On December 26, 2016, Vera Rubin died at the age of 88. She is the scientist whose team made observations of anomalous stellar velocities in the spiral arms of galaxies that provided a great impetus to the dark matter hypothesis.
The idea of dark matter had been around from around the first quarter of the 20th century but received a major boost in the late 1970s with more careful measurements by Rubin and co-workers using new technology of the speeds of stars in the spiral arms of galaxies. These more precise data could not be explained using existing, well-established theories of gravity based on the observed amount of familiar matter that we knew about but could be explained by postulating that galaxies were immersed in a halo of matter that we had not detected as yet that extends well beyond the visible edge of the galaxies. The mass of this dark matter creates the gravitational force that causes the anomalous velocities. Subsequent measures of stellar phenomena have bolstered the idea of such matter existing.
The problem is that three decades of searching have not been able to produce any direct detection of dark matter particles. If this matter is so abundant and all around us, why have we not observed it? This is indeed a pressing question in the field of astrophysics and cosmology and a multiplicity of theories have emerged that seek to explain the amount of dark matter, why we have not observed it so far, and what possible experiments might be performed to directly detect these dark matter particles. Clearly, if they exist, their interactions with ordinary matter must be extremely weak since we are immersed in it and yet do not feel it, and the major searches currently underway involve setting up heavily shielded, highly sensitive detectors in deep underground mines to minimize the possibility of other cosmic particles falsely triggering the detectors.
The problem is that as the detectors get more and more sensitive, they will start to pick up other faint signals that could mask any dark matter signals that might be produced. Some astrophysicists are suggesting that it is close to the time to reject dark matter as a failed hypothesis.
However, many scientists believe time is running out for the hunt, which has lasted 30 years, cost millions of pounds and produced no positive results. The LZ project – which is halfway through construction – should be science’s last throw of the dice, they say. “This generation of detectors should be the last,” said astronomer Stacy McGaugh at Case Western Reserve University in Cleveland, Ohio. “If we don’t find anything we should accept we are stuck and need to find a different explanation, perhaps by modifying our theories of gravity, to explain the phenomena we attribute to dark matter.”
Other researchers reject this view: “Theory indicates we have a really good chance of finding dark matter particles,” said Chamkaur Ghag, chair of the Dark Matter UK consortium. “This is certainly not the time to talk of giving up.”
This is an excellent example of an interesting issue that I go into in my forthcoming book The Great Paradox of Science: Its surprising success against all odds (the title is a work in progress too and keeps evolving!) about how we decide in science that a search for something that we believe should exist but that has come up negative should be abandoned. In other words, when should the absence of evidence be taken as evidence of absence? It is a fascinating question and I go into it in some depth in my book, and I alluded to it in an earlier post where I provided an except of the introductory chapter that introduces the problem of proving a negative, that something does not exist.