Around this time of year, we often pause and take stock of where we are on many levels: emotionally, financially, professionally, politically and, at least here at 13.7, cosmically.
Where do things stand these days, when it comes to the universe?
It is a remarkable feat of modern science that we have, during the past 50 years or so, learned an enormous amount about the cosmos. This has happened around our local neighborhood — from the planets of the solar system all the way to demoted Pluto, their moons, planets around other stars. And it has extended out to the far reaches of space and time — nascent galaxies billions of light-years away from us, radiation from when the first atoms were forming a mere 380,000 years after the Big Bang itself.
Just think that around 1500, if you had asked an educated European about the universe, the answer would have been very similar to what the Greeks had been saying before the days of Christ: A static Earth is at the center of everything; the moon, sun and planets revolve around it in perfect circular orbits, the stars sit at the limit of space, closing the cosmos. Outside are God and the elect in a timeless existence.
The telescope would change all that, first in the hands of Galileo, who pointed it up to the sky in 1609 to reveal a very different picture. He saw change and asymmetry in a sky that the ancients had thought perfect and immutable: moons orbiting Jupiter, sunspots, "ears" on Saturn, mountains and craters on our own moon. The telescope, as other tools would do in the history of science, was instrumental to change a worldview that a combination of common-sense observations and faith had sustained for thousands of years.
Fast forward to the 20th century, and again our views of the cosmos were set for a radical overhaul. Much had been learned in the intervening centuries, especially about different types of objects out there in space, "planetary nebulae" and other colorful clouds that the late 18th-century astronomer William Herschel compared to a garden of "the greatest variety of productions." Ever more powerful telescopes, like the one Herschel built with a 4-ft. mirror, allowed for a deluge of discoveries. He himself found a new planet, Uranus, and produced a catalog with 2,500 nebulae.
The skies were seen as the dynamic stage of an ongoing drama of change and transformation, the very opposite of the old static view.
What we see of nature colors our perceptions of reality. The universe, of course, has always been what it is; but our views of it are always changing, thanks to the inventiveness of scientists and the amazing tools that help us amplify our myopic perception of the world. We should keep this in mind as we venture into more uncertain realms of knowledge.
In 1924, Edwin Hubble used the 100-in. Mount Wilson telescope to redefine our place in the cosmos. The Milky Way, our home galaxy, was not, as most thought then, the only one out there, but one among myriads of others, separated from one another by millions of light-years. And in 1929, he made this view even more amazing, by revealing that these galaxies are moving away from one another. This is the discovery of the cosmic expansion, that space, as Albert Einstein had predicted, is stretchy, and can carry objects with it as corks floating along rivers. Under this view, galaxies are not like shrapnel from a primeval explosion but hitchhikers on a cosmic expansion where space itself stretches in time. Play the movie backward and we get to the Big Bang, the moment in the past where all the stuff that makes up the cosmos was squeezed into a tiny volume.
In the past 90 years or so, this view became much more nuanced. First, we have confirmation that this was indeed the case, that the cosmos was very dense and hot early on, and that it has been expanding ever since. Modern numbers, collected by NASA's mission WMAP and by the European Space Agency mission Planck, date the universe at 13.8 billion years. (As if to prove the ongoing nature of scientific discovery, it was 13.7 billion years when this commentary blog started at NPR on Dec. 16, 2009.). To get to this level of precision, these missions, and many other ground-based observations, have been pushing technology to the limit, obtaining a wealth of data that leaves no doubt that our current view of the universe rests on firm ground: The universe is dynamic and it has a history that had a beginning in the distant past.
The devil, as always, is in the details. What kinds of matter and radiation fill the universe? That, to our knowledge, is the oldest question philosophy has asked and that we continue to explore with our evolving tools. We now know that the stuff we are made of — electrons, protons, neutrons — makes up only about 4 percent of the total. All the stars you see with the naked eye, the nebulae that so enchanted Herschel, the billions of galaxies and globular clusters and gas clouds out there, make for a tiny fraction of what fills the cosmos.
The rest is still an unknown, even if we can organize our ignorance into two groups.
Some 26 percent of it is what we call dark matter, stuff that doesn't shine but has mass and thus gravity and collects around galaxies like an invisible cloak. We "see" its effects, as this excess of invisible matter bends light going through galaxies, and also makes galaxies spin faster than they would without it. Dark matter is also essential to provide the seeds for galaxies to grow, making Herschel's vision of a cosmic garden even more compelling.
The challenge with dark matter is that we can't find it. We have been hunting it for decades, with detectors on the surface of the Earth and underground in deep mines, attempting to collect it as they collect other kinds of particles and radiation falling from the skies, with detectors in satellites orbiting Earth, and from collisions in particle accelerators, such as the Large Hadron Collider at CERN, the European Center for Nuclear Physics, where the Higgs boson was found in July 2012. Many theorists have hoped that more particles of nature exist beyond those of the "Standard Model" — the collection of all known fundamental particles of nature — and that some of those new players would be dark matter particles. Unfortunately, all these efforts failed, so far, to produce any reliable hints that these particles exist, leading some to question the whole dark matter business.
The rest, all 70 percent of it, is what we call dark energy, discovered as recently as 1998. This is the most mysterious of all, a truly strange material (if we can call it that) that has the uncanny property of stretching space as a kind of repulsive force. Its net effect is to push galaxies away from one another at a fast clip, causing an acceleration of the cosmic expansion. A gargantuan effort known as the Dark Energy Survey has been afoot since 2013, with the goal to map hundreds of millions of galaxies, detect thousands of bright supernovae to use as distance markers, and to find patterns of cosmic structure in the distribution of galaxies that can give clues to the nature of dark energy with unprecedented precision. We still don't know what it is, but we are definitely learning a lot about the universe as we try to find out.
As a species, we can be proud of our remarkable scientific prowess; we devise amazing tools of exploration that amplify our view of reality and, in the process, revise and redefine our place in the cosmos. This is an ongoing effort, a narrative we build slowly, gathering data and ideas that stretch our imagination. To think that only 400 years ago Galileo was dealing with heavy censorship from the Roman Inquisition (1616 was the year he was admonished by Cardinal Bellarmine and convinced to abandon his opinions about a sun-centered cosmos — which he did, at least temporarily) and that, now, here we are, pursuing mysteries at the cutting-edge of knowledge.
Although we can't know how the search for dark matter and dark energy will turn out, we can be sure that our cosmic views are going to be up for grand new revisions in the near future. And this is precisely what makes science so exciting.
Marcelo Gleiser is a theoretical physicist and writer — and a professor of natural philosophy, physics and astronomy at Dartmouth College. He is the director of the Institute for Cross-Disciplinary Engagement at Dartmouth, co-founder of 13.7 and an active promoter of science to the general public. His latest book is The Simple Beauty of the Unexpected: A Natural Philosopher's Quest for Trout and the Meaning of Everything. You can keep up with Marcelo on Facebook and Twitter: @mgleiser