Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe

by Lisa Randall

Ten times more bacterial cells than human cells live inside us and help with our survival.

dark matter—though an insignificant fraction of our bodies—accounts for about 85 percent of the matter in the Universe.

However, when large amounts of dark matter aggregate into concentrated regions, its net gravitational influence is substantial, leading to measurable influences on stars and on nearby galaxies.

The big lesson of physics over the centuries is how much is hidden from our view.

Dark matter is not dark—it is transparent. Dark stuff absorbs light. Transparent things, on the other hand, are oblivious to it. Light can hit dark matter, but neither the matter nor the light will change as a result.

Dark energy is not matter—it is just energy. Dark energy exists even if no actual particle or other form of stuff is around. It permeates the Universe, but doesn’t clump like ordinary matter. The density of dark energy is the same everywhere—it can be no denser in one region than another.

Dark energy also remains constant over time. Unlike matter or radiation, dark energy does not become more dilute when the Universe expands. This is in some respects its defining property. The dark energy density—energy not carried by particles or matter—remains the same over time. For this reason, physicists often refer to this type of energy as a cosmological constant.

Early in the Universe’s evolution, most of the energy was carried by radiation. But radiation dilutes more quickly than matter so matter took over eventually as the largest energy contribution. Much later in the Universe’s evolution, dark energy—which never diluted whereas both radiation and matter did—came to dominate and now constitutes about 70 percent of the Universe’s energy density.

Perhaps one can say that physicists studying “the dark” are participating in a Copernican revolution in a more abstract form. Not only is the Earth not physically the center of the Universe, but our physical makeup is not central to its energy budget—or even to most of its matter.

To account for all that extra matter, Zwicky proposed the existence of what he named dunkle Materie, which is German for dark matter and sounds either more ominous or sillier depending on how you pronounce it.

Pie chart illustrating the relative amounts of energy stored in ordinary matter (atoms), dark matter, and dark energy. Notice that dark matter is 26 percent of the total energy density, but makes up about 85 percent of the energy of matter because matter includes only the atom and dark matter contributions, but not that of dark energy.

No one can experimentally validate or rule out conjectures that apply beyond the horizon.

“Nothing” is so special that without an underlying reason, you wouldn’t expect it to characterize the state of the Universe.

You might not always find what you are looking for, but you don’t randomly find nothing.

This is possible only because matter dominates over antimatter—there is a matter-antimatter asymmetry.

But before you become too disappointed by this omission, let me reassure you that you would probably find any answer to this question unsatisfactory. Either the Universe was around an infinite length of time or it started at some particular time. Both answers can seem disturbing, but those are the options.

This is why cosmologists sometimes say that dark matter is cold, which means that it’s not hot and relativistic and doesn’t act like radiation.

where there is light, there is dark too.

About 50 tons of extraterrestrial material enters the Earth’s atmosphere every day, carried by millions of small meteoroids. And none of us are affected in any noticeable way.

My collaborators and I are chiefly theoretical particle physicists.

I’m generally wary of name changes, which in business or politics are often used to divert attention from the real issues.

An astrophysics colleague joked that planets—like senior faculty—clear nearby orbits. Dwarf planets would then be more like postdoctoral fellows, who work independently but nonetheless have offices close to the graduate students—who, like asteroids, are less well formed.

But the Universe is a complex place in which time, dedication, and technology are essential to successfully sorting out the nature of the objects it contains.

Greek meteoreon—meaning “high in the sky”—and logos—the word for “knowledge—weather studies had already usurped it.

You might be lucky and find a meteorite in the vicinity of a meteoroid impact, but you are more likely to see them in laboratories, museums, or the houses of sufficiently obsessive, lucky, or wealthy people.

If you ever have the opportunity to travel to the Italian city of Padua, be sure to visit the Scrovegni Chapel.

These trails of brightly shining dust and gas are the origin of their name, which comes from a Greek word that translates as “wearing long hair.”

This book is about the seemingly abstract stuff such as dark matter that I study, but it is also about the Earth’s relationships to its cosmic surroundings.

On the Origin of Ironmasses,

Bilbao, Spain, I was very lucky to have a physics colleague tell me about the Flysch Geoparque in the nearby town of Zumaia.

I was relieved when the interviewers were satisfied with my response, which was that students absorb ideas and process them to create new ones that they then send out into the world to restart the cycle—much as stars absorb interstellar material to create heavy elements, which they then eject back into space to be processed anew. When molecular material is expelled, dispersed throughout the interstellar medium, and aggregated in dense clouds where some fraction reenters star-forming regions, the distribution pattern is not entirely different from the creation, dissemination, and progression of ideas.

Our goal is to identify what we might be missing for lack of attention. Model builders like me try to imagine what might be out there that experimenters haven’t yet looked for or realized could be within their grasp.

I’ll let nature decide.

Xogenesis, playing with the idea of dark matter as the unknown quantity, X.

The best science should always encompass, or at the very least be consistent with, the broadest possible range of observations. The real question is what most effectively solves the entire set of unexplained phenomena.

My second concern about Occam’s Razor is just a matter of fact. The world is more complicated than any of us would have been likely to conceive.

Science too should have a properly set table—one that allows us to address the many phenomena that we observe. Although scientists tend to prefer simple ideas, they are rarely the whole story.

Just as a beer’s small-percentage alcohol content affects carousers far more than the rest of the drink, ordinary matter, though carrying a small percentage of the energy density, influences itself and its surroundings much more noticeably than something that just passes through.

What if the world of dark matter—if not equally rich—is reasonably wealthy too?

Yet most people persist in assuming that their perspective and their conviction of our importance are in keeping with the external world.

“What is the speed of dark?”

Dark objects or dark life could be very close—but if the dark stuff’s net mass isn’t very big, we wouldn’t have any way to know.

“Shadow life,” exciting as that would be, won’t necessarily have any visible consequences that we would notice, making it a tantalizing possibility but one immune to observations.

Radical departures should be accepted only when they explain phenomena that older ideas fail to accommodate. In only very rare instances are new ideas truly necessary to explain observations.

In today’s era of “big data,” the best places to look for distinctive dark matter properties could well be seemingly ordinary astronomical data sets.