The Expanding Universe

FAS Astronomers Blog, Volume 31, Number 4.

This is part 3 of a 5-part series on the Universe (The Visible Universe, The Dark Universe, The Expanding Universe, The Microscopic Universe, and The Multiverse).

For a long time, people thought that the universe was static and everlasting. It was here. It had always been here. And it always will be here. However, astronomers discovered that the universe is expanding, and if we trace that expansion back in time, it also had a beginning.

We know this because of three important discoveries.

1. Albert Einstein’s general theory of relativity works for an expanding (or contracting) universe, but not a static one. Various folks including Einstein, Willem de Sitter, Alexander Friedmann, and Georges Lemaitre solved Einstein’s equations and came up with a mathematical description of an expanding universe.
2. Edwin Hubble, along with Vesto Slipher and Milton Humason, measured the doppler shift of light coming from distant galaxies. They found that the light from most galaxies is “red shifted” and, as such, these galaxies are all moving away from us. They also found that the farther a galaxy is from us, the faster it is moving away – something that is captured as Hubble’s law.
3. Arno Penzias and Robert Wilson were measuring the microwave radiation coming from different parts of the sky. They found a strange hum in their data that seemed to come from everywhere. It turned out that this hum was the Cosmic Microwave Background (CMB) – radiation left over from a hot and energetic early universe.

The universe isn’t expanding into something; it is just expanding. The best analogy is the surface of a balloon or a loaf of bread with nuts (many people use raisin bread, but I like nuts better). If you draw some small circles on a balloon and then blow it up, all the circles will move away from each other as the balloon is expanding. As you bake your loaf of bread, all the nuts will move away from each other as the bread expands. The universe is expanding in much the same way.

So, let’s find out some more about our expanding universe.

The Beginning of the Universe

The universe began with something called the Big Bang. It wasn’t really an explosion in the traditional sense. It wasn’t big and there really wasn’t a bang. In fact, the term Big Bang was coined by astronomer Fred Hoyle who didn’t believe that the universe had a beginning. He might have been somewhat sarcastic when he used the term (although I think he claimed he wasn’t). However, the Big Bang has stuck with us, and everyone uses it today.

We don’t know what was there before, but around 13.8 billion years ago, something happened. First there was nothing, then there was an extremely hot and dense ball of energy. Initially everything was condensed into the size of a subatomic particle, and the four fundamental forces of nature were combined into a single unified force.

Astronomers think that the universe then went through a period of cosmic inflation. This is when the universe expanded by a huge factor in an infinitesimal fraction of a second. In doing so, it became the size of a grapefruit. Inflation stopped and from then on, the universe began a long steady expansion.

Shortly after the Big Bang, the universe experienced many changes (see The History of the Universe). The four fundamental forces of nature (Strong, Weak, Electromagnetic, and Gravity) quickly split off from the single unified force. As the universe cooled, energy was converted into matter. First electrons and quarks formed. Then quarks combined into protons and neutrons, which became atomic nuclei. These nuclei combined with electrons to form atoms of hydrogen, some of which, in turn, fused into helium. All the hydrogen we see today is from the Big Bang. So next time you drink some water (H₂O), you’re drinking a little bit of the early universe.

The universe continued to cool. For a while, it was too hot for light to escape. Photons of light just bounced around and the universe was an opaque cloud that we can’t see through even today. Eventually, the universe cooled enough so that light could escape. We see this as the cosmic microwave background (CMB), which is light from the early universe that has stretched out and cooled down to a temperature of 2.7^o above absolute zero.

As an aside, you might wonder why the picture of the CMB looks like an oval and not a circle. That’s because it is a curved image mapped onto a flat space. The mapping used what is called the Mollweide projection, which preserves equal areas. See an earlier article, Mapping the World, for more about this.

Today, the visible universe extends across a diameter of around 92 billion light years, and it’s continuing to expand as you read this. We ourselves are not expanding, nor are the planets, stars, or galaxies. It is space that is expanding, and this expansion is pushing galaxies away from each other. We see this because the light from distant galaxies is “red shifted.” That is, the wavelength of light we see is stretched out as most galaxies are moving away from us. The farther a galaxy is from us, the faster it is moving away. This expansion rate is expressed using Hubble’s constant, which is around 70 (km/s)/Mpc. You can find more about Hubble’s constant in an earlier article on Cosmic Distances.

