MARCUS CHOWN
In terms of our understanding of the Universe, some things just don’t add up. Which means either our measurements are wrong, or our theories are…
Marcus explains what cosmologists think might fill the puzzling gap that sits between our theories of the Universe and our observations of it.
The standard model of our Universe may be showing some cracks. Several fundamental cosmological observations are contradicting each other. For instance, the Universe appears to be expanding 10 per cent faster than it should be, according to observations of the leftover heat from the Big Bang. It’s perfectly possible that the contradictions will disappear as our estimates of cosmic parameters improve. But it’s also possible that the contradictions won’t go away and that our fundamental picture of the Universe is about to undergo a radical revision, perhaps to include invisible, ‘dark’ components as complex as atoms, stars and galaxies.
ADDITIONAL INGREDIENTS
Cosmology is the ultimate science. It deals with the birth, evolution and fate of the Universe. The standard model consists of several ingredients: the Big Bang, dark matter, dark energy and inflation.
First, take the Big Bang. Astronomers can see that the galaxies – the basic building blocks of the Universe of which the Milky Way is one – are flying away from each other in the aftermath of a titanic explosion. They also observe that the Universe is permeated by relic heat – the cosmic background radiation. Together, these two observations tell astronomers that the Universe was smaller and hotter in the past. In fact, according to the standard picture, the Universe was born in a blisteringly hot fireball 13.82 billion years ago and has been expanding ever since, with the galaxies congealing out of the cooling debris.
But the basic Big Bang picture requires a few extra ingredients, because it conflicts with observations. First, and most seriously, it predicts that we should not exist.
According to the basic Big Bang, when matter emerged from the primordial fireball it was spread extremely smoothly. Thereafter, regions that were slightly denser than average pulled in matter faster with their stronger gravity. The result of this process, akin to the rich getting ever richer, were the galaxies we see today. The problem is, it would have taken longer than 13.82 billion years to assemble galaxies as massive as our Milky Way.
Cosmologists fix the problem by bolting invisible dark matter on to the basic Big Bang. Invisible dark matter outweighs the visible stars and galaxies by a factor of six, and its extra gravity speeded up galaxy formation.
The second way in which the basic Big Bang picture conflicts with observations is that it predicts that cosmic expansion should be slowing down. Gravity acts like a web of elastic between the galaxies, braking their rush from each other. But in 1998, astronomers discovered that, contrary to all expectations, the expansion is speeding up.
They fix this by bolting dark energy onto the basic Big Bang, something that’s invisible, fills all of space and has repulsive gravity. It’s dark energy that’s speeding up cosmic expansion.
The third way in which the Big Bang picture conflicts with observations is that Universe has the same temperature everywhere – the temperature of the cosmic background radiation. That temperature is 2.726K (absolute zero is 0K). Early on in the Big Bang, regions that are today on opposite sides of the sky were too far apart to equalise their temperatures.
Cosmologists fix this by postulating that, early on, the Universe was far smaller than expected. It must therefore have expanded faster to achieve its current size in 13.82 billion years. In fact, the Universe, in its first split-second, is believed to have undergone an expansion so violent that it’s been likened to the explosion of an H-bomb. This is compared with the stick of dynamite of the Big Bang expansion, which took over when the initial ‘inflation’ ran out of steam.
There you have it. The standard model of cosmology = the Big Bang + inflation + dark matter + dark energy. Technically, it goes by the name of ‘Lambda-CDM’. Whereas the Big Bang + inflation is implicitly assumed, Lambda is shorthand for dark energy and CDM for cold dark matter, with the ‘cold’ meaning that its components move sluggishly so gravity can concentrate them into clumps.
SOMETHING’S WRONG
The first way Lambda-CDM conflicts with observations involves clusters of galaxies. According to the cold dark matter model, gravity will cause dark matter to clump on a range of scales, including those smaller than a galaxy cluster. Later, ordinary matter (which forms into stars) is pulled in. These ‘subhalos’ may have plenty of stars, but some subhalos may have no stars, or so few stars that they’re invisible. There is a way to reveal them, however.
A team led by Dr Massimo Meneghetti of the National Astrophysics Institute in Bologna, Italy, observed 11 galaxy clusters with the Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope in Chile. They examined the light of more distant galaxies and how it was distorted, or ‘gravitationally lensed’, due to passing by the invisible subhalos. To the team’s surprise, the lensing by subhalos was much stronger than expected, indicating that they’re very compact. This conflicts with the cold dark matter model, which maintains the subhalos should be much puffier.
