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

by Lisa Randall

At this point, we know only that dark matter and ordinary matter interact via gravity.

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

Dark matter passes right through our bodies—and resides in the outside world as well.

billions of dark matter particles pass through each of us every second. Yet no one notices that they are there. The effect of even billions of dark matter particles on us is minuscule.

Dark matter doesn’t interact with light—at least to the extent that people have been able to probe so far. Dark matter is not made out of the same material as ordinary matter—it’s not composed of atoms or the familiar elementary particles that do interact with light, which is essential to everything we can see.

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.

Dark matter affects the expansion of the Universe, the path of light rays passing to us from distant objects, the orbits of stars around the centers of galaxies, and many other measurable phenomena in ways that convince us of its existence.

Without dark matter, there wouldn’t have been enough time to form the structure that we now observe. Clumps of dark matter seeded the Milky Way galaxy—as well as other galaxies and galaxy clusters. Had galaxies not formed, neither would have the stars, nor the Solar System, nor life as we know it.

Electromagnetic radiation appears as light at visible frequencies, but can also appear as radio waves or ultraviolet radiation, for example, when outside the limited range of frequencies that we can see.

Since human senses are all based in electromagnetic interactions of some sort, we can’t directly detect dark matter in the usual ways.

Dark energy is not matter—it is just energy.

It is very different from dark matter, which collects into objects and will be denser in some places than in others.

Dark energy also remains constant over time. Unlike matter or radiation, dark energy does not become more dilute when the Universe expands.

physicists often refer to this type of energy as a cosmological constant.

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.

Without knowing the answer in advance, physicists would have estimated that the dark energy density should be an astounding 120 orders of magnitude bigger. The question of the small size of the cosmological constant has flummoxed physicists for years.

Since these velocities are determined by the mass inside a galaxy, they serve as proxies for a measurement of a galaxy’s mass.

The dark matter between an emitting object and the viewer bends light.

electrically neutral bound states of positively charged nuclei and negatively charged electrons—didn’t exist. Electrons and nuclei could stably combine into atoms only after the temperature dropped below the atomic binding energy.

Above that temperature, radiation would separate protons and electrons and would thereby blow atoms apart. Because of all the charged particles early on, radiation permeating the Universe initially didn’t travel freely. Instead, it scattered off the many charged particles that the early Universe contained.

This was early enough in the Universe’s lifetime—about 380,000 years after the Big Bang—that structure hadn’t yet formed.

The amount of dark matter determined the strength of the gravitational potential that pulled stuff in, resisting the outward force of the radiation. This influence helped shape the oscillations, allowing astronomers to measure the total dark matter energy density that was present at the time.

In an even more subtle effect, dark matter also influenced how much time elapsed between when matter began to collapse (which happens when the energy density in matter exceeds that in radiation) and the time of recombination—when stuff began to oscillate.

In other words, dark matter carries five times the energy of ordinary matter, meaning it carries 85 percent of the energy of matter in the Universe.

Because dark matter and ordinary matter affect in different ways the perturbations in the radiation background—the radiation that survives today from the time of the Big Bang—data about this background confirmed the existence of dark matter and furthermore measured how much of it is present.

However, dark energy—the mysterious form of energy that the previous chapter discussed, which is present in the Universe but not carried by any form of matter—influences these fluctuations too.

Type Ia supernovae result from the nuclear explosions of white dwarfs, which are the seemingly innocuous end states of some stars’ evolution after they have burned up all the hydrogen and helium at their core via thermonuclear fusion.

Because the white dwarfs that explode to produce them all have the same mass, type Ia supernovae all shine with about the same brightness, making them what astrophysicists refer to as standard candles.†

So if astronomers measure both the speed with which a galaxy is receding and the galaxy’s luminosity, they can determine the expansion rate of the Universe in which the galaxy is carried, as well as how far away the galaxy is. Armed with this information, they can determine the Universe’s expansion as a function of time.

