MACS J1149.5+2223 is a cluster of galaxies in the constellation Leo. The acronym MACS stands for the Massive Cluster Survey, which was one of the surveys that discovered this cluster [1]. The letter J and the digits indicate the coordinates of this object. I could not find a good estimate for the total number of galaxies within the cluster, but it's probably in the thousands. The total mass of the cluster is somewhere around 2 and 3 quadrillion times the mass of the Sun [2, 3], or around or above 1000 times the mass of the Milky Way, and the cluster has a diameter of about a few million light years [4], although its a bit difficult to actually state where the edge of a cluster of galaxies is. The light from the cluster MACS J1149.5+2223 has taken 5.4 billion years to reach us [5], but the distance in light years is not exactly equivalent to the light travel time for complex reasons that I'm not going to get into.
A few different techniques can be used to find clusters of galaxies. The classical technique is to stare at images of the night sky and look for locations with lots of galaxies. This is the technique that George Abell used in the mid-twentieth century [6]. However, in spite of the fact that George Abell was an expert in staring at things, he could only find a limited number of clusters this way.
Another technique is to look for the X-ray emission assocaited with the very hot gas in between the galaxies within these clusters. In the 1990s, an X-ray telescope named ROSAT made maps of the X-ray emission from the entire sky, and a couple of surveys working with the X-ray data from that telescope, including the Massive Cluster Survey (MACS), published results at about the same time identifying the object now known as MACS J1149.5+2223 [1, 7]. As a side note, I was a student at the University of Hawaii at the same time that Harald Ebeling and Pat Henry at the University of Hawaii were working on the MACS survey.
Anyway, MACS J1149.5+2223 is not just a nice spherical cloud of galaxies orbiting within a nice spherical blob of X-ray gas. The cluster is actually a little elongated and lop-sided, and the gas appears to be moving around somewhat chaotically, with at least one giant shock wave passing through the gas within the cluster [4]. This along with the motions of the galaxies within the cluster indicates that MACS J1149.5+2223 was made through the collision of three smaller clusters around one billion years before the point in time that we are seeing the cluster [2]. (As a reminder, the light from the cluster has taken 5.4 billion years to reach us.) Give it a couple more billion years, and, if no other collisions take place, maybe everything will calm down within the cluster.
One of the really cool things about MACS J1149.5+2223 is that it is so massive that the gravitational forces from the cluster can bend the light from more distant galaxies behind the cluster [7]. This phenomenon is called gravitational lensing. In MACS J1149.5+2223, the gravitational lensing has has caused some of the background galaxies to appear in multiple places in the sky as seen from Earth. The light from these background galaxies may also appear brighter than it would without the lensing. This phenomenon can be used to measure the mass of the cluster, which is very useful given that the chaotic nature of the cluster makes it difficult to try to use the orbits of the galaxies within the cluster to determine its mass.
However, MACS J1149.5+2223 has not just gravitationally lensed the light from a bunch of boring background galaxies. Some of those galaxies are actually really really far away. One galaxy, which was named MACS1149-JD, is so far away that we are seeing the galaxy about 500 million years after the Big Bang, and the light has taken about 13.2 billion years (or most of the lifetime of the universe) to reach the Earth [8]. MACS1149-JD was either the most distant galaxy known or close to being the most distant galaxy known in the universe when it was discovered in 2012, it is still among the most distant known galaxies in the universe, and it is only visible because the gravitational lensing effects in the cluster MACS J1149.5+2223 have boosted the brightness of the galaxy. Astronomers have looked at the galaxy MACS1149-JD in detail to understand how galaxies formed in the early universe.
However, this really distant galaxy, as impressive as it is, only wins the runner-up reward in terms of the most interesting things that were gravitationally lensed by MACS J1149.5+2223. The first place award goes to a gravitationally lensed supernova behind the cluster. Discovered by a group led by Patrick L. Kelly, the supernova first appeared in November 2014 in four locations that formed a cross-shaped pattern in one location within the cluster [9]. After a little over a year, the supernova appeared in a fifth location outside the cross pattern but sort-of nearby [10]. This is not only because the gravitational lensing bent the light from the supernova in multiple different ways but also because the time it took the light to travel these different paths was different, with the light taking less time to travel down the four paths that made the cross-shape and more time to travel down the path to the fifth location.
This object was named Supernova Refsdal after the Swedish astronomer Sjur Refsdal, who had written a paper in 1964 stating that, if anyone is ever lucky enough to find a supernova affected by gravitational lensing, it can be used to measure the Hubble constant [11]. Because of the expansion of the universe, most galaxies appear to be moving away from our own, and galaxies that are further away appear to be moving faster than galaxies that are nearby. The Hubble constant relates the speed at which galaxies are moving away from the Milky Way to their distance. Sjur Refsdal's idea was that a supernova produces a lot of light for a very brief period of time, but when that light is gravitationally lensed, the light from different locations on the sky will take different amounts of time to reach Earth, and the difference in time between when the supernova appears in each different location is directly related to the Hubble constant.
It literally took 50 years after Sjur Refsdal's paper before anyone found a supernova, more specifically Supernova Refsdal in MACS J1149.5+2223, where they could apply his theory to actually measure the Hubble constant. Patrick Kelly's group published two different estimates of the Hubble constant based on different sets of models describing the gravitational lensing. One set of models gave a Hubble constant of 64.8 km/s/Mpc, while the other set gave 66.6 km/s/Mpc [12].
For comparison, the Hubble constant has also been measured using two other techniques. Using the brightnesses of "ordinary" unlensed nearby Type Ia supernovae, astronomers have calculated the Hubble constant to be 73.0 km/s/Mpc [13]. (I talked about one of these Type Ia supernovae in Episode 118. Go listen to that episode if you want more information on how the brightnesses of unlensed supernovae are used to measure distances.) The other technique for calculating the Hubble constant has involved modelling the cosmic microwave background radiation, which is the afterglow left over from the Big Bang. That has been giving a Hubble constant of 67.4 km/s/Mpc [14].
The people who work on the brightnesses of Type Ia supernovae and the people who work on the cosmic microwave background radiation have spent quite a bit of time trying to figure out whose results were correct [15]. The Hubble constant from the Supernova Refsdal is more consistent with the Hubble constant from the cosmic microwave background. This would be rather interesting, as it would indicate that people are modelling the cosmic microwave background well but that people's models of Type Ia supernovae might be a little off. Quite honestly, I've always been just a little worried about the supernovae results regarding the expansion of the universe, and these results from Supernova Refsdal make me feel a little more worried. In any case, I think these results are quite revolutionary and could potentially lead to scientific consensus on the exact value of the Hubble constant.