“IT WAS, without a doubt, the single most exciting thing WMAP found.” That’s a big claim, but astrophysicist Jerry Ostriker at the University of Cambridge, is not backing down. Last year, the Wilkinson Microwave Anisotropy Probe, a satellite that examined the radiation left over from the big bang, made a surprise discovery about the history of the universe.
Immediately after the big bang, the universe was full of a blindingly hot sea of electrons and positively charged hydrogen and helium ions. The intense heat meant that these particles had too much energy to combine into neutral atoms, and it took 380,000 years before the universe was cool enough for electrons and ions to recombine. Some of the atoms and molecules jostling about in the young universe then coalesced into the first stars, and the radiation from these stars began to strip the electrons away from surrounding atoms again – a period known as “reionisation”.
But according to WMAP’s analysis, reionisation started when the universe was just 200 million years old. That’s about 700 million years earlier than astronomers’ previous best guess, according to Princeton University, New Jersey’s David Spergel, one of the architects of the WMAP mission.
Advertisement
The result has sent theorists scrambling for explanations: what could have been lurking in the young cosmos that had enough power to ionise its gas? Although astronomers have always had the first stars as prime suspects, they are now starting to wonder if there might be some other explanation. Were the first black holes responsible? Could it even have been down to dark matter?
Finding the answers to these questions would certainly fill some blank pages in our history of the universe. And this early chapter is crucial, according to Piero Madau of the University of California, Santa Cruz. “Reionisation sets the stage for everything that happens afterwards,” he says.
Consider how reionisation helped determine the sizes of galaxies, for example. The material that is ultimately incorporated in galaxies has to cool before a galaxy can coalesce – too hot and the constituents will bounce around too violently for gravity to pull them together. Because reionisation heats things up, it makes it difficult for anything to stick together unless the gravity is strong enough. So the only way a pre-galactic cloud can condense into a galaxy in the presence of intense ionising radiation is by being so large that its gravity overcomes the energy pushing it apart. Reionisation, notes Benedetta Ciardi of the Max Planck Institute in Munich, is what gave us a universe without really small galaxies.
So why were scientists’ calculations about when reionisation started so far out of whack? For a start, everything we thought we knew was based on extrapolation. Estimates were based on the analysis of light from distant quasars, which are powered by black holes that turn matter into radiation. But the quasars that indicate a period of reionisation formed only around a billion years after the big bang. Deducing how long before that the first stars – the first possible sources of reionising radiation – might have formed has always been tricky. “WMAP has given us our first chance to look back that far,” Spergel says.
Although WMAP couldn’t see the first stars directly, it could measure the polarising effect they had on the photons of the cosmic microwave background, the radiation left over from the big bang. That effect is what enabled WMAP to date the onset of reionisation (see “Rolling back the years”).
These first stars are certainly great candidates to kick off reionisation. They would have formed from hydrogen and helium, the only ingredients available at the time (elements heavier than helium, which astronomers call “metals”, did not exist until later when they were forged in the cores of stars). But these stars had to be big – big enough for their gravity to overcome the pressure of the hot gas. Tom Abel of Pennsylvania State University has carried out computer simulations of their formation and found that the first stars would have been quite massive: of the order of 30 to 300 suns.
According to calculations by Harvard astrophysicist Abraham Loeb, these stars would have a surface temperature of about 100,000 kelvin, hot enough to churn out ultraviolet photons capable of tearing electrons from hydrogen. A star of roughly 100 solar masses could ionise 10 million solar masses worth of hydrogen, Loeb says. “That means you only need to convert 1/100,000 of all the gas in the universe into such stars in order to ionise everything.”
Though these stars look like ideal reionisers, there’s a big problem with this scenario. These oversized monsters could start the reionisation process, but they probably couldn’t carry it on for long enough to ionise all the matter in the young universe. That’s because their ionising power would have hindered more stars from forming.