The End of the Universe

We’re not sure what will eventually happen to the universe. A lot of it depends on how dense it is, which in turn determines its geometry (flat, open/hyperbolic, or closed/elliptic). The visible universe appears to be flat. However, we don’t know what’s beyond our horizon. The universe could continue on forever or it could curve back in on itself. Based on this, the universe will either continue to expand and eventually a deep freeze will result, or it will eventually collapse in on itself resulting in a big crunch. There’s more to this in another article, Geometry, Omega, and the Universe.

The Formation of Stars

During the early days of the universe, first generation (population III) stars began to form. These early stars were huge and made mostly of hydrogen. When they ran out of hydrogen, they exploded in what we call a supernova. Supernovae explosions created many of the heavier elements we see today. Second generation (population II) stars then formed from remnants of these early stars. The newer stars burned hydrogen, but they also burned heavier elements creating even more heavier elements. If they were large enough, they too exploded in supernovae. Eventually, 3^rd generation (population I) stars formed, including our Sun.

Over time gravity pulled the remnant material from supernovae explosions into nebulae consisting of gas, dust, and ice. The Orion nebula is one such nebula that can be easily seen from the Earth. Population I stars, such as the Sun, form within these nebulae.

We think the star formation process goes something like this. A supernova explosion disturbs a cloud of gas, dust, and ice within a nebula. Gravity slowly takes over. Over a long period of time, gravity causes the cloud of material to collapse into a large ball made mostly of hydrogen. Gravity continues to compress the ball of hydrogen and it becomes extremely dense and hot. Eventually, the core becomes so hot that nuclear fusion reactions begin to fuse hydrogen into helium. This creates a huge amount of energy, and a star is born.

Stars tend to form together, but drift apart over time. We can see some young stars in open clusters such as the Pleiades. More often than not, stars also form in pairs and over half of the stars have companions. The Sun is an exception, and, as far as we know, it is a solo star.

More on this process and stars in general can be found in an earlier article about stars.

The End of Stars

All stars will eventually use up their supply of hydrogen. Don’t worry, the Sun has another 4 to 5 billion years before it uses up its hydrogen. The outer layers of small stars like the Sun will then expand into a red giant while the core will shrink into a white dwarf. Larger stars will expand into a supergiant and then explode as a supernova. The remaining material left over after the supernova will collapse into either a neutron star or a stellar sized black hole depending on the mass of the star.

The Formation of Planets and Moons

After a star is formed, a small fraction of the star forming material remains. This material swirls around the newly formed star and flattens into a disk. Over time, gravity again takes over, and the disk begins to clump into spheres. These spheres eventually coalesce into planets.

• The heat of the star burns off much of the gas and ice in the inner part of the disk – inside the frost line. So, only dust is left and small rocky planets form.
• Outside of the frost line, gas and ice are still plentiful. Here large gas and ice giant planets form.

Most moons are found in the outer part of the Solar System. There is more left-over material after the formation of the large outer planets. This material swirls around each of the planets and, over time, gravity collects it into spheres that become icy moons like those we see around the outer planets in our Solar System.

There are three moons in the inner part of the Solar System: The Earth’s moon along with Phobos and Deimos (moons of Mars). Phobos and Deimos are most likely asteroids that were captured by Mars’ gravity. The Earth’s moon is a bit unusual. Most astronomers adhere to the Giant Impact Theory, which says a Mars sized object called Theia smashed into the Earth and knocked material out into space. The material swirled around the Earth and eventually coalesced into our moon.

After the Sun, planets, and moons formed, there was still some material remaining. We find this material as chunks of rock and ice in the asteroid belt between Mars and Jupiter, the Kuiper belt outside of the orbit of Neptune, and the Oort cloud way out around one light year from the Sun.

Creation of the Elements

All of the hydrogen and much of the helium found today along with a few other trace elements were created in the Big Bang. The rest were formed within stars. All stars “burn” hydrogen, transforming it into helium. Once their hydrogen supply is gone, large stars successively “burn” helium and then other elements (carbon, oxygen, …) eventually creating lead as they expand into supergiants. They then explode as supernovae during which heavier elements such as gold, silver, and uranium are formed.

Many of these heavy elements, such as uranium and radium, are unstable. We say they are radioactive – they emit radiation and transform into lighter elements. As an example, uranium decays through a long process that eventually results in lead. The time over which an element decays into something else is called its half-life. This is the time required for half of its mass to be transform into the lighter element.