Here is something that I think every human being should do every once in a while, as part of life, and as one of those things that make life worth living and gets us wondering about the universe and what it means to us and our place in it: Travel to a location far from any major city or other sources of terrestrial light--preferably to the top of a mountain somewhere, surrounded by natural habitat, rather than villages or towns. Stay up after dark, and spend some time looking at the night sky. If you've chosen the location well, on a cloudless, moonless night you should see magnificent things you rarely see from "normal" locations: such as the Andromeda nebula--a faint cloudlike patch that is the nearest galaxy to our own, at 2.2 million light years away; or the brilliant Pleiades, also called the Seven Sisters--a cluster of bright young stars draped in a bluish haze: the cloud from which these stars are being formed. Or, in the southern hemisphere, you would see the small and the large Magellanic clouds: satellites galaxies to our own Milky Way home; or the eery Southern Cross, and the dark patch of sky called the Coal Sack (a dense cosmic cloud obscuring stars), and near it the Jewel Box--a shimmering, multicolored star cluster.
Admiring the immensity of the universe around you, with all its visual wonders, ask yourself: What percentage, what fraction of the night sky is taken up by bright objects, as compared with the darkness that surrounds everything? And the answer you will come up with will probably be: a small percentage--most of the universe I am seeing is just darkness, accentuated here and there by distant bright objects. It turns out that, by coincidence, that "small percentage" of the totality of the night sky taken up by light-emitting objects is roughly the percentage of the entire universe that is taken up by matter as we know it: stars, gas and dust clouds, and planets. The rest of the universe is taken up by two kinds of entities that we do not understand and which, for us, are totally "dark": dark energy and dark matter. The matter-as-we-know-it universe comprises less than 5 percent of the total universe around us. About 70 percent of the mass-energy in the entire universe is taken up by the totally mysterious force called "dark energy"; and about 25 percent of the universe is taken up by the equally-mysterious "dark matter." What are these two entities?
To really understand the comparison of these quantities, we need to think in the same units. Einstein's famous formula tells us that mass and energy are really the same thing, so we can think of the universe as being filled with energy, and measured in units of pure energy. Some of the energy in the universe is in the form of mass, and the rest of it is sheer energy. The largest portion of this mass-energy of the entire universe is taken up by the mysterious force called "dark energy," whose purpose is to push the universe outward, countering and overcoming the gravitational pull of all the combined massive components of the universe. Dark matter, on the other hand, is matter--rather than sheer energy--but matter we can't see or feel or detect in any direct way other than through its gravitational pull on other matter.
In 1917, a year after Einstein published his greatest masterpiece: the general theory of relativity, he attempted to apply his gravitational theory to the universe as a whole. Einstein was unaware of just how amazingly powerful his own field equation of gravitation really was--for it foretold the expansion of the universe! But unfortunately (or fortunately--as we will see--by a twist of fate), Einstein had more faith in the astronomers of his time than he had in his own equation! Astronomers told Einstein that the universe was static--just sitting there, as it had, according to their reckoning, from time immemorial. The problem was that the only galaxy observable to the naked eye, Andromeda, and others seen through the telescopes of the time, were all believed to be part of our own galaxy. Since the Milky Way was not seen to expand, astronomers told Einstein that, no, the universe was not expanding.
So what Einstein did was to add a term called lambda (after the Greek letter he used to represent it) as a multiplier of one of the elements in his equation, and what this term--now called the "cosmological constant"--did was to "hold back" the universe from expanding. In 1929, Edwin Hubble published a paper with what has become known as "Hubble's law" about the expansion of the universe, based on a study of the recession rates (based on the Doppler redshift in the spectrum of light) of very distant galaxies observed with the 100-inch telescope of the Mount Wilson Observatory in southern California. As a result, Einstein threw away the cosmological constant with disgust, never to speak of it again.
Fast forward almost 70 years, and at the Lawrence Berkeley Laboratory in northern California, a bright young astrophysicist named Saul Perlmutter is directing a survey of the sky of unprecedented dimensions, called the Supernova Cosmology Project. (At the same time, a team based at Harvard is doing similar work.) Perlmutter and his team use high-power telescopes from Chile to Hawaii to the Canary Islands and to space to search for a particular kind of cosmic explosion called a Type Ia supernova, which happens to have very standard light-curve characteristics (it is believed to look the same wherever it happens) and to take place roughly once a century in any galaxy. By scanning thousands of faraway galaxies, the team is able to correlate the redshift--meaning the speed of recession from us--for a given galaxy against its distance from us as evaluated using the light-curve of the supernova taking place within that galaxy. The results of the study are so stunning that they abruptly change our entire thinking about the universe.
A Type Ia Supernova in a Distant Galaxy and its Light Curve as Observed on Earth. Photo Courtesy of the Lawrence Berkeley Laboratory.
