Bakersfield Night Sky - August 18, 2018

Bakersfield Night Sky - August 18, 2018
by Nick Strobel

Last weekend was the first good meteor shower of 2018, the famous Perseids. It was the first good meteor shower because all the other meteor showers this year have had the moon to contend with. Although there was no moon, we had smoke and haze from the various fires preventing us from having a truly dark sky at the observing location the Kern Astronomical Society used near Frazier Park. I hope the skies will be clearer at the Dark Sky Festival that will be held September 7-9 at Sequoia National Park. KAS will have their telescopes there for the general public to view. The cost is just the entrance fee to Sequoia.

Tonight is the free public star party at Barnes & Noble (on California just west of Hwy 99). KAS will have their telescopes out to look at the moon, planets, star clusters, and bright nebulae that can be seen even under our light-polluted skies. Observing runs from sunset to 10 PM (maybe start even before sunset for the moon and Venus).

At the August meeting of KAS, we heard from Josh Simon, a research astronomer of the Carnegie Observatories in Pasadena. He shared with us his work on one piece of the dark matter mystery that has confounded astronomers since the 1930s when Fritz Zwicky coined the term to describe the extra gravity force making galaxies in galaxy clusters move about much faster than they should. Members raised a number of questions for Dr. Simon that basically came down to “how do we know it's real (and not some mistake in our calculations)” and “how do we  know dark matter is some exotic stuff that isn't ordinary matter in black holes, planets, or other hard-to-see objects?” After talking about some of the strange properties of dark matter, astronomers certainly understand the skepticism many in the general public express and frustration in trying to understand something you can't directly sense.

Well, I don't think I can completely explain the mysteries of dark matter in the space of this column but let me try a brief summary anyway and ask you to go to my Astronomy Notes website for further explanation. Dark matter is material not made of ordinary atoms (no protons, neutrons, or electrons) and it does not block or reflect light or even ordinary matter. It doesn't respond to electromagnetism, so light will pass right through it and ordinary matter passes right through it as well (and vice versa). However, like the elusive neutrinos that also can pass through ordinary material very easily, we think that dark matter is not absolutely perfect in this ghostlike quality and that occasionally (very, very occasionally!), it will bump into a nucleus of an atom in a way that can be detected. This is how the neutrino was finally detected several decades ago. All sorts of experiments are trying to find dark matter particles in the same way. Other experiments are searching for signs of dark matter particles colliding with each other to produce things we can see like gamma rays. Nobel prizes await the team that succeeds and the competition for the discovery is intense.

Dark matter does respond to gravity and that's how we know it exists—from its gravitational effect on things we can see. Unfortunately, that gravitational effect is the only way we've been able to “see” it so far, and since gravity doesn't depend on composition, it's darn hard to figure out what dark matter is made of. There are multiple, independent observations that show there's something producing extra gravity. Many are of the form of seeing bright things (stars, galaxies, hot gas) move faster than they should. Another set of observations uses gravitational lensing (an effect of Einstein's General Relativity that bends the path of light) to measure the amount of mass in galaxies and galaxy clusters and find it's way more than what can be ordinary stuff.

Other observations use the amount of deuterium (a heavy form of hydrogen) left over from the first few minutes after the Big Bang to tell us how much ordinary matter there can be in the entire universe. Although we don't know for sure what started the Big Bang, the physics of a universe filled with hot gas and radiation (like the insides of hot stars) right after the Big Bang has been understood very well for many decades. The observations of primordial deuterium tell us that the upper limit of ordinary matter is much smaller than the gravity needed to make the stars and galaxies move as they do. Dark matter has to be five times the amount of ordinary matter. Furthermore, observations of the lumps in the cosmic microwave background (produced when protons and electrons got together for the first time to make atoms) also tell us that the amount of dark matter must be five times more than ordinary matter. SO: there are multiple, independent lines of evidence for dark matter and that it's not just ordinary matter current technology can't detect yet. Do a search for “dark matter” on the Astronomy Notes website for more details. Unlike the world of politics, the real physical universe doesn't have to fit our views of how we want it to be, so we're stuck with the dark matter mystery. It also means that the real universe is lot more interesting than what we could dream up on our own!

Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website