"We love the Universe. It's great!"
THE DAILY ASTRONOMER
Thursday, July 28, 2016
Dark Galaxy (Parts I and II)
Hello!
So, here's the rub. The e-mail system I'm using at the planetarium has gone a bit off the rails.
Presently, we're attempting to fix the problem, which will likely prove a bit tricky until we can find someone with enough stamina to constantly turn the crank connected to my medieval computer. However, all is not lost, as I am equipped with a more modern machine at home. So, I am sending the DA late. However, it is a repeat.
When the new DA school year begins anew on Sept 1, all will be in proper working order.
No repeats.
No delays
No excuses
No worries
No frets, frowns or furrowed brows.
And, now that the DA has fallen into your inbox like to much manna from Heaven, your lives -which had been trapped in suspended animation- can resume.
Yours hopelessly,
Edward
PART I
Today we revisit the adage "If it is not observed, it is inferred." Therein lies astronomy's true miracle: the astronomer's ability to ascertain an invisible object's existence merely by noting how it interacts with the visible material in its proxmity. Astronomers observe the light* that visible objects either produce (stars) or reflect (moon and planets.) This light provides them with copious information about the body, be it its chemical composition, discernible through analysis of dark lines within its spectrum,** or its motion relative to nearby bodies. It is through a celestial body's motions, of course, that an astronomer can deduce the existence of objects that neither emit nor reflect light. The most important example of this inference through indirection observation involves dark matter, the mysterious and opaque material believed to comprise nearly twenty five percent of the physical Universe.
We start with the simple notion that our solar system and billions of other star systems revolve around the galactic nucleus: the galaxy's center. By our lethargic terrestrial standards, these stellar motions are quite rapid, often exceeding 200 miles a second. Unlike solar system body movements, which are comparatively simple to model as they result principally from the Sun's gravitational influence, galactic stellar motions are more complex. The central galactic region likely harbors a supermassive black hole and is certainly home to myriad stars, clusters and prodigious amounts of dust. This abundant material exerts a gravitational influence on the galaxy's orbiting stars, though astronomers are still trying to determine the precise nature of the motions and how interactions with all the galaxy's matter affects stellar orbital velocities.
What is known, however, is that the visible matter is not solely responsible for the observed motions. The galaxy, itself, and the innumerable galaxies within its vicinity and those farther afield, must all contain vast quantities of unseen matter. This conclusion, first reached in the 1930's by Swiss astronomer Fritz Zwicky, is based on observed motions of galaxies and of stars within galaxies. All of which move far more quickly than they should be moving if the only matter within the stars and galaxies are visible. There is a direct relation between the a system's mass -and, by extension, the gravitational potential energy- and the kinetic energy of the system's particles. It is called "the Virial theorem," a particularly complex little relation that, at its base, enables physicists to relate the speed of a system's components with the system's mass. By knowing one, one can calculate the other. The greater the gravitational potential energy (or matter) the greater the kinetic energy (velocity.)
Astronomers have measured the speeds of stars within the galaxy***. These speeds are quite high and, by the estimations through the Virial theorem, astronomers have realized that much more matter must be present within the galaxy to produce such high stellar speeds. They have concluded that ninety percent of the galaxy's material must consist of 'dark matter.' The physics is quite clear on this point. The problem involves the missing matter's nature and location. Where is it is and of what is it composed?
These matters pertaining to dark matter have not yet been resolved. For the last few decades - as long as the notion of dark matter became widely accepted- astronomers have tried to develop the means by which to detect the unseen material. For scientists accustomed to observing visible matter, such a task as finding dark matter is no mean feat. These efforts have yielded few results. However, this lamentable, but hardly surprising lack of success, might have finally come to an end, as some astronomers might have finally found evidence of dark matter.
