Supernova replay reveals expansion rate of the Universe – 05/28/2023 – Sidereal Messenger

Supernova replay reveals expansion rate of the Universe – 05/28/2023 – Sidereal Messenger


The long-awaited replay of a supernova provided an independent method for measuring the expansion rate of the Universe. The result, published in the journal Science, is especially important at a time when cosmology is experiencing a crisis, with different strategies providing mutually incompatible results for the so-called Hubble constant.

So called in reference to the astronomer Edwin Hubble (1889-1953), who in 1929 demonstrated that the cosmos was expanding and made a first estimate of this parameter, it is basically the number that describes the rate of growth of the Universe.

The estimate made from the cosmic background radiation –the oldest light in circulation, a kind of echo of the Big Bang–, precisely measured by the European Planck satellite, suggests something like 67 km/s/Mpc. Translating the unit: each megaparsec of space (the equivalent of 3.26 million light-years) grows 67 km with each passing second. The result is based on the Standard Model of cosmology, which includes ingredients like dark energy and dark matter and is now our best explanation for the beginning and evolution of the Universe.

But another well-established method for measuring the expansion rate involves using supernovae of a certain type (Ia) as standard candles. All are expected to have the same intrinsic brightness, which allows it to immediately translate into distance. At the same time, the distortion of its light due to the pull away (the so-called “redshift”) gives the recession velocity. Together they make it possible to estimate the Hubble constant. In recent times, the estimate has been consolidated as something around 74 km/s/Mpc.

It’s not a huge difference. It would change the estimated age of the Universe by something like 20 million years – quite a bit compared to the 13.8 billion years since the Big Bang. However, the level of precision of the two measurements prevents both of them from being right. If the derivative of the cosmic microwave background is right, there is something wrong with our understanding of supernovae; if the derivative of supernovae is correct, there is something wrong with our model of the evolution of the Universe. Hence the need for other independent methods to arrive at the Hubble constant, in order to break the impasse.

A prime opportunity came with the Refsdal supernova, which was seen exploding across the sky in 2014 — in four simultaneous images. That’s because the colossal explosion took place at the bottom of a large cluster of galaxies, which produced a gravitational lens (a phenomenon in which bodies with great gravity bend rays of light) capable of multiplying their image.

The researchers led by Patrick Kelly, from the University of Minnesota (USA), modeled the lens of the cluster and predicted that there would be a new supernova replay, in 2015 – which actually happened.

Now, putting it all together, and estimating the paths taken by light in each of the supernova images, with the two most consistent models of the cluster and its corresponding gravitational lensing, they extracted a new estimate of the Hubble constant: 66.6 km/h s/Mpc – much closer to measuring the cosmic microwave background, thus reinforcing the standard model of cosmology.

It is an encouragement, but it is not yet the end of the crisis, due to the margin of error. It will take more observations like this one, of other gravitationally lensed supernovae, to tip the scales definitively to one side.

This column is published on Mondays in the printed version, in Folha Corrida.

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