“BUT THE BASIC BIG BANG PICTURE REQUIRES A FEW EXTRA INGREDIENTS BECAUSE IT CONFLICTS WITH OBSERVATIONS”
“We need to know if this anomaly can be caused by the way we analyse our data or the way we make our theoretical predictions,” says Meneghetti. “If we fail to explain it, the only option will be to revise the model.” One possibility is that the dark matter isn’t made of what we think it’s made from. Favoured candidates are massive, weakly interacting particles that interact with ordinary matter only through gravity. Such weakly interacting massive particles, or WIMPs, aren’t part of the Standard Model of particle physics but are predicted by a speculative theory called ‘supersymmetry’.
“Maybe the dark matter consists of particles that interact in different ways to WIMPS,” says Meneghetti. “Possible alternatives include a new type of neutrino called a ‘sterile neutrino’, another class of particles called ‘axions’, or even primordial black holes, formed just after the Big Bang.”
TOO SMOOTH
The second conflict between LambdaCDM and observations concerns the clumpiness of matter on a large scale. A team led by Prof Koen Kuijken at the Leiden Observatory in the Netherlands looked at the distribution of 31 million extremely faint galaxies in the latest data release of the European Kilo-Degree Survey (KiDS). The KiDS collaboration used the Very Large Telescope’s Survey Telescope in Chile to observe two large swathes of sky.
Specifically, Kuijken’s team looked at how the light of these galaxies was gravitationally lensed by the matter between them and Earth, enabling its distribution. It discovered that matter was spread 8.3 per cent smoother than predicted by the cold dark matter model, which takes the very small variations in the density of the Universe shortly after the Big Bang – revealed by the cosmic background radiation – and calculates how these would have been enhanced by gravity over the past 13.82 billion years. Once again, the anomaly might go away with a better analysis of the data or a modification of the cold dark matter model. Or it could be telling us the model is fundamentally wrong.
The third conflict between Lambda-CDM and observations, known as the ‘Hubble tension’, concerns the Hubble constant, a measure of the current expansion rate of the Universe. There are two ways to measure it and they’re contradicting each other.
One way is to look at subtle variations in the temperature of the cosmic background radiation across the sky. These were imprinted on the radiation by the ‘fluid’ of matter and radiation at the beginning of time as it sloshed about like water in a Universesized bath. It’s possible to extract from these sloshing modes all the key cosmological parameters. The European Planck satellite, for instance, found that the Universe is 4.9 per cent atomic matter, 26.8 per cent dark matter and 68.3 per cent dark energy.
Crucially, such observations also reveal the Hubble constant in the early Universe and this can be extrapolated to the present time. And herein lies the problem: the extrapolated value is about 10 per cent smaller than the Hubble constant observed today.
A key thing to bear in mind is that the Hubble constant deduced from the cosmic background radiation is very precise because the physics is simple and well understood. But measurements
of the Hubble constant in today’s Universe are cruder and fraught with problems.
Such measurements involve finding objects that are believed to always have the same intrinsic luminosity, such as Cepheid variables and Type 1a supernovae. Like standard 100W lightbulbs strung across a field at midnight, such ‘standard candles’ reveal their relative distance by their relative faintness. The trouble is that the physics of such stars is not well understood and may not be as standard as we hope. So, it could be that these measurements of the distance of standard candles as they’re dragged away from us by cosmic expansion are in error and will eventually yield a Hubble constant in line with that from the cosmic background radiation.
On the other hand, it could be that nature is telling us something new about the Universe. “The ‘standard model of cosmology’ is an admission of ignorance,” admits Prof Abraham Loeb of Harvard University. “We label components whose nature we don’t know as ‘dark matter’ and ‘dark energy’. Since we don’t know what they are, it’s a very crude model that could easily be an oversimplification of reality.”
Loeb points out that dark matter might not be a fluid of one type of dark matter particle. “There may not be a single dark matter particle but rather a mix of particles of different masses and interactions,” he says. Dark matter could be complex, just like ordinary matter, which is composed of quarks and electrons that are assembled into 92 naturally occurring elements.
In addition, dark matter particles might behave in complex ways. For instance, they might decay over the age of the Universe, reducing their gravitational pull and thus taking the brakes off cosmic expansion. Such a boost to the cosmic expansion rate would relieve the Hubble tension.
One possible way to confirm or refute the Hubble tension is with ‘standard sirens’ rather than standard candles. Gravitational waves are vibrations of space-time akin to sound waves and the merger of neutron stars is believed to create standard sirens, like the foghorns of lighthouses. The quieter the sound, the further away the siren. “Gravitational wave sources offer the most robust method to resolve the uncertainties we currently have,” says Loeb. The hope is that such techniques will show whether the current contradictions between different observations are real.
The standard model of cosmology is relatively simple, despite its multiple invisible components. But its simplicity may be blinding us to reality, which may be more complex. “Nature,” cautions Loeb, “is under no obligation to comply with the simplest version.”
“WE NEED TO KNOW IF THIS ANOMALY CAN BE CAUSED BY THE WAY WE ANALYSE OUR DATA OR THE WAY WE MAKE OUR THEORETICAL MODELS”