Knocking on Heaven's Door - Lisa Randall [63]
We know dark matter exists because of its gravitational effects. But without seeing it directly, we won’t know what it is. Is it composed of many tiny identical particles? If so, what is the particle’s mass and how does it interact?
We might, however, soon learn much more. Remarkably, the LHC might in fact have the right energy to make particles that could be the dark matter. The key criterion for dark matter is that the universe contains the right amount to exert the measured gravitational effects. That is, the relic density—the amount of stored energy that our cosmological models predict survives to this day—has to agree with that measured value. The surprising fact is that if you have a stable particle whose mass corresponds to the weak energy scale that the LHC will explore (again via E = mc2) and whose interactions also involve particles with that energy, its relic density will be in the right ballpark to be dark matter.
The LHC could therefore not only give us insights into particle physics questions, but also give us clues to what is out there in the universe today and how it all began, questions that are incorporated into the science of cosmology, which tells us how the universe has evolved.
As with the elementary particles and their interactions, we understand a surprising amount about the universe’s history. Yet also as with particle physics, some very big questions remain. Chief among these difficult questions are these: What is the dark matter?, What is the even more mysterious entity called dark energy?, and What drove a period of exponential expansion of the early universe known as cosmological inflation?
Today is a tremendous time for observations that might tell us the answers to these questions. Dark matter investigations are at the forefront of the overlap between particle physics and cosmology. Dark matter’s interactions with ordinary matter—matter we can make detectors from—are extremely weak, so weak that we are still looking for any evidence of dark matter aside from its gravitational effects.
Current searches therefore rely on the leap of faith that dark matter, despite its near invisibility, nonetheless interacts weakly—but not impossibly weakly—with matter that we know. This isn’t merely a wishful guess. It’s based on the calculation mentioned above that shows that stable particles whose interactions are connected to the energy scale that the LHC will explore have the right density to be dark matter. We hope that even though we haven’t yet identified dark matter, we have a good chance of detecting it in the near future.
However, most cosmology experiments don’t take place at accelerators. Dedicated outward-looking experiments on Earth and out in space are primarily responsible for addressing and advancing our understanding of potential solutions to cosmological questions.
For example, astrophysicists have sent satellites into space to observe the universe from an environment not obscured by dust and physical and chemical processes on or near the Earth’s surface. Telescopes and experiments here on Earth give us additional insights in an environment scientists can more directly control. These experiments in space and on Earth are poised to shed light on many aspects of how the universe has come to be.
We’re hoping that a sufficiently strong signal in any of these experiments (which we will describe in Chapter 21) will let us decipher the mysteries of dark matter. These experiments could tell us the nature of dark matter and illuminate its interactions and mass. In the meantime, theorists are thinking hard about all possible models of dark matter and how to use all these detection strategies to learn what dark matter really is.
DARK ENERGY
Ordinary matter and dark matter still do not provide the sum total of the energy in the universe