Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

Reconciliation required that either those distant supernovas behaved unlike their nearer brethren, or they were as much as fifteen percent farther away than where the prevailing cosmological models had placed them.

Standard candles simplify calculations immensely: since the supernovas all have the same wattage, the dim ones are far away and the bright ones are close by.

But there’s a second way to measure the distance to galaxies: their speed of recession from our Milky Way—recession that’s part and parcel of the overall cosmic expansion. As Hubble was the first to show, the expanding universe makes distant objects race away from us faster than nearby ones. So, by measuring a galaxy’s speed of recession (another simple task), one can deduce a galaxy’s distance.

It turned out that the supernovas were splendid standard candles, surviving the careful scrutiny of many skeptical investigators, and so astrophysicists were left with a universe that had expanded faster than we thought, placing galaxies farther away than their recession speed would have otherwise indicated. And there was no easy way to explain the extra expansion without invoking lambda, Einstein’s cosmological constant.

Here was the first direct evidence that a repulsive force permeated the universe, opposing gravity, which is how and why the cosmological constant rose from the dead. Lambda suddenly acquired a physical reality that needed a name, and so “dark energy” took center stage in the cosmic drama, suitably capturing both the mystery and our associated ignorance of its cause.

The most accurate measurements to date reveal dark energy as the most prominent thing in town, currently responsible for 68 percent of all the mass-energy in the universe; dark matter comprises 27 percent, with regular matter comprising a mere 5 percent.

The shape of our four-dimensional universe comes from the relationship between the amount of matter and energy that lives in the cosmos and the rate at which the cosmos is expanding.

Since both mass and energy cause space-time to warp, or curve, omega tells us the shape of the cosmos. If omega is less than one, the actual mass-energy falls below the critical value, and the universe expands forever in every direction for all of time, taking on the shape of a saddle, in which initially parallel lines diverge. If omega equals one, the universe expands forever, but only barely so. In that case the shape is flat, preserving all the geometric rules we learned in high school about parallel lines. If omega exceeds one, parallel lines converge, and the universe curves back on itself, ultimately recollapsing into the fireball whence it came.

Ω = 0.3. As far as observers were concerned, the universe was “open” for business, riding a one-way saddle into the future.

A fundamental by-product of this update to the big bang was that it drives omega toward one.

the update predicted three times as much mass-energy as observers could find.

Omega does equal one, just as the theorists demanded of the universe, even though you can’t get there by adding up all the matter—dark or otherwise—as they had naively presumed. There’s no more matter running around the cosmos today than had ever been estimated by the observers. Nobody had foreseen the dominating presence of cosmic dark energy, nor had anybody imagined it as the great reconciler of differences.

The closest anybody has come is to presume dark energy is a quantum effect—where the vacuum of space, instead of being empty, actually seethes with particles and their antimatter counterparts. They pop in and out of existence in pairs, and don’t last long enough to be measured. Their transient existence is captured in their moniker: virtual particles. The remarkable legacy of quantum physics—the science of the small—demands that we give this idea serious attention. Each pair of virtual particles exerts a little bit of outward pressure as it ever so briefly elbows its way into space.

Whatever dark energy turns out to be, we already know how to measure it and how to calculate its effects on the past, present, and future of the cosmos.

As a consequence, anything not gravitationally bound to the neighborhood of the Milky Way galaxy will recede at ever-increasing speed, as part of the accelerating expansion of the fabric of space-time. Distant galaxies now visible in the night sky will ultimately disappear beyond an unreachable horizon, receding from us faster than the speed of light. A feat allowed, not because they’re moving through space at such speeds, but because the fabric of the universe itself carries them at such speeds. No law of physics prevents this.

hydrogen

Out of the ninety-four naturally occurring elements, hydrogen lays claim to more than two-thirds of all the atoms in the human body, and more than ninety percent of all atoms in the cosmos, on all scales, right on down to the solar system.

Helium

One of the pillars of big bang cosmology is the prediction that in every region of the cosmos, no less than about ten percent of all atoms are helium, manufactured in that percentage across the well-mixed primeval fireball that was the birth of our universe.

Lithium

Another prediction of big bang cosmology is that we can expect no more than one percent of the atoms in any region of the universe to be lithium.

carbon

oxygen

Both oxygen and carbon are major ingredients of life as we know it.

Silicon

In the end, we expect carbon to win because it’s ten times more abundant than silicon in the cosmos. But that doesn’t stop science fiction writers, who keep exobiologists on their toes, wondering what the first truly alien, silicon-based life forms would be like.

sodium

Turns out, while all light pollution is bad for astrophysics, the low-pressure sodium lamps are least bad because their contamination can be easily subtracted from telescope data.

Aluminum

Polished aluminum makes a near-perfect reflector of visible light and is the coating of choice for nearly all telescope mirrors today.

Titanium is 1.7 times denser than aluminum, but it’s more than twice as strong. So titanium, the ninth most abundant element in Earth’s crust, has become a modern darling for many applications, such as military aircraft components and prosthetics that require a light, strong metal for their tasks.

iron

most important element in the universe.

iron’s odd distinction comes from having the least total energy per nuclear particle of any element. This means something quite simple: if you split iron atoms via fission, they will absorb energy. And if you combine iron atoms via fusion, they will also absorb energy.

gallium

technetium

osmium

platinum,

ir...

Iridium is also the world’s most famous smoking gun. A thin layer of it can be found worldwide at the famous Cretaceous-Paleogene (K-Pg) boundary† in geological strata, dating from sixty-five million years ago.

Iridium is rare on Earth’s surface but relatively common in six-mile metallic asteroids, which, upon colliding with Earth, vaporize on impact, scattering their atoms across Earth’s surface.

einsteinium

Phosphorus

Selenium

palladium

mercury,

Thorium

uranium

With ninety-two protons packed in its nucleus, uranium is widely described as the “largest” naturally occurring element, although trace amounts of larger elements can be found naturally where uranium ore is mined.

neptunium