happening around us.

B. Static Universe

Before the theory of the Big Bang was formulated, Western scientists assumed a static universe: a universe that has always been there and will always remain as it is.
However, mathematical calculations based on Newton's law of gravity show that such a universe would collapse in the distant future.
Newton recognized that problem, but made the following argument:
If matter were evenly distributed in infinite space, there would be no center to which matter would fall.

C. Einstein's Theory Of Relativity

Albert Einstein (1879-1955) also assumed a static universe, but his general theory of relativity showed conclusively that the universe must expand or collapse. He then postulated in mathematical terms the cosmological constant to counter that likely collapse.
(The cosmological constant is expressed with the Greek capital letter lambda: Λ)

D. Expanding Universe

The Dutch astronomer Willem de Sitter (1872-1934) came up with a different model of the universe. He observed the sun and other stars using telescopes and examined the color shifts. All the analyses of color shifts in photographs taken with telescopes are called spectroscopic observations. He analyzed the redshift of light rays. When light waves are stretched, they tend to become slightly redder in color. So in the case of an expanding universe: when white light moves towards our Earth from a distance, the waves become slightly stretched, therefore redder in color, because the universe is elongating.
Based on these types of observations, he predicted in 1918 a redshift that was proportional to the distance between the Earth and a celestial body. De Sitter's theoretical model contained no matter, but did contain an expanding universe. De Sitter's idea is now relevant again in the modern inflation theory of the Big Bang.

Image A: static versus dynamic universe.
According to some scientists a static universe will eventually collapse. A dynamic universe might continue to expand.

E. Other Contributions

Independently of De Sitter, Alexander Friedmann (1888-1925) found solutions to the mathematical equations of general relativity, which described an expanding universe.

Building on De Sitter's universe vision, the Belgian Roman Catholic priest Georges Lemaître (1894 - 1966) also published mathematical solutions for a theory describing a dynamic universe.

By the way, some scientists suspected that there was nothing at all before the Big Bang, not even time. So, one cannot even speak of a 'before the Big Bang' reality.

F. Redshift and the Doppler Effect

William Huggins (1824-1910) was the first scientist to actually discover that the light from some galaxies was redshifted; this showed that these galaxies were moving away from us. This phenomenon is called the Doppler effect. He published calculations that Sirius (a bright star in our sky) is moving away from us at a speed of more than 20 miles per second.

What is redshift? This is the phenomenon that light changes color (becomes a little redder) when the light waves are stretched slightly. Red light has a longer wavelength and blue light has a shorter wavelength. When a celestial body that emits white light in our direction moves away from us, the wavelengths of light are stretched slightly. This is because this light is moving slightly away from us as it moves towards us! However, because light moves toward us many times faster than the celestial body moves away from us, the light waves reach us a fraction of a second later, and the waves are a bit stretched. White light therefore becomes slightly redder in color.
Conversely, if the same celestial body were to move towards Earth, the researchers would see that the light became slightly bluer in color. The waves are then slightly compressed, causing the wavelength to become shorter.

G. Hubble's Law

However, Edwin Hubble (1889-1953) was the researcher who, together with Milton Humason, calculated a linear relationship between the redshift of distant galaxies and their distance from our planet. This relationship is now known as 'Hubble's law' (with the mathematical 'Hubble constant' named after him).

H. General Conclusion

According to certain observations, the universe could be expanding.
Some laws of physics can describe this spatial expansion mathematically.