Cosmologists know that the universe is expanding, and that the expansion is getting faster by the moment, propelled by a mysterious dark energy that has so far eluded detection or description by scientists.

The story of these discoveries, Robert Kirshner said at the AAAS Annual Meeting, includes luminaries both human and cosmic. Supernovae, the massive and bright star explosions that he studies, were crucial to piecing together this new understanding. But, Kirshner also talked about the people, including two of his graduate students who won the Nobel Prize, whose work gradually revealed an accelerated universe.

In his Plenary Lecture, the Harvard University astrophysicist led listeners through 13.7 billion years of cosmic evolution, and 100 years of research—and even managed to eat a modest slice of pumpkin pie along the way. 

Robert Kirshner speaking about The Accelerating Universe at the AAAS Annual Meeting. Credit: Atlantic Photography Boston

More about that pie in a moment, but the scientific story starts in 1917, when “Albert Einstein thought the Milky Way was more or less equal to the universe,” Kirshner said. Einstein also thought the universe was unchanging, and his equations describing it included a repulsive force that he called the cosmological constant. The cosmological constant was Einstein’s way of explaining why the stilled universe did not collapse in on itself from the gravitational pull of all the stars and other matter it contained.

Around the same time, Harvard College Observatory astronomer Henrietta Swan Leavitt showed that the luminosity of certain “variable stars” was related to fluctuations in their brightness. The longer the period between fluctuations, the more luminous the star was. These variable stars became the first “standard candles” to light the way in exploring the size and dynamics of the universe, Kirshner noted. 

Researchers already knew that stars should become increasingly dim the farther they were away from Earth. With variable stars now offering a standard measurement of brightness, astronomers could calculate exactly how far these stars might be, depending on how dim they were. 

The discovery confirmed that the universe was bigger than the Milky Way, but it also revealed another surprising fact to astronomer Edwin Hubble in 1929. When he and others closely examined the wavelengths of light coming from these stars, they were able to see shifts in the wavelengths that correspond to direction and speed. The stars, Hubble concluded, were moving away from each other and the universe was expanding.  

Leavitt’s variable stars are too dim to be seen at the far edges of this expanded universe, and now Type 1a supernovae, among the brightest objects in the sky, are the new standard candles. They burn “four billion times as bright as the Sun for about a month,” Kirshner said. But “they are really, really rare. There’s only about one per century in a galaxy.”

Kirshner and his graduate students Brian Schmidt and Adam Riess would have to use computers and increasingly better cameras to scour thousands of galaxies for the supernovae, but their efforts paid off. In 1998, they reported that the expansion of the universe is accelerating. Schmidt and Riess, along with Saul Perlmutter, won the 2011 Nobel Prize in Physics for the discovery.

The cosmological constant, which Einstein had disavowed when Hubble discovered that the universe was expanding, has new relevance in modern cosmology. Something has to be giving the universe its “springy quality,” Kirshner said, and it might just be Einstein’s repulsive force.

The cosmological constant could represent what astrophysicists call “dark energy.” Dark energy may make up as much as 73% of the total universe, with unseen cold dark matter making up an additional 23%. Only 4% of the universe is made up of visible matter, a fact that Kirshner illustrated by slicing and eating a sliver of pie on the AAAS stage. It may not look like much, he joked, “but it’s the tastiest component of the universe.”

What does the future of an accelerated universe hold? In four billion years, the Milky Way Galaxy is expected to collide with the neighboring Andromeda Galaxy. After that, Kirshner said, the distances between the Milky Way and all other galaxies could be so great that the Milky Way will appear as Einstein first considered it: a universe unto itself.