In this post we are going to learn more about what the universe is, what galaxies are and how they are different. So if you want to know what the difference is between a galaxy and the universe then read on.
What Is the Universe?
In order to clarify the difference between any two things you first have to determine what they are exactly so let’s start with the universe. The concept of a universe is something that is all of time and space. It contains everything that is in existence and is believed to have been brought into existence by the Big Bang event.
How the Universe Was Created
In order to answer the question of why galaxies are flat we need to take a closer look at how our universe likely formed. The theoretical Big Bang event is a concept that exists in physics which describes a possible version of our universe expanding from an initial state of high density and temperature. The work of mathematicians and astronomers has combined for decades in trying to prove or disprove this theory based on our observable universe.
Expanding Outwards
Key to the idea of the Big Bang is an understanding that the universe is gradually expanding. In 1912 observations by Vesto M. Slipher indicated redshifting of the light from remote galaxies. This led to the theory that the galaxies may be receding from the earth or basically moving further away.
A decade later using Einstein’s field equations Alexander Friedmann was able to provide theoretical evidence that the universe was indeed expanding.
The Big Bang Process
It Starts with Cosmic Inflation
The theory suggests that at one point all the matter in the universe was held together in an area of infinite density and temperature. This was prior to the Big Bang which in truth was more of an inflation outwards. Around 13.8 billion years ago it is thought that the universe began to expand outward from this dense mass.
The universe is thought to have expanded faster than the speed of light for a fraction of a second. This is a theoretical period known as cosmic inflation. Scientists aren’t sure what initiated this inflation or what may have powered it.
Cosmologists believe that inflation may explain a number of observable aspects of the universe as it is today. These include its flatness or lack of curvature.
Next the Big Bang and Nucleosynthesis
When this cosmic inflation stopped, the energy that was driving it is thought to have transferred to matter and light. This new abundance of matter and light is what is described as the Big Bang. In the second following the Big Bang it is theorized that the universe consisted of an extremely hot primordial soup of light and particles. This hot soup is estimated to have been 18 billion degrees Fahrenheit or 10 billion degrees Celsius.
Within minutes, an era known as nucleosynthesis began which saw protons and neutrons colliding to produce the earliest elements. These would include:
- hydrogen
- helium
- lithium
- beryllium
After the first 5 minutes it is thought that most of today’s natural helium had been formed. The Initial expansion and cooling of the universe had also reached a point to where no further element formation took place.
The universe however was still too hot for the atomic nuclei of these elements to capture electrons which would be necessary for them to form complete atoms.
Recombination
It would take 380,000 years for the universe to cool enough for atomic nuclei to capture electrons. This is a period referred to by astronomers as the epoch of recombination. There would be two major effects of this.
The first effect was that the cosmic fog of free electrons was cleared allowing the universe to become transparent. This would also allow light to travel freely over great distances.
The second effect was that the formation of these first atoms produced light energy. We can still detect the glow of the formation of these first atoms to this day. This glow is known as the cosmic microwave background.
The Universal Dark Ages
In human history we refer to dark ages as periods in which a nation sees no new advances. It is a period of little advancement in knowledge and perhaps even decline. Cosmically speaking the universe would enter a dark age after the cosmic microwave background was formed.
The hydrogen atoms would absorb the shorter wavelengths of the existing light and as yet stars had not started to form. Over the next 200 million years our universe remained dark. It was simply a sea of hydrogen, helium and other trace elements in atomic form.
The First Stars
The gases of our universe were not spread out evenly and there would be so called clumps that had denser clouds of gas. As the clumps grew they would become denser and more compact. At the center of these dense clumps temperatures became hotter eventually leading to nuclear fusion.
This nuclear fusion essentially lit the stars which would have roughly been 30 – 300 times the size of our own star the sun. It would take several million years but eventually these new stars would collect together into galaxies.
Finally the Reionization Period
In the early lives of stars their light still could not travel far, relatively speaking, due to it being scattered by the dense gas clouds that surrounded them. However over time the ultraviolet light emitted by the stars would break down these clouds.
The UV light did this by ionization of the gas atoms breaking them into their constituent electrons and protons. With the progression this reionization allowed starlight to travel further and unobstructed. Within the first billion years the light given off by stars had transformed nearly all of the gas making the universe transparent and as we see it today.
Is There More Than One Universe?
It is certainly a popular topic in science fiction that other universes exist but it is also a theory within scientific studies as well. Theories exist that our universe is just one of many disconnected universes. There are many reasons for such theories to come about although at present no proof has been established that other universes exist outside of our own.
What Is a Galaxy?
A galaxy is basically a system of stars, stellar remnants, gasses, dust and dark matter which is bound together by gravity. Over time planets may form as can other bodies within the confines creating versions of what we have in our own galaxy the Milky Way.
It is estimated that there may be over 200 billion galaxies in observable space and range on average between 3,000 – 30,000 light years in diameter. Our galaxy, the Milky Way, is roughly 87,400 light years in diameter meaning at light speed it would take us over 87,000 years to cross from one side of the galaxy to the other.
Why Are Galaxies Flat?
Invariably when we view galaxies through things like the Hubble telescope they appear stretched out and flattened in shape. If our universe is vast, why do galaxies appear this way? The answer is actually quite simple according to NASA.
Galaxies start life as huge balls of slowly rotating gas. As they begin to slowly collapse in on themselves their rotation becomes faster and faster. This turns the once ball-shaped mass of gases into a more stretched out pizza like shape. In fact you might even think of it spinning a fresh pizza dough in the way it stretches and flattens.
Essentially anything that may try to escape upwards or downwards is pulled back into the ensuing disc of the galaxy. The rotation is a very complex version of conservation of angular motion. A more basic version of this same theory would be spinning around in an office chair with your arms and legs out and then pulling your limbs in to reduce friction.
The galaxy compresses more as it speeds up and speeds up more as it is compressed. This creates the flattening effect we see.
It is important to note that some galaxies are flatter than others. Some may bulge out more while others may still be almost spherical. This does not mean these other galaxies will not eventually flatten because the same forces and laws of physics tend to apply.
What Is the Difference Between Galaxies and the Universe?
The terms universe and galaxy share one common point, they are both terms related to space. Beyond this however the key difference between the two can be compounded by considering a galaxy as an apple and the universe as a fruit bowl.
Basically galaxies are part of the universe. Everything that exists does so within the universe as this is the term we give to the vast expanse of space and time that surrounds us. The galaxies just make up small portions of this space and contain stars, planets, asteroids and other bodies in space.
Final Thoughts
To try and further clarify the difference between galaxies and the universe, galaxies are part of space and there are billions of them. The universe is space, everything that exists is found in the universe. So in theory if we had a spaceship fast enough we could leave a galaxy but we would never be able to leave the universe.