3 issues

[Jurek]

Hi, yet at school I was thinking about several questions which so far have not found a satisfactory answer. Probably not once one has it erected. Perhaps here is a good opportunity to clarify these issues?

1. Centre of the Universe:

We observe far objects rushing away from us in any direction we do the observations what suggests, the move away from each other and consequently from some alleged center i.e. area or point the matter was concentrated earlier. On the other hand, the further the objects we observe the closer to the beginning of the Universe we are. The most distant observations are dated back 14 billion years, which is relatively close (100 million years as ) to the hypothetical Big Bang as I remember. The paradox is that our observation in any direction - especially in the opposite ones - brings us to the same starting point from which on the other hand objects are moving away. All this points on the edge of the observable Universe are the same starting point?

2. The accelerated expansion of the Universe :

Observations indicate that the further the objects are observed the bigger speed they move with away. However, this also means that the sooner after the beginning of Universe was the speed was higher and the later, the speed becomes lower. This observation suggests the matter slows down and not accelerates as it is said, doesn`t it?

3. Age of the Universe and our position:

We are observing most distant objects from about 14 trillion years of light what also is estimated at a little less than the age of the Universe. The speed of light is the largest and - as assumeed - impassable. However, in order to observe the most distant objects spaced 14 trillion years of, the first we (i.e. the matter we are built of) had to depart from them. How could the matter we are built of be able to travel the distance light travels in 14 trillion years, which is almost the assumed age of the universe since the speed of matter can only be a little fraction of the speed of light?

[CaptainPanic]

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Moderator Note

 

[update]

I had closed this thread, and requested that the 3 issues be discussed separately. One of our experts on the forum suggested however that this can be discussed in a single thread, and therefore the thread has been re-opened.

 

I apologize for any inconvenience.

 

Edited September 1, 2014 by CaptainPanic

[Strange]

I wouldn't call myself an expert ... And now I have to attempt an answer!

 

I will give a quick answer now, as I don't have much time, and then add more detail later. All of these questions revolve around understanding the concept of "expanding space".

 

 

We observe far objects rushing away from us in any direction we do the observations what suggests, the move away from each other and consequently from some alleged center i.e. area or point the matter was concentrated earlier.

 

An important point here is that this is due to everything (on sufficiently large scales) moving away from everything else. As a result, wherever you are in the universe, you will see the same thing: everything moving away from you.

So we are are in the center of our "observable universe". But so is every other point in space. But the universe as a whole doesn't have a centre. One way of thinking about this is to "wind the clock back" and imagine the universe shrinking instead of expanding. All the gaklaxies in the universe will get closer and closer ... until they are all compressed into the same tiny space. So everywhere is where the big bang started. Everywhere is the "centre".

 

 

The paradox is that our observation in any direction - especially in the opposite ones - brings us to the same starting point from which on the other hand objects are moving away. All this points on the edge of the observable Universe are the same starting point?

 

I don't really follow this. The most distant galaxies we can see were already a few billion light years away when the light we see now was emitted. They are now about 45 billion light years away (I think).

 

 

2. The accelerated expansion of the Universe :

Observations indicate that the further the objects are observed the bigger speed they move with away.

 

Note that this is not the accelerating expansion. This is just expansion. It is a simple consequence of geometry.

 

Consider a number of galaxies separated by the same distance (far enough apart that the expansion of space is significant and is the same between all of them).

At time 0, they are 1 unit apart:
A.B.C.D.E.F

After some time they are 2 units apart:
A..B..C..D..E..F

After the same time again, they are 3 units apart:
A...B...C...D...E...F

And so on:
A....B....C....D....E....F

Now, if we look at the distance between B and C, for example, it increases by 1 at every time step. But the distance between B and D increases by 2 at every step. So the distance between B and D is increasing twice as fast as the distance between B and C; i.e. the speed of separation is twice as great.

Choose any pairs of galaxies and you will see that apparent the speed of separation is proportional to the distance between them. Take two objects far enough apart and the speed of separation will be greater than the sped of light. (But that is OK, because the speed of light limit is a local thing, whereas these objects are in different frames of reference - see below.)

 

Now, more detailed observation and analysis showed that at some point in the past, this rate of expansion increased and so the speed of separation at any given distance increased.

 

 

How could the matter we are built of be able to travel the distance light travels in 14 trillion years, which is almost the assumed age of the universe since the speed of matter can only be a little fraction of the speed of light?

 

This is an important, but quite subtle, point. Firstly, note that the "speed limit" comes from Special Relativity which is an approximation that only applies locally. So something nearby cannot move through space faster than the speed of light.

 

However, the expansion of space is described by (and is an inevitable consequence of) General Relativity. In this case, the expansion of space between two points is not limited. As we can see from the description above you can always choose two points which are sufficiently far apart that the total amount of space between them will cause them to appear to be separating faster than light. This has always been true. There are now, and always have been, galaxies we can see which are moving away at greater than light speed.

 

Hope that helps a bit. It is quite a confusing subject!

 

More answers and detail here: http://www.astro.ucla.edu/~wright/cosmology_faq.html

[swansont]

There is no center — expansion means all points are moving away from all other points (ignoring local motion and gravitational effects). You would see the same kind of expansion from any point.