SBS 1520+530 is another object where the digits correspond to the coordinates of the object in the outdated 1950 coordinate system. The letters SBS stand for the Second Biurakan Survey, which was a survey performed in the 1980s and 1990s using the facilities at the Biurakan Astrophysical Observatory in what was once part of the Soviet Union but which is today Armenia. Additionally, the survey performed follow-up observations of some objects using the 6m telescope at the Special Astrophysical Observatory in what was one part of the Soviet Union but which is today the Caucasus Mountains in southern Russia. This survey used prisms to produce spectra of various objects in the sky that produced excess ultraviolet emission (because Armenians were really interested in astronmomical objects that produced excess ultraviolet emission) or objects that had interesting spectral lines [1].
What people involved with the survey noticed was unusual about SBS 1520+530 is that it consisted of two point-like objects located very close to each other on the sky, and the light from these objects appeared to be redshifted such that the wavelengths of light observed on Earth were 2.885 times longer than the original wavelengths of light [2]. These results indicated that the two objects were at relatively large distances (which correspond to the light travelling 11.5 billion years to reach the Earth but which, for complicated reasons, do not exactly correspond to distances of 11.5 billion light years).
People concluded that the two objects were actually two different images of the same object that had been created by gravitational lensing, and a paper led by V. H. Chavushyan announcing this discovery was published in 1997 [2]. In the gravitational lensing in SBS 1520+530, gravity from a galaxy in front of SBS 1520+530 is bending the light in such a way that, as seen from Earth, SBS 1520+530 appears in two different locations on opposite sides of the galaxy in front of it. A lot of people (including me) are currently studying gravitational lenses, and we currently know about lots and lots of these systems. (In fact, we know about so many gravitational lenses that I am not going to try to guess how many have been found.) Back in 1997, however, gravitational lensing was an extremely rare phenomenon, so it was really noteworthy to find any gravitationally lensed object anywhere.
However, that is not where the excitement ends with SBS 1520+530. The object is also a quasar, a type of galaxy with an active galactic nucleus consisting of a black hole millions or billions of times the mass of the Sun, a disk of gas falling into that black hole, and jets of ionized gas that emerge above the poles of the black hole because the infalling gas gets deflected away from the black hole by the system's magnetic fields. Like many other quasars, the rate at which gas is falling into SBS 1520+530 is variable, and this means that the amount of gas that emerges in the jets is also variable and that the amount of light that comes out across the electromagnetic spectrum is also variable.
What is special about SBS 1520+530 compared to most other quasars is that, because gravitational lensing causes the light to take two different paths to reach to Earth, any variations in light from the quasar will be seen in one location before the other. The difference in time between the variations in light from one image of the quasar and the other image is about 124 days [3]. This information on the relative differences in the length of the two different paths is related to the physical size of the lens, and this along with the apparent size of the lens on the sky and the redshift allows for calculating the Hubble constant.
This constant describes how, because of the expansion of the universe, galaxies further away from Earth appear to be moving faster than galaxies closer to Earth. For example, a galaxy that appeared to be moving 2000 km/s away from Earth would be expected to be twice as far as a galaxy that appeared to be moving 1000 km/s away from Earth. Back in the 1990s and early 2000s when I was a PhD student, astronomers were vigorously debating what this value was, and people were measuring values falling somewhere in the 50-100 km/s/Mpc range (which means that, for a galaxy moving 1000 km/s away from Earth, it could be located at a distance anywhere from 10 to 20 Mpc). Everyone was on the lookout for new opportunities for measuring the Hubble constant, and variations in the light seen from gravitationally lensed quasars like SBS 1520+530 provided a new way to calculate the Hubble constant. So, in 2002, a group of people led by Ingunn Burud published an analysis calculating the Hubble constant using the variations in light from SBS 1520+530, and they came up with a value of 51 km/s/Mpc [4]. That is, very surprisingly, on the low end of the range of possible values for the Hubble constant.
Since 2002, people have come closer to an agreed upon value for the Hubble constant, but they aren't there quite yet. The two most notable methods people have relied on in 2026 to calculate the Hubble constant are models of the variations in the Cosmic Microwave Background Radiation produced by the Big Bang, which have yielded a Hubble constant of 68.22 km/s/Mpc [5], and measurements using very distant supernovae, which have yielded values of 73.5 km/s/Mpc [6]. Although the difference in the Hubble constant between these two methods is only about 7.5%, people are still getting really worked up about that last few percentage difference. I suppose, back in the 1990s, the difference in the possible range of values of the Hubble constant could be related to the difference in the range of prices between fast food restaurants and fine dining, while the debate today is equivalent to the question of exactly how much extra to tip at a mid-range family restaurant.
Anyhow, that Hubble constant of 51 km/s/Mpc originally reported for SBS 1520+530 seems especially low compared to these modern estimates. I found a 2015 paper that revised the Hubble constant derived from SBS 1520+530 as 59.0 km/s/Mpc [3]. That's still low compared to modern values. However, that paper also made measurements of the Hubble constant from other gravitationally lensed quasars and obtained an average value of 68.1 km/s/Mpc, which is more in the ballpark of what we now expect. A more recent paper repeated this analysis with even more gravitationally lensed quasars and got 71 km/s/Mpc [7], which falls right between the cosmic microwave background value and the supernova value.
This debate is going to on for a while just like people are going to continue to debate whether to tip more or less at a restaurant because the service was fast and the chicken burger was good but the fries were only lukewarm. However, I personally don't feel compelled to get involved in that debate. Arguing over a 7.5% disagreement in the measurement of a constant isn't my thing. On the other hand, I am working with a lot of gravitational lens data right now, so I could always wander into the debate sheerly by accident and, in a few years time, find myself invovled in a deeply heated argument at a conference somewhere in France or California or someplace like that.