Let’s rewind the clock back…oh, I don’t know, let’s say a hundred years. It was 1917, and Einstein had just developed his general theory of relativity.
It was a masterpiece, giving us our modern day view of the gravitational force.
And like anybody curious about gravity, Einstein decided to apply his new equations to the evolution of the universe.
I mean, why not? On average the universe is electrically neutral, so the electromagnetic force isn’t going to tell you a whole lot about how things operate at large scales. Einstein didn’t know about the strong and weak nuclear forces (to be fair, nobody did) but those are both short-range interactions anyway.
If you want to do cosmology, the story begins with gravity. If you take a random collection of stuff and call it “a universe,” it’s perfectly valid to ask what that random collection of stuff will do.
To his surprise, Einstein found that relativity naturally predicted an evolving, dynamic universe, one that naturally either expanded or contracted. But the prevailing wisdom at the time was that the universe was static – it had always been the same throughout cosmic history.
In a rare move, Einstein yielded to observations and added a “cosmological constant,” denoted by the Greek letter Lambda.
The equations of relativity naturally allowed this constant. It’s like a background gravitational effect that just exists throughout the universe, even when there’s nothing inside it.
And this effect can be either positive or negative, either background attraction or repulsion that’s built into spacetime itself.
Einstein tuned his parameter to make the background gravity cancel out the gravitational antics of all the matter and stabilize the universe.
Just a few years later, Edwin Hubble would discover that the universe is expanding, and other theorists, like Russian cosmologist Alxander 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.”
Faster 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.
In the 1980’s and 90’s we had developed a sophisticated cosmological model, which we somewhat arrogantly called the Standard Model of Cosmology (physicists have a penchant for calling cohesive, collaborative, consensus models “Standard”’). The discovery of accelerated expansion forced us to throw away that model.
In its place is our current best description of the history of the universe since the big bang. It’s called LCDM cosmology. The Lambda is for the cosmological constant, also known as dark energy. The CDM is for cold dark matter, the form of matter that dominates the mass of almost every galaxy. The CDM bit is a different episode, as today we’re focusing on the Lambda.
The LCDM is enormously successful. It’s a relatively simple model: just a few free parameters and a couple 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.
Written by Paul Sutter/Universe Today.
