Ask Dr. SETI ®
by Dan Duda
from the October 2022 issue of Penn Central,
the monthly newsletter of Central PA Mensa,
used by permission
When scientists observe a phenomenon that they don’t understand, they develop a hypothesis to try to explain what they see. In the 1970s, astronomer Vera Rubin observed that the volume of visible matter inside galaxies couldn’t provide sufficient gravity to hold them together. So why don’t galaxies fly apart? That question sent shock waves through the cosmological community.
And Dark Matter is the term they developed to rationalize Rubin’s finding. According to Rubin, in a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That’s probably a good number for the ratio of our ignorance-to-knowledge. We’re out of kindergarten, but only in about third grade.
Let’s go back to gravity. We know that mass (matter) generates gravity. Objects are attracted to each other; the larger the mass, the more powerful the attractive force. That led scientists to expect that the speed of universal expansion should be slowing down due to all the mass. In the early 1900s, one thing was fairly certain about the expansion of the universe ... gravity was certain to slow the expansion as time went on. But with the finding that the concentrated mass of a galaxy isn’t enough to hold it together, let alone the whole universe, the dark matter hypothesis nicely filled the gap.
So let’s focus on the rate of expansion of the entire universe. Until recently cosmologists knew that the rate of expansion was decreasing due to gravity, including the effects of dark matter. It just made sense. However, cosmologist Saul Perlmutter won a Nobel Prize for his discovery that instead of slowing, the speed of expansion is increasing.
That discovery sent a second wave of shock into the cosmological community. How’s that for generating the need for a new hypothesis? And another “dark” term emerged. “Dark energy” entered the scientific lexicon. Regarding dark energy, more is unknown than is known. We know how much dark energy there is because we know how it affects the universe’s expansion. Other than that, it is a complete mystery. With this finding, we’ve graduated to Rubin’s fourth grade.
Now, we’re refining our concepts about these mysterious dark forces, and “quintessence” enters our vocabulary. This has been proposed to help us focus on the dark mysteries (but we still don’t really understand what’s going on).
Quintessence is believed to be a substance that is the opposite of the force of gravity (in the interest of brevity allow me to refer to gravity as a force). Instead of being attractive, quintessence is repulsive. (Keep in mind that we’re not talking about human characteristics). As numerous observations and experiments reshape the field, many cosmologists are exploring the possibility that the vast majority of the energy in the universe is in the form of a hitherto undiscovered substance called ‘quintessence’.
The choice of this name has classical precedence. In ancient Greece it meant the fifth element after ‘air, earth, fire and water.’ So, this promotes us to Rubin’s fifth grade.
The constant changes in scientific thinking, especially in cosmology, make its study irresistible, at least to me. Whether or not we’re getting closer to understanding reality is questionable, but the possibility is endlessly intriguing. To quote Paul Davies, “For me, science is already fantastical enough. Unlocking the secrets of nature with fundamental physics or cosmology or astrobiology leads you into a wonderland compared with which beliefs in things like alien abductions pale into insignificance.”
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this page last updated 1 October 2022
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