Universe Today | Brian Koberlein | 2023 May 17
The cosmos is expanding at an ever-increasing rate. This cosmic acceleration is caused by dark energy, and it is a central aspect of the evolution of our universe. The rate of cosmic expansion can be expressed by a cosmological constant, commonly known as the Hubble constant, or Hubble parameter. But while astronomers generally agree this Hubble parameter exists, there is some disagreement as to its value.Image Credit: NASA, ESA, S. Rodney (JHU) and the FrontierSN Team;
- This image shows the appearances of the Refsdal supernova. The uppermost circle shows the position of the supernova as it could have been seen in 1998. The lowermost circle shows the galaxy which lensed the supernova four time — a discovery made in late 2014. The middle circle shows the position of the reappearing supernova in 2015.
T. Treu (UCLA), P. Kelly (UC Berkeley) and the GLASS Team;
J. Lotz (STScI) and the Frontier Fields Team; M. Postman (STScI)
and the CLASH Team; and Z. Levay (STScI)
The parameter is usually measured in terms of kilometers per second per megaparsec. This means that if we looked at a galaxy a megaparsec away (about 3.3 million light-years), then the speed at which the galaxy moves away from us in kilometers per second would be the value of the Hubble parameter. The bigger the value of the parameter, the faster the universe is expanding.
There are lots of ways to measure the Hubble parameter, but they generally fall into two categories. One general method uses the cosmic microwave background. While the cosmic microwave background is almost a perfect blackbody, there are small fluctuations in its temperature. The scale of these fluctuations tells us how much the universe has expanded, which in turn tells us the rate of cosmic expansion. This approach gives a Hubble parameter of about 67 – 68 (km/s)/Mpc.
The other approach looks at distant supernovae. One type of supernova known as Type Ia has a fairly uniform maximum brightness. So if you know the distance to a Type Ia supernova, you can compare its apparent brightness to its actual brightness, and calculate cosmic expansion. This relies on knowing the distance to the supernova’s galaxy, which relies on a complex set of distance calculations known as the cosmic distance ladder. This approach gives a Hubble value of about 71 – 75 (km/s)/Mpc. There is a third approach, using astrophysical masers emitted from the accretion disks of black holes, but so far this has had mixed success.
The upshot of all this is that two very good, very accurate measures of cosmic expansion give contradictory results. The precision of these results is good enough that we now know one or both of them must be wrong. It’s known as the Hubble tension problem. One solution to this problem would be to find a new way to measure expansion that doesn’t rely on the cosmic distance ladder or the cosmic microwave background. The maser approach might succeed in time, but recently a team has presented a fourth approach. One that involves a supernova and a bit of gravitational lensing.
... for something like a supernova, gravitational lensing could let us observe the supernova multiple times. This is exactly what happened with a supernova nicknamed Refsdal.
The supernova was first observed in 2014. ... The Refsdal supernova happened to be located in a gravitationally lensed galaxy. When astronomers realized this, they used computer models to predict when the supernova would appear again. They estimated it should appear again sometime in 2015-2017, and sure enough in 2015 it appeared again. This let astronomers predict other appearances. By 2018, astronomers had confirmed half a dozen appearances of SN Refsdal, which leads us to a new way to calculate cosmic expansion. ...
First-of-its-kind Measurement of the Universe’s Expansion Rate
Weighs in on a Longstanding Debate in Physics and Astronomy
University of Minnesota | College of Science & Engineering | 2023 May 11
Constraints on the Hubble Constant from Supernova Refsdal's Reappearance ~ Patrick L. Kelly et al
- Science (online 11 May 2023) DOI: 10.1126/science.abh1322
- arXiv > astro-ph > arXiv:2305.06367 > 10 May 2023
Time Delay and Magnification Measurements ~ Patrick L. Kelly et al
- Astrophysical Journal 948(2):93 (2023 May 10) DOI: 10.3847/1538-4357/ac4ccb
- arXiv > astro-ph > arXiv:2305.06377 > 10 May 2023