Exploring Universe Expansion, Dark Matter and Gravity

[K Rool]

I have some questions that I have been wondering about for a while. I'm not a physicist, and I don't know the math behind anything. Just a guy who is thinking up some questions. Just think of me as a student asking some questions. I know these questions can be answered and have been thought of. Some random guy like me isn't going to shake the foundations of astronomy. Sorry if I have a lot.

Why do we think entire Universe is expanding? Why can't the Big Bang/Inflation just be a big explosion/inflation in only a small part of the Universe?

Why do we think space came with the Big Bang/Inflation? Is there a reason why we think space didn't exist before everything?

Galaxies rotations have us think there is Dark Matter at play. Why do we think that? (Stupid question) Is it possible gravity acts different at larger scales? Do Galaxy clusters rotate, and if they do, do they show evidence of dark matter as well?

You don't have to put any of your answers in Layman's terms. Speak to your hearts content.

 

[Dickfore]

I have some questions that I have been wondering about for a while. I'm not a physicist, and I don't know the math behind anything. Just a guy who is thinking up some questions. Just think of me as a student asking some questions. I know these questions can be answered and have been thought of. Some random guy like me isn't going to shake the foundations of astronomy. Sorry if I have a lot.

Why do we think entire Universe is expanding? Why can't the Big Bang/Inflation just be a big explosion/inflation in only a small part of the Universe?

Why do we think space came with the Big Bang/Inflation? Is there a reason why we think space didn't exist before everything?

Galaxies rotations have us think there is Dark Matter at play. Why do we think that? (Stupid question) Is it possible gravity acts different at larger scales? Do Galaxy clusters rotate, and if they do, do they show evidence of dark matter as well?

You don't have to put any of your answers in Layman's terms. Speak to your hearts content.

The answers to all your questions are model dependent and impossible to be answered precisely without the use of the necessary math that is involved in the construction of those models. Physics is a science written in the language of mathematics. What you are asking is for us to translate this language into common English language. What makes you think all the concepts from mathematics have a term in the common English language? For example, some tribes from the Amazon have a counting system with only the concepts of one and many? How do you explain in their language that two is different from three, then?

 

[K Rool]

There's no possible way? Not even a, "Well [insert name here] found that this...and [insert name here] discovered that."

 

[Dickfore]

Reliance on authority is not a scientific method. It doesn't matter who said it, what it matters is how he proved it.

 

[K Rool]

Introductory astronomy and space television shows seem to do it fine. They don't show any math. Is it possible you could say, "Well, math shows..." so on and so forth.

 

[The riddler]

I'll try to answer each of your questions as simply as possible and I am probably not going to use math.

Why do we think entire Universe is expanding?
The reason why we think the universe is expanding is because stars and galaxies are moving away from us. We know this due to redshift, this is where light emitted by stars/galaxies moving away from us is (from our point of view) stretched out showing a lower frequency than that of stars that are static. This is also known as the doppler effect.

Why can't the Big Bang/Inflation just be a big explosion/inflation in only a small part of the Universe?
To describe my answer to this question accurately it would help to know the shape of the universe itself. There are many different theories about the shape of the universe, such as the theory universe is infinetly big or the theory it is simple a 4 dimensional sphere. But in either of these theories and probably a lot of others the idea that the Big Bang/Inflation happened in all the universe due to the reason below.

The way in which the universe is expanding gives the idea that the universe has no centre. I like simplifying how i think of the universe by instead imagining a giant balloon, imagine within the skin of the balloon are stars with orbiting planets which in turn contain tiny humans that can only see and move inside the 2 dimensional rubber surface of the balloon. As this balloon is filled with more and more air the rubber surface expands causing the stars to move away from each other. The fact that every star inside this rubber balloon is moving away from every other star at exactly the same speed means this universe cannot be assigned a centre (not 2 dimensionally anyway).

The only difference between our universe and the balloon universe is that ours has an extra dimension. Imagine that on a small part of the balloon the the Big Bang happened causing the creation of stars and so on. This small part of the balloon begins expanding causing stars to move away from each other. Unlike in the other Universe where the whole universe is expanding and stars are moving away from each other with no centre, this universe has a centre and thus the different stars are moving away from each other at different speeds depending on how close they are to the centre.

Evidence shows that the expansion of the universe is dragging stars/galaxies away from each other with the exact same force, this indicates that the exapansion has no centre and thus is affecting the whole universe and not just a small part of it.

Why do we think space came with the Big Bang/Inflation?
We think space came with the Big Bang because there theoretically was no universe, space, or absolutley anything else before it happened.

Is there a reason why we think space didn't exist before everything?
We believe that the Big Bang was the beggining of time and Einsteins theory of relativity states that both space and time are interlinked, suggesting they were created together.

Galaxies rotations have us think there is Dark Matter at play. Why do we think that?
Although we do not know exactly what dark matter is we assume it's there because it causes stars on the outer edges of galaxies to move faster than they should by Einsteinian laws of physics.

Is it possible gravity acts different at larger scales?
I really don't know, its possible but its probably unlikely under our current laws of physics.

Do Galaxy clusters rotate, and if they do, do they show evidence of dark matter as well?
Galaxy clusters do rotate and the fact that galaxies contain dark matter probably effects the rotation of the clusters.

I hopes this helps :)

 

[Ich]

You already got some good answers, here'my take:

Why do we think entire Universe is expanding? Why can't the Big Bang/Inflation just be a big explosion/inflation in only a small part of the Universe?

As long as this "small part" is much bigger than the observable universe, this may be the case.

Why do we think space came with the Big Bang/Inflation? Is there a reason why we think space didn't exist before everything?

We actually know nothing about before inflation. Time may have started there, or not.
It's rather the simple math models ("FRW universe") that end at the Big Bang. Maybe in reality there was something before, maybe not.

Galaxies rotations have us think there is Dark Matter at play. Why do we think that?

Because our theory of gravity should be quite reliable at these small scales, because it works, and because there's a lot of other evidence that hints in the same direction.

Is it possible gravity acts different at larger scales?

Yes. But nobody figured out how such a deviation could mimick dark matter.

Do Galaxy clusters rotate, and if they do, do they show evidence of dark matter as well?

DM was hypothesized initially by Zwicky to explain exactly the behaviour of galaxy clusters. Rotation curves, BAO, WMAP, rotation curves, lensing, all came later and supported the hypothesis.

 

[K Rool]

Yes. But nobody figured out how such a deviation could mimick dark matter.

Well, I was just thinking there is no dark matter. Gravity doesn't act the way it does at quantum levels. Perhaps these galaxies are rotating the way they do because gravity doesn't work the way it should at that level. Just a thought. I'm uneducated in the whole subject. I'll still believe in dark matter, though. Innocent until proven guilty. Thanks for the answers, guys.

 

[Nabeshin]

Well, I was just thinking there is no dark matter. Gravity doesn't act the way it does at quantum levels. Perhaps these galaxies are rotating the way they do because gravity doesn't work the way it should at that level. Just a thought. I'm uneducated in the whole subject. I'll still believe in dark matter, though. Innocent until proven guilty. Thanks for the answers, guys.

What you are essentially describing is MOND. I personally am not much of a fan, but you can read up on it here:
http://en.wikipedia.org/wiki/Modified_Newtonian_dynamics