The Week in Space and Physics: The Hubble Constant
The Week in Space and Physics: The Hubble Constant
On the Hubble constant, Fomalhaut's rings, a huge cosmic explosion and hints of catastrophe around Saturn
Vesto Melvin Slipher noticed it first. Over several years in the 1910s he focused his attention on galaxies, measuring, for the first time, the speed at which they were moving. Almost all of them, he realised, were moving away from us, receding at great speed into the cosmic void.
That was odd, but things got even stranger a decade later when Edwin Hubble started measuring the distances to these galaxies. Not only were most galaxies receding, but, he discovered, how fast they were doing so correlated almost perfectly with how far away they were. The more distant a galaxy was, the faster it was moving away from us.
He encapsulated this discovery with a value we now know as the Hubble constant. Multiply the distance of a galaxy by this constant and you get, more or less, the speed at which it is receding. Hubble estimated the constant to have a value of 500 km/s/Mpc, meaning that an object will recede 500km/s faster for every megaparsec - around three million light years - it is from us.
Hubble’s discovery had huge implications for our understanding of cosmology. Solutions to Einstein’s equations had earlier hinted that the universe was expanding, but the idea was controversial. Even Einstein thought it must be wrong, introducing a “cosmological constant” to keep the universe static. Yet Hubble had found the opposite - the motion of the galaxies proved the universe really was expanding, with its rate set by his constant.
Einstein regarded the discovery as something of a humiliation, apparently going so far as to call his earlier work a “great blunder”. Still, Hubble had also erred in his work, miscalculating the distances to the galaxies he measured. When astronomers realised this, they found the law still held, but that the value of the constant was far lower than Hubble had calculated.
Exactly what the value is, however, is still something of a puzzle. One approach to measuring it, based on observations of supernovae, has settled on a result around 73 km/s/Mpc. Yet other approaches, using observations of the early universe, have found another value, close to 67 km/s/Mpc. This difference has created something of a crisis in astronomy, with researchers struggling to explain why their methods result in different values.
Recently, however, astronomers announced results from another technique. Back in 2014 they spotted a supernova erupting in a distant galaxy. Yet they also found that a different galaxy had warped the light rays coming from it, causing shifts in both space and time. That meant astronomers caught several visions of the supernova, but also that some light coming from it was delayed: an effect which created an illusion of repeated explosions over time.
The length of this delay depended on the supernova, the intervening galaxy and on the Hubble constant. By carefully observing the first two, astronomers were thus able to precisely measure the constant. They found a value close to 67 km/s/Mpc - which agrees well with measurements from the early universe.
This new measurement strengthens the case for a lower value of the Hubble constant. But it doesn’t yet resolve the question of why different methods find different values. Solving that may yet take a deeper rethink of cosmology.
The Rings of Fomalhaut
The star Fomalhaut is one of the brightest in the sky. That’s partly because it is close - lying twenty-five light years from Earth - but also because it is large, roughly twice the size of the Sun.
Over the past two decades, observations of Fomalhaut with Hubble and other telescopes have revealed a ring of debris around the star. In 2008, astronomers even thought they’d discovered a planet circling the star, though later on this planet seemed to disappear. More likely the “planet” was actually a cloud of dust thrown up by some kind of collision around the star.
Still, all this has hinted that Fomalhaut is forming a planetary system. That made it a natural target for the James Webb Space Telescope, which recently imaged the star in greater detail than ever before.
The James Webb revealed a complex pattern of rings around the star. Instead of the one ring seen before, Fomalhaut is now known to have at least three rings around it, with gaps swept clean between them.
These rings are possibly belts of small rocks, akin to the asteroid belt and Kuiper belt around the Sun. In our solar system the asteroid belt is sculpted by Jupiter, and it is plausible that planets are doing similar work around Fomalhaut. If so, this may be the first real evidence of planets around the nearby star.
Astronomers also spotted a bright dust cloud towards the edge of the system. This, they speculate, may be the result of another collision between planets in the system, resulting in a huge cloud of debris.
Fomalhaut is a young star, around four hundred million years old. That means the planetary system is likely still forming - and may in some ways resemble our own solar system in its youth. Collisions between planets were once commonplace. We believe, for example, that the Moon formed when an object the size of Mars smashed into the early Earth.
Observations of Fomalhaut over the past few decades have rarely been short of drama. Now, with the James Webb, we are likely to get even more detailed views of a young planetary system in action. That, surely, promises to be exciting.
A Vast Cosmic Explosion
Every now and then, a star will somewhere encounter a black hole. The result is messy, involving things like “spaghettification” and “pancake detonations”. Astronomers, being rather more serious, like to call these events “tidal disruption events”, the technical term for a star being ripped apart by a black hole.
Such violent events usually result in a burst of energy, which gradually fades away as the black hole consumes the remains of the star. They are rare, but occur often enough for astronomers to have detected a few dozen over the past few years.
In April 2021 astronomers picked up an event that looked rather like a tidal disruption event. Checks soon revealed that the object in question had been gradually brightening for at least a year. Over that time, the researchers reckon, the event released a huge amount of energy, far more than even the brightest supernova.
This could be the tidal disruption of a huge star, researchers say, although it seems unlikely. Such events should be extremely rare and the odds of spotting one with our current telescopes are almost impossibly low.
Instead, they think it’s more likely we’re actually seeing a black hole consume a large molecular cloud. These clouds can be vast - dozens of times larger than the solar system - and sometimes contain stars in the process of being born. As bits of this cloud fall into the black hole, the researchers speculate, surges of energy are echoing through the cloud, creating the explosions seen from Earth.
Saturn’s Moons Hint at Ancient Catastrophe
Jupiter may be the largest planet, but Saturn now holds the title for the most moons. Astronomers recently announced the discovery of dozens of new moons around the planet, bringing the total to 145.
Most of these moons, however, are little more than large rocks a few kilometres across. In truth, many probably started out as asteroids, before they happened to be captured by Saturn’s powerful gravitational pull. Others, though, may have come from a larger moon that shattered some time in Saturn’s past.
Some speculate that Saturn’s rings, which seem too unstable to last for long, were created when an icy moon ventured too close to Saturn and was torn apart. Patterns in the orbits of the newly found moons hint that they, too, could have originated in such a catastrophe.