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.


The oldest problem in astronomy – solved?

It’s probably the oldest problem in astronomy: what’s the origin of high-energy cosmic rays? Finally, the question might have been solved.

Victor Hess discovered cosmic rays back in 1912, but it proved incredibly difficult to identify the astrophysical source of these bullets. The obstacle to progress was the fact that charged cosmic rays – whether protons or atomic nuclei – don’t follow a straight-line path from source to Earth. Instead, the paths get bent and twisted by magnetic fields in space. Just because a cosmic ray appears to come from a particular direction of sky doesn’t mean it really did come from that direction. It seems to be an insurmountable problem.

But we are now in the era of multi-messenger astronomy! And that allows astronomers to answer questions that once seemed impossible.

The key to unlocking the cosmic ray mystery is that the violent events that generate high-energy charged particles will also generate neutrinos. And neutrinos do follow a straight-line path from source to Earth: because they interact solely via the weak force their paths aren’t bent by magnetic fields, and they don’t get absorbed or scattered by intervening matter. Neutrinos can act as tracers of high-energy cosmic rays. Of course, the same properties that make them useful tracers also make them incredibly difficult to detect: indeed until recently, apart from a diffuse neutrino background,  astronomers had managed to confirm only two astrophysical sources of neutrinos: the Sun and SN1987A (the latter being a relatively close supernova). The IceCube observatory, however, now has good evidence for a third source: TXS 0506+056. And this might have solved the mystery of high-energy cosmic rays.

In September 2017, IceCube – a neutrino telescope consisting of detectors buried in a cubic kilometer of South Pole ice – spotted a neutrino with an energy of 290TeV. (That’s 40 times more energetic than the particles accelerated by the LHC.) Astronomers could trace it back to a source in the direction of Orion. IceCube sent out an alert to observatories around the world, and several of them – Fermi, MAGIC, HAWC and others – detected an increase in gamma-ray activity from the same patch of sky. The culprit was TXS 0506+056 – a blazar that’s about four billion light years away.

Artist's depiction of a blazar

A blazar is an active galactic nucleus in which one of the jets points directly at Earth. Charged particles are deflected by magnetic fields, but neutrinos and EM radiation can head straight towards Earth. Needless to say, this artist’s depiction is not to scale! (Credit: IceCube/NASA)

A blazar is an active galactic nucleus – the compact central region of a galaxy where a supermassive black hole sucks material onto an accretion disk and spews out radiation in two opposing relativistic jets. When we see a blazar, we just happen to be looking directly down one of the jets. It’s quite a thought: four billion years ago the central black hole of a galaxy hurled neutrinos and charged particles and gamma radiation towards Earth. Magnetic fields steered the charged particles away from us. But the neutrinos and gamma rays made it to Earth. And, in September 2017, IceCube detected one of those neutrinos.

The “Case of the High-Energy Cosmic Rays” isn’t entirely closed. Astronomers would want to see more examples before they can be sure that active galactic nuclei are the source. But the observation is very, very suggestive.

And, as with all else in science, the answer to one question raises others: Can other objects besides active galactic nuclei produce high-energy cosmic rays? What is the exact mechanism whereby these particles are produced? And what is the source of the most powerful cosmic rays – are blazars responsible for them too? Now that we are in the age of multi-messenger astronomy, an age in which we can observe astrophysical events not only across the entire electromagnetic spectrum but also with gravitational wave telescopes and neutrino telescopes … well, the answers might start to come more quickly.

A new dawn for astronomy

We should be grateful that we’re alive to see the birth of a new type of astronomy.

On 16 October 2017, astronomers from the LIGO-VIRGO collaboration announced that they had detected — for the first time — gravitational waves from the collision of two neutron stars. The detection was made two months earlier, on 17 August so the event was given the name GW170817. But the announcement was of something much, much more exciting than another observation of gravitational waves (tremendously exciting though that is in itself — it is, after all, only the fifth observation of gravitational waves!)

Information from all three LIGO/VIRGO detectors enabled astronomers to localise GW170817 to a patch of sky in the constellation Hydra. However, about 1.74 seconds after detectors were shaken by gravitational waves, the Fermi Space Telescope registered the detection of gamma rays from a gamma-ray burst — the event GRB 170817A, which was in the same patch of sky as the merger. The chances of observing GW170817 and  GRB 170817A at the same time and in the same place were tiny — unless, of course, they were the same event!

