A little more than a century ago, Albert Einstein shifted his focus from gravity to the fate of the entire universe. In 1917, soon after developing his theory of general relativity, Einstein began applying its equations to cosmology. Because general relativity transformed our understanding of gravity, it was natural to ask what the theory could reveal about the universe on the largest possible scale.
Gravity was the logical place to begin. On average, the universe is electrically neutral, so electromagnetic forces do not control its behavior across cosmic distances. Einstein also knew nothing about the strong and weak nuclear forces—no one did at the time—but these interactions operate only across extremely short ranges.
Einstein’s unexpected dynamic universe
In cosmology, gravity governs how matter behaves over vast distances. If we imagine all the matter in the universe as a single system, Einstein’s equations can describe how that system changes over time.
What Einstein discovered was surprising. General relativity does not naturally describe an eternally static universe. Instead, its equations point toward a dynamic cosmos that must either expand or contract. This contradicted the prevailing scientific view of the early 20th century: that the universe was static and remained essentially unchanged throughout its history.
To preserve a stable universe, Einstein added a term known as the cosmological constant, represented by the Greek letter lambda (Λ). General relativity allows this term to act as a gravitational influence built into space-time itself, even in empty space. Depending on its value, the cosmological constant can produce either an attractive or repulsive effect. Einstein selected a value that would balance the gravitational pull of matter and keep the universe stable.
That solution did not last long.
Evidence for an expanding universe changes everything
Within a few years, Edwin Hubble’s observations showed that the universe is expanding. At the same time, theorists such as Russian cosmologist Alexander Friedmann interpreted Einstein’s equations more literally and developed mathematical models that helped establish the foundation of Big Bang cosmology.
Einstein eventually abandoned the cosmological constant. He later described introducing it as his “biggest mistake.”
Then another major surprise emerged.
In 1998, two independent teams of astronomers were investigating a long-standing disagreement about the amount of matter in the universe. Different observations produced widely different estimates, with some suggesting relatively little matter and others indicating much more.
Astronomers already knew that the universe was expanding. Because matter generates gravity, scientists expected that gravity would gradually slow the expansion. By measuring how quickly the expansion was changing, researchers hoped to determine how much matter the universe contained.
Instead, they found the opposite.
The universe is accelerating
The expansion of the universe was not slowing down. It was accelerating.
The observations indicated that the universe contained relatively little matter, and even that matter could not explain the measured acceleration. Something was driving cosmic expansion faster and faster over time.
The simplest explanation was a familiar one: Einstein’s cosmological constant. A background energy associated with space itself could produce the repulsive effect responsible for the accelerating expansion of the universe. Decades after Einstein abandoned the idea, his supposed mistake returned as the leading explanation for new astronomical evidence.
Dark energy reshapes modern cosmology
During the 1980s and 1990s, cosmologists developed an increasingly detailed framework for explaining the universe, commonly known as the Standard Model of Cosmology. However, the discovery of accelerating cosmic expansion showed that the model needed to be revised.
The updated framework became the leading explanation for how the universe has evolved since the Big Bang: ΛCDM cosmology, also called Lambda-CDM.
Lambda refers to the cosmological constant, which is commonly associated with dark energy. CDM stands for cold dark matter, a form of matter believed to account for most of the mass in nearly all galaxies. Cold dark matter is a subject of its own; here, the focus is on lambda and the nature of dark energy.
The remarkably successful model also has serious problems.
ΛCDM has been extraordinarily successful. It is also remarkably simple, relying on a small number of adjustable parameters and a limited set of assumptions within the framework of general relativity.
Despite its simplicity, the model explains a wide range of observations, including the history of cosmic expansion, the cosmic microwave background, baryon acoustic oscillations (BAO), galaxy formation, and the growth of large-scale cosmic structures.
ΛCDM has become one of the most extensively studied and rigorously tested models in modern science.
And yet, it may not be the final answer.
Source: www.sciencedaily.com