Physical cosmology: the story on HearLore

Physical cosmology

In 1917, Albert Einstein published a paper on relativistic cosmology that described a universe which was static, finite, and unbounded, much like the surface of a sphere. He had added a cosmological constant to his field equations specifically to counteract the attractive force of gravity, ensuring the universe would not collapse under its own weight. This model, known as the Einstein model, was the prevailing view of the cosmos for decades, yet it was fundamentally unstable to even the smallest perturbations. History would later reveal that Einstein himself had made a significant error in judgment, famously calling the introduction of the cosmological constant his greatest blunder after evidence emerged that the universe was actually expanding. The static universe was a mathematical convenience that failed to account for the dynamic reality of the cosmos, a reality that was about to be uncovered by astronomers who were beginning to look beyond the Milky Way.

The Expanding Cosmos Revealed

The 1920s marked a seismic shift in our understanding of the universe, driven by the work of Vesto Slipher and Edwin Hubble. Slipher had interpreted the red shift of spiral nebulae as a Doppler shift indicating they were receding from Earth, but the distances to these objects remained a mystery. In 1929, Hubble provided the observational basis that changed everything, showing that these spiral nebulae were actually galaxies outside our own Milky Way. He discovered a relationship between the redshift of a galaxy and its distance, now known as Hubble's law, which demonstrated that galaxies are receding from Earth in every direction at speeds proportional to their distance. Although Hubble's numerical factor relating recessional velocity and distance was off by a factor of ten due to a lack of knowledge about Cepheid variable stars, the implication was undeniable: the universe was expanding. This discovery forced physicists to abandon the static model and consider theories of an evolving cosmos.

The Primeval Atom and The Steady State

As the evidence for expansion mounted, two competing theories emerged to explain the origin and evolution of the universe. Georges Lemaître, a Belgian Roman Catholic priest, independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed that the universe began with the explosion of a primeval atom, a concept that would later be dubbed the Big Bang. This theory was further developed by George Gamow, who argued that the universe evolved from a hot, dense state. In stark contrast, Fred Hoyle proposed the steady state model, which suggested that new matter was continuously created as galaxies moved away from each other, keeping the universe roughly the same at any point in time. For a number of years, support for these theories was evenly divided, but the discovery of the cosmic microwave background in 1965 by Arno Penzias and Robert Woodrow Wilson lent strong support to the Big Bang model. The steady state theory, which required the continuous creation of matter, could not explain this radiation, and it eventually fell out of favor among the scientific community.

Common questions

What did Albert Einstein publish in 1917 about the universe?

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.

When did Edwin Hubble prove the universe is 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.

Who discovered the cosmic microwave background radiation in 1965?

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.

What percentage of the universe is dark energy according to modern cosmology?

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.

When did Alan Guth propose the theory of cosmic inflation?

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.

What are the possible future scenarios for the universe involving dark energy?

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.