Expanding Ever Faster: The Discovery that Changed the Fate of the Universe
An astonishing result that amazed the world of astronomy, the discovery that the expansion of the universe is increasing in speed is today recognized as a major contribution to science. Astronomers Beatriz Barbuy (Brazil. São Paulo) and Silvia Torres-Peimbert (Mexico. Mexico City) both laureates of the L’Oréal-UNESCO program “For Women in Science”, reflect on the importance of this work.
How will it all end? What is the ultimate fate of the cosmos? The answer to these questions was announced almost simultaneously, in 1998, by two competing groups of astrophysicists. And they were the exact opposite of what everybody had expected. There were thought to be two possible scenarios: either the universe would fall back in on itself, or it would drift lazily apart into eternity. No one had really considered a third option, but Brian Schmidt and Adam Riess of the High-z Supernova Search team, and Saul Perlmutter, head of the Supernova Cosmology Project, had come to the same remarkable conclusion: the expansion of the universe was actually accelerating.
Professor Beatriz Barbuy, astrophysicist at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo in Brazil, and 2009 laureate of the L’Oréal-UNESCO “For Women in Science” program, admits she was astonished when she found out. Astronomers had known since Edwin Hubble’s 1917 discovery that the universe was expanding. But, rather than expecting to find proof of acceleration, “everybody was looking for evidence of deceleration.” Theoreticians had long thought the universe would either collapse in a “Big Crunch” scenario, or else expand forever, albeit more and more slowly. The amount of mass in the universe would determine which fate prevailed, and either case implied a slowing down of the outward expansion.
One theoretician who considered the question was Einstein himself. His Theory of General Relativity predicted these two possible outcomes. And, yet, he wasn’t comfortable with the idea of a non-static universe. To reconcile his equations with his understanding of the cosmos, Einstein added an element—the cosmological constant—that would mathematically counteract the effects of expansion. This allowed his equations to reflect his understanding of the universe as a stable system. However, when Hubble showed that galaxies were receding from Earth, Einstein was forced to admit that the universe was expanding. He regretted tinkering with his equations and called the cosmological constant his greatest mistake. Today, though, we see it differently: Although Einstein’s reasons for adding the cosmological constant may have been flawed, his prediction of an extra force acting on the fabric of the universe has come back into favor since 1998 with the revelation that expansion is, in fact, speeding up. According to Professor Barbuy, some would call his most serious blunder one of the greatest discoveries of our times.
This enthusiasm is echoed by Silvia Torres-Peimbert, Professor of Astrophysics at the National University of Mexico, and one of five recipients of 2011 L’Oréal-UNESCO “For Women in Science” award. The word “exciting” comes up again and again as she talks about the discovery of accelerating expansion.
“It was quite a breakthrough. It’s something we can hardly understand. We thought we understood a good fraction of the universe, but now…! Instead of being discouraging, though, I find it very exciting.”
The excitement surrounding the increasing speed of expansion is due to the implications it holds for our fundamental understanding of the universe. It was already understood long ago that the cosmos is expanding, as a consequence of the Big Bang. If this expansion is accelerating, though, there must be something pushing it. A force of some kind, something we are not able to feel in our everyday lives, something we have yet to identify. This is the famous dark energy, estimated to make up some 75% of all energy and matter in the universe. Beatriz Barbuy marvels at the fact that “we have absolutely no clue what it is. It’s probably something that we don’t understand, a new kind of physics.”
Whatever its nature, dark energy may be a force that does not act continuously. It was active at the beginning of the universe, but then, around six billion years ago, it seems the cosmos slowed down for a while in its outward growth. It has picked up the pace again since then, however, as we know from Perlmutter, Schmidt and Reiss’s measurements.
The teams had set out to find evidence that the expansion of the universe was slowing down. Their method involved analyzing the light from supernovae—giant explosions of stars—in distant galaxies. The particular type they study, type IA supernovae, begins with a double-star system. One, called a white dwarf, is a very dense star, with the weight of the Sun in a body the size of the Earth. Its strong gravitational force draws away the gas of the other, until its mass reaches a tipping point; the core of the star becomes so hot that it triggers a runaway fusion reaction, resulting in a massive explosion. The amount of light released—sometimes as much as emitted by an entire galaxy—peaks in the first few weeks and then peters out, in a known, predictable fashion. “We have no details about the event, we just receive the light and have to understand from that,” explains Professor Torres-Peimbert.
Using this light, the two teams identified more than 50 type IA supernovae and took images of the same stellar explosions, three weeks apart. It was a complex business analyzing light that had traveled more than a third of the way across the universe, correcting for the effects of interstellar dust, and determining the distance and speed of the galaxies in question. When the painstaking adjustments and calculations were complete, both teams looked at their results and…paused. If the expansion of the universe were slowing down, as everyone expected, the supernovae should be getting brighter with time. What they found was the opposite: the light was growing fainter. The supernovae, within their distant galaxies, were moving farther away, and faster than before.
The finding was “immensely unexpected”, in the words of Torres-Peimbert, but each team had the reassurance that their competitor had come to the same conclusion. Not only was the universe not going to slow down into a comfortable equilibrium, but it would continue hurtling apart, ever faster, in all directions. Which means that every bit of matter may be spread so thin that the universe will end up a dark, freezing, lonely place. “But that won’t happen for 100 billion years,” Professor Barbuy points out. “So, there’s no problem.”
In the meantime, we have a lot to learn. The discovery that the expansion is accelerating actually created many more questions than it answered. Our universe has revealed itself to consist largely of dark energy, something we know nothing about. All the matter and energy that we see, feel, and understand amounts to only 4 or 5% of the universe. The remaining 20% is thought to be dark matter. Researchers at CERN (European organization for nuclear research), among others, are working to identify its nature and Beatriz Barbuy believes that, eventually, we will succeed. Silvia Torres-Peimbert feels lucky to be living and working at a time when our view of the universe is changing so much.
“In astronomy, you’re never certain. You need a degree of open-mindedness, to be willing to change your paradigm.”
That’s good advice, particularly in a universe that is continually challenging our assumptions and may very well defy all expectations, once again.
Beatriz Barbuy received the 2009 L’Oréal-UNESCO Award for Latin America “For her work on the evolution of the stars from the birth of the universe to the present time.”
Silvia Torres-Peimbert received the 2011 L’Oréal-UNESCO Award for Latin America “For her work on the chemical composition of nebulae which is fundamental to our understanding of the origin of the universe.”