New Eyes on the Universe

Book cover - New Eyes on the Universe

Astronomers will soon have access to some tremendous new instruments — from radio telescopes through to gamma-ray telescopes, as well as new completely new types of observatory. Exciting times ahead!

The book’s subtitle is “Twelve cosmic mysteries and the tools we need to solve them” so in this category I also post about dark energy, dark matter, dark ages et al.

Another twist in the dark matter mystery

In New Eyes on the Universe I gave only passing mention to the Large Underground Xenon (LUX) dark matter experiment. LUX was clearly going to be an important player in the search for dark matter, but while I was writing the book the experiment was still in its commissioning phase. Yesterday, LUX presented results from its first three months of operation. (For those who haven’t read the book, LUX employs 370kg of liquid xenon cooled to about 160K and shielded by water in a search for WIMP dark matter. If a WIMP collides with a xenon nucleus then the photons and electrons emitted as the nucleus recoils can be detected. In order to shield the xenon from cosmic rays and other background radiation, experimenters have placed the detector a mile underneath the Black Hills of South Dakota.)

The first thing to note is that LUX is now the world’s most sensitive detector currently searching for WIMP dark matter. Richard Gaitskell, a spokesperson for LUX, described its sensitivity with a footballing analogy. Imagine a 75000-strong crowd of football fans, each clapping twice a second: the number of claps is what  the detector was hearing each second while it was on the surface. That’s a tremendous cacophony. When the detector was placed a mile underground it was  as if the clapping fell to a rate of one clap per minute. That reduction in background is necessary: LUX is trying to ‘hear’ the equivalent of a sigh…

The second thing to note is that LUX is sensitive to WIMPs across a wide range of possible masses. There have been tantalizing hints by other dark matter experiments of WIMPs having a relatively low mass of around 8.6 GeV; many models based on supersymmetry, on the other hand, predict WIMPs with a mass of 35 GeV or more. LUX is sensitive to both low- and high-mass WIMPs.

And the results of the first 90 days of LUX operation? Well, the LUX data are consistent with the detector having seen zero dark matter particles during that time. As a LUX team member put it: “We’ve seen nothing better than anyone else.” The problem is, if an 8.6 GeV WIMP particle did indeed exist, as hinted at by CDMS, then LUX should have seen 1550 of them during those first 90 days. It seems impossible to reconcile these latest results with the existence of a low-mass WIMP.

The LUX results don’t prove the non-existence of dark matter, of course, and before reaching any conclusions we will really need to wait for the next LUX report: that will present an analysis of the first 300-days of operation. But the LUX results do put the dark matter mystery squarely in the spotlight: it’s becoming imperative that we learn just what dark matter is.

Bert and Ernie – dark matter candidates?

At the time New Eyes on the Universe was published, the only confirmed sources extraterrestrial neutrinos were the Sun and SN1987A. The view of the sky afforded by neutrino telescopes was rather dull.

That view of the neutrino sky is beginning to change. The IceCube SouthPole Neutrino Observatory – a “telescope” consisting of particle detectors buried in one cubic kilometre of Antarctic ice – has detected 28 neutrinos with an energy in excess of 30 TeV (a teraelectronvolt is 1012 eV). Two of these neutrinos, dubbed Bert and Ernie, had energies in excess of 1 PeV (that’s 1015 eV) – far in excess of energies available at the Large Hadron Collider.

Artist's impression

An artist’s impression of the array of optical sensors, buried in Antarctic ice, that form the IceCube telescope. If a high-energy neutrino interacts with an oxygen atom in the ice, a charged particle can be produced that will be moving through the ice faster than light itself can travel through the ice. A cone of Cerenkov radiation, with its characteristic blue hue, will be produced – and it’s this radiation that the sensors detect. (Credit: IceCube Collaboration/NSF)

It’s possible that Bert and Ernie were produced by high-energy cosmic rays smashing into Earth’s atmosphere, but an extraterrestrial origin for these neutrinos does seem more likely than not. And If IceCube has indeed detected high-energy neutrinos from the depths of space the question becomes: what was their source? That’s where things get interesting. If they came from some violent astrophysical source then astronomers have a telescope that lets us study them. Or perhaps they came from the decay of dark matter particles – a suggestion made in a recent preprint by Arman Esmaili and Pasquale Serpico (Are IceCube neutrinos unveiling PeV-scale decaying dark matter?). Whatever the source of Bert and Ernie turns out to be, it seems certain that IceCube truly is giving us some new eyes through which to view the universe.

