The Week in Space and Physics: A hint of MOND?
On cosmology without dark matter, falling rockets, neutrinos from beyond the galaxy and the Falcon Heavy.
Dark matter, many astronomers will assure you, certainly does exist. So confident are they in their belief that they have mapped out its presence, finding its ghostly fingerprint on galaxies across the night sky. They have built models of the universe’s evolution from the Big Bang to the modern day, and discovered that only dark matter, added in the right proportions, can explain what we see today. They have even found odd galaxies that lack dark matter – clear evidence, they say, that the invisible stuff dominates the cosmos.
Its initial discovery came quite by accident. Vera Rubin, an American astronomer, took to studying the quiet fringes of galaxies in the 1970s. She never expected to find anything interesting – the edges of galaxies are usually calm, sedate places. Yet the more she looked, the more she found they were moving too fast. This, under the law of gravity as we know it, could only be explained if there was some extra invisible matter lingering around the galaxies.
That there had to be an awful lot of this matter – roughly ten times more than the visible kind – is nowadays widely accepted. And, as later studies found, adding dark matter into other models of the cosmos turned out to resolve a lot of other problems. The alternative – that there was something wrong with the law of gravity – was mostly ignored.
That’s partly because the laws of gravity work extremely well everywhere else. But it’s also true that the modifications needed to get rid of dark matter look rather inelegant, as though the numbers have been fiddled to get the answers just right. Dark matter, simply put, is a neater explanation. Yet even so, an alternative theory of gravity has slowly, if controversially, emerged over the years: an idea now known as Modified Newtonian Dynamics, or MOND for short.
It happens that the two theories make different predictions about how clusters of stars should move around the galaxy. As they orbit the galactic core, some stars within those clusters tend to either fall behind or race ahead of the main group, stretching out into two tails. The standard laws of gravity say these two tails should be equal, with roughly the same number of stars in each.
Yet, as a recent paper reported, this doesn’t seem to be the case in reality. One of the tails – the one in front – is always bigger; exactly as MOND happens to predict. This, then, may be the first clear evidence in favour of MOND, and against the theory of dark matter. If more such evidence emerges, the impact on physics will be enormous. As one of the researchers behind the paper noted, it would imply that almost all of modern cosmology is wrong – and that physicists need to start over almost from scratch.
China’s Falling Rockets
On Friday morning Spain briefly closed its airspace. Barcelona airport, one of the busiest in Europe, fully halted operations, an action that delayed hundreds of flights. The reason for all this disruption was something highly unusual: an out-of-control rocket plunging back to Earth.
The rocket had been launched days earlier from China, carrying the third and final module of China’s new space station. In this mission it was successful. The module docked, as planned, with the station on Tuesday. Yet the core stage of the rocket was also left in orbit, tumbling chaotically as it skimmed the Earth’s atmosphere.
This has happened before. Indeed, the design of the Long March 5B rocket makes it almost inevitable. The core stage is bulky, weighing some twenty-five tons, and lacks any kind of guidance systems. Once the launch is over, then, the stage is simply discarded as debris and left to fall back to Earth.
The size and weight of the core module means big chunks of it can survive falling through the atmosphere: a descent that inevitably follows a few days after launch. In each of the three previous launches of the Long March 5B parts of the core stage have smashed into the Earth: twice falling across populated areas of Africa and Asia.
So far, fortunately, no one has been hurt. Indeed the odds are relatively good that nobody will ever get hurt – most of the planet, after all, is ocean; and even the continents are filled with vast tracts of uninhabited land. But the risk is not zero, and most space agencies prefer not to take any chances of an accident.
Many countries have thus criticised China’s lax approach to the safety of their rockets. After a similar re-entry in 2021, NASA boss Bill Nelson labelled the Chinese irresponsible. Europe, by taking the drastic step of closing its airspace, seems to be sending a similar message.
Fortunately, Friday’s falling rocket passed safely over Europe. Tracking data suggests it fell somewhere in the south Pacific, around half an hour after flying through Spanish airspace. Next time we may not get so lucky.
Neutrinos From Far Far Away
The IceCube observatory at the South Pole last week reported picking up dozens of neutrinos coming from a nearby galaxy. The discovery marks one of the first times astronomers have been able to precisely pinpoint the source of neutrinos coming from outside our galaxy.
Neutrinos are famously elusive particles. Vast numbers of them pass through the Earth every day, yet as they rarely interact with atoms we detect just a tiny fraction of them. Only with dedicated detectors, like that installed at the South Pole, have we been able to build up a picture of where they are coming from.
Most, of course, come from the Sun. Yet astronomers have also picked up thousands coming from more distant objects. Mapped on the sky, these neutrinos appear randomly scattered, a detail that doesn’t give researchers much information about where, precisely, they originate.
Yet by compiling a decade’s worth of observations by the IceCube observatory, astronomers found a bright spot in the neutrino sky. Over that decade roughly eighty neutrinos seem to have come from the nearby NGC 1068 galaxy. This is a bright galaxy, with an active black hole at its core.
Such galaxies are known as blazars. Beams of energy shooting out from their black holes make them look extremely bright, though this brightness can change rapidly. NGC 1068 is one of the closest known blazars, and therefore one of the better studied ones. The neutrinos are probably coming from its active black hole; a detail that astronomers hope will allow them to probe it with new tools.
The Falcon Heavy Returns
Elon Musk made plenty of headlines last week following his purchase of Twitter. Slightly less noticed was the first launch of his Falcon Heavy rocket in three years. Onboard was a secret satellite owned by the US Space Force, which the Falcon Heavy sent straight into a geostationary orbit twenty-two thousand miles high.
The Falcon Heavy has long been an oddity in SpaceX’s line of rockets. Far more powerful than the oft used Falcon 9, the rocket is capable of lifting much more hardware into orbit. Yet demand for it seems to be lacking, and SpaceX has thus focused on perfecting the launch and landing of the Falcon 9 instead.
There are a few more missions lined up for the Falcon Heavy, including the Europa Clipper and the Psyche asteroid mission. Yet the relative lack of demand does raise questions about how much commercial need there is for SpaceX’s even larger rocket, Starship.
There's a lot that the seemingly silly idea of MoND gets right. Have you read any of Stacy McGaugh's blog Triton Station?