Our July meeting was addressed by Roger O’Brian, who gave us a talk with the title Dark Energy and Dark Matter. Miriam writes:
Why don’t stars escape from their galaxies? Why doesn’t dark matter fall into black holes?
Questions, conundrums and WIMPs all featured in this lecture. Roger said he could not promise to provide any answers and finished up with a query for us: ‘Any ideas?’
An explanation for why stars do not escape from galaxies is the presence of undetectable mass which is not seen, not luminous and therefore dark. There are several theories for what dark matter is: gas, dust, large planets, dim stars, stellar remnants or MACHOs (massive compact halo objects). One by one Roger dismissed these theories: gas is easy to detect but there is not enough of it, dust blocks out the light and anything else is difficult to see, dim stars have a low mass so too many would be required; large planets would form new stars; stellar remnants would need to be spread throughout the galaxy.
The other idea put forward is that dark matter is something else, and perhaps should be called Exotic Matter. WIMPS were mentioned here – weakly interacting massive particles. Roger showed images of various objects and said that observational techniques tell us that dark matter must be exotic.
There are two basic problems: Firstly, dark matter cannot be seen so must be highly transparent and it is therefore impossible to detect and secondly the bigger the scale investigated the more dark matter is required to explain its influence. However, scientists have found no evidence for dark matter in a large volume around the Sun.
Despite its incomprehensibility, there seems to be a lot of dark matter about! Is there another explanation such as a gravitational anomaly? Theories vary and there is no consensus.
What do you think?