HAWC eyes the Moon

In New Eyes on the Universe I give a brief mention to the High-Altitude Water Cherenkov Observatory (HAWC). When I was writing the book, HAWC had not yet started its mission. Last month HAWC published its first image. It wasn’t a particularly striking image – just the shadow of the Moon – but it was a start. Once HAWC is complete, we can expect to see some discoveries.

Cosmic-ray shadow of the Moon

The Moon’s cosmic-ray shadow, as seen by HAWC. (Credit: HAWC Collaboration)

HAWC is a gamma-ray observatory. It’s the world’s largest such observatory and, even though the observatory is not yet complete, it already holds the record for detecting the highest-energy light ever seen on Earth: HAWC can detect gamma rays with energies up to 100TeV, which is many trillions of times more energetic than visible light. By capturing such high-energy photons, HAWC will enable astronomers to learn more about phenomena such as pulsars, supernovae and feeding black holes.

Image of HAWC array

The HAWC Observatory in Mexico consists of an array of Cherenkov detectors – water-filled steel tanks in which photomultipliers detect radiation emitted by charged particles passing through the water. (Credit: HAWC Collaboration)

Construction of HAWC began in 2009, at an altitude of 4100 meters on the flanks of the Sierra Negra volcano near Puebla, Mexico. At the time of writing the observatory consists of 30 Cherenkov detectors: each detector is a water-filled, corrugated steel tank some 4m high and 7.3m in diameter. Inside each tank are four photomultiplier tubes that detect the cascade of particles created when high-energy gamma rays and cosmic rays crash into molecules in Earth’s atmosphere. (The photomultipliers don’t detect these particles directly. Rather, they detect the Cherenkov radiation that is emitted whenever fast-moving charged particles pass through the water more quickly than light itself can pass through the water.) By comparing signals from the different detectors, astronomers can reconstruct some of the properties of the incoming radiation that generated the particle cascade. By August of this year, about 100 of the detectors will be fully functional and HAWC will commence continuous observations of the sky. By 2014, HAWC will consist of 300 Cherenkov detectors. It will complement beautifully the existing gamma ray facilities such as MAGIC (in the Canary Islands) and HESS (in Namibia).

A hint of dark matter?

Yesterday Sam Ting, the 1976 Nobel prize winner, announced the first results from his Alpha Magnetic Spectrometer (AMS-02) experiment. The results hint at a possible dark matter signal. But it remains only a hint.

The story begins with the observation that the visible stuff in the universe – stars, galaxies, and so on – are all made of matter. We see no evidence for large amounts of antimatter. However, small amounts of antimatter are constantly being created when high-energy cosmic rays scatter off particles in the interstellar medium, or when the intense electromagnetic field surrounding pulsars produce electron-positron pairs. So Earth is certain to be hit by antimatter particles, and in particular by positrons, that have been created by events within our galaxy.

Now, astrophysicists are interested in measuring the so-called positron fraction that hits Earth. (The positron fraction is the ratio of positrons to the total number of electrons and positrons.) If the main production mechanism for positrons is the scattering of high-energy cosmic rays off particles in the galactic disk, an assumption that until recently seemed quite reasonable, then the positron fraction would decrease with energy since there are other processes that generate high-energy electrons without accompanying positrons. In 2008, however, the PAMELA experiment measured a rise in the positron fraction between 10 GeV and 100 GeV. The Fermi satellite later confirmed this excess number of positrons, and it showed that the rise extended up to 200 GeV. So what is going on? Well, one explanation for the rising positron fraction is that positron generation by a few nearby pulsars could be the cause. But there’s another possibility.

If dark matter exists then sometimes, simply because there’s so much of the stuff, dark matter particles and antiparticles will meet and annihilate. In some models, dark matter annihilation can give rise to excess numbers of positrons. Furthermore, the dark matter signal must take a particular form: it will be isotropic (in other words, the positrons will come equally from all directions in space) and the rising positron fraction will have a sharp cut-off after a certain energy is reached (an energy that is determined by the dark matter particle mass, since the particles can’t give rise to positrons that are more energetic than themselves). So after the PAMELA and Fermi results there was hope that we might be seeing hints of a dark matter signal, but the data were not clear enough to draw any conclusions. It wasn’t even certain that the positron excess was real.