A massive star will break up and ionise all the molecular hydrogen around it. But molecular hydrogen is essential to the star-formation process because it is an important coolant. So once the first stars appear there will be a significant lag until other stars can form in sufficient quantity to reionise all the remaining gas in the universe – or at least a sizeable fraction of it.
“Feedback from the first stars could stop the process before you reionise everything,” Spergel says. Even in Abel’s simulation, which seems to suggest that the earliest stars might have appeared just 100 million years after the big bang, there simply wouldn’t have been enough of them around to have reionised the universe at the time WMAP suggests it was done. “You need something else to finish the job,” Spergel says.
So what is that “something else”? Perhaps it was the black holes that formed when the first stars died. In a paper to be published in the Astrophysical Journal, Madau, Martin Rees of the University of Cambridge, and three colleagues argued that black holes of 100 solar masses or more would be much more powerful sources of ionising radiation than stars. Stars emit radiation through thermonuclear fusion, whereas these “mini-quasars” do it by accretion – the matter that spirals into them becomes superheated as it draws closer to the event horizon, throwing out vast amounts of radiation. Quasar accretion is nearly 15 times more efficient than stellar fusion at turning mass into energy, Madau claims. “If we’re right, black holes will dominate the radiation emission relative to stars.”
To find out for sure, we first need to know whether there were enough of these black holes around. And that depends on the masses of the earliest stars.
Astrophysicists have long known that massive stars can collapse to form black holes. But some stars end their lives with an almighty bang, exploding as supernovae. According to Ciardi, if one of the first, giant stars has a mass that falls between 40 and 140 suns, or more than 260 suns, it should form a black hole when it dies. If it is between 140 and 260 solar masses, it will explode in a supernova.
But determining the precise mass distribution of those first stars is going to be difficult. It means developing simulations that can take a cloud of star-forming gas and follow its evolution from the birth of the stars to their deaths. Present simulations, even state-of-the-art ones like Abel’s, fall well short of that.
But there are other ways to gather clues. The Swift X-ray telescope that is scheduled to go into orbit later this year could provide information about the abundance of the first black holes. It should witness about 100 gamma-ray bursts per year, some of which will have happened when the universe was much younger. The latest thinking is that each burst marks the formation of a new black hole. “If the first stars make black holes, there’s a good chance we could see them with Swift,” says Abel.
And the James Webb Space Telescope, the successor to Hubble that is slated for launch in 2011, should be able to spot supernovas from the first stars. “We can’t see the first stars themselves, even if they’re a million times brighter than the sun,” Abel says. “But a supernova can be a billion times brighter than the sun, and that makes all the difference.”
These observations should help us understand what proportion of the first stars collapsed to form black holes. By then we should also have other clues from radio-frequency observations of the universe.
Neutral hydrogen emits radiation at a wavelength of 21 centimetres, but this gets red-shifted to longer wavelengths by the universe’s expansion, more so the farther out you go – and hence the farther back in time you look. This can reveal a variety of details about reionisation. For starters, it can tell us about what is producing the ionising radiation. A universe ionised mostly by quasars looks very different from one ionised predominantly by stars. That’s because quasars emit X-rays that travel farther and permeate the universe more uniformly, than the ultraviolet rays produced by massive stars. And so quasars will produce larger ionised regions than stars.
“If you think of the universe as Swiss cheese,” says Ostriker, “ultraviolet rays would first ionise the holes, the bright spots around stars, whereas X-rays would tend to ionise the cheese – the much larger, low-density regions in between.” Radio maps of hydrogen emissions should reveal the size and distribution of the ionised regions in the early universe, helping to answer the question of whether stars or quasars were responsible for the bulk of the job.
