Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Monday, April 1, 2019

Ghost galaxies

I love a mystery.

I think part of it is that a truly unsolved question is a blow to our complacent attitude that we know all we need to know about the universe.  It's all too easy to walk around thinking we understand everything around us, to be fooled into the misapprehension that the cosmos is simple and obvious.

Anyone with a science background already has a significant dent in this notion, but it runs far deeper than that.  I think the hardest punch to my own certainty came when I started looking into the concepts of dark matter and dark energy, something like thirty years ago.  The presence of dark matter was first discovered back in the 1970s by Vera Rubin, Kent Ford and Ken Freeman, when they found that the rotational speed of every galaxy they looked at was impossible given the amount of visible matter present.  The discovery of dark energy came from a different source -- data from telescopes studying the cosmic microwave background radiation found that the universe was expanding at a far greater rate than it "should" have been given the measured mass/energy we knew about.  As peculiar as it sounds, the expansion could be explained if there was an invisible sort of energy -- later christened "dark energy" -- that exerted a repulsive force.

What I haven't told you is that by the most recent measurements, dark energy comprises 68% of the total mass/energy density in the observable universe, and dark matter 27%.  If you're wondering if that means what you think it means, you're right.

Ordinary ("baryonic") matter only makes up 5% of the mass and energy of the universe.

Worse still, that 95% has thus far been undetectable except by indirect measurements -- a bit like seeing the footprint of the bear but not the bear itself.  Every attempt to observe either dark matter or dark energy directly has been an abject failure.  So although there is speculation about the nature of this ubiquitous stuff, at present what we know for certain about it is exactly zero, other than the fact that it exists.

Of course, some people even doubt the latter bit, and remind us of the "aether" -- a substance proposed in the 19th century to explain peculiarities about the propagation of light.  Like dark matter and dark energy, the aether resisted all attempts to elucidate its nature.  We now know why that is -- it doesn't exist.  It took a clever experiment by Albert A. Michelson and Edward W. Morley to show that its existence was impossible, and no less than Albert Einstein to explain how light could propagate in the absence of a medium.  I've heard more than one scientist compare dark matter and dark energy to the aether -- and suggest that we're still waiting for this century's Einstein to blow us all away with an elegant explanation of the data we have, with no need for mysterious undetectable substances permeating the universe.

Messier 51, the Whirlpool Galaxy [Image is in the Public Domain, courtesy of NASA/JPL]

But at the moment, we're still stuck with them, and the bizarre observations keep rolling in to further deepen the mystery.  Just last week, a study was published in Astrophysical Journal Letters about some highly peculiar galaxies that seem to be entirely devoid of dark matter.  Called ultradiffuse galaxies, they are as large as an ordinary galaxy, but have such a low matter density that you can see right through them.  Measurements of rotational rates and matter distributions support the conjecture that they have the characteristics they do because -- for some unknown reason -- they have almost no dark matter at all.

"I spent an hour just staring at the Hubble image," said Pieter van Dokkum of Yale University, who was part of the study that discovered ultradiffuse galaxies.  "It's so rare, particularly these days after so many years of Hubble, that you get an image of something and you say, 'I've never seen that before.'  This thing is astonishing: a gigantic blob that you can look through.  It's so sparse that you see all of the galaxies behind it."

"We hope to next find out how common these galaxies are and whether they exist in other areas of the universe," said the paper's lead author Shany Danieli, also of Yale.  "We want to find more evidence that will help us understand how the properties of these galaxies work with our current theories.  Our hope is that this will take us one step further in understanding one of the biggest mysteries in our universe — the nature of dark matter."

Which is certainly a laudable goal.  The idea that the lion's share of the stuff of the universe has up till now resisted every attempt to learn about its characteristics is frustrating, to say the least.  But it does fascinate me, despite the frustration.  It illustrates to me how much more we have to learn about the universe we live in, and that when it comes to our understanding of science, we're still at the starting line.  Perhaps Socrates put it best when he said, "The more I know, the more I realize how little I know."


This week's Skeptophilia book recommendation combines science with biography and high drama.  It's the story of the discovery of oxygen, through the work of the sometimes friends, sometimes bitter rivals Joseph Priestley and Antoine Lavoisier.   A World on Fire: A Heretic, an Aristocrat, and the Race to Discover Oxygen is a fascinating read, both for the science and for the very different personalities of the two men involved.  Priestley was determined, serious, and a bit of a recluse; Lavoisier a pampered nobleman who was as often making the rounds of the social upper-crust in 18th century Paris as he was in his laboratory.  But despite their differences, their contributions were both essential -- and each of them ended up running afoul of the conventional powers-that-be, with tragic results.

The story of how their combined efforts led to a complete overturning of our understanding of that most ubiquitous of substances -- air -- will keep you engaged until the very last page.

[Note:  If you purchase this book by clicking on the image/link below, part of the proceeds will go to support Skeptophilia!]

No comments:

Post a Comment