Enter Sam Ting’s AMS-02 experiment.

AMS-02 is a cosmic ray detector on board the International Space Station. The detector was put in place by astronauts on 19 May 2011, and since then it has detected 30 billion cosmic rays. It has been able to measure the positron fraction to higher energies than any previous detector and with a precision that is much, much better than anything that has gone before. Yesterday, Ting announced the results of an analysis of the first 10% of data from AMS-02.

Artist view of AMS-02 installed on the attached S3 location on the main truss of the ISS

An artist’s view of AMS-02 installed on the attached S3 location on the main truss of the ISS (Credit: NASA/JSC)

The first result is that the excess seen by PAMELA and Fermi is real: the positron fraction increases from about 5% at 10 GeV to about 15% at 350 GeV. That in itself is significant, and requires an explanation. Second, the excess seems to be isotropic: if it turns out to be truly isotropic then that would tend to disfavour pulsars as being the source of the excess. Third, there are suggestions – and these are nothing more than tantalising hints – that AMS-02 might be seeing the start of a cut-off.

The positron fraction as a function of positron energy. Compare the AMS-02 error bars with previous experiments! (Credit: AMS collaboration)

The positron fraction as a function of positron energy. Compare the AMS-02 error bars with previous experiments! (Credit: AMS collaboration)

So are we seeing the effects of dark matter? As the years go by, and AMS-02 improves and extends the energy spectrum positron fraction yet further, astrophysicists might decide that the dark matter explanation is the only viable one. But at present it’s far too early to make any claims: the AMS-02 results announced yesterday are interesting, but nothing more.

The AMS paper has been published in Physical Review Letters.

Planck results are here!

This morning an ESA press conference presented results from an analysis of the first 15 months of data from the Planck mission. The results are exquisite, and it’s clear that Planck will be as important for cosmology as its predecessors COBE and WMAP. Cosmologists will be poring over the data for years to come.

I’ll give more detail in future posts, but for the moment here are just two items.

First, the most detailed picture yet of the early universe:

Planck's stunning new map of the universe

Planck’s stunning new map of the universe (Credit: ESA)

Second, some of the stand-out points from this morning’s presentation:

  • The universe is slightly older than we previously thought (about 80 million years older in fact): it’s 13.82 billion years old.
  • Planck measures the Hubble constant to be 67 km s-1 Mpc-1. This is slightly smaller than most other recent estimates. Curious!
  • The energy inventory of the universe isn’t quite what we thought it was: there’s slightly more dark matter than previously thought and slightly less dark energy. The universe is currently made up of 4.9% normal matter, 26.8% dark matter and 68.3% dark energy.
  • On small scales, the standard cosmological model (which includes inflation) agrees supremely well with the observed cosmic microwave background. The standard cosmological model is in good shape.
  • There are hints, based on observations of the largest angular scales, of physics beyond our current theories. In particular: (i) the sky in the southern hemisphere is ever so slightly warmer than the sky in the northern hemisphere; (ii) large-scale temperature fluctuations are weaker than expected; and (iii) there’s a cold spot in the universe, in the constellation Eridanus, that’s much larger than our models would predict. Gaining an understanding of these anomalies is going to lead to some really interesting ideas over the next few years.

Black holes in a spin

After less than 9 months in orbit the NuSTAR X-ray telescope, which I discussed in a previous blog post (“A NuSTAR is born“), has produced an important scientific result: it has teamed up with the venerable XMM-Newton to make the first definitive measurement of the spin rate of a black hole.

It’s not easy to measure the spin of a black hole. The key to the measurement is the fact that a rotating accretion disk of gas forms around a black hole, and the gas in the disk gets extremely hot and emits X-rays as it spirals around. However, the disk can get closer to a black hole if the hole is spinning, and thus the X-ray emissions are more strongly affected by the gravity of a spinning black hole than by a non-spinning black hole. Thus if you measure the gravitational redshift in the X-ray emission from an accretion disk, it should be possible to tell whether the black hole is spinning.

Previous measurements from XMM-Newton on supermassive black holes have suggested that those black holes it investigated did indeed have a high spin rate. However, the XMM-Newton results were not conclusive. XMM-Newton measured the X-ray spectrum in the 0.5-10 keV range and at these energies there is another possible explanation for the observations: it’s possible that absorbing layers of surrounding gas clouds mimic the spectrum that would come from a rapidly spinning black hole.

