The online astronomy office hours from the UofA continue apace. Every week Prof. Chris Impey answers ex tempore a mix of questions from planetary science to the fate of the universe from a thirsty audience across the globe. A large Indian contingent stays up until the wee small hours of their morning to join in. Part of the fun is pausing the video and trying to figure out the answer from basic considerations before resuming. It is fun to be right but more instructive to be wrong. I’ve been moved to send in three questions over the past couple of sessions and all have been answered.
When in relation to the Big Bang did dark matter originate?
The third video from Corning’s Museum of Glass shows that the path to science is not always smooth and that learning from mistakes is the norm. The original 200 inch pyrex disk for the Palomar primary did not go according to plan and had to be recast. The second attempt succeeded and even so, it took ten years of painstaking grinding and polishing at Caltech before it was ready for use.
But the early universe was very hot, very dense, and gravitationally very different from the comfortable-to-us 1 g we experience today on the surface of the earth. Einstein has convincingly shown that spacetime is accordingly divorced from that human experience. Clocks, for example, are affected by gravity and satnav constellations have to take this into account. Did the first three minutes flow the same way three minutes flow in the here and now? I sent that question to Chris Impey’s online office hour and he kindly answered. It is a tantalizing response and one that will require substantial further study to fully appreciate – perhaps finally diving into the guts of GR. It makes me wonder even more intensely why we anthropomorphize those intervals the way we do.
The mere detection of gravitational waves two years ago was cause for celebration and, for those of us skeptical of LIGO, eating of crow. Now gravitational wave detections regularly cue electromagnetic observatories on the ground and in space with tighter integration to come.
Youtuber skydivephil puts the camera on several researchers active in developing the next generation GW systems and the ever more ambitious cosmological probing that these observatories will enable.
Skydivephil and the unnamed narrator are self-effacing providing few details about themselves, not even their names in the nonexistent credits. They also have enviable access to many leading physicists and institutes, largely on the theoretical side. The style is simple: Let the speaker speak. It is a refreshing antidote to the modern space documentary which highlights the doom-and-gloom with an explosion and visual effect every fifteen seconds. Whatever one may think about string theory, loop quantum gravity, or their alternatives, it is refreshing to hear about them from the purveyors. Here’s the “Before the Big Bang” playlist with an assortment of views on modern cosmology (note that the episodes are in reverse chronological order.)
Pianist Nahre Sol delightfully explains sixteen levels of pianistic complexity in about ten minutes. That doesn’t mean there are only sixteen but, damn, what a lower bound for the recreational pianist to aspire to!
It took me a year and an email to understand how merging neutron stars could generate heavy elements. For this you need protons and from where do these protons come. The year was for intermittent research and the email due to failing in that research. As it happens, neutron stars are chock full of particulate goodness, far more than their name implies. On the one hand it is good to know how it works, on the other hand it shows that my research skills could use some improvement.
The Event Horizon Telescope team announces its major discovery following two intense and quiet years of data analysis on top of a longer period of development. A nice testament to aperture synthesis and international collaboration as the rest of the world spirals into madness. Damn!!!
Soon after LIGO‘s first detection of a black hole-black hole merger, the astronomical community was hinting about a potentially more scientifically exciting event within the interferometer’s grasp: The merging of two neutron stars. When two dark objects coalesce, the product is unsurprisingly dark. Colliding neutron stars on the other hand might emit light of some kind and the collision product need not necessarily be a black hole. More intriguingly, so-called kilonovae resulting from neutron star collisions have been proposed as the actual origin in our universe of many elements heavier than iron, challenging the conventional wisdom of these coming from supernovae.
Here’s a prescient talk by Prof. Brian Metzger of Columbia University and coiner of the term ‘kilonova’ on the consequences of neutron star binary mergers. He discusses their signatures in the gravitational wave record and across the electromagnetic spectrum to their ultimate role in nuclear synthesis. Given at Harvard on 16 March 2017, it is quite accessible for a technical colloquium presentation. A mere five months later on 17 August 2017, LIGO and its European counterpart VIRGO indeed detected the merger of two neutron stars and set of a flurry of observational activity across the globe and in space which confirmed at least qualitatively the predictions by Metzger and his group.
The details are still confusing. For example, we can assume that it takes a long time for two neutron stars to form, presumably from the death as a supernova of each of a large, but not too large, binary pair. These violent events will eject a lot of material into the interstellar medium. The neutron stars then spiral slowly and combine, releasing a lot of neutrons to stick to light elements, transmuting them up the periodic table through the r-process. But, where do these light elements come from if the ejecta from each of the progenitor stars has had a very long time to spread? (*)
Harvard’s Edo Berger has a concise summary of the multimessenger gold rush incited by the event in a special issue of Astrophysical Journal Letters. Many of the papers are free to download. As an aside, I was acquainted with Edo when he was an undergraduate physics student at UCLA while I was a researcher in the same department. I had no idea then he’d become one of the Dukes of Earl of high energy astrophysics.
(*) Addendum 20 April 2019: After a year of futility in not finding an answer to this question, I emailed Prof. Metzger and asked. In a prompt and gracious reply he said that the ejecta from the merging neutron stars create the seed nuclei required for the r-process. There are sufficient protons (10-30%) in the ejecta to form nuclei of mass number ~100 within milliseconds. These then absorb further neutrons within the constraints of beta decay to create very heavy elements within a few seconds. So, it seems that neutron stars aren’t neutrons all the way down!
30 May 2020: New video source; prior channel was deleted.