A rung on the cosmic measuring ladder

There’s a grand puzzle lying at the heart of the expansion of our Universe. Actually, there’s several great puzzles, not least the mystery of what dark energy, which is causing the expansion to accelerate, is. However, even more puzzling are our measurements of the Hubble constant, which describes the current expansion rate of the cosmos.

By measuring the Hubble constant by using ‘local’ standard candles such as Cepheid variables and type Ia supernovae in nearby galaxies, astronomers obtain a value of about 73 kilometres per second per megaparsec (a megaparsec is a million parsecs, with one parsec being equivalent to 3.26 light years). However, when measuring the Hubble constant by mapping the properties of the cosmic microwave background radiation to the standard model of cosmology, astronomers get a value of about 69 kilometres per second per megaparsec. The puzzling thing is that neither measurement should be incorrect – they are both being made to accuracies of less than two per cent.

Is there some unknown systematic error that is bringing up this discrepancy, or is there something fundamentally amiss with our understanding of physics and the expansion of the Universe?

To try and rectify the problem, astronomers are working to get as accurate a value of the Hubble constant as they can, to try and completely rule out systematic errors. To that end, astronomers are refining the accuracy of our measurements of Cepheid variables in other galaxies, such as Markarian 1337 pictured here, and then using those refinements to calibrate the distance measurements to type Ia supernovae that might occur in the same galaxy. Those calibrated measurements can then be applied to even more distant type Ia supernovae in galaxies too far away for any Cepheids to be seen.

Markarian 1337 is therefore a rung on the cosmic ladder, at a distance of 120 million light years away in the constellation of Virgo.