A paper in today’s Nature (“A rapidly spinning supermassive black hole at the centre of NGC 1365” by Guido Risaliti and co-workers) describes how data from NuSTAR and XMM-Newton have been combined to measure the spin rate of the supermassive black hole powering the active galactic nucleus of NGC 1365. Using NuSTAR, Risaliti and his team were able to take a spectrum of photons with energies in the range 3-80 keV. At these extremely high energies the signal is clean, and the data allow a direct comparison between the two possibilities: either a thick layer of gas blankets the accretion or else the black hole is spinning rapidly. It turns out that the “gas-absorbtion” explanation doesn’t work, at least for NGC 1365: for this to be the explanation of the observed spectrum the active galactic nucleus would have to be so luminous that radiation pressure would blow it to smithereens.

Picture of NCC 1365

The Great Barred Spiral galaxy (NGC 1365) is about 56 million light years away in the constellation Fornax. (Credit: NASA)

It turns out that the supermassive black hole at the centre of NGC 1365 is rotating with at least 84% of its maximum permitted value. And why should we care? Well, astronomers would really like to know how these supermassive black holes evolved and how they affected the evolution of their parent galaxies: did the black holes become supermassive by a gradual process of eating randomly moving clouds of gas and matter, or did they grow by gorging in just a few, gigantic events? If the former is the case, the black hole would rotate slowly; if the latter is the case, the black hole would rotate quickly. In NGC 1365, at least, it seems that the black hole grew to be large in just a few feeding events. If the same sorts of measurements can be made on other galaxies, astronomers will have a much clearer idea of how supermassive black holes and their parent galaxies evolved.

Smaller than Mercury

That astronomers can find exoplanets at all is still a source of wonder to me. That they can find Earth-sized planets is astonishing. But a paper published in today’s issue of Nature is almost miraculous: A sub-Mercury-sized exoplanet, by Thomas Barclay and many others, describes the discovery of an exoplanet that has a radius that’s just 0.3 times that of Earth. It’s smaller than Mercury, in other words.

Kepler 37b, as it’s name implies, was found from data taken by the Kepler mission. The parent star, Kepler-37, is interesting because it’s the densest star in which solar-like oscillations have been detected. Just as a measurement of the frequencies of a musical instrument allows you to determine some of the properties of that instrument, the characteristic “ringing” of a star allows astronomers to determine some of the star’s properties with great accuracy. In this case astronomers were able to determine the radius of Kepler-37 with great precision, and this in turn allowed them to determine the radius of its planets with precision. Transit signals suggest that Kepler-37 has three planets. Kepler 37d has a radius about 1.99 times that of Earth’s; Kepler 37c has a radius about 0.74 times that of Earth’s; and Kepler 37b has a radius just 0.3 times that of Earth’s. It’s not much bigger than our Moon – and Kepler detected it!

Artist's impression of Kepler 37b

An artist’s impression of Kepler 37b (Credit: NASA)

For every 200 stars that Kepler studies you’d expect to see the transit signal in the data of perhaps one star. So the fact that the astronomers were able to identify this sub-Mercury-sized object does rather tend to suggest that small planets are extremely common.

Weighing a black hole

At the heart of every galaxy there seems to be a supermassive black hole; that much astronomers are pretty sure about. What’s much less well understood is the possible connection between the mass of the central black hole and the size of the galaxy’s bulge; there are hints that supermassive black holes and galaxies might co-evolve, but the details are unknown. One of the barriers towards understanding this possible relationship is the difficulty in measuring black hole masses. It’s not impossible to measure the mass of a supermassive black hole: you can track stars as they orbit the black hole, for example, and use their motion to deduce the mass. Or, even better, you can track the motion of ionised gas clouds. In a few rare cases, as I mentioned in Measuring the Universe, it’s possible to observe maser emission from the central regions and from this measure a mass. These methods only work for nearby galaxies, however.

Timothy Davis, Martin Bureau, Michele Cappellari, Marc Sarzi and Leo Blitz, in a paper published in the 30~January~2013 issue of Nature (A black-hole mass measurement from molecular gas kinematics in NGC4526) have developed a new technique for measuring the mass of a supermassive black hole and used it to estimate the mass of the central black hole in NGC4526.

