Our Dark Universe: How We Found It

For most of human history, we believed that what we saw, heard, and felt was all that made up the real world.  This changed around the beginning of the 20th century when modern science began to make uncomfortable discoveries such as that by Wilhelm Conrad Roentgen whose discovery of X-rays revealed that even more of a reality that was invisible and undetectable by our ordinary senses, or by our "common sense."  

Roentgen won the first Nobel Prize for physics in 1901, and subsequent Nobel Prizes have continued to show how our normal world is really composed of many strange new worlds as revealed by microscopes, telescopes, X-rays, and gravity waves.  

What we once imagined as being strange ("goblins, ghosts, and things that go 'bump' in the night") are nothing when compared to the strangeness that science has revealed.  As the British evolutionary biologist J.B.S. Haldane noted, the universe is not only queer "but even queerer than we can suppose."

And now it turns out that "everything on Earth, everything ever observed with all of our instruments, all normal matter,"  represents only about 5 percent of reality.  The remaining 95 percent is composed of something called Dark Matter and Dark Energy, which are undetectable except by their effects.  The road to this discovery began with a series of investigations made in the 1970s by Vera Rubin, a researcher at the Carnegie Institute in Washington, DC.  

Rubin's work focused on the dynamics of stars within spiral galaxies, how the gravity within galaxies affects the stars in the galaxy.  She was measuring the speed of the stars in various parts in a spiral galaxy (ours) by examining that spectra of light emitted by the stars.  A star that is moving away from the observer will show that its spectrum will shift toward the red end of the spectrum while one moving toward the observer will shift to the blue end.  The shift will be proportional to the star's speed.  (The Doppler Effect:  the change in frequency or wavelength of a wave (or other periodic event) for an observer moving relative to its source.)

According to our understanding of the effect of gravity on the motion of stars, those at the center of a spiral galaxy (where there is more mass) should rotate faster than those father from the galaxy's center.  (The phenomenon is known as the "galactic rotational curve.") Strangely Rubin's measurements showed that stars farther from the center of the galaxy were rotating as fast as those nearer the center.  

Rubin and a colleague checked their data by examining the motion of stars in 60 other spiral galaxies and found the same outcomes.  The outcomes revealed that there was a "galaxy rotational problem."  Rubin solved it by using the rotational speed of the stars she studied in order to calculate how much mass was needed to account for the gravity needed for the stars to attain their observed rotational speed.  Her calculations revealed that the galaxies must contain about 10 times more mass than could be accounted for by the visible stars, and concluded that 90 percent of the mass in the galaxies she tested was invisible.  "What you see in a spiral galaxy is not what you get," Rubin observed.  (AMNH, 2000)

Rubin's results were treated with skepticism until, when in the 1990s, astronomers began to calculate what they anticipated would be the deceleration of the Universe's expansion.  The surprise was that the Universe appeared to be accelerating instead.  

Every mass in the universe attracts every other mass proportionally to the product of their masses and inversely proportional to the square of the distance between them.  Calculations of the gravity needed to account for how objects are held in galaxies and galaxy clusters, reveals that the visible universe ("everything on Earth, everything ever observed with all of our instruments, all normal matter") accounts for only about 5 percent of the mass needed.  (You can see the calculations required to support these conclusions here.)

The remaining "roughly 68%" of the universe is dark energy while 27% is dark matter.  (NASA, Universe)

So the normal matter, what has been studied deeply is not actually normal.  What is normal, the "dark" matter and "dark" energy interacts gravitationally just like ordinary matter does--clumping into galaxies and galaxy clusters.  It is "dark" because it doesn't interact in any way that we can so far detect with light.  "It is not made up of atoms and doesn't carry an electron charge." (Randall, 2017)

This is science, so investigations continue.  

Brian Koberlein, an astronomer at Oberlin College, explains the facts that support the Dark Matter/Dark Energy theory.  

Since the 1920s discrepancies have been found that are explained by that either our understanding of gravity is wrong or that there is more mass in the universe than we can see.  

Since our current gravitational model does seem to work and alternatives that have been proposed have been disproved by observation, scientists are led to conclusion that the second clause in the previous sentence is true:  there is more mass in the Universe than we can see.  

Next the proposition that there are examples of mass that only interacts weakly with light is true.  Neutrinos have mass and weakly interact with light so there must be additional types of dark matter.  Koblein concludes with the statement that we "know...how much Dark Matter and Dark Energy, there is in the universe, as well as its distribution among the galaxies.  Dark Matter is not just a name we use to hide our ignorance."  


Resources:
Kobelein, Brian (2017). Dark Matter Works. Retrieved from https://briankoberlein.com/page/2/?s=dark+matter
 
NASA Universe. Dark Energy, Dark Matter. Retrieved from https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy
 
Randall, Lisa (2017). "Why Vera Rubin Deserved a Nobel Prize
NASA (2017). Dark Energy, Dark Matter. Retrieved from https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy
 
Vera Rubin (2000). Retrieved from http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_rubin.htm
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