Astronomers around the world were notified, and about 70 telescopes were trained on Hydra. And so, in the weeks following the merger, astronomers were able to observe the after effects at all the different electromagnetic wavelengths, from gamma rays and X-rays all the way down through visible and radio. This is the most studied event in the history of astrophysics!

It’s difficult to keep up with what this event tells us. To pick three items at random, from the combined observations we now know that:

  • colliding neutron stars power short gamma-ray bursts
  • elements heavier than iron, such as gold and platinum, form in these collisions
  • gravitational waves travel at the speed of light

But it’s the promise of seeing more of these events that is so exciting, because they will enable us to learn so much more about the universe. A merger of two neutron stars is a “standard siren“, which gives us a new method of directly measuring cosmic distances — and thus of measuring the Hubble constant; we can use neutron star mergers to investigate relativity; we can learn much more about the behaviour of matter in these intense conditions; the possibilities are huge. We have now entered the era of multi-messenger astronomy — and it’s going to be wonderful!

 

A fourth detection of gravitational waves

On 27 September 2017, astronomers announced the fourth detection of gravitational waves. This was the first time the Italian-based VIRGO observatory has detected gravitational waves; the same waves, of course, were picked up by the two LIGO observatories.

The VIRGO observatory in Italy consists of two 3km-long arms, which together form an interferometer. (Credit: VIRGO Collaboration)

We are now entering the era of gravitational wave astronomy. When the number of observatories detecting a gravitational wave signal increases from two to three, scientists can glean much more information about the source. For example, they can gain information about the polarization of the wave. (It looks as if the general relativistic prediction about the number of polarizations is correct; Einstein wins again.) And, as the image below demonstrates, they can localise the source much more accurately. (Astronomers pointed 25 optical telescopes in the direction suggested by the LIGO/VIRGO discovery, but saw nothing – as was to be expected if the source of the gravitational waves was the collision of a pair of inspiralling black holes.)

When VIRGO works in tandem with the two LIGO observatories, astronomers can localise a source with much more accuracy. (Credit: VIRGO Collaboration)

So what was the source of this gravitational wave event? It seems to have been the merger of a pair of black holes, with mass 31 and 25 times that of the Sun, about 1.8 billion light years away in the constellation of Eridanus. The merger created a single black hole with a mass 53 times that of the Sun; thus three solar masses was radiated away in gravitational waves. That’s a huge amount of energy. LIGO and VIRGO caught a tiny fraction of it.

We now have data from four events. That’s too few to start drawing conclusions, but these four events are interesting. They all come from the merger of two quite large-mass black holes, as the table below shows:

Event Mass of BH1 (solar masses) Mass of BH2 (solar masses)
GW 150914 36 29
GW 151226 14 8
GW 170104 31 20
GW 170814 31.5 25

As I made clear in my book New Eyes on the Universe, I was sure that gravitational waves from merging black holes would soon be found. LIGO found them slightly more quickly than I anticipated, but I wasn’t surprised by the announcement. But I am surprised at the masses of the black holes that are involved: these seem to me to be on the high side. As more events are discovered in the coming years, it will be interesting to see whether these four events are typical.

The origin of ultra-high-energy cosmic rays

Every so often Earth’s atmosphere gets hit by a charged particle (typically an atomic nucleus) with an energy greater than 1 EeV (1018 eV) — in other words, an ultra-high-energy cosmic ray. One of the longest-standing problems in astronomy is the origin of these ultra-high-energy cosmic rays.

°The reason it’s difficult to pinpoint where these particles come from is that the Milky Way’s tangle of magnetic fields sends electrically charged particles in wild spirals. Any directional information about the origin of these charged particles get destroyed. Cosmic rays thus hit Earth equally from all directions, and that begs the question: do these high-energy particles originate within our galaxy or do they have an extragalactic origin? In a recent paper, astrophysicists at the Pierre Auger Laboratory claim to have solved the mystery: ultra-high-energy cosmic rays have an extragalactic origin.

Surface detector

One of the many water-Cherenkov surface detectors that make up the Auger cosmic ray telescope.

The Auger Collaboration looked at 30000 of the very highest-energy particles. Because of their high energies, these particles undergo less deflection than the billions of low-energy cosmic rays that constantly bombard Earth. The Collaboration tracked these particles back, and found that there was an excess of particles coming from a patch of sky 120° away from the centre of the Milky Way. Furthermore, this patch of sky contains a high density of nearby galaxies.