The team’s idea is to measure the motion of molecular gas clouds by observing emission from carbon monoxide (CO). They tested their idea by observing the CO(2-1) emission line from gas in the lenticular galaxy NGC 4526, which is about 17 Mpc away from us in the Virgo cluster. The wavelength from such CO emission is 1.3 mm, which means that millimetre-wavelength telescopes are required. The team used the 23-telescope Combined Array for Research in Millimetre Astronomy (CARMA) to observe the emission, and from their observations deduced a central black hole mass of about 450 million solar masses.


The lenticular galaxy NGC4526 (Credit: NASA)

What’s interesting about this paper is not the result itself, but the possibilities it opens. The team needed 100 hours of observing time with CARMA to get their result. With ALMA, however, the same job would take less than an hour: observations become possible that were previously impractical. When it becomes fully operational ALMA, using this technique, may well provide us with an understanding of how galaxies and their supermassive black holes live together.

Earths galore

The Kepler mission team announced exciting new results on 7 January 2013, at the 221st meeting of the American Astronomical Society. As I’ve explained in other posts (such as this one here), the team aims to detect exoplanets by staring at more than 150,000 stars in a fixed part of the sky. The Kepler telescope looks for regular dips in the brightness of these stars, variations that might be caused by the presence of a transiting planet.

The technique used by the Kepler team has been tremendously successful. On 7 January Christopher Burke, a Kepler scientist at the SETI Institute, announced the discovery of 461 new planetary candidates. So, as of the time of writing, Kepler has found 2740 potential planets orbiting 2036 stars. This is a hugely impressive number, when you consider that it wasn’t such a long time ago that people were debating whether exoplanets existed at all and, if they did, whether it would be possible to detect them.

The sizes of the planetary candidates discovered by Kepler(Credit: NASA/Kepler)

The sizes of the planetary candidates discovered by Kepler
(Credit: NASA/Kepler)

The AAS presentation that really caught the attention of the world’s press, however, was that given by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics. Fressin has tried to estimate the fraction of stars that possess of Earth-sized planets, based on the Kepler data. Kepler will inevitably see only a small fraction of exoplanets because the transit technique only picks up those planetary systems in which the orbital plane is more or less side on to our view. If the orbital plane is slanted by more than a few degrees to our line of sight, the planets we won’t see the planets transit and there’ll be no drop in brightness. On the other hand, Kepler is seeing regular brightness fluctuations that are not due to transits; for example, a non-variable star might be extremely close in our line of sight to a regular variable, which would give a similar signal to a transit. After Fressin had corrected for both these effects he arrived at the following estimate: one in six stars host an Earth-sized planet. If that is indeed the case, there are about 17 billion Earth-sized planets in our Galaxy and it’s certain that some of these will be in the habitable zone.

If we’re looking to explain the Fermi paradox, we now know that we can’t invoke a paucity of Earths!

CHEOPS go-ahead

I’m old enough to remember a time when some people thought we’d never discover planets beyond the solar system. It’s not that long ago that astronomers first managed to confirm the existence of a few exoplanets. Techniques improved, and the number of known exoplanets started to increase rather rapidly. Then the Kepler mission was launched, and the number of candidate exoplanets simply rocketed (at the time of writing, Kepler has identified 2321 exoplanetary candidates). That’s impressive progress.

Now that astronomers have identified so many exoplanets, it becomes possible to design a mission that can study those bodies in more detail. That’s precisely what the CHEOPS mission will do. (Yes, CHEOPS is yet another acronym. This one stands for CHaracterising ExOPlanets Satellite.) ESA have selected CHEOPS for study as the first “small” or S-class mission.

If all goes well, CHEOPS will launch in 2017 and study stars that we know have planets around them. The satellite will monitor a star’s brightness, looking for the characteristic dip in brightness as a planet transits. This measurement will allow astronomers to determine the radius of the transiting planet; if the planet’s mass is already known from other measurements then astronomers will be able to calculate the planet’s density. This in turn will provide clues about the planet’s internal structure. CHEOPS will tell us a lot about the formation and evolution of planets with a similar mass to Earth. And, just as Kepler has provided targets for CHEOPS to study, CHEOPS in turn will provide targets for follow-up study by the next generation of powerful telescopes such as E-ELT.

An artist's impression of CHEOPS

An artist’s impression of CHEOPS. The satellite will be placed in a Low Earth Sun-Synchronous orbit at an altitude of 800km. Its 33cm telescope will observe in the range 400-1100nm.
(Credit: University of Bern)