Questions about expansion

The non amateur stuff. Hawking, black holes, that sort of thing

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johana
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Questions about expansion

Post by johana »

Hi,about the expanding universe:
I have some questions that I do not get any answers for. Can somebody please help.
1. Why are the objects closer to earth(or our galxie) moving away slower than those far away? What do we have to do with the whole situation.
2. What force is eccelerating all that mass in space. If it is black matter, why are they evenly spread around the visible galaxy to achieve such even acceleration in all directions.
3. If you go backwards in time and reverse the expansion, then you end up with all matter in one heap, how do you start the expansion from there.
Thanks,
johana.
big_kev
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Post by big_kev »

Hi Johana,

In answer to your questions.

1. The universe is expanding in all directions, its a bit like dots on a balloon , as the balloon expands all dots move away from each other and the ones further away move faster.
No dot on the balloon is special, whichever dot you start from it will appear the same.

2. The force accelerating this is "dark energy" , which is also known as "vacuum energy" ....this is still hypothetical at the moment but is the accepted reason.
Please note that "dark energy" is not related to "dark matter".

3. The accepted reason for the original expansion is "inflation" whereby the universe increased in size from something very small to something very much bigger in a very very small fraction of a second.
"inflation" is very complicated and you may wish to read further about it.
brian livesey
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Post by brian livesey »

The analogy of the expanding ballon with dots on it moving apart, can be a bit misleading.
Objects in the universe have their own motion , so not everything is "moving apart". For example, the Andromeda galaxy, M31, is on a collision course with our galaxy.
Galaxies are stll merging, regardless of universal expansion.
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Deimos
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Post by Deimos »

brian livesey wrote:The analogy of the expanding ballon with dots on it moving apart, can be a bit misleading.
Objects in the universe have their own motion , so not everything is "moving apart". For example, the Andromeda galaxy, M31, is on a collision course with our galaxy.
Galaxies are stll merging, regardless of universal expansion.
I disagree and think the balloon analogy is a good one. The expansion of the Universe (as per the balloon) applies except where local gravitation binds groups of objects together. In the case of Andromeda (and other galaxies in the local group and other galaxies in their own clusters, etc.) there are gravitational forces which outweigh the expansion.

Ian
brian livesey
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Post by brian livesey »

I'm sure you'll appreciate, Ian, that gravitational forces impart motion. How else could galaxies merge without individual movement?
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Deimos
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Post by Deimos »

brian livesey wrote:I'm sure you'll appreciate, Ian, that gravitational forces impart motion. How else could galaxies merge without individual movement?
Exactly, as I said, the balloon analogy is good, but it does not apply to gravitationally bound objects (same as expansion of the Universe), as it does not apply to the length of my car, etc., etc.

Ian
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Post by brian livesey »

In popular science, the dots on the balloon are represented as galaxies moving apart, giving the false impression that all galaxies are moving away from each other.
As we know, this is an oversimplification.
brian
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Post by Davej »

Hi,
Reguarding the "balloon" analogy where everything is moving apart, what is left behind ? ie inside the "balloon" and at the centre where the "big bang" occurred :?
ATB
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Deimos
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Post by Deimos »

Davej wrote:Hi,
Reguarding the "balloon" analogy where everything is moving apart, what is left behind ? ie inside the "balloon" and at the centre where the "big bang" occurred :?
ATB
Dave
The balloon analogy is a 2 dimensional analogy to illustrate stretching. Some people extend the analogy so that rather than points on the surface there are sticky labels (which don't stretch). But it is reduced dimensions.

Ian
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Post by joe »

Davej wrote:Hi,
Reguarding the "balloon" analogy where everything is moving apart, what is left behind ? ie inside the "balloon" and at the centre where the "big bang" occurred
Hi Dave,

For me, this is one of the crucial points about the BB and the balloon analogy; there is no "inside the balloon" - disregard it. The skin of the balloon represents three dimensions. Imagine the balloon being many millions of light years in diameter. The surface would then appear flat to an onlooker standing on one of the spots.

It's easy to imagine the skin representing length and breadth - take off in one direction and you eventually end up where you started. Imagine now that the skin has depth. You can not only travel along the surface but up and down "in" the surface. If you travel up or down, you eventually end up where you started - just as you did when you traveled in the other dimensions of length and breadth.

