StarTalk S16E76: How We Discovered That Dark Energy is Tearing Us Apart

Hey StarTalkers! Season 16, Episode 76 saw Neil, Paul and guest Professor Adam Riess take a deep dive into arguably the most mysterious phenomenon in all of physics. Professor Riess described how his team helped to discover what is known today as “dark energy.”

Discovering Invisible Forces in Our Universe, with Adam Riess

But what exactly did they discover? If you’re looking to dig a little deeper, here’s what his Nobel Prize-winning paper actually says. 

The Background: Einstein’s Biggest Blunder

When Einstein first set out to apply his theory of general relativity to cosmology, he made it a static universe. This was done through a now-infamous mathematical term called the “cosmological constant,” a little extra push on the universe’s gas pedal to counteract the effect of gravity.

He would later call this decision his “biggest blunder.”

But, it actually turned out to be kind of right.

Riess’s Paper, Simplified

There are two big players when it comes to the expansion of the universe: 

• Mass density Ω_M: More mass means more gravity, which slows the expansion of the universe.

• The “cosmological constant” Ω_Λ: This is the “vacuum energy” of the universe – the cosmic gas pedal driving the expansion.

Adam Riess and his team’s Nobel-winning 1998 paper basically proved that the “cosmological constant” is real. But its size revealed that the universe is almost tearing itself apart.

Riess’ team pinned down these values by looking at Type 1a supernovae. These exploding stars can serve as “standard candles.” Just like a candle will appear dimmer as you move further away, standard candles help us determine cosmic distances.

These supernovae are rare, but with a wide-angle search, the team were able to identify 10 Type 1a supernovae in the redshift range z = 0.3 to z = 0.6. Riess describes this as like winning the lottery not out of luck, but because you bought all the tickets.

The distance to these supernovae (aka the “luminosity distance”) can be used to find our crucial values from a scary-looking formula containing our key parameters Ω_M and Ω_Λ. With some clever math, they were able to turn their observations into estimates.

What They Found: The Rate of Expansion is Increasing

The only valid solution had a positive “cosmological constant.” They were able to determine with over 99.7% confidence that Ω_Λ must be greater than zero. And the vacuum energy increases with distance, so the expansion of the universe is getting faster, not slower.  

So yes, the universe isn’t actually static. But Einstein’s “biggest blunder” is still the way we describe what’s happening to this day.

Most amazing of all, we still have basically no idea why this is happening. Dark energy remains one of the greatest mysteries in all of science.