Astronomers are charting the impact of dark matter and dark energy on the Universe by measuring over 1,500 supernovae

In 2011 Perlmutter, Schmidt and Reiss were awarded the Nobel Prize in Physics for their discovery that the universe is not only expanding, but accelerating. The work supported the idea of ​​a universe full of dark energy and dark matter and was based on observations of distant supernovae. In particular, Type Ia supernovae, which have consistent light curves, we can use as standard candles to measure cosmic distances. Now a new study of more than 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 lightcurve measurements from 1,550 Type Ia supernovae, spanning two decades of observations and a cosmic timespan of 10 billion years. It is the most comprehensive study of dark energy supernova measurements ever undertaken. The data set covers the transition from the early Universe dominated by dark matter to the modern Universe dominated by dark energy. Thus, it confirms the effects of both. The data set is so detailed that it also gives us a measure of the Hubble parameter with an accuracy of five sigma, which eliminates systematic errors in our measurements. Based on this data, we know that we live in a universe that is about two-thirds dark energy and one-third matter and dark matter.

But this is where it gets weird. Over the years we have measured the effects of dark energy and dark matter in a variety of ways. In addition to supernova observations, we also see the effects of fluctuations in the cosmic background, the accumulation 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 can be seen most clearly in the discrepancies in the Hubble parameter values.

The Hubble parameter or Hubble constant is a measure of the rate at which the universe is expanding. Since 2001 we have known that the Hubble parameter is about 64 to 80 (km/s)/Mpc, giving an age of the universe between 12.5 and 15.6 billion years. At the time, we were quite uncertain about the exact value. Since then our measurements have become more precise and the value has narrowed down to around 70 (km/s)/Mpc or 14 billion years. The problem is that supernovae measurements give a value of more than 70, while cosmic background measurements give a value of a little less than 70. This disagreement is known as the Hubble voltage, and it was hoped that better observations would solve the problem. This latest study confirms that it is both real and not going away.

The team used the Pantheon+ data to examine two different outcomes. The supernova measure Pantheon+ SH0ES gives a Hubble parameter of 72 – 74 (km/s)/Mpc. The cosmic background measure, Pantheon+Planck, gives a Hubble parameter of 66 – 68 (km/s)/Mpc. Both are very precise and contradict each other. The study confirms that the Hubble voltage is real. There is no measurement error and we cannot say that one or the other is wrong.

This study basically threw down the figurative gauntlet at the theorists’ feet. Given that our Big Bang, Dark Energy, and Dark Matter universe model is confirmed, how do you resolve this observational disagreement? The short answer is we don’t know, but it will be an intriguing puzzle to find out.

Relation: Brout, Dillon et al. “The Pantheon+ Analysis: Cosmological Constraints.” The Astrophysical Journal 938.2 (2022): 110.