The Week in Space and Physics: Dark Energy and Black Holes
The Week in Space and Physics: Dark Energy and Black Holes
On the question of dark energy, a pair of rogue black holes, nuclear rockets and commercial space stations
Could cosmologists finally have solved the question of dark energy? For years they have been troubled by measurements showing the cosmos expanding at an ever faster rate; an expansion that has no explanation in the known laws of physics. To resolve it cosmologists have proposed all sorts of exotic ideas, speculating about new forces of nature and hypothetical particles.
A second problem, at first seemingly unrelated, surrounds the supermassive black holes found at the heart of most galaxies. For years researchers have puzzled about how they formed and got so big. In theory, of course, they could have done this by feeding upon stars and planets. Yet, in practice black holes probably can’t do this fast enough to reach such enormous sizes.
To find out more, a team of researchers led by the University of Hawaii took a closer look at supermassive black holes. They focused their attention on a type of galaxy that stopped producing new stars early on. They should, therefore, have provided little food for black holes to feed on. By comparing these galaxies at distinct stages of their evolution, they were able to see how much their black holes have grown over time.
As thought, they have grown by much more than the available supply of stars should have allowed. They could instead be growing by more subtle means - perhaps feeding on hard to see gas clouds or merging as galaxies collide. Yet the researchers think this is unlikely to be the full story. Instead they invoke the idea of dark energy, tying it directly to the growth of black holes.
Key to this is a concept known as “vacuum energy”. Though a vacuum is, by definition, empty space, quantum laws tell us that “virtual” particles are constantly popping in and out of existence. These particles are short-lived yet hang around long enough to interact with other more stable particles. They thus create a slight influence that physicists have successfully measured in the past.
Einstein’s laws, some think, could allow for black holes to fill with vacuum energy. If they do, theory predicts that black hole growth and the expansion of the universe will link. As black holes grow, then, they produce a pressure that pushes the universe to expand. That sounds an awful lot like dark energy, at least as cosmologists have imagined it.
That, so far, is just speculation. Yet when the researchers ran the numbers, they found vacuum energy can explain why black holes have grown faster than should have. And, intriguingly, they also found that the pressure all this vacuum energy creates closely matches the observed expansion of the universe.
If true, this is a deep discovery, neatly solving two big problems in cosmology at once. It could even shed light on a third mystery: giving us the first glimpse of what lies beyond the event horizon of a black hole.
A Pair of Stray Black Holes
At first it looked like an error. A stray beam of light, perhaps, or a cosmic ray smashing into the camera at exactly the right moment. Such things, after all, are not uncommon in astronomy, and lines so straight and thin do not often appear in galaxies.
Yet a closer inspection showed the image was accurate. The nearby galaxy, RCP28, really did have a curious object within it: a long thin line that appears to stretch for dozens of light years across its disk. Intriguingly, a similar line also appeared on the other side of the galaxy, almost as a mirror image of the first.
What could explain such an unnatural looking object, if not an error? The team behind the discovery, which contains astronomers from the US, Australia and Canada, think the lines are actually shock waves spreading through gas clouds. As they do they trigger a glow of radiation and then, later, the formation of new stars.
Indeed, the astronomers spotted signs of new stars emerging from the lines; stars that seem to get progressively older along the line. That suggests the shock waves were triggered by an object shooting through the galaxy. Based on the length of the lines, and the ages of the stars, they calculated that, whatever they are, they must be moving at over one thousand kilometres per second.
Because there are two lines, on opposite sides of the galaxy, it seems likely a single event spat these two objects out from the galactic core. That, in turn, points to an interesting conclusion: they are supermassive black holes, hurled out of the core after a violent encounter.
Most galaxies contain a black hole in their centres. When galaxies collide and merge, then, two supermassive black holes can encounter one another. Often they will begin a delicate dance, spiralling warily around one another in the combined cores of their galaxies.
If a third black hole enters the picture, perhaps from another merging galaxy, then the situation can rapidly become chaotic. Indeed, models suggest that the chaos can send two of the black holes shooting off into space, while one lucky survivor remains in the core.
In this case, the lines may be forming in the aftermath of such an encounter. Unfortunately, the team could not find clear signs of the black holes themselves, instead noting where they might be hiding. Other explanations may be possible, but the most likely scenario seems to be that these really are stray supermassive black holes.
NASA’s Nuclear Option
If Elon Musk is to be believed, Starship will power humanity towards Mars. His new rocket is, indeed, enormous; capable of lifting over a hundred tonnes of material towards the Red Planet. Yet Starship is also based on chemical fuel systems, which means it must also carry vast amounts of fuel as it goes.
Plans for Starship thus envision refuelling the spacecraft in orbit, meaning two must launch for every one that heads to Mars. Is there a better way? Engineers have long pondered the idea of putting nuclear engines on spacecraft, an approach that could also allow for faster trips to the Red Planet.
NASA now seems to be considering the idea seriously. In January the agency announced a collaboration with DARPA, a research agency, to demonstrate the use of nuclear engines for spaceflight between the Earth and the Moon. This, they say, could open the way for nuclear powered flight towards Mars.
A demonstration of the engine has thus been pencilled in for the end of this decade. For now it is more likely to be a learning exercise: figuring out how, exactly, to build nuclear engines for space. Should it succeed - still a big question - then the future of Mars may not belong to Starship after all.
Space Stations: The Next Generation
With the end of the International Space Station looming, NASA is turning its attention towards a set of commercial stations expected to replace it. These, though smaller and less capable than the current Space Station, will be privately operated at far lower cost.
NASA last week released two documents outlining how they hope to use these stations. They foresee booking around three to four thousand hours of astronaut time a year, representing the efforts of two NASA astronauts stationed onboard. These astronauts will carry out more than one hundred experiments every year and require at least twenty-four cubic metres of space for storage.
Interestingly, they expect the operators of these private space stations to provide their own crews of astronauts. If so, that would represent a significant move towards the commercial use of space - and be the first sustained presence of private crews in orbit.