The case seems to be settled: ultra-high-energy cosmic rays have an extragalactic origin. They come from nearby galaxies. Furthermore, since they don’t come from the Milky Way we can probably assume that the galaxies they do come from are somehow different to the Milky Way. Perhaps they originate in a galaxy such as Centaurus A, which contains relativistic jets powered by a supermassive central black hole. Further study will surely pinpoint the precise origin of these enigmatic particles.

FAST completed

Time rolls by. When I wrote New Eyes on the Universe I pictured FAST – the Five-hundred-meter Aperture Spherical radio Telescope – as an instrument for the mid-term future. Construction of the telescope began in March 2011 but I thought its remote location in a karst sinkhole in Guizhou Province, combined with the technical difficulty of operating such a large device, would cause innumerable delays. In September 2016, however, the telescope saw first light. Chinese scientists are now calibrating the telescope, and by 2019 it should be doing astronomy.

FAST is a staggering telescope. It consists of 4450 triangular reflecting panels that combine to form a collection area more than twice as large as the famous 300m Arecibo telescope in Puerto Rico. If it works to its design specifications, FAST will be twice as sensitive as Arecibo, observe three times more sky than Arecibo, and be able to survey the sky 5–10 times faster than Arecibo.

Such a vast instrument has the potential to make a number of discoveries in pure astronomy, but a further intriguing possibility lies in its use for SETI: scientists interested in the search for extraterrestrial intelligence will be able to “piggyback” on pure science projects and comb the data for signals.

FAST

FAST lies in a natural basin in the Guizhou Province of China. The telescope is undoubtedly an impressive astronomical instrument, but its name is a slight misnomer: although the reflector diameter is 500m only a 300m diameter circle is used at any one time. (Credit: Asianewsphoto)

MeerKat

On Saturday 16 July 2016 the MeerKat radio telescope, currently under construction in South Africa, formally commissioned its first 16 dishes. The same day, astronomers released the first image taken by these 16 dishes: the image revealed 1300 galaxies in a small region of space where only 70 galaxies had previously been detected.

These 16 MeerKat dishes represent only one quarter of the final contingent: the complete telescope will contain 64 dishes. And the 64-dish MeerKat will be only one element of the multi-component Square Kilometer Array (SKA).

This first image from a quarter-completed Meerkat gives a hint at what a tremendously powerful instrument the SKA is going to be!

First light mages from MeerKat

A “first light” image from the MeerKat radio telescope. The central image is a montage. The two right-hand panels show galaxies containing central supermassive black holes.(Credit: MeerKat/SKA South Africa)

Thirty Meter Telescope to move?

In July 2009, the governing board of the Thirty Meter Telescope (TMT) chose Mauna Kea mountain on Hawaii as the site of the $1.4bn facility. In October 2014, however, at the telescope’s ground-breaking, native Hawaiians held a protest. They regard the Mauna Kea summit as sacred and they object to the number of telescopes there. (Mauna Kea is one of the best places in the world to put a telescope, which is why astronomers have put 13 telescopes there. The TMT would be the 14th, and by far the largest.)

The TMT board are now considering other possible locations for the telescope. Possible options include Chile and the Canary Islands. It will be interesting to see where on Earth this vast telescope will be built.

TMT

An artist’s impression of the TMT which, if it is built, would allow astronomers to peer back to the earliest stages of the universe. (Credit: TMT Observatory)

The Edge of the Sky, by Roberto Trotta: A book review

Last month I finished a first draft of an article about AMS-02 for the Yearbook of Astronomy 2016. (As I write this, 2016 seems a long way away – but the lead time for this volume is long and, besides, I have other writing commitments this year. I wanted to get a draft version out of the way while I had chance.) The target audience for the Yearbook consists of amateur astronomers – people who are deeply interested in astronomy and cosmology but who don’t necessarily have a scientific or mathematical training. Much of my article, therefore, is taken up by explaining the meaning of various technical words and phrases – “positron fraction”, “neutralino annihilation”, “primordial antimatter” and so on. Even the name of the experiment – Alpha Magnetic Spectrometer – requires explanation. It took me several thousand words to explain why Sam Ting and his AMS might, or might not, have seen something of great scientific interest.

When I heard that Roberto Trotta had written The Edge of the Sky – a book about cosmology using only the thousand most frequently used English words – I thought he must either be barking mad or else a member of the Oulipo (you know, those authors who decide to write novels while blindfolded and using only letters that appear on the left-half of a keyboard, or something equally arbitrary and constraining). I thought it would be a disaster. It turned out to be one of the most charming, fresh and inventive books of popular science that I’ve read.

