Short answers, pulled from the story.
Albert Einstein published a paper on relativistic cosmology in 1917 that described a static, finite, and unbounded universe. He added a cosmological constant to his field equations to counteract gravity and prevent the universe from collapsing. This model was later called his greatest blunder after evidence showed the universe was expanding.
Edwin Hubble provided the observational basis for the expanding universe in 1929. He discovered a relationship between the redshift of galaxies and their distance, now known as Hubble's law. This discovery demonstrated that galaxies are receding from Earth in every direction at speeds proportional to their distance.
Arno Penzias and Robert Woodrow Wilson discovered the cosmic microwave background in 1965. This radiation is a thermal black-body spectrum with a temperature of 2.7 kelvins today. It provided the first direct evidence that the universe had a hot, dense beginning.
Modern cosmology reveals that dark energy constitutes 73% of the total mass-energy content of the universe. Dark energy permeates all space and is responsible for the accelerating expansion of the universe. This phenomenon was confirmed in 1999.
Alan Guth of MIT first proposed the theory of cosmic inflation to resolve problems in the very early universe. The theory addresses the horizon problem and the absence of magnetic monopoles. Inflation drives the universe to flatness and exponentially dilutes monopoles.
The current cosmological epoch suggests the universe will continue expanding forever due to dark energy. Possible outcomes include a Big Rip that tears the universe apart or a Big Freeze where the universe becomes cold and dark. The exact outcome depends on the properties of dark energy.