PART II
In our last riveting episode, we rapidly reviewed how astronomers determined that dark matter exists. "Dark matter," for the benefit of those subscribers who didn't find the episode quite that riveting, is the blanket term applied to all material that doesn't emit or reflect light. Detecting such material proved a considerable challenge for astronomers who have been accustomed to studying celestial objects through their electromagnetic emissions (e.g. visible light, radio waves, UV rays). Scientists realized that such material must exist in abundance through its gravitational interactions with visible matter. For instance, the stars in our galaxy move far more quickly than they would if the galaxy consisted solely of the stars and other objects we either directly observe, or know to exist, such as the super massive black hole with the galactic nucleus. The stellar motions directly correspond to the galactic material through a complex, yet handy relation, called the "virial theorem." It tells us that the kinetic motion of any system's particles increase with increasing matter. Astronomers can measure the velocity of stars within the galaxy and thereby have estimated that approximately ninety percent of the Milky Way's material must consist of dark matter. Moreover, cosmologists estimate that dark matter comprises twenty-five percent of the Universe entire.
As dark matter represents a significant proportion of the Universe, locating the unseen material and then identifying it has become a profoundly important issue in modern astrophysics. Unfortunately, discerning the nature of matter that one cannot scrutinize through traditional means (i.e. studying its light) presents considerable difficulties. However, astronomers have surmounted such challenges before. (We recall the early 19 century view that humans would never know a star's chemical composition.) Now, hundreds of researchers are embroiled in a determined effort to ascertain precisely what makes up a quarter of everything in the Universe.
While they have made no definitive discoveries as of yet, a recent experiment conducted on the International Space Station might yield promising results. The device used for this experiment is the Alpha Magnetic Spectrometer, a highly sophisticated (billions of dollars worth of sophisticated) series of instruments designed to detect and identify deep space cosmic rays: those emanating from far intergalactic and extra-galactic sources. The collection of these rays could allow astronomers to indirectly detect dark matter, itself.
To understand the relation between cosmic rays and dark matter, we must introduce a new concept, that of the WIMP (Weakly interacting massive particles.) As their name suggests, they are highly elusive because they rarely interact with material. For instance, neutrinos produced by the Sun's core nuclear reactions, distant supernovae, and even from the Universe's infancy, are passing through you at this very moment. Billions of them every second. As they don't interact with you, you don't notice them at all. And, they move extremely fast. Try this: Blink. The neutrinos that entered your body when you closed your eyes already exited the opposite point of Earth by the time you opened them.
WIMPS could very represent a large proportion of matter in the galaxy and Universe. Though they don't "interact" for the most part, they could still exert a strong gravitational influence owing to their staggering abundance. Think analogously of those harmless little water droplets that can form a shore-assaulting tidal wave if you get enough of them in one place. The problem, which isn't trivial, is their detection. That is where the cosmic rays become necessary.
While WIMPS don't interact with much of anything, generally, they can interact with each other. Such interactions or collisions, should produce two types of particles: electrons, and their positively charged counterparts, positrons. These highly energetic 'rays' are far easier to detect than WIMPS.
*We use "light" in reference to all electromagnetic radiation, which includes, but is not limited to, the visible light spectrum.
**We’ll be brief. Imagine you divide light into its component colors (Roy, Orange, Yellow, Green, Blue Indigo, Violet) through a prism or diffraction grating. A star emits light at all wavelengths, but chemicals within the star's outer atmosphere absorbs light at some of these wavelengths, thereby producing a sequence of dark lines. Each chemical has its own 'signature' sequence within the spectrum. By observing this dark line pattern, an astronomer can identify chemicals within a star.
***Now, we won't be brief:
How do you measure a star's 'space velocity?' We first have to explain that such a velocity consists of two components: radial velocity and transverse velocity. Radial velocity refers to a star's velocity along our line of sight: a radial velocity is positive if the object approaches; and negative if it recedes. Transverse velocity refers to motion along one's line of sight. Somebody running side to side in front of you exhibits a high transverse velocity, but a cannon ball fired at your chest has a zero transverse velocity.
Astronomers can measure a star's radial velocity by measuring how its light is elongated or compressed as it either moves away from us or approaches. This elongation/compression is called the "Doppler Effect," a phenomenon we notice in sirens that have a high pitch when they approach (compression) and a lower pitch when they move away (elongation.)
Astronomers can also observe a star's 'proper motion' defined as a star's angular displacement relative to the other stars. If astronomers know a star's distance, and also the proper motion, they can determine a star's transverse velocity. By combining the radial and transverse velocities, the astronomers measure the star's velocity through space.