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Astronomie - Signal detected from cosmic dawn

1.03.2018

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The radio spectrometer at Murchison is a little smaller than a table-tennis table

Scientists say they have observed a signature on the sky from the very first stars to shine in the Universe.

They did it with the aid of a small radio telescope in the Australian outback that was tuned to detect the earliest ever evidence for hydrogen.

This hydrogen was in a state that could only be explained if it had been touched by the intense light of stars.

The team puts the time of this interaction at a mere 180 million years after the Big Bang.

Given that the cosmos is roughly 13.8 billion years old, it means the first stars lit up a full nine billion years before even our own Sun flickered into life.

Dr Judd Bowman of Arizona State University, US, is the lead author on the scholarly paper describing the observation in the journal Nature.

He told BBC News that the discovery's great significance meant his group had to be absolutely sure no mistakes were made.

"We first started seeing signs in our data back in late 2015. And we've really spent the last couple of years trying to think of all sorts of possible alternative explanations, and then rule them out one by one," he said.

"This is the first time any team has been able to present evidence for the detection of this signal and hopefully it will go down as a milestone for this type of astrophysical observation."

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Image copyrightNSF
Image captionArtwork: The first stars were likely hot blue giants that burned brilliant but brief lives

For decades, scientists have sought evidence for the very first population of stars.

Theory says they would have grown out of the cold hydrogen gas that filled the Universe following the Big Bang. They were very probably unwieldy giants that burned brilliant but brief lives, before then exploding and seeding the Universe with all the chemical elements that make life possible.

We cannot see these ancient behemoths directly with current technology, but we can, say astronomers, capture an indirect proof of their activity.

Their ultraviolet light should have altered the great clouds of hydrogen surrounding them, exciting the gas into a state that made it absorb background radiation at a very specific radio frequency of 1.4 gigahertz.

The challenge for the table top-sized telescope at the Murchison observatory in Western Australia was to try to pick out this signal in the sea of radio noise coming from the sky.

Part of the issue was knowing where exactly to search on the spectrum, given that the expansion of the Universe over the eons would have stretched the signal to much lower frequencies.

But the team eventually found it in the region of 78 megahertz. And knowing what we do about the rate of cosmic expansion, the shift in frequency gives an origin for the starlight-hydrogen interaction of 180 million years after the Big Bang.

'New window'

Of significant interest is the strength of the signal which is well above expectation.

This suggests, say commentators, that the hydrogen gas was a lot colder than previously supposed.

One exciting explanation for this is if the hydrogen atoms in the early cosmos had some direct interaction with so-called "dark matter".

This unseen "stuff" is postulated to exist because of the way its gravitational influence is seen to affect the rotation of galaxies. But its substance is unknown because no other type of interaction has yet been observed.

The Murchison observation may be the first hint that such interactions are possible and the news therefore is likely to galvanise efforts to make the first detections of dark matter particles.

Paolo de Bernardis is a professor of astrophysics at the University of Rome "La Sapienza", and was not connected with the research. He said the signal was extremely interesting.

"If confirmed, it's a breakthrough," he told BBC News. Quite why the signal was so strong, we would have to indulge our fantasies until follow-up experiments could probe it further, he added. "But I find this measurement really exciting."

Prof Brian Schmidt, who won the Nobel Prize for detecting an acceleration in the expansion of the Universe, described the discovery as potentially "revolutionary". And Prof Karl Glazebrook from Swinburne University of Technology, Australia, said astronomers worldwide would now be holding their breath until the result was confirmed by an independent experiment.

"If it is, then this will open the door to a new window on the early Universe and potentially a new understanding of the nature of dark matter," he explained.

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A history of the Universe

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Image copyrightESO
  • Observations indicate the Big Bang occurred about 13.8bn years ago
  • After which, conditions cooled and neutral hydrogen atoms formed
  • The period before the first stars is often called the 'Dark Ages'
  • When the first stars ignite, they start to change their environment
  • These giants also forge the first heavy elements in big explosions
  • 'First Light', or 'Cosmic Renaissance', is a key epoch in history
  • The Renaissance likely peaks around 560m years after the Big Bang

Quelle: BBC

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Astronomers detect light from the Universe’s first stars

Surprises in signal from cosmic dawn also hint at presence of dark matter.
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A radio telescope in Western Australia has detected a sign of the light from the Universe’s first stars.Credit: Commonwealth Scientific and Industrial Research Organisation

Astronomers have for the first time spotted long-sought signals of light from the earliest stars ever to form in the Universe — around 180 million years after the Big Bang.

