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.