Ghostly galaxies burned off mysterious cosmic fog

Deep space

Ghostly galaxies in the distant universe are almost certainly the culprits behind a mysterious change in intergalactic gas that allows us to see across the cosmos. Although these galaxies are too faint to be spotted by current telescopes, future instruments could soon reveal their presence.

About 300,000 years after the big bang, the hydrogen that filled the universe cooled and became neutral and opaque, plunging everything into the so-called cosmic dark ages. Any visible wavelengths from early stars were quickly absorbed by the gas, which formed a cosmic fog that persisted for almost a billion years.

Some type of radiation must have broken up the neutral hydrogen atoms into electrons and protons in a process called reionisation, which ultimately made the universe transparent. But whether galaxies would have been numerous and bright enough back then to produce this radiation was uncertain.

Now the latest observations from the Hubble Ultra Deep Field 2012 survey (UDF12), presented this week at a meeting of the American Astronomical Society in Long Beach, California, have suggested that galaxies could, indeed, have turned the universe clear.

“This is the last uncharted piece of cosmic history,” said UDF12 team member Richard Ellis of the California Institute of Technology in Pasadena, California.
Staring intently

Ellis and colleagues used the Hubble Space Telescope to stare at one spot in the sky for 100 hours – twice as long as in previous surveys – and used a filter that made the telescope more sensitive to faint, distant objects. “For the first time with Hubble, we can do this in a systematic way,” Ellis says.

In December the team reported that they had spotted seven new galaxies hailing from when the universe was between 380 million and 600 million years old, right in the middle of the period when reionisation was under way. Since then, the team has analysed the radiation from these galaxies.

Using spectral colour as a yardstick of stellar age, James Dunlop and Alexander Rogers of the Institute for Astronomy in Edinburgh, UK, found that the UDF12 galaxies contain surprisingly old stars. “What we’re seeing are the second generation of stars,” Rogers said at the meeting. “They’re already mature – and must have been around for 100 million years.”

Older stars do not pump out as much ionising radiation as young ones, so these galaxies, at the limit of Hubble’s vision, could not have done the job by themselves.

The team then needed to figure out how many faint galaxies from this era may have gone undetected. Caltech’s Matthew Schenker and colleagues used statistical modelling, based on known galactic populations, to show that there must be exponentially more faint galaxies in the early universe than bright ones – enough to supply the radiation needed.

“We can say confidently that galaxies can do the job, but the faintest galaxies that do most of the work are just below the limits of the UDF12 project,” said team member Brant Robertson of the University of Arizona in Tucson.
Happy ending

“We’re pretty certain it’s galaxies now,” agrees Steven Finkelstein of the University of Texas at Austin, who was not involved in the new work. Other possible candidates for reionisation, such as colliding dark matter particles, had been all but ruled out by earlier observations.

“I think it’s a happy ending,” Ellis says. “Reionisation is a normal process produced by things we can see, and not yet another dark something that we don’t understand.”

The ghost galaxies will probably be detected by Hubble’s successor, the James Webb Space Telescope, which is expected to launch later this decade. If James Webb does not manage to see them, that would present a puzzle, says Ellis. “We’d need an additional source of radiation, whether annihilating particles or whatever else.” But he said he would be very surprised if the faint galaxies did not turn up.

Introduction: Cosmology

 

CosmSpaceologists study the universe as a whole: its birth, growth, shape, size and eventual fate. The vast scale of the universe became clear in the 1920s when Edwin Hubble proved that “spiral nebulae” are actually other galaxies like ours, millions to billions of light years away.

Hubble found that most galaxies are red shifted: the spectrum of their light is moved to longer, redder wavelengths. This can be explained as a doppler shift if the galaxies are moving away from us. Fainter, more distant galaxies have higher red shift, implying that they are receding faster, in a relationship set by the hubble constant.

The discovery that the whole universe is expanding led to the big bang theory. This states that if everything is flying apart now, it was once presumably packed much closer together, in a hot dense state. A rival idea, the steady-state theory, holds that new matter is constantly being created to fill the gaps generated by expansion. But the big bang largely triumphed in 1965 when Arno Penzias and Robert Wilson discovered cosmic microwave background radiation. This is relic heat radiation emitted by hot matter in the very early universe, 380,000 years after the first instant of the big bang.
Space-time curve

The growth of the universe can be modelled with Albert Einstein’s general theory of relativity, which desribes how matter and energy make space-time curve. We feel that curvature as the force of gravity. Assuming the cosmological principle (that on the largest scales the universe is uniform), general relativity produces fairly simple equations called Friedmann models to describe how space curves and expands.

