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.

Season 16 Episode 76 – A Short History of Expanding Universe Models

Hey StarTalkers! Following up on the previous post, this post delves a little deeper into the historical impact of Professor Adam Riess’s research and the discussion in Season 16, Episode 76.

Discovering Invisible Forces in Our Universe, with Adam Riess

Professor Riess’s work was the final nail in the coffin of many older cosmological models, proving that dark energy exists and issuing in a new era of cosmology. The ΛCDM model became the dominant model of cosmology and – despite some issues – it remains that way today.

Einstein’s Static Universe

Einstein basically kick-started modern theoretical cosmology in his 1917 paper based on his theory of general relativity. He used the “cosmological constant” to make the universe static, balancing out the effect of gravity.

This had a lot of problems, though. Just after publishing, Einstein’s friend Willem de Sitter pointed out a major issue. Even in a completely empty universe, it predicted that a “test particle” would move. He asked, rhetorically, “has this inertia?”

The “Steady State” Universe

In 1948, Hermann Bondi, Thomas Gold and Fred Hoyle invented what’s called “steady state cosmology.” In their model, the universe expands, but it rests on the assumption of the “perfect cosmological principle.” This means that the universe has to look the same on large scales at all times.

With an expanding universe, the only way this is possible is if there is a constant influx of new matter. And that’s what they did – they proposed a “creation field” which churned out new matter as needed.

Notably, they included the cosmological constant as a fundamental constant of nature.

The Expanding Universe

Models of the universe involving expansion first got going with Alexander Friedman in 1922, and Georges Lemaître later discovered the same solutions to Einstein’s field equations. In 1928, Lemaître and Howard Robertson made an initial estimate for the speed of expansion.

Just a year later, Edwin Hubble would publish his famous equation linking the observed redshift of cosmic objects to their distance from the observer.

This made it clear that the universe really was expanding, and attention shifted accordingly. Even Einstein got involved again in 1932, proposing the “Einstein-de Sitter” model, which removed the cosmological constant, incorporated expansion and arguably even predicted dark matter.

But Adam Riess’s 1998 discovery showed that these early pioneers didn’t go far enough. Not only is the universe expanding, the expansion is accelerating.

ΛCDM to the Rescue?

After this long, winding journey, cosmologists have settled on the ΛCDM model of the universe. This includes the cosmological constant – Λ, lambda, to account for Riess’s result – and also “cold dark matter.” This has been very successful, overall, but issues like “Hubble tension” persist.

But one thing still seems clear: the expansion of the universe – dark energy – is real.