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University of Pittsburgh

October 27, 2016

Science 2016: Eye on the cosmos

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What’s the universe made of? What do we know about dark matter and dark energy? And does it matter, if we’re living in a computer-simulated universe anyway?

Such were the questions explored by University physics and astronomy faculty, with colleagues from Carnegie Mellon, at the Oct. 21 “Eye on the Cosmos” session of Pitt’s annual science research showcase, this year titled Science 2016: Game Changers.

Michael Wood-Vasey, Pitt physics and astronomy faculty member, measures the distance supernovas have traveled since their explosions to track our expanding universe and help determine the effect of dark energy on this expansion.

Shortly after the Big Bang 13.8 billion years ago, which propelled the expansion of the original universe outward from something less than the size of an atom, this expansion began to decelerate. But five billion years ago — as astronomers have calculated by looking back in time to more distant objects — there was an acceleration that began to move visible objects away from each other again. The universe, being infinite, is not itself expanding, but galaxies are getting measurably farther apart.

The accelerating force, Wood-Vasey said, is called “dark energy, because that is our indication of our ignorance.” We as yet know nothing about this force, apart from its apparent effects on visible matter, although by current estimates it accounts for 70 percent of the energy in the universe. It was posited because the gravity of all visible matter, which had enough attractive power to create stars and their planets, is not powerful enough to stop this current expansion.

Science 2016 exhibit of undergraduate research posters. Photo: Tom Altany/Photographic Services

“Dark energy is determining so much about our universe and what it is doing,” he added, “and we need to understand so we can understand our whole universe.”

Supernovas, being bright, are used to measure how fast and far galaxies are traveling away from each other. Thus far, astronomers have measured the light from 1,000 supernovas.

“We need to do 10,000, maybe 100,000 [supernovas] to figure out what this dark energy is,” Wood-Vasey said. With the construction of the Large Synoptic Survey Telescope underway in Chile, the Dark Energy Science Collaborative — with which Pitt and CMU faculty are involved — soon will greatly aid this effort. “You’ll see results of this in the next four-five years,” he said.

The finding that the universe is currently accelerating its expansion was a startling and unbelievable research result when it was first announced years ago, Wood-Vasey acknowledged.

Are there “any other crazy theories” that will turn out to be true? he was asked by an audience member.

“Unfortunately, no,” he said. “We need to keep trying. We need to keep stumbling around. Some day there’ll be a brilliant idea … and I hope I’m still around for this.”

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Matthew Walker, a CMU physics faculty member, is using dark matter — the other largely unknown piece of the cosmic puzzle — to detect some of the universe’s smallest galaxies.

About 22 percent of the universe is dark matter, and it holds the universe together, Walker said. Dark matter, like dark energy, cannot be perceived directly, but must be inferred from its gravitational pull on surrounding visible objects.

Walker uses simulations of universe formation, with the assumption of dark matter’s existence, that result in the formation of Milky Way-type regions in space. He also uses the Clay telescope at Las Campanas Observatory in Chile to observe the spectra of light emitted by nearby dwarf galaxies’ stars to estimate their chemistry, size and speed. He has discovered 100,000 times more mass in some of these dwarf galaxies than may be accounted for by their stars and other visible matter.

Using these findings, he has concluded that dwarf satellite galaxies abutting the Milky Way, some with just a few hundred stars, are made “almost entirely of dark matter,” he said.

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Jeffrey Newman, a Pitt physics and astronomy faculty member, is trying to determine the true color of our galaxy, the Milky Way, to help show its origins and composition.

Galaxies are collections of stars, dark matter, gases and dust, all held together by gravity; our sun is one of hundreds of billions of stars in the Milky Way.

Many galaxies share some form of the Milky Way’s spiral shape, with a bulbous center and a thinner disk of trailing arms, looking like two fried eggs placed back to back, Newman said. Others are simpler globe shapes and still others are clumpy, irregular accretions of all of this interstellar matter.

Galaxies appear more blue if they are dominated by younger blue stars; older red stars give galaxies a redder hue. Yellow stars, such as our sun, are middle aged. Thus a galaxy’s overall color in this continuum can indicate its age.

“Where the Milky Way falls has been a bit of a puzzle, and that’s because we’re in the middle of it,” says Newman: The earth and its sun are in one of our galaxy’s spiral arms, and our view of the central Milky Way is partly obscured by gas and dust clouds.

Using the Sloan Digital Sky Survey Telescope in New Mexico, with which Pitt has been involved since the 1990s, scientists have collected detailed light spectra for about a million galaxies to find analogs to the Milky Way: galaxies with the same number of stars and rate of star formation as the Milky Way.

Such comparisons have concluded that our galaxy is, essentially, white: technically, either one of the reddest blue galaxies in the universe or one of the bluest red galaxies.

“We found our galaxy is very appropriately named,” Newman concluded. “We’re in a typical galaxy,” with both older red stars formed less than a million years after the Big Bang, and other stars still forming today.

Don’t mistake the white of the Milky Way in our night sky for its true color, Newman said. Most stars appear white to us because our eyes see low levels of light in black and white. If you want to see the Milky Way’s true white color, he explained, look at an accumulation of spring snow, with its finer flakes, an hour after sunrise.

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Of course, none of these astronomical findings may matter — we, and our universe, might just be simulations in the supercomputers of technologically superior beings, Carnegie Mellon physics faculty member Rupert Croft said in his presentation.

Such notions are taken seriously, he explained. Last month, Bank of America Merrill Lynch sent a letter to investors noting that there’s a 50 percent chance we’re living in such a simulation; Elon Musk, head of Tesla and SpaceX, has said he believes it’s even less likely our reality is real.

All of this is based on a 2003 paper from Oxford philosophy faculty member Nick Bostrom. It postulated that, to be conscious, one does not need to have a physical body; that consciousness eventually can be simulated within computers; and that “post-human civilizations” eventually will do exactly that.

Since there are many simulations possible, but only one reality, the conclusion was inevitable, Croft said: “We’re most likely to be a simulated human.”

Croft’s astronomy work uses powerful simulations that are far from being able to model such things. He employs Blue Waters, the largest National Science Foundation supercomputer, to conduct cosmological simulations, modeling how the universe progressed from the Big Bang to today’s observable world — how its tiny quantum fluctuations evolved into gravity, with dense regions becoming denser and empty regions becoming emptier, as other forces, from gas dynamics to radiation, came into play.

The largest matter and forces in the universe are easiest to simulate, so Croft and colleagues have begun with superclusters of galaxies and now are down to modeling the formation of dwarf galaxies. Simulating planets and people eventually will be possible, he said — the latter by the year 2300, according to the latest predictions.

We’ll be using supercomputers to form single model atoms just a hundred years later, he said.

“The physics is standard — it will happen,” he added. “The universe is complicated, but supercomputers can handle it.”

—Marty Levine 

Filed under: Feature,Volume 49 Issue 5

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