All the recent data from the Planck space observatory and the Sloan Digital Sky Survey suggest there is just enough dark energy to continue the universe’s expansion, but not enough to keep it accelerating forever. This conclusion points toward the Big Freeze, or “heat death” of the universe. The most up-to-date science leads us to the conclusion that our universe—and Robert Frost’s—is more likely to end in ice than in fire. That, however, assumes that what we believe about dark energy is true. Considering that dark energy itself is a phenomenon cloaked deeply in mystery, such assumptions may yet prove untenable.
The entire observable universe is 93 billion light-years. The Milky Way is in the center, but it is too small to be seen in this diagram, above left.
What Is the Shape of the Universe?
Most casual observers would assume that the cosmos is a space that expands into infinity, but the answer is not as simple as gazing into a starry sky and hazarding a measurement. Einstein’s theory of general relativity, when paired with estimates of the relative amounts of matter and energy in the cosmos, allows for only one possible solution—the universe is infinite.
General relativity requires that the universe remain the same throughout (homogeneity) and appear the same in all directions (isotropy). Therefore, the shape of the universe is the result of the push and pull of gravity and dark energy. This may sound familiar. The same characteristics determine the universe’s three possible fates: the Big Crunch, the Big Rip, and the Big Chill.
Just as a universe with an energy density less than its gravitational pull will eventually collapse in on itself (the Big Crunch scenario), the same gravity will overcome dark energy to mold the universe into a sphere. A spherical universe implies that there is a finite amount of space (just as there is a finite amount of surface on a sphere), that two lines appearing parallel will eventually converge (just as lines of longitude on Earth converge as they approach the poles from the equator), and that by traveling far enough we can return to our original position.
Conversely, a universe with an energy density greater than its gravitational pull will exhibit the opposite geometry, better resembling a saddle than a sphere. In such a universe, the overwhelming force of dark energy pulls the universe into an inverted curve where initially parallel lines will gradually diverge. Much like the previous scenario, this universe is still finite.
However, just as cosmologists are fairly confident that the cosmos will not end its life in a Big Rip or Big Crunch, they are equally confident that the geometry of the universe is neither spherical nor saddle-shaped. When both gravity and dark energy reach a balance in their effect on the cosmos, the math implies that the universe will simply stretch out forever as an infinite flat plane. In this universe, two initially parallel lines remain parallel forever, and we will never be able to return to our starting point by traveling any distance in the same direction.
It is worth noting that confidence in this measurement depends on the correctness of Einstein’s assumptions about homogeneity and isotropy as well as the accuracy of the current understanding of dark matter. These assumptions underlie the standard models of cosmology, but should they prove even marginally inaccurate, we could be living in a much different universe indeed.
Can We Deflect Asteroids?
Will an asteroid hurtling through space some day crash into Earth and cause massive damage? The odds are small, but they’re real. Deflecting an incoming asteroid might seem like the stuff of science fiction, but scientists say it can be done—if the asteroid is detected in time. Given enough prior knowledge, former U.S. astronaut Ed Lu says, governments could launch one or more spacecraft into the threatening asteroid and change its path enough so that it would miss Earth. These “kinetic impactors,” Lu says, could even divert an asteroid the size of the one that brought down the dinosaurs.
Lu is one of the co-founders of the B612 Foundation, a non-profit organization that monitors asteroids and other Near Earth Objects (NEO) and studies how to protect Earth from them. Its goal is to fund the building and launch of a space telescope named Sentinel. The telescope, scheduled for launch in 2018, will map all the asteroids around Earth. Realizing the threat of an asteroid collision is real, in 2013 the United Nations called for the creation of the International Asteroid Warning Network. The goal is to bring together scientific organizations and nations with active space programs so they can share knowledge about the asteroid threat.
Why Is the Milky Way a Spiral?
The shape of our galaxy is nothing special. Among the other clusters of stars that can be easily observed from our corner of the universe, a few are blobby and egg-shaped, but more than two-thirds are so-called “disc galaxies” whose stars have settled into flat orbits, as if traveling along the surface of a giant vinyl record. Almost every disc galaxy looks at least a bit like ours, with stars that group together into spiral arms.
What causes the spirals? “A galaxy is constantly bombarded by satellite galaxies,” says Chris Purcell of West Virginia University. When one galaxy passes by or through another, the resulting forces can send a shock wave throughout its structure, bunching stars together in spindly shapes that rotate around the center. “It’s essentially a vibration that travels gravitationally throughout the disc,” Purcell explains. As a galaxy ages, these perturbations tend to mount, and the disc goes from being thin, circular, and relatively homogeneous to thicker and more distorted. It’s a natural process, says Purcell: “These galaxies are not only trying to turn themselves into spirals; they are constantly getting banged into by things that are turning them into spirals.”
