# Inflation (Big Bang Theory) question

I have read some stuff on Inflation recently, and in one of the papers the author talks about how the galaxy looks the same on opposite sides of the horizon. My question is; what the hell is he talking about? Is there some dividing line in the middle of the sky , or something?

If you’re talking about what I think you’re talking about, scientists looked in a bunch of directions, and they always saw the same thing. So you can pick any dividing line you want, and the universe will look the same on both sides.

Since the universe is a certain finite age, about 15 billion years at current estimates, we can’t see anything that is more then 15 billion light years away, as the light traveling from it will not have reached us yet. Thus the observable universe forms a sphere with a radius equal to the age of the universe times the speed of light, around the observer. The line that seperates us from the observable universe is what cosmologists usually mean when they refer to a “horizon”.

Since the universe is a certain finite age, about 15 billion years at current estimates, we can’t see anything that is more then 15 billion light years away, as light traveling from this distance will not have reached us yet. Thus the observable universe forms a sphere with a radius equal to the age of the universe times the speed of light, around the observer. The line that seperates the observable universe from the unobservable universe is what cosmologists usually mean when they refer to a “horizon”.

Then, if a person says that the Universe (not the Galaxy, which is much, much smaller) looks the same on the other side of the horizon, it means that there’s nothing special about that boundary, as far as the Universe as a whole is concerned, and we just think it’s special because of the way we see it. Someone on the other side wouldn’t see the exact same things that we see, but they’d see about the same sort of distribution of galaxies and such, and their very own horizon around it all.

First of all, I am not an astronomer.

I am not sure what you are referring to, but I notice that you said the galaxy looks the same no matter where we look. I suspect that is not correct–after all, we have been able to resolve the shape of our galaxy, and know that we reside in one of the spiral arms. So the galaxy definitely does not look the same if we look in different directions.

On the topic of the Universe, things are a little tricky. The most important thing to keep in mind is that all these discussions involve the visible Universe. Who knows what lies beyond? We can’t even be sure space and time are viable concepts out there.

The Universe is generally accepted as being between 10 and 20 billion years old. 15 billion years is a popular number among many scientists.

The Big Bang is easily explained. The Big Bang states that the Universe began at a single point and expanded outwards (in other words, the most powerful explosion ever).

If you look at the Universe in any given direction, yes, it does look the same. More or less anyway, since there is some debate about galactic clusters. The Universe has the same temperature, density, and appearance, and we therefore say it is homegenous.

Inflation goes hand in hand with the Big Bang, and is the concept used to explain why the Universe is homogenous. At one time, the story of inflation goes, our visible Universe expanded very rapidly from a smaller size. The result is that the same attributes (temperature and density) were reatined throughout the Universe in spite of the incredible distances from one edge of the visible Universe to the other (this is also strong evidence for the Big Bang).

Unfortunately things get really complicated here. If the furthest objects (like Quasars) we can see are 15 billion light years away, and therefore 15 billion years old, here’s a few questions that bother me:

1. where the heck are they now?? In 15 billion years, they could have ended up anywhere, and our Universe could actually be vastly bigger than we suspect.

The objects next to the horizon are 15 billion years old, and 15 billion light years away. Since the Big Bang postulates that all matter and energy resided in one single point:

1. how fast did those objects cover the distance?? They didn’t magically appear on the border of the visible Universe, so how did they get there and how quickly?

2. Were they or were they not moving at the speed of light?

If they were moving at the speed of light during inflation, and they are 15 billion light years away, and their light has taken 15 billion light years to reach us, and the objects had to somehow cover 15 billion light years BEFORE they got to where they were 15 billion years ago, then perhaps these objects are actually 30 billion years old (15 billion light years away, plus an observable distance of 15 billion light years away). If they were moving at a much lower velocity the Universe must be older.

It’s enough to give a non-astronomer a headache.