The Week in Space and Physics: Timescapes
On an end for dark energy, new plans for the Mars sample return, how Pluto got its moons, and the Parker Solar Probe.
In 1915, Albert Einstein wrote down a new theory of space and time. It proved a success: after a first experimental proof in 1919, Einstein’s equations went on to reshape our understanding of the cosmos and predict everything from black holes to gravitational waves.
But it also predicted something else, something Einstein found unsettling. The cosmos his equations described was not a stable one. It must, they said, either expand or contract; and under the action of gravity this instability seemed doomed to one day result in utter collapse.
To get rid of this troubling idea, Einstein added an extra term to his equations to balance out the attraction of gravity, and so keep the universe stable. But then, when Hubble discovered the cosmos really was expanding, Einstein realised he had erred. Adding the extra term had been, he later said, his greatest blunder.
In 1998, measurements of supernovae changed things again. They showed that not only is the universe expanding, but that this expansion is accelerating over time. That was unexpected; and no theory of physics, Einstein’s included, could explain why this should happen. To fill the gap, researchers created a new force, “dark energy”, and then set about trying to figure out what it might be.
A quarter century on, the situation seems little changed. We know dark energy must be powerful, since it is somehow strong enough to overcome the gravitational pull of the entire universe. Yet it must also be subtle enough to have so far evaded detection on Earth. No experiment in any lab has ever found a sign of its influence, and no theory has yet emerged with a convincing explanation of where it comes from.
Since 2007, however, David Wiltshire – a professor at the University of Canterbury in New Zealand – has suggested that dark energy might be an illusion. His theory of “timescapes” argues that time flows at radically different speeds in different parts of the universe. Einstein’s theory says this is possible: clocks, it tells us, run slower in strong gravitational fields. And if this is happening on a large enough scale, it could create effects that look like dark energy.
On the one hand, this is a neat idea that resolves the question without any pesky new forces. But on the other, there is something called the cosmological principle. This argues that matter is evenly spread across the cosmos. Of course we know this isn’t exactly true - galaxies hold a lot of matter, while the space between them is relatively empty. Yet on a large enough scale these lumps seem to even out, so that the average density of the cosmos is roughly constant.
For timescapes to work, though, time must flow about thirty-five percent faster in voids than in galaxies. That means abandoning the cosmological principle, which many physicists are reluctant to do with good reason. And, so far at least, observations of galaxies and voids do not suggest they differ enough in density to cause such a big difference in the flow of time.
Wiltshire recently published a paper showing timescapes are able to explain the 1998 supernova measurements that gave birth to dark energy. But he hasn’t yet shown that we should abandon the cosmological principle. Data from observatories like Euclid and the upcoming Nancy Grace Roman Telescope might give the data needed to do that — or, they might do the opposite.
Timescapes are, as a recent article on the topic argued, huge if true. But that is still a very big if.
Mars Sample Return
Ever since the Perseverance rover landed on Mars in 2021, it has been drilling for and collecting samples of Martian rocks. It stores these in tubes, and has deposited them at a number of “depots” along its path. One day, if these samples can be brought back to Earth, they might reveal deep secrets about Mars’ past.
The question, however, is how to do this. NASA’s original architecture for sample return was complex, involving a series of spacecraft that would go to Mars, fly around and pick up the samples, bring them back into orbit and then return them to Earth. All this would take years. Before the project was put on hold, NASA had expected to bring them back no earlier than 2040.
In April last year, as the price tag soared to well above ten billion dollars, NASA decided to go back to the drawing board. A fresh approach was needed, they said, one that would be quicker and cheaper. Cost was perhaps not the only concern - China has plans for a sample return mission of its own, and it would, frankly, be embarrassing for NASA if China got their hands on Mars rocks first.
NASA was supposed to unveil the new plan by the end of 2024. Instead they have demurred, proposing two possible options and then suggesting a choice between them could be made later this year. The first option, which would cost about seven billion dollars, makes use of a sky crane like that which landed both the Curiosity and Perseverance rovers.
The other, which is only vaguely described, would cost a little less and make use of a commercial heavy lander. What that might be is not said, but presumably it involves some variant of SpaceX’s Starship.
Pushing the final decision to later in the year means it will be dealt with by the new administration – and mooted NASA director Jared Isaacman is likely to have ideas about how this should be done. He has already indicated he is keen to involve more commercial partners, and is close to SpaceX boss Elon Musk. Don’t be surprised, then, if the new plan helps push Starship towards the Red Planet.
When Charon Met Pluto
How did Pluto get its large moon Charon? For a long time planetary scientists assumed the pair formed much like the Earth and Moon: that’s to say, after a massive collision between Pluto and another world, out of which Charon formed.
A recent study found, however, that this explanation is flawed. Instead of forming through a violent collision, it argues the two probably fused in a brief “kiss” before pulling apart and ending up in orbit about one another.
Initially both Pluto and Charon were separate worlds, charting their own paths through space. At some point they collided: an event that past models suggested was violent enough to destroy and reform the two worlds.
Yet those models did not account for the internal strength of Pluto and Charon. When the new study included that strength they found it was more likely the two briefly fused together, creating an odd-looking object rather like Arrokoth, another trans-neptunian world. But because they would have been spinning fast, the connection did not last.
After a few hours the two would have separated, leaving Charon in orbit around Pluto. The models found this scenario could recreate the circular orbit Charon actually follows. Debris from the collision could also have been thrown out – perhaps forming the four other small moons around Pluto.
These are models, of course, and so the findings rely on the assumptions made by the researchers. They could be wrong. But there are ways to test this idea. Notably, Pluto and Charon would have mostly kept their initial properties during the collision, rather than mixing. As a result they should – if the idea is correct – look geologically distinct in character. Future visitors to the outer solar system might be able to look for evidence of this.
Parker Solar Probe
The Parker Solar Probe survived its close approach to the Sun, NASA announced at the start of the year. On Christmas Eve the probe flew through the Sun’s corona, touching a distance less than four million files from the surface of the star. That made it the closest artificial object ever to the Sun.
At the same time it became the fastest ever spacecraft built by humans. As it flew past the Sun it reached a speed of 427,710 mph, slightly faster than a twentieth of one percent of the speed of light.
The probe is now heading away from the Sun towards the orbit of Venus. Over the coming weeks operators will downlink the data it collected, and then start the work of analysing it to better understand our star. Parker will next approach the Sun on March 22, when it should repeat its Christmas Eve feat.
I am curious if the Solar Wind burp the Sun did the other day would have affected the Parker Solar Probe if it was there?