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DARK ENERGY

Dark energy is even more mysterious, and its discovery in the 1990's was a complete shock to scientists. Previously, physicists had assumed that the attractive force of gravity would slow down the expansion of the universe over time. But when two independent teams tried to measure the rate of deceleration, they found that the expansion was actually speeding up. One scientist likened the finding to throwing a set of keys up in the air expecting them to fall back down-only to see them fly straight up toward the ceiling.

Scientists now think that the accelerated expansion of the universe is driven by a kind of repulsive force generated by quantum fluctuations in otherwise "empty" space. What's more, the force seems to be growing stronger as the universe expands. For lack of a better name, scientists call this mysterious force dark energy.

Unlike for dark matter, scientists have no plausible explanation for dark energy. According to one idea, dark energy is a fifth and previously unknown type of fundamental force called quintessence, which fills the universe like a fluid.

Many scientists have also pointed out that the known properties of dark energy are consistent with a cosmological constant, a mathematical Band-Aid that Albert Einstein added to his theory of general relativity to make his equations fit with the notion of a static universe. According to Einstein, the constant would be a repulsive force that counteracts gravity, keeping the universe from collapsing in on itself. Einstein later discarded the idea when astronomical observations revealed that the universe was expanding, calling the cosmological constant his "biggest blunder."

Now that we see the expansion of the universe is accelerating, adding in dark energy as a cosmological constant could neatly explain how space-time is being stretched apart. But that explanation still leaves scientists clueless as to why the strange force exists in the first place.

DARK MATTER

A simulation of the dark matter distribution in the universe 13.6 billion years ago

The visible universe—including Earth, the sun, other stars, and galaxies—is made of protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, orbaryonic, matter makes up less than 5 percent of the mass of the universe.

The rest of the universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent).

Scientists have not yet observed dark matter directly. It doesn't interact with baryonic matter and it's completely invisible to light and other forms of electromagnetic radiation, making dark matter impossible to detect with current instruments. But scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.

For instance, according to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a galaxy's visible matter is concentrated. But observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk. This puzzling result makes sense if one assumes that the boundary stars are feeling the gravitational effects of an unseen mass—dark matter—in a halo around the galaxy.

Dark matter could also explain certain optical illusions that astronomers see in the deep universe. For example, pictures of galaxies that include strange rings and arcs of light could be explained if the light from even more distant galaxies is being distorted and magnified by massive, invisible clouds of dark matter in the foreground-a phenomenon known as gravitational lensing.

Scientists have a few ideas for what dark matter might be. One leading hypothesis is that dark matter consists of exotic particles that don't interact with normal matter or light but that still exert a gravitational pull. Several scientific groups, including one at CERN's Large Hadron Collider, are currently working to generate dark matter particles for study in the lab.

Other scientists think the effects of dark matter could be explained by fundamentally modifying our theories of gravity. According to such ideas, there are multiple forms of gravity, and the large-scale gravity governing galaxies differs from the gravity to which we are accustomed.

BLACK HOLES

Black holes are the cold remnants of former stars, so dense that no matter—not even light—is able to escape their powerful gravitational pull.

While most stars end up as white dwarfs or neutron stars, black holes are the last evolutionary stage in the lifetimes of enormous stars that had been at least 10 or 15 times as massive as our own sun.

When giant stars reach the final stages of their lives they often detonate in cataclysms known as supernovae. Such an explosion scatters most of a star into the void of space but leaves behind a large "cold" remnant on which fusion no longer takes place.

In younger stars, nuclear fusion creates energy and a constant outward pressure that exists in balance with the inward pull of gravity caused by the star's own mass. But in the dead remnants of a massive supernova, no force opposes gravity—so the star begins to collapse in upon itself.

With no force to check gravity, a budding black hole shrinks to zero volume—at which point it is infinitely dense. Even the light from such a star is unable to escape its immense gravitational pull. The star's own light becomes trapped in orbit, and the dark star becomes known as a black hole.

Black holes pull matter and even energy into themselves—but no more so than other stars or cosmic objects of similar mass. That means that a black hole with the mass of our own sun would not "suck" objects into it any more than our own sun does with its own gravitational pull.

Planets, light, and other matter must pass close to a black hole in order to be pulled into its grasp. When they reach a point of no return they are said to have entered the event horizon—the point from which any escape is impossible because it requires moving faster than the speed of light.

Small But Powerful

Black holes are small in size. A million-solar-mass hole, like that believed to be at the center of some galaxies, would have a radius of just about two million miles (three million kilometers)—only about four times the size of the sun. A black hole with a mass equal to that of the sun would have a two-mile (three-kilometer) radius.

Because they are so small, distant, and dark, black holes cannot be directly observed. Yet scientists have confirmed their long-held suspicions that they exist. This is typically done by measuring mass in a region of the sky and looking for areas of large, dark mass.

Many black holes exist in binary star systems. These holes may continually pull mass from their neighbouring star, growing the black hole and shrinking the other star, until the black hole is large and the companion star has completely vanished.