If you throw an object up in the air, it will go up, slow down, stop at the apex of its flight, and then fall back to Earth. Until 1998, many physicists believed that this was what would eventually happen to the entire universe. The thinking has been that the Big Bang provided an impetus for all mass to be "thrown" outwards, and that when the effects of that powerful event were over, the expansion of the universe, discovered by Hubble and his collaborators, would slow down because of gravity and--just as an object thrown into the air falls back down by the pull of Earth's gravity--the universe will re-collapse onto itself as all matter in it exerts the force of gravity on all other matter. The theory had a certain "hopefulness" to it, too, because it implied that one day in the distant future, after a universal collapse, another universe would be reborn out of the ashes of our own in a new Big Bang.
But Perlmutter and his colleagues, and the competing group at Harvard, whose results were also ready in 1998, proved exactly the opposite: the universal expansion is not slowing down at all--actually, it is accelerating! Now, from basic physics you probably remember Newton's law, which says that force equals mass times acceleration, so if the universe is accelerating its expansion--long after the Big Bang had taken place--then there must be some mysterious force that is acting on the universe as a whole and making it accelerate its expansion. Such a force would do what Einstein's old cosmological constant did to Einstein's equation--in an opposite kind of way: instead of stopping an expansion, it would be accelerating it. Thus Einstein's lambda was back with a vengeance and the "dark energy" that is evident through its acceleration of the cosmic expansion is now associated with the cosmological constant, lambda, and some physicists even call it "quintessence," after the ancient Greek concept. The moral is that not only the universal expansion will never end, but the universe will die a death of excessive "thinness" as space becomes ever more diffuse of matter, stars die out, and galaxies disappear. In 2011, Perlmutter and his colleagues were awarded the Nobel Prize in physics.
When I interviewed him about his monumental discovery, Perlmutter told me: "Imagine a lattice in three dimensions. At each corner of the lattice there is a galaxy. Now, the lattice itself expands, so that a galaxy at the next corner expands away from our corner, and one farther away from us appears to move at yet a greater speed, and so on." To this picture of cosmic expansion we add the fact that this rate of expansion is actually increasing through time, rather than slowing down as we might have expected. To learn more about the dark energy that pushes space outwards would require further astronomical and satellite observations since the range of the problem is that of hundreds of millions to billions of light years (within our cosmic neighborhood we cannot discern the expansion).
In the early 1930s, the famous Dutch astronomer Jan Oort (after whom the Oort cloud of comets surrounding the Sun is named) and, independently, the Swiss-American astronomer Fritz Zwicky, came to the same inescapable conclusion: No matter how you reckon in, there has to be more mass in the universe than is evident from accounting for all the normal matter. They came to this conclusion by noticing that the gravitational pull of matter as we know it could not account for astronomical phenomena--the orbital velocities of stars, in the case of Oort, and the orbital velocities of entire galaxies, in the case of Zwicky. Since then, any "normal" theory to account for the missing mass has fallen flat: neutrinos, abundant as they may be, cannot do it; neither can giant black holes; nor can the laws of mechanics be found to be wrong. So today we know that most of the matter in the universe is, in fact, "dark"--in the sense that it doesn't interact with or emit light--and completely mysterious to us. But nobody has ever "seen" the dark matter, right?
When I interviewed the renowned American Nobel Prize winning physicist Steven Weinberg about dark matter and asked him how we know that it really exists, he told me: "We can see it!--go online and search for images of the Bullet Cluster of galaxies. There, you can actually 'see' the dark matter."
The Bullet Cluster, courtesy: NASA
The red clouds in the picture are hot "normal matter" that is emitting x-rays. The blue cloudy regions, however, are composed of dark matter and they are "seen" through a unique technique due to Albert Einstein, called "gravitational lensing." Like a lens, gravity can bend light, as Einstein has taught us, and in this case, the light of faraway galaxies is gravitationally "lensed" by the invisible dark matter in this cluster of colliding galaxies. The red part and the blue part of the picture thus represent two very different kinds of matter: regular matter seen through its emission of detectable x-rays, and dark matter, seen only through the fact that it acts gravitationally on light from a source that is behind this cluster. The Bullet Cluster is the first example of actually seeing the effects of dark matter. Other galaxy clusters, such as Abell 520, also show evidence of large amounts of dark matter present. But what is this dark matter?
By definition, dark matter is something that is very different from the matter we know. It only interacts with other matter gravitationally--so it is something that has not been accounted for in our particle physics theory, called the Standard Model. The last elementary particle of the Standard Model is the famous Higgs boson, whose discovery was announced by CERN on July 4 of this year, and whose properties are now being analyzed to make sure that it is, indeed, the long sought-after Higgs. Another theory in physics, called supersymmetry (abbreviated as SUSY) posits the existence of many other kinds of particles, which are "symmetrically" related to the known particles of the Standard Model. None of these have ever been discovered, but if any of them are ever found, they will constitute strong candidates for the mysterious dark matter. This is one good reason why we must continue our efforts to find new particles through research in powerful particle accelerators such as the Large Hadron Collider, and through other experiments.