According to these models, the shape of the universe could be like the surface of a sphere, or curved like the surface of a saddle. But in fact, observations suggest that it is poised between the two, almost exactly flat. One explanation is the theory of inflation. This states that during the first split second of existence, space expanded at terrifying speed, flattening out any original curvature. Then today’s observable universe, grew from a microscopic patch of the original fireball. This would also explain the horizon problem – why it is that one side of the universe is almost the same density and temperature as the other.

The universe is not totally smooth, however, and in 1990 the COBE satellite detected ripples in the cosmic microwave background, the signature of primordial density fluctuations. These slight ripples in the early universe may have been generated by random quantum fluctuations in the energy field that drove inflation. Topological defects in space could also have caused the fluctuations, but they do not fit the pattern well.

Those density fluctuations form the seeds of galaxies and galaxy clusters, which are scattered throughout the universe with a foamy large-scale structure on scales of up to about a billion light years. All these structures form because gravity amplifies the original fluctuations, pulling denser patches of matter together.
Dark matter

In simulations, however, visible matter does not supply enough gravity to create the structure we see: it has to be helped out by some form of dark matter. More evidence for the dark stuff comes from galaxies that are rotating too fast to hold together without extra gravitational glue.

Dark matter can’t be like ordinary matter, because it would have made too much deuterium in big-bang nucleosynthesis. When the universe was less than 3 minutes old, some protons and neutrons fused to make light elements, and cosmologists calculate that if there had been much more ordinary matter than we see, then the dense cauldron would have brewed up a lot more deuterium than is observed.

Instead, dark matter must be something exotic, probably generated in the hot early moments of the big bang – maybe particles such as WIMPs (weakly interacting massive particles) or lighter axions, or, less likely, primordial black holes. An alternative to dark matter is modified Newtonian dynamics, or MOND, a theory in which gravity is relatively strong at long range.
Dark energy

Another dark mystery emerged in the 1990s, when astronomers found that distant supernovae are surprisingly faint – suggesting that the expansion of the universe is not slowing down as everyone expected, but accelerating. The universe seems to be dominated by some repulsive force, or antigravity, which has been dubbed dark energy. It may be a cosmological constant (or vacuum energy) or a changing energy field such as quintessence. It could stem from the strange properties of neutrinos, or it could be another modification of gravity.

The WMAP spacecraft put the standard picture of cosmology on a firm footing by precisely measuring the spectrum of fluctuations in the microwave background, which fits a universe 13.7 billion years old, containing 4% ordinary matter, 22% dark matter, and 74% dark energy. WMAP’s picture also fits inflationary theory. However, a sterner test of inflation awaits the detection of cosmic gravitational waves, which the rapid motions of inflation ought to create, and which would leave subtle marks on the microwave background.

The density of dark energy is far smaller than the vacuum energy predicted by quantum theory. That is seen as an extreme example of cosmological fine tuning, in that a much larger value would have torn apart gathering gas clouds and prevented any stars from forming. That has led some cosmologists to adopt the anthropic principle – that the properties of our universe have to be suited for life, otherwise we would not be here to observe it.
Unanswered questions

The biggest questions are still unanswered. We do not know the true size of the universe, even whether it is infinite or not. Nor do we know its topology – whether space wraps around on itself. We do not know what caused inflation, or whether it has created a plethora of parallel universes far from our own, as many inflationary theories imply.

And it is not clear why the universe favours matter over antimatter. Early in the big bang, when particles were being created, there must have been a strong bias towards matter, which the standard model of particle physics cannot explain. Otherwise matter and antimatter would have annihilated each other and there would be almost nothing left but radiation.

The fate of the universe depends on the unknown nature of dark energy and how it behaves in the future: galaxies might become isolated by acceleration, or all matter could be destroyed in a big rip, or the universe might collapse in a big crunch – perhaps re-expanding as a cyclic universe. The universe could even be swallowed by a giant wormhole.

And the true beginning, if there was one, is still unknown, because at the initial singularity all known physical theories break down. To understand the origin of the universe we will probably need a theory of quantum gravity.