The Milky Way would seem to be somewhat early in the process, as evidenced by its slender figure. But things are changing: Several of the other, smaller galaxies are now (on a cosmological timescale) bumping up against us. One of these is the Sagittarius Dwarf. “It turns out that it’s on the opposite side of the galaxy from us,” Purcell says, “and so it’s hitting the disc from underneath.” Purcell’s simulations suggest that these collisions could account for the spiral that we see today.
An even more extreme collision could be in our future. “We’re on our first in-fall toward Andromeda,” Purcell warns. “It’s going to destroy both discs and turn the entire system into an elliptical blob.” But let’s not get ahead of ourselves: That crash is still more than a billion years off.
Wrapped up in our own daily trials, it’s hard to imagine life outside of earth, let alone in an alternative universe. But according to some scientists, there may be an infinite number of alternative universes, also called multiverses, and finding them is not only possible, but probable.
Will We Find Other Universes?
There are several theories of the multiverse. One comes from the “many worlds” interpretation of quantum physics by Hugh Everett. In 1955, over a bottle of sherry while a student at Princeton University, Everett considered the implications of quantum physics. At the elementary level (protons and electrons), each particle exists in a superposition of different locations, velocities, and orientations of its spin, but when measured by scientists there is a definitive result. Somehow our unique world emerges in a system that has a multitude of possibilities at the quantum, or nanoscopic, level. In this theory, every possible outcome in the universe exists simultaneously in other universes. For example, if you shoot a basketball and miss, there is a parallel reality in which your basketball slides right through the net. This alternative universe doesn’t occupy a physical space, but is instead a co-existing, abstract reality. However, for many physicists, understanding and proving this alternative reality is too far afield.
Another type of multiverse is conceivable through a theory called inflation. In the first moments after the Big Bang, the universe expanded exponentially, traveling faster than the speed of light. Some theorists suggest that random quantum fluctuations in the early universe caused this inflation to stop in some regions but not in others. In places where inflation stopped, pocket universes formed, where atoms, star
s, and even planets could assemble. Our universe may even be one of the myriad of pocket universes. Recently, this theory gained momentum as physicists behind the Bicep2 telescope in Antarctica found ripples in the space-time fabric of the cosmos called gravitational waves. The unique pattern in the sky reinforced the inflation theory.
But traveling to one of these alternate universes may be impossible. Each pocket universe would exist as a bubble, with its own laws of physics. The bubbles are connected, but in between them, eternal inflation is still stretching space-time faster than the speed of light. Even if we could somehow travel faster than light, the journey would be rough. As Anthony Aguirre, a physicist at the University of California at Santa Cruz, explains, “You also have to survive the inflation in between that would want to inflate every atom in your body. It’s not very practical.”
Could We Live on Mars?
What happens when the human population outpaces the resources of our planet? Many people wonder if moving humankind to another planet is possible. Scientists agree that of all the other planets in our solar system, Mars would be the most habitable. But that’s not saying much. If the goal is to create a self-sustaining Martian world, life will be difficult and dangerous.
Mars has some similarities to Earth. Its axial tilt is about the same, so Mars experiences similar seasons; however, its orbital eccentricity is much larger, so the length of the seasons varies and a year lasts almost twice as long as on Earth. The length of a day is about the same. The desert terrain is similar to some regions on Earth. However, despite these similarities, Mars is a completely hostile environment. There is no breathable air and very little air pressure. Lower gravity presents problems for prolonged settlement. Temperatures vary widely: While they may climb as high as 70 degrees Fahrenheit (21 degrees Celsius), in some places they drop as low as minus 225 degrees (-142 degrees Celsius). The soil is toxic, and radiation from the Sun is deadly. All this may seem a little too out-of-this-world, but some scientists and a few entrepreneurs hope to make the dream of life on Mars a reality.
Technology will be a major player in a successful Mars settlement. Residents will require constant pressurized and heated environments. Luckily, the planet provides a few raw materials, such as soil, to make concrete. Mars is home to several large caves, which would screen settlers from radiation. Residents may be able to grow plants after removing harsh chemicals from the toxic soil. Water on the fourth planet from the Sun is available, but the atmosphere is too thin for liquid water to exist for long. Instead, water is trapped just under the surface of the polar regions. Extracting water would be vital for drinking, growing food, and producing oxygen.
Space is opening up to the private sector, and a few companies are taking one small step toward life on Mars. Elon Musk, the founder of SpaceX, a space exploration and technology company, aims to build a colony of 80,000 people. SpaceX announced plans to put humans on Mars as early as 2026, 10 years ahead of NASA. But getting to Mars isn’t as difficult as landing, surviving on the planet, or even returning to Earth. Musk told CNBC, “the thing that matters long term is to have a self-sustaining city on Mars, to make life multiplanetary,” indicating that Mars could be a refuge in case we outgrow our current planet. Given Earth’s dwindling resources, that could be sooner than we think.
The shadowy ring in this galaxy cluster, captured by the Hubble Space Telescope, is evidence of dark matter, a mysterious substance that pervades the universe.
What Is Dark Matter Made Of
As far back as the 1930s, evidence for the existence of a “dark matter” in the universe began to emerge.
