The Week in Space and Physics: The Neutrino Galaxy
The Week in Space and Physics: The Neutrino Galaxy
On viewing the Milky Way with neutrinos, the worlds of TRAPPIST-1, the gravitational wave background and Euclid
When we talk about astronomy, we often reach instinctively for light. When discussing Betelgeuse, for example, we speak of its red colour, a detail which reveals its nature as a supergiant star. Shifts in how strongly it shines give away its age and its impending demise, an event that promises to be one of the brightest celestial events of all time.
The physics of the last century revealed other ways in which we can monitor the universe. Einstein’s theories taught us to look for subtle effects of gravity, as is now done by the observatories searching for gravitational waves sweeping over the Earth. Research into fundamental particles uncovered the existence of the neutrino, a fleeting particle that, like the photon, floods the universe.
Unlike photons, however, we can barely see them. Neutrinos rarely bother to interact with the atoms and molecules that make up our world, leaving them all but invisible to us. Despite this, they seem to be everywhere, created by many different sub-atomic processes. Nuclear reactors on Earth generate them as they run; the Sun - itself a vast nuclear furnace - produces a constant stream of them. Violent events like supernovae create even more.
Neutrinos are thus constantly washing over the Earth. Yet detecting them is difficult. When one strikes an atom, it creates a telltale signature. Of the untold trillions passing through the Earth every second, however, few will actually hit an atom in the right place to create that signature. The vast majority of them, indeed, can pass right through the Earth, through the entire galaxy, as though those obstacles were simply not there at all.
One of the most sensitive neutrino detectors built lies in the ice under the South Pole. The IceCube Observatory, as it is known, is made up of thousands of detectors buried in the ice sheets, searching for the signs of passing neutrinos. Every year it picks out around one hundred thousand neutrinos with enough sensitivity to figure out where in the sky they came from.
Only a few neutrinos have been linked to individual events. One high energy neutrino came from an energetic galaxy some four billion light years away. Two others possibly came from stars ripped apart by black holes. Over the years, however, the observatory has picked up enough neutrinos to start giving us a general sense of what the neutrino sky looks like.
Just as certain parts of the sky appear brighter in visible light - galaxies, for example, or giant stars - so do some parts appear bright in neutrinos. These areas will not always be the same - just because something shines brightly does not mean it also creates a lot of neutrinos. Indeed the differences are part of what interests astronomers. By studying the sky in neutrinos, instead of photons, we hope to see things that were hitherto unknown.
Remarkably our galaxy - one of the brightest objects in visible light - is barely visible with neutrinos. Neutrino surveys typically pick out handfuls of bright galaxies far away, but few neutrinos from the Milky Way. Recently, however, a team of researchers managed to use data from IceCube to put together a neutrino map of the Milky Way.
The contours of that map are, it is true, rather vague. That is to be expected: neutrino astronomy is still in its infancy, and our detectors are still improving. In the future we should be able to make better maps - maps, indeed, that allow us to clearly pick out the sources of neutrinos within our own galaxy. This map, then, is more a hint of things to come, rather than a finished product itself.
Eyes on the TRAPPIST System
Forty light years away lies a small star known as TRAPPIST-1. Around it, astronomers discovered in 2016, are seven rocky planets. Models hint that several of these planets could be warm enough to host liquid water, a feature that most scientists take as a prerequisite for life.
Since the planets are close enough to be easily observed, TRAPPIST-1 and its planets have thus been a key target of the James Webb Space Telescope. Earlier this year researchers released data from TRAPPIST-1b, the innermost world of the system. That revealed a rocky world similar to Mercury or the Moon, with no substantial atmosphere around it at all.
Now data from TRAPPIST-1c, the next planet out, has been released. Some researchers had thought the world might look like Venus, with a thick toxic atmosphere and soaring temperatures. Yet the data seems to rule this out. If TRAPPIST-1c has an atmosphere it must be a faint one, so tenuous as to be invisible to the James Webb. That means, in all probability, that this is another dead world.
That’s a disappointment, but perhaps not a huge surprise. Stars like TRAPPIST-1 tend to pass through a turbulent phase early in their lives, giving out flares that strip the air from nearby planets. That, at least, may have been the fate of the two innermost worlds of this star system.
Planets further out, however, might have managed to cling on to their atmospheres. Researchers will be examining each of them with the James Webb in the months and years to come, hoping to learn more about their characters. Indeed, it is these more distant planets that are also more interesting. If they have atmospheres they may also have water, and all the things that come with it.
Still, researchers are also prepared for the opposite scenario: that all these worlds are dead; long-since stripped of their air. That would suggest that small stars like TRAPPIST-1 are inhospitable to life, thus narrowing the search for Earth-like planets elsewhere in the galaxy.
A Hint of the Gravitational Wave Background
In recent years physicists have picked up gravitational waves coming from colliding black holes. Yet theory predicts that black holes should also give out weaker waves as they spiral towards one another, creating a kind of gravitational wave background. So far signs of these waves have been scant. They are, indeed, too small for even our most sensitive gravitational wave observatories to spot.
With that in mind, researchers turned to another technique - pulsar surveys - to try to pick them up. Pulsars spin rapidly, emitting regular bursts of energy as they do so. So regular are these bursts that when they were first spotted, in the 1960s, some thought they must have an artificial - even alien - origin. That proved not to be the case, but the regularity of pulsars has proven useful for many kinds of observation.
By timing the signals coming from dozens of pulsars over many years, researchers were recently able to spot fluctuations in the rate at which pulses arrived. After comparing data from several pulsars, they then linked those fluctuations to distortions caused by passing gravitational waves.
This, they say, points to the first signs of the gravitational wave background. Still, the evidence is not quite enough to claim a certain discovery. For that the researchers will need more data, which should be forthcoming over the next few years. If that pans out, they also hope the data will be good enough to track the origin of those waves, letting them pick out otherwise invisible black holes.
Euclid Launches
The European Space Agency launched Euclid, a new space telescope, on Saturday last week. Once it reaches its final destination, one million miles from Earth, Euclid will start mapping the positions of galaxies up to ten billion light years away.
This, scientists are hoping, will show the distribution of matter throughout the cosmos. That should help them trace the roles dark matter and dark energy have played in the universe, thereby shedding light on these two mysteries of physics. In this way Euclid could reveal much about these rather mysterious entities.
Euclid will take four weeks to reach the Lagrange point where it will be stationed. After that operators will take six months to commission and calibrate the observatory before it begins work. Euclid should be in action for at least six years, mapping out about a third of the celestial sphere in that time.