There is no centre because the "place" where the BB occurred is the whole of the universe. It has expanded, therefore, the place where the BB occurred is everywhere.
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Post by Davej »

Hi Joe,
Thanks very much for the explanation, I think I was taking the balloon analogy too literal (ie just two dimensional (thanks also to Deimos)).
I just find it a bit hard to comprehend that the universe isn't expanding into space (which doesn't yet exist), it is creating it as it expands.
All the best.
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Cliff
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Post by Cliff »

Dear Johana et al(L)
When I was a younger I was absolutely sold on the Big Bang. although if I recall correctly when I very first got interested in astronomy the term "Big Bang" had not yet then been invented.
I understand it was an advocate of the so-called steady state universe, Fred Hoyle who about 1950 coined the phrase Big Bang meaning to refer to the expansion of the Universe idea in a derogatory way, on a radio programme.
However, in my simplistic way of thinking the Big Bang sounded exciting so it had to be right.
Irrespective of me then knowing very little of what was then known, since that time , and even though I like to think I know more now than I used to, cosmology has meantime got a lot more complicated and I feel further behind now than I ever did.
With regards the expanding balloon idea, I can see it might be helpful for some in trying to understand Big Bang theory. However, in my own case having sort of grown up with the Big Bang and loving it (particularly when I first got interested in astronomy) I was happy with my own simpe ideas about it from the (my) start.
That said I never needed the "expanding balloon" concept, and now I tend to agree with Brian, that it is a bit (or a lot) too simplistic and may do more confusing harm than good.
I now quite confused about the development of the Universe and sceptical about cosmology, half believing several apparently somewhat different theories or ideas.
I think the need to introduce "Inflation" was the first thing that reduced my confidence in the Big Bang theory.
Another thing in particular that bugs me is String Theory according to which there may (perhaps even must) be eleven or possibly more dimensions.
Now a big old fashioned side of me cringes at such an apparently Sc-Fi nonsnsical idea. It has occurred to me that the "two dimensional expanding balloon model" supposedly representing three (or possibly even four) dimensions may have contributed to the idea if we can use a two dimensional model to illustrate 3 or 4 dimensons, then why couldn't there actually be even more dimensions.
One part of me says that is nonsensical rubbish, but another part says well suppose it might be possible that there are many more than three or four dimensions.
Me not understanding String Theory (or theories) maths but accepting the maths is probably correct doesn't help.
So I am something of a fence sitter, but if I could only have a single choice, I'd opt for only four dimensions, up, down, in (or out) and time.
As for Dark Matter and Dark Energy I do my best to keep an open mind but believing humans will never properly understand cosmology and might even be getting it very wrong.
Best of luck from Cliff
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Basics

Post by ukmtk »

This is my relatively poor understanding of current thought. What one must remember is that science is quite exact. What generally happens is that people come up with ideas (theories) and then propose tests for those theories. Some tests prove the theories others make the theory doubtful, leading to a change in the theory (or even replacing it).

The two greatest theories in modern physics are: (1) general relativity and (2) the standard model of particles. These have been tested to great degrees and have proved very resilient (tested to parts in a billion or better).

The reason that string theory is being proposed is a unification theory. The standard model and general relativity disagree at the extremes (i.e. small scale for general relativity and large scale for the standard model).

String theory used to require 26 dimensions! This was narrowed down to 11 (10 space + time). But there are some physicists who are not overly happy with string theory and think it is a bit like the emperor's clothes!

The reasons for dark energy + dark matter are experimental. A test was devised to see if the universe is open or closed. I.e. will it expand forever or collapse back on itself? The experiment showed that it would expand forever. It also appeared to show that the expansion was accelerating. This was totally unexpected! One of the occasions where experiment disagrees with the theory. :D

The dark matter is required because it turns out that galaxies + clusters of galaxies are more massive than what appears to be in them (i.e. all the stars). So the assumption is that there is something out there that we can't see but it is big and massive. There are various theories as to what the dark matter is made of and various experiments being done to work out what it is.

What I always do is think how far the understanding of the universe has come in just over 100 years (end of the 19th century to today). If you want to see how it has changed read something like "The Story of the Heavens" by R.S.Ball. This was the knowledge of astronomy at the end of the 19th century. It was one of the first astronomy books I read as a kid (from a second hand shop). There are some parts of it that would now make you laugh.
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Post by joe »

Cliff wrote:I'd opt for only four dimensions, up, down, in (or out) and time.
Isn't that only three? You're trying to confuse us even more, Cliff. :D
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Cliff
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Post by Cliff »

Dear Joe
Don't (a)cross me.
from the invisible snowman
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