Swiss astronomer Fritz Zwicky measured the velocities of several galaxies in the Coma cluster, a group of more than 1,000 identified galaxies, and concluded that many of them were moving so fast that they should have escaped the gravitational pull of the other galaxies. Zwicky, and other astronomers noticing the same phenomenon, concluded “that something we have yet to detect is providing these galaxies with additional mass, which generates the extra gravity they need to stay intact. This “something” is invisible—hence the nickname “dark matter.”
But exactly what is dark matter, and what is it made of?
NASA notes that we’re “more certain what dark matter is not than we are what it is.” Dark matter does not take the form of stars and planets we can see, yet it constitutes about 27 percent of all the matter in the universe. It is not made of baryonic matter, the protons, electrons, and neutrons that make up regular space matter such as stars, planets, rocks, and gas clouds. It does not absorb, emit, or reflect light—the very reason it is extremely difficult to see. In fact, we can only infer its existence based on its gravitational effects on the motions of galaxies and stars.
So what is dark matter made of? The most common view is that dark matter is composed of weakly interacting massive particles, or WIMPS. These particles interact weakly with baryonic matter via gravity. WIMPS have as much as 100 times the mass of a proton, but their weak interactions with baryonic matter make them nearly impossible to see.
Other nonbaryonic candidates include neutralinos, hypothetical heavy particles; the smaller neutrinos, subatomic particles without charge; and photinos, a hypothetical subatomic particle. Some scientists believe that dark matter may be composed of bodies of baryonic matter that emit little light and drift through space unattached to any single solar system. Because they emit no light, these bodies, called massive compact halo objects, or MACHOs, would be difficult to detect.
A clearer understanding of the composition of dark matter could help scientists better understand the nature of our universe—especially, how galaxies hold together.
A gamma ray burst is the brightest event in the universe, theoretically visible from billions of light-years away.
What Causes Gamma Ray Bursts
Imagine a single blast of energy powerful enough to destroy the equivalent of a thousand Earths in a second. Explosions of that magnitude happen in the universe every day, thanks to gamma ray bursts. Scientists didn’t know these extreme bursts of energy existed until the 1960s, when satellites designed to monitor nuclear weapons tests on Earth picked up the phenomenon.
A long the electromagnetic spectrum of energy—which includes radio waves, ultraviolet waves, and visible light—gamma rays are the most powerful. A gamma ray burst is a focused stream of energy that can last from just a few seconds to several minutes. Just one 10-second burst releases more energy than our Sun will produce over its 10-billion-year lifetime. Most bursts occur outside the Milky Way in galaxies with many massive stars.
Today scientists have two main theories to explain what might cause a gamma ray burst. One idea involves neutron stars. If two of these massively dense stars orbit each other and their orbits start to decay because of gravitational pull, they collide. That collision creates a black hole. Before some of the stars’ matter tumbles into the black hole, it releases energy that some scientists think produces a gamma ray burst.
The second theory gives hypernovas credit for the bursts. The “death” of a star with a mass 10 times greater than the Sun’s creates an explosion called a supernova. The death of an even more massive star creates a hypernova explosion. Hypernovas might cause some gamma ray bursts. Scientists also consider that both theories might be accurate: Neutron star collisions create short bursts, and hypernovas create longer ones. Or another process the scientists haven’t considered could explain all the bursts.
Knowing what causes gamma ray bursts may not be as important as understanding how they could affect Earth. In 2014, astronomers Tsvi Piran and Raul Jimenez calculated fairly high odds that a past gamma ray burst caused a mass extinction of life on Earth. Long-ago bursts might also explain why humans have not yet found life on other planets.
Next stop: 11,740,608th floor. Housewares! Lingerie! Geostationary orbit!
Is a Space Elevator Possible
New developments in nanotechnology have scientists hopeful that the idea of a space elevator
should no longer be relegated to the imagination of futurists and sci-fi authors. In fact, several organizations, including NASA and Google X, have recently investigated this latest vision of rocket-free space flight.
It is technically possible that we can use Earth’s own rotation as a means to deliver people and cargo to orbit. By constructing a tether 60,000 to 90,000 miles (96,000 km to 145,000 km)long, with a counterweight at one end and the other anchored to a point along the equator, we could theoretically use Earth’s approximately 1,000 miles per hour (1,609 km/h) rotational velocity to keep the tether suspended (just as a rope with a rock tied at one end remains taut as you spin it around). This tether could then act as a conduit for sending elevators up and down. Any cargo released at a high point along the elevator would remain in orbit.
While theoretically possible, there are several problems with this approach. The biggest is tensile strength: A long enough cable will break under its own weight. Steel cables falter under their own weight at about 15 miles (24 km); Kevlar can hold up at about 10 times that length, but still well short of the 60,000 miles (96,000 km) required for a space elevator. Materials engineers are placing their hopes in carbon nanotubes, tiny carbon structures that, when woven together, exhibit enormous tensile strength.
100 Mysteries of Science Explained Page 4
