The Universe's first light—the earliest we can peer back in time—is the Cosmic Microwave Background, produced some 350,000 years after the Big Bang. It's the product of electrons pairing up with protons to form hydrogen atoms, releasing light in the process. From there, however, the Universe went dark until the formation of the first stars and galaxies, hundreds of thousands of years later.
Understanding the end of the cosmic dark ages can help us figure out the processes that built the Universe we currently occupy. For now, however, the wavelengths that would allow us to do so have remained out of reach. But researchers are building a new generation of telescopes that will help us reach back to this remote time, and they described their progress at the meeting of the American Association for the Advancement of Science.
The formation of hydrogen left the Universe electrically neutral. From there on out, most of the Universe's light was produced by hydrogen atoms—and typically absorbed by other atoms nearby. It wasn't until a sufficient population of stars formed that the light they produced put hydrogen back into an ionized state, allowing light to pass great distances unhindered. Thus, this "epoch of reionization" made the Universe transparent and accessible to our astronomical eyes. And, by studying it, we can examine the first large-scale structures that formed in the Universe.
As Saleem Zaroubi of Europe's LOFAR telescope put it, reionization marks the onset of astrophysics. We don't fully understand the source of the radiation that drove it, nor the rate at which it took place. It's expected to have been a gradual process, in part because the galaxies that drove reionization aren't evenly distributed, and in part because it would have taken time for the photons that drove reionization to penetrate the surrounding gas.
Unfortunately, studying reionization is a bit of a challenge. The photons have been traveling so long that some light produced by hydrogen with a wavelength of 21cm now has shifted to over two meters long. On the long journey to Earth, it has had to pass through other radio-emitting material both inside and outside our galaxy, as well as the Earth's ionosphere (a region of the upper atmosphere). Its detection then has to compete with our own radio emissions.
The James Webb Space Telescope is being designed to operate outside the ionosphere and peer back in time using wavelengths that should reveal the Universe's first galaxies. But, before it launches, we will have some ground-based hardware that could help us see this critical period for the first time.
LOFAR
The LOFAR telescope that Zaroubi described uses what's called "long baseline interferometry" to generate enhanced images: telescopes are spread out over a large area, and the precise timing of photon arrival allows the reconstruction of images with much higher resolution than is possible with the individual parts. In LOFAR's case, that includes hardware at sites in the UK, Sweden, and France, along with six in Germany and three in Poland. But the core telescopes, along with the compute facilities, are located in the Netherlands.
To build it, Zaroubi said, you have to level an area the size of a football pitch to the precision of two centimeters—pretty challenging when the tractors that are doing the leveling leave tread imprints five centimeters deep. There were other complications; "the Netherlands, for those who don't know, is a swamp," Zaroubi told the audience while showing a slide of a backhoe partially submerged in the muck. ("We still are looking for the driver," he quipped.)
But the swamp has been overcome, and LOFAR is beginning to take data. Researchers measure the strength of these signals in Kelvin, and Zaroubi said that the signals are only a few milliKelvin, while the foreground noise is over 5K. Right now, they're in the process of characterizing that noise in order to subtract it. And that's produced some surprises. While showing an image of some data, Zaroubi pointed to large-scale features and said, "You see these things? We have no idea what they are."
The researchers have been able to determine that the data lines up with the inventory of ionized dust in our galaxy as identified by the Planck telescope. But they're not sure why it would generate the signal they're seeing.
MWA and PAPER
While LOFAR is trading off lots of foreground noise for easy access to infrastructure, two other projects—which are centered in remote areas—have escaped human-generated radio signals. Miguel Morales of the University of Washington talked about his work on the Murchison Widefield Array in Australia. Located in a remote region of the Australian outback, it's so radio-quiet that the MWA can watch the International Space Station pass overhead using nothing but reflected FM radio signals—and determine the station's direction based on its doppler shift.
Passing satellites also pose problems, as do features in the Earth's ionosphere, which sees a big burst of activity after dusk—data cables can also insert other noise. Its operation so far has produced four Petabytes of data—or, as Morales put it, a metric tonne of USB sticks. That data goes through two independent software pipelines designed to recognize interference and background noise—"the Universe is very creative with finding ways of fouling you up," Morales said. And a surprising amount of the analysis involves printing out processed data to allow actual humans to look it over and pick out potential problems.
With the current level of analysis, Morales said you can start seeing possible patterns in the data from the Epoch of Reionization in just three hours of data. But, as with the other telescopes, more data will be needed before a full analysis will tell us anything.
The MWA's location was chosen in part because of plans to build part of the Square Kilometer Array telescope there. The other site is going to be in South Africa, and Richard Bradley of the US' National Radio Astronomy Observatory was at the meeting to talk about a telescope being built there: PAPER, for Precision Array for Probing the Epoch of Reionization.
PAPER's focus is going to be on the signal produced by all that neutral hydrogen, some of which is still around. It'll use that to look for the discontinuities caused by the first stars and galaxies in the Universe. Right now, however, the focus is still on the engineering challenges, and Bradley described how the telescope's receivers had to be shielded from its electronics in order to prevent those from interfering with the signal. There's also a danger of feedback as any signals are amplified and then sent down cabling, which can create similar signals that are picked up by the receiver again.
While none of the telescopes are ready for a LIGO-style announcement, they're already taking data and will undoubtedly refine their approach in the years between now and when the James Webb is lifted into orbit. Hopefully, they'll provide a sufficient picture of the Universe's first structures, so we'll know what to look for with the Webb.
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