STATIC AND EXPANDING MODELS OF THE UNIVERSE

Albert Einstein, a German-American physicist, introduced a theory of the universe grounded in general relativity, where gravity manifests as a curvature in four-dimensional space. His solution suggested that the universe couldn't remain static; it either had to expand or contract. Among the non-static theories, the one proposed by Russian mathematician Alexander Friedmann is now widely accepted. The destiny of the Friedmann universe hinges on the average matter density in the cosmos. If matter density is relatively low, the gravitational pull among galaxies remains weak, leading to perpetual expansion indefinitely. Conversely, if matter density exceeds a critical threshold, expansion slows and eventually reverses into contraction, culminating in the collapse of the entire universe. The outcome of this collapsed universe is uncertain; one hypothesis suggests it could undergo an explosive event, spawning a new expanding universe, which in turn collapses, perpetuating the cycle indefinitely.

The Steady-State Theory

British astronomers Hermann Bondi, Thomas Gold, and Sir Fred Hoyle introduced a radically different concept of the universe. They suggested that the universe's expansion-induced decrease in density is counteracted by a perpetual generation of matter, thus preserving the current state of the universe indefinitely. However, the steady-state theory has fallen out of favor among the majority of cosmologists.

The Big Bang Theory

Russian-American physicist George Gamow proposed the idea that the universe originated from an immense explosion now known as the Big Bang. This theory laid the groundwork for comprehending the earliest phases of the universe and its subsequent development. The initial high density would trigger a rapid expansion of the universe, leading to the cooling and condensation of hydrogen and helium into stars and galaxies.

According to Gamow's hypothesis, radiation lingering from the Big Bang would have cooled to approximately 3 Kelvin (about -270 °C/-454 °F) by the present day. This cosmic background radiation was first observed in 1965, widely regarded by most astronomers as confirming the validity of the Big Bang theory. Inflationary theory addresses challenges in Gamow's original concept by incorporating recent advancements in particle physics.

The ultimate fate of the universe, whether it will continue to expand indefinitely or eventually contract, remains uncertain. One proposed method to tackle this uncertainty involves determining the average matter density in the universe. When considering the mass of individual galaxies multiplied by their number, it amounts to only 5 to 10 percent of Friedmann's critical value. However, when factoring in the mass of galactic clusters multiplied by their quantity, the average density approaches the critical value. This variance hints at the existence of unseen matter—referred to as dark matter—outside galaxies but within clusters. Until the mystery of this missing mass is elucidated, this approach to determining the universe's destiny will remain inconclusive.

Black Holes

A black hole is a theoretical entity characterized by extreme density and an immensely powerful gravitational field, preventing anything, including electromagnetic radiation, from escaping it. Consequently, it appears completely devoid of light. In 1994, astronomers utilizing the Hubble Space Telescope discovered compelling evidence for the existence of a black hole. Their observations revealed the presence of an object weighing between 2.5 billion to 3.5 billion times the mass of the Sun at the core of the M87 galaxy.

The concept of black holes was initially formulated by German astronomer Karl Schwarzschild, building upon the framework of German-American physicist Albert Einstein's theory of general relativity. According to general relativity, gravitational forces profoundly warp space and time in the vicinity of a black hole. Time experiences significant dilation relative to distant observers and comes to a standstill within the black hole itself.

Black holes can originate from the remnants of spent stars. As nuclear fusion reactions cease in a star's core, the pressure generated by its heat no longer counteracts gravitational collapse. If the mass of the core exceeds approximately 1.7 times that of the Sun, it undergoes collapse, leading to the formation of a black hole.