Their observations led to the remarkable conclusion that some unanticipated energy source was accelerating the rate of the Universe’s expansion. Dark energy fits the bill, since its gravitational influence makes the Universe expand at an increasingly rapid rate over time. Along with measurements in the cosmic microwave background, the supernovae measurements established the existence of dark energy.

all measurements to date agree with predictions.

Pythagoras of Samos in the sixth century B.C. might have been the first to use kosmos to apply to the Universe.

Cosmology is about big inquiries—nothing less than how the Universe began and subsequently developed into its current state.

The origin of the asymmetry remains one of the important unsolved problems in cosmology.

Unlike its very beginning, which no known theory can describe, the Universe’s evolution only a tiny fraction of a second after its beginning hewed to established laws of physics.

Earlier in the Universe, when things were denser and gravitational effects were stronger, the Universe expanded far more rapidly than it does today.

To convert to what is perhaps a slightly more familiar measure of distance, a parsec is about 3.3 light-years.

The age of the Universe is now known to within a couple of hundred million years, and measurements have continued to improve. When I wrote my first book it was 13.7 billion years old but we now believe it to be a bit older—13.8 billion years from the so-called Big Bang.

According to the Big Bang theory, the very early Universe originated 13.8 billion years ago as a hot, dense fireball consisting of many interacting particles with temperature higher than a trillion trillion degrees.

Physicists call the hot, dense gas that filled the Universe in its early stages radiation. For cosmological purposes, radiation is defined as anything that moves at relativistic speeds, which means at or very close to the speed of light.

To count as radiation, objects have to possess so much momentum that their energy far exceeds the energy stored in their mass.

As space expanded, the radiation and particles that permeated the Universe became more dilute and cooled down. They behaved like hot air trapped inside a balloon, which becomes less dense and cooler as the balloon expands. Because each energy component’s gravitational influence affects the expansion differently, the study of the Universe’s expansion over time allows astronomers to disentangle the separate contributions of radiation, matter, and dark energy. Matter and radiation dilute with the expansion but radiation, which redshifts to lower energy—much like a siren decreases in frequency as it moves away—dilutes even more rapidly than matter. Dark energy, on the other hand, doesn’t dilute at all.

The residual amounts of different elements were set by the relative number of protons and neutrons as well as by how quickly the required physical processes took place relative to the speed at which the Universe expanded. So the predictions of nucleosynthesis (as this process is known) test the theory of nuclear physics as well as the details of the Big Bang expansion. In a significant confirmation of both the Big Bang theory and nuclear physics, observations agree with predictions spectacularly well.

The Universe was originally filled with both charged and uncharged particles. But at this later time, the Universe had cooled sufficiently that positively charged nuclei combined with negatively charged electrons to form neutral atoms. From that time forward, the Universe consisted of neutral matter, which is matter that carries no electric charge.

In the absence of charged matter to deflect them, photons could traverse the Universe unhindered. This meant that radiation and light from the early Universe could reach us directly, essentially independent of any more complicated evolution in the Universe that might occur later on. The background radiation we see today is the radiation that existed 380,000 years into the Universe’s evolution.

Numerical simulations confirm these predictions on the largest scales, with dark matter correctly accounting for the density and shape of structure in the Universe.

Dark matter forms a diffuse spherical halo, whereas ordinary matter can collapse into a disk, such as the familiar disk of stars of the Milky Way plane.

Because angular momentum is conserved, gaseous regions cannot collapse as efficiently in the radial direction (as defined by rotation) as in the vertical one.

Like the other hundred billion stars in the Milky Way disk, the Sun circles around the galaxy at a speed of about 220 kilometers per second. At this speed, it takes about 240 million years for the Sun to orbit the galactic center. Since the galactic plane is less than 10 billion years old, the stars in the plane have made less than 50 revolutions in that time. It’s enough time for the system to homogenize some gross features, but really it’s not all that many trips around.

About 50 tons of extraterrestrial material enters the Earth’s atmosphere every day, carried by millions of small meteoroids.