Extremely large black holes may exist at the centre of some galaxies—including our own Milky Way. These massive features may have the mass of 10 to 100 billion suns. They are similar to smaller black holes but grow to enormous size because there is so much matter in the center of the galaxy for them to add. Black holes can accrue limitless amounts of matter; they simply become even denser as their mass increases.

Black holes capture the public's imagination and feature prominently in extremely theoretical concepts like wormholes. These "tunnels" could allow rapid travel through space and time—but there is no evidence that they exist.

OUR SOLAR SYSTEM

What Is The Solar System?

The Solar System is made up of all the planets that orbit our Sun. In addition to planets, the Solar System also consists of moons, comets, asteroids, minor planets, and dust and gas.

Everything in the Solar System orbits or revolves around the Sun. The Sun contains around 98% of all the material in the Solar System. The larger an object is, the more gravity it has. Because the Sun is so large, its powerful gravity attracts all the other objects in the Solar System towards it. At the same time, these objects, which are moving very rapidly, try to fly away from the Sun, outward into the emptiness of outer space. The result of the planets trying to fly away, at the same time that the Sun is trying to pull them inward is that they become trapped half-way in between. Balanced between flying towards the Sun, and escaping into space, they spend eternity orbiting around their parent star.

How did the Solar System formed?

This is an important question, and one that is difficult for scientists to understand. After all, the creation of our Solar System took place billions of years before there were any people around to witness it. Our own evolution is tied closely to the evolution of the Solar System. Thus, without understanding from where the Solar System came from, it is difficult to comprehend how mankind came to be.

Scientists believe that the Solar System evolved from a giant cloud of dust and gas. They believe that this dust and gas began to collapse under the weight of its own gravity. As it did so, the matter contained within this could begin moving in a giant circle, much like the water in a drain moves around the center of the drain in a circle.

At the center of this spinning cloud, a small star began to form. This star grew larger and larger as it collected more and more of the dust and gas that collapsed into it.

Further away from the center of this mass where the star was forming, there were smaller clumps of dust and gas that were also collapsing. The star in the center eventually ignited forming our Sun, while the smaller clumps became the planets, minor planets, moons, comets, and asteroids.

A Great Storm

Once ignited, the Sun's powerful solar winds began to blow. These winds, which are made up of atomic particles being blown outward from the Sun, slowly pushed the remaining gas and dust out of the Solar System.

With no more gas or dust, the planets, minor planets, moons, comets, and asteroids stopped growing. You may have noticed that the four inner planets are much smaller than the four outer planets. Why is that?

Because the inner planets are much closer to the Sun, they are located where the solar winds are stronger. As a result, the dust and gas from the inner Solar System was blown away much more quickly than it was from the outer Solar System. This gave the planets of the inner Solar System less time to grow.

Another important difference is that the outer planets are largely made of gas and water, while the inner planets are made up almost entirely of rock and dust. This is also a result of the solar winds. As the outer planets grew larger, their gravity had time to accumulate massive amounts of gas, water, as well as dust.

The Solar System Has Over 100 Worlds

It is true that there are only eight planets. However, the Solar System is made up of over 100 worlds that are every bit as fascinating. Some of these minor planets, and moons are actually larger than the planet Mercury!

Others, such as Io, have active volcanoes. Europa has a liquid water ocean, while Titan has lakes, rivers, and oceans of liquid Methane.

The Asteroid Belt & The Kuiper Belt

You have probably heard about the Asteroid Belt. This band of asteroids sits between the orbits of the planets Jupiter and Mars. It is made up of thousands of objects too small to be considered planets. Some of them no larger than a grain of dust, while others, like Eros can be more than 100 miles across. A few, like Ida, even have their own moons.

The Inner Ring is The Asteroid Belt
& The Outer Ring is The Kuiper Belt

Further out, beyond the orbit of the minor planet Pluto, sits another belt known as the Kuiper Belt. Like the Asteroid Belt, the Kuiper Belt is also made up of thousands, possibly even millions of objects too small to be considered planets. A few of these objects, like Pluto, are large enough that their gravity has pulled them into a sphere shape.

These objects are made out of mostly frozen gas with small amounts of dust. They are often called dirty snowballs. However, you probably know them by their other name... comets.

Every once in a while one of these comets will be thrown off of its orbit in the Kuiper Belt and hurled towards the inner Solar System where it slowly melts in a fantastic show of tail and light.

THE MILKY WAY

The Milky Way is a barred spiral galaxy, about 100,000 light-years across. If you could look down on it from the top, you would see a central bulge surrounded by four large spiral arms that wrap around it. The Milky Way contains two significant minor arms, as well as two smaller spurs. One of the spurs, known as the Orion Arm, contains the sun and the solar system. The Orion arm is located between two major arms, Perseus and Sagittarius.

The Milky Way does not sit still, but is constantly rotating. As such, the arms are moving through space. The sun and the solar system travel with them. The solar system travels at an average speed of 515,000 miles per hour (828,000 kilometers per hour). Even at this rapid speed, the solar system would take about 230 million years to travel all the way around the Milky Way.

