SPT will chart far-flung galaxy clusters for details about the universe's size, age and origin
Somewhat amazingly, there is still plenty of dark in the sky. Even with 100 billion stars in our galaxy and some 100 billion other galaxies in the observable universe, we can still see a lot of space between them all, to the very beginning of time.
Carlstrom explained that as we look deeper into space, further back in time, eventually we get to the point when the universe was ionized. This is the cosmic microwave background, nearly uniform radiation coming from every direction in the sky, a spherical puzzle that defines the edge of the observable universe.
“What this tells us is that if you go back beyond 10 billion years, or something like that, there are no galaxies. That doesn’t mean there aren’t seeds of galaxies and something happening. But it’s important to realize that it’s back in time. ... This doesn’t tell you that the universe is finite. It just tells us, time-wise, that there’s a tale to be told.”
The bewildering aspect of the cosmic microwave background was that the intensity of the radiation is virtually the same from every direction, even though their sources are billions of light-years apart. An explanation for this was finally developed in the 1980s with the theory of inflation.
“Remember Einstein had his cosmological constant,” Carlstrom said. “His equations show that if you had what was called a false vacuum, some uniform energy density of the vacuum, then you would get this exponential expansion.”
That is what the inflation theory states, that in an incredibly brief time – about 10-30 of a second – there was an expansion far greater than the speed of light and our universe was created from a small subatomic region. Inflation would explain why the background is so uniform – what the astronomers term smooth – because it all once “pertained to a very, very small part of causally connected space.”
Inflation gave scientists enough information to predict the average energy density of the universe, something they determined to be about three hydrogen atoms per cubic meter. This presented another problem. All of the visible matter in the universe and the much larger amount of dark matter totaled to an average of only about one hydrogen atom per cubic meter.
“What we find is that there’s some other component that in the present day accounts for about 70 percent of the universe,” Carlstrom said.
“It’s called dark energy. The best model for dark energy, the one that looks most compelling although we don’t understand it, is Einstein’s cosmological constant, energy of a vacuum.”
Learning more about dark energy is what led to the design of the South Pole Telescope, he said.
“The whole big-bang scenario looks as though it’s a pretty good explanation of what happened, and that’s a huge step forward for us,” said Padin, project manager for the construction of the SPT. “Twenty years ago, nobody was really sure if a process like that occurred. They weren’t really sure how to test it. So, we’ve had an exciting 20 years here, and I hope it continues and we actually figure out if this really is the right story.”
SPT will target uncharted, far-flung galaxy clusters. Once these clusters are located, they can be studied for additional clues to the size, age and origins of the universe.
Beginning this austral winter, Padin and his winter-over team of Zak Staniszewski and Allan Day will employ the Sunyaev-Zel’dovich effect to find these galaxies. To adequately survey most of the viewable sky will take about three winter seasons, even though the SPT is 5,000 times faster than previous telescope experiments, Carlstrom said.
The cosmic microwave background emits radiation at a uniform level, but it has been discovered through the Sunyaev-Zel’dovich effect that galaxy clusters can augment those photons ever so slightly. Carlstrom said there is about a 1 percent chance that a photon that escaped from the cosmic microwave background might interact with an electron while passing through a galaxy cluster.
Such a photon would have very little energy by the time it reached the cluster and the much more energetic electron would scatter the photon, distorting the background spectrum and creating a hot spot that would not ordinarily be there.
“With a telescope like the South Pole Telescope,” Carlstrom said, “we can look at these objects. In fact, we can survey the sky looking for these objects and we can find them, no matter where they are, because they distort the background.”
Galaxy clusters are never small things, he said, and a large enough telescope can always resolve them. In fact, it does not matter how far away they are, the effect is the same, but it is very subtle. He said the SPT’s camera, using multiple exposures of the sky, should be able to resolve temperature differences as slight as a millionth of a degree Celsius, hovering just above absolute zero, the lowest temperature theoretically obtainable.
Carlstrom said researchers are going to gather the information they can about galaxy clusters and “that will let us solve the cosmic time of this tug-of-war of [visible and dark] matter trying to pull things together or dark energy trying to cause things to expand. We want to test those theories of dark energy. That’s our first experiment.”
Padin suggested the excitement, for him, is in the search, even if the theory tested ends up being wrong.
“It’s just more exciting, in a sense, when you come up with a measurement that is completely at odds with what you expect. That causes a rethink, and sometimes that’s the key to unleashing a huge amount of progress.”
The last 20 years, he said, things have turned out pretty much as expected, and that’s satisfying, “but some of the folks working in theoretical cosmology ... were way too pleased with themselves at this point so they’re almost insufferably happy at this stage.”
NSF-funded research: John Carlstrom, University of Chicago.
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