In Trotta’s book, the Milky Way becomes the White Road, electrons are referred to as Very Small Drops and antimatter becomes Sister Drops. (So I guess the positron, which I write about in my Yearbook article, would be the Sister Drop of the Very Small Drop.) These and other word choices are wonderful and lead to a surprising clarity of expression.

But is it really possible to describe the complexity of modern science using this approach. Well, I tried to explain the importance of AMS-02 using polysyllabic words to replace other, more technical, polysyllabic words. In The Edge of the Sky a space-based detector such as AMS-02 becomes a flying Far-Seer in the sky. The heroine of Trotta’s book wonders whether dark matter will first show up in such a flying Far-Seer in the sky or in one of the big ears in the rock (in other words, one of the multitude of underground detectors such as LUX or DAMA), in the huge eye in the ice (the IceCube Neutrino Observatory) or in the Big Ring in the ground (the Large Hadron Collider). This approach is possible. It works, and it works beautifully.

The experience of reading The Edge of the Sky is strange and rather hypnotic. I think everyone can learn something from it. This book is a must-read. (A sentence that uses the 28th, 455th, 13th, 85th, 88th and 317th most-used English words according to Project Gutenberg.)

Kepler’s latest haul

The Kepler mission continues to astound. In May 2013 it suffered a malfunction that ended its primary mission: the spacecraft needs three reaction wheels in order to point its telescope with accuracy, and when the second of four such wheels failed the mission was over. However, a team of scientists and engineers developed a method for using the pressure of sunlight as a third “virtual” reaction wheel: the method works (albeit with reduced capabilities and with the need for a prodigious amount of signal analysis). Kepler is back in business.

In December 2014, astronomers announced that Kepler had spotted a new planet (HIP 116454b), a discovery that was subsequently confirmed by other telescopes. HIP 116454b is a super-Earth: it has about 12 times the mass of Earth and is about 2.5 times larger. It orbits close to a K-type orange dwarf. (The Allen Telescope Array has observed this planet, looking for signals in the range 1000 to 2250 MHz.  It didn’t hear anything.)

And further analysis of existing pre-malfunction Kepler data has taken its tally of exoplanet discoveries past 1000. The latest batch includes the most Earth-like planet found to date: Kepler 438b is just 12% larger than Earth.

Kepler continues to produce surprises!

 

 

 

BICEP2 turns to dust?

I have just finished checking page proofs of an article that will appear in the 2015 Yearbook of Astronomy. The article is entitled “Ripples from the start of time?” and it discusses what had the potential to be one of the most important and exciting cosmological discoveries in decades: B-mode polarization of the cosmic microwave background. Earlier this year, the BICEP2 experiment claimed to have found just such polarization and argued that their measurements could only be explained in terms of primordial gravitational waves – ripples of space made large by inflation, an event that took place when the universe was only a trillionth of a trillionth of a trillionth of a second old.

The BICEP2 team made a bold claim, and bold claims require a lot of solid evidence before they can be accepted by the scientific community. Soon after the team made their announcement of B-mode polarization, scientists raised doubts about the interpretation of the measurements (though not of the measurements themselves: everyone acknowledges that the scientists involved here are extremely capable astronomers). One problem was that the BICEP2 experiment observed the sky at only one frequency: when you have only one data point, any curve can be made to go through it. When you observe the cosmic microwave background you need measurements at several different frequencies before you can be sure that your signal really does come from the distant cosmos and not somewhere nearby. A second, more pernicious, problem was that it was not at all clear that the BICEP2 team had properly accounted for dust.

The issue is that dust grains in the galaxy can polarize light – and in particular it can give rise to a B-mode pattern of polarization. If the BICEP2 team had underestimated the amount of dust emission then their interpretation of their observed signal had to be under suspicion. It’s why I added a question mark to the title of my article: BICEP2 might have been seeing a signal from the dawn of time, but it might not.

A recent paper by the Planck collaboration suggests that the BICEP2 result might well have been the result of dust. It turns out that there is much more dust in the area of sky observed by BICEP2 than was originally thought. That in turn means that the BICEP2 results are entirely consistent with observations of dust. This doesn’t mean that B-mode polarization of the cosmic microwave background does not exist, nor even that BICEP2 didn’t spot such polarization; but it does mean that we don’t need to invoke inflation in order to explain the BICEP2 results.

A joint paper by the Planck and BICEP2 teams, due for publication later this year, should clarify the situation further. But at the moment it seems that our dreams of being able to look back to the very start of the universe must be put on hold. Shame. Those dreams were beautiful while they lasted