In 2011, the Nobel Prize in Physics was awarded to Perlmutter, Schmidt and Reiss for their discovery that the universe is not just expanding, it is accelerating. The work supported the idea of a universe filled with dark energy and dark matter, and it was based on observations of distant supernovae. In particular, Type Ia supernovae, which have consistent light curves, which we can use as standard candles to measure cosmic distances. Now, a new study of over 1,500 supernovae confirms dark energy and dark matter, but also raises questions about our cosmological models.
The study is based on datasets known as Pantheon+ and SH0ES. It contains 1,701 light curve measurements of 1,550 Type Ia supernovae spanning two decades of observations and a cosmic period of 10 billion years. This is the most comprehensive investigation of dark energy supernova measurements ever. The dataset covers the transition from the early universe, which was dominated by dark matter, to the modern universe dominated by dark energy. Thus, it confirms the effects of both. The dataset is so detailed that it also gives us a measure of the Hubble parameter with five sigma precision, which rules out systematic errors in our measurements. Based on this data, we know that we live in a universe that is roughly two-thirds dark energy and one-third matter and dark matter.
But here’s where things get weird. Over the years we have measured the effects of dark energy and dark matter in many ways. In addition to supernova observations, we also see the effects of fluctuations in the cosmic background, clustering of galaxies over time, gravitational waves, and even microwave laser light. They all tell a similar story of a universe dominated by dark matter and dark energy. But they don’t tell exactly the same story. This is seen most clearly in the discrepancies between Hubble parameter values.
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The Hubble Parameter, or Hubble Constant, is a measure of the expansion rate of the universe. Since 2001, we know that the Hubble parameter is around 64 to 80 (km/s)/Mpc, which gives the universe an age of between 12.5 and 15.6 billion years. At the time, our uncertainty as to the exact value was quite large. Since then, our measurements have become more precise and the value has reduced to around 70 (km/s)/Mpc, or 14 billion years. The problem is that supernovae measurements give a value above 70, while cosmic background measurements give a value a little below 70. This disagreement is known as the Hubble strain, and it was hoped that better observations would solve the problem. This latest study confirms that it is both real and not going away.
![](https://oponame.com/wp-content/uploads/2022/10/Astronomers-map-the-influence-of-dark-matter-and-dark-energy.jpg)
The team used data from Pantheon+ to look at two different results. The supernova measurement, Pantheon+ SH0ES, gives a Hubble parameter of 72 – 74 (km/s)/Mpc. The cosmic background measurement, Pantheon + Planck, gives a Hubble parameter of 66 – 68 (km/s)/Mpc. Both are very specific and contradict each other. The study confirms that the Hubble tension is real. This is not due to measurement error, and neither can be said to be wrong.
This study has essentially thrown the figurative gauntlet at the feet of theorists. Given that our model of the big bang, dark energy and dark matter universe is confirmed, how do you resolve this disagreement in the observations? The short answer is we don’t know, but it will be an exciting mystery to figure out.
Reference: Brout, Dillon, et al. “The Pantheon+ Analysis: Cosmological Constraints.” The Astrophysical Journal 938.2 (2022): 110.
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