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Cosmic Tug-of-War

Large Dark Matter Halos Favor Growth of Larger Early Galaxies

Nov. 20, 2015


Few scientific questions are as fundamental, or fascinating, as the origin of the Universe. And we can see the early Universe. The farthest galaxies from us are so far away that it takes light rays about 13 billion years to reach us.

Our newest telescopes are, in essence, time machines that will see the light that these galaxies created just a few hundred million years after the Big Bang.

Hy Trac of Carnegie Mellon University and Renyue Cen of Princeton University lead a team of cosmologists whose simulations on PSC’s Blacklight supercomputer predict that the largest early galaxies would tend to win a cosmic tug of war in galaxy formation, making it harder for smaller ones to develop. Such predictions help the big-ticket telescopes know what phenomena to look for, making them more productive.


Trac and Cen’s team uses a two-phase simulation: First, an “N-body” simulation, including only dark matter and the force of gravity, creates a framework. Adding ordinary matter and radiation then refines a more realistic, “radiation- hydrodynamic” simulation.

The latest version of the N-body simulation, run in November 2013, required almost 10 terabytes of computer memory—roughly the amount of information in all the Library of Congress’ printed books. With the largest amount of “shared memory” available to academic researchers—32 terabytes. With the help of XSEDE ECSS staff at PSC, the scientists determined that PSC's Blacklight system would be ideal for such a calculation.

The vastness of the simulation is striking. With 70 billion particles of dark matter, it encompasses an area 300 million light years across, containing 100,000 times the mass of our Milky Way galaxy. And it’s just a first step in creating a more complex radiation-hydrodynamic simulation, now under way.

"Our simulations will guide the next-generation observations by the James Webb Space Telescope and others. Ultimately the dialog between simulation and observation will test the standard cosmological model and the theory of galaxy formation at the epoch of reionization.—Renyue Cen, Princeton University



Left image, top: Vast filaments of dark matter (black)—which account for 85 percent of the mass in the Universe—form dark- matter halos where they intersect in the early Universe.The halos’ gravity attracts cosmic gas.

Middle: As the gas collects and heats, the first stars and galaxies light up (white), creating an outward burst of ionizing radiation that partly counteracts the inward pull of gravity.

Bottom: Only in the largest halos is gravity strong enough to continue collecting gas, creating large galaxies (white) in the process. Smaller halos lose the cosmic tug of war, so the smaller galaxies they would have formed (ghosted blue) are less likely to get started.

Right image: The scientists' visualization shows small deviations in the echoes of the Big Bang that hint at how early galaxies came into being.

Distortion of the cosmic microwave background predicted by the team's latest radiation-hydrodynamic simulation. (above-right)