I was explaining red shifts of galaxies to a colleague at work earlier this week when they asked me how fast the universe is expanding. Although I have made a conscious effort to remember the figure in the past I was unable to recall it. I knew there was a 7 in it and parsecs were involved but basically I failed and said I’d look it up once I’d finished internally cursing myself. Having just done so it appears there is an interesting controversy surrounding the number.
In the past month 3 papers have been published to the arXiv all of which try to pin down the exact figure, known as the Hubble Constant after Edwin Hubble who first discovered that the universe is expanding in the 1920s. The number that prevailed in recent years, the one I couldn’t remember, was 73 kilometres per second per megaparsec, a megaparsec being about 3.3 million light years. For scale, the Milky Way is only about 100,000 light years in diameter and so would expand by about 2 km/sec. Remember, we’re not talking about things just drifting away from each other here, this is literally the rate at which the fabric of space-time is being stretched.
This figure was arrived at by measuring a whole lot of cepheid variable stars and type 1a supernovae. Cepheid variables are very cool and worthy of a blog post of their own so I won’t fully explain them here; basically, though, they are a type of star that pulses at a certain rate dependent upon how bright they are. Type 1a supernovae are what’s known as a standard candle in cosmological circles. They always explode with the same amount of force and give off the same amount of light.
Both of these phenomena allow us to have objects in the universe that we can precisely know the brightness of. If we know the absolute brightness of something and compare it to the apparent brightness from earth then we can determine how far away that object is as the brightness of the light will diminish as determined by the inverse square law. Simple trigonometry then allows us to calculate how far apart two such stars are and measuring the red shift in the light from them gives us a figure for the rate of expansion of the universe. Back in April a paper was published that further narrowed the error bars on this measurement; they put it at 73.24+/-1.74 km/sec/Mpc.
However, in the past couple of months two more papers have come out that disagree with this finding. The first was from the Planck Collaboration and gave a figure of 67 km/sec/Mpc. Full disclosure: I don’t get this paper. It’s beyond me. I understand that they measured the cosmic microwave background radiation and, from that, they were able to determine the expansion rate of the universe but I can’t explain it any further than that.
The final paper is similarly problematic in that I don’t understand it. Let me give you a quote:
Using the anisotropic clustering of the pre-reconstruction density field, we measure the product DMH from the Alcock-Paczynski effect and the growth of structure, quantified by fσ8(z), from redshift-space distortions.
So I’m sure you’ll forgive me. In a nutshell, though, they measured 1.2 million galaxies, analysed how they clustered and from that they were able to determine the pressure waves present in the early universe. Then there is a load of algebra and they get a figure of 67 km/sec/Mpc. This closely agrees with the Planck data and contradicts the cepheid/supernova data.
I cannot even begin to pretend I know which of the two groups are correct. What does seem clear though is that this is a genuine discrepancy, it isn’t wriggle room where they might both be right once we get better data; at least one of them is wrong. If I were to give in to impulse I’d say the 73 figure is correct. But that would solely be because I understand how that one works and the others make me want to weep onto my calculator. I’d be falling for the argument from personal ignorance.
The leading possible explanations, from people that know what they’re talking about, are that one or more of the groups are wrong or that there is something about Dark Energy, the mysterious force we know almost nothing about other than it makes up more than two thirds of the universe, that we don’t fully understand yet. I’m going with the latter if for no other reason than it’s a lot more cool.