Philip Hopkins, a theoretical astrophysicist at the California Institute of Technology in Pasadena, likes to prank his colleagues. An expert in simulating the formation of galaxies, Hopkins sometimes begins his talks by projecting images of his creations next to photos of real galaxies and defying his audience to tell them apart. “We can even trick astronomers,” says Hopkins, a leader of FIRE, the Feedback in Realistic Environments simulation. “Of course, it’s not a guarantee that the models are accurate, but it’s sort of a gut check that you’re on the right track.”
For decades, scientists have tried to simulate how the trillions of galaxies in the observable universe arose from clouds of gas after the big bang. But in the past few years, thanks to faster computers and better algorithms, the simulations have begun to produce results that accurately capture both the details of individual galaxies and their overall distribution of masses and shapes. “The whole thing has reached this little golden age where progress is coming faster and faster,” says Tiziana Di Matteo, a numerical cosmologist at Carnegie Mellon University in Pittsburgh, Pennsylvania, and a leader of the BlueTides simulation.
As the fake universes improve, their role also is changing. For decades, information flowed one way: from the astronomers studying real galaxies to the modelers trying to simulate them. Now, insight is flowing the other way, too, with the models helping guide astronomers, says Stephen Wilkins, an extragalactic astronomer at the University of Sussex in Brighton, U.K., who works on BlueTides. “In the past the simulations were always trying to keep up with the observations,” says Wilkins, who is using BlueTides to predict what NASA’s James Webb Space Telescope will see when it launches in 2020 and peers deep into space and far back in time. “Now we can predict things that we haven’t observed.”
For example, the models suggest that the earliest galaxies were oddly pickle-shaped, that wafer-thin spiral galaxies are surprisingly rugged in the face of collisions, and that to explain the evolution of the universe, galaxies must form stars far more slowly than astrophysicists expected.
The simulations also sound a cautionary note. Some cosmologists hope galaxy formation will ultimately turn out to be a relatively simple process, governed by a few basic rules. However, modelers say their faux universes suggest that, like maturing teenagers, galaxies are unpredictable. It’s hard, for example, to tell why one turns into a graceful spiral but another evolves into a blob. “It’s clear from everything that we’ve done that the physics of galaxy formation is incredibly messy,” Wilkins says.
Before you can cook up a universe, you need to know the ingredients. From various measurements, cosmologists have deduced that just 5% of the mass and energy of the cosmos is ordinary matter like that in stars and planets. Another 26% consists of mysterious dark matter that, so far, appears to interact only through gravity—and presumably consists of some undiscovered particle. The remaining 69% is a form of energy that stretches space and is speeding up the expansion of the universe. That “dark energy” may be a property of the vacuum of space itself, so physicists call it the cosmological constant, denoted lambda (Λ).
Cosmologists also know the recipe’s basic steps. The universe sprang into existence in the big bang as a hot, dense soup of subatomic particles. Within a sliver of a second, it underwent an exponential growth spurt called inflation, which stretched infinitesimal quantum fluctuations in the particle soup into gargantuan ripples. Slowly, dense regions of dark matter coalesced under their own gravity into a vast tangle of clumps and filaments known as the cosmic web. Attracted by the dark matter’s gravity, gas settled into the clumps, also called haloes, and condensed into the fusing balls of hydrogen called stars. By 500 million years after the big bang, the first galaxies had formed. Over the next 13 billion years, they would drift on cosmic gravitational tides and grow by merging with one another.
Computer simulations helped develop that theory. In the 1980s they showed that to form clumps large enough to bind the observed clusters of galaxies, dark matter particles had to be slow moving and cold. The basic theory, which assumes a cosmological constant, became known as Λ cold dark matter (ΛCDM). As the theory grew more refined, so did the simulations. By 2005 the Millennium simulation, led by researchers at the Max Planck Institute for Astrophysics in Garching, Germany, produced a rendering of the cosmic web whose structure closely matched how the galaxies are strewn through space in clusters, threads, and sheets.
Millennium and similar simulations suffered from a fundamental shortcoming, however. They modeled the gravitational interactions of dark matter alone, which are easy to simulate because, as far as scientists know, dark matter flows through itself without friction or resistance. Only once the haloes formed did the programs insert galaxies of various sizes and shapes, following certain ad hoc rules. In such simulations, “The fundamental assumption is that the galaxies occupy the haloes and don’t do anything to them,” says Yu Feng, a cosmologist at the University of California (UC), Berkeley. “The interaction is all one way.”
Now, modelers include the interactions of ordinary matter with itself and with dark matter—processes that are far harder to capture. Unlike dark matter, ordinary matter heats up when squeezed, generating light and other electromagnetic radiation that then pushes the matter around. That complex feedback reaches an extreme when gas clouds collapse into glowing stars, stars blow up in supernova explosions, and black holes swallow gas and spew radiation. Critical to the behavior of galaxies, such physics must be modeled by using the equations of hydrodynamics, which are notoriously difficult to solve, even with supercomputers.