The signal is a fingerprint left on background radiation by hydrogen that absorbed some of this primordial light. The evidence hints that the gas that made up the early Universe was colder than predicted. This, physicists say, is a possible sign of dark matter’s influence. If confirmed, the discovery could mark the first time that dark matter has been detected through anything other than its gravitational effects.

“This is the first time we’ve seen any signal from this early in the Universe, aside from the afterglow of the Big Bang,” says Judd Bowman, an astronomer at Arizona State University in Tempe who led the work, which is published in Nature1 on 28 February. “If it’s true, this is major news,” says Saleem Zaroubi, a cosmologist at the University of Groningen in the Netherlands. Other teams will need to confirm the signal but, so far, the finding seems to be robust, he says. “It’s very exciting stuff. This is a period in the Universe’s history we know very little about.”

Star sparks

Physicists think that the Big Bang, 13.8 billion years ago, generated an ionized plasma, which cooled rapidly as the Universe expanded. After about 370,000 years, this soup began to form neutral hydrogen atoms. Over time — and under gravity’s influence — these clumped together forming stars that ignited. This transition is known as the cosmic dawn (see ‘Dawn’s early light’).

Light from these stars would now be so faint that detecting it with Earth-based telescopes is near impossible. But astronomers have long hoped to see it indirectly: the light would have subtly shifted the behaviour of the hydrogen that once filled the space between stars. This change would have allowed hydrogen gas to absorb radiation from the cosmic microwave background (CMB) — the afterglow of the Big Bang — at a characteristic radio wavelength of 21 centimetres, which leaves a dip in the intensity of the CMB.

To search for the signal, the team used a radio telescope called the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), based at the Murchison Radio-astronomy Observatory in Western Australia. Because our own galaxy and human-generated FM radio generate waves in the same band as the signal, spotting the dip meant carefully filtering out these more powerful sources. But Bowman and his colleagues soon found the predicted signal at roughly the frequency they expected. And despite being a puny 0.1% drop in the radiation, it was still twice the magnitude predicted. The finding was so stark that the researchers spent two years checking that it didn’t come from an instrumental effect or noise. They even built a second antenna and pointed their instruments at different patches of sky at different times. “After two years, we passed all of these tests, and couldn’t find any alternative explanation,” says Bowman. “At that point, we started to feel excitement.”

Radiation from this period arrives stretched out by the expansion of the Universe, meaning the band in which the signal was found gives away its age. This allowed the team to date the latest onset of the cosmic dawn to 180 million years after the Big Bang. The signal’s disappearance gives away a second milestone — when more-energetic X-rays from the deaths of the first stars raised the temperature of the gas and turned off the signal. Bowman’s team puts that time around 250 million years after the Big Bang.

Understanding these primordial stars is important not only because they shaped the matter around them, but also because their explosive deaths created the soup of heavier elements, such as carbon and oxygen, from which later stars formed, says Bowman. “If we really want to understand the cosmic ladder of our origins, this is a critical step to understand,” he says.

 

Cosmic cradle

While the signal appeared at an expected frequency, its strength was utterly unexpected, says Rennan Barkana, a cosmologist at Tel Aviv University in Israel. “I was actually quite amazed,” says Barkana, who has published a second, related paper in Nature2. He says the strength suggests that either there was more radiation than expected in the cosmic dawn, or the gas was cooler than predicted. Both would be “very strange and unexpected”, he says.

The only explanation that makes sense to Barkana is that the gas was cooled by something. That points to dark matter, he says, which theories suggest should have been cold in the cosmic dawn. The results also suggest dark matter should be lighter than the prevailing theory indicates, says Barkana. This could help to explain why physicists have failed to observe dark matter directly, in experiments stretching over decades. If that’s true, we have to design new kinds of experiments to see it, he adds.

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For now, the cosmic-dawn signal is tentative. But other experiments are lined up to investigate it. Most radio astronomers had been looking for other hydrogen signals from a later period in the Universe’s history. One such experiment in development, the Hydrogen Epoch of Reionization Array, an international radio-telescope project based in South Africa’s Karoo desert, is now being adapted to detect signals at the wavelengths explored by Bowman’s team. He hopes that it could replicate his results during the next few years.

Other experiments, such as LOFAR (Low-Frequency Array), a large system of radio antennas spread over five European countries, should be able to go a step further and map how the intensity of the signal fluctuates across the sky. And if the cause of the strong signal is dark matter, that should be visible as a distinctive pattern. “We’re eager for another instrument to confirm it,” says Bowman.

We’ve been trying to study the period when stars first formed for 35 years, says Martha Haynes, an astronomer at Cornell University in Ithaca, New York. “I’m excited to think that we have finally detected the signal sought for so long.”

Quelle: nature

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