Radio maps of interstellar hydrogen gas might add yet another detail. By observing the abundance of neutral hydrogen at different red shifts, astronomers could get a clearer picture of how quickly reionisation took place. It might show the neutral hydrogen signal gradually declining as time went on, suggesting that reionisation occurred gradually too. If, on the other hand, the neutral hydrogen signal dropped rapidly to zero, that would suggest rapid reionisation. There may even have been two separate reionisation peaks, or perhaps even multiple peaks.
“There are various possible histories, and we don’t know which one is correct,” says Loeb. But we could have some answers relatively soon: later this decade, for example, data will start rolling in from the Low Frequency Array, a group of radio telescopes covering one square kilometre in the Netherlands.
However, there are a few issues to sort out before astronomers can determine the relative importance of stars and black holes in reionisation. For instance, the early quasars couldn’t shine unless there was enough gas around to feed the black holes within, and no one can yet guarantee that these hungry beasts would have had enough of that food.
There is also still some considerable uncertainty in WMAP’s measurements. Although the team is fairly convinced that reionisation started 200 million years after the big bang, the margin of error in the data means reionisation could have happened anywhere between 100 and 400 million years after the big bang. If it happened as early as 100 million years afterwards, cosmologists will have to invoke totally new physics to explain this, as there seems to be no way that conventional ionisation sources, such as stars or black holes, would be present in sufficient numbers to do it that early.
One possible explanation would be the decay of some kind of dark matter. Physicists think that the decay of “heavy sterile neutrinos, which are tens of millions of times heavier than normal neutrinos, could be responsible, The trouble is, no one is sure they exist. However, the WMAP team is working on a more refined analysis of the data, and Lyman Page, who heads the Princeton component of the collaboration, says they soon expect to pin down the onset of reionisation to somewhere between 150 and 200 million years after the big bang, which would confirm their latest ideas.
So far, just about everything else that WMAP found – information about dark matter, dark energy, the universe’s age and shape – is “pretty much what we guessed already”, says Ostriker. But its findings about reionisation have challenged our picture of the universe’s youth and thrown into question just what created its architecture. As new data comes in and we fill some of the gaps in our understanding, astronomers and cosmologists might finally be in a position to write the definitive history of the cosmos.
Rolling back the years
How can astronomers deduce that “reionisation”, the epoch when radiation from the first stars began to kick the electrons off the universe’s atoms, began just 200 million years after the big bang?
Ionisation leaves free electrons floating through the cosmos. Also moving through the cosmos were photons of the cosmic microwave background, the vestigial light from the big bang. When a CMB photon bumps into one of the free electrons, this “scattering” changes the photon’s light: it becomes polarised, meaning that its electric field oscillates in a particular direction.
The Wilkinson Microwave Anisotropy Probe looked at the abundance of polarised photons in the CMB, and from this the WMAP team inferred the “optical depth” of the universe. That is a measure of how difficult it is for a CMB photon to move without being scattered, which is in turn a measure of the density of free electrons in the universe. The team then determined how long it would take to produce this amount of scattering.
Essentially they did this by adding up the scattering that could have taken place over time, working backwards until they had enough scattering to account for the observed optical depth.
It’s a tricky calculation because the number of free electrons has diminished over this time period, and the size of the universe has been increasing. The WMAP researchers simplified the situation somewhat by assuming that the universe had always remained fully ionised, and so the number of free electrons remained constant. Though it might seem a rather drastic over-simplification, it’s not actually so bad. That’s because the rapid expansion of the universe is the dominant factor. Its immense size now means that, even assuming the universe was still fully ionised, the free-electron density between now and, say, a billion years ago, is so low that it would produce only a tiny fraction of the observed scattering.
The epoch between 1 billion and 5 billion years ago gives a similar story. The electron density was higher because the volume of space was smaller, but it still only contributes a tiny amount of scattering to the CMB photons. Working back through history, taking into account the changing size of the universe, the WMAP astronomers added up all the likely scattering until they got to a total that was equivalent to the optical depth measured by WMAP. The amount of history they had to cover in order to get this result took them right back to just 200 million years after the big bang.