This Is The Most Exciting Crisis in Cosmology
For as long as there has been a Universe, space has been expanding. It winked into existence roughly 13.8 billion years ago, and has been puffing up ever since, like a giant cosmic balloon.
The current rate of this expansion is called the Hubble constant, or H0, and it’s one of the fundamental measurements of the Universe.
If you know the Hubble constant, you can calculate the age of the Universe. You can calculate the size of the Universe. You can more accurately calculate the influence of the mysterious dark energy that drives the expansion of the Universe. And, fun fact, H0 is one of the values required to calculate intergalactic distances.
However, there’s a huge problem. We have several highly precise methods for determining the Hubble constant… and these methods keep returning different results for an unknown reason.
It could be a problem with the calibration of our measurement techniques – the standard candles and standard rulers we use to measure cosmic distances (more on those in a moment). It could be some unknown property of dark energy.
Or perhaps our understanding of fundamental physics is incomplete. To resolve this might well require a breakthrough of the kind that earns Nobel Prizes.
So, where do we begin?
The basics
The Hubble constant is typically expressed with a seemingly unusual combination of distance and time units – kilometres per second per megaparsec, or (km/s)/Mpc; a megaparsec is around 3.3 million light-years.
That combination is needed because the expansion of the Universe is accelerating, therefore stuff that’s farther away from us appears to be receding faster. Hypothetically, if we found that a galaxy at 1 megaparsec away was receding at a rate of 10 km/s, and a galaxy at 10 megaparsecs appeared to be receding at 100 km/s, we could describe that relation as 10 km/s per megaparsec.
In other words, determining the proportional relation between how fast galaxies are moving away from us (km/s) and how far they are (Mpc) is what gives us the value of H0.
If only there was an easy way to measure all this.
Cosmologists have devised a number of ways to arrive at the Hubble constant, but there are two main methods. They involve either standard rulers, or standard candles.
Standard rulers and their signals
Standard rulers are based on signals from a time in the early Universe called the Epoch of Recombination. After the Big Bang, the Universe was so hot and dense, atoms couldn’t form. Instead, there existed only a hot, opaque plasma fog; after about 380,000 years of cooling and expansion, that plasma finally started recombining into atoms.
We rely on two signals from this period. The first is the cosmic microwave background (CMB) – the light that escaped the plasma fog as matter recombined, and space became transparent. This first light – faint as it is by now – still fills the Universe uniformly in all directions.
Fluctuations in the temperature of the CMB represent expansions and contractions in the early Universe, to be incorporated into calculations that let us infer our Universe’s expansion history.
The second signal is called the baryon acoustic oscillation, and it’s the result of spherical acoustic density waves that propagated through the plasma fog of the early Universe, coming to a standstill at the Epoch of Recombination.
The distance this acoustic wave could have travelled during this timeframe is approximately 150 megaparsecs; this is detectable in density variations throughout the history of the Universe, providing a ‘ruler’ whereby to measure distances.
Standard candles in the sky
Standard candles, on the other hand, are distance measurements based on objects in the local Universe. These can’t just be any old stars or galaxies – they need to be objects of known intrinsic brightness, such as Type Ia supernovae, Cepheid variable stars, or stars at the tip of the red giant branch.
“When you’re looking at the stars in the sky, you can measure their positions left and right really precisely, you can point at them really precisely, but you can’t tell how far away they are,” astrophysicist Tamara Davis, from the University of Queensland in Australia, told ScienceAlert.
“It’s really difficult to tell the difference between something that’s really bright and far away, or something that’s faint and close. So, the way people measure it is to find something that’s standard in some way. A standard candle is something of known brightness.”
Both standard rulers and standard candles are as precise as we can get them, which is to say – very. And they both return different results when used to calculate the Hubble constant.
According to standard rulers, that is, the early Universe, H0 is around 67 kilometres per second per megaparsec. For the standard candles – the local Universe – it’s around 74 kilometres per second per megaparsec.
Neither of these results have an error margin that comes even close to closing the gap…