Shedding Light on the Dark Side of the Universe


From Plato’s claim that the world is made up of earth, water, air, and fire, to the discovery in the past century of protons, neutrons, quarks and ever more exotic particles, humans have made a lot of progress in figuring out what their world is made of. It may seem shocking, then, that in the past 15 years or so, scientists have realized that the "stuff" making up all the atoms in all the galaxies, stars, planets, and humans we've ever observed only constitutes ~5% of the Universe. While we might be close to pinning down the nature of part (~27%) of the missing stuff, the nature of the dominant (~68%) component of the Universe, called “dark energy,” is still incredibly elusive. Josh Meyers talked to Josh Frieman (Fermilab) after the KIPAC@10 dark energy session.



What do we mean by “dark energy?”

Why do we think there’s something strange about 68% of the Universe? In 1998, two teams of astronomers announced the expansion of the Universe is accelerating, a result that led to the 2011 Nobel Prize in physics for members of both teams. This surprising observation runs contrary to expectations that the mutual gravitational attraction between galaxies will slow down the expansion. Einstein’s theory of gravity, “general relativity,” allows for such a contrary accelerating Universe, but only given the existence of a cosmic fluid with exotic properties. This fluid, proposed to explain the observed acceleration, is what scientists refer to as dark energy.

While a name is a good start, we still don’t know very much about dark energy. Fortunately, its properties lead to several observable effects in the Universe. One of these, alluded to above, is that dark energy affects how the Universe expands. Measuring how the size of the Universe has changed over time is one way scientists can determine the quantity and nature of dark energy. Another consequence we can see is that, since dark energy competes with the gravity of dark matter, changing the amount and properties of dark energy will change the evolution of structure and therefore the number of galaxies and clusters of galaxies we expect to see at different times. Current and future astronomical surveys can help determine the nature of dark energy by observing these two effects.

Probes of dark energy

Finding, counting, and weighing clusters is one way to measure the amount of clumpiness in the Universe and probe dark energy. The best means of weighing clusters is to observe how the light of background objects is bent around them, a technique called gravitational lensing. The more massive the cluster, the more the light bends. Clusters also contain about the same ratio of regular matter to dark matter as the Universe as a whole. This regular matter can be measured using X-rays, providing another probe of the Universe’s components, and helping to distinguish between the effects of dark matter and dark energy.

Gravitational lensing effects are not limited to galaxy clusters. The structure of the matter in the Universe at all scales bends the light of distant galaxies. Using this effect, called cosmic shear, scientists can create a 3D map of the matter in the Universe. The clumpiness of this map, and in particular how the clumpiness of the map changes with distance (and therefore time), can tell us about dark energy and how it evolves.

Combining probes such as cosmic shear and galaxy clusters (and several others), allows one to measure the expansion history and growth of cosmic structures simultaneously to develop a more consistent theory of dark energy. Using multiple probes also helps prevent any single probe’s weakness from affecting the analysis.


One goal of cosmologists is to develop a theory of how the dark energy fluid behaves in detail. The simplest description of dark energy is that it’s the vacuum energy of empty space (in quantum field theory, a vacuum is never really empty, but instead consists of particles and anti-particles continuously emerging and annihilating). This explanation is consistent with the observed properties of dark energy, but for one caveat: the most direct predictions suggest there should be an absurd factor of 10120 times more vacuum energy than we actually measure! For comparison, there are only about 1080 electrons in the entire observable Universe.

One possible solution to this conundrum is known as the anthropic principle. The idea is that maybe the quantity of vacuum energy could be different in different pockets of a “multiverse,” one of which is our own Universe. Some calculations suggest that complex life can only form in pockets with particular values of the vacuum energy. The fact that we observe a small amount of vacuum energy may simply be related to the fact that we (that is, humans) are around to observe it. Another possibility, however, is that we just haven’t been clever enough yet to develop a physical mechanism to explain the small amount of vacuum energy.

New observations

A few weeks ago, an ambitious new project called the Dark Energy Survey (DES) began taking data. Over the next five years, this survey will image about 1/8th of the sky to sensitivities never before possible over such a wide footprint. DES will weigh one hundred thousand galaxy clusters and capture data from about two hundred million galaxies to use for cosmic shear calculations. The completed survey will deliver an unprecedented amount of information related to dark energy and the composition of the Universe.


You can watch all the talks in this session on the KIPAC youtube channel.

You can also read more about KIPAC@10 on the conference blog home page.