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  • Writer's pictureLuke

The Cosmic Horizons

My last blog post went over the history of how our current model of the universe came to be. This is not the end of the story by far. We will forever be in this story as it's hard to see our understanding of the universe will ever being complete. I’d also suppose that there are many commonly held beliefs today that are just as wrong as the earth being in the center of the solar system.
Artist's logarithmic scale conception of the observable universe with the Solar System at the center, inner and outer planets, Kuiper belt, Oort cloud, Alpha Centauri, Perseus Arm, Milky Way galaxy, Andromeda galaxy, nearby galaxies, Cosmic Web, Cosmic microwave radiation and Big Bang's invisible plasma on the edge.

I mentioned the observable universe at the very end of my last post. Observable universe is to say we limit our universe down to just what we can see. The basic idea is that if the universe is ~13.8 billion years old, and nothing moves faster than the speed of light, we should only be able to see things that are ~13.8 billion light years away (the distance light travels in a year).

It’s a little more complex the observable universe actually has a radius of ~46 billion light years. This is larger than the what we would expect knowing age of the universe because the universe is also expanding. How I like to think of this is say you are baking bread with raisins in it, when you put the dough in the oven you have raisins spaced out 1cm from each other. If you pick any one raisin you will have the closest raisin is 1 cm away, next one is 2 cm away, next is 3 cm, and so on. Now when you bake the bread it doubles in size so now the closest one is 2 cm away the next one is 4 cm away, next is now 6 cm away, and so on.

photo from NASA
As the universe expands things that are farther away move farther away faster. in this picture one raisin is 5cm the other 10cm when baked the bread becomes twice as big moving the near raisin 5cm (to 10cm away) while moving the far raisin 10 cm (to 20cm away) over the same amount of time. The raisins represent galaxies.

We can see ~46 billion light years in every direction because when that light left the most distant objects they were only ~13.8 billion light years away. This gives us a limit to the universe, a horizon ~46 billion light years away (a sphere with a radius of ~46 billion light years). We’ve been able to map this horizon and call it the “Cosmic Microwave Background Radiation”

The Cosmic Microwave Background Radiation (or CMB) is the first light that was able to be released during the big bang. The CMB was discovered by Arno Penzias and Robert Wilson in the 1960s. While working at Bell labs Penzias and Wilson, had noise coming in from a large horn shaped antenna no matter where they pointed it, they evicted some pigeons that were nesting in the antenna, and still had the noise. They determined that the radio signals they were picking up had to be from outside the galaxy but didn’t have an idea of what it could be. Later Penzias heard of the work of Robert H. Dicke who had predicted the CMB and realized it matched the noise he was picking up in his antenna. If you ever pick up static or noise in a radio receiver a small part of it is the background noise is the CMB.

Image from NASA
WMAP (Wilkinson Microwave Anisotropy Probe) image of the CMB (Cosmic microwave background radiation). This is a 2D projection of a sphere just like a world map you can imagine this as a globe with the earth as the smallest speck in the core.

I’ve always found it interesting that we started with the ancient Greeks thinking the stars were just on a sphere that was centered on the earth, and today’s understanding has us in the center a much larger sphere called the CMB. This is the edge of the observable universe which makes it always centered on the observer. If we were Observing from Saturn it would be a sphere with Saturn in the center, if we went to a different planet around a different star in a different a galaxy it would still just be a sphere surrounding us. Even you have a different observable universe centered around you that no one else can observe.

There’s no way to see passed this sphere but there’s nothing that would indicate that the universe is somehow different outside of what we can observe. No one thinks of this sphere in the same way the early astronomers thought of the sphere of the stars was the limit of the universe, but the similarity in their geometry is an odd coincidence.

There’s many things we know we don’t understand: dark energy, dark matter, dark flow, or dark fluid. All of these “dark” things describe different phenomenon that we can see the effects off but don’t know the source

I will save talking about each of these in detail for a later post but since I touched on the universe expanding this brings up “dark energy” nicely.

The universe is actually expanding faster and faster (accelerating). You would think gravity which pulls all things together would be slowing the expansion but our observations tell us the expansion is accelerating. For the expansion to over come gravity and to accelerate it needs to be getting energy from somewhere. Dark energy is the unknown energy source that is expanding our universe. If you think back to the raisin bread analogy, dark energy is like the yeast that makes the bubbles that ultimately makes the bread expand.

Image from NASA
To accelerate the expansion of our universe at the rate it is there must be a lot of Dark Energy. So much that it's thought to be the majority of what our universe is made of!

Our models of the universe are still limited by our technology and what we can see from where we are in the universe. If you look at a map of all the known galaxies you will notice it’s hourglass shaped, and the bottom is a little more sparse than the top.

The hourglass shape is because we are viewing the universe from within the milky way. It’s kind of like if you think about looking at the world being in the middle of a sheet of glass you can see up and down very well but looking edge on everything is obscure and distorted. Looking edge on in our milky way there’s many stars but if you look at any directions away from the plane of our galaxy we see fewer. If we want to look at other galaxies the best way to do so is in the directions where there’s less nearby stars to get in the way.

There's nothing to indicate the dark areas are different than what we can see. No one thinks this shape represents how our universe looks. The hourglass shape just shows off our cosmic blind zones. This is similar to how we assume what's outside of the CMB horizon is the same as what we can see within it.
Our galaxy like this lends lets us look thru the thin axis easily but it's hard to see anything looking through it edge on.

Why there’s so many more in the top part of the hour glass than the bottom is due to the geography of earth. There’s more land, people, and resources in the northern hemisphere that the sky above North America, Asia, northern Africa and Europe is much better cataloged than the sky above South America, southern Africa, Australia, and Antarctica. All of these factors: only being able to see the observable universe, the vast dark holes in our understanding of our universe, and the limitations in our current technology give us our current understanding of the universe. There are many things waiting to be discovered. I feel everyone has something to add to this story, aiding in development of our understanding of the universe.

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