Dark energy revived Einstein’s discarded cosmological constant and reshaped the leading model of the universe.
In 1917, Albert Einstein had recently completed his general theory of relativity, the framework that transformed how physics understands gravity. He soon applied those equations to a much larger question: how the universe changes over time.
Gravity was the natural place to begin. The universe is electrically neutral on average, so electromagnetism cannot explain its behavior across the largest scales. Einstein also knew nothing about the strong and weak nuclear forces (to be fair, nobody did), but both act only across extremely short distances.
Cosmology therefore begins with gravity. Once physicists gather matter and energy into something they call “a universe,” they can use Einstein’s equations to ask how it should evolve. The answer surprised him. General relativity naturally described a dynamic cosmos that either expanded or contracted, while the dominant scientific view held that the universe was static and had remained unchanged throughout its history.
Einstein forces the universe to stand still
Rather than accept what his equations implied, Einstein adjusted them to match the observations and assumptions of his time. He introduced a “cosmological constant,” represented by the Greek letter Lambda, which general relativity already allowed. The constant acts as a gravitational influence built into spacetime itself, even in otherwise empty space. Depending on its value, it can produce attraction or repulsion. Einstein selected a value that counterbalanced the gravitational pull of matter, allowing his model universe to remain stable.
Just a few years later, Edwin Hubble would discover that the universe is expanding, and other theorists, like Russian cosmologist Alexander Friedmann, would take Einstein’s equations at their face value and provide the theoretical backing for the Big Bang theory. As for Einstein himself, he would later remark to friends that the addition of the cosmological constant was his “greatest blunder.”
The expected slowdown becomes acceleration
Fast forward to 1998. Two teams of astronomers set out to settle a decades-long debate about how much matter was in the universe. Some observations said there was barely any matter at all, and others said there was a lot. The universe was expanding, but the matter inside of it should be slowing down that expansion – by measuring the deceleration, they could get a grip on this amount of stuff and settle the debate.
Which they did… sort of. Instead of finding a deceleration, they measured an acceleration. There was still not very much matter in the universe, but even what IS there is not enough to decelerate the expansion.
As for that acceleration, the simplest way to explain is to, you guessed it, invoke the cosmological constant, a fundamental background anti-gravity effect that simply exists in the universe. Decades after Einstein scrubbed away his blunder, the constant came roaring back to life as the best explanation for the data.
Dark energy replaces the old model
By the 1980s and 1990s, cosmologists had built a detailed framework known, somewhat boldly, as the Standard Model of Cosmology (physicists have a penchant for calling cohesive, collaborative, consensus models “Standard”’). The discovery that the universe’s expansion is accelerating made that model obsolete.
Its replacement is our best current account of how the universe has evolved since the Big Bang: LCDM cosmology. Lambda represents the cosmological constant, commonly associated with dark energy. CDM stands for cold dark matter, which provides most of the mass in nearly every galaxy. Cold dark matter is a separate topic, so the focus here is Lambda.
A successful model may still fail
The LCDM is enormously successful. It’s a relatively simple model: just a few free parameters and a couple of assumptions working within a framework of general relativity. And with that model, we are able to explain SO MUCH about the universe: its expansion history, the appearance of background radiation, the BAO feature, the growth of galaxies and large structures, and on and on and on. It’s one of the most well-studied and well-tested theories in ALL of science.
And it’s almost certainly wrong.
Adapted from an article originally published in UniverseToday.
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Full article can be found at: https://scitechdaily.com/einsteins-greatest-blunder-may-be-breaking-cosmology/
