Clever ways of observing invisible stuff
Imagine a new form of energy, or perhaps a new constant of Nature, filling the whole Universe and yet so mysterious that it cannot be studied in any laboratory experiment. Imagine it having the most puzzling properties, such as a negative pressure which is accelerating the expansion of the Universe. Imagine something so strange that Einstein himself, who first saw the possibility of its existence, later reportedly called it ?my greatest blunder?. This is one of the greatest unanswered questions of contemporary physics: the nature of dark energy.
About a decade ago, cosmologists where stunned by a study looking at immensely powerful explosions of distant stars at the end of their life, called supernovae. A supernova explosion is so bright that for a brief time it can outshine the entire galaxy to which it belongs, and therefore it can be observed billions of light years away. To cosmologists, supernovae are very interesting objects because they act as beacons of known luminosity, which signpost the expansion history of our Universe. By measuring the dimming of their light, we can infer how much the Universe expanded since the supernova went off. The amazing discovery was that in the last couple of billions years the expansion of the Universe did not slow down, as one would expect if it only contained matter and ordinary radiation (i.e., light). Rather, the expansion had been accelerating, under the influence of an unknown repulsive effect. The first hint for the existence of dark energy had been found.
Since then, cosmologists have worked hard to devise other inventive ways to pin down dark energy. If supernova explosions serve as “standard candles”, then perhaps one could use “standard rulers”, as well. In theory, the recipe is simple: find a “cosmic yardstick”, i.e. some astrophysical object of known length, and observe it at different epochs in the cosmic history. Its apparent size will then tell us about how much the Universe has expanded in between, and in particular if dark energy has given the Universe an extra stretch. In practice, such a cosmic yardstick must be of immense length, but fortunately nature has provided us exactly with such a tool, in the form of so called “acoustic oscillations”.
This is the name cosmologists give to sound waves in the primordial Universe, when the temperature was high enough that only a hot plasma of elementary particles could exist. When the Universe cooled down sufficiently, the ripples of the sound waves got frozen, and it is precisely around the peaks that galactic structures tended to form preferentially. Therefore, the separation between galaxies today should show a preferred distance of about 500 million light years: a perfect cosmic yardstick! The reason why we know precisely how long the cosmic yardstick is today is that we have measured it in the very distant past, thanks to observations of the cosmic microwave background, the relic radiation of the Big Bang. In fact, the distance between a peak and a trough of the primordial sound wave is reflected in the separation between hot and cold spots in the microwave background, and this can be used to calibrate the yardstick. The existence of acoustic oscillations has now been proved, and in the future observations of millions of nearby and distant galaxies will reconstruct the length of the yardstick deep in the cosmic past.
What about the possibility of using our knowledge of the growth of galactic structures to track down dark energy? Here a major hindrance is that most of the mass in the Universe is in the form of dark matter (about 5 times more dark then luminous matter). But this can actually be used to our favour, by exploiting the fact that the gravitational attraction of massive objects – both luminous and dark – deflects the path of light passing nearby. Thus the image of a distant galaxy as observed on Earth will be distorted, because the light has been slightly deviated from its path by the gravitational effect of the dark and luminous matter between the galaxy and us. This is what in cosmological jargon is called “gravitational lensing”. By analyzing a great number of images of galaxies, we can reconstruct the distribution of matter at different epochs in the cosmic history. The theory of gravitational collapse can then be used to link these observations to dark energy. For instance, the presence of dark energy slows down the growth of structures, and this will be apparent from future gravitational lensing observations.
Combination of evidence from all those different techniques points strongly towards the existence of dark energy, perhaps in the form of Einstein’s cosmological constant. Faced with one of the deepest mysteries of modern physics, cosmologists are now planning a new generation of bigger, faster, and more accurate surveys, which in the next decade will enable us to shed new light on the dark side of the Universe.