Curled around the center of the galaxy, the spiral arms contain a high amount of dust and gas. New stars are constantly formed within the arms. These arms are contained in what is called the disk of the galaxy. It is only about 1,000 light-years thick. 

At the center of the galaxy is the galactic bulge. The heart of the Milky Way is crammed full of gas, dust, and stars. The bulge is the reason that you can only see a small percentage of the total stars in the galaxy. Dust and gas within it are so thick that you can't even peer into the bulge of the Milky Way, much less see out the other side.

Tucked inside the very center of the galaxy is a monstrous black hole, billions of times as massive as the sun. This supermassive  black hole may have started off smaller, but the ample supply of dust and gas allowed it to gorge itself and grow into a giant. The greedy glutton also consumes whatever stars it can get a grip on. Although black holes cannot be directly viewed, scientists can see their gravitational effects as they change and distort the paths of the material around it, or as they fire off jets. Most galaxies are thought to have a black hole in their heart. 

The bulge and the arms are the most obvious components of the Milky Way, but they are not the only pieces. The galaxy is surrounded by a spherical halo of hot gas, old stars and globular clusters. Although the halo stretches for hundreds of thousands of light-years, it only contains about two percent as many stars as are found within the disk.

Dust, gas, and stars are the most visible ingredients in the galaxy, but the Milky Way is also made up of dark matter. Scientists can't directly detect the material, but like black holes, they can measure it based on its effect on the objects around it. As such, dark matter is estimated to make up 90 percent of the mass of the galaxy.

GALAXIES

Galaxies are sprawling space systems composed of dust, gas, and countless stars. The number of galaxies cannot be counted—the observable universe alone may contain 100 billion. Some of these distant systems are similar to our own Milky Way galaxy, while others are quite different.

Galaxies with less than a billion stars are considered "small galaxies." In our own galaxy, the sun is just one of about 100 billion stars.

Galaxies are classified into three main types: spiral galaxies, elliptical galaxies, and irregular galaxies.

Spiral galaxies, such as the Milky Way, consist of a flat disk with a bulging center and surrounding spiral arms. The galaxy's disk includes stars, planets, dust, and gas—all of which rotate around the galactic center in a regular manner.

This spinning motion, at speeds of hundreds of kilometers per second, may cause matter in the disk to take on a distinctive spiral shape like a cosmic pinwheel. Some spiral galaxies obtain even more interesting shapes that earn them descriptive names, such as sombrero galaxies.

Elliptical galaxies are shaped as their name suggests. They are generally round but stretch longer along one axis than along the other. They may be nearly circular or so elongated that they take on a cigarlike appearance.

Elliptical galaxies contain many older stars, up to one trillion, but little dust and other interstellar matter. Their stars orbit the galactic center, like those in the disks of spiral galaxies, but they do so in more random directions.

The universe's largest known galaxies are giant elliptical galaxies, which may be as much as two million light-years long.

Galaxies that are not spiral or elliptical are called Irregular galaxies. Irregular galaxies appear misshapen and lack a distinct form, often because they are within the gravitational influence of other galaxies close by.

EXPANDING UNIVERSE

 In the 1920's, astronomer Edwin Hubble discovered the universe was not static. Rather, it was expanding, a find that revealed the universe was apparently born in a Big Bang.

After that, it was long thought the gravity of matter in the universe was certain to slow the expansion of the universe. Then, in 1998, the Hubble Space Telescope's observations of very distant supernovae revealed that a long time ago, the universe was expanding more slowly than it is today. In other words, the expansion of the universe was not slowing due to gravity, but instead inexplicably was accelerating. The name for the unknown force driving this accelerating expansion is dark energy, and it remains one of the greatest mysteries in science.

BIRTH OF THE SUN

The sun formed within a cloud of gas in a spiral arm of the Milky Way galaxy. A vast disk of gas and debris that swirled around this new star coalesced into planets, moons, and asteroids.

SUPERNOVA

Within galaxies, as stars were being born, others died...often in enormous cataclysmic explosions. These explosions, called supernovae, are important to the evolution of galaxies because they distribute all the common elements such as oxygen, carbon, nitrogen, calcium, and iron into interstellar space. Explosions of especially massive stars also create and distribute heavier elements such as gold, silver, lead, and uranium. The supernova pictured here is of a smaller type, used by astronomers to determine distance. This one appears to us now as it looked when the universe was about five billion years old.

BIRTH OF STARS AND GALAXIES

As time moved forward, the pull of gravity exerted its influence on the early universe. It amplified slight irregularities in the density of the primordial gas. Even as the universe as a whole continued to expand, pockets of gas became more and more dense. Stars ignited within these pockets. Groups of stars then became the earliest galaxies. Modern telescopes can detect these primordial galaxies as they appeared when the universe was only one billion years old, just 7 percent of its present age

THE BIG BANG

The universe began with a vast explosion that generated space and time, and created all the matter and energy in the universe. Exactly what triggered this sudden expansion remains a mystery. Astronomers believe it involved a runaway process called "inflation," in which a peculiar type of energy that existed in the vacuum of space was suddenly mobilized. The inflationary expansion ended only when this energy was